THE CLIMATIC SIGNIFICANCE OF LATE ORDOVICIAN–EARLY SILURIAN BLACK SHALES (Invited Presentation)

The Ordovician-Silurian transition (~455–435 Ma) is characterised by repeated climatic perturbations, concomitant with major changes in the oceanic redox state, as best exemplified by the periodic deposition of black shales. The relationship between climatic evolution and oceanic redox cycles, however, remains largely debated. Here, using an ocean-atmosphere general circulation model accounting for ocean biogeochemistry (MITgcm), we investigate the potential for the development of anoxia immediately before, during and after the latest Ordovician Hirnantian (445–444 Ma) glacial peak. Our results are compared with recent sedimentological and geochemical data, including the thorough black shale compilation of Melchin et al. (2013).

We show that the pre-Hirnantian time-slice (late Katian), typified by the deposition of black shales in tropical settings, represents an unperturbed ocean redox state, with regional organic carbon burial driven by surface primary production.

During the Hirnantian, ocean circulation strengthens in our model in response to climate cooling, thus favouring ventilation and oxygenation of the deep ocean. Should a Hirnantian Anoxic Event be confirmed, our experiments suggest that cooling itself is no valid explanation and other mechanisms should be invoked.

Our simulations indicate that the perturbation of the oceanic circulation induced by the release of freshwater, in the context of the post-Hirnantian deglaciation, does not sustain over sufficiently long geological periods to cause the Rhuddanian oceanic anoxic event. Input of nutrients to the ocean, through increased continental weathering and the leaching of newly-exposed glaciogenic sediments, may instead constitute the dominant control on the spread of anoxia in the early Silurian.

Reference:

Melchin, M. J., Mitchell, C. E., Holmden, C., Štorch, P. 2013. Environmental changes in the Late Ordovician-early Silurian: Review and new insights from black shales and nitrogen isotopes. Geological Society of America Bulletin 125 (11-12), 1635–1670, doi:10.1130/B30812.1.