Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmpp32a..04c&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #PP32A-04
Other
4901 Abrupt/Rapid Climate Change (1605), 4902 Anthropogenic Effects (1803, 4802), 4924 Geochemical Tracers, 4948 Paleocene/Eocene Thermal Maximum
Scientific paper
The concept of an Oceanic Anoxic Event (OAE) was first suggested by Schlanger and Jenkyns in 1976. It was based on the observation that stratigraphically restricted but globally extensive carbon-rich horizons are developed in pelagic successions of Aptian-Albian and Cenomanian-Turonian age, intervals now known as OAE 1a (the `Selli' event) and OAE 2 (the `Bonarelli' event), respectively. These authors suggested that the widespread formation of organic-rich facies for a relatively short time resulted from O-depletion throughout the world's oceans, and they highlighted the differences between global anoxia and anoxia that was confined to isolated regions. The Toarcian OAE was subsequently added to this list, and represents the only Jurassic OAE (Jenkyns, 1985) recognised so far. A range of other major environmental changes are associated with OAEs, including worldwide C-isotope excursions, global warming, mass extinctions and major evolutionary changes on land and in the oceans. Recent Mo-isotope data for the Toarcian OAE reveal a clear relationship between the redox state of the oceans and astronomically-paced changes in δ13C (Pearce et al., Fall AGU 2006). Based on a detailed critical comparison between the environmental and geochemical changes that occurred during the Paleocene-Eocene thermal maximum (PETM) and the Toarcian OAE, we propose that there is overwhelming evidence for the PETM being an OAE. For example, both events involved: C-isotope excursions of up to ~-5 to ~-7 ‰ that lasted ~200 ka with several shifts each of ~-1‰ to ~-3‰ occurring in ~1 ka; the deposition of C-rich sediments throughout the world's oceans; sudden global warming; species extinctions and major evolutionary changes; and evidence for profound changes in climate and weathering. The geochemical and isotopic changes during the two events show a strong control by astronomical forcing, whilst the first of the sudden shifts in δ13C values were preceded by more gradual decreases. The environmental changes during the PETM and Toarcian OAEs are consistent with the sudden influx of large amounts of isotopically `light' C, which we argue most likely involved the dissociation of large quantities of methane hydrate at C release rates of ~4 Gt/a during the initial stages. The recognition that the PETM fulfils the criteria for being an OAE provides a clear generic link between this event and the earlier Mesozoic OAEs. This observation should enable us to understand more readily an otherwise disconnected series of events, separated by great lengths of time, which each exerted major and sometimes similar environmental and evolutionary influences. For example, the dissociation of methane hydrate has also been suggested as a possible cause of OAEs 1a and 2. The major OAEs all appear to have been linked in some way with the eruption of a Large Igneous Province, which may have started a series of events that led to the development of global oceanic anoxia.
Coe Angela L.
Cohen Seth A.
Kemp D.
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