Earth’s global Ag, Al, Cr, Cu, Fe, Ni, Pb, and Zn cycles

Jason N. Rauch1 and Jozef M. Pacyna2,3

Received 11 September 2008; revised 6 January 2009; accepted 23 January 2009; published 2 April 2009.

[1] The stocks and flows of the global silver, aluminum, chromium, copper, iron, nickel,lead, and zinc cycles quantify over 98% of the total mass of metal mobilized by humanactivity at the turn of the 21st century. Iron and aluminum, representing >95% by mass ofall metals mined, are for the first time assessed for global anthropogenic emissions toair, water, and land. Anthropogenic activity has significantly perturbed Earth’s naturalbiogeochemical cycles, attested by the ‘‘grand nutrient’’ cycles of carbon, nitrogen,phosphorous, and sulfur and further revealed here by the ‘‘anthrobiogeochemical’’ cyclesof metals. We demonstrate that humans today mobilize about half the metal mass of theseglobal elemental metal cycles.

Citation: Rauch, J. N., and J. M. Pacyna (2009), Earth’s global Ag, Al, Cr, Cu, Fe, Ni, Pb, and Zn cycles, Global Biogeochem.

Cycles, 23, GB2001, doi:10.1029/2008GB003376.

1. Introduction

[2] Though metals serve as ‘‘technological nutrients’’without which modern society could not function, they havenot received the same attention on the global scale as the‘‘grand nutrients’’ C, N, S, and P [Falkowski et al., 2000;Galloway et al., 2004; Rodhe, 1992; Smil, 2000]. Only a fullglobal cycle of copper, inclusive of natural and anthropogenicstocks and flows, has been constructed quantitatively [Rauchand Graedel, 2007]. Rather, previous studies have addressedparticular metal stocks and flows, the most holistic of whichhave examined natural atmospheric trace metal emissions[Lantzy and Mackenzie, 1979; Nriagu, 1989], anthropogenictrace metal emissions to land, air, and water [Nriagu andPacyna, 1988], andmovement of metal through the industrialecosystem [Graedel et al., 2005; Johnson et al., 2005, 2006;Wang et al., 2007].[3] These works suffer from narrow system boundaries,

failing to fully illustrate relative anthropogenic and naturalflow magnitudes, natural reservoirs that are not in mass bal-ance, and the degree to which human activity has perturbedthe natural cycling of metals. The intent of our analysis isto both highlight these gaps in knowledge and to provide acomprehensive view of Earth’s metal systems. Al, Fe, Zn,and Cu are chosen for analysis as these metals represent over95% of the total metal mass mined for human consumption.This analysis also includes Pb, Cr, Ag, and Ni, the only othermetallic elements for which enough data are presently avail-able for constructing comprehensive global cycles.[4] Quantifying Earth’s metal cycles provides an under-

standing of the comparable magnitudes of metal appropriatedby humans and by the natural environment, and the associ-

ated uncertainty illustrates potential priority areas for futureresearch. The cycles presented can be a useful tool forassessing the apportionment of metals when analyzing theiramounts in the environment. One may assess the changesto the environment caused by any change of anthropogenicdrivers, such as industry production, population increase, orincreased demand for energy and food production. The resultsreported here also integrate, update, and expand upon previ-ous anthropogenic and natural global emission inventories[Nriagu and Pacyna, 1988].

2. Quantification

[5] Given the scope of the analysis, various literature, datasources, and methods of calculation are required to quantifyeach metal cycle. Many of the methods and sources we useherein are presented in further detail in an analysis of theglobal copper cycle for the mid-1990s [Rauch and Graedel,2007]. We define the system boundary as Earth and its orbitalbodies. All flows >1 Mg/a between reservoirs are explicitlyquantified. Mass balance of atmospheric, water, and biomassreservoirs, assuming no net accumulation due to high turn-over rates, was applied to estimate flow magnitudes. Resultsare for the year 2000, except where comprehensive data arenot available, as is for Ag (1997) and Al (2002). Estimateduncertainties are absolute errors calculated by propagation ofthe estimated or standard errors associated with the sourcedata. Averages are arithmetic unless otherwise noted. Al-though certain results are limited by informed approxima-tions, the values and methods to calculate these valuesrepresent the present state of understanding. The entireexercise provides a diagrammatic representation of Earth’sglobal metal cycles, combining the endogenic reservoirs ofthe core, mantle, and consolidated crust, the natural exogeniccycles of the pedosphere, hydrosphere, and biosphere, theanthropogenic exogenic reservoirs from production to wastemanagement, and Earth’s atmosphere and orbit. Note thatthis study builds upon the methodology and framework ofa previous review of Earth’s mid-1990s global copper cycle

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1Yale School of Forestry and Environmental Studies, Yale University,New Haven, Connecticut, USA.

2Norwegian Institute of Air Research, Kjeller, Norway.3Chemistry Faculty, Gdansk University of Technology, Gdansk, Poland.

Copyright 2009 by the American Geophysical Union.0886-6236/09/2008GB003376$12.00

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[Rauch and Graedel, 2007]. Only those quantification meth-ods with significant revision are outlined here.

2.1. Data

[6] Many calculations are simply a generally acceptedglobal mass multiplied by a global average metal concentra-tion, though deriving these underlying two values can becomplex. Generally accepted masses used or derived havebeen reported previously [Rauch and Graedel, 2007]. Asummary of the metal concentrations used in these calcula-tions is presented in Table 1.

2.2. Select Revised Methods

2.2.1. Natural Reservoirs of Metal Stocks[7] For Earth’s crust, we divide crust into consolidated

(igneous, metamorphic, and sedimentary bedrock) and un-consolidated (regolith) rock, subdividing consolidated crustfurther into oceanic and continental crust. The mass totalsof the subdivided continental crust were taken to be 5.1 �1015 Gg (upper), 4.9� 1015 Gg (middle), and 8.1� 1015 Gg(lower), derived from bulk crust mass estimates and aver-age depth and density estimates of each section of crust[Christensen and Mooney, 1995; Mackenzie et al., 2004;Rudnick and Gao, 2003; Taylor and McLennan, 1995]. The

total mass of the oceanic crust (6.6 � 1021 kg) has also beenestimated [Mackenzie et al., 2004]. Metal concentrations inthe upper, middle, and lower continental crust were takenfrom [Rudnick and Gao, 2003]. The concentration of metal inthe oceanic crust is available for most metals [Wedepohl andHartmann, 1994]. However, Pb concentration is taken as anN-MORB average [Hofmann, 1988], and Ag concentrationis taken from basalt compositions [McDonough and Sun,1995]. The consolidated crust contains potentially recover-able resources of metal, most size estimates of which comefrom the United States Geological Survey commodity reports,with the exception of Cu [Gordon et al., 2006] and Ag[Reese, 1992].[8] While many estimates exist for total global living bio-

mass [Smil, 2002], we use 1800 Pg [Whittaker, 1975]. Thisliving biomass can be disaggregated between terrestrialphytomass (800 Pg C) [Falkowski et al., 2000] and marinephytomass (3 Pg C) [Holmen, 1992]. We convert Pg C to Pgusing a 2.22 conversion ratio [Smil, 2000]. Since trees con-stitute 80% of the world’s biomass [Kindermann et al., 2008],metal concentrations in terrestrial plant drymatter are derivedas a biomass weighted mean of the metal concentrationsin foliage (4%), twigs and branches (8%), bark (8%), wood(60%), and roots (20%) [Adriano, 2001; Demirbas, 2003;

Table 1. Global Mean Metal Concentrationsa

Reservoir Cu Zn Fe Pb Ag Ni Cr Al Source(s)b

Core 125 0 822000 0.4 0.15 50400 8400 0 1,2Mantle 30 56 62700 0.03 0.006 1960 2500 11300 3,4,5Bulk continental crust 27 72 52000 11 0.056 59 140 42000 6Upper continental crust 28 67 39000 17 0.053 47 92 41000 6Oceanic crust 78 78 70000 0.49 0.04 150 320 83000 7,8,9Continental sediment 40 65 40000 17 1.1 40 74 71000 10,11Oceanic sediment 75 86 40000 20 0.13 71 79 32000 10,12Loess 21 63 24000 13 - 27 59 67000 11,13Soil 39 48 33000 27 2.6 25 130 62000 10,14Coal 12 29 9100 14 0.23 14 16 12000 10,15,16,17Crude oil 0.091 0.32 2.5 0.30 0.0001 10 0.19 0.5 15,18Rivers (particulate) 100 250 48000 35 0.07 90 100 89000 19,20,22,23Rivers (dissolved) 0.0015 0.0006 0.053 0.000055 0.0003 0.00065 0.00085 0.081 19,20,21Freshwater lakes (dissolved) 0.00058 0.0003 0.0011 0.000036 0.000012 0.0006 0.00025 0.0036 24,25,26,27Groundwater (dissolved) 0.0046 0.014 6.8 0.001 0.0000018 0.003 0.0007 0.012 28,29,30,31Greenland ice 0.00006 0.00022 0.015 0.000028 0.0000006 0.000058 0.000038 0.12 32,33,34,35,62,63Antarctic ice 0.000008 0.000030 0.0071 0.000005 0.0000002 0.00015 0.000007 0.026 36,37,38,39,40,41,42,64,65,66Ocean (dissolved) 0.00016 0.00035 0.00011 0.000002 0.0000025 0.0005 0.00026 0.00086 23,43,44,45Terrestrial biomass (dry weight) 4.8 36 193 9.4 0.094 1.1 0.94 166 46,47,53,54,55,56,57,58,59,60,61Agricultural crops (dry weight) 5.1 26 60 1.1 0.053 0.91 0.066 42 46,47,52Marine organisms (dry weight) 12 39 160 8.7 0.22 1.4 1.8 62 43Atmosphere 0.0000008 0.000002 0.0004 0.000001 0.00000002 0.0000005 0.0000009 0.0006 67Micrometeorites 93 52 300000 4.5 0.082 19000 3300 9300 48,49Moon 6.4 3.1 110000 0.000037 0.009 6500 1700 39000 50Man-made satellites 69000 27000 76000 86 9000 8000 23000 456000 51

aConcentration are in mg/g.bNotation is as follows: 1,McDonough [1998]; 2, Allegre et al. [1995]; 3, Salters and Stracke [2004]; 4, Rehkamper and Hofmann [1997]; 5,Workman and

Hart [2005]; 6, Rudnick and Gao [2003]; 7, Wedepohl and Hartmann [1994]; 8, Hofmann [1988]; 9, McDonough and Sun [1995]; 10, IAEA Database ofNatural Matrix Reference Materials; 11, McLennan [1995]; 12, Plank and Langmuir [1998]; 13, Poulton and Raiswell [2000]; 14, Han et al. [2002]; 15,Bertine and Goldberg [1971]; 16, Clarke and Sloss [1992]; 17, Finkelman [1999]; 18, Stigter et al. [2000]; 19,Martin and Whitfield [1983]; 20,Martin et al.[1993]; 21,Gaillardet et al. [2003]; 22, Poulton and Raiswell [2002]; 23, Lisitsyn et al. [1982]; 24,Nriagu et al. [1996]; 25, Rossmann and Barres [1988]; 26,Sterner et al. [2004]; 27, Beaubien et al. [1994]; 28, Hussam et al. [2003]; 29, Minnesota Pollution Control Agency [1999]; 30, Page [1981]; 31, Sanudo-Wilhelmy et al. [2002]; 32,Barbante et al. [1997]; 33,Hong et al. [1996a]; 34,Osterberg et al. [2006]; 35, Snyder-Conn et al. [1997]; 36, Batifol et al. [1989];37, Boutron and Patterson [1986]; 38, Boutron et al. [1990]; 39,Gabrielli et al. [2005]; 40,Hanappe et al. [1968]; 41,Hong et al. [2005]; 42, Planchon et al.[2001]; 43, Li [1991]; 44, Nozaki [1997]; 45, Quinby-Hunt and Turekian [1983]; 46, Adriano and Adriano [2001]; 47, Kabata-Pendias and Pendias [1984];48, Kaye and Laby [1995]; 49, Richardson et al. [2001]; 50, Morgan et al. [1978]; 51, Opiela [2008]; 52, Sujari and Bowen [1987]; 53, Watmough et al.[1998]; 54,Harju et al. [1997]; 55,Wolterbeek et al. [1996]; 56, Reimann et al. [2007]; 57,Madejon et al. [2004]; 58, Tyler [2004]; 59,Demirbas [2003]; 60,Nissen and Lepp [1997]; 61, Smith and Siccama [1981]; 62, Hong et al. [1994]; 63, Hong et al. [1996c]; 64, Planchon et al. [2002]; 65, Van de Velde et al.[2005]; 66, Marteel et al. [2008]; 67, Calculated from this paper.

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Harju et al., 1997; Kabata-Pendias and Pendias, 1984;Madejon et al., 2004; Nissen and Lepp, 1997; Reimannet al., 2007; Smith and Siccama, 1981; Tyler, 2004;Wolterbeeket al., 1996; Watmough et al., 1998]. This approach isnecessitated by the uneven distribution of metal content intrees, where lower concentrations exist in the majority of thebiomass contained in wood, but can be overwhelmed by theminority of biomass in other tree parts that contain highermetal concentrations, as has been observed for Pb [Smithand Siccama, 1981]. Average metal concentrations in marineorganisms are applied separately [Li, 1991]. In our analysis,all dead biomass is considered part of the soil mass.[9] We adopt here the theory that the Moon is depleted in

volatile elements and enriched in refractory elements, andutilize the resulting estimated bulk average elemental concen-trations [Morgan et al., 1978]. These values are multiplied bythe generally accepted mass of the Moon (7.4 � 1022 kg).2.2.2. Anthropogenic Reservoirs of Metal Stocks[10] We report the mass of metal residing in the anthropo-

genic life cycle stage of metal production either directly fromindustry statistics or calculated from available data. Govern-ment stockpiles, being relatively illiquid, are grouped herewith producer stocks for many metals. All values are for theyear 2000 except Ag (1997) and Al (2002). Cu stocks inproduction include those reported at ‘‘producers’’ and in gov-ernment stockpiles of refined copper [International CopperStudy Group, 2004] plus unrefined copper (blister anode,ores, and concentrates) [World Bureau of Metal Statistics,2002]. Zn and Pb stocks in production include refined Zn andPb at ‘‘producers’’ plus government stockpiles [InternationalLead and Zinc Study Group, 2003]. Production stocks ofFe available for the United States (USGS iron ore statistics),Japan (Japan Iron and Steel Federation), Europe (EuropeanConfederation of Iron and Steel Industries), and South Africa(South African Steel and Iron Institute) are scaled to a globalstock estimate on the basis of relative iron and steel produc-tion statistics available from the International Iron and SteelInstitute. Ni stocks include those at ‘‘producers’’ plus govern-ment stockpiles [International Nickel Study Group, 2003].Government stockpiles of Cr exist for Japan (estimated froma stated 60 day supply) and the United States (USGS sta-tistics) and are added to an estimate of Cr stocks at producers.We estimate Cr stocks at producers by averaging (1) a scaledestimate of global Cr producer stocks based upon U.S.producer stocks and relative global Cr production (USGSstatistics) and (2) an assumed average 10 week supply ofchromite ore at producers and statistics of global annualchromite ore consumption [Pariser, 2002]. Ag stocks includereported global end of year refinery stocks [American Bureauof Metal Statistics, 2001] plus government stockpiles [CPMGroup, 1998]. Al (unwrought) inventory at producers isreported by the International Aluminum Institute.[11] We combine metal stocks at fabrication and manufac-

turing facilities with metal in the market. The metal marketincludes metal exchanges, merchants, and treasury stock-piles. A formalized metal market exists for all metals exam-ined except Cr and Fe. Again, we derive values directly fromindustry statistics or calculated from available data; all valuesare for the year 2000 except Ag (1997) and Al (2002). Notethat ‘‘consumers’’ in metal industry statistics refers to the

manufacturers and fabricators that purchase refined metalfrom ‘‘producers.’’ Cu at the fabrication stage is that which isreported as at ‘‘consumers’’ and is added to Cu at merchantsand exchanges [International Copper Study Group, 2004].Pb and Zn follow a similar approach, with stocks at ‘‘con-sumers’’ (adjusted estimate from reported value to accountfor ‘‘other countries’’ not reporting, as based upon previousyears estimates) added to merchant and exchange stocks[International Lead and Zinc Study Group, 2003]. GlobalFe stocks at fabrication are scaled from reported ‘‘consumer’’stocks in Japan (Japan Iron and Steel Federation), Europe(European Confederation of Iron and Steel Industries), andSouth Africa (South African Steel and Iron Institute) on thebasis of the apparent consumption of iron and steel in thesecountries relative to global apparent consumption (statisticsfrom the International Iron and Steel Institute). Ni fabricationand market stocks are those reported at ‘‘consumers’’ plus theinventory on the London Metal Exchange [InternationalNickel Study Group, 2003]. Ag manufacturing stocks arethose reported as industry inventory [CPMGroup, 1998] lessthat which is already accounted for in production stocks.Market exchange bullion and other bullion repositories[CPM Group, 1998] are added to these Ag manufacturingstocks. We sum scrap, process, and semifab stocks and dis-tributor inventories as reported by the International Alumi-num Institute, then subtract out our independent estimateof Al stock in waste management to arrive at the stock of Alin fabrication. We then add this value to Al market stocks,as reported by the International Aluminum Institute for theLondonMetal Exchange (end of year inventory) and the NewYork Mercantile Exchange (warehouse stocks), and by mar-ket analysts for Japanese port stocks (L. T. O’Carroll andT. P. Hayes, Monthly aluminum update: September 2004,26 pp., BB&T Capital Markets, 2004).[12] All values for in-use metal stocks are for the year 2000

except Ag (1997) and Al (2002). The global in-use stocks ofCu and Zn were calculated from the accumulated differenceover time between inflows (new purchases) and outflows(discards). We adopt this top-down methodology that isexplained and applied in depth elsewhere [Gordon et al.,2006; Kapur, 2004; Spatari et al., 2005], applying it here toCu [Lifset et al., 2002] and Zn [Gordon et al., 2004] globalconsumption data and utilizing estimated average categoricallifetimes [van Beers and Graedel, 2007]. The summationof the annual net accumulation (or depletion) of Cu and Znbegins in 1700, capturing essentially all the Zn and Cu incontemporary use. With a start date of 1900, global Pb in-usestocks have also been estimated [Mao and Graedel, 2009].This top-down methodology is fairly accurate despite metalconsumption in ancient times [e.g., Hong et al., 1996b], asthe combination of product lifetimes measured in decadeswith the different uses of metals in the modern industrial eraresult in the vast majority of preindustrial age metal havingbeen discarded through recycling, landfilling, or environ-mental dissipation. The global in-use stock of Fe is linearlyinterpolated from estimates made for 1985 [Baccini andBrunner, 1991] and 2005 [Muller et al., 2006]. Al in-usestocks have been estimated by the European AluminumAssociation [European Aluminum Association, 2004]. Al-though a global estimate of in-use stocks was made for Ag in

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1991 [Charles River Associates, 1992], the estimate for Japanappears to be eight times larger than a subsequent estimatemade for the year 2000 [Murakami, 2006]. Given themagnitude of disagreement of in-use silver stocks, here weaverage the two estimates for Japan and then scale to a globalvalue on the basis of the proportion of in-use silver stocksrepresented in Japan in 1991. Although the result is appliedfor 1997, it essentially stands as an estimate for the end of the20th century. We take studies conducted in New Haven,Connecticut [Rostkowski et al., 2007] and Stockholm,Sweden [Sorme et al., 2001] and extrapolate per capita Niand Cr in-use stocks to the globe on the basis of the ratio ofPb per capita in-use stocks in the developing and developedregions of the world [Mao and Graedel, 2009] and globalpopulation. Ag, Ni, and Cr values thus exist as rough order ofmagnitude estimates and further in-depth studies are requiredto accurately assess these global in-use stocks.2.2.3. Natural Flows of Metal[13] Yearly sulfur emissions from energetically (explosive)

and quiescently degassing volcanoes [Bluth et al., 1993] andaverage metal to sulfur ratios from quiescently degassingbasaltic and andesitic volcanoes are the data used to estimateglobal emissions of metals [Hinkley et al., 1999]. Because ofthe present uncertainty as to how these metal to sulfur ratiosapply to energetic degassing, we average available estimatesof global volcanic metal emissions to the atmosphere [Hinkleyet al., 1999; Lambert et al., 1988; Lantzy and Mackenzie,1979; Le Cloarec and Marty, 1991; Nriagu, 1989; Pacyna,1986; Patterson and Settle, 1987; Schmidt and Andren, 1980;Zoller, 1984] (adjusting for noted errors [Mather et al., 2003;Patterson and Settle, 1988]) for our reported estimate ofglobal emissions. Metal emissions into the oceans also occurowing to degassing and hydrothermal fluids [Rubin, 1997].Emissions of Fe and Al into the oceans have also been exam-ined [Le Cloarec and Marty, 1991].[14] The flow of metal in subducting oceanic sediments

has been quantified [Plank and Langmuir, 1998], with theexception of Ag. Utilizing the same methodology [Plank andLangmuir, 1998], we calculate Ag in regolith subductionutilizing Ag oceanic sediment concentrations from the Inter-national Atomic Energy Agency (IAEA) Database of NaturalMatrix Reference Materials. The total mass of subductingconsolidated crust, 60 Pg/a [Mackenzie et al., 2004], is multi-plied by the typical metal concentrations of ocean crust.60 Pg/a of oceanic crust and 4.5 Pg/a of continental crust areestimated to be produced from the mantle each year [Reymerand Schubert, 1984]. Metal concentrations utilized are thoseaforementioned for crustal metal stock estimation. Lithifica-tion of continental and oceanic regolith also produces crust.For our purposes we assume constant freeboard with nochange in crustal volume [Schubert and Reymer, 1985] andthat 75 ± 20%, or the majority of sedimentary rocks, are ofmarine origin [Blatt, 1992]. With these two assumptions weconstruct a mass balance calculation wherein the mass ofsediment deposited to the seafloor and continental margins,less the sedimentary mass that is subducted into the mantle,is equal to the mass of regolith lithified. We employ ourassumption that this mass flow accounts for 75% of thesedimentary rock production, with the other 25% occurringowing to lithification of continental regolith. Assuming

overall consolidated crust mass balance between weatheringand lithification, the mass of metal weathered into regolithcan also be estimated.[15] We adjust a rough estimate of sediment mass mobi-

lized by freshwater, 60 Pg/a, of which about 40 Pg/a isredeposited [Stallard, 1998], to 65 Pg/a to agree with recentestimates of about 25 Pg/a of sediment deposited to theoceans. These total mass flows are multiplied by estimatedaverage metal concentrations in sediment [McLennan, 1995].We add to the continental regolith mass mobilized byfreshwater the difference between the mass of continentalcrust weathered and that lithified. The sediment redepositedonto terrestrial land is adjusted according to an assumed massbalance (inputs = outputs) of the freshwater reservoir.[16] It has been estimated that about 20% of sediment

