T235 Ocean Chemistry through the Precambrian and Paleozoic II

Wednesday, 10 November 2004
1:30 PM - 5:30 PM

THE REDOX STATE OF DEVONIAN OCEANS: MO ISOTOPE EVIDENCE

Paper number: T235-11
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

We examine Mo stable isotopes in Devonian black shales to study the extent of ocean anoxia during this time. This application of Mo isotopes is based on recent findings that δ97/95Mo of seawater reflects the relative magnitudes of oxic vs. euxinic sinks for Mo (1,2) and that changes in global ocean redox should be reflected in δ97/95Mo in euxinic sediments (1). This approach was recently applied to mid-Proterozoic oceans (3).

Black shales of the Oatka and Geneseo Fms. of western New York provided an excellent dataset of complementary redox-related parameters, including organic carbon and redox metal concentrations, Fe/Al, Ti/Al, Mo/Al ratios and degree-of-pyritization (DOP) values (4,5). Prior work indicates the Geneseo Fm. was deposited under less intensely reducing conditions than the Oatka Fm. Hence, this study tests the fidelity of black shales to record δ97/95Mo of contemporaneous seawater.

δ97/95Mo was measured by MC-ICP-MS. Replicate chemistries are reproducible within 0.1 per mil. Oatka Fm. δ97/95Mo values are 0.95-1.30, with most values >1.1. Geneseo Fm. values are 0.68-0.93. There is no correlation between Mo or organic C concentrations and δ97/95Mo, but δ97/95Mo and DOP values covary.

These data lead to three conclusions: (a) covariation of δ97/95Mo and DOP, suggestive of non-quantitative removal of Mo in suboxic to weakly anoxic settings, cautions against use of sediments from weakly euxinic settings (e.g., DOP < 0.6) as recorders of contemporaneous seawater δ97/95Mo; (b) conversely, if mean ocean δ97/95Mo for a particular time is independently known, δ97/95Mo in euxinic sediments constrains local depositional redox conditions; (c) by analogy with modern Black Sea euxinic sediments which record modern seawater δ97/95Mo, high-DOP sediments of the Oatka Fm. imply Devonian seawater δ97/95Mo of ~ 1.3 per mil. This value is only moderately lighter than today’s (1.5 per mil), suggesting the global extent of Devonian ocean anoxia was only moderately greater than today.

Acknowledgements: Dr. Jugdeep Aggarwal’s analytical assistance is appreciated.

References: (1) Barling et al. (2001) Earth Planet. Sci. Lett. 193, 447; (2) Siebert et al. (2003) Earth Planet. Sci. Lett. 211, 159; (3) Arnold et al. (2004) Science 304, 87; (4) Werne et al. (2002) Am. J. Sci. 302, 110; (5) Sageman et al. (2003) Chem. Geol. 195, 229.