North-Central Section - 50th Annual Meeting - 2016

Paper No. 29-14
Presentation Time: 8:00 AM-12:00 PM

CONTROLS ON TRACE METAL ENRICHMENT IN EARLY PROTEROZOIC MARINE SEDIMENTS OF BRAZIL AND SOUTH AFRICA


PARR, Kayla S., Department of Geology, University of Cincinnati, Cincinnati, OH 45221, ALGEO, Thomas J., Department of Geology, University of Cincinnati, 500 Geology-Physics Building, University of Cincinnati, Cincinnati, OH 45221-0013 and BEKKER, Andrey, Department of Earth Sciences, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, parrks@mail.uc.edu

Trace-metal enrichment in marine sedimentary units older than ~0.7 Ga has received little attention to date. In this study, we investigated five Early Proterozoic units from South Africa and Brazil ranging in age from ~2.45 to 2.2 Ga. These units represent a wide range of redox facies, from oxic to anoxic, as shown by Corg:P ratios from <10 to >400. Total organic carbon (TOC) content ranges from 0.1 to 3.1%, and barium (Ba) enrichment factors (EFs) range from ~1 to 18. These two parameters show strong correlation (r = +0.63), suggesting that both are valid proxies for organic productivity, and Ba-EF correlates strongly with Corg:P ratios (r = +0.67), suggesting that redox conditions were controlled mainly by productivity.

Within this environmental framework, we assessed patterns of enrichment of redox-sensitive trace elements (RSTEs), with concentrations normalized to average upper continental crust (AUCC; McLennan, 2001). A number of trace metals show little enrichment, with EFs close to 1.0 for all redox facies. This group includes Mo, Co, Cr and V. A few elements show modest enrichment (~2×), including U and Ni. A third group shows stronger enrichment (~3-5×), including Cu, Pb and Zn. The lack of enrichment in certain elements (especially Mo) might be attributed to the presence of ferruginous-anoxic rather than euxinic-anoxic bottom-water conditions, the former failing to generate particle-reactive thiomolybdates. Contemporaneous ferruginous conditions are suggested by Fe/Al ratios (0.75-1.25) that are distinctly higher than that for AUCC (0.43). It is likely that Early Proterozoic seawater was depleted in Mo, Co, Cr and V relative to modern seawater. Cu, Pb and Zn are chalcophile elements, which show weak correlation (r) of 0.12 to 0.60 with non-acid-volatile sulfur, suggesting that their uptake may have been controlled by solid-phase substitution in pyrite. Given the absence of H2S in the water column, such uptake must have occurred within sediment porewaters in which organic decay generated H2S, a process that would have limited total trace-metal enrichment.