Isotopic Fingerprints of Anthropogenic Molybdenum in Lake Sediments

Sep 12, 2012 - Gwyneth W. Gordon,. § and Ariel D. ... copper deposits (Lake Vose) or through combustion of coal and oil also containing Mo (Lake Tant...
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Isotopic Fingerprints of Anthropogenic Molybdenum in Lake Sediments Anthony Chappaz,*,†,‡ Timothy W. Lyons,† Gwyneth W. Gordon,§ and Ariel D. Anbar§,∥ †

Department of Earth Sciences, University of California, Riverside, California, United States Institute for Great Lakes Research, Department of Chemistry, and Department of Earth and Atmospheric Sciences, Central Michigan University, Mount Pleasant, Michigan, United States § School of Earth and Space Exploration and ∥Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, United States ‡

S Supporting Information *

ABSTRACT: We measured the molybdenum isotope compositions (δ98Mo) of well-dated sediment cores from two lakes in eastern Canada in an effort to distinguish between natural and anthropogenic contributions to these freshwater aquatic systems. Previously, Chappaz et al.1 ascribed pronounced 20th-century Mo concentration enrichments in these lakes to anthropogenic inputs. δ98Mo values in the deeper sediments (reflecting predominantly natural Mo sources) differ dramatically between the two lakes: −0.32 ± 0.17‰ for oxic Lake Tantare and +0.64 ± 0.09‰ for anoxic Lake Vose. Sediment layers previously identified as enriched in anthropogenic Mo, however, reveal significant δ98Mo shifts of ±0.3‰, resulting in isotopically heavier values of +0.05 ± 0.18‰ in Lake Tantare and lighter values of +0.31 ± 0.03‰ in Lake Vose. We argue that anthropogenic Mo modifies the isotopic composition of the recent sediments, and we determine δ98Moanthropogenic values of 0.1 ± 0.1‰ (Lake Vose) and 0.2 ± 0.2‰ (Lake Tantare). These calculated inputs are consistent with the δ98Mo of molybdenite (MoS2) likely delivered to the lakes via smelting of porphyry copper deposits (Lake Vose) or through combustion of coal and oil also containing Mo (Lake Tantare). Our results confirm the utility of Mo isotopes as a promising fingerprint of human impacts and perhaps the specific sources of contamination. Importantly, the magnitudes of the anthropogenic inputs are large enough, relative to the natural Mo cycles in each lake, to have an impact on the microbiological communities.

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INTRODUCTION

Quebec City. Lake Vose is located within 25 km of RouynNoranda, where an important copper smelter has operated since 1927 (recent production of 194 300 tons·year−1 copper anode in 2010; www.xstratacopper.com). The drainage basins of these two oligotrophic lakes are uninhabited and have not been affected by lumbering or wildfires for at least 100 years. Therefore, the only appreciable anthropogenic inputs arrive via the atmosphere. The hypolimnion of modern Lake Tantare remains oxygenated through the year, whereas Lake Vose becomes seasonally anoxic in summer, as it was during our sampling (Table 1). Chappaz et al.1 were able to reconstruct the historical records of atmospheric molybdenum (Mo) fluxes (JatmMo) to the two lakes (Figure 2) and to tie those inputs of anthropogenic Mo to either a regional smelter (Lake Vose) or more diffuse long-range pollution (Lake Tantare). We initiated this study to see if Mo isotope relationships are consistent with

Two headwater lakes located in the province of Quebec, Eastern Canada, were sampled between June 2001 and September 2003 (Figure 1; see ref 1 for further details). Lake Tantare is located in an ecological reserve 40 km northwest of

Received: Revised: Accepted: Published:

Figure 1. Map showing the location of Lakes Tantare and Vose. © 2012 American Chemical Society

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thiomolydate ions (MoO4−xSx2−) and early uptake of these highly reactive species by iron sulfide minerals and organic matter.16−18). The geochemical behavior of molybdenum has also been examined in the context of its isotopic composition. Lighter isotopes of Mo (95Mo) are adsorbed preferentially onto iron (Fe) and manganese (Mn) oxyhydroxides19 relative to heavier isotopes of Mo (98Mo), with fractionations for Mn phases of approximately −3.0‰20 and somewhat smaller effects ranging from about −2.2‰ to −0.8‰ for iron oxyhydroxides, depending on the specific Fe(III) mineral phase involved.21 Previous work has revealed an extended range of δ98Mo values in natural marine settings described ambiguously as “suboxic”. The defining characteristic for these sites of intermediate redox, in terms of Mo cycling, is the presence of H2S in the pore waters beneath sulfide-free but typically oxygen-lean bottom waters. The isotopic range for these locations, from −0.5‰ to 1.3‰, can be explained by a combination of known oxic processes (adsorption onto iron and manganese oxyhydroxides with well-known isotope effects) and still poorly studied isotopic fractionation under sulfidic conditions in the pore fluids,22 including those tied to transitions among the thiomolybdate intermediates at relatively low dissolved sulfide concentrations.11 Molybdenum isotope behavior under sulfidic conditions remains a topic of investigation, including fractionations among the thiomolybdate species.23 However, sediments accumulating in euxinic marine settings, modern and ancient, can capture the δ98Mo of seawater.10,24 Recent studies have revealed that δ98Mo values in the world’s rivers are positive, with an average modern value of 0.7‰ ± 0.5‰.25−27 By measuring δ98Mo in sediments collected below the surface layers in these two Canadian lakes, we hope to identify specific isotopic signatures associated with Mo burial by well-known processes, particularly Mo adsorption on iron oxyhydroxides. These data provide the preanthropogenic baseline conditions. Against this backdrop of natural Mo cycling, our ultimate aim is to trace anthropogenic Mo inputs to otherwise pristine lacustrine settings using combined concentration and isotopic data, with the hope that addition of the isotopic tracer will ultimately allow for quantitative fingerprinting of specific contaminant sources. Our study provides a first window to that potential and the range of data needed to do it well. We selected three samples at the maximum peak of anthropogenic Mo influence during the 1970s and 1980s, identified hereafter as “anthropogenic Mo”. A second group, referred to as “natural Mo”, was collected below 15 cm to represent deposition predating the smelter and other appreciable inputs tied to human activity. Using simple mass balance arguments within the framework of our well-constrained, preanthropogenic baseline, we can calculate the diagnostic δ98Mo of those inputs. An analogous lead (Pb) isotope approach was successful in tying Pb contamination in sediments from Lake Vose to the nearby smelter.28 Previous studies exposed dramatic molybdenum, rhenium, and thallium inputs to the two lakes via smelting activity and more diffuse anthropogenic sources, such as those tied to increasing urbanization of Montreal and Quebec City.1,29,30 Given the important enzymatic role of Mo in biological processes, particularly as related to nitrogen cycling,31 Mo augmentation to otherwise Mo-poor lacustrine settings and the degree of bioavailability of this delivery must be considered in studies of modern and future lake ecology. Among other benefits, improved understanding of the Mo source relation-

Table 1. Locations and Characteristics of Study Sites Lake Tantare location geologic region altitude (m) sampling depth (m) hypolimnion at sampling time pH (water column) sediment mass accumulation ratea (mg·cm−2·year−1)

47°04′ N, 71°32′ W Can. Shield 450 15 oxic 5.3−5.6 3.5−6.7

Lake Vose 48°28′ N, 78°50′ W Can. Shield 350 17 anoxic 6.4−6.7 7.8−16.4

a

It is important to note that sediment mass accumulation in Lake Vose is approximately twice that in Lake Tantare.

Figure 2. Historical records of anthropogenic Mo fluxes in Lakes Tantare and Vose (figure modified from Chappaz et al.1). The flux of anthropogenic Mo, JanthMo, is calculated as JatmMo = ([{Mo}deposited − {Al}meas ({Mo}/{Al})crust]ω)(210PbIatm/210PbIsed), where {Al}meas is the measured concentration of sedimentary Al, ({Mo}/{Al})crust is the Mo/Al molar ratio in the Earth’s crust, ω is the sediment mass accumulation rate, 210PbIatm is the cumulative atmospheric input of 210 Pbun in the area of study, and 210PbIsed is the inventory of 210Pbun measured in the sedimentary column at the sampling site. Such a correction implies that the processes that control Mo and 210Pb deposition to the sediments act similarly. See Chappaz et al.1 for additional details.

