HR: 1340h
AN: PP13A-1031    [Abstracts]
TI: A geochemical modelling study of the evolution of the chemical composition of seawater linked to a global glaciation
AU: * Le hir, G
EM: Guillaume.Lehir@lsce.ipsl.fr
AF: LSCE, CNRS-CEA-UVSQ, Gif-sur-Yvette, 91191, France
AU: Goddéris, Y
EM: godderis@lmtg.obs-mip.fr
AF: LMTG, CNRS,Observatoire Midi-Pyrénées, Toulouse, 31000, France
AU: Donnadieu, Y
EM: Yannick.Donnadieu@lsce.ipsl.fr
AF: LSCE, CNRS-CEA-UVSQ, Gif-sur-Yvette, 91191, France
AU: Ramstein, G
EM: Gilles.Ramstein@cea.fr
AF: LSCE, CNRS-CEA-UVSQ, Gif-sur-Yvette, 91191, France
AB: The Snowball Earth theory initially proposed by Kirschvink (Kirschvink, 1992) to explain the Neoproterozoic glacial episodes, suggested that the Earth was fully ice-covered at 720 My (Sturtian episode) and 640My (Marinoan episode). In the absence of continental weathering due to the spreading of large ice sheets (Hyde et al 2000, Donnadieu 2003), the main processes changing the seawater composition are those occurring at the seawater - oceanic crust interface. Using a numerical model of carbon-alkalinity global cycles, we study oceanic crust - seawater interactions to quantify the environmental changes caused by a global glaciation wherein the atmosphere is highly enriched in CO2. We propose a scenario wherein the ocean becomes acidic (pH~6) during global glaciations, even if the carbonate dissolution occurs. During the glacial episode, the oxygen consumption by hydrothermal activity tends to enhance the anoxia of the deep ocean. The quick transition from ice-house to greenhouse conditions and the associated temperature rise causes an extended hypoxia in the surface ocean. The intense continental weathering, in the aftermath of the glaciation, deeply affects the seawater composition inducing rapid changes in terms of pH and alkalinity. According to our modelling results the short-lived and large post-glacial perturbations are more critical than glacial environmental changes. Instead of the glaciation itself, these results tend to suggest that the deglaciation might have played the role of environmental filter proposed in the classic snowball Earth theory.
DE: 1040 Radiogenic isotope geochemistry
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460)
DE: 4870 Stable isotopes (0454, 1041)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting