14C Food Crust Analysis on Mesolithic Pottery

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Faculty of Physics and Astronomy

University of Heidelberg

Diploma thesis

in Physics

submitted by

Bente Philippsenborn in Heide, Germany

2008


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Hard Water or High Ages?

14C food crust analysis on Mesolithic potteryfrom Northern Germany

This diploma thesis has been carried out by Bente Philippsen

at the Institute for Environmental Physics

under the supervision of Prof. Werner Aeschbach-Hertig.


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Abstract

The aim of this thesis is the investigation of the hardwater effect and its implications for the dating of thefirst occurrence of pottery in Northern Germany. The hardwater effect denotes the effect of a high carbonatecontent of freshwater on radiocarbon dating: the dissolved minerals can lead to spurious, too high agesof samples from freshwater systems. The possibility of the hardwater effect in food crusts on pottery isinvestigated for two sites of the Late Mesolithic Erteblle culture. The first radiocarbon dates from the foodcrusts from these sites were surprisingly high. I will examine if the pottery really is so old or if the hardwatereffect is responsible for this. The samples I thus have to analyze are very small. Instrumental developmentwas needed for the dating of these samples. On-line combustion combined with stable carbon and nitrogenisotope measurements is developed to minimize the number of steps in the preparation and measurement ofa sample so a reduction of contamination may be achieved. Also the conversion of the CO2 from combustedsamples to graphite, the form of carbon in which the radiocarbon content can be measured, is examined and

some improvements are suggested and tested.

Abstract

Das Ziel dieser Arbeit ist die Untersuchung des Hartwassereffekts und seines m oglichen Einflusses auf dieDatierung der fruhesten Keramik in Norddeutschland. Der Hartwassereffekt bezeichnet die Auswirkungeneines hohen Karbonatgehalts von Flussen und Seen auf die Radiokarbondatierung: die gelosten Minerale

konnen zu falschen, zu hohen Altern von Proben aus Suwassersystemen fuhren. Die Wahrscheinlichkeitdes Hartwassereffekts in Speiseresten auf Keramik, in der Suwassernahrung zubereitet wurde, wird furzwei norddeutsche Fundstellen der spatmesolithischen Ertebllekultur untersucht. Die ersten Radiokarbon-datierungen von Speiseresten auf Keramik dieser Fundorte ergaben erstaunlich hohe Alter. Ich werde unter-suchen ob die Keramik wirklich so alt ist oder ob der Hartwassereffekt dafur verantwortlich ist. Die Proben,die ich dafur analysieren muss sind sehr klein. Um diese sehr kleinen Proben datieren zu konnen, ist instru-mentelle Weiterentwicklung notig. Zum Beispiel soll Fraktionierung minimiert werden. Die Verbindung derProbenverbrennung mit der Messung von stabilen Isotopen wurde entwickelt um die Anzahl der Probenauf-bereitungsschritte zu minimieren. Dieses kann zu einer Reduzierung von Verunreinigungen der Probe fuhren.Die Umwandlung des CO2 zu Graphit, der Form von Kohlenstoff in der der Radiokarbongehalt gemessenwerden kann, wird untersucht. Einige Verbesserungen dieses Prozesses werden vorgeschlagen und getestet.


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Preface

The influence of the hardwater effect on pottery dating, the necessary methodological and instrumentaldevelopments, and the influence the hardwater effect has on the dating of the first pottery in NorthernGermany will be presented in this thesis. The hardwater effect is the influence of dissolved carbonate onsamples from freshwater systems. It is a long-known and broadly accepted phenomenon. However, thepossibility of a hardwater effect in one sample type is still under debate. Food crusts on pottery in whichfreshwater food such as fish was cooked could as well show spurious ages if the freshwater food came fromwater systems with a high carbonate content. So far, this effect has not been agreed on. I have thereforechosen two sites next to freshwater rivers in Northern Germany where a hardwater effect on food crusts

on pottery is probable. I have taken a large number of archaeological samples as well as examined recentfreshwater samples in order to obtain statistically significant results. As the food crust and fishbone samplesfrom these sites are very small, development in sample processing methods was necessary. A device combiningsample combustion for AMS 14C dating with on-line stable isotope measurements was advanced and tested.This device has the potential of reducing the number of steps in sample preparation, the contamination risk,and the total sample size.

Due to various problems with the ion sources and tandem accelerator, some of the samples could not bemeasured before this work had to be finished. A new ion source was constructed and it was planned thatmy samples could be measured using this new ion source. Because of several delayals, it was not possibleto start the new ion source for standard 14C datings before this thesis had to be finished. Therefore, theold ion source was used for my samples. There are two disadvantages with that: First, the old ion sourceis not able to produce enough ion beam current for dating when the samples are very small. Second, theion source is only optimised for samples graphitised with cobalt at 700C. My small archaeological samples

could thus not be measured at all. Normal-sized samples graphitised with iron at 550C are measured witha far higher uncertainty than would be the case in the new ion source.

Working in the intersection of physics and archaeology

The topic lies in the intersection of physics and archaeology, an area which is also called archaeometry.

Archaeology is the study of the human past through material remains (Hayashida 2003).

Physics is the study of matter and energy and how they work with each other(http://simple.wikipedia.org/wiki/Physics on February 26, 2008).

Archaeometry is thus the (e.g. physical or chemical) analysis of material remains to study the human past.

Archaeometry is another term for archaeological science and comprises different scientific technologies whichare applied in archaeology. This can be chemical and physical dating methods, provenance studies of arte-facts, study of the distribution and use of artefacts, reconstruction of past landscapes, climate and envi-ronment, dietary studies of humans and animals, remote sensing and geophysical surveys, and conservationtechniques (Tite 2003a).

This work will deal with two aspects of science applied to archaeology: radiocarbon measurements fordating and stable isotope measurements on food crusts for dietary studies. Instumental development

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and examination of the existing sample preparation methods will form the physical part of the work. Atthe same time, it will also deal with the archaeological background on a scientific basis, bearing in mindthe appeal of Dunnell (1993): The attraction, particularly from the physical sciences side, often seemsto be to the non-scientific archaeology of the Sunday newspaperss feature section, and there will be nobasis for integrating archaeometry and archaeology until archaeometrists focus their attention on scientificarchaeology. Tite (2003a) also emphasizes this fact: To ensure that archaeometry remains relevant to

archaeology, it is essential that only real archaeological questions are addressed. This in turn necessitatesthe maintenance of a substantial dialogue between archaeometrists and archaeologists together with a holisticapproach that goes beyond reconstruction to a full interpretation within the specific archaeological contextunder investigation. On the other hand, archaeologists should be careful not to depend too much onscientific dates. It is tempting to use for example radiocarbon dates as a calendar for prehistory, thus givingarchaeologists the role of historians. But it has to be kept in mind that reporting the archaeological phaseof a site is often more useful for comparing findings in a larger region than expressing the time horizon incenturies BC. Absolute chronology may be the aim of archaeology, but it must never be the backbone in away that a calendar is the backbone of history (Fischer 1976).

The topic presented in this thesis was especially interesting to me as I had gained insight both in physicsand prehistoric archaeology in my studies. During archaeological studies and in physics projects, I couldlearn about physical dating methods and especially radiocarbon dating. In archaeology, I specialized in theNeolithic in Northen Germany and South Scandinavia, particularly in the Mesolithic-Neolithic transition.

This made it easier for me to take both the physicists and the archaeologists point of view when discussingthe sites I examined and their importance in the cultural development of the region. I hope therefore that Ican avoid some of the possible mistakes that happen when two differnent disciplines meet.

Outline of thesis

The first chapter presents an overview over the methods applied. After an introduction to the basic principlesof AMS radiocarbon dating and stable isotope analyses I will describe the method of pottery dating. In thesecond chapter, I will present the development of enhanced techniques for the preparation of small samples.The third chapter will deal with the sites from where I got the samples as well as their geological andarchaeological background. In the same chapter, the events being dated in this thesis will be put into thecontext of cultural development at that time. The fourth chapter will finally deal with the archaeological

samples and the recent test samples which all were used to determine the hardwater effect for the two rivers.Many of the physical terms may be unknown to archaeologists and, the other way round, physicist are likelyto be unfamiliar with the archaeological terms. A glossary at the end of this thesis may contribute to abetter understanding for specialists of both fields without lengthening the text.

