Isotopic Fingerprints of Anthropogenic Molybdenum in Lake Sediments

We measured the molybdenum isotope compositions (delta Mo-98) 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. delta Mo-98 values in the deeper sediments (reflecting predominantly natural Mo sources) differ dramatically between the two lakes: -0.32 +/- 0.17 parts per thousand for oxic Lake Tantare and +0.64 +/- 0.09 parts per thousand for anoxic Lake Vose. Sediment layers previously identified as enriched in anthropogenic Mo, however, reveal significant delta Mo-98 shifts of +/-0.3 parts per thousand, resulting in isotopically heavier values of +0.05 +/- 0.18 parts per thousand in Lake Tantare and lighter values of +0.31 +/- 0.03 parts per thousand in Lake Vose. We argue that anthropogenic Mo modifies the isotopic composition of the recent sediments, and we determine delta Mo-98(anthropogenic) values of 0.1 +/- 0.1 parts per thousand (Lake Vose) and 0.2 +/- 0.2 parts per thousand (Lake Tantare). These calculated inputs are consistent with the delta Mo-98 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.