Variations in trace metal concentrations and Sr, Nd isotopic compositions in sediments from two contrasting settings in the Eastern Arabian Shelf: Implications for provenance and paleoclimate reconstruction

Abstract

Major and trace elements concentrations as well as Nd and Sr isotopic ratios were measured in sediments from two gravity cores (GC17 and GC19) deposited in the high-sediment flux regime of the Eastern Arabian Shelf (EAS). The core-sampling locations are close to the mouths of the geochemically and isotopically well-characterized Rivers Mandovi and Zuari and are overlain by water columns of contrasting redox conditions. Given the available knowledge of bottom water oxygen concentrations in the studied locations, the compositions of the Mandovi and Zuari sediments as well as the lithology of the drainage basin, the goal of this study was to test the efficacy of different paleo-redox proxies and provenance indicators in a high-sediment flux regime of the ocean.The variations of immobile elements like Al, Ti, Th, and Zr in the studied samples suggest lack of sediment reworking and sorting. Provenance proxies such as Th/Co, La/Sc ratios and variations in La-Th-Sc concentrations suggests a dominantly silicic source rock of granitic to granodioritic composition for these sediments, which is consistent with the dominant lithology of the drainage basins of the Mandovi and Zuari Rivers. Minor contributions from a mafic source (basalt or metabasalt) cannot be ruled out. The chemical index of alteration (CIA) and plagioclase index of alteration (PIA) values of the samples suggest low to moderate weathering of the source rocks. While the Sr isotopic compositions of the GC17 and GC19 sediments overlap with that of the gneissic basement rocks from the Mandovi and Zuari drainage basins, the εNd(0) of the sediments (average −13.72) show significantly more radiogenic values compared to the gneissic basement rocks (average −36). This difference in the Nd isotopic compositions is explained by early selective weathering and removal of rock-forming minerals like plagioclase in the source rocks which results in the supply of radiogenic Nd isotopic composition bearing sediments to the ocean. This argument is consistent with the overlapping εNd(0) of the GC17 and GC19 sediments and suspended particulate matter (SPM) of the Mandovi and Zuari Rivers. Although, the Nd isotopic compositions overlap, the less radiogenic Sr isotopic compositions of the GC17 and GC19 sediments compared to the SPM of the Mandovi and Zuari Rivers suggests the influence of organic matter, formation of authigenic clay and carbonates, as well as the contribution of Sr from submarine groundwater discharge.Trace metal concentrations and ratios were measured in the bulk sediments as well as in the leached authigenic and residual detrital-rich fractions of selected samples. For the GC19 sediments overlain by oxygenated waters, these geochemical proxies correctly infer their deposition under oxic conditions. In contrast, most of these proxies, barring those involving U and Mo, erroneously infer oxic depositional conditions for the GC17 sediments which are overlain by an oxygen minimum zone (OMZ) of the eastern Arabian Sea. We believe that the high sedimentation rate and large terrigenous flux possibly prohibits the equilibration of some of these trace metals in the sediments with the ambient seawater. Our study suggests that U and Mo-based proxies for inferring paleo-redox conditions are robust even under high sedimentation conditions. Overall, this study provides useful insights into the behaviour of redox-sensitive elements and provenance indicators in a continental margin setting impinged by an OMZ.