Another Post Humanity Geological Signal

Anthropogenic osmium in rain and snow reveals global-scale atmospheric contamination

1. Cynthia Chena,
2. Peter N. Sedwickb and
3. Mukul Sharmaa,1

-Author Affiliations

1.
aRadiogenic Isotope Geochemistry Laboratory, Department of Earth Sciences, Dartmouth College, 6105 Sherman Fairchild Hall, Hanover, NH 03755; and
2.
bDepartment of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Avenue, Norfolk, VA 23529

1.

Edited by Karl K. Turekian, Yale University, New Haven, CT, and approved March 27, 2009 (received for review November 19, 2008)

Abstract

Osmium is one of the rarer elements in seawater, with typical concentration of ≈10 × 10−15 g g−1 (5.3 × 10−14 mol kg−1). The osmium isotope composition (187Os/188Os ratio) of deep oceans is 1.05, reflecting a balance between inputs from continental crust (≈1.3) and mantle/cosmic dust (≈0.13). Here, we show that the 187Os/188Os ratios measured in rain and snow collected around the world range from 0.16 to 0.48, much lower than expected (>1), but similar to the isotope composition of ores (≈0.2) that are processed to extract platinum and other metals to be used primarily in automobile catalytic converters. Present-day surface seawater has a lower 187Os/188Os ratio (≈0.95) than deep waters, suggesting that human activities have altered the isotope composition of the world's oceans and impacted the global geochemical cycle of osmium. The contamination of the surface ocean is particularly remarkable given that osmium has few industrial uses. The pollution may increase with growing demand for platinum-based catalysts.

Footnotes

* 1To whom correspondence should be addressed. E-mail: mukul.sharma@dartmouth.edu

There's another signal of our presence that we'll leave in the rocks long after we're gone. And, no, I don't support the anthropocene idea.