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

The Early Palaeozoic Icehouse (Late Ordovician-Early Silurian) represents a non-uniformitarian solution to the Earth's carbon budget, with marked glaciations occurring at a time characterised by elevated levels of atmospheric CO2. Unlike the modern oceans, where short term changes in atmospheric CO2 are accommodated by a carbonate buffer and long term changes in atmospheric CO2 are reflected in carbonate sedimentation, the chemical oceanography of the Early Palaeozoic Icehouse (EPI) was markedly different. Elevated atmospheric CO2 prohibited significant carbonate deposition, whilst periods of marine anoxia, characterised by the deposition of black shales, were common. Thus, the marine organic carbon reservoir may have strongly influenced long-term changes in atmospheric CO2 during the EPI, and we suggest that the deposition of black shales represented a major sink for atmospheric CO2.

Though the extent of ice formation during this EPI is poorly constrained, several major tillites and dimicrites were deposited, indicative of warmer intervals within the icehouse, suggesting global sea-level change was of glacio-eustatic origin. Rather than dealing with the GCM-derived forcing mechanisms, this paper aims to address the role of black shale deposition and CO2 draw-down with regard to glacioeustasy within the relatively stable EPI environment.

Sequence stratigraphy reveals that periods of extensive black shale deposition in epicontinental seas correspond to transgressions, whereas regressions are characterised by deposition of bioturbated facies in such settings, which allows changes in lithofacies to be related to the Early Palaeozoic carbon cycle. Assuming temperature is a function of atmospheric CO2, and that relative sea-level can serve as proxy for temperature, we infer that the deposition of transgressive black shales may act as a negative feedback mechanism, drawing down CO2 and preventing the onset of runaway greenhouse conditions.