Contrasting sulfur geochemistry and Fe/Al and Mo/Al ratios across the last oxic-to-anoxic transition in the Cariaco Basin, Venezuela

Abstract

An abrupt transition from oxic to anoxic–sulfidic (euxinic) marine bottom waters occurred in the Cariaco Basin in response to increasing productivity resulting from the late Pleistocene post-glacial rise in sea level and corresponding increase in surface-water nutrient availability. The microlaminated sediments of the euxinic interval, which span the last ∼14.5 ky, suggest a predominance of water-column (syngenetic) pyrite formation based on (1) high pyrite sulfur (S py) concentrations in the surficial sediment layers, (2) values for degree-of-pyritization (DOP) that generally do not increase appreciably with increasing burial, (3) ratios of total iron (Fe T) to Al that are elevated above the continental baseline recorded in the underlying oxic sediments, and (4) S py isotope trends that largely mimic the δ 34S HS − of the modern water column. Intermediate DOP values in the microlaminated deposits and Fe T/Al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of Fe-bearing siliciclastic accumulation. As predicted from the relative rates of siliciclastic delivery, Fe T/Al and DOP data lie between end-member values observed in the modern Black Sea. As viewed broadly, Fe T/Al and DOP relationships in euxinic sediments reflect the balance between syngenetic Fe scavenging and temporal and spatial gradients in siliciclastic input. Pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon (C org). However, the upper 80 cm of the Fe-rich, C org-poor, bioturbated sediment show evidence for a strong diffusional HS − overprint from the overlying, Fe-limited euxinic marine environment. This post-glacial transition manifests in pyrite overprints that are strongly 34S-depleted relative to those in restricted, presently euxinic marine settings elsewhere in the world, such as the Black Sea, where the sedimentary sequence spanning the last glacial–interglacial transition begins with a shift from freshwater to C org-poor oxic marine deposition and thus dominantly sulfate diffusion. Trends for Mo/Al ratios in the microlaminated sediments suggest that Mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits. Organic matter plays a role by enhancing HS − production and/or by providing a substrate for Mo scavenging. Significant Mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite HS −-rich pore waters as recorded in the heavy iron sulfide overprint. We have not, however, proven that high sulfide concentrations within the water column are required for enhanced Mo sequestration in sediments.