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

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.