HR: 0800h
AN: OS11B-0511    [Abstracts]
TI: Anthropogenic Osmium in Precipitation
AU: * Chen, C
EM: cynthia.chen@dartmouth.edu
AF: Department of Earth Sciences, 6105 Fairchild Hall, Dartmouth College, Hanover, NH 03755, United States
AU: Sedwick, P N
EM: peter.sedwick@bios.edu
AF: Bermuda Institute of Ocean Sciences, BIOS, St. George's, GE01, Bermuda
AU: Sharma, M
EM: mukul.sharma@dartmouth.edu
AF: Department of Earth Sciences, 6105 Fairchild Hall, Dartmouth College, Hanover, NH 03755, United States
AB: Here we report the Os isotopic composition for precipitation from Hanover (NH), Soda Springs (CA) and the Ross Sea (Antarctica) as determined by negative thermal ionization mass spectrometry. All samples yielded non- radiogenic Os isotopic compositions. Snow and rain samples from Hanover, NH had Os concentrations of 0.8 - 12.2 fg/g (1 fg/g = 1E-15 g/g) and 187Os/188Os from 0.16 - 0.24. Snowpack from the high Sierra Nevada (Central Sierra Snow Laboratory, Soda Springs, CA) yielded Os concentration and isotopic composition of 3.6 fg/g and 0.21, respectively; Antarctic snow deposited above first year pack ice had [Os] = 0.8 fg/g and 187Os/188Os = 0.42. The isotopic ratios indicate that potential natural sources of Os to the atmosphere, such as continental mineral aerosols (187Os/188Os = 1.26) and seawater (187Os/188Os = 1.05) do not contribute bulk of Os to the precipitation. Instead, the isotopic ratios are identical to the platinum ores from the Merensky Reef in the Bushveld Igneous Complex, South Africa and Noril'sk Ni-Cu sulfide deposit associated with the Siberian Flood Basalts, Russia. These two deposits produce greater than 95 percent of the total Pt, Pd and Rh consumed annually primarily by the automotive industry. We infer that anthropogenic Os contribution dominates the isotopic composition of precipitation. The similar and non-radiogenic Os isotopic compositions observed in precipitation from disparate locations suggest that contamination of the troposphere with anthropogenic Os may be global in scale. We think that processing of ore to extract Pt, Pd, and Rh from PGE ores (PGE: group of six closely related elements Os, Ir, Pt, Pd, Rh, and Ru), which involves smelting and converting at high temperature and in the presence of oxygen, releases the volatile, toxic compound OsO4 into the troposphere, where it is mixed and then scavenged by precipitation, thus explaining both the non-radiogenic isotopic composition and the high and variable Os concentrations of Os in the precipitation. The magnitude of this contamination is likely sufficient to alter the global Os cycle and impact the isotopic composition of Os in the surface ocean.
DE: 1030 Geochemical cycles (0330)
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4808 Chemical tracers
DE: 4875 Trace elements (0489)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting