The Osmium Isotopic Composition of Seawater: Past and Present

Abstract

The osmium isotopic composition of seawater (187Os/188Os) reflects a balance between radiogenic continental sources and unradiogenic mantle and extraterrestrial derived sources. Reconstruction of this value has allowed us to unlock vital information about a series of Earth system processes, both today and in Earth’s geological past. This body of work looks to reconstruct the 187Os/188Os of seawater for past and present oceans using the 187Os/188Os composition of shales and macroalgae (seaweed) respectively.
The 187Os/188Os composition of Icelandic (0.16 to 0.99) and Japanese (0.16 to 1.09) macroalgae are highly variable, and reflect the mixing between multiple sources. The 187Os/188Os of Icelandic coastal waters is dominated by seawater and local river catchments, and has been utilised to trace the influence of basaltic weathering on the global Os cycle. The 187Os/188Os of Japanese coastal waters is dominated by seawater and river catchments draining Miocene-Holocene continental rocks or anthropogenic sources, and has been utilised to trace mankind’s impact on the global Os cycle. The 187Os/188Os profiles of shales from the Silurian Ireviken, Mulde, Lau and Klonk biovents are similar to those previously recorded for the Hirnantian glaciation. This data suggests the Silurian has been punctuated by several glaciations associated with fluctuations in global temperatures, sea-level and the carbon cycle. When combined with the Li isotopic (δ7Li) composition of carbonates, this study suggests glacial processes caused large changes in oxidative and silicate weathering.
This study has successfully utilised macroalgae as a proxy for the 187Os/188Os of seawater and proven it can become a powerful tracer of Earth system processes and human activity. This study has also redefined the Silurian as an icehouse, and suggests the long term decline in atmospheric CO2, due to orogeny, land-plant diversification, volcanic- arc degassing and/or paleogeography, was reversed by periodic glaciations which acted to enhance oxidative weathering whilst suppressing silicate weathering.