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Journal of Archaeological Science 35 (2008) 2686–2697

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Journal of Archaeological Science

journal homepage: ht tp: / /www.elsevier .com/locate/ jas

Shell-gathering from mangroves and the seasonality of the Southeast AsianMonsoon using high-resolution stable isotopic analysis of the tropical estuarinebivalve (Geloina erosa) from the Great Cave of Niah, Sarawak: methods andreconnaissance of molluscs of early Holocene and modern times

Mark Stephens a,*, David Mattey b, David D. Gilbertson c, Colin V. Murray-Wallace d

a Department of Geography, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UKb Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UKc School of Geography, University of Plymouth, Plymouth, Devon, PL4 8AA, UKd School of Earth and Environmental Sciences, University of Wollongong, NSW 2522, Australia

a r t i c l e i n f o

Article history:Received 27 March 2007Received in revised form 25 April 2008Accepted 28 April 2008

Keywords:Geloina erosaGreat Cave of NiahLaser ablationMonsoonSoutheast AsiaStable isotopes

* Corresponding author. Tel.: þ44 01484 546822.E-mail address: m_stephens2004@yahoo.co.uk (M

0305-4403/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.jas.2008.04.025

a b s t r a c t

This paper describes a reconnaissance, high-resolution, study of stable isotopes (d18O and d13C) frommodern shells of the estuarine bivalve Geloina erosa, and those dated to the early Holocene that wereharvested by people from mangroves near the Great Cave of Niah on the north coast of Borneo. Thisreconnaissance study provides high-resolution palaeoclimatic-palaeohydrological information concern-ing early human activity in the region and the past character of the Southeast Asian Monsoon. Laserablation continuous flow isotope ratio mass spectrometry (LA-CF-IRMS) on modern shells of Geloinaerosa revealed ‘saw-tooth’ stable isotopic profiles that bear a close resemblance to peaks and troughs oftrends in recent local rainfall, including the 1998 El Nino drought, highlighting the potential of Geloinaerosa for reconstructing seasonality of the Southeast Asian Palaeomonsoon. LA-CF-IRMS analysis ofprehistoric shells of Geloina erosa held in the Harrisson Archives of the Sarawak Museum revealed cy-clical shifts in d18O with similar amplitudes of variation as found in modern shells of Geloina erosa. As atthe present day, this probably reflects the changing seasonality of the monsoon rains with shifts tonegative d18O values during periods of high runoff. Lighter mean d18O values of the prehistoric shells,however, may indicate a greater annual surplus of rainfall and possibly consistent with the early Holo-cene strengthening of the summer monsoon at that time. The similarity of the last growth incrementd18O of the prehistoric shells to their mean d18O profile values suggests that gathering took place duringtimes of moderate runoff.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

This paper presents a reconnaissance, high-resolution, study ofstable isotopes (d18O and d13C) of modern and prehistoric shellsdated to the early Holocene of the mollusc Geloina erosa collectedfrom estuarine muds in mangroves bordering the Kuala (Estuary)Niah at the wet monsoonal northern coast of Sarawak in MalaysianBorneo (Fig. 1a and b).

It has three objectives:

(1) to explore the extent to which it is possible to recognise thepresent seasonal monsoonal climate using laser ablation con-tinuous flow isotope ratio mass spectrometry (LA-CF-IRMS)

. Stephens).

All rights reserved.

analysis of the growth profiles of modern shells from man-groves, in combination with the results of earlier stable isotopicstudies of waters in the Niah catchment (Stephens, 2005; Ste-phens and Rose, 2005);

(2) to establish if LA-CF-IRMS studies of d18O and d13C can identifyanalogous profiles within shells harvested as food and thendiscarded in the West Mouth of the Great Cave of Niah duringthe early Holocene; and

(3) if so, is it possible to identify existence and finer details ofa monsoonal climate in part of the early Holocene (w8.6 14C kyrBP; and then expand the analysis to discover the time of yearwhen these molluscs were harvested for food.

The Southeast Asian Monsoon is a key component of the Earth’sclimate system, characterised by periodic heavy rains and thendrier seasons that are associated with seasonal changes inwind direction over Australasia–Asia. Detailed, high-resolution,

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–2697 2687

palaeoclimatic information on the Southeast Asian Monsoon isscarce. This also limits understanding of many aspects of earlyhuman activity, especially concerning human subsistence, dispersaland palaeobiogeographical reconstruction in this tropical region(Gilbertson et al., 2005; Hunt and Rushworth, 2005; Stephens et al.,2005; Barker et al., 2007; Hunt et al., 2007), as well as the validationand application of global climate models (e.g. COHMAP, 1988;Hoffmann and Heimann, 1997; Kutzbach, 1981).

Potentially rich sources of information about the historic andprehistoric past of this monsoonal region exist in the region asa result of the large-scale excavations of the West Mouth of theGreat Cave of Niah in Sarawak, Malaysian Borneo (Figs. 1c, 2a, 3) byTom and Barbara Harrisson between 1954 and 1967 (Harrisson,1970). This work created the Niah Cave Archive that is maintainedby The Sarawak Museum and overall the Archive reflects aroundw50 ka of human activity in and around this cave in lowlandtropical rainforest. Whilst the cave is perhaps best known becauseit contains human skeletal and artifactual evidence for some of the

Fig. 1. (a) Niah in the context of Southeast Asia. (b) The Sungai Niah basin with locationof collection of the two live Geloina erosa molluscs (on 22 April 2002). (c) The WestMouth of the Great Cave of Niah.

earliest anatomically modern humans in Southeast Asia (Barkeret al., 2002, 2007), the Archive contains the shells of estuarinemolluscs, including the bivalve Geloina erosa, that were harvestedin prehistory from muds accreting in mangrove swamps. The size(c. 8 cm length) and thickness (c. 0.6 cm) of the shells of the estu-arine bivalve Geloina erosa from the Archive were thought partic-ularly suitable for high-resolution studies using LA-CF-IRMS, due totheir large size, thickness and abundance in the Archive (Medway,1960; Stephens, 2005).

A limitation of this particular source of evidence, however, isthat it was produced by an excavation strategy of horizontal ‘‘spits’’,typically shallow rectangular ‘‘slices’’, that in large part involved thecomplete removal of sediments (Fig. 2). This excavation did notrecognise the complex three-dimensional stratigraphy and strati-fication that often involved steeply dipping sediments as well asepisodes of erosion or non-deposition (Gilbertson et al., 2005; Huntet al., 2007). As a result, the correlation of these archived shells withthe surviving remnants of the sequence that have been dated byAMS 14C of in situ charcoal and amino acid racemisation (AAR)determinations of the shells of in situ Bellamya javanica (Stephensand Murray-Wallace, in press) requires the judgement illustrated inFig. 3. As a result, this paper is primarily concerned with examiningan analytical approach.

The Great Cave is now w12 km in a direct route through wetlowland rainforest to the shore of the South China Sea, and 3–4 kmeast of the extensive mangroves that line the estuary of the Sungai(River) Niah (Hazebroek and Morshidi, 2001). During the mid-Holocene back-swamp mangroves reached the northern margins ofthe Great Cave of Niah (Hunt and Rushworth, 2005). The number ofother occasions when mangroves were close enough to be har-vested for food through history and prehistory is currently underinvestigation; the precise distances that people travelled to reachthe mangroves in the early Holocene are unknown.

A fundamental issue of concern within the ancient human ge-ography and coastal geomorphology of this coastal zone involvesunderstanding seasonality of subsistence and site occupation. Oneof the most promising methods for determining seasonality ofharvesting of shellfish is that of oxygen isotope analysis of the lastgrowth increments of mollusc shells from coastal shell middens(e.g. Shackleton, 1969, 1973; Bailey et al., 1983; Deith, 1983, 1985;Deith and Shackleton, 1988; Godfrey, 1988; Kennett and Voorhies,1996; Andrus and Crowe, 2000; Mannino et al., 2003). Estuarineshells are particularly suitable for reconstructing seasonality sinceoxygen isotope analysis of these shells typically produces ‘saw-tooth’ growth profiles, that reflect strong seasonal variations inrunoff (e.g. Leng and Pearce, 1999; Aguirre et al., 1998; Andrus andCrowe, 2000; Stephens, 2001; Khim et al., 2003).

Laser ablation of shell growth profiles offers the possibility toanalyse these geochemical archives at high spatial and temporalresolution (e.g. Fuge et al., 1993; Schettler and Pearce, 1996; Priceand Pearce, 1997; Andrus and Crowe, 2000; Vander Putten et al.,2000; Lazareth et al., 2003). Laser ablation facilitates sampling ofinner parts of individual growth layers as demonstrated in thispaper and reduces the problems of diagenesis and damage asso-ciated with analysing the periphery of outer shell layers (Tolandet al., 2000). X-ray diffraction (XRD) analyses have also been per-formed on a range of shell species excavated from the West Mouthto inform on the preservation of shell mineralogy and the resultsare also described here.