that inference. If so, isotopic data might provide a new way to identify and quantify the sources of Mo contamination in natural settings. Strong enrichments (10−100×) in Mo compared to average crustal rock have been used widely as evidence for past euxinia.2−6. Euxinic oceans or lakes are defined as being anoxic and containing dissolved H2S in the water column. More recently, the isotopic composition of Mo (δ98Mo) in euxinic black shales has shown promise as a proxy for the relative areal extents of oxic versus euxinic bottom waters in the ancient deep ocean.7−9 In efforts to develop, refine, and apply this tracer, abundant δ98Mo data have been generated in diverse modern and ancient marine settings, including the Black Seathe largest and most commonly cited modern analogue for ancient euxinia.3,7,10−12 Mechanisms leading to Mo sequestration into oxic sediments are relatively well understood, but the situation is less clear for euxinic settings. Under oxic conditions, otherwise soluble MoO42− is adsorbed onto iron and manganese oxyhydroxides.1,13,14 For euxinic settings, laboratory experiments15 suggest that most Mo is initially sequestered via conversion to 10935

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ships will speak to its bioavailability. Overall, the inclusion of Mo isotope analyses improves our understanding of natural and anthropogenic controls on Mo cycling in these settings.

(Johnson Matthey Chemical, Specpure Stock 38719, Lot 012793; δ98Mo = −0.05 ± 0.05, n = 16, 2 SD). Additional details are provided in the Supporting Information.

MATERIALS AND METHODS Study Sites and Sampling. Divers using 9.5 cm internal diameter butyrate tubes collected sediment cores. The cores were extruded within 2 h of collection and sectioned at 0.5 cm intervals to 10 cm (Lake Vose) or 15 cm (Lake Tantare) depth and then at 1 cm intervals to the bottom of the cores. These sediments were stored in polyethylene containers and kept at 4 °C until they were returned to the laboratory, where they were freeze-dried. A previous complementary study yielded the vertical distributions of Mo, Fe, Mn, sulfide, sulfate, organic carbon, major ions, and pH in pore water from both lakes by use of peeper sampling devices, as well as trends for solid Mo, acid-volatile sulfide, Fe, Mn, Al, organic C, 210Pb, and 137Cs in sediment cores from the same sites. These cores used in our Mo isotope study were previously reported.1 The analytical specifics for Mo are described below; all other details are reported in Chappaz et al.1 Analyses. Total Molybdenum Concentration. Sediments were ground, homogenized, and digested completely with HNO3, HClO4, and HF. Acids were evaporated to near dryness, and the residues were redissolved in 0.2 N HNO3 solution.32 Mo concentrations in the bulk digests were measured with an inductively coupled plasma quadrupole mass spectrometer (ICP-MS; Thermo Instrument X7) with external calibration. Specifically, we used certified materials MESS-3 and PACS-2 from the National Research Council of Canada (NRCC) to determine precision (0.7% for Mo; n = 10) and accuracy (7.1% for Mo). Molybdenum Isotope Compositions. Molybdenum isotope data were measured at the W.M. Keck Foundation Laboratory for Environmental Biogeochemistry, School of Earth and Space Exploration, Arizona State University (ASU). Aliquots of the digested sediment cores were spiked with the ASU2 double spike enriched in 97Mo and 100Mo. Samples were then purified with standard ion-exchange procedures.33 Measurements were made on a Thermo Neptune MC-ICPMS using an Elemental Scientific Apex-Q with a 50 or 100 μL/ min nebulizer and 100 cycles of 4.2 s. Measured ion beams included 91Zr, 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo, 99Ru, and 100 Mo. Data reduction was performed off-line with an iterative Matlab routine to remove outlier cycle data, calculate the instrumental fractionation, and correct for minor contributions from Zr. All samples had 97Mo/98Mo ratios between 3.1 and 4.3, very close to the theoretical ideal value of 3.3 and well within the range of validated standard values. The range of acceptable concentrations was confirmed with standards between 5 and 50 ppb; all analyzed sample solutions were between 10 and 25 ppb. Standards were run every two samples, with frequent secondary standards. The bracketing standard used was the ASU in-house ICP solution, RochMo2 (Johnson Matthey Chemical, Specpure ICP-MS standard, Stock 35758, Lot 802309E). We further checked reproducibility by running secondary standards during the same period that we analyzed our samples. Those standards included SDO-1 (USGS Devonian Ohio Shale; δ98Mo = 1.12 ± 0.05, n = 16, 2 SD), NIST Mo (NIST 3134, Lot 891307; δ98Mo = 0.33 ± 0.03, n = 17, 2 SD), UMd Mo (Johnson Matthey Chemical, Lot 013186S; δ 98Mo = 0.16 ± 0.03, n = 16, 2 SD), and KyotoMo

RESULTS AND DISCUSSION Chappaz et al.1 described the Mo geochemistry in Lakes Tantare and Vose in detail. Briefly, the surface-most sediments of Lake Tantare are enriched in Mo as a result of adsorption onto iron oxyhydroxides produced at the sediment−water interface by iron recycling (Figure 3). Unlike most marine

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Figure 3. Depth profiles of total Mo concentrations in dried sediments and δ98/95Mo measurements for selected samples.

systems, the low pH values (∼5.7) preclude the formation and preservation of authigenic manganese oxyhydroxides in the sediments of this lake.29 The concentrations of bulk solid-phase Mo decreased below this surface layer, reached a maximum at 5.5 cm depth primarily through peak anthropogenic Mo inputs, and then decreased to a constant background value of 24 ± 1 nmol·g−1, which is similar to the natural abundances (10−21 nmol·g−1) seen deeper in the core. The maximum peak at 5.5 cm depth is attributed primarily to combustion of coal and oil used intensively in the Greater Quebec City area for domestic and industrial purposes from the beginning of the 20th century to the 1970s.1 Furthermore, molybdenite has been used intensively in many industrial processes since the 1920s. MoS2 is an efficient catalyst for the hydrogenation of coal.34 MoS2 is also employed for petrochemistry as a catalyst to remove sulfur in oil,35 and many engine manufacturers use MoS2 as a lubricant.36 These different sources of anthropogenic Mo tied to industrial activities contribute Mo pollution to the atmosphere37 that can disperse locally and regionally. The inferred decrease in the atmospheric flux of Mo (JatmMo) from the 1970s to about 1995 is likely the result of a decrease in coal combustion, as well as lower industrial emissions in Canada and the United States in general, as a result of more stringent regulations and improved technology. Intermittently anoxic conditions in Lake Vose limit the accumulation of FeIII and MnIV phases at the sediment−water interface (SWI), and thus the solid Mo profile did not show the sharp surficial enrichment seen in Lake Tantare. Mo concentrations increased with depth below the SWI, reached a maximum at 6.5 cm depth, and decreased to nearly a constant 10936

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value of 11 ± 1 nmol·g−1 below 15 cm (Figure 3). Here, too, we can tie the Mo peak to anthropogenic Mo, in this case derived from the nearby smelter. Temporal variations observed in sediment Mo concentrations are consistent with the history of operation of the Rouyn-Noranda smelter, which began in 1927 and increased progressively in its production. Trace element emissions were reduced in the early 1980s due to tighter regulations and introduction of steps designed to scrub the emissions of metals.1 We can assume that the measured sediment Mo concentration at both lakes represents the sum of Mo in the settling particles deposited at the sediment surface (which includes the anthropogenic Mo particles in the upper sediment layers) and that of any Mo added to or removed from the solid phase by diagenetic reactions during sediment burial. Anthropogenic Mo is delivered as tiny particulates directly by wind transport or through erosion and riverine transport from the surrounding catchment. There is no evidence for remobilization of this Mo in the sediments during diagenesis. Previous diagenetic modeling1 showed that natural authigenic Mo enrichment in Lake Tantare represents about a third of the total Mo concentration measured in the first centimeter below the SWI and only a tenth of that between 1 and 10 cm. Based on the same pore water modeling approach, there is no evidence for authigenic enrichment at any depth in Lake Vose.1 In summary, only a small fraction of the excess Mo above the detrital natural background seen in the upper 3 and 8 cm in Lake Tantare is due to diagenesis; the majority of the excess is an anthropogenic contribution as shown in Figure 2. In Lake Vose, all of the excess between the SWI and 15 cm is anthropogenic. Molybdenum isotope values measured in sediment samples identified as anthropogenic Mo (i.e., dominated by anthropogenic fluxes) show an average δ98Mo of 0.05‰ ± 0.18‰ in Lake Tantare and 0.31‰ ± 0.03‰ in Lake Vose (Figure 3 and Table 2; 2 SD). Deeper in the sediments, δ98Mo values display a range from −0.38‰ to −0.22‰ (average δ98Mo = −0.32‰

± 0.17‰) in Lake Tantare and from +0.61‰ to +0.69‰ (average δ98Mo = +0.64‰ ± 0.09‰) in Lake Vose. These results allow us to estimate the isotopic composition of the contamination at both sites and, more generally, to define an approach for fingerprinting sources of contamination in many natural settings.

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MOLYBDENUM SOURCES AND MASS BALANCE RELATIONSHIPS The plans for this isotopic study arose only after the initial concentration data revealed evidence for strong anthropogenic inputs.1 Unfortunately, we were not able to return to the field to collect additional samples to constrain the pieces missing in the isotope mass balance, such as riverine inputs, dissolved Mo in the water column, and atmospheric particulates. As a consequence, our assessment of natural (preanthropogenic) Mo relationships in the two lakes required assumptions described below. We are confident that our approach is appropriate given the data available; however, the need for assumptions could easily be minimized or avoided altogether in future studies of this type by including a broader range of sample types, including the riverine and water column solid and dissolved fractions. Importantly, however, our focus is on anthropogenic inputs, which are readily evaluated from our existing data. Natural Molybdenum (Lower Sediment Layers). We first highlight the sediment samples assumed to represent only natural Mo inputs to Lake Tantare (17.5, 22.5, and 27.5 cm) and Lake Vose (20.5, 25.5, and 29.5 cm). These data, generated to constrain the preanthropogenic baseline, are very different in the two lakes, as we would expect given their contrasting redox conditions in the hypolimnion. The three deep samples from oxic Lake Tantare yielded negative δ98Mo values, ranging from −0.2‰ to −0.4‰. In contrast, samples from deeper in the core of intermittently anoxic Lake Vose displayed positive δ98Mo ranging from +0.6‰ to +0.7‰ (mean = +0.64‰ ± 0.09‰) (Table 2). Because Chappaz et al.1 clearly demonstrated the absence of authigenic Mo enrichment in Lake Vose sediments, we can write the following equation:

Table 2. Molybdenum Isotope Data for Lakes Tantare and Vose depth (cm)

Mo (nmol/g)

3.25 4.25 5.25

74.7 68.4 69.3

17.5 22.5 27.5

6.25 6.75 7.75 20.5 25.5 29.5

a

22.7 25.5 22.0

30.2 28.5 35.7 10.2 12.7 10.2

δ98/95Mo (‰)

Lake Tantare −0.02 0.01 0.15 0.05 −0.38 −0.36 −0.22 −0.32 Lake Vose 0.32 0.30 0.29 0.31 0.61 0.61 0.69 0.64

Vose 98 Lake Vose δ 98 Mo Lake natural sediment measured = δ Mo settling particles

SD (‰)a

Lake Vose = δ 98 Modetrital

0.02 0.13 0.04

(1)

Vose where δ98MoLake natural sediment measured = ∼+0.6‰. Also, because this is a headwater lake, the particulate and dissolved weathering components derive from the same local catchment geology, rather than reflecting inputs from rivers transporting material from outside the catchment. Neubert et al.27 found strong isotopic overlap between bedrock lithologies and dissolved Mo in the associated rivers in analogous small catchments in China, India, and Switzerland, suggesting that there is no net fractionation (solid + liquid) during weathering and transport in a small catchment, such as Lake Vose, where transport is efficient (=quantitative). As in our system, the authors argued that the Mo budget of the associated streams, for dissolved and particulate inputs, was dominated by local weathering of outcrops in the catchment. Therefore, we assume

± 0.18b 0.04 0.06 0.01 ± 0.17b 0.08 0.04 0.03 ± 0.03b 0.02 0.04 0.06 ± 0.09b

Vose 98 Lake Vose δ 98 Mo Lake natural sediment measured = δ Mo settling particles b

2 SD based on samples analyzed in triplicate on separate days. 2 SD based on samples from three preceding different depths.

Lake Vose Vose = δ 98 Modetrital = δ 98 Mo Lake water column

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(2)

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minerals present. In general, though, the agreement is consistent with previous arguments1 asserting, on the basis of metal concentrations, that authigenic Mo observed in the deeper sediments of Lake Tantare reflects adsorption onto iron oxyhydroxides. Our encouraging results could also be confirmed through a better understanding of the specific uptake process in our lakes (e.g., the full quantitative range of the minerals involved) and any associated fractionations, as well as the isotopic composition of the riverine inputs of dissolved and particulate Mo. Future studies should seek to characterize all sources and sinks in the lake of interest. Nevertheless, our approach shows the value of thoughtful approximations and provides an important template for future efforts. Anthropogenic Molybdenum (Upper Sediment Layers). Here we discuss the sediment samples assumed, on the basis of our previous analysis of metal concentrations, to be representative of anthropogenic Mo in Lake Tantare (3.25, 4.25, and 5.25 cm) and Lake Vose (6.25, 6.75, and 7.75 cm). The main anthropogenic source(s) shifted the δ98Mo away from the preanthropogenic background. More specifically, δ98Mo values in Lake Tantare are shifted isotopically heavy in the upper sediment layers (with an isotopic offset of ca. +0.3‰), while those in Lake Vose show a negative isotopic shift relative to the preanthropogenic baseline (with an offset of ca. −0.3‰). On the basis of these data, we can estimate the δ98Mo value of the anthropogenic Mo via an isotopic mass balance that considers the relative natural and anthropogenic inputs and their isotopic compositions and does not require the assumptions used in our treatment of the natural Mo cycle. For this we use the following equation:

More specifically, the isotopic composition of dissolved Mo in the water column of Lake Vose would be a close match to the δ98Mo of the riverine particulate and dissolved inputs because of the small uptake of dissolved Mo in the lake relative to its rapid flushing rate. It is worth noting that the positive δ98Mo of the sediments in Lake Vose (+0.64‰ ± 0.09‰) is similar to the average of what others have described for dissolved Mo in many other rivers distributed globally (+0.7‰),25−27 which suggests our assumptions are reasonable. We attribute the negative δ98Mo values observed in deeper sediments of Lake Tantare to fractionation during Mo adsorption onto iron oxyhydroxides. Ferrihydrite and lepidocrocite have been identified as the principal iron oxyhydroxides present in the surface sediments of this lake.38 The experiments of Goldberg et al.21 yielded a preferential adsorption of light Mo to iron oxyhydroxides with a fractionation range (Δ98Mo) of −0.8‰ to −2.2‰. Because Lake Tantare is also a “headwater” lake, we assume that the particulate and dissolved weathering components derive from the same local catchment geology. As for Lake Vose, the isotopic composition of the detrital sediment would be a close match to the dissolved Mo in the catchment rivers and therefore to the dissolved Mo in the lake water column. This leads us to a simple expression: Lake Tantare 98 Lake Tantare Tantare δ 98 Mosettling = δ 98 Mo Lake particles = δ Modetrital water column

(3) Tantare where δ98MoLake is assumed to be similar to the δ98Mo detrital value measured in the deeper sediments of Lake Vose (averaging ∼+0.6‰). This is a reasonable assumption because the two lakes share generally the same Precambrian Shield catchment geology, and fractionations during weathering and transport of the dissolved and detrital phases in small catchments are fairly efficient (=quantitative). We are more comfortable with this approach than using the global average of +0.7‰ for dissolved Mo in rivers because our lakes are small headwater systems. Importantly, however, this value is very close to our +0.6‰ average. In summary, we can write the following equation:

δ 98 Momeasured = Cδ 98 Moanthropogenic + Dδ 98 Monatural

which can be rewritten to determine δ Moanthropogenic: δ 98 Moanthropogenic =

δ 98 Momeasured − Dδ 98 Monatural C

(6)

where C represents the fraction of anthropogenic Mo (0.67 and 0.65 for Lakes Tantare and Vose, respectively), and D (= 1 − C) represents the fraction of natural Mo (0.33 and 0.35 for Lakes Tantare and Vose, respectively). Natural Mo refers to the combined detrital and authigenic inputs, as constrained by the lower, preanthropogenic portions of the two cores. The results from both lakes are very consistent and give δ98Mo values of +0.2‰ ± 0.2‰ (Lake Tantare) and +0.1‰ ± 0.1 ‰ (Lake Vose) for the anthropogenic Mo. This result implies that δ98Moanthropogenic is consistent with the observation that δ98Mo values for molybdenites from porphyry copper deposits average +0.1‰ ± 0.3‰.39 The isotopic range for molybdenite ores is rather wide, from roughly −0.4‰ to +2.0‰,39,40 with average δ98Mo values for molybdenites from porphyry copper deposits and from other deposit types of +0.1‰ ± 0.3‰ and +0.7‰ ± 0.4‰, respectively (Figure 4). Importantly, our calculations for anthropogenic inputs are independent of any uncertainties remaining in our modeling of the natural Mo relationships discussed in the previous section. As a first impression, it might seem surprising to find such similar δ98Mo values for both lakes, given that one has a proximal point source of contamination (smelter), while the second is located in an ecological reserve well removed from any direct human activities and thus only under the influence of diffuse (nonpoint) long-range pollution. A reasonable explanation is that the copper ore processed at the smelter close to

Tantare δ 98 Mo Lake natural sediment estimated Fe oxyhydroxides Lake Tantare 98 = Aδ 98 Mosettling particles + Bδ Moadsorbed

(5)

98

(4)

where A (0.67) represents the fraction of detrital Mo. As Tantare 98 Lake Tantare discussed, we let δ98MoLake = +0.6‰, settling particles = δ Modetrital which is also the dissolved Mo baseline from which any authigenic fractionation occurs. B (1 − A = 0.33) represents the Fe oxyhydroxides fraction of authigenic Mo, and δ 98 Mo absorbed = 98 δ Moauthigenic = −0.5‰ [i.e., +0.6‰ − (1.1‰ ± 0.2‰)]. Because experimental fractionation data are not available for lepidochrocite, we calculated δ98Moauthigenic by applying the fractionation factor for ferrihydrite (1.1‰ ± 0.2‰) determined by Goldberg et al.21 because it is the dominant Fe(III) mineral phase detected in the sediments of Lake Tantare.38 The method used to determine A and B is thoroughly described in Chappaz et al.1 Our calculated value for Lake Tantare δ98Monatural sediment meas is +0.3‰ ± 0.2‰. Our average measured value of −0.32‰ ± 0.17‰ for δ98Mo in Lake Tantare deeper sediments (referred to as natural Mo) agrees Tantare reasonably well with this calculated δ98MoLake natural sediment meas but is limited by the incomplete knowledge of the fractionation factors associated with Mo uptake by the full range of Fe 10938

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ASSOCIATED CONTENT

S Supporting Information *

Two tables listing MC-ICP-MS measurements and secondary standards. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (989) 744-4388; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C. thanks C. Gobeil and A. Tessier for their assistance during the sampling. G.W.G. thanks Steven Romaniello for development of the Matlab routine for the Mo double spike data reduction routine. Funds from the Agouron Institute and the Geobiology and Low-Temperature Geochemistry Program of the U.S. National Science Foundation supported the contributions of A.C. and T.W.L. Analyses were supported by a grant from the NASA Astrobiology Institute.

Figure 4. Plot of the δ98/95Mo of molybdenite (MoS2) from different types of ore deposits. Figure modified from ref 40.

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Lake Vose contained appreciable molybdenite, and any industrial, non-point-source inputs might have derived initially from the same or analogous Mo-bearing ores. Recall that Mo is used extensively in the processing of oil and coal. The diffuse dispersal mechanism would have occurred via combustion of Mo-bearing coal and/or oil.1 These details demand additional study, but the strong predictive potential of our tracer is already clear. Implications. If it is assumed that specific anthropogenic Mo sources can be fingerprinted for their isotopic compositions and that magnitudes of anthropogenic enrichment can be assessed by concentration and isotope data, we can fingerprint the contribution of the specific contaminant sources via a mass balance approach and perhaps estimate the relative contributions when multiple contaminants are likely. There are, nonetheless, additional considerations. For example, Scheiderich et al.41 applied a similar approach based on δ98Mo measurements of Chesapeake Bay sediments before and after the development of significant human activities around Chesapeake Bay. Unlike our study, there was no clear boundary between sediments with and without the signatures of human impact because of extensive physical mixing of the sediment column. Fortunately, such overprints would be readily apparent through the incorporation of others tracers, such as 210Pb, whose relatively rapid half-life (22.2 years) can illuminate sediment ages and accumulation rates and vertical extents of sediment homogenization. Our mass balance approach would permit assessments of the relative contributions of multiple contaminants to a single site. For example, contributions from ore minerals locally sourced via smelters could be unmixed from other, more diffuse, longrange sources (oil−coal combustion processed with MoS2). Furthermore, we could constrain the timelines for these processes precisely through parallel use of well-constrained isotopic (210Pb and 137Cs) chronometers. Future studies may allow us to extend this approach to other trace metal isotope systems, such Cr, U, Zn, W, and Cd.

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