Sedimentary trace element records over the last 200 kyr from within and below the northern Arabian Sea oxygen minimum zone

Abstract

Two piston cores (PCs), containing two full glacial cycles, were retrieved at Murray Ridge. One core (PC463) contains sediment deposited at 920 mbsl within, the other (PC464) has sediment deposited at 1470 mbsl below the current oxygen minimum zone, OMZ (O 2: ≤ 2 μM). Variations in the organic carbon (OC) content correlate well between the two cores; high OC contents correspond with periods of high insolation and are accompanied by a stable and intensive OMZ. The lithogenic fractions are similar and have a rather constant composition. The V, Mn, Co, Cu, Se, Zn and Sb contents in these fractions are in fair agreement with the values in standard shales (SS). The Ni content of the lithogenic fraction is higher than in SS due to its high content in the regional eolian input, and the Mo, Re and U contents are higher as a result of syngenetic and(or) diagenetic overprints. In both cores, Mn and Co have a negative, the other elements a positive correlation with OC. Four processes, contributing to sedimentary enrichment in trace elements, can be envisaged: 1) delivery, as minor or trace components, in hydrogenous and biogenic particles, 2) scavenging in the water column, 3) (partial) retention in the sediment during decomposition of host phases, 4) diffusion into the sediment across the sediment–water interface and subsequent immobilization in response to reduction or precipitation as sulfides. Vanadium, Mo, Re, and U were found to be clearly enriched, and Ni, Cu, Zn and Se to be possibly enriched as a result of these processes. Enrichment or depletion in trace elements in PC463 relative to PC464 was tested as a proxy for bottom-water oxygen (BWO) conditions during periods of maximum accumulation rates of organic matter. Only Mo, Re and Re/Mo appear to be unambiguous proxies for differences in depositional conditions between sites 463 and 464. Using these proxies, it was found that the OMZ expanded to at least the depth of site 464 from 155 to 135 ka.