Friday, October 28, 2016

There was Significantly Less Sediment Laid Down During the Cryogenian NeoProterozoic (Snowball Earth) Glaciations


Partin et al


During the Sturtian and Marinoan "snowball Earth" episodes, ice cover is thought to have extended from polar to tropical latitudes. We test the supposition that such an extreme glacial climate, not repeated in the subsequent ∼635 m.y. of Earth history, would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans. With >500 sediment accumulation rates to characterize Sturtian and Marinoan deposits, we find median accumulation rates at least four to 15 times slower than expected for Phanerozoic glaciomarine deposits as characterized by >10,000 rates. Our comparison is conservative with respect to time span, latitude, and distance from the ice margin. Phanerozoic accumulation rates decrease systematically when averaged over longer time spans. Comparisons were drawn, therefore, at 5 and 57 m.y. time spans to match minimum Marinoan and Sturtian durations, respectively. Cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin. After accounting for time span, snowball Earth deposits at low latitude are found to be thinner than would be expected either for high-latitude Cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice-rafted debris. The rate discrepancy is not readily attributed to overestimates of the total Marinoan or Sturtian durations. If sediment fluxes during warm melt intervals did approach Phanerozoic rates, these intervals must have occupied a much smaller proportion of snowball Earth episodes than in younger glacial climates.

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