Acknowledgements

First of all I would like to thank the AMS 14C Dating Center of the University of Aarhus and its director,Jan Heinemeier, for a warm welcome and the opportunity to work on an exciting topic. I was in thefortunate position to benefit both from the laboratorys equipment and its know-how as well as from itscontacts to scientists of other disciplines. The laboratory technicians Hanne Jakobsen, Ann-Berith Jensenand Vibeke Jensen taught me how to use the equipment, helped me patiently whenever I was confronted with

problems. Together with the graduate students Torben Ankjr and Christina Maria Lutz they created agood working atmosphere. With Marie Kanstrup, I had inspiring discussions about archaeological andstable-isotope related matters. Egon Jans and Klaus Grossen helped me to build up the new graphitisationsystem. Klaus Bahner, although constantly nagging about adverse conditions, measured my samples in thebest possible way at the accelerator together with his team of accelerator operators. Prof. Dr. Claus vonCarnap-Bornheim, director of the Archaeological Museum of Schleswig-Holstein in Schloss Gottorf, paid 30of the datings and the foundation Prof. Werner Petersen-Stiftung in Kiel, Germany, provided the funds

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for this. The choice of the topic gave me the chance to interdisciplinary co-operation with the team ofarchaeologists from the Archaeological Museum. Sonke Hartz was an enthusiastic co-worker and made thefinds from Schlamersdorf available to me. He also organized the pottery experiments together with HarmPaulsen and Aikaterini Glykou and introduced me to a number of helpful people: Manfred Pfeiffer fromNeustadt (Holstein) provided the equipment for the fishing attempts in the Alster. Dr. Rainer Brinkmannfrom Schlesen and Dr. Mattias Brunke from Flintbek helped establishing the contact to Dipl.-Biol. Dennis

Grawe and Dipl.-Ing. Markus Vainer from Institut BIOTA GmbH who finally caught fish for us in the Alster.Winfried Dobbrunz from Bad Oldesloe provided us with fish from the Trave. A very warm thank you isaddressed to all the archaeologists from Schleswig for receiving me and my project so cordially. My dearfriend Birte Kruse provided a home for me during my visits to Schleswig and accommodated me even whenI returned from the pottery experiments, sooty and smelling of smoke. My flatmate Oline Laursen madesure that I was well provided with victuals for long days at the university and that it always was niceto come home. This thesis was supervised by Prof. Werner Aeschbach-Hertig in Heidelberg. He providedtheoretical knowledge both about the physical aspects of this thesis and about the formalities of a diplomathesis. Henrik Kjeldsen was my supervisor inArhus and helped me with all technical and theoretical aspectsof my work. I want to thank him for support and encouragement throughout the time of research and writingas well as for interesting scientific discussions. I want to thank my parents who made it possible for me tostudy a subject of my own choice and who supported me at all times. As English is not my native languageI am thankful for all the help I received concerning the language - both grammar and spelling as well as

choice of words and the correct use of expressions. The dict.leo.org-community provided a great databaseof English expressions and fine nuances of acceptations of words as well as surprisingly fast answers to allmy questions about the English language. Christopher Dege supported me in all my plans and was alwaysthe first reader of my texts. He helped me a lot with critical statements from a linguists point of view andcorrected most of the errors in my manuscript. He was also very helpful in all questions concerning thetypesetting system LATEX which was used for producing this paper. I am solely responsible for all remainingerrors.

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Contents

1 Methodology 1

1.1 Radiocarbon dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.1 The history of radiocarbon dating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.1.2 Error sources and correction factors in radiocarbon dating . . . . . . . . . . . . . . . . 41.1.3 Dendrochronology and the calibration of the radiocarbon time scale . . . . . . . . . . 5

1.2 Accelerator mass spectrometry (AMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.1 Reporting of 14C data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.2.2 Reservoir Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.2.3 How the hardwater effect works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.2.4 The hardwater effect in archaeological material . . . . . . . . . . . . . . . . . . . . . . 21

1.3 Isotopic fractionation and stable isotope analysis . . . . . . . . . . . . . . . . . . . . . . . . . 211.3.1 13C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221.3.2 15N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

1.4 Food residues on pottery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251.4.1 Terminology and general remarks on prehistoric pottery . . . . . . . . . . . . . . . . . 261.4.2 Dating of pottery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261.4.3 Analyses on food crusts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2 Small sample preparation 31

2.1 Sample pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.2 Graphitisation of small samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.2.1 Experiments with the existing graphitisation system . . . . . . . . . . . . . . . . . . . 352.2.2 Establishing a new graphitisation system with smaller reactors . . . . . . . . . . . . . 422.2.3 On-line combustion with EA measurements . . . . . . . . . . . . . . . . . . . . . . . . 48

3 The sites 56

3.1 Geography and research history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.2 Geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.2.1 Water hardness and 14C in the Trave and Alster . . . . . . . . . . . . . . . . . . . . . 603.3 Erteblle and Funnel Beaker culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

3.3.1 EBK research history and Kkkenmddinger . . . . . . . . . . . . . . . . . . . . . . . 613.3.2 The environment of the Erteblle culture . . . . . . . . . . . . . . . . . . . . . . . . . 623.3.3 The Erteblle economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.3.4 Erteblle pottery and tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.3.5 Transition to farming: From Mesolithic to Neolithic or from Erteblle to Funnel Beaker

culture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

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4 The samples from Schlamersdorf and Kayhude 71

4.1 Selection and pretreatment of samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.1.1 Water samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 714.1.2 Recent fish and molluscs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.1.3 Recent food crusts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.1.4 Old food crusts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.1.5 Bone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804.2 Dating and stable isotope measurement results . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.2.1 Recent samples from the river Trave . . . . . . . . . . . . . . . . . . . . . . . . . . . . 844.2.2 Archaeological samples from Schlamersdorf (Trave) . . . . . . . . . . . . . . . . . . . . 874.2.3 Recent samples from the river Alster . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904.2.4 Archaeological samples from Kayhude (Alster) . . . . . . . . . . . . . . . . . . . . . . 914.2.5 Comparison with marine samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.3 Comparison of the sites, discussion and conclustion . . . . . . . . . . . . . . . . . . . . . . . . 95

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Chapter 1

Methodology

In this thesis, I have worked with two physical methods which are routinely applied to archaeological research.The first is radiocarbon dating, a physical dating method that is based on the radioactive decay of the 14Catom. The second is stable isotope measurement, in this case carbon (13C) and nitrogen (15N). Stable isotopemeasurements are used for examining the marine, terrestrial, or freshwater origin of archaeological samples

and for food reconstruction of prehistoric populations. Both methods will be presented here, including theirhistorical development, the principles, applications, and limitations.

1.1 Radiocarbon dating

Radiocarbon or 14C dating is a method of age determination of the last uptake of carbon in a sample, forexample in the form of carbon dioxide from the atmosphere. Therefore, it is in principle suitable for thedating of all organic material. The method can also be used for some inorganic material like carbonates thatare formed from atmospheric CO2. Radiocarbon dating makes it possible to examine ancient cultures fromall over the world which before could not be dated. A reason for the lack of dates can be that writing wasnot known in that culture as is, by definition, the case in all prehistoric cultures. Other reasons include thatthe writing is not yet deciphered as for Maya Yucatan and Etruscan Tuscany or that time was unimportant

in an otherwise literate culture such as in India (White 1976). In a culture where time is unimportant,no dates are bequethed. Radiocarbon dating is probably the technique that has had greatest impact onarchaeology, of particular importance being the investigation of the chronology for the development andspread of agriculture across the world (Tite 2003a). The introduction of radiocarbon dating as a datingmethod for archaeology was therefore called radiocarbon revolution.

Carbon has three isotopes, 12C, 13C, and 14C. The Carbon-14 isotope is radioactive with a half-life of5730 years. Because of this constant decay rate, it is possible to use 14C for the measurement of the timethat passed since 14C was last uptaken by a sample. 14C decays in the reaction

14C 14N + + .

As time passes, the amount of14C decreases while the amount of14N increases. The 14N is here marked withan asterisk to indicate that it is radiogenic, i.e. originating from the radioactive decay of another element.For calculating an age, one could therefore measure both the 14C- and 14N-amount present in a sample. The

more 14N there is present compared to 14C, the more time has passed. Unfortunately, the concentration ofthe decay product 14N in a sample can not be measured. 14N is ubiquitous as atmospheric air consists to78% of nitrogen, and 99.634% of natural nitrogen atoms are 14N. The small amount of14N being formed ina sample is, as all nitrogen under standard conditions, in gas phase. It thus mixes with atmospheric nitrogenand can not be separated from the 14N that is present everywhere on earth.

The fact that 14C decays with a constant rate, can nevertheless be used for measuring time. This becomes

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clear when we take a look at the exponential decay law that describes the decay of radiocarbon (equation 1.1).

N=N0et

. (1.1)

From this equation, the time since the last uptake of carbon can be calculated. When the initial concentrationof 14C, N0, and its mean life is known, time since death t can be calculated from the following equationafter measuring the present 14C concentration N:

t= lnN0N

. (1.2)

This equation is derived from the decay law. The mean life of radiocarbon has been measured with sufficientprecision. The present 14C concentration in a sample can be measured with methods which are discussedbelow. Thus, only the initial 14C concentration of the sample, N0, is missing before we can calculate theage of a sample. How is it possible to find the initial 14C concentration, that was present in a sample anunknown time ago?

