Evidence for rapid weathering response to climatic warming during the Toarcian Oceanic Anoxic Event

Theodore R Them,Richard M Friedman,Jeremy D Owens,Benjamin C Gill,Darren R Gröcke,David Selby

doi:10.1038/s41598-017-05307-y

Abstract

Chemical weathering consumes atmospheric carbon dioxide through the breakdown of silicate minerals and is thought to stabilize Earth’s long-term climate. However, the potential influence of silicate weathering on atmospheric pCO2 levels on geologically short timescales (103–105 years) remains poorly constrained. Here we focus on the record of a transient interval of severe climatic warming across the Toarcian Oceanic Anoxic Event or T-OAE from an open ocean sedimentary succession from western North America. Paired osmium isotope data and numerical modelling results suggest that weathering rates may have increased by 215% and potentially up to 530% compared to the pre-event baseline, which would have resulted in the sequestration of significant amounts of atmospheric CO2. This process would have also led to increased delivery of nutrients to the oceans and lakes stimulating bioproductivity and leading to the subsequent development of shallow-water anoxia, the hallmark of the T-OAE. This enhanced bioproductivity and anoxia would have resulted in elevated rates of organic matter burial that would have acted as an additional negative feedback on atmospheric pCO2 levels. Therefore, the enhanced weathering modulated by initially increased pCO2 levels would have operated as both a direct and indirect negative feedback to end the T-OAE.

Highlights

The chemical weathering of rocks constitutes a negative and stabilizing feedback to Earth’s long-term (108–109 yr) climate by consuming atmospheric CO2, modulating the greenhouse effect and, in turn, global temperatures[1,2,3]
To resolve whether the transient increases in 187Os/188Os observed across the T-OAE were a global signal, we have investigated the osmium isotope record from the Lower Jurassic Fernie Formation of the Western Canada Sedimentary Basin located in present-day western Alberta (Fig. 1)
The 187Os/188Osi values decrease after the Toarcian carbon isotope excursions (CIEs) and asymptotically approach ~0.4 (Fig. 3; see Supplemental Information)

Summary

Introduction

The chemical weathering of rocks constitutes a negative and stabilizing feedback to Earth’s long-term (108–109 yr) climate by consuming atmospheric CO2, modulating the greenhouse effect and, in turn, global temperatures[1,2,3]. The addition of mantle-derived CO2 and thermogenic CH4 derived from the emplacement of the LIP9–11 and subsequent releases of CH4 from marine clathrates[12, 13] and terrestrial environments[14, 15] to the oceans and atmosphere are the proposed drivers of the T-OAE warming and carbon cycle perturbations These perturbations are recorded in sedimentary successions as pronounced negative carbon isotope excursions (CIEs), which occurred during a long-term trend to more positive carbon isotope values. A previous osmium isotope study of the T-OAE interval from a sedimentary succession in the Cleveland Basin of Yorkshire, United Kingdom indicates that, during the event, there was a concomitant, transient increase of 187Os/188Ossw values by 0.720 (Fig. 2) This record was originally interpreted to be the result of an increase in continental weathering rates of 400 to 800%20. A recently published osmium isotope record across the T-OAE from the Mochras borehole[23], located in nearby Wales, displays a much less pronounced excursion of 0.4 during the T-OAE interval (Fig. 2), which further suggests that geochemical changes recorded in the Cleveland Basin were likely influenced by regional climatic and oceanographic dynamics[18, 24, 25]

Results

Discussion

Conclusion