Laser ablation also allows a comparatively rapid rate of analysis(Spotl and Mattey, 2006) and therefore makes it a suitable tech-nique for a reconnaissance-level study. In this work we have useda laser ablation technique to obtain analyses at 700 mm spatialresolution (Spotl and Mattey, 2006) for a reconnaissance study ofmodern and early Holocene shells of the estuarine bivalve Geloinaerosa, to obtain high-resolution palaeoclimatic-palaeohydrological

Lunch

table

-2.72

-1.13

-3.66

-2.13

-3.80

-3.32

-2.63

-3.510.0

-3.38

-2.26

-2.23

-3.30

-0.81-1.84

-2.27

-1.44-1.79

-0.10

-0.76 -0.09

-1.88

-2.02-1.95

0.26

-2.64

-1.97

-2.75

-2.77

-1.70

-1.75

2.17

3.23

3.46

2.79

4.30

4.29

6.15

5.52

5.39

5.21

6.23

6.68

6.707.03

9.078.74

8.95

9.51

9.65

9.28

8.628.39

8.23

9.76

8.28

7.08

7.25

7.56

7.37

6.21

3.98

2.56

5.14

-0.97

N

Boulder

Hut

0 metres 10

Fissue in cave wall

See inset (b.) for detail

of Harrisson’s excavations

10

b

X/V2A

X/E1

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E/X2

Fissure incave wall

Section 10.02

X/W1

W/3B

W/E2

X/V1

Wedge of remainingUnit 4 sediments underoverhang and adjacentto cave wall

a

metres0

Fig. 2. (a) The archaeological zone in the north part of the entrance of the West Mouth of the Great Cave of Niah. Extensive grids of excavation trenches and pits are shown and arelargely the work of Tom and Barbara Harrisson between 1954 and 1967. Spot heights (in metres) indicate the topography of the remaining deposits. The location of section 10.02 ishighlighted and is detailed in Fig. 3. (b) Plan view of Harrisson’s excavation trench X/V1 and adjacent trenches in the NW corner of the West Mouth. The location of section 10.02 isshown, which details Lithofacies 4 deposits (see Fig. 3) adjacent to Harrisson’s trench X/V1. Note: the original excavation grid of Harrisson has been placed in its ‘true’ position in thecave mouth by deduction based upon reference to remaining sections and baulks.

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–26972688

information concerning early human activity in the region and thepast character of the Southeast Asian Monsoon.

2. Geography of the Great Cave of Niah and Kuala Niah

The Great Cave of Niah is situated in a region of dense wettropical lowland rainforest and is developed in the massif known asthe Gunong Subis (N 3� 480; E 113� 470), mainly of limestones of theTangap Formation of Miocene age (Banda and Heward, 2000;Hazebroek and Morshidi, 2001). The West Mouth of the Great Caveof Niah is a vast cave entrance (60 m high and 180 m wide) thatopens about 12 m up the cliff-like side of a bedrock-floored gorge,which separates the massif containing the Great Cave from themain massif of the Gunong Subis. The main Harrisson excavationsare on the north side of the West Mouth, immediately inside theentrance to the cave (Fig. 1c; and see Barker et al., 2002, 2007 forfurther detail).

The stable isotope chemistry of waters in the Sungai and KualaNiah reflect many aspects of the physical geography of the area(Hazebroek and Morshidi, 2001; Stephens and Rose, 2005). TheGunong Subis lies within the catchment of the Sungai Niah, whichhas its source in the northern range of the Dulit Mountains, some60 km inland. At its mouth, the Kuala Niah has a maximum width ofw150 m where it flows into the South China Sea. Several small

tributary streams of the Sungai Niah originate/flow through theGunong Subis, including the Sungai Subis and the Sungai Sekaloh.Inland, the main river flows largely through dense, mixed wetdipterocarp rainforest although some small urban centres havedeveloped along the river (Fig. 1b).

Close to the sea, the Kuala Niah is lined with a variety of man-grove species; drier terrain inland being increasingly given over toplantations of oil palm. Behind the offshore sand bar with itsfringing Casuarina, and on the alluvial basin of the Sungai Niah,there are extensive peat swamp forests where drainage is impeded(Wall, 1966). A survey by Blaber et al. (1997) in 1994 of the physicalcharacteristics of 23 estuaries in Sarawak and Sabah, including thenearby Kuala Sibuti (Fig. 1b), found that all estuaries in north-central Sarawak have tidal ranges of <2 m, relatively lowzooplankton biomasses and variable salinities that are largelycontrolled by seasonal changes of freshwater inflow. The SungaiNiah is tidal and as with the other rivers of north-central Sarawaksalinity varies on a (daily) tidal basis. During the wet season therivers become very fresh and conversely during the drier season,salinity is higher.

Sarawak is situated in the hot and wet humid tropics and has anequatorial climate. Climatic patterns in Sarawak are directly relatedto atmospheric pressure variations in Southeast Asia, affected bythe large Asian and Australian land masses. The heating capacity

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Fig. 4. Meteorological parameters measured from 01 February 1996 to 22 April 2002in north-central Sarawak: (a) rain (Niah, Rangkaian Hidrologi Sarawak); (b) tempera-ture (Miri, Malaysian Meteorological Service (MMS)); (c) relative humidity (Miri,MMS). The MMS station at the coastal town of Miri is 50 km north east along the coastfrom Niah. Rainfall is highly variable when compared with temperature and relativehumidity; rainfall peaks around the northeast monsoon (typically November to March)and is reduced during the southwest monsoon (typically June to September).

24-36"

(61-91.5 cm)

36-48"

(91.5-122 cm)

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(30.5-61 cm)

0-12"

(0-30.5 cm)

Harrisson excavations (1954-1962)

Gradual sedimentary unit boundary

Cobble-rich layer

AMS 14C date (8,630 ±45 BP) (OxA-11549)

AMS 14C date (19,650 ±90 BP) (OxA-11550)

Key:

Limestone

wall overhang

Red painted line (drawn byHarrisson to show originalsediment surface level)

7.5YR 4/4 brown, bedded sands and silts (L. 2)

10YR 5/4 yell. brown, crudely bedded silts (L. 4)

0 10 20 30 40 50 cm

Fig. 3. Scale drawing looking 120� EES at an exposure of Lithofacies 4 and Lithofacies 2in the West Mouth (section 10.02, Fig. 2b). Approximate location of Harrisson’s exca-vation levels for trench X/V1 is shown. Geloina erosa shells were analysed in this studyfrom the levels of 0–12, 12–24 and 24–36 inches. However, since Harrisson excavatedhorizontal levels across inferred dipping strata of Lithofacies 4, shells sampled be-tween 0 and 36 inches could be from the same sedimentary layer and hence the sameage. Star symbols represent AMS 14C dates on charcoal from sampling contexts 1015and 1020 (samples taken from the adjacent section 10.01) (Barker et al., 2002).

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–2697 2689

contrast between land mass and ocean drives monsoon circulationand produces extreme seasonality in wind direction and rainfallover Southeast Asia. Greatest warming in the tropics producesa belt of low pressure due to warm, rising air and is termed theintertropical convergence zone (ITCZ). The strongest precipitationin the tropics can be found along the ITCZ. During the northernhemisphere winter, the combination of high pressure in China andlow pressure over Australia pushes the ITCZ further south, whichhas the effect of bringing the northeast (winter) monsoon(November to March) over Sarawak. The converse is true during theNorthern Hemisphere summer with low pressure over Asia andhigh pressure over Australia and the ITCZ migrating north. At thistime, Sarawak is influenced by the southwest (summer) monsoon(June to September). The northeast monsoon brings the heaviestrainfall of the year (Fig. 2a) and consequent maxima in river runofflevels in Sarawak at this time (Blaber et al., 1997; Staub et al., 2000).This pattern is complicated by El Nino events, however, whichcause drought in the region (Nakagawa et al., 2000). In contrast tothe variability of rainfall in Sarawak, temperature is constantlywarm with only small deviation around the annual mean of w27 �C

(Fig. 4b) (Walsh, 1982). In accordance with the high rainfall andtemperatures, humidity is constantly high at around 85% (Fig. 4c).

It is clear from this analysis, and especially the meteorologicalparameters shown in Fig. 4, that monsoon rainfall is the mostseasonally variable parameter and a key control on the hydrologicalresponse at Niah. In this paper, rainfall data collected from Niah arecompared with the stable isotopic profiles recorded in the modernshells of Geloina erosa.