Dating is made possible because the 14C-content of the atmosphere is reasonably constant. The atmo-spheres 14C content in the past can thus be deduced from its recent 14C content. This constancy is due tothe way of14C formation: The radioactive decay is compensated by the constant production of14C by cosmicrays in the upper layers of the atmosphere through the reaction 14N(n,p)14C. There is thus an equilibrium

between formation and decay which results in a reasonably constant 14

C concentration in the atmosphere.The formation of 14C is here described in more detail: A neutron which is produced by cosmic rays reactswith Nitrogen-14 which consists of 7 protons and 7 neutrons to form a Carbon-14 nucleus with 6 protons and8 neutrons, and a proton. 14C is the radioisotope with the highest atmospheric production rate: 2.2 atomscm2sec1 (Kocharov 1992). About 2/3 of the 14C production take place in the stratosphere and about 1/3in the troposphere. Through the following reaction, 14C and an oxygen molecule O2 react to CO2:

14C + O2 14CO + O2

14CO + OH 14CO2 + H

It resides for about 10 years in the stratosphere. In the form of CO2, the14C is being distributed throughout

the atmosphere and then for example built in by plants through photosynthesis. During its life, an organismincorporates carbon from this well mixed atmospheric reservoir. This is how the constant atmospheric 14Cconcentration leads to a constant 14C concentration in a living organism. When the 14C content of a living

plant is measured now, we can assume that a plant living at any time in the past had the same 14C contentwhile it was alive. With the organisms death, the carbon uptake ends and the 14C that decays is not beingreplaced by new 14C. So, the 14C content of the sample decreases according to the exponential decay law(equation 1.1, see figure 1.1).

After these explanations, it is clear how the initial 14C concentration N0 of a sample is estimated. Inreality, though, the situation is a little more complex than it was described here, because there are variationsin the production rate of14C. How these variations were discovered will be described in section 1.1.1. Thesevariations in atmospheric 14C do not nullify the method of 14C dating, as one can correct for them. Howthis correction is made will be described in section 1.1.3.

Now the method of measuring the present 14C concentration in a sample has to be discussed. Theconcentration of 14C can either be measured directly via accelerator mass spectrometry (see section 1.2) orindirectly via decay counting. This is referred to as the conventional method. In the first case, N andN0denote concentrations of 14C; in the second case, they denote activities. The 14C concentration of a sampleis measured as the 14C/13C ratio when accelerator mass spectrometry is used. Details will be presented laterin section 1.2. For decay counting, the number of 14C atoms that decay per time unit is measured. Thenumber of decaying 14C atoms is proportional to the total number of14C atoms present in the sample. Withmeasuring the mass of the sample, one can convert the measured number of 14C atoms to a concentration,the 14C/12C ratio. In both cases, one can not be sure to have counted all 14C atoms that are present in thesample. The measured 14C concentration in the sample is thus being compared to the 14C concentration ofa standard material with known age. If for example only 10% of the samples 14C atoms can be detected,

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Figure 1.1: The radioactive decay of 14C

then also only 10% of the 14C atoms in the standard material can be detected. The limited detection ratioscancel thus each other out. Section 1.2.1 describes in detail how the age of a sample is calculated from themeasured 14C concentrations of the sample and the standard.

1.1.1 The history of radiocarbon datingAfter the basic principles of radiocarbon dating have been explained above, I have now chosen a historicalperspective to shed light on the development from the discovery of 14C to the routine measurements oftoday. It will be shown that some of the basic assumptions stated above have proven inaccurate, but thatthis always led to new knowledge and only made the method of radiocarbon dating more reliable in the end.An overview over the early years of radiocarbon dating is given by Suess (1992). A lot of the informationpresented here as been extracted from his work.

The first list of radiocarbon ages for unknown samples appeared in 1951 (Arnold and Libby 1951) afterthe idea had been proposed by Willard Libby in 1946. Two key points lead to the state of knowledge whichprovided the basis for 14C dating. On the one hand, the unstable 14C carbon isotope was discovered in1937 (Ruben and Kamen 1941). It was observed that it was produced by cosmic rays and that it had ahalf-life between 1,000 and 25,000 years, thus fitting archaeological time scales (Korff and Danforth 1939).

Later, the half-life was determined more precisely to 5568 years, which was used for the first datings. Thishalf-life had finally to be specified over again: In 1962, a more precise half-life of 14C was determined - 5730instead of 5568 years. Even though the half-life was not known exactly, the theoretical knowledge about the14C atom and its deccay was hence available in the 1940s. On the other hand, a few weak counting sourceshad been constructed, and Libby had invented the screen-wall counter. The practical basis was consequentlyalso given. Note that this counter measured radioactiviy. It was thus capable of measuring the number ofdecaying 14C atoms and give the activity of a sample. From 1952 to 1955, 7 radiocarbon dating laboratories

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were established. One of them has to be named specifically: In 1953, the USGS (US Geological Survey)Radiocarbon Dating Laboratory started work with acetylene counters under the leadership of Hans E. Suessand carried out 200 measurements in the first two years. Their most important results were the dating ofthe maximum extent of the North American ice sheet, the result that Homo neanderthalensis survived inselected areas until about 30,000 years ago and the Suess effect which was named after its discoverer. TheSuess effect is the anthropogenic drop in 14C activity in air which occurred during the industrial revolution,

when14

C-free CO2 from fossil-fuel combustion was added to the atmosphere. This drop in atmospheric 14

Cconcentration is one of the reasons for the fact that the initial 14C concentration of a sample in the past isnot the same as the present 14C concentration.

In 1959, de Vries demonstrated the variability of atmospheric14C over the past centuries. The atmopheric14C concentration is thus not only altered by the Suess effect, that diluted the 14C concentration, but thereare additional variations. Those variations are called wiggles. In 1971, Houtermans found out from 350radiocarbon samples that had been dendrochronologically dated that these radiocarbon variations correspondto a line spectrum with a prominent 200-year line. It took a long time until the existence of those wiggleswere widely accepted, and still they are not fully understood. Dendrochronology is the method of tree ringdating. This method provides samples of a known age for testing radiocarbon dating - just as Houtermansdid. When 14C-dating a tree ring of known age, one can therefore calculate the atmospheric 14C activity ofthe time when the tree ring was formed. When this is done for a long stretch of time reaching back in thepast, one can give the initial 14C activity for each year. The plot of this information is called the calibration

curve, as it can be used for calibrating a measured 14C concentration to obtain a calendar age. This will bedescribed in section 1.1.3.

1.1.2 Error sources and correction factors in radiocarbon dating

As mentioned in section 1.1.1, the 14C concentration of the atmosphere is not completely constant. Thevariations in 14C production that cause the above-mentioned wiggles are due to variations in sun activityand the earths magnetic field that affect the cosmic radiation. As the cosmic radiation produces neutronsthat, together with 14N, form 14C, the 14C production rate varies according to the variations in cosmicradiation intensity. Two other important effects are anthropogenic: The Suess effect that was described insection 1.1.1 and the bomb pulse. The Suess effect is the depletion of 14C in the atmosphere because of thecombustion of fossil, 14C-free fuels since the industrial revolution.

The bomb pulse(see figure 1.2) denotes the increase of14C in the atmosphere after H-bomb tests during

the 1960s up to twice the normal activity in the mid-1960s and its decrease since then. The 14C concentrationin the present atmosphere is decreasing exponentially, but with a much smaller half-life than the 14C half-lifeof 5730 years. Not only the decay of 14C reduces the atmospheric 14C concentration. The excess 14C isalso incorporated in the biosphere and in the oceans and so gradually removed from the atmosphere. Themean life of this decay is around 17 years, so that its half-life is approximately 12 years. Because of thisrapid decrease, the high 14C concentrations in samples from the last few decades can be used as a tracerand for high-precision radiocarbon dating (see e.g. Lynnerup, Kjeldsen, Heegaard, Jacobsen, and Heinemeier(2008)).

Variations in the initial 14C activity or content can be calibrated with radiocarbon dating of tree rings ofknown age. The plot radiocarbon age over calendar years is called the calibration curve. Unfortunately, insome periods there are ambiguities in that curve, so that it can not be decided which calendar age a certain14C concentration belongs to. Calibration will be explained in section 1.1.3.

As we have seen so far, some basic assumptions were made for radiocarbon dating. These are summarizedin the following list (after Browman (1981)):

1. 14C decays exponentially at a known rate, and the initial measurement of that rate, Libbys half-lifeof 556830a, is adequate enough to make reasonable computations of age

2. activity or 14C content can be measured with acceptable accuracy3. secular constancy of 14C concentration in the atmosphere at one point of the earth4. simultaneity: the 14C concentration is the same for any two points on the earths surface

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Figure 1.2: The bomb pulse in atmospheric CO2 and in the dissolved inorganic carbon in near-surfaceseawater. The annual total testing is given in megatons (Clark and Fritz 1997).

