T68 The Ordovician Earth: Integrated Perspectives on the Fossil and Rock Records

Sunday, 22 September 2019
1:30 PM - 5:30 PM
Room 228AB, North Building

THE CLIMATIC SIGNIFICANCE OF LATE ORDOVICIAN–EARLY SILURIAN BLACK SHALES

Paper number: T68-3
Program: Technical Sessions
Time: 2:00 PM – 2:20 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-336415

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.