3. Modern ecology and shell growth of Geloina erosa

Geloina erosa (Solander) is also known as Polymesoda andCyrena. It is a large edible bivalve mollusc and is common inmangroves of Southeast Asia (Morton, 1984). The species is infaunaland occurs on the landward side of mangrove swamps arounda drainage network of streams and pools that may vary widely insalinity according to the state of tide and the amount of rainfall(Frith et al., 1976; Morton, 1976). It has adapted to be able to surviveemersion for many weeks through aerial respiration (Depledge,1983) and during periods of drought the mollusc collects

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–26972690

groundwater and filters detrital food via the pedal gape in burrows(Morton, 1976). Diurnal studies by Morton (1975) of the closelyrelated Geloina proxima have shown that filtering for food beginsimmediately in response to immersion either by tidal or rain waterwith maximal feeding occurring during intermediate salinity. Thisfeeding response reflects the adaptation of Geloina to the temporaldynamics of the mangrove environment. For Geloina erosareproduction occurs over a single extended phase during summer(June–August) that coincides with the higher summer tempera-tures (Fig. 4b) (Morton, 1985).

In bivalves, the body (‘veliger’) is encased by a shell containingtwo valves hinged together. Geloina erosa produce valves made ofaragonite. These grow outward from the umbo to the ventralmargin and it is this accretionary growth that provides the envi-ronmental record through time (Rhoads and Lutz, 1980) (Fig. 5).These molluscs can produce several layers of material including theouter organic periostracum and inner shell layers. The perios-tracum isolates the shell edge during growth and helps protect theshell from dissolution in acidic waters, although damage some-times occurs in the very acidic mangrove soils (Morton, 1984). TheLA-CF-IRMS analyses were carried out on Geloina erosa shellswithin their upper growth layer as a result of its thickness and thisbeing the last layer deposited by the mollusc before harvesting.

4. Factors affecting d18O and d13C in the shells of tropicalestuarine molluscs

Previous stable isotopic (d18O and d13C) analyses of modern watersin the Niah catchment by Stephens (2005) and Stephens and Rose(2005) found spatial and seasonal variations consistent with theseasonality of the Southeast Asian Monsoon and regional-scalehydrological processes (e.g. Cobb et al., 2007). Aquatic molluscs thatgrow their shells in these waters may also record these environmentalparameters and this highlights the potential for palaeoenvironmentalreconstruction (Stephens, 2001; Stephens et al., in press).

Both theoretical calculations (Urey, 1947) and constrainedexperiments (McCrea, 1950; Epstein et al., 1953; O’Neil et al., 1969;Grossman and Ku, 1986; Kim and O’Neil, 1997) have shown that the

Youngest, upper growthlayer is sampled

VM

Direction of shell growth

a

Fig. 5. Sketch showing a cross-section profile of the major growth bands within a Geloina erothin section images (a and b) viewed under cross-polarised light, reveal the detail of the grow3–4 cm towards the VM and so sampling is concentrated here (Fig. 6).

d18O of a slowly precipitating carbonate is controlled both by thed18O of the water in which precipitation occurs and the prevailingwater temperature. The d18O of water in tropical estuarineenvironments, in particular the Kuala Niah, is particularly variabledue to an interplay of factors such as the variable isotopic signal ofrainfall, evaporation, residence time, groundwater contribution andtidal exchange with seawater (Stephens and Rose, 2005). In wettropical areas seasonal temperature changes are minimal and it isthe heavy rainfalls that control fractionations of 18O and 16O,termed the ‘amount effect’ (Dansgaard, 1964; Gat, 1996; Araguas-Araguas et al., 1998). The monsoon rainfalls have a major influenceon seasonal variations of salinity within rivers, with shifts toisotopically lighter values during the wet season.

Source and transport history of rains are also important factorsthat must be considered when interpreting d18O records (e.g.Aggarwal et al., 2004). Major moisture sources for Sarawak duringthe winter monsoon are the tropical western Pacific Ocean and theSouth China Sea whereas during the summer monsoon moisturefrom the Indian Ocean dominates (see Aggarwal et al., 2004, andreferences therein for more detail on moisture transport patterns inthe Asian monsoon region). These water bodies had heavier d18Oduring the last glacial period up until the Holocene by which timeextensive deglaciation had occurred in the Northern Hemisphere.Stable isotope analysis of dated groundwaters in South andSoutheast Asia suggests that the overall structure of monsooncirculation and moisture transport patterns have remained rela-tively stable over the last 20 ka (Aggarwal et al., 2004). This body ofobservation suggests there is potential in the wet lowland tropicsfor both the reconstruction of the isotopic composition of the waterthat the mollusc used to grow its shell, and to make inferencesabout previous monsoon rainfall conditions (e.g. Kennett andVoorhies, 1995).

Studies of carbon isotopes provide useful information such asbiological cycling within the drainage catchment (e.g. Keith et al.,1964; Keith and Parker, 1965), vegetation type (C3 vs. C4 plants)(e.g. Surge et al., 2003) and the diet of molluscs (e.g. Metref et al.,2003), depending on what carbon source the mollusc used to buildits shell. Carbon sources for aquatic molluscs are dissolved

Umbo

b

sa shell; growth occurs outwards from the umbo towards the ventral margin (VM). Twoth lines; image b shows that the most recent (upper) layer becomes thicker in the final

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–2697 2691

inorganic carbon (DIC) and/ or metabolic carbon. Fritz and Pop-lawski (1974) and Hickson et al. (1999) report DIC as being theprimary control of shell d13C. Tanaka et al. (1986), however, showedthat up to 85% of mollusc shell carbonate can originate from met-abolic carbon. Indeed, where a large negative shift exists in d13Cshell relative to the DIC, metabolic CO2 derived from respiration, iscommonly attributed as a major factor (e.g. Fastovsky et al., 1993;Dettman et al., 1999; Vander Putten et al., 2000; Owen et al., 2002;Mueller-Lupp et al., 2003). Linked to changes in the metabolism ofa mollusc are ontogenetic factors such as feeding, growth, old ageand reproductive activity (Erlenkeuser and Wefer, 1981; Krantzet al., 1987; Wefer and Berger, 1991; Vander Putten et al., 2000).Consequently, shell d13C variations might be influenced primarilyby metabolism rather than reflect the carbon isotopic trend of DICin the external aquatic environment. Hu et al. (2002) have dem-onstrated the dominant presence of C3 plants throughout theglacial and Holocene periods for north Borneo.

5. Methods

5.1. Shell collection and dating: Harrisson and theNiah Cave Archive

Shells of Geloina erosa were selected for study from trench X/V1,which was excavated by Harrisson in 1958 (Table 1, Fig. 3). TrenchX/V1 was chosen in particular since it could be related to the sur-viving stratigraphy; a position close to the fissure (Fig. 2b) andadjacent to a component of a major lithofacies termed Lithofacies 4in the Late Quaternary stratigraphy of the West Mouth (section10.02, Fig. 3; Gilbertson et al., 2005). The suggested contempora-neity between deposits removed during excavation of trench X/V1with the Lithofacies 4 deposits (Fig. 3; Stephens, 2005) has sincebeen confirmed through amino acid racemisation (AAR)determinations on shells of Bellamya javanica from both X/V1 andLithofacies 4 (Stephens and Murray-Wallace, in press). AMS 14Cdating of charcoal from the Lithofacies 4 deposits (Fig. 3) indicatesdeposition of Lithofacies 4 during the Late Pleistocene and earlyHolocene (Gilbertson et al., 2005; Stephens, 2005). PreliminaryAAR determinations on 12 ‘‘archived’’ shells of Bellamya javanicafrom 0 to 48 inches depth below the original sediment surface inarea X/V1 indicate an age range of latest Late Pleistocene to mid-Holocene (Stephens and Murray-Wallace, in press) and are in broadagreement with the AMS radiocarbon age of 8630 � 45 14C years BPon charcoal from the top of Lithofacies 4 (Fig. 3).

Shells of Geloina erosa were taken from each sampling level of X/V1 that was found in the Archive; at excavation spit-depths reportedby T. Harrisson as 0–12,12–24, 24–36 and 36–48 inches. These shellswere put into labelled plastic bags and kept in the cool store at RoyalHolloway. No shells of Geloina erosa were found between 48 and72 inches of X/V1 in the Archive. The organic periostracum was notpreserved on any of this archaeological shell material. The shells ofGeloina erosa were analysed using LA-CF-IRMS from the spit-depths0–12, 12–24 and 24–36 inches of X/V1 (Table 1).

Preliminary AAR determinations of MIG3 (X/V1 24–36 inches)and MIG1 (X/V1 0–12 inches) produced aspartic acid D/L ratios of

Table 1The sampling context of the Geloina erosa shell samples analysed using stable iso-topes in this study

Sample code Context

MOG1 Collected live on 22/4/02 from S. NiahMOG2 Collected live on 22/4/02 from S. NiahMIG1 Harrisson Archive trench X/V1 0–12 inchesMIG2 Harrisson Archive trench X/V1 12–24 inchesMIG3 Harrisson Archive trench X/V1 24–36 inches

0.57 � 0.06 and 0.40 � 0.03, respectively (Stephens and Murray-Wallace, in press). D/L values may provide a useful indication of therelative ages of the shells but to estimate numerical ages the rate ofracemisation must be determined through calibration to radiocar-bon dated shells (e.g., Sloss et al., 2004).

