Response of trace metal redox proxies in continental shelf environment: The Eastern Arabian Sea scenario

Abstract

Major and trace elements (viz. Al, Si, Ca, Na, Fe, Ti, K, S, Mn, V, Cu, Zn, Mo, Ni, Th and U) along with organic carbon and nitrogen content were analyzed in the sediment samples of a gravity core (GC-08/SK-291; 12�34'N: 74�11.47'E) which lies within the upper edge of intense oxygen minimum zone (OMZ) of Eastern Arabian Sea. The aim was to determine how the geochemistry of the redox-sensitive elements (viz. V, Cu, Zn, Mo, Ni, and U) was influenced by the suboxic water column and bottom water condition and whether the sediments show a unique signature of such redox condition. The weathering intensity was also determined and the provenance of sediments has been inferred to discern the uniformity in sediment supply in the studied location. The chemical index of alteration (CIA), Chemical Index of Weathering (CIW) and Al-Ti-Zr ternary diagrams suggest low to moderate source area weathering of granodioritic to tonalitic source rock composition, which is similar to the regional geology of the continental mass nearby. The relative variations of major elements such as Si, Al and Ca suggest that the terrigenous fractions in the studied sediments are diluted by marine carbonates, unlike by biogenic opal, which is the case for most shelf regions of the world experiencing upwelling. The relative distributions of total organic carbon (TOC), total sulfur (TS) and total nitrogen (TN) suggest that the organic matter in the studied sediments is primarily of terrestrial origin irrespective of its presence in the deeper part of the shelf. We adopted the multi-proxy procedure for analyzing the redox condition prevalent during the deposition of sediments (e.g., enrichment of redox-sensitive elements, authigenic U, and ratios of trace metals such as U/Th, V/Cr, V/Mo, Ni/Co and (Cu+Mo)/Zn). The most striking result is a fully oxic signature reflected consistently by all redox proxies despite the occurrence of studied location within the OMZ of Arabian Sea. The redox proxies estimated from the bulk composition as well as the marine fraction of trace metals, do not reveal any significant difference in the depositional condition. The seasonal variation of O<inf>2</inf> concentration seems to be the dominant factor which controls the response of the trace metals to the water column anoxia in the studied location. The major implication of the present study is that the trace element redox proxies alone are not always suitable to discern the redox condition of deposition in continental shelf sediments which are characterized by high sediment input from continents and significant seasonal variation of O<inf>2</inf> concentration in the water column. � 2015 Elsevier Ltd.