WERE TRANSGRESSIVE BLACK SHALES A NEGATIVE FEEDBACK MODULATING GLACIOEUSTASY IN THE EARLY PALAEOZOIC ICEHOUSE?

Abstract

The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian, c. 455-425 Ma) was a remarkable event in the Earth's climatic history, marked by extensive glaciations occurring at a time of elevated atmospheric CO2. The oceanography of the Early Palaeozoic Icehouse was markedly different from that of modern oceans, with frequent episodes of oceanic anoxia and high concentrations of CO2 which may have acidified the oceans and restricted carbonate burial. Thus, the marine organic carbon reservoir may have more strongly influenced long-term changes in atmospheric CO2 than at present. We suggest that deposition of black shales represented a major sink for atmospheric carbon. Sequence stratigraphy reveals that widespread black shale deposition occurred in transgressions, whereas regressions are characterized by deposition of bioturbated facies, allowing changes in lithofacies and deep-water redox conditions to be related to the Early Palaeozoic carbon cycle. Assuming increased temperature is a function of increased atmospheric CO2, and that glacioeustatic sea-level can serve as a proxy for temperature due to changing ice volume, we infer that the deposition of transgressive black shales may have acted as a negative feedback mechanism, drawing down CO2 and preventing the onset of runaway greenhouse conditions.