MULTIPLE EXTREME GLACIATIONS IN THE PALAEOPROTEROZOIC: COUPLED CLIMATE BIOGEOCHEMICAL DYNAMICS

We explore hypotheses for multiple Palaeoproterozoic glaciations due to coupled climate biogeochemical dynamics.

We assume that Archean climate was warm due to a methane greenhouse; in anoxic conditions the biogenic methane source is balanced by photochemical loss, and resulting methane concentrations are high [Kasting et al., 2001]. Methane photolysis is accompanied by hydrogen loss to space, an irreversible loss of reducing material, which corresponds to a source of atmospheric oxygen assuming an early origin of oxygenic photosynthesis. However, oxidation of a large stock of reduced material may act as a sink for free oxygen, effectively buffering the atmospheric redox state. Only when the reservoir of reduced material is suffciently depleted will atmospheric oxygen rise, accounting for the delay between the origin of photosynthesis and the rise of oxygen [Catling et al., 2001]. Accompanying the Great Oxidation, methane concentrations would collapse, causing massive global cooling and triggering deep glaciation through ice albedo feedback.

Standard models of recovery from low-latitude glaciation invoke cessation of silicate weathering carbon dioxide sink, but continued volcanic source, resulting in an intense greenhouse [Kirschivink, 1992]. However, negative feedback via silicate weathering [Walker et al., 1981] would result in a single steady state temperature following deglaciation, which is not consistent with geological evidence of multiple glaciations [e.g. Young, 2002, Bekker et al., 2001]. Possible explanations for the apparent glacial cycles are investigated in simple models of the Earth System; these include re-establishment of a methane greenhouse during interglacials, with renewed collapse causing another glaciation, or further alteration of the carbon cycle during deep glaciation.

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