Why no Phanerozoic Snowball Earth? Changes in biogeochemical cycling across the Precambrian-Cambrian transition

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• Dr Graham Shields-Zhou (UCL)
• Tuesday 26 November 2013, 16:30-17:30
• Harker 1 seminar room, Department of Earth Sciences.

If you have a question about this talk, please contact Dr. Alex Liu.

The Neoproterozoic Era is best known for the climatic extremes of the Cryogenian Period, during which fledging metazoan life began to evolve, leading eventually to the Cambrian Explosion. Climatic fluctuations and biological innovations can both be related to carbon cycle perturbations and so it is perhaps no surprise that the defining events of the ensuing Precambrian-Cambrian transition are bracketed by the onset and demise of enigmatic negative carbon isotope excursions. It appears that highly negative C-isotopic shifts and large amplitude isotopic swings ceased once modern marine ecosystems and pervasive bioturbation had become firmly established by about 520 Ma. The disappearance of negative excursions after 520 Ma might suggest a key role for methanogenesis which would arguably have declined following the onset of bioturbation. However, the decrease in amplitude and frequency of isotopic shifts suggests that negative feedbacks on nutrient cycles became tighter as newly evolved animals revolutionised ecosystems and biogeochemical cycling. In this presentation, I would like to consider how biological and environmental changes and in particular the introduction of pervasive bioturbation helped to strengthen the relationship between benthic oxidation state and P-retention. This tighter P-O relationship catalysed negative feedbacks which since that time have helped stabilize productivity, climate and atmospheric composition. One possible consequence of bioturbation was to constrain the impact of changing nutrient fluxes on atmospheric oxygen. More speculatively, it could be argued that carbon dioxide levels were buffered to higher levels as a consequence of increased P burial efficiency, thus avoiding any future recurrence of the climatic extremes of Snowball Earth.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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