Geology Seminar Series - Dr Michael Babechuk - Precambrian paleosols as archives of ancient Earth surface processes and atmospheric oxygenation

Wed., Nov. 17, 2021 4:00 p.m. - Wed., Nov. 17, 2021 5:00 p.m.

Location: https://uregina-ca.zoom.us/j/91336364774?pwd=VGhxSzB1OWNNSkg0akJhK0gwYkxRQT09

You are invited to the Geology Virtual Seminar. All are welcome!

Topic: Precambrian paleosols as archives of ancient Earth surface processes and atmospheric oxygenation

Speaker: Dr. Michael Babechuk

Abstract:

Paleosols are rare in the geological record, but, when preserved, provide an important glimpse into chemical weathering reactions near the ancient atmosphere-lithosphere interface. Studies throughout the late 1980s to early 2000s defined criteria to recognize true paleosols, and discovered a transition in Fe speciation/redox behaviour that parallels other terrestrial geological markers of Earth’s Great Oxidation Event (e.g., disappearance of detrital pyrite and uraninite, and appearance of red beds). In the last two decades, our nuanced understanding of atmospheric redox has expanded exponentially, largely from the reading of redox proxies in marine sedimentary rocks. In comparison, paleosol research has lagged behind in applying a wider suite of new trace element and isotopic proxies. In cases where such an approach has been applied, there is increasing discrepancy in inferences regarding paleo-environmental and paleo-atmospheric redox when extrapolating signatures from the paleosol scale. It is not fully resolved whether these discrepancies reflect local paleo-environmental redox variations, reveal differential proxy response at lower but accumulating levels of atmospheric O2, or hint at evidence of contrasting hot spots of oxygen production (e.g., shallow marine vs. terrestrial).

This presentation will document some recent progress on studies of three mafic paleosols: the ca. 2.7 Ga Mt. Roe paleosol, the ca. 2.45 Ga Cooper Lake paleosol, and the ca. 1.85 Ga Flin Flon paleosol. A research approach beginning with the examining pre-requisites for paleosol recognition using (e.g., linking parent to weathered rock via immobile element ratios) and then merging traditional major element chemical weathering proxies with newer proxies such as stable Cr isotopes is demonstrated as promising, but also reveals additional complexities at the paleosol scale. It is proposed that some previously reported redox signatures are a product of localized non-redox weathering effects. As such, a case to bring a multi-proxy toolkit to all paleosol studies and carefully evaluate signatures at different scales and different hydrological zones of weathering profiles is advocated.