Precambrian and Phanerozoic postglacial processes

From their investigation of the Late Ordovician Soom Shale, Gabbott et al. (2010, p. 1103 in this issue of Geology) have produced an extremely detailed account of important relationships between sedimentological and organic processes during the dying phases of the short-lived Hirnantian glaciation in South Africa. They suggest that eolian input played a key role in stimulating growth of phytoplankton, so that bottom sediments were characterized by unusual aggregates of silt-size grains intimately associated with organic material. These organic-rich silty layers and lenses occur together with inorganic mudstone laminae deposited from nepheloid plumes and weak turbidity currents thought to be associated with fl uvial input. In seeking an explanation for the remarkable preservation of fossilized metazoan organisms in the Soom Shale Lagerstatte, they suggest that organic productivity, stimulated by introduction of eolian material, may have played a pivotal role in the development of eutrophication and anoxic bottom conditions. This interesting account draws on disparate lines of evidence to bring into focus a rare picture illustrating interactions between sedimentological and organic processes during a very special period accompanying the rapid demise of an ice sheet. Gabbott et al.’s detailed investigation begs the question of what evidence we have regarding conditions following older, possibly much more prolonged and extensive, glaciations, when the composition of Earth’s atmosphere was probably different and organic evolution was at a much more primitive stage—for example, when there were no land plants or metazoans. In this different world, was deglaciation accompanied by the development of anoxic conditions? Was there signifi cant eolian input because of the dearth of terrestrial plants? How may the Snowball Earth hypothesis (SEH) contribute to our understanding of ancient postglacial events? How do Neoproterozoic banded iron formations and postglacial “cap carbonates” fi t into the picture, and how do we accommodate the accumulating evidence from stable isotopes? There are many questions, but few satisfactory answers. The following is an attempt to briefl y consider some aspects of postglacial phenomena throughout geological time. Most studies of Quaternary and recent glacial deposits are, for practical reasons, carried out in terrestrial settings (Martini, 1997), but the majority of ancient glacial sedimentary rocks are marine. Inasmuch as they can be accurately read from the fragmentary sedimentary rock record, climatic conditions on Earth appear to have fl uctuated dramatically throughout its long history. In fact, it could be argued that most organic evolution (including the emergence of our own species) occurred in response to climatically induced environmental pressures. Among the most easily recognized paleoclimatic signals are those of ancient glaciations. The criteria are simple and few: the presence of striated rock or sediment surfaces beneath suspected glacial deposits; the occurrence of widespread diamictites—conglomerates with clasts scattered through an abundant matrix; dropstones, which are large, isolated rock fragments in fi nely bedded sedimentary rocks; and ancient varved deposits that record annual freeze-and-thaw cycles in ancient lakes. There are many caveats and more sophisticated criteria, but the fi eld observations are critical. Existing data suggest that, during its ~4.6 g.y. history, signifi cant por

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