35 Correlating the Ordovician: Honoring the Career and Contributions of Stig M. Bergström (Posters)

Friday, 8 April 2022
8:00 AM - 12:00 PM
Junior Ballroom B

USING PAIRED CARBON AND SULFUR ISOTOPES FROM CARBONATE ROCKS TO CONSTRAIN THE TIMING OF ANOXIA DURING THE LATE DEVONIAN IN THE GREAT BASIN REGION, USA: IMPLICATIONS FOR CHEMOSTRATIGRAPHIC CORRELATIONS

Paper number: 35-4
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The Devonian period is known both for the proliferation of forests on land and as a time of several extinctions of marine life. These extinctions occurred during the Late Devonian across the Frasnian-Famennian boundary (FFB) and Devonian-Carboniferous boundary. Though the Late Devonian (FFB) mass extinction is one of the “big five” mass extinctions of the Phanerozoic, the main driver(s) are still not fully understood. Global marine ocean anoxia was likely an important driver as the occurrence of black shales and positive carbon isotope excursions (δ13C) occurred during extinction pulses. However, black shales are not globally present, and δ13C values are prone to alteration and may not be a reliable proxy for anoxia if used alone. Thus, δ13C trends should be coupled with other geochemical proxies for anoxia to pinpoint when anoxia occurred and how that compares to biodiversity trends.

In this study stable carbon and sulfur isotopes (δ34S) – measured from carbonate associated sulfate (CAS) and pyrite – were analyzed from four bio-stratigraphically constrained sections in order to test whether evidence for anoxia occurs in the Great Basin region (western USA) across the FFB. Sulfur can be a valuable proxy to identify anoxic periods in carbonate rocks, especially in conjunction with δ13C trends, as both isotopic systems will be affected by anoxia as organic matter and pyrite burial rates increase. Because alteration processes affect each system differently, diagenetic overprints can be identified and not confused with secular changes in seawater values. In the four studied sections, evidence for anoxia is preserved, and in some sections may slightly post-date the FFB.

For example, at Bactrian Mountain positive excursions in δ34SCAS and δ34Spyrite (8‰ and 30‰, respectively) reach peak values above the FFB (mid-late triangularis conodont zone). However, at the Coyote Knolls west section, peak positive δ13C and δ34S excursions appear to occur just above the FFB and the magnitude of the excursions are much smaller than at Bactrian Mountain (2‰ and 10‰ for δ34SCAS and δ34Spyrite, respectively). These trends indicate that anoxia occurred during the Late Devonian, but further work needs to be done to unravel why the magnitudes vary in our studied sections and why the peak excursions differ in timing between the studied sections.

Geological Society of America Abstracts with Programs. Vol. 54, No. 4

doi: 10.1130/abs/2022NC-375068

© Copyright 2022 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.