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Possible effects of evaporite weathering and deposition on oxygen and climate

Thursday MAY 28 – 4 PM CET (Zoom Webinar)

Graham Shields
UCL Earth Sciences, University College London

ABSTRACT: Non-steady state evaporite dynamics have been overlooked in studies of the long-term carbon cycle. In this talk I would like to show how imbalances between evaporite mineral weathering and deposition rates can affect Earth’s oxygen budget via the coupling of sulfate dissolution (and deposition) with pyrite burial (and weathering), leading to surplus (or deficient) oxidising capacity in the Earth system. I would also like to explore how evaporite dynamics may affect climate through the process of carbonate sedimentation. Above steady-state calcium sulfate deposition represents a net transfer of calcium ions away from the carbonate carbon sink, increasing ocean pH and lowering atmospheric pCO2. Conversely, dissolution of calcium sulfate minerals provides the ocean with Ca2+ ions unaccompanied by carbonate alkalinity, lowering ocean pH and strengthening greenhouse forcing. A predictable association of evaporite deposition with rifting and of weathering with orogenesis implies that non-steady state evaporite dynamics may act to buffer against tectonically driven warming and cooling trends, respectively. However, the sporadic and regional nature of basin-scale evaporite deposition means that imbalances between weathering and deposition might also have the potential to induce climate change.

BIO: Graham Shields is Professor of Geology at UCL, where he uses geochemical and isotopic tracers to study the composition of past oceans and atmosphere to unravel how the surface environment has 'co-evolved' with life through crucial junctures in Earth history. His focus is on rocks deposited between about 1000 and 500 million years ago, when complex multicellular life first began to dominate ecosystems. His Precambrian Research Group at UCL develops proxies to trace biogeochemical fluxes and related feedbacks that govern oxygen, carbon dioxide and nutrient budgets on Earth. During the Neoproterozoic, the Earth experienced profound climate change, deep ocean oxygenation and tectonic upheaval, events that are closely related to the biological revolutions which led the way to our modern Earth system.

Feedback diagram illustrating the effects of evaporite weathering on ocean oxygenation