Is the marine osmium isotope record a probe for CO2 release from sedimentary rocks?

Abstract

The marine osmium (Os) isotope composition shows some significant variability during the past 250,000yr, which also correlates with climate sensitive proxies, e.g. oxygen isotopes. This correlation led to the assumption that the marine Os may in part contain information about past changes of continental weathering as a direct response to large-scale events, such as Northern hemisphere glaciation–deglaciation cycles. Here we test this hypothesis by analyzing marine Os isotope records applying an inverse modeling approach, with a particular focus on variable weathering rates of Os-rich lithologies, such as organic-rich and sulfide-rich sedimentary rocks. Assuming loess represents the Os isotope composition of average continental silicate crust, weathering thereof would produce an Os isotope composition that is close to the modern day seawater value. Therefore, silicate weathering is unlikely to exert any significant control on the marine Os isotope composition. Instead, the isotopic variability is thus more likely to be controlled by weathering of rock types enriched in Os and with an isotope composition distinct from silicates and seawater. Weathering of these Os-rich lithologies is closely linked to high rates of physical erosion and is thus expected to show a strong response to large-scale rock-grinding glaciation events. By combining a model of variable weathering rates to an Os-cycle model we present a new mechanistic model that is able to explain the Late Pleistocene marine Os isotope variability and we propose that this variability is best explained by a changing continental Os fluxes derived from the dissolution of aforementioned Os-rich lithologies. The weathering of such rocks promotes a set of weathering reactions that releases CO2 into the atmosphere, and we contend that the marine Os isotope record may be suited to gauge relative changes in the magnitude of these processes. The modeling results presented here suggest that variable weathering rates of organic-rich sedimentary deposits could play an important role in the long-term atmospheric CO2 budget and the interplay with CO2 consumption by silicate weathering.