In the absence of direct radiocarbon dating we therefore use thecalibrated data from another species by which to produce pre-liminary age estimates for Geloina erosa. Murray-Wallace andKimber (1988) analysed specimens of Anadara (Tegillarca) granosafrom Princess Charlotte Bay in northern Queensland; the calibratedradiocarbon age is 7430 � 130 years cal BP and the aspartic acid D/Lratio was 0.52 � 0.01 with a mean annual temperature (MAT) of26 �C. Since Sarawak has a very similar MAT of 27 �C and assuminga similar racemisation rate between Anadara (Tegillarca) granosaand Geloina erosa, we speculate that: MIG3 (X/V1 24–36 inches) isof early Holocene age and MIG1 (X/V1 0–12 inches) is of mid-Holocene age, in agreement with AAR determinations on Bellamyajavanica from 0 to 36 inches of X/V1 (Stephens and Murray-Wallace, in press).

Although the AAR determinations of MIG3 (X/V1 24–36 inches)and MIG1 (X/V1 0–12 inches) appear to have stratigraphic consis-tency with increasing D/L ratio with depth, the age of MIG2 (12–24 inches) may be anything in age from the early to mid-Holocenedue to Tom Harrisson’s sampling strategy of excavating horizontalslices – spits – across inferred dipping strata (Fig. 3) creatinga mixing of shells of various ages in the Archive.

5.2. Shell collection: modern analogues

In order to gain some understanding of the stable isotopicrecord of the prehistoric shells of Geloina erosa we analysed modernshells of Geloina erosa and compared the isotopic record of these tomodern meteorological information. Collection on 22 April 2002was with the guidance of the local naturalist and archaeologist, thelate Edmund Kurui. This work was made difficult due to high tideand local hazards. A day of intense searching through the man-groves produced only two live Geloina erosa specimens from theirmodern natural habitat; just below the sediment surface in theback-swamp areas of mangroves that border the Kuala Niah (Fig. 1b,Table 1). In the research base centre at Niah, the shells were openedby boiling in water and the soft parts were removed with a pair oftweezers.

5.3. XRD analysis of shell

XRD analyses were conducted to analyse the mineralogy ofa range of molluscan species from different excavation contextsof the West Mouth Archive to determine if there were any tracesof chemical alteration (or ‘diagenesis’) and therefore whether theshells were suitable for stable isotopic analysis. XRD analyses wereperformed on the following shells: Anadara granosa (E/A2 24–36 inches); 15 Bellamya javanica (X/V1 0–48 inches and from ourown investigation of Lithofacies 4); Crassostrea sp. (X/V1 24–36 inches); Cyclophorus borneensis (E/B1 0–12 inches); Pila scutata(E/A2 24–36 inches); and two Geloina erosa (D/E3 24–30 inches andE/A 0–12 inches).

Powdered shell material was adhered onto a silicon mount us-ing isopropenol and an x-ray beam was defracted off the spinningmount to the detectors. Results came in the form of elementalpeaks: all shell (except for Crassostrea sp.) produced major peaks of26.2� and 27.2� (see Stephens, 2005 for a graphical display of theresults) typical of aragonite (Hardy and Tucker, 1988). Crassostreasp. (oyster) had a major peak at 29.5�, representing calcite, as ex-pected for oysters. Diagenetic alteration of aragonitic shells typi-cally involves conversion to the more stable polymorph calcite; no

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–26972692

sign of diagenetic alteration was therefore detected in any of theshells reported here.

5.4. Sample preparation for stable isotope analysis

In order to analyse the axis of maximum growth of the shellsusing LA-CF-IRMS, the shells had to be held in a sturdy upright(growth bands facing upwards) position. Following cross-sectionalsawing (1.75 mm diamond-bladed saw) along the axis of maximumgrowth, a strip of shell (c. 0.6 cm wide) was sawn from one of thevalves of Geloina erosa. The strip of shell was then cleaned of anyadhering soil or sediment through mechanical agitation in anultrasonic bath.

Organic material was then mechanically separated from theshell with a scalpel, and further removed through oxidationfollowing the addition of 10% H2O2 in a glass beaker. The beakerwith the shell and 10% H2O2 was placed on a hotplate (in a fumecupboard) to accelerate the oxidation process and was left until theshell strip was fully bleached of organic discolourations. Thisprocedure was particularly important for the modern shells thathad an intact organic periostracum. The LA-CF-IRMS sample holderrequired a sample <4 cm diameter; Geloina erosa shells are typi-cally >4 cm and so the shell strips were broken into two or threepieces (Fig. 6).

Two methods were tested for mounting the shell strips for laserablation analysis along a longitudinal traverse. The initial attemptwas to encase the shell in a resin, and thus stabilise the shell edgesthat may be fragmented by laser heating (Spotl and Mattey, 2006).A rectangular foil ‘boat’ was made and the shell strips placed side byside in the boat with their internal growth banding facing upwards.Polyester resin was then mixed with 1% hardener and carefullypoured into the foil boat so as not to contaminate the upper surfaceof the shell to be analysed. The resin was poured to just below theupper surface of the shell and then left to set (w1 h). When set theblock was polished to create a flat surface ready for analysis.

A second, less time-consuming method was to grind flat thebase of the shell strips and these were then simply glued to a cov-erslip with epoxy resin (Fig. 6). Prior to this, coverslips were groundto <4 cm diameter so as to fit into the LA-CF-IRMS sample holder.The upper surfaces of the shell strips to be analysed were thencarefully ground flat with a diamond lapping plate and a 600 silicacarbide slurry (w25 mm grit). The upper surfaces of the shell stripswere finally polished with an aluminium oxide slurry (w1 mm grit).Polishing allowed observation with the naked eye of the shell’s

Fig. 6. LA-CF-IRMS analysis of Geloina erosa (sample MIG2) mounted on a coverslipbase. Laser ablation sampling pits are visible and spaced 700 mm apart to give the bestspatial resolution with minimal overlap of thermal halos. The halos are obscured byrings of calcium oxide ejecta and are less than 400 mm in diameter. Only the first 3–4 cm of the shells from the VM could be analysed as the last growth layer typicallybecomes thinner than 400 mm after this, after which sampling of other growth layersmay occur. The three pits that are surrounded by black halos are caused by pyrolysis oflocal traces of organic carbon.

major internal growth layers. Laser ablation of the shell strips thatwere adhered to a coverslip did not produce fragmentation of theedges of the shell and so this was the preferred preparationmethod. All shell samples were analysed using the coverslip baseexcept for MOG1 which was encased in resin. Prepared shellsamples were placed into the sample holder in the sample chamberready for analysis by LA-CF-IRMS.

5.5. Stable isotope analysis

The automated laser ablation system at Royal Holloway usesa 25 W CO2 laser to heat targets contained in a sample chamber thatis on-line via a gas chromatograph to a Micromass Optima stableisotope mass spectrometer. Laser heating causes thermal de-carbonation liberating CO2 which is transferred into the massspectrometer using helium gas. The system and operating condi-tions are described in Spotl and Mattey (2006) who discuss thelimitations on spatial resolution caused by a halo of partiallyreacted carbonate surrounding the ablation pit. Laser power andpulse duration are optimised such that the halos in carbonatetargets are less than 400 mm in diameter and the net linear spatialresolution in this study was 700 mm (Fig. 6).

The shell samples were placed inside the sample chamber alongwith a tablet of Carrara Marble used as a drift monitor. CO2 wasthermally released from carbonate using a 1000 ms laser pulse at40% power and CO2 swept through an 80 cm packed GC column andinto the mass spectrometer for isotopic analysis. The Carrara Mar-ble monitor was analysed 20 times at the beginning and end of eachlaser run. Drift correction by linear interpolation was then appliedto the shell analytical data. Blocks of 20 analyses of Carrara Marbleshow reproducibility typically better than �0.15& for d13C and�0.25& for d18O.

Laser data were normalised to V-PDB by classical stable isotopeanalysis of drilled samples taken from the opposing shell strip sawnfrom each of the modern shells. Carbonate powders were sampledusing a hand-drill fitted with a 1 mm bit and analysed by digestionon orthophosphoric acid using an automated common acid bathsystem on-line to a Prism dual inlet mass spectrometer (DI-MS) atRoyal Holloway. Analytical reproducibility is typically better than0.05& for d18O and d13C. The grand averages of drilled data wereused to normalise the laser data to the V-PDB scale. Comparison ofdata between d18O of the two modern shells analysed with LA-CF-IRMS and DI-MS showed a negative offset in the LA-CF-IRMS by c.3& consistent with previous work using the system (Spotl andMattey, 2006) and a comparison of hand drilled and corrected LA-CF-IRMS data are shown in Fig. 7.

-10

-9

-8

-7

-6

-5

-405101520253035

Distance from VM (mm)

‰ V

-P

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MOG1R drilled

MOG1 laser ablation 3pt running meanMOG1 laser ablation

Fig. 7. d18O profiles of modern Geloina erosa shell sample MOG1 using laser ablation(MOG1) vs. drilled carbonate (MOG1R). Normalisation to the V-PDB scale requiresa correction of 3& (see text for discussion). A three point running mean (dark line) isfitted to the laser ablation profile (MOG1) to compare at similar resolution to thedrilled profile.

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–2697 2693

6. Results and discussion

6.1. Modern stable isotopic shell profiles and comparisonto rainfall data

The two modern specimens of Geloina erosa exhibit ‘saw-tooth’shape stable isotopic profiles with broad co-variation between d18Oand d13C for each modern shell (Fig. 8). Both modern shells alsohave similar mean d18O (MOG1, �6.7&; MOG2, �6.4&) and d13Cvalues (MOG1, �9.9&; MOG2, �9.5&), respectively (Fig. 9). Peaks(negative shifts of d13C and d18O) in the stable isotopic profiles ofboth modern shells exhibit a general decreasing trend (becomeisotopically heavier) towards the ventral margin (Fig. 8). Theseasonal peaks of rainfall associated with the northeast (winter)monsoon at Niah from mid-1998 to 2002 also exhibit a similardecreasing trend (Fig. 8). From March 1997 to March 1998,

0

100

200

300

400

500

600

700

Feb Jul De Ma Oct Mar Au Jan Jun NoMonth (1996-2002

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tal m

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th

ly rain

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Distance from ventral m

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El Niño drought

Decreas

rainfall

Fig. 8. Comparison of modern isotopic profiles of two modern Geloina erosa shells with moisotopic scale is used with which to compare the peaks and troughs of rainfall. Rainfall peaoccurs during the southwest monsoon (typically June to September).

however, relatively very little rain fell at Niah. A similar trough(isotopic enrichment) occurs between 35 and 30 mm in the stableisotopic profile of MOG2 (Fig. 8). The d18O value of the last growthincrement of MOG2 (�5.5&) is more positive than the mean d18Ovalue for the shell profile of MOG2 (�6.4&). The d18O value of thelast growth increment of MOG1 (�8.7&), however, is more nega-tive than the mean d18O value for the shell profile of MOG1(�6.7&).

The broad co-variation between d18O and d13C in each modernshell implies a common forcing factor controlling the seasonalfractionations of oxygen and carbon isotopes in the estuary, such asthe heavy northeast monsoon rains of November to March in Sar-awak. Negative shifts in d18O are typical of heavy monsoon rainsassociated with the gradual rainout of moist air masses movinginland forced by monsoon circulation (e.g. Dansgaard, 1964;Araguas-Araguas et al., 1998). The subsequent flooding reduces

v Apr Se Feb Jul De)

051015argin (mm)

ing peaks in seasonal

from mid-1998 to 2002

3 pt running mean

Shell collection periodMean

5 pt running mean

Shells collected

during below

average rainfall

conditions

Shell collected

during below

average runoff

conditions

δ18Oδ13C

dern rainfall for Niah. Note the 1997–1998 El Nino drought marker horizon. A reversedks around the northeast monsoon (typically November to March) and reduced rainfall

MOG1

MOG2

MIG1 (0-12")

MIG2 (12-24")

MIG3 (24-36")

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Sam

ple id

.

-11-10-9-8-7-6-5-4δ18

O (‰ V-PDB) δ13C (‰ V-PDB)

Fig. 9. Mean and standard deviation of the d13C and d18O profiles of the modern and prehistoric midden shells of Geloina erosa analysed in this study.

3 pt running mean

Shell collection period

δ18O

Mean δ18O

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Distance from ventral margin (mm)

MIG1 (X/V1 0-12")

MIG2 (X/V1 12-24")

MIG3 (X/V1 24-36")

Shells collected

during moderate

runoff

δ13C

Fig. 10. Stable isotopic profiles of three shells of Geloina erosa sampled from theHarrisson Archive, trench X/V1, 0–36 inches. The isotopic value of the last growthincrement gives information on the timing of collection; in these three cases duringtimes of moderate runoff.

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–26972694

seawater influence and salinity (e.g. Staub et al., 2000) and pro-duces negative d13C DIC values in the estuary (Stephens and Rose,2005). In addition, negative d13C peaks in the these shells may bethe result of increased metabolic CO2 derived from feeding (e.g.Vander Putten et al., 2000) associated with the runoff of nutrientsand soil particles (Kazungu et al., 1989). Geloina erosa does not,however, appear to use metabolically-derived CO2 to build its shellduring summer months when reproductive activity occurs (Mor-ton, 1985) as negative shifts in d13C would also occur in this period.

This isotopic pattern has been observed for other estuarinemolluscs and also attributed to seasonal mixing of freshwater andseawater in the estuary with the negative isotopic shifts related topulses of freshwater (e.g. Leng and Pearce, 1999; Tripati et al., 2001;Khim et al., 2003). The decreasing trend in the peaks of the stableisotopic profiles might reflect decreasing runoff. This inference issupported by the similar decreasing trend of seasonal rainfallrecorded at Niah from mid-1988 to 2002 (Fig. 8). The low rainfall atNiah from March 1997 to March 1998 is in response to the El Ninoevent that caused prolonged dry conditions in Sarawak (Nakagawaet al., 2000). The similar isotopic enrichment trough in MOG1 maythus be a marker of the El Nino drought.

The higher than average d18O value for the last growth in-crement of MOG2 is what would be expected since it was collectedduring a period of low rainfall (Fig. 8). This indicates that d18Oanalysis of the last growth increments of shell profiles of Geloinaerosa have the potential to reveal information on the timing ofcollection in prehistoric times at Niah. However, the last growthincrement of MOG1 displays a lower than average d18O value, in-correctly indicating that this shell was collected during a period ofhigher than usual rainfall/runoff. The analysis of further modernshells is required to resolve this discrepancy.

6.2. Prehistoric data

The three prehistoric shells all exhibit ‘saw-tooth’ shape, stableisotopic profiles as found in their modern counterparts. The pre-historic and modern shells also have similar amplitudes of variationin d18O with standard deviation of each shell c. 1& (Fig. 9). Themodern and prehistoric shells have d18O values that overlap withinstandard deviations of each other although the prehistoric shellsare around 0.5& lighter (Fig. 9). A further notable difference be-tween the modern and prehistoric shell isotopic profiles is the lackof co-variation between d18O and d13C in the prehistoric shells(Fig. 10). In addition the prehistoric shells of Geloina erosa displaymuch smaller d13C standard deviation and have typically 2–3&

lighter mean d13C values than those of the modern shells (Fig. 9).The d18O values of the last growth increments of the three pre-historic shells (MIG1, �7.4&; MIG2, �7.1&; MIG3, �6.9&) arerelatively similar to the mean d18O of the individual shell profiles(MIG1, �6.9&; MIG2, �7.2&; MIG3, �7.2&), respectively (Fig. 10).

The similar ‘saw-tooth’ shape d18O profiles and similar ampli-tudes of variation in d18O between the prehistoric and modernshells of Geloina erosa suggest similar environmental processestook place in the past as at the present time (e.g. Aggarwal et al.,2004). The cyclical shifts in d18O observed in these molluscs datingto the early Holocene therefore probably reflect the seasonality ofthe monsoon rains at those times with shifts to negative valuestaking place during periods of high runoff. The lighter mean d18O

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–2697 2695

values of the prehistoric shells possibly indicate a greater annualsurplus of rainfall compared to modern times due to the ‘amounteffect’. Lighter than modern d18O values have also been found inprehistoric shells of the freshwater Bellamya javanica from 0 to48 inches of X/V1 and also from excavations of Lithofacies 4 directlyassociated with the radiocarbon age of 8.6 14C kyr BP in the WestMouth (Stephens, 2005; Stephens et al., in press).

An early Holocene age has been tentatively suggested in thispaper for the prehistoric Geloina erosa shells based on the evidenceof their sampling position (Fig. 3), the associated AMS 14C dates, andby direct AAR determinations; in consequence a climatic scenarioof increased overall annual rainfall would seem appropriate for thistime period. Higher than modern levels of runoff have been inter-preted for the early Holocene in north Borneo from lighter d18Oseawater values (lower salinity) in the southern South China Seabetween 11and 8 kyr cal BP (Steinke et al., 2006). This appears to belinked to the early Holocene maximum in summer monsoonprecipitation modelled using orbital parameters by Kutzbach(1981). Such an increase in early Holocene rainfall is not recorded inoxygen isotope profiles of stalagmites from Gunung Buda NationalPark in Sarawak (Partin et al., 2007), although the site is locatedconsiderably further inland, w100 km ENE from Niah.