entering the ocean makes its way to the deep ocean seafloor,the remainder being deposited in estuaries and continentalmargins [McLennan, 1995]. This ratio is applied to the metalflow inputs to the ocean. The values for continental marginand seafloor deposition are adjusted for (see relevant sectionsfor these calculations) sewage sludge dumping, net atmo-spheric influx, and hydrothermal emissions, assuming themetal stocks in the ocean reservoir itself are in steady state.[17] Atmospheric emissions from biomass occur owing

to biogenic emission or combustion. We multiply averageterrestrial biomassmetal concentrations by the total estimatedglobal emission of biogenic particulates (1000 Tg/a) [Jaenicke,2005] to arrive at the mass of metals emitted. Utilizing themethodology and data available in an analysis of metalemissions from burning in savanna and tropical rain forests[Yamasoe et al., 2000], we compute and scale Cu, Zn, Fe, Ni,Cr, and Al emissions to the total amount of vegetative matteron the basis of total global dry matter mass burned [Andreaeand Merlet, 2001]. We proceed with the same approach butuse different data sources for metal concentrations in partic-ulate emitted for Ag [Vincze et al., 2002] and Pb [Battye andBattye, 2002]. This independent calculation is averaged withother available estimates (scaled to total biomass burnedwherenecessary) to produce a best estimate [Klee and Graedel,2004; Lantzy and Mackenzie, 1979; Nriagu, 1989; Pacyna,1986; Richardson et al., 2001; Schmidt and Andren, 1980;Zoller, 1984].We subtract from this total biomass burned thatwhich is estimated to be burned owing to agricultural activityto arrive at metal emitted from natural biomass burning.[18] Certain metals are known to become enriched in sea

spray (Cu, Zn, Pb [Richardson et al., 2001] and Fe, Al [Weiselet al., 1984]) while others are not (Ni [Richardson et al.,2001], Cr [Schneider, 1985], and Ag [Arimoto et al., 1990]).While we report what has been calculated from observations,note that the extent of enrichment remains a matter of somedebate, given contamination concerns and the variability dueto sampling techniques [Hunter, 1997]. Following the calcu-lation outlined elsewhere [Richardson et al., 2001], we utilizemetal enrichment factors, seawater concentrations derivedpreviously for estimating ocean metal stocks, sea-salt [Weiselet al., 1984] and seawater [Duce et al., 1976] Na concentra-tions, and total sea-salt emissions [Gong et al., 2002] to cal-culate metal emission in sea spray. We average our results forCu, Zn, Pb, Fe, and Al with other estimates [Weisel et al.,1984] to arrive at a best estimate. The total sea salt flow to the

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atmosphere is about 1 � 1013 kg/a, of which 98.5% is de-posited back into the ocean, with only the remaining 1.5%deposited onto land [Gong et al., 2002].2.2.4. Anthropogenic Flows of Metal[19] For fossil fuel combustion, atmospheric emissions

from mobile sources such as diesel and gasoline are consid-ered negligible for the metals under analysis, with theexception of Pb [von Storch et al., 2003]. Rather, the vastmajority of emissions arise from stationary coal combustionsources.We estimate the total global emission of metals usingglobal coal consumption and the average metal concentra-tions in coal used in the stock estimate. Our results can becompared with other recent estimates [Klee and Graedel,2004]. Although most of this emission is collected in pollu-tion control equipment and/or disposed of as bottom ash, afraction is emitted to the atmosphere. Utilizing the sameapproach for Cu, Zn, Ni, Pb, and Cr emission estimates madefor the mid-1990s [Pacyna and Pacyna, 2001], we updateatmospheric emissions to the year 2000. Pb emissions in-clude that from petroleum combustion by mobile sources.For Ag, total emissions are fractionated between air and landemissions using data from an Ag emission study [Eckelmanand Graedel, 2007]. Fe and Al emissions assume 10% oftotal emissions enter the atmosphere given the emissioncontrols [Lantzy and Mackenzie, 1979].[20] In our analysis framework for flows related to agri-

cultural crops, manure fertilizer is an internal cycle withinagricultural stock, so is not quantified explicitly. However,artificial fertilizers and pesticides contain trace amounts ofmetals which flows we do quantify.We treat the source of thisflow as arising from production, although it could be arguedthat artificial fertilizers and pesticides should be sourced fromfabrication and manufacturing. For artificial fertilizers, weassume the global yearly consumption is applied to agricul-tural use in the same year. Armed with this assumption, weuse global consumption statistics for major nitrogen, phos-phorous, and potash fertilizers as provided by the Interna-tional Fertilizer Association. We arithmetically averagefertilizer metal concentrations obtained from a literaturereview [Adriano, 2001; Bowhay et al., 1997; Franklin et al.,2005; Iskander, 1994;McBride and Spiers, 2001;Moolenaarand Lexmond, 2000; Rauf et al., 2002; Raven and Loeppert,1997; Senesi and Polemio, 1981; Weinberg Group, 2000;Environmental Protection Agency (EPA), 1999]. We esti-mate the metal flow in each individual fertilizer (urea, triplesuperphosphate, potassium chloride, etc.) and then sum. Areview of the Pesticide Action Network (PAN) databasereveals that only Al, Cu, and Zn had significant applicationvolumes as of the year 2000, all in fungicides. No globalstatistics exist regarding consumption of individual pesti-cides, and the only data available for the United States are forthe years 1992 and 1997 from the National Center for Foodand Agricultural Policy pesticide use database. We employthe 1997 U.S. data for pesticide consumption, and use pes-ticide molecular weights to estimate Al, Cu, or Zn masscontained. Al is a major constituent of fosetyl aluminum, Cuis used mostly in copper sulfate and copper hydroxide, andZn is a component of ziram, metiram, and mancozeb. Wescale the U.S. metal mass flow to a global value on the basis

of the relative consumption of total fungicides in the year2000 [Fishel, 2007].[21] We determine the uptake of metal by crop growth

by assuming the total agricultural biomass stock turns overonce per year. One of the major caveats of this assumptionconcerns the difficulty in assessing how much of metaluptake occurs from the soil and how much from appliedfertilizers. It appears that on average 10% of Cu, 28% of Zn,and 5% of Pb in agricultural crops is derived from artificialfertilizer input [Moolenaar, 1999]. We are unaware of otherstudies from which these fractions can be determined, so weaverage the above fractions together as an approximation forother metals. These percentages can be interpreted as repre-senting that fraction of fertilizer applied that is taken up byagricultural crops. Consumption of crops by human ingestionresults in a flow of metal into use; we use the estimate that 8%of agricultural biomass is actually consumed [Wirsenius,2003]. For Cu, Zn, and Pb we average this result with con-sumption estimates from Dutch [Moolenaar and Lexmond,2000] and Finnish [Moolenaar, 1999] studies that we scaleto global values on the basis of total cropland. The metalreturned to the soil is determined by a mass balance of theoverall flows of inputs and outputs.[22] Quantified technological cycles (flows such as min-

ing, production to fabrication, consumption into use, dis-cards) for the year 2000 exist for Pb [Mao et al., 2008], Fe[Wang et al., 2007], Ni [Reck et al., 2008], Cu [Gerst andGraedel, 2007], and Cr [Johnson et al., 2006]. Ag [Johnsonet al., 2005] data are available for the year 1997. We alsoobtained global cycle data for Al (year 2002 from theEuropean Aluminum Association) and derived Zn data onthe basis of the preceding technological cycle methodologies.[23] The masses of metal mined, metal released back to the

environment in the forms of tailings and slag, and metal sentto fabrication have been determined in former assessments ofthe global technological cycles ofmetals [European AluminumAssociation, 2004; Gerst and Graedel, 2007; Graedel et al.,2005; Johnson et al., 2005, 2006; Mao et al., 2008; Recket al., 2008;Wang et al., 2007] (where Zn has been updated to2000 values). Atmospheric emissions occur both from min-ing activity itself and from the smelting production phase ofmetal ore processing. For atmospheric mining emissions, weaggregate recorded (all metals but Fe) mining metal emis-sions for the United States (Toxic Release Inventory (TRI)),Canada (National Pollutant Release Inventory (NPRI)), andAustralia (National Pollutant Inventory (NPI); Al and Ag notrecorded). We scale by employment these emissions to thetotal mining sector in each country using the ratio ofemissions to number of employees, a ratio derived fromCanada’s NPRI data.We then scale to global emissions on thebasis of total global employment in mining as published bythe International Labour Organisation. Although likely anunderestimate given the better pollution controls in devel-oped countries, the emissions calculated from this approachare within the range of previous global emission estimates[Nriagu and Pacyna, 1988]. This same method is used toestimate water emissions from mining and metal productionactivity. Atmospheric emissions for Ag and Al from produc-tion also use this approach. However, for atmospheric emis-sions from production of Cu, Cr, Zn, Pb, andNi we are able to

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use a more robust methodology [Pacyna and Pacyna, 2001],updating results to the year 2000. For total atmosphericemissions of Fe from mining and production, we scale tothe year 2000 an estimate for the year 1980 of globalproduction emissions [Malakhov and Makhon’ko, 1990] onthe basis of the average relative change of emissions of theother seven metals in our analysis. No previous estimates forglobal water emissions of Fe could be found, so we estimatewater emissions on the basis of the average relative propor-tion of water emissions to mass of metal ore mined of theother seven metals we assess.[24] The masses of metal sent to use, recycled back to

production, and discarded from fabrication have been deter-mined in the previous global technological cycles of metals[European Aluminum Association, 2004;Gerst and Graedel,2007; Graedel et al., 2005; Johnson et al., 2005, 2006; Maoet al., 2008; Reck et al., 2008; Wang et al., 2007] (where Znhas been updated to 2000 values). Scrap produced at thisstage is also often recycled in a recursive, internal flow ofmetal back into fabrication and manufacturing, but this flowis not required by our framework. Fabrication emissions toair, land, and water are calculated using the same approach asproduction emissions, on the basis of TRI, NPRI, and NPIdata. Air emissions for Cr, Cu, Ni, Pb, and Zn again utilize amore robust methodology [Pacyna and Pacyna, 2001], withupdated 2000 data. Fe emissions to land and water areestimated on the basis of the relative ratio between metalemissions and metal mass flow into fabrication averagedacross the other sevenmetals in our analysis. For atmosphericemissions of Fe in fabrication, we assume the proportion ofmass emitted relative to Al is the same as the proportion ofmass emitted between Fe and Al in production.[25] Discards from use are either managed (to waste

management) or unmanaged (emissions). Metal discards towaste management have been assessed [European AluminumAssociation, 2004; Gerst and Graedel, 2007; Graedel et al.,2005; Johnson et al., 2005, 2006; Mao et al., 2008; Recket al., 2008;Wang et al., 2007] (where Zn has been updated to2000 values). Emissions from use are often described asdissipative losses; these losses are very difficult to quantify,let alone disaggregate among air, water, and landmedia. Untilfurther research expands our understanding of these dis-sipative losses, we believe we should only make a generalemission estimate for each metal, where emissions to variousmedia are not disaggregated. This general emission estimateis represented in our model by a flow to land. On the basis ofpreliminary evidence with copper [Rauch and Graedel,2007], we estimate that no more than about 1% of the flowof metal into use is dissipated. We apply this assumption toall metals in our analysis except Fe, Al, and Pb. For thesemetals, independent estimates of dissipative emissions exist[European Aluminum Association, 2004; Mao et al., 2008;Wang et al., 2007].[26] Recycling of metal from waste management back to

fabrication, to production, and that metal sent to landfill havebeen quantified in the technological cycle of these metals[European Aluminum Association, 2004;Gerst and Graedel,2007; Graedel et al., 2005; Johnson et al., 2005, 2006; Maoet al., 2008; Reck et al., 2008; Wang et al., 2007] (where Znhas been updated to 2000 values). Air emissions from discard