5. the atmosphere is the only source of carbon in living organisms6. isotopic integrity after organisms death, no material added,14C/12C changes only by radioactive decay7. all organisms are in isotopic equilibrium with the atmosphere i.e. no fractionation occurs

But we have also seen that these assumptions are not completely appropriate, so corrections have to beapplied:

1. the half-life of 14C is 573040 a instead of 556830 a, and as data is still published with Libbyshalf-life, ages have to be multiplied by 1.029

2. background corrections and measurement of standards for setting up the instruments; compensationof differences between laboratories because of differences in preparation and evalutation of data

3. calibration curve (see section 1.1.3) because of long term variations, caused by geomagnetic field inten-

sity, and short term variations, heliomagnetic modulation of 14

C production, of the atmospheric 14

Ccontent, which is additionally altered by the Suess effect and bomb tests4. latitude and altitude effects: samples from the southern hemisphere are 40-80 years older than those

from the north. Because of different reservoir exchange ratios, the ocean surface on the southernhemisphere is bigger.

5. water plants and animals take carbon from the water; error sources and corrections: see section 1.2.26. contaminants that entered the sample after the organisms death have to be removed7. fractionation takes place on all steps of the food chain, beginning with photosynthesis and has to be

corrected through 13C/12C measurements (see section 1.3)

1.1.3 Dendrochronology and the calibration of the radiocarbon time scale

Because of the wiggles in the atmospheric 14C content (see section 1.1.1 and 1.1.2), the assumption of a

constant atmospheric 14

C concentration is wrong and the radiocarbon ages have to be calibrated to correctfor the variations in atmospheric 14C content. For calibrating the radiocarbon ages, one needs samples ofknown age. Mostly, dendrochronologically dated wood is used for that. The calibration curves used for thecorrection of the radiocarbon ages are generated by plotting the wood radiocarbon ages versusthe calibrated(cal.) ages. After a description of dendrochronology and a short overview over its history, the history ofits application for calibration of radiocarbon ages will be given. The spectrum of the variations will beexplained as well as the impact calibration had on archaeology.

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Dendrochronology, or tree ring dating, is still the most precise dating method for archaeology. When thereis a sufficient number of rings (100-150 for oak) and the bark ring is preserved, a dating precision of 0.5 to 1year can be achieved. With preserved sapwood, the precision becomes 5-30 years, but when only heartwoodis preserved, dendrochronology gives a terminus post quem for the felling of the tree. Dendrochronologywas the only scientific method of precise dating until radiocarbon dating was invented. The first successfulattempts of dendrochronological dating were made by A. E. Douglass in 1906. In 1953, conifers in the White

Mountains of California were found that were older than 4000 years, so that Edmund Schulman, a co-workerof Douglass, was able to construct an unbroken 4600-year sequence. Since a 7104-year tree-ring record waspresented by C. W. Ferguson in 1969, dendrochronology became important not only for dating but also forexamining the long-term variations of atmospheric 14C and for geophysical research. In 1957, Hessel de Vriesbegan measuring dendrochronologically dated wood samples after natural variations of the atmospheric 14Clevel were reported by K. O. Munnich from German oak samples dated by Bruno Huber. The variations are inthe order of magnitude of about 120 (Sternberg 1992). J. C. Houtermans used methods of Fourier analysisin his above-mentioned work for calculating a spectrum of the variations. He suggested both an approx.200- and 2000-year period. Later research has supported these suggestions and added other frequencies, forexample a periodicity of 10,000 - 12,000 years, to the spectrum (Sternberg 1992). Although some of theshorter periods (11, 88, and 210 years) have been identified with solar activity (Damon and Jirikowic 1992),the radiocarbon spectrum is by no means fully understood (Sonett 1992). The variations are so hard tounderstand because there are many possible sources interacting. They influence either the production of14C

in the atmosphere or cause changes in the carbon/radiocarbon geochemical system (Sternberg 1992):

Radiocarbon is produced when neutrons produced by cosmic rays interact with 14N in the atmo-sphere; thus, it is affected by the galactic primary cosmic-ray flux, modulation of this flux by theheliomagnetic field and by the geomagnetic field, or by production due to solar cosmic rays. Sys-tematic changes include changes in geochemical reservoir sizes or exchange rates between them,or in the amount of 14C in the reservoir or system as a whole.

Unfortunately, the variations in the radiocarbon production are not reflected unaltered in the radiocarboncontent of tree rings: 14C studies require the consideration of complex transport processes damping outfast processes and shifting the phase (Kocharov 1992).

The introduction of calibration had such a big impact on dating that it is called the second radiocarbonrevolution. The second radiocarbon revolution involved the re-interpretation of European prehistory

following the introduction of calibration curves, when events in Europe were found to be earlierthan (notjust uncomfortably close to) their supposed progenitors in the Near East (Tuniz, Zoppi, and Barbetti 2003).On the basis of conventional 14C dates it had previously been assumed that European prehistoric culturesgenerally postdate Middle East cultures (Becker 1992).

It has to be paid attention that calibration is applied in the right way, appropriate to the event beingdated. When dating one event by dating many samples associated with that event, the samples have to becalibrated together. When the samples are calibrated individually, the age distribution for the event becomesbroad, and it becomes the broader the more samples are measured (Ottaway 1986).

1.2 Accelerator mass spectrometry (AMS)

We have witnessed two revolutions so far: The first radiocarbon revolution was the introduction of radiocar-bon dating. The second radiocarbon revolution was the introduction of calibration and the re-interpretationof datings. There is a third event which is called a radiocarbon revolution: the introduction of acceleratormass spectrometry. For understanding why this is a revolution, we have to remember that radiocarbonin the first decades was done via decay counting (see section 1.1.1). Only the 14C atoms decaying duringthe measurement period could be detected. The sample masses required for this technique were about 1 gcarbon. Although 1 g of carbon already is a small amount, not all types of samples could be dated. Thecarbon yield of different sample materials differs a lot, so that in some cases far more than only few grams

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of original sample were needed. Very small samples could thus not be dated, and some other samples aretoo valuable for allowing the removal of for example 100 g sample material.

In a modern sample, the average fraction of 14C decaying per day equals only 3.3 107 of the amountpresent. If the number of 14C atoms present could be counted instead of only the number of decaying ones,the sample size could be reduced drastically. This is exactly what accelerator mass spectrometry does. Itis in principle a very easy technique: The carbon atoms are extracted as ions from the sample, they are

accelerated, separated from each other, and counted. The separation and counting of isotopes of differentmasses is called mass spectrometry. This is where the name accelerator mass spectrometry comes from: itis a mass spectrometric measurement with the help of an accelerator. The accelerator is needed to producehigh enough ion energies to make a separation of different ions with almost equal masses possible.

First of all, we will follow a 14C atom on its way from the sample to the final detector for showing howAMS works in principle. Background levels and precision of AMS will be addressed after that. The impactof AMS on archaeology and its advantages over decay-counting will be explained thereafter. A short historyof AMS will end this section. A lot of the information presented here can be found in Gove (1992).

From sample to detector

All laboratories have their specific design of an AMS setup. I will try to explain the principles as general aspossible, but when more details are needed, I will refer to the setup that is installed at the 14C AMS dating

laboratory at Aarhus University. In summary, the carbon ions are extracted from the sample, acceleratedwith 0.5-10 MV, separated according to their momentum, charge and energy and finally counted by an iondetector after identification by nuclear mass and charge. Accelerator mass spectrometry must be able toseparate nuclides of almost equal mass, sort out interfering molecules and measure the abundances of ionsin different orders of magnitude.

The sample is placed into the ion source in the form of elemental carbon, i.e. graphite. A beam of caesiumatoms is targeted at the sample which causes negative carbon ions to leave the sample surface. They areaccelerated towards a positive electrical potential. The most important mass-14 component that disturbsthe 14C measurement is already removed: 14N does not form negative ions and is therefore not present inthe ion beam.

When the ions are formed, they have different energies. The ion energy is the sum of the energy theyobtained from the acceleration towards the positive potential, which is equal for all of them, plus the kineticenergy they obtained from the ionization. Only ions with the right energy are desired to enter the accelerator.

The ion beam is therefore electrostatically deflected. Only ions with the desired energy Eare moving on thetrajectory of radius r :

r=Mv2

Q

E

Q (1.3)

with M= nuclear mass, Q = charge, v = velocity and E= energy of the ion and = electric field. Thekinetic energy of an ion is E= 12mv

2.We have thus obtained a beam of ions with equal energy. Ions with mass 12, 13 and 14 are now selected

for sequential or simultaneous injection into the accelerator. The so-called injection magnet selects the ionswith the desired mass. In a magnetic field, ions are deflected according to equation 1.4.

ME

Q2 (Br)2 (1.4)

withB = magnetic field. Only ions with a specific MEQ2 are deflected to the circular path with radiusr. As allions have the same charge (-1 from the ion source) and as the above-mentioned electrostatical filter selectedions with equal energy, one can also say, Only ions with a specific massare deflected to the circular pathwith radiusr. With varying the magnetic field intensity B , one can now chose between the ions with mass12, 13, or 14. As this magnet is used for injecting beams of ions of different masses, it is called injectionmagnet. Many AMS laboratories cannot accelerate the large 12C currents and only inject mass 13 and

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Figure 1.3: A negative ion beam enters the tandem accelerator, loses electrons in the stripper material andleaves the accelerator with a charge of 3+.

14. The mass-14 beam consists mainly of 12CH2 and 13CH. 14C is present in this mass-14 beam, but its

percentage is negligible as the other carbon isotopes are far more abundant.The ion beam enters now the accelerator. Tandem accelerators are used in this case. The negative

ion is attracted by a positive potential in the middle of the accelerator. Typical acceleration voltages are2MV for Tandetrons and 6MV for Van de Graaff accelerators. In the middle of the accelerator, a so-calledstripper material is installed. Collisions with the stripper material, gas or foil, remove several electrons, sothat the resulting carbon ions are positively charged. +3 is the most common case for 2MV accelerationand +4 for 6MV. Figure 1.3 shows a sketch of the tandem accelerator. When losing 3 or more electrons,molecules are not longer stable. 12CH2 and

13CH, for example, are removed from the mass-14 beam. Thestripper removes thus the interfering molecules. After the accelerator, another magnetical analysis (afterequation 1.4) is necessary: ChargeQ and energyEof the ions is different due to the electron stripping andacceleration. Also the massM of some constituents of the ion beam can be different, as molecules weredestroyed.

Velocity selectors or Wien filters consist of magnetic and electric fields at right angles to each other andperpendicular to the direction of the incident ions so that only ions with a specific velocity are not displaced:

v2 = 2 EMQ2

2B2 (1.5)

Afterwords, the ions have to be counted. More abundant isotopes can be counted in faraday cups. Theseare devices that just measure the charge that accumulates on them. Less abundant ions are counted withparticle detectors. These have an additional advantage: The rate of the energy lossdE/dx identifies thenuclear chargeZ:

dE

dx

Z2

v2 (1.6)

The identification of the nuclear charge is only possible when the abundances of the different ions are limited.When plotting the count ratio of a particle detector, a picture similar to figure 1.4 emerges. The final energyis the total energy minus the energy loss. One would expect that only 14C enters the particle detector sothat only a 14C peak could be observed. This is not the case because ambiquities can occur. It is for example

possible that particles that are removed from the ion beam again enter the beam after small-angle scatteringon residual gas particles (i.e. on gas particles that are not removed although the system is evacuated). Infigure 1.5, the whole procedure for the mass-14 beam is summarized.

Precision and background

High-quality AMS measurements can reach a precision in pmC determination of about 0.2 to 0.3%. Theaccelerator background, that is the amount of 14C atoms that are registered although the sample is 14C

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Figure 1.4: Count ratios of 14N, 12C, 13C and 14C in a particle detector as a function of total energy andfinal energy.

Figure 1.5: Following the mass-14 beam through the AMS measurement procedure

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free, can go down to a 14C level according to 60-70,000 years. Dating is normally limited by the chemicalpreparation background to 50,000 years but can be better with special techniques (Tuniz, Zoppi, and Barbetti2003). There are different background sources in different steps of the sample preparation and measurementprocess after Kirner, Taylor, and Southon (1995):

1. Machine background: 14C detected when the sample is 14C-free

(a) Detector anomaly: 14C pulse registered when no 14C ion is present

(b) Ion identification anomaly: particle of same mass/energy ratio as 14C reaches the detector

(c) Beam-line contamination

2. Combustion/acidification background

(a) Materials contamination from materials in the combustion/acidification tube

(b) Tube contamination

3. Graphitisation background

(a) Materials contamination (e.g. catalyst)

(b) Reaction tube contamination

4. Pseudo 14C-dead sample background

(a) Sample erroneously assumed to contain no 14C

(b) 14C introduced into material that contains no 14C

Detector anomalies, or electronic noise, can be measured by collecting a spectrum for several days withoutinjecting any particles into the accelerator (Beukens 1992). Charge recombination in the detector createsa tail from the 12C and 13C peaks which underlies the 14C peak. Nuclear physics techniques of spectrumanalysis can be used to cope with this problem of a tail, but the reduced statistical precision limits thebackground level (Beukens 1992).

The ion source can also introduce contamination, because only about 10% of the samples carbon atomsis turned into negative ions and the remaining 14C atoms are deposited somewhere in the ion-source re-

gion (Beukens 1992).Kirner, Taylor, and Southon (1995) also observed that the way of sample storing has an effect on the

background value. A geologic graphite sample that was powdered and encapsulated under argon had a 14Cage of 69,000 BP while samples of the same material that were powdered and encapsulated in air had ages of58-60,000 BP. The materials contamination from graphitisation could for example been estimated by pressingpure catalyst into a target and then measuring it in the accelerator. The disadvantage of this method isthat the ion beam current in this case is too small and instable for a general statement (Vandeputte, Moens,Dams, and van der Plicht 1998).

Impact on archaeology and advantages over decay-counting

The most important background in decay counting, the cosmic radiation, is thus eliminated in AMS. It wasanticipated in the beginning of AMS that the background reduction could make the dating of samples up

to 100,000 years old possible. This is not the case, though, because contamination-free samples can not beprepared (Kirner, Taylor, and Southon 1995). The introduction of AMS reduced the required sample masseswith a factor of approx. 1/1000 and is therefore sometimes called the third radiocarbon revolution (Tuniz,Zoppi, and Barbetti 2003). Samples that before were too small or too valuable could now be dated. It is nowfor example possible to select only the best samples from a skeleton, minimizing problems with degradationand contamination. When a bone is reasonably preserved, 14C dating and stable isotope analysis is possiblewithout destroying the object (Arneborg, Heinemeier, Lynnerup, Nielsen, Rud, and Sveinbjornsdottir 1999).

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Only the capital costs and the greater complexity of AMS hardware (Gove 1992) have so far preventedAMS from completely supplanting decay-counting facilities, although the number of AMS laboratories is stillincreasing.

The development of AMS had a strong impact on archaeology, alone by the reduction of required samplemass to 1/1000 (Harris, Grove, and Damon 1987). A reduction of sample mass in conventional measurements,the use of small-counter facilities, had the disadvantage of long measurement times: several days for one

sample. AMS with measurement times about half an hour to a few hours per sample is thus much moreeffective. The small required sample mass makes it possible to date objects that were too small or toovaluable to be dated with the conventional method. Especially when dating the introduction of agriculturein different areas, AMS is the only possible dating method that can directly date the key material, singlecereal grains. Those plant remains are too small to be dated conventionally and too mobile to be datedstratigraphically or via associated finds (Harris, Grove, and Damon 1987).

The advantages of using an accelerator for 14C mass spectronomy are the following:

no interference with 14N because it does not form negative ions the samples are smaller and more easily prepared than for decay counting it is possible to accelerate all 3 C isotopes, so in principal both the date from 14C/12C or 14C/13C as

well as additional information from 13C/12C is available

It is possible to measure other radionuclides beyond 14C with the accelerator. The advantage of AMS is

that it can detect long-lived cosmogenic radioisotopes in the presence of vastly larger quantities of their stableisotopes. Many radionuclides which are produced in measurable amounts in the atmosphere or environmenthave decay constants matching temporal scales relevant to the history ofhominidae (Tuniz, Zoppi, andBarbetti 2003). While 14C can reach 50,000 years ago, when Homo sapiens sapiensstarted colonizing vastregions of our planet, can 10Be and others reach back to 5 million years when Australopithecusappeared. Thedifferent detectable radioisotopes can not only be used for dating. A plenitude of applications in hydrology,geoscience, materials science, biomedicine, sedimentology, environmental sciences and many other fieldsemerged as soon as the AMS detection capabilities of the appropriate isotopes were demonstrated. Oneexample is the measurement of water flow rates with the 36Cl bomb pulse: 36Cl was produced in nuclearweapon tests in the 1950s by neutrons interacting with the chlorine in the seawater. It was injected into thebiosphere at a level which was two orders of magnitude above the pre- and postbomb test ambient levels.

History of AMSIn 1977, two independent approaches using particle accelerators were taken, one with a cyclotron and one witha tandem Van de Graaff electrostatic accelerator. In May 1977, 14C in an organic sample, barbecue charcoal,was measured via AMS for the first time. The team at the University of Rochester (USA) demonstratedthat negative 14N ions are unstable (Gove 1992). Thus, the most important disturbing factor in 14C massspectronomy, the 14N isotope with nearly the same mass, could be eliminated. NH molecular ions are theonly nitrogen species left after the negative ion source (Beukens 1992). Only three weeks later after thissuccess, another group from the Canadian Simon Fraser University detected 14C in a AD 1880-90 woodsample at McMaster Universitys accelerator, also in Canada. Purportedly, neither the Rochester nor SimonFraser group was aware of the other groups efforts at that time. It could be said that the time was justripe for the development of AMS, after the first accelerator mass spectrometric detections of 3He alreadytook place in 1939 and the tandem accelerator employing negative ions had been invented by Luis Alvarezin 1951 (Gove 1992). Anyway, is took some time until a tandem accelerator was used for 14C dating. Itwas shown in 1977 that if three or more electrons are removed from a neutral mass 14 molecule like 12CH2,the molecule dissociates in a Coulomb explosion and the resultant fragments are swept aside before reachingthe final detector. Thus, another source of interferences with 14C could be eliminated. For these firstattempts of AMS radiocarbon dating, existing accelerators were used, but later small tandem acceleratorswere specifically designed for AMS, because the high terminal voltages of the big accelerators were notnecessary. At the end of the 1970s, 14C was measured with completely acceptable sensitivity using smalltandem accelerators with terminal voltages around 2 MV, because all that was required was a negative ion

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energy high enough to have a reasonable probability of producing charge 3+ ions in the terminal stripper toensure the elimination of mass 14 molecules. The first AMS 14C datings were made in 1978.

