On the evolution of the oxygen minimum zone in the Arabian Sea during Holocene time and its relation to the South Asian monsoon
Published:January 01, 2002
Michael Staubwasser, Peter Dulski, 2002. "On the evolution of the oxygen minimum zone in the Arabian Sea during Holocene time and its relation to the South Asian monsoon", The Tectonic and Climatic Evolution of the Arabian Sea Region, P. D. Clift, D. Kroon, C. Gaedicke, J. Craig
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The evolution of the oxygen minimum zone within the permanent thermocline of the Arabian Sea (AS) during early and mid-Holocene time was reconstructed from a laminated sediment core taken from the Pakistani continental margin (316 m water depth). A trace metal proxy for water column ventilation (authigenic U) was extracted by principal component analysis from a large dataset of inorganic and total organic carbon (TOC) measurements. This proxy is compared with preservation of lamination and paired benthic-planktonic 14C data. The latter record the Δ14C depth gradient in the AS and may provide a sensitive indicator for ventilation by enhanced surface convection. Laminated sediments were preserved between 10 and 7.5 ka bp on the Pakistani continental margin and accumulated authigenic U independently from TOC accumulation. The inferred reducing conditions in the AS thermocline are in agreement with high palaeoproductivity in the western AS upwelling region. Century-scale variability in northern AS surface hydrography (recorded as δ18O in planktonic foraminifera) is reflected in the accumulation of authigenic U on the Pakistani margin. The agreement of AS surface conditions, which generally reflect the South Asian monsoon (SAM), with ventilation of the OMZ confirms a dominant influence of the SAM and summer monsoon upwelling in particular on AS thermocline ventilation during early Holocene time. However, the preservation of laminated sediments off Pakistan and palaeoproductivity in the western AS disagree before 10 ka cal. bp, and between 7.5 and 5.5 cal. ka bp. Here, the absence of lamination indicates better ventilation of the thermocline, whereas palaeoproductivity in the upwelling region was high. This suggests that other factors may also have contributed in variable proportions to AS thermocline ventilation. At present, these factors include lateral advection of oxygenated Central Indian Water and ventilation by winter surface convection in the northern AS.
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The Tectonic and Climatic Evolution of the Arabian Sea Region
Over long periods of time the tectonic evolution of the solid Earth has been recognized as the major control on the development of the global climate system. Tectonic activity acts in one of two different ways to influence regional and global climate: (i) through the opening and closing of oceanic gateways and its effect on the circulation patterns in the global ocean; (ii) through the growth and erosion of orogenic belts, resulting in changes in oceanic chemistry and disruption of atmospheric circulation. The Arabian Sea region has several features that make it the best area for studies of climate and palaeoceanographic responses to tectonic activity, most notably in the context of the South Asian monsoon and its relationship to the growth of high topography in the adjacent Himalayas and Tibet.
The Tectonic and Climatic Evolution of the Arabian Sea Region brings together a collection of recent studies on the area from a wide group of international contributors. The paper range from high resolution, Holocene palaeoceanographic studies of the Pakistan margin to regional tectonic reconstructions of the ocean basin and surrounding margins throughout the Cenozoic. Marine geophysics, stratigraphy, isotope chemistry and neotectonics come together in a multidisciplinary approach to the study of interactions of land and sea. while much work remains to be done to understand fully the tectonic and climatic evolution of the Arabian Sea, a great deal has been achieved since the last major review, as detailed in the 26 contributions. This volume is essential reading for palaeoceanographers, sedimentologists and geophysicists. It will also be interest to structural geologists and those working in the petroleum industry.