management result from municipal solid waste (MSW) andsewage sludge (SS) incineration. For emissions for the year2000 we assume little change in emission controls, resultingin an increase of emissions by 10% over mid-1990s levels[Pacyna and Pacyna, 2001]. For Al, Fe, and Ag we estimateemissions using average emission factors forMSW [Law andGordon, 1979] and SS [EPA, 1995] incineration, utilizing ouradjusted estimates for total MSW (85 Tg/a) and SS (8 Tg/a)incinerated [Pacyna and Pacyna, 2001]. Water emissionsresult from sewage sludge management and domestic waste-water discharge. Dumping occurs to both oceans and fresh-water. We estimate 50 Tg/a dry weight of total sewage sludgeproduction globally on the basis of a review of regionalestimates [Chang et al., 2002; Epstein, 2002; Krauss andPage, 1997] scaled up by population and previous globalestimates [Eckelman and Graedel, 2007; Nriagu andPacyna, 1988]. Metal concentrations in sewage sludge areobtained from an arithmetic average of published literaturevalues [Alonso et al., 2006;Berrow andWebber, 1972;Feiler,1980; Furr et al., 1976a, 1976b; Georgopoulos et al., 2001;Gutenmann et al., 1994; Karvelas et al., 2003; Lisk et al.,1992; Mumma et al., 1983, 1984; Scancar et al., 2000;Spiegel et al., 1985]. To apportion sewage sludge dischargebetween freshwater and ocean reservoirs, we use an oceandumping rate of of 12 Pg/a dry weight [Ross, 1995]. Weestimate domestic wastewater use as ‘‘withdrawal’’ minus‘‘consumption’’ as reported in available world regions esti-mates [Shiklomanov, 1999]. Metal discharge in wastewater isthe difference between the metal mass influent to wastewatertreatment and metal mass effluent discharge. Influent watermetal concentrations are taken from a collection of studies forEurope [Busetti et al., 2005; Lesage et al., 2007; Rule et al.,2006; Sorme and Lagerkvist, 2002], the United States [Shaferet al., 1998; EPA, 1991], Africa [Odiyo et al., 2005], and Asia[Chino et al., 1991; Lim et al., 2006; Yadav et al., 2002] withcare that only wastewater from domestic sources is consid-ered. For regions for which data are not available we use aglobal average of all available data. We calculate metalremoval in wastewater treatment using UN statistics on thecountry distribution of wastewater treatment types (no treat-ment, primary, secondary, tertiary) and associated metalremoval efficiencies [Busetti et al., 2005; Grafton WaterPollution Control Facility, 2003; Isaac et al., 1997; da SilvaOliveira et al., 2007; Oliver and Cosgrove, 1974; EPA,2004]. The resulting mass flows are available for betweentwo and seven regions on all 6 continents, as defined by theoriginal UNESCO data, and are summed for reporting globalemissions. Emission to oceans versus freshwater are dividedaccording to the same fraction as sewage sludge emissions.

3. Results

[27] The results in Gigagrams (Gg) (a thousand metrictonnes) are presented in global cycle diagrams, colored toreflect the estimated uncertainty (Figures 1 and 2). Numericalvalues of the uncertainty are available in the rainbow plotslocated in the auxiliary material affiliated with this paper(Figures S1 and S2).1

1Auxiliary materials are available in the HTML. doi:10.1029/2008GB003376.

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Figure 1. Earth’s global metal cycles for (a) aluminum (Al), (b) iron (Fe), (c) copper (Cu), and (d) zinc(Zn). These four metals constitute over 95% of the total mass of all metals extracted annually for humanutilization. Anthropogenic data are for the year 2000 except for Al (2002). The line thicknesses arelogarithmically proportional to the flow magnitude, and colors indicate the percent uncertainty.

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Figure 1. (continued)

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Figure 2. Earth’s global metal cycles for (a) chromium (Cr), (b) nickel (Ni), (c) lead (Pb), and (d) silver(Ag). These four metals are the additional metals for which global cycles could be constructed given theavailable data. Anthropogenic data are for the year 2000 except for Ag (1997). The line thicknesses arelogarithmically proportional to the flow magnitude, and colors indicate the percent uncertainty.

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Figure 2. (continued)

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[28] Although Fe and Al undergo crustal subduction andproduction at the same mass flow rate (4–5 � 106 Gg/a), Febecomes increasingly more abundant with depth (Figures 1aand 1b). Cu exceeds Pb abundance in the crust by a factor offive (Figures 1c and 2c), yet the subduction and productionmass flow rate of Pb (�70–80 Gg/a) is almost 2 orders ofmagnitude less due to the much lower concentrations of Pbin the oceanic basalts that dominate the mass of crustal pro-duction and subduction.[29] Groundwater (1 � 1013 Gg) [Gleick, 2000] contains

the majority of metal mass in freshwater bodies, despiterelatively lower concentrations. Metal masses contained inrivers, lakes, ice, wetlands, and soil moisture are all relativelyinsignificant. Al is the most abundant of the studied metals inthe oceans (1.4 � 106 Gg), followed by Ni, Cr, Zn, and Cu,with Fe (4.8� 104 Gg) relatively depleted given the oxidativeconditions (Figures 1a–1d and 2a and 2b).[30] Nature mobilizes metal mass on Earth’s surface