1.2.1 Reporting of 14C data

Stuiver and Polach (1977) developed guidelines for the reporting of 14C data that still are in use. The

following remarks about calculating and reporting of 14

C data can be found in their article. To find outwhich measured 14C activity or concentration belongs to which age, standard materials of known age haveto be measured. To ensure that results from different laboratories are comparable, all should use the samestandard material. Now, all laboratories report their data either directly related to NBS [Natural Bureau ofStandards] oxalic acid or indirectly by using a substandard which is directly related to the NBS oxalic acid.The internationally accepted radiocarbon dating reference value is 95% of the activity in 1950 AD of the NBSoxalic acid normalized to13C=-19 PDB (see section 1.3.1 for an explanation of13C fractionation and the-notation). The activity of this standard does not change, because it is the activity measured in a certainyear (1950 AD), although the activity of oxalic acid is changing with time. It is called absolute internationalstandard activity, AISA. Most laboratories use the activity AON, which is 95% of the measured net oxalicacid activity / count rate:

AON= 0.95AOX(1 2(19 + 13C)

1000 ). (1.7)

The expression in brackets accounts for fractionation. The factor 2 considers that fractionation for 14C isabout twice the fractionation for 13C. AsAONdepends on the year of measurement, y, it has to be correctedfor decay between 1950 AD and the year y of actual counting date. The absolute international standardactivity AISA is thus

Aabs= AONe(y1950) (1.8)

with= 18267years1. is based on the 5730 years half-life. Terrestrial samples are normalized to 13C =

-25 VPDB, the postulated main value of terrestrial wood:

ASN=AS(1 2(25 + 13C)

1000 ) which is an approximation of the more preciseASN=

AS0.9752

(1 + 13C1000 )

2. (1.9)

For the calculation of the radiometric age of a sample, the assumption is made that the atmospheric 14C

level was constant in all past times and that is by definition equal to Aabs after normalizing to -25. Theaget of a sample it thus

t= 8033 lnASN(1950)

AON(1950)with a half-life of 5568 years and thus a mean life of 8033 years. (1.10)

t is independent from the year of measurement, because both sample and oxalic acid lose their 14C at thesame rate. The years calculated with this formula are called conventional radiocarbon ages in years BP(before present) with present = 1950 AD.

Another method of giving radiocarbon values is percent modern carbon, pmC:

pmC =ASNAabs

100% = ASN

AONe(y1950)100%. (1.11)

Here is y the year of oxalic measurement and = 18267a1 is based on the 5730 a half-life. The pmC

is sometimes defined differently, as ASNAON

100% without the factor correcting for decay since the year ofmeasurement. For archaeological samples, there is almost no difference between the two definitions of pmC,as the time since measurement is negligible compared to the age of the sample. However, when reporting forexample biomedical samples, it should clearly be stated if the correction for decay since measurement hasbeen applied or not.

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old CO2 with a low 14C content: The ocean can be divided into two parts, the surface water and the deep

water. Both are well mixed individually, and there is little exchange between them. In ocean water, the13C value of dissolved inorganic carbon is 1, so that there is a difference of 26 between the atmospherewith 13C = -25 and the ocean (fractionation and the -notation will be explained in section 1.3.1). Adifference in the 13C/12C ratio of 26 means a difference of approximately 52 or 5.2% in 14C/12C, so thatthe ocean, compared to the atmosphere, should have a 14C activity of 105.2% (Lanting and Van der Plicht

1996). Rapid exchange with the atmosphere, though, only takes place in the surface water, so that theseconsiderations do not apply to the deep ocean. There is 102 times as much deep water as surface water. Inaverage, the proportion of carbon that circulates down into the deep water is about 102 times as great asthat which rises up. The 14C in the deep water has more time to decay and thus the activity of deep wateris less than the activity of surface water (Olsson 1976b). Via photosynthesis, plants incorporate the carbonfrom this CO2. From the phytoplankton via the zooplankton and fish, this old carbon finally also endsin food for animals living on land. One example are recent polar bears from Svalbard and East Greenlandwho had 14C ages of 48070 and 49545 years, respectively. The same effect can be found in humans - aperson who eats much fish will naturally have lower 14C activity than a person who eats mainly terrestrialfood (Olsson 1976b). This effect perhaps is conserved as a charred residue in a cooking pot, when the fishhad been cooked. The radiocarbon age of this food residue now is older than its actual age.

Corrections of the marine reservoir effect are made by subtracting the reservoir age of the ocean fromthe measured radiocarbon age of the sample. The reservoir age is about 400 years for the North Alantic

according to measurements and model calculations; see Stuiver, Pearson, Branziunas (1986). Reservoir agesof different parts of the oceans can vary strongly depending on sea currents and local seabed/coast shape.The age differences between surface ocean and atmosphere are generally greater at high latitudes than inthe tropics (Gillespie and Polach 1976). For finding out if a sample has marine origins that necessitatereservoir corrections, the content of the stable carbon isotope 13C is measured. 13C values differ significantlyfor organic material of marine and terrestrial origin. When 14C dating human bones, the percentage ofmarine food has to be estimated for calculating a marine reservoir correction. One example is the stableisotope analysis and 14C dating of Greenland vikings. The endpoint for a 100% terrestrial diet of13C =-21 is easy to find as there are many populations who solely live on terrestrial products. The endpoint fora 100% marine diet is harder to find because no human population a priori can be expected to have a 100%marine diet (Arneborg, Heinemeier, Lynnerup, Nielsen, Rud, and Sveinbjornsdottir 1999). Therefore, the13C value of -12.5 of the most marine individuals that the authors were aware of, Thule culture Eskimos,was used. The13C value of the viking bones is an indicator of the percentage of marine food and thus of

the extent of reservoir correction that has to be applied. When correcting the 14C dates in this way, theages of the human bones fit well historically assumed ages and the ages of associated terrestrial material liketextiles. In this case, a simple linear mixing model could be applied as the population on Greenland mainlylived on proteins from meat and dairy products because agriculture was not possible in the cold climate onGreenland (Arneborg, Heinemeier, Lynnerup, Nielsen, Rud, and Sveinbjornsdottir 1999).

There are different sources of14C depleted carbon in freshwater environments. The most important is thedissolution of geologic carbonates, which because of their high age are almost 14C free. Other explanationsinclude inputs of old soil humus, residence time of for example ground water in an aquifer, or vital effectsof the species studied (Culleton 2006). Carbon can also be dissolved from rocks other than carbonates, forexample from vulcanic glasses. This accounts for example for high reservoir ages in Icelandic groundwatersalthough there are no carbonaceous rocks on Iceland. The effect of CO2 from the decomposition of vulcanicglasses can be corrected with measuring the boron concentration in the water, as carbon and boron arereleased in stoichiometric relation from the glass (Sveinbjornsdottir, Heinemeier, and Arnorsson 1995). Inlakes with a large depth to surface ratio in combination with good wind protection, the CO 2 exchange ratiobetween the atmosphere and the lake water is low and can also lead to a14C depletion in the water (Hakansson1976).

If the 14C depletion of the freshwater system is caused by dissolved bicarbonates and carbonates, thefreshwater reservoir effect is also called hardwater effect. The water hardness is defined as the concentrationof the alkaline earth metal ions, predominantly calcium (Ca) and magnesium (Mg). They originate often

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from carbonates. The water hardness is therefore an indicator of the carbonate concentration and thus ofthe amount of 14C-dead material in the water. Hard water can be found in regions where limestone andsandstone are dominating.

The effect of dissolved bicarbonates on the radiocarbon age has already been anticipated during the firstradiocarbon datings, when Godwin (1951) examined dates from British lake deposits:

Too great an age may result from introduction of inactive Carbon, as with bicarbonate derivedfrom ancient limestones, brought into a lake, where it is fixed in organic compounds or precipitatedthrough biological activities.

In 1954, Deevey et al. examined a hardwater lake and dated water samples as well as plant samples, alsofrom terrestrial plants. The water samples had an apparent age of 2200 years, and also the clams and plantsfrom the lake which use bicarbonate as a carbon source had comparable ages. Deevey, Gross, Hutchinson,and Kraybill (1954) conclude: Great care must be clearly be exercised in the future when material fromhighly calcareous regions is being examined.