The lack of co-variation, however, between d18O and d13C in theprehistoric shells indicates that the seasonally large runoff eventsas inferred from the d18O record do not appear to be in tune withthe cyclical shifts of the d13C record. This may be due to a change inbiological processes within the molluscs and that shell d13C varia-tions might be influenced primarily by metabolism rather than thecarbon isotopic trend of DIC in the external aquatic environment.Palaeoenvironmental interpretation of d13C may therefore beproblematic and studies of a greater number of modern Geloinaerosa shells are obviously required to further understand the factorsaffecting the d13C signal.

The general similarity between the last growth increment d18Oand the mean d18O profile values of the prehistoric shells mayindicate that the shells were mostly gathered in times of moderaterunoff. Moderate runoff levels may have provided optimal condi-tions for collecting Geloina erosa since under dry conditions it isknown to burrow deeper into mangrove sediments to gain access togroundwater (Morton, 1976). Conversely, during times of highrainfall and flooding, our own field work showed that movingthrough these mangroves and gathering these inter-tidal molluscscan be difficult and (sometimes) dangerous in the deeper waters.

7. Conclusions

A coherent body of theory, geographical information and labo-ratory methods has been presented that underpins this attempt tostudy the stable isotopes of oxygen and carbon in the shells of thetropical estuarine bivalve mollusc Geloina erosa, obtained fromsediments in modern mangroves and prehistoric midden fromexposures within the Great Cave of Niah, by the hot, wet andmonsoonal northern coast of the island of Borneo.

Analysis of a small number of modern shells of Geloina erosarevealed ‘saw-tooth’ stable isotopic profiles with broad covariationbetween d18O and d13C. Comparison of the peaks and troughs of themodern isotopic profiles with recent local rainfall data revealeda striking resemblance, including the 1998 El Nino drought markerhorizon. Major peaks (negative isotopic shifts) in d18O and d13C aresuggested to be the results of large pulses in freshwater runoff intothe mangrove swamps caused by to the heavy northeast (winter)monsoon rains and subsequent reductions of influence of seawaterand salinity within the estuary.

It is emphasised, however, that the trends in these geochemicaldata may just be coincidence and many more replicates arerequired to produce a statistically significant data-set. It is also

recommended that future studies of Geloina erosa use microdrillingbecause of the greater degrees of accuracy, precision and samplingresolution that are possible with that technique. However, thereremains a clear thrust within the outcomes of the reconnaissanceLA-CF-IRMS exploration presented here that highlights the poten-tial of Geloina erosa for providing a new and powerful insight intothe past seasonality of the Southeast Asian Palaeomonsoon.

Three prehistoric midden shells were studied from the Harris-sons’ excavations of the West Mouth of the Great Cave of Niah andan early Holocene age is provisionally indicated based on theirsampling position and associated AMS 14C dates and by direct AARdeterminations. Laser ablation analysis revealed cyclical shifts ind18O with similar amplitudes of variation as found in modern shellsof Geloina erosa. As at the present day, this probably reflects thechanging seasonality of the monsoon rains with shifts to negatived18O values during periods of high runoff. Lighter mean d18O valuesof the prehistoric shells, however, may indicate a greater annualsurplus of rainfall and possibly consistent with the early Holocenestrengthening of the summer monsoon at that time (Kutzbach,1981).

The similarity of the last growth increment d18O of the pre-historic shells to their mean d18O profile values suggest that theprehistoric molluscs were gathered by people during times ofmoderate runoff and then taken back to the Great Cave. Our ownexperience of working in these mangroves suggests that suchmoderate runoff levels, as opposed to low or high runoff conditions,would have allowed optimal conditions for gathering Geloina erosa.Under dry conditions this species is known to burrow deeper togain access to groundwater, whilst in times of high water levels,moving through these mangroves and gathering these molluscs aretasks that are demonstrably difficult and sometimes dangerous.

Acknowledgements

This paper is dedicated to the memory of Edmund Kurui, As-sistant Curator for the Sarawak Museum, who sadly passed awayduring the completion of this paper. This work, along with much ofthe work of the Niah Cave Project, would not have been possiblewithout Edmund’s great skill and endeavour in the field – not leastin guiding our collecting work in the estuary and in its fringingmangroves. This work forms part of the Niah Cave Project fundedprincipally by the Arts and Humanities Research Board of theUnited Kingdom. We thank the Chief Minister’s Department ofSarawak for permission to undertake the fieldwork at Niah, and thestaff of the Sarawak Museum, especially its Director Sanib Said, andits Assistant Director Ipoi Datan, for their support and encourage-ment, and to Dr Richard mani Banda of the Geological Survey ofMalaysia for the informed first reconnaissance of the estuary and itsmolluscs. Many thanks are due to Graeme Barker (now at theUniversity of Cambridge) for co-ordinating the project and devisingour fieldwork logistics and to the Director of Excavations, TimReynolds. David Gilbertson is the Co-ordinator of the Environ-mental Studies. We are indebted to Jim Rose (Royal Holloway,University of London) who designed the stable isotope studies atNiah, helped with fieldwork and provided useful discussion andcomments on an earlier draft. Thanks are extended to Neil Hollo-way for help preparing the shells of Geloina erosa for laser ablationanalysis and thin sectioning in the Department of Geology at RoyalHolloway. Rangkaian Hidrologi Sarawak provided the rainfall dataand the Meteorological Service of Malaysia made available therecords of temperature and relative humidity. We are grateful toMike Andrews and David Alderton for help with XRD analysis at theUniversity of Reading and Royal Holloway, University of London,respectively. Lindsay Lloyd-Smith (University of Cambridge)provided information on the Harrisson excavation grid and in its‘true’ position in the West Mouth; Tim Absalom (Cartographer at

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–26972696

School of Geography at the University of Plymouth) redrew the planof excavations in the West Mouth. We thank the useful commentsof the anonymous reviewers that have helped to improve the paper.Finally, we would like to thank both John Taylor (Natural HistoryMuseum, London) for useful discussions on the identification andecology of Geloina erosa and Melanie Leng (NERC IsotopeGeosciences Laboratory, Nottingham) for helpful comments on theisotopic data in this paper.

References

Aggarwal, P.K., Frohlich, K., Kulkarni, K.M., Gourcy, L.L., 2004. Stable isotope evi-dence for moisture sources in the Asian summer monsoon under present andpast climate regimes. Geophysical Research Letters 31, L08203.

Aguirre, M.L., Leng, M.J., Spiro, B., 1998. Variation in isotopic composition (C, O andSr) of Holocene Mactra isabelleana (Bivalvia) from the coast of Buenos AiresProvince, Argentina. The Holocene 8 (5), 613–621.

Andrus, C.F.T., Crowe, D.E., 2000. Geochemical analysis of Crassostrea virginica asa method to determine season of capture. Journal of Archaeological Science 27,33–42.

Araguas-Araguas, L., Froehlich, K., Rozanski, K., 1998. Stable isotope composition ofprecipitation over southeast Asia. Journal of Geophysical Research 103 (D22),28721–28742.

Bailey, G.N., Deith, M.R., Shackleton, N.J., 1983. Oxygen isotope analysis and sea-sonality determinations: limits and potential of a new technique. AmericanAntiquity 48 (2), 390–395.

Banda, R.M., Heward, F., 2000. The General Geology of the Niah Caves Area. Un-published report of the Minerals and Geosciences Malaysia Department, PO Box560, 93712. Kuching, Sarawak, Malaysia.

Barker, G., Barton, H., Beavitt, P., Bird, M., Daly, P., Doherty, C., Gilbertson, D., Hunt, C.,Lewis, H., Manser, J., McLaren, S., Paz, B., Piper, P., Pyatt, B., Rabett, R.,Reynolds, T., Rose, J., Rushworth, G., Stephens, M., 2002. Prehistoric foragers andfarmers in southeast Asia: renewed investigations at Niah Cave, Sarawak. Pro-ceedings of the Prehistoric Society 68, 147–164.

Barker, G., Barton, H., Bird, M., Daly, P., Datan, I., Dykes, A., Farr, L., Gilbertson, D.,Harrisson, B., Hunt, C., Higham, T., Kealhofer, L., Krigbaum, J., Lewis, H.,McLaren, S., Paz, V., Pike, A., Piper, P., Pyatt, B., Rabett, R., Reynolds, T., Rose, J.,Rushworth, G., Stephens, M., Stringer, C., Thompson, J., Turney, C., 2007. The‘human revolution’ in lowland tropical Southeast Asia: the antiquity and be-havior of anatomically modern humans at Niah Cave (Sarawak, Borneo). Journalof Human Evolution 52, 243–261.

Blaber, S.J.M., Farmer, M.J., Milton, D.A., Pang, J., Boon-Teck, O., Wong, P., 1997. Theichtyoplankton of selected estuaries in Sarawak and Sabah: composition, dis-tribution and habitat affinities. Estuarine, Coastal and Shelf Science 45,197–208.

Cobb, K.M., Adkins, J.F., Partin, J.W., Clark, B., 2007. Regional-scale climate influenceson temporal variations of rainwater and cave dripwater oxygen isotopes innorthern Borneo. Earth and Planetary Science Letters 263, 207–220.