mostly by water transport, biomass growth and death, andeolian action. Note that potential biogenic emissions ofmetals by volatile natural hydrocarbons are not included inour analysis, as there is no experimental evidence that naturalvolatile organic compounds of isoprene and derivative ter-penes complex with metal ions to act as a significant vector ofmetal emissions, despite past extrapolation of plausibleassumptions to global estimates [Nriagu, 1989]. Groundwa-ter has a large standing mass, but flows slowly enough thatriver mass transport (25 Pg/a) [Syvitski et al., 2003] domi-nates movement of material to the oceans. Although wind-blown dust is the largest medium for atmospheric metaltransport [Jickells et al., 2005], volcanic emissions are largerthan natural biomass combustion for Fe, Ni, Cr, and Al, butsmaller for Cu, Pb, Ag, and Zn (Figures 1 and 2). Much moreFe than Al is released directly into the oceans (�700x) andatmosphere (�2x) by mantle-derived outgassing. Sea sprayemission is also a significant flux for Cu, Pb, Ag, and Zn(Figures 1c and 1d and 2c and 2d).[31] Metals constitute less mass in most anthropic reser-

voirs than in natural reservoirs. However, U.S. GeologicalSurvey estimates of global resources for Ag, Cu, Ni, and Znare only 1 order of magnitude larger than derived estimates ofglobal in-use metal stocks [Gerst and Graedel, 2008]. Allmetals examined are more abundant in global in-use stocksthan in Earth’s living biomass.[32] Significant (>1 Mg/a) flows of metal from anthropic

sources to natural sinks occur through emissions to air, water,and land from metal production, fabrication, and discardmanagement, and emissions to air and land from fossil fuelcombustion, agricultural biomass, and metal use. Our resultsindicate that the largest anthropogenic emissions to air arisefrom fossil fuel combustion for Al, Fe, and Pb (Figures 1a and1b and 2c). For Ag, Cr, Cu, Ni, and Zn, the largest airemissions are from metal production (Figures 1c and 1d and2a, 2b, and 2d).[33] Coincident with these pollution flows, we find that

large imbalances exist in both the natural and anthropogenicparts of these metal cycles. The yearly growth rates of in-usemetal stocks at the turn of the 21st century range from around2% for Zn and Fe to around 6% for Ni and Cr. The higher rateof accumulation of Ni and Cr can be attributed to the high

global demand for stainless steel in durable products. Deple-tion of crustal ore resources contributes to this net in-usestock accumulation. Whereas flood control and hydroelectricdams reduce denudation rates, and agricultural activity andurban run off increase such rates [Syvitski et al., 2003], a netaccumulation of metal mass in continental margin sedimentsoccurs for all metals except Fe. This result assumes no netchange in crustal mass during the Phanerozoic. Previousestimates agree, suggesting total mass flow in the Holocenewas 50% of what it is with today’s human impacts [Millimanand Syvitski, 1992].

4. Discussion and Conclusion

[34] Comparison to previous research focused upon par-ticular stocks or flows indicates general agreement with ourresults. The atmospheric mass of metals, calculated frommass balance of inputs and outputs and average residencetimes, agrees with previous estimates [Lantzy andMackenzie,1979], though Pb has been reduced owing to significantreductions in its use in gasoline combustion. The minorreduction in Cu and Zn atmospheric stock can be attributedto advances in pollution control technology in production;the trend in atmospheric emission is evident over time[Nriagu and Pacyna, 1988; Pacyna and Pacyna, 2001].The atmosphericmetal concentrations derived from this work(Table 1) are on the lower end of the range of measuredatmospheric concentrations [Wiersma and Davidson, 1986].Despite successful reductions in hydrologic and atmosphericemissions, society’s increasing consumption of metal hasincreased both in-use stocks and waste stream discards. Asa result, metal emissions to land have increased by a factorof 1.5 to 3 since 1983 [Nriagu and Pacyna, 1988].[35] While the process of translocating metal from below

ground to human built infrastructure has seen an overallreduction over the past few decades in the wasteful andpotentially harmful dissipative emissions to environmentalmedia, the continued increase in mass movement from oreto in-use stocks has repercussions for the future of environ-mental emissions. Though dissipative emissions from in-usestocks have traditionally been minor relative to upstreamemissions, the chances of increased emissions from in-usestocks is greater with the increasing size of this reservoir. Thistrend necessitates continued work in measuring and moni-toring nonpoint metal emissions, with information on loca-tions, sources, and sinks of emissions related to the in-use lifestage of metals.[36] The increasing size of the in-use stock reservoir also

has repercussions on the future sources of metal resources, asmore metal will likely be sourced from the discards of in-usestocks through recycling. This recovery will not necessarilyreduce the burden on raw ore extraction, for most raw oresremain highly concentrated relative to in-use stocks, and oreis required to provide for increasing levels of consumptionand compensate for dissipative losses. However, stabilizationof the world’s population will likely provide the first steptoward bringing the world’s metal cycles into balance.Emphasis on understanding the amounts, forms, and spatialdistributions of in-use metal stocks will be important as this

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reservoir becomes an increasingly more significant locationand source of metal resources.[37] Our results overall indicate that human action has

become a significant geomorphic agent in metal mobiliza-tion. Total natural and anthropogenic flows presently existat approximately the same order of magnitude across mostmetals investigated (Figures 1 and 2). The largest total massmovements in the natural system (crustal production andsubduction, sediment erosion and denudation, and oceandeposition) exceed by an order of magnitude or more thelargest anthropogenic mass movements (mining extraction,metal production, and metal consumption) only for Al andFe. Cu is the only metal in which anthropogenic flows exceednatural flows by an order of magnitude or more (Figure 1c).This order of magnitude equivalency compares with thegrand nutrients, where the mobilization of C by humanactivity is less than an order of magnitude of natural flows,N and S are approximately the same, and P mobilization ex-ceeds that of nature by half an order of magnitude [Falkowskiet al., 2000]. Such perturbation of Earth’s natural globalmetal cycles is resulting in the reallocation of metal massfrom surface sediments to continental margins and from orebodies to in-use stocks.

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�������������������������J. M. Pacyna, Norwegian Institute of Air Research, P.O. Box 100, N-2027

Kjeller, Norway.J. N. Rauch, Yale School of Forestry and Environmental Studies, Yale

University, New Haven, CT 06511, USA. (jason.rauch@yale.edu)

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