In 2006, B. J. Culleton measured paired samples of charcoal and shells from lake sites in Californiaand found a 340 years discrepancy between them as well as a greater variability of ages among the shellsthan among the charcoal. This might be an evidence for genus-specific reservoir corrections, maybe basedon habitat differences (Culleton 2006). A freshwater reservoir effect also explains the difference betweenarchaeological and radiocarbon-based chronology for the Catacomb cultures of the North-West Caspian

steppe (Shishlina, Van der Plicht, Hedges, Zazovskaya, Sevastyanov, and Chichagova 2007). Seeds andfishbones that were found in the same grave had an age difference of 640 60 14C-years, and the15N valuesof human bone indicated that aquatic protein accounted for 70% of the total protein supply so that a reservoircorrection of about 400 years has to be made for the human bone radiocarbon dates. More examples for thefreshwater reservoir effect in archaeological material will be given in section 1.2.4.

The occurence of the hardwater effect on food crusts on pottery was first proposed in 2003 (Fischer andHeinemeier 2003). The results of Fischer and Heinemeier (2003) have been doubted, though. Apparently, thenumber of dated samples was small enough to explain the age difference between terrestrial and freshwatersamples with statistical methods so that the hardwater effect was not necessary as an explanation. Insteadof a real radiocarbon age difference, Hart and Lovis (2007) explain the measured values with a single outlierand statistical variations. The basics of the hardwater effect will be explained in section 1.2.3.

In coastal areas as for example the Danish fjords, a mixing of two carbon reservoirs can be observed:the marine and the freshwater reservoir. Danish Baltic Sea areas like the Skagerrak-Kattegat and the Belts

show the same reservoir age as the North Sea and North Atlantic: about 400 a (see a description of themarine reservoir effect above). In contrast to that, the Danish fjords have higher and more scattered agesbetween 400 and 900 years and are thus not part of the uniform marine reservoir. The variability in thereservoir ages of the fjords can best be explained with different concentrations of old dissolved carbonate inthe soil (Heier-Nielsen, Heinemeier, Nielsen, and Rud 1995).

1.2.3 How the hardwater effect works

The mechanisms leading to the hardwater are explained in Clark and Fritz (1997) and Fontes and Garnier(1979) where a lot of the information presented here can be found. Normally, there is no reason for datingriver water, because its age is known as modern. It is only used for estimating the hardwater effect thatinfluences the age of freshwater organisms. Most research in order to understand the mechanisms thatproduce the high water ages has therefore been done in the area of groundwater dating. Groundwaterdating is an important part of water resources management. For human use over a long period of time, onlygroundwaters that are recharged constantly provide sustainable solutions. Using groundwaters that are notactively recharged is mining (Clark and Fritz 1997), because the groundwater reservoir will be exhaustedafter a certain period of water extraction. Dating the water is a method to decide whether a groundwaterreservoir is actively recharged or not.

The hardwater effect makes it complicated to date groundwaters because the disolved carbonate makesthe initial activity of the water smaller than 100 pmC. pmC is the abbreviation for percent modern carbon

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and is a measure of 14C activity. 100 pmC is the activity of the atmospheric carbon before the beginning ofthe bomb tests. For estimating the correct initial 14C activity, model calculations had to be done and themechanisms of solution and transport of the different carbonaceous species had to be understood. Ground-water dating is therefore an important field of study also regarding the hardwater effect in other freshwatersystems. The measurement of the tritium content of a groundwater is a possibility for deciding if the wateris affected by the hardwater effect. The half-life of tritium is only 12.26 years so that tritium-containing

groundwater must be post-bomb, i.e. relatively young water. A hardwater effect in groundwater has forexample been reported by Boaretto, Thorling, Sveinbjornsdottir, Yechieli, and Heinemeier (1998) from Hvin-ningdal in Denmark. Tritium values showed that the water was post-bomb water, but its 14C content was30 to 100 pmC.

There are two sources of dissolved carbon in groundwaters and so two sources for an explanation ofhigh 14C ages in young water: relatively active carbon comes from the soil zone and less active carbon is ofinorganic origin. The carbon of inorganic origin is itself a mixture of active carbon from soil gas CO2 andold carbon from carbonate in the subsurface. It is also called DIC, as an abbreviation for dissolved inorganiccarbon. This will be described first. Later, I will explain the effect ofDOC, which means dissolved organiccarbon. DOC and DIC 14C values can differ significantly in groundwater, with DOC in most cases beingyounger, but are often in the same order of magnitude (Long, Murphy, Davis, and Kalin 1992). DOC canin single cases also lead to a spurious, too young radiocarbon age of the water, when young DOC entersold water. Carbon transformations are linked through acid-base and redox reactions which are most often

mediated by bacteria (St-Jean 2003). Bacteria act as catalysts and can in these cases influence isotopic ratiosbecause they prefer to break the looser molecular bonds. The hardwater effect is expected to be greaterin running, i.e. river water, than in stagnant water like lake water. If there is not a noticeable meltwatercomponent, river water consists largely of groundwater (Lanting and Van der Plicht 1996). This is anotherreason why groundwater research methods are being discussed here. The definition of DIC comprises fourspecies:

Carbon dioxide CO2

Carbonic acid H2CO3

Bicarbonate anion or hydrogencarbonate HCO3

Carbonate anion CO23

The first two species, CO2 and carbonic acid, are often summed up as CO2 because the carbonic acid onlyexists in aqueous solution. The bicarbonate anion HCO3 is an amphoteric substance. That means, it canboth act as acid and base because it both can donate and accept protons. Bicarbonate is the conjugate baseof carbonic acid H2CO3 and the conjugate acid of the carbonate ion CO

23 :

CO23 + 2H2O HCO

3 + H2O + OH H2CO3+ 2OH

H2CO3+ 2H2O HCO

3 + H3O+ + H2O CO

23 + 2H3O

+ (1.15)

The concentration of HCO3 and CO23 together is called carbonate alkalinity. Alkalinity denotes the con-

centration of dissolved species which act as proton acceptors and buffer pH, i.e. consume acidity.In the soil, root respiration and the decay of organic material release CO2. In contact with water, a

small amount of this CO2 forms carbonic acid (H2CO3) which can dissolve lime from the surrounding rock.The dissolved lime hardly contains any 14C because of the high age of the lime stone. Carbonic acid is thepredominant acid in natural waters and most responsible for rock weathering (Langmuir 1997). Water plantsconvert CO2 and water to glucose and oxygen and use the glucose as an energy source and a basic materialfor other organic matter. When an extensive amount of the CO2 in the water has a high age, then the plantwill accordingly show a spurious, high radiocarbon age. Correspondingly, fish that feed on these plants andhumans that eat these fish, will also show too old radiocarbon ages. This effect could also be transferred tothe food crusts on pottery in which a considerable amount of aquatic food was cooked. This freshwater partof the carbon cycle will be examined more detailed below.

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Figure 1.6: The pathway and associated fractionation of 14C and 13C in CO2 during photosynthesis, respi-ration in soils, and dissolution by groundwaters (Clark and Fritz 1997).

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Fresh groundwater originates exclusively from precipitation, which in most cases percolates through soiland geosphere. The CO2 in the root zone, which is dissolved by rainwater, has a recent

14C activityand its 13C is approximately -25. The amount of CO2 that can dissolve depends on temperature, initialwater pH and the partial pressure of CO2. Although rainwater contains some CO2 from the atmopshere,the groundwaters radiocarbon signal is dominated by the carbon from the soil zone. In the deeper subsoilexchange takes place between this dissolved CO2 and fossil marine carbonate (e.g. limestone) which has

no 14

C activity and 13

C = +1 (see above). When finally equilibrium is reached, CO2 in groundwaterhas only half the recent 14C activity and its 13C is c. -12 (Lanting and Van der Plicht 1996). Otherprocesses like the dissolution of extra CO2 and exchange processes in the unsaturated zone can alter the14C and 13C concentrations additionally. To correct the measured 14C concentration, the 13C value of thewater can be measured. If assuming that fossil carbonate has a 13C value of about 0 and CO2 from theroot zone has -25, the corrected 14C activity of the water is

Ad= Am25

13Cm(1.16)

with the measured activityAm. The activityAd has now to be taken to calculate the age, as the uncorrectedmeasured activity Am would overestimate the age (Boaretto, Thorling, Sveinbjornsdottir, Yechieli, andHeinemeier 1998). The 14C age measurement can also be corrected by estimating the initial 14C activiyAinthat originates from the dilution of the atmospheric activity Aatm with fossil carbonate:

Ain= Aatm13Cm

25 (1.17)

Boaretto, Thorling, Sveinbjornsdottir, Yechieli, and Heinemeier (1998) discovered thatAm is correlatedwith the oxygen concentration of the water. One can therefore assume that besides the dissolution ofcarbonates, the oxidation of old organic carbon in the soil dilutes the 14C concentration, as will be explainedbelow when discussing DOC.

The existence of a three phase system consisting of gaseous CO2 and aqueous and solid carbon al-lows significant isotopic exchange. Isotopic exchange is for example represented by the following reaction:14CO2+H

12CO3 12CO2+H

14CO3 . This process is responsible for 14C contents significantly higher than

those indicated by the single dissolution process in modern groundwater. It is known since Munnichs re-search in 1957 (Fontes 1992). Each stage of the series of chemical reactions is isotope fractionating: when

CO2 gets dissolved in water, when it forms carbonic and bicarbonic acid and when the CO2

3 forms solidcarbonate. Furthermore, each single compound will exchange with the other coexisting carbon species. How-ever, because of differences in reaction rates and large variations in the relative amounts of coexisting carbonspecies, isotopic exchange reactions may not reach equilibrium for each compound.

The inorganic carbon is mainly controlled by acid-base reactions. The distribution of the three speciesof dissolved inorganic carbon, CO2 (dissolved CO2), HCO

3 (bicarbonate ion) and CO23 (carbonate ion),

is largely a function of pH (St-Jean 2003). The three DIC species are related in the following pH controlledchemical equilibrium:

CO2+ H2O H2CO3 H+ + HCO3 2H

+ + CO23 (1.18)

The most important component of inorganic carbon in natural waters is bicarbonate. It is produced inthe reaction CO2 + H2O + MeCO3 Me(HCO3)2 with Me = Ca

2+, Mg2+ or 2Na+. Dissolved CO2 orcarbonate, depending on the pH, generally accompanies bicarbonate in the unsaturated zone and in theaquifer. The higher the CO2 concentration is in the soil atmosphere, the lower is the initial pH. The lowpH is then buffered by mineral weathering in soil and upper bedrock. The most common and most effectivebuffering reaction is calcite dissolution with Me = Ca2+ in the reaction above. Carbonate dissolution is oftenautomatically considered as an index of14C dilution and reported as hardwater effect. But single examplesshow exceptions from this general rule (Fontes 1992) so that the measurement of the water hardness is notsufficient to trace the dilution of 14C content in the water.

Carbonate is not the only mineral that gets dissolved in natural waters; silicate is another importantcomponent. There is a major difference between carbonate and silicate dissolution regarding the 14C/12C

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of DIC. For systems in contact with the atmosphere, each equivalent of Ca, Mg, Na, and K ions dissolvedfrom silicate minerals is accompanied by the addition of 1 mole of atmospheric CO2 (see equation 1.20).For carbonate, 1/2 mole of atmospheric CO2 is added to the system for each equivalent of Ca and Mgdissolved so that 1/2 of the total CO2 is

14C-free while the other half has atmospheric concentration (seeequation 1.19). In contrast to that, no 14C-free HCO3 is added in the case of silicate solution (Broecker andWalton 1959). The only change to the carbonate system is the increase in pH that is associated with silicate

dissolution. This increase in pH shifts the distribution of DIC species to the HCO

3 field. If this occursunder non-saturated conditions in an open system, additional CO2 will be dissolved from the soil zone. Inany case, the DIC from silicate weathering is derived solely from soil CO2 .

CaCO3+ H2O = Ca++ + HCO3 + OH

OH + CO2 = HCO

3

CaCO3+ H2O + CO2 = Ca++ + 2HCO3

(1.19)

In the following example of silicate dissolution, the plagioclase feldspar anorthite CaAl2Si2O8 is dissolved.

CaAl2Si2O8+ H2O = Ca++ + 2OH + Al2O3+ 2SiO2

2OH + 2CO2 = 2HCO

3

CaAl2Si2O8+ H2O + 2CO2 = Ca++ + 2HCO3 + Al2O3+ 2SiO2

(1.20)

These reactions are based on the assumption that nearly all the dissolved CO2 in fresh-water bodies is inthe bicarbonate ion form (Broecker and Walton 1959). After the bicarbonate is formed, its 14C/12C ratio iscontrolled by the second factor, the exchange ratio of CO2 with the atmosphere. In igneous areas, silicatesolution is dominant so that the majority of dissolved bicarbonate has atmospheric origin. Rivers which areflowing over sedimentary rocks dissolve both silicate and carbonate minerals (Broecker and Walton 1959).

The 14C/12C ratio of the dissolved bicarbonate in fresh waters is thus controlled by

1. the ratio of the amount of carbonate to the amount of silicate dissolved2. the rate of exchange with CO2 in the atmospheric reservoir

CO2 exchange rates for rivers can be as high as 100 moles/m2, whereas CO2 exchange rates in inland lakes

are about 5 moles/m2 (Broecker and Walton 1959).Age distortion can also be introduced by dissolution of secondary calcite precipitated under open-system

conditions. Fractionation between dissolved HCO

3 and solid calcite is probably very small. When calcitethat emerged from dissolved HCO3 is than dissolved again, it is hardly noticeable in the

13C content of theTDIC, and may lead to apparent aging of the TDIC (Fontes 1992). TDIC means total dissolved carbon.Thus the age of the DIC changes without changes in the 13C content that may indicate the age distortion.One example for this overestimation of 14C ages can be found in Aeschbach-Hertig, Stute, Clark, Reuter,and Schlosser (2002). 14C can further be diluted by sulphate reduction and methanogenesis, 2CH2O CO2+ CH4.

CO2 produced from oxidation of organic matter most likely will alter the 14C activity of the DIC as

CO2 is one of the DIC species, no matter what its origin may be. A lower pH that is associated with thehigher concentrations of dissolved CO2 will additionally dissolve calcite if present and so further alter the14C activity in the DIC (Long, Murphy, Davis, and Kalin 1992). As Boaretto, Thorling, Sveinbjornsdottir,Yechieli, and Heinemeier (1998) noticed, is the 14C concentration of the water after correction for carbonatedissolution correlated to the oxygen content of the water. For estimating the extent of oxidation of old organic

matter in the soil, the highest measured oxygen concentration in the water body is taken as the initial oxygencontentration [O2]in. We assume that the missing oxygen was used for the oxidation of organic matter, C org+ O2 = CO2. The fractionFof the total inorganic carbon in the water that was derived from the oxidationof organic matter is thus calculated to

F = 1.38[O2]in [O2]

[CO2,total] (1.21)

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where 1.38 is the ratio of the molecular weights. The 14C activity of the water when corrected for carbonatedissolution and organic carbon oxidation is thus

Acorr = Am25

13Cm+ 25F. (1.22)

The dissolved organic carbon DOC is the result of decomposition of higher organic matter, such as

plants, in the soil. Its particle size is smaller than 0.45 m. Organic carbon with particle sizes bigger than0.45m can also be present in groundwaters and is called particulate organic carbon. The DOC originatesfrom chemical and physical changes in soil organic matter (SOM) that becomes soluble (St-Jean 2003) andis stored in the soil waters. The concentration of DOC in soil moisture can reach a maximum of 10 to100mgC/L in the root zone. This concentration drops off towards the water table. Groundwaters oftenhave less than 1 to 2 mgC/L DOC, although groundwaters in certain environments can recharge with muchhigher DOC concentrations (Clark and Fritz 1997). DOC species comprise the so-called humic substancesthat are responsible for the dark colour of soils and some waters. Humic and fulvic acid are both humicsubstances and they are often the reason for contamination with younger carbon in archaeological samples.

DIC can be extracted from the water sample by acidifying it. The procedure used for my water sampleswill be described in detail in section 4.1.1. DOC can be extracted afterwards when the DIC is removed.One isolation technique for DOC is photo-oxidation, e.g. irradiating 2L of water with a 1200W UV-lampafter adding Na2S2O8 to expedite oxidation. In another isolation technique, water is passed through a

series of columns packed with pre-cleaned resin materials to collect DOC of different sizes (Long, Murphy,Davis, and Kalin 1992). In 10 to 120L filtered water, liquid chromatography separation of hydrophobicsubstances, fulvic and humic acids, is possible (St-Jean 2003). Whatever method is used, DOC analysis isregarded as difficult and time-consuming and requires large samples (St-Jean 2003). High molecular weightsubstances, for example humic acids, are insoluble below a pH of 2. They precipitate when DIC is removedby acidification, and that makes them more resistant to oxidation (St-Jean 2003).

When measuring the reservoir age of a water system, it has to be taken into account that the 14C contentof the water can be subject to seasonal or secular changes. The dating of one water sample only, taken ata special time, can therefore be not more than an estimate of the reservoir effect (Geyh, Schotterer, andGrosjean 1998). Lakes are subject for the biggest changes because in them there is no continuous flow ofwater. Bicarbonate for example is probably periodically concentrated in lakes by evaporation (Culleton2006). A very slowly running river or a lake can regain recent activity due to exchange with atmosphericCO2. The extent of the hardwater effect is for example dependent on the surface-to-volu

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14C Food Crust Analysis on Mesolithic Pottery