The Search for Dark Ice on Snowball Earth
Abstract
There is now irrefutable geological evidence that tropical ice sheets flowed directly into Earth’s oceans at 716 Ma (Sturtian glaciation) and again at 635 Ma (Marinoan glaciation). Yet, the sparse fossil record suggests that the ocean was never entirely shuttered by a limitless ice shelf (‘sea glacier’). This dilemma has encouraged numerical simulations in search of stable climate states that satisfy the geological evidence, yet maintain open water and/or dark (thin) ice around the paleoequator. Global paleogeographic model maps for 716 and 635 Ma now exist, but dynamically coupled ice sheet - sea glacier - ocean - atmosphere general circulation models are computationally futuristic. In the snowball Earth scenario, terminal ice-sheet drainage is triggered by collapse of the tropical sea glacier and the loss of the buttress it long provided for the world’s many large ice sheets. Because terminal ice-sheet drainage dominates the existing sedimentary record, evidence for open water (e.g. wave ripples) in periglacial marine deposits cannot disprove a previously unlimited sea glacier. A more promising approach is to seek glacial atmospheric proxies, on the premise that the greenhouse radiative forcing required for glacial termination is strongly dependent on the tropical surface albedo. Evidence for large changes in surface temperature and pCO2 at the 635-Ma termination have been inferred from carbon and boron isotope records, respectively, and supercritical pCO2 levels, had the tropical ocean surface been dark, are implied by extreme mass-independent Δ17O anomalies in glacial and postglacial sulfate deposits. Meanwhile, a biostratigraphic history richer than hitherto imagined is beginning to emerge for the interval from ca 820 to 580 Ma. But as yet, the fossil record reveals no specific biological response to either of marine life’s darkest hours. From genomics, we learn that genes thought to have evolved at the origin of animal multicellularity are far more primitive, meaning that under selective pressure from environmental collapse, gene cooption rather than gene evolution may have been critical for the early evolution of metazoa.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.A34A..01H
- Keywords:
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- 0444 BIOGEOSCIENCES / Evolutionary geobiology;
- 1030 GEOCHEMISTRY / Geochemical cycles;
- 4900 PALEOCEANOGRAPHY;
- 5225 PLANETARY SCIENCES: ASTROBIOLOGY / Early environment of Earth