HR: 08:00h
AN: PP21A-01 INVITED    [Abstracts]
TI: Understanding Oceanic Anoxic Events: An Integrated Geochemical Approach
AU: * Cohen, A S
EM: a.s.cohen@open.ac.uk
AF: The Open University, Department of Earth Sciences, Milton Keynes, MK7 6AA, United Kingdom
AU: Coe, A L
EM: a.l.coe@open.ac.uk
AF: The Open University, Department of Earth Sciences, Milton Keynes, MK7 6AA, United Kingdom
AU: Kemp, D B
EM: Dave.Kemp@Neftex.com
AF: Neftex Petroleum Consultants Ltd, 115BD Milton Park, Abingdon, OX14 4SA, United Kingdom
AU: Pearce, C R
EM: c.r.pearce@open.ac.uk
AF: The Open University, Department of Earth Sciences, Milton Keynes, MK7 6AA, United Kingdom
AB: Discrete intervals of widespread organic carbon accumulation, termed Oceanic Anoxic Events (OAEs), occurred at a few relatively brief intervals during the Mesozoic. Recent studies have shown that these events took place at the same time as other substantial environmental changes that included global warming, ocean acidification, and unusually high levels of species extinctions. However, many factors relating to the behaviour of the Earth System during OAEs remain unclear. These include: The primary driving mechanism(s) - was there one common mechanism or were OAEs the result of different processes; the spatial and temporal extent of seawater anoxia during OAEs; the precise effects on marine and terrestrial biota; variations in atmospheric CO₂ and global temperature; and the mechanism and timescale of Earth's recovery process. The records of environmental change during OAEs are best preserved in marine deposits, with continental shelf sections being particularly well studied. The combined use of geochemical, sedimentological and palaeontological observations indicates a complex interplay of factors. Significant advances in our understanding of OAEs have taken place in the last decade or so using new geochemical and isotopic proxies and a high- resolution, multidisciplinary approach. For example, Sr- and Os-isotope data indicate that rates of chemical weathering increased markedly during the Toarcian (Early Jurassic) OAE, whilst Mo-isotope data suggest that the areal extent of seawater anoxia fluctuated during the OAE despite the persistence of euxinic conditions in some regions. The pattern of Mo-isotope data for the Toarcian contrasts strongly with new Mo-isotope results from the Kimmeridge Clay Formation (Late Jurassic), when anoxic conditions were confined to European epicontinental seas and were likely to have resulted from very different primary causes. Cyclostratigraphic analysis has been used to provide a temporal framework for the timescale of OAEs at sub- Milankovitch resolution. Evidence from a number of OAEs indicates that sudden and major changes during these events sometimes occurred in no more that a few hundred years. These abrupt changes appear to correspond with the crossing of certain critical thresholds in the Earth System. The rate and magnitude of environmental change during OAEs, in parameters such as atmospheric CO₂, global temperature increase, etc., have been shown to have been broadly similar to present day rates of change. The study of OAEs can thus potentially inform us about the possible longer-term consequences of anthropogenic environmental change.
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4901 Abrupt/rapid climate change (1605)
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting