Following on from their linking carbon-isotope excursions associated with Neoproterozoic diamictite-cap carbonate sequences to methane release (see Methane and Snowball Earth in Earth Pages, December 2001), Martin Kennedy, Nicholas Christie-Blick and Anthony Prave turned to the d13C values in the diamictites for which a glaciogenic interpretation forms the main plank of the Snowball Earth hypothesis (Kennedy, M.J. et al. 2001. Carbon isotope composition of Neoproterozoic glacial carbonates as a test of paleoceanographic models for snowball Earth phenomena. Geology, v. 29, p. 1135-1138). Complete ice cover of the oceans would chemically isolate ocean water from the atmosphere, and would effectively shut down the organic sinks for atmospheric carbon dioxide. While they operate, the exclusion of 13C relative to lighter carbon by organisms drives up d13C in sea water, to be preserved in carbonate sediments. The Snowball Earth model predicts negative d13C, approaching the -5‰ of the mantle, in carbonates deposited during all-enveloping glacial epochs. However, few researchers have made the measurements needed to test that part of the hypothesis.
Kennedy and co-workers show from three such diamictite sequences that the carbonate precipitated as cement in them has consistently positive d13C. Although that does not disprove the existence of glaciation at tropical latitudes, it is not consistent with the dreadful scenario of totally ice-bound oceans devoid of life. Nor, for that matter, is there any evidence from strontium isotope variations in carbonate cap rocks for the massive continental weathering that the Snowball Earth devotees propose as a means of escape from the eventual hot-house that build up of volcanic CO2 emissions to release Earth from the mothers of all cold snaps would create. Expect interesting news in later Earth Pages of how the greatest Earth science debate of the 21st century develops.