COHMAP, 1988. Climate changes of the last 18,000 years: observations and modelsimulations. Science 241, 1043–1052.

Dansgaard, W., 1964. Stable isotopes in precipitation. Tellus 16, 436–468.Deith, M.R., 1983. Seasonality of shell collecting determined by oxygen isotope

analysis of marine shells from Asturian sites in Cantabria. In: Grigson, C.,Clutton-Brock, J. (Eds.), Animals and Archaeology. 2. Shell Middens, Fishes andBirds. International Series 183. BAR, Oxford, pp. 67–76.

Deith, M.R., 1985. Seasonality from shells: an evaluation of two techniques forseasonal dating of marine molluscs. In: Fieller, N.R.J., Gilbertson, D.D., Ralph, N.G.A. (Eds.), Palaeobiological Investigations Research Design, Methods and DataAnalysis. International Series 266. BAR, Oxford, pp. 119–130.

Deith, M.R., Shackleton, N.J., 1988. Oxygen isotope analyses of marine molluscs fromFranchthi Cave. In: Shackleton, J.C. (Ed.), Marine Molluscan Remains fromFranchthi Cave. Indiana University Press, Bloomington/Indianapolis, IN, pp.133–156.

Depledge, M.H., 1983. Physiological responses of the Indo-Pacific mangrove bivalve,Geloina erosa (Solander, 1786) to aerial exposure. In: Morton, B., Dudgeon, D.(Eds.), Proceedings of the Second International Workshop on the Malacofaunaof Hong Kong and Southern China, Hong Kong. Hong Kong University Press,Hong Kong.

Dettman, D.L., Reische, A.K., Lohmann, K.C., 1999. Controls on the stable isotopecomposition of seasonal growth bands in aragonitic fresh-water bivalves(Unionidae). Geochimica et Cosmochimica Acta 63 (7/8), 1049–1057.

Epstein, S., Buchsbaum, R., Lowenstam, H.A., Urey, H.C., 1953. Revised carbonate-water isotopic temperature scale. Bulletin of the Geological Society of America64, 1315–1326.

Erlenkeuser, H., Wefer, G., 1981. Seasonal growth of bivalves from Bermudarecorded in their O-18 profiles. Proceedings of the 4th International Coral ReefSymposium, Vol. 2, Manila, pp. 643–648.

Fastovsky, D.E., Arthur, M.A., Strather, N.H., Foss, A., 1993. Freshwater bivalves(Unionidae), disequilibrium isotopic fractionation, and temperatures. Palaios 8,602–608.

Frith, D.W., Tantanasiriwong, R., Bhatia, O., 1976. Zonation and abundance of mac-rofauna on a mangrove shore, Phuket Island. Research Bulletin Phuket MarineBiological Center, 1–37. No. 10.

Fritz, P., Poplawski, S., 1974. 18O and 13C in the shells of freshwater molluscs andtheir environments. Earth and Planetary Science Letters 24, 91–98.

Fuge, R., Palmer, T.J., Pearce, N.J.G., Perkins, W.T., 1993. Minor and trace elementchemistry of modern shells: a laser ablation inductively coupled plasma massspectrometry study. Applied Geochemistry (supplementary issue no. 2),111–116.

Gat, J.R., 1996. Oxygen and hydrogen isotopes in the hydrologic cycle. AnnualReview of Earth and Planetary Sciences 24, 225–262.

Gilbertson, D., Bird, M., Hunt, C., McLaren, S., Pyatt, B., Rose, J., Stephens, M., 2005.Past human activity and geomorphological change in a guano-rich tropical cavemouth: the Late Quaternary succession in Niah Cave, Sarawak. Asian Perspec-tives 44 (1), 16–41.

Godfrey, M.C.S., 1988. Oxygen isotope analysis: a means for determining theseasonal gathering of the pipi (Donax deltoides) by Aborigines in prehistoricAustralia. Archaeology in Oceania 23, 1–16.

Grossman, E.L., Ku, T.-L., 1986. Oxygen and carbon isotope fractionation in biogenicaragonite: temperature effects. Chemical Geology (Isotope Geoscience Section)59, 59–74.

Hardy, R.G., Tucker, M.E., 1988. X-ray powder diffraction of sediments. In: Tucker, M.E.(Ed.), Techniques in Sedimentology. Blackwell Scientific Publications, Oxford,pp. 191–228.

Harrisson, T., 1970. The prehistory of Borneo. Asian Perspectives 13, 17–45.Hazebroek, H.P., Morshidi, A.K.B.A., 2001. National Parks of Sarawak. Natural History

Publications, Kota Kinabalu, Sabah.Hickson, J.A., Johnson, A.L.A., Heaton, T.H.E., Balson, P.S., 1999. The shell of the

Queen Scallop Aequipecten opercularis (L.) as a promising tool forpalaeoenvironmental reconstruction: evidence and reasons for equilibriumstable-isotope incorporation. Palaeogeography, Palaeoclimatology, Palae-oecology 154, 325–337.

Hoffmann, G., Heimann, M., 1997. Water isotope modelling in the Asian monsoonregion. Quaternary International 37, 115–128.

Hu, J., Peng, P., Jia, G., Fang, D., Zhang, G., Fu, L., Wang, P., 2002. Biological markersand their carbon isotopes as an approach to the palaeoenvironmentalreconstruction of Nansha area, South China Sea, during the last 30 ka. OrganicGeochemistry 33, 1197–1204.

Hunt, C.O., Rushworth, G., 2005. Cultivation and human impact at 6000 cal yr BP intropical lowland forest at Niah, Sarawak, Malaysian Borneo. QuaternaryResearch 64, 460–468.

Hunt, C.O., Gilbertson, D.D., Rushworth, G., 2007. Modern humans in Sarawak,Malaysian Borneo, during Oxygen Isotope Stage 3: palaeoenvironmentalevidence from the Great Cave of Niah. Journal of Archaeological Science 34,1953–1969.

Kazungu, J.M., Dehairs, F., Goeyens, L., 1989. Nutrients distribution patterns in Tudorestuary (Mombasa, Kenya) during rainy season. Kenya Journal of Sciences SeriesB 10, 47–61.

Keith, M.L., Anderson, G.M., Eichler, R., 1964. Carbon and oxygen isotopic compo-sition of mollusk shells from marine and fresh-water environments.Geochimica et Cosmochimica Acta 28, 1757–1786.

Keith, M.L., Parker, R.H., 1965. Local variation of 13C and 18O content of molluskshells and the relatively minor temperature effect in marginal marine envi-ronments. Marine Geology 3, 115–129.

Kennett, D., Voorhies, B., 1995. Middle Holocene periodicities in rainfall inferredfrom oxygen and carbon isotopic fluctuations in prehistoric tropical estuarinemollusc shells. Archaeometry 37 (1), 157–170.

Kennett, D., Voorhies, B., 1996. Oxygen isotopic analysis of archaeological shells todetect seasonal use of wetlands on the southern Pacific coast of Mexico. Journalof Archaeological Science 23, 689–704.

Khim, B.K., Krantz, D.E., Cooper, L.W., Grebmeier, J.M., 2003. Seasonal discharge ofestuarine freshwater to the western Chukci Sea shelf identified in stable isotopeprofiles of mollusc shells. Journal of Geophysical Research Oceans 108 (C9),3300.

Kim, S., O’Neil, J.R., 1997. Equilibrium and nonequilibrium oxygen isotope effects insynthetic carbonates. Geochimica et Cosmochimica Acta 61, 3461–3475.

Krantz, D.E., Williams, D.F., Jones, D.S., 1987. Ecological and paleoenvironmentalinformation using stable isotope profiles from living and fossil molluscs.Palaeogeography, Palaeoclimatology, Palaeoecology 58, 249–266.

Kutzbach, J.E., 1981. Monsoon climate of the Early Holocene: climate experimentwith the Earth’s orbital parameters for 9000 years ago. Science 214, 59–61.

Lazareth, C.E., Vander-Putten, E., Andre, L., Dehairs, F., 2003. High-resolution traceelement profiles in shells of the mangrove bivalve Isognomon ephippium: a re-cord of environmental spatio-temporal variations? Estuarine, Coastal and ShelfScience 57, 1103–1114.

Leng, M.J., Pearce, N.J.G., 1999. Seasonal variation of trace element and isotopiccomposition in the shell of a coastal mollusk, Mactra isabelleana. Journal ofShellfish Research 18 (2), 569–574.

Mannino, M.A., Spiro, B.F., Thomas, K.D., 2003. Sampling shells for seasonality:oxygen isotope analysis on shell carbonates of the inter-tidal gastropod Mon-odonta lineata (da Costa) from populations across its modern range and froma Mesolithic site in southern Britain. Journal of Archaeological Science 30, 667–679.

McCrea, J.M., 1950. On the isotope chemistry of carbonates and a palae-otemperature scale. Journal of Chemical Physics 18, 849–857.

Medway, Lord, 1960. Niah shell – 1954–8 (a preliminary report). Sarawak MuseumJournal 9 (15–16), 368–379.

Metref, S., Rousseau, D.-D., Bentaleb, I., Labonne, M., Vianey-Liaud, M., 2003. Studyof the diet effect on d13C of shell carbonate of the land snail Helix aspersa inexperimental conditions. Earth and Planetary Science Letters 211, 381–393.

M. Stephens et al. / Journal of Archaeological Science 35 (2008) 2686–2697 2697

Morton, B., 1975. The diurnal rhythm and the feeding responses of the Southeast-Asian mangrove bivalve, Geloina proxima Prime 1864 (Bivalvia: Corbiculacea).Forma et Functio 8, 405–418.

Morton, B., 1976. The biology and functional morphology of the SE Asian mangrovebivalve Polymesoda (Geloina) erosa (Solander, 1786) (Bivalvia: Corbiculidae).Canadian Journal of Zoology 54, 482–500.

Morton, B., 1984. A review of Polymesoda (Geloina) Gray 1842 (Bivalvia: Corbicu-lacea) from Indo-Pacific mangroves. Asian Marine Biology 1, 77–86.

Morton, B., 1985. The reproductive strategy of the mangrove bivalve Polymesoda(Geloina) erosa (Bivalvia: Corbiculacea) in Hong Kong. Malacological Review 18(1–2), 83–89.

Mueller-Lupp, T., Erlenkeuser, H., Bauch, H.A., 2003. Seasonal and interannualvariability of Siberian river discharge in the Laptev Sea inferred from stableisotopes in modern bivalves. Boreas 32 (2), 292–303.

Murray-Wallace, C.V., Kimber, R.W.L., 1988. A review of amino acid racemisationdating and its application to Australian Quaternary marine mollusks – currenttrends and future prospects. In: Prescott, J.R. (Ed.), Archaeometry: AustralasianStudies 1988. Department of Physics and Mathematical Physics, University ofAdelaide, pp. 6–21.

Nakagawa, M., Tanaka, K., Seng, L.H., 2000. Impact of severe drought associatedwith the 1997–1998 El Nino in a tropical forest in Sarawak. Journal of TropicalEcology 16 (3), 355–367.

O’Neil, J.R., Clayton, R.N., Mayeda, T.K., 1969. Oxygen isotope fractionation in di-valent metal carbonates. Journal of Chemical Physics 51, 5547–5558.

Owen, R., Kennedy, H., Richardson, C., 2002. Experimental investigation into par-titioning of stable isotopes between scallop (Pecten maximus) shell calcite andsea water. Palaeogeography, Palaeoclimatology. Palaeoecology 185, 163–174.

Partin, J.W., Cobb, K.M., Adkins, J.F., Fernandez, D.P., Clark, B., 2007. Millennial-scaletrends in west Pacific warm pool hydrology since the Last Glacial Maximum.Nature 449, 452–455. doi:10.1038/nature06164.

Price, G.D., Pearce, N.J.G., 1997. Biomonitoring of pollution by Cerastoderma edulefrom the British Isles: a Laser ablation ICP-MS study. Marine Pollution Bulletin34, 1025–1031.

Rhoads, D.C., Lutz, R.A., 1980. Skeletal records of environmental change. In:Rhoads, D.C., Lutz, R.A. (Eds.), Skeletal Growth of Aquatic Organisms. PlenumPress, New York, pp. 1–19.

Schettler, G., Pearce, N.J.G., 1996. Metal pollution in extinct Dreissena polymorphacommunities, Lake Breitling, Havel Lakes system, Germany: a laser ablationinductively coupled plasma mass spectrometry study. Hydrobiologia 317,1–11.

Shackleton, N.J., 1969. Marine mollusca in archaeology. In: Brothwell, D., Higgs, E.S.(Eds.), Science in Archaeology. Thames and Hudson, New York, pp. 407–414.

Shackleton, N.J., 1973. Oxygen isotope analysis as a means of determining season ofoccupation of prehistoric midden sites. Archaeometry 15 (1), 133–141.

Sloss, C.R., Murray-Wallace, C.V., Jones, B.G., Wallin, T., 2004. Aspartic acid race-misation dating of mid-Holocene to recent estuarine sedimentation in NewSouth Wales, Australia: a pilot study. Marine Geology 212, 45–59.

Spotl, C., Mattey, D., 2006. Stable isotope microsampling of speleothems forpalaeoenvironmental studies: a comparison of microdrill, micromill and laserablation techniques. Chemical Geology 235 (1–2), 48–58.

Staub, J.R., Among, H.L., Gastaldo, R.A., 2000. Seasonal sediment transport anddeposition in the Rajang River delta, Sarawak, East Malaysia. Sedimentary Ge-ology 133, 249–264.

Steinke, S., Chiu, H.-I., Yu, P.-S., Shen, C.-C., Erlenkeuser, H., Lowemark, L., Chen, M.-T.,2006. On the influence of sea level and monsoon climate on the southern SouthChina Sea freshwater budget over the last 22,000 years. Quaternary ScienceReviews 25, 1475–1488.

Stephens, M., 2001. Molluscs and stable isotopes in Borneo. Sarawak MuseumJournal 56 (77), 91–92.

Stephens, M., 2005. Reconstructing Late Quaternary palaeoenvironments fromdeposits in the West Mouth of the Great Cave of Niah, Sarawak, Borneo. Un-published PhD thesis, University of London.

Stephens, M., Murray-Wallace, C.V. Amino-acid racemisation analysis of shells fromthe Harrisson Archive: preliminary results, in: Barker, G., Gilbertson, D., Rey-nolds, T. (Eds.), The Archaeology and Environmental History of the Niah Caves,Sarawak: Excavations 1954–2004, vol. 2. McDonald Research Institute,University of Cambridge, in press.

Stephens, M., Rose, J., 2005. Modern stable isotopic (d18O, d2H, d13C) variation interrestrial, riverine, estuarine and marine waters from north-central Sarawak,Malaysian Borneo. Earth Surface Processes and Landforms 30 (7), 901–912.

Stephens, M., Rose, J., Gilbertson, D., Canti, M., 2005. The micromorphology of cavesediments in the humid tropics: Niah Cave, Sarawak. Asian Perspectives 44 (1),42–55.

Stephens, M., Mattey, D., Rose, J., Gilbertson, D. Stable isotope analysis of shells fromthe West Mouth: palaeoenvironments, palaeoseasonality and harvesting, in:Barker, G., Gilbertson, D., Reynolds, T. (Eds.), The Archaeology and Environ-mental History of the Niah Caves, Sarawak: Excavations 1954–2004, vol. 2.McDonald Research Institute, University of Cambridge, in press.

Surge, D., Lohmann, K.C., Goodfriend, G.A., 2003. Reconstructing estuarine condi-tions: oyster shells as recorders of environmental change, Southwest Florida.Estuarine Coastal and Shelf Science 57, 737–756.

Tanaka, N., Monaghan, M.C., Rye, D.M., 1986. Contribution of metabolic carbon tomollusc and barnacle shell carbonate. Nature 320, 520–523.

Toland, H., Perkins, B., Pearce, N., Keenan, F., Leng, M.J., 2000. A study ofsclerochronology by laser ablation ICP-MS. Journal of Analytical Atomic Spec-trometry 15, 1143–1148.

Tripati, A., Zachos, J., Marincovich Jr., L., Bice, K., 2001. Late Paleocene Arctic coastalclimate inferred from molluscan stable and radiogenic isotope ratios. Palae-ogeography, Palaeoclimatology, Palaeoecology 170, 101–113.

Urey, H.C., 1947. The thermodynamic properties of isotopic substances. Journal ofthe Chemical Society, 562–581.

Vander Putten, E., Dehairs, F., Keppens, E., Baeyens, W., 2000. High resolutiondistribution of trace elements in the calcite shell layer of modern Mytilus edulis:environmental and biological controls. Geochimica et Cosmochimica Acta 64(6), 997–1011.

Wall, J.R.D., 1966. The Bekenu-Niah-Suai area, Sarawak, Malaysia. An analysis of theenvironment, a detailed appraisal of the soils and the technique of soil map-ping, and an assessment of the agricultural potential. Unpublished PhD thesis,University of Reading.

Walsh, R.P.D., 1982. Climate. In: Jeremy, A.C., Kavanagh, K.P. (Eds.), Gunung MuluNational Park, Sarawak: an account of its environment and biota being theresults of the Royal Geographical Society/Sarawak Government Expedition andSurvey 1977–1978, Part I. Sarawak Museum Journal 30 (51, Special Issue no. 2),29–67.

Wefer, G., Berger, W.H., 1991. Isotope paleontology: growth and composition ofextant calcareous species. Marine Geology 100, 207–248.