Intrashelf basin record of redox and productivity changes along the Arabian margin of Neo-Tethys during Oceanic Anoxic Event 1a

Abstract

The biotic, environmental, climatic, oceanic, and sea-level perturbations during the Early Aptian Oceanic Anoxic Event (OAE) 1a have been extensively documented from both deep- and shallow-marine deposits worldwide. However, there has been relatively little comparative assessment of the simultaneous interplay among organic carbon burial, redox conditions, terrigenous output, and productivity, leading to a lack of precise constraints on these relationships. Here, we use analyses of stable carbon isotopes (δ13Corg, δ13Ccarb, and Δ13C), total organic carbon (TOC), detrital proxies (Al, Si, Ti, K), redox-sensitive (RSTE: U, V, Mo) and productive-sensitive (PSTE: P, Cu, Ni) trace elements from a continuous, predominantly carbonate succession of the Kazhdumi Intrashelf Basin to evaluate the culprits for the OAE1a-associated changes in bottom-water oxygenation, organic-rich layer formation, and biotic shifts along the Arabian margin of the Neo-Tethys. Concentrations of Al-normalized RSTE and TOC values indicate that the bottom water conditions ranged from oxic prior to and at the onset of the OAE 1a (carbon-isotope segments C2 to basalmost C4 sensu Menegatti et al., 1998), to anoxic-suboxic but not euxinic (Mo < 25 ppm) during the lower C4 through C5 + C6 segments, and then returned to oxic-suboxic in the remaining C5 + C6 segment. The increase in Al-normalized PSTE coupled with TOC concentrations in the basal C4 is coeval with a change from predominantly orbitolinid-ostreid to planktic foraminifera-radiolarian biota. The periodically high productivity, driven both by the surface-water productivity as well as by phosphorus recycling from the sediments, continued through the C5 + C6 segments as evidenced by matching Si/Al and PSTE peaks (Cu/Al and Ni/Al). The study sheds new light on the causes of variations in bottom-water deoxygenation, organic content, nutrient availability, and biotic shifts in semi-restricted, relatively deep (>100 m), continental-margin basins during major oceanic perturbations.