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SUMMARY:Why no Phanerozoic Snowball Earth? Changes in biogeochemical cycli
 ng across the Precambrian-Cambrian transition - Dr Graham Shields-Zhou (UC
 L)
DTSTART:20131126T163000Z
DTEND:20131126T173000Z
UID:TALK45609@talks.cam.ac.uk
CONTACT:Dr. Alex Liu
DESCRIPTION:The Neoproterozoic Era is best known for the climatic extremes
  of the Cryogenian Period\, during which fledging metazoan life began to e
 volve\, leading eventually to the Cambrian Explosion. Climatic fluctuation
 s and biological innovations can both be related to carbon cycle perturbat
 ions and so it is perhaps no surprise that the defining events of the ensu
 ing Precambrian-Cambrian transition are bracketed by the onset and demise 
 of enigmatic negative carbon isotope excursions. It appears that highly ne
 gative C-isotopic shifts and large amplitude isotopic swings ceased once m
 odern marine ecosystems and pervasive bioturbation had become firmly estab
 lished by about\n520 Ma. The disappearance of negative excursions after 52
 0 Ma might suggest a key role for methanogenesis which would arguably have
  declined following the onset of bioturbation. However\, the decrease in a
 mplitude 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 lik
 e to consider how biological and environmental changes and in particular t
 he introduction of pervasive bioturbation helped to strengthen the relatio
 nship between benthic oxidation state and P-retention. This tighter P-O re
 lationship 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 nutri
 ent fluxes on atmospheric oxygen. More\nspeculatively\, it could be argued
  that carbon dioxide levels were buffered to higher levels as a consequenc
 e of increased P burial efficiency\, thus avoiding any future recurrence o
 f the climatic extremes of Snowball Earth.
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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