Reconstructing hypoxia on the eastern Arabian sea continental shelf during the past ∼7400 years: A multi-proxy approach

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Shallow-sea environmental variations in the eastern Arabian Sea (EAS) have been related to monsoon variability, poorly ventilated intermediate water, and primary productivity changes. To understand these conditions, benthic foraminiferal assemblages, oxygen isotope ratios, and total organic carbon content have been examined in sediment Core SK291/GC13 from the EAS. We aim to retrieve decadal to centennial time-scale changes in past oxygenation and paleoproductivity during the last 7400 calibrated years before the present (cal yr BP). Our proxy data suggest that the oxic benthic foraminifera species were dominant during ∼7400–6000 cal yr BP. The change from oxic to dysoxic occurred during ca. 6000–3500 cal yr BP, and transition from dysoxic to suboxic environmental conditions happened at ca. ∼3500 cal yr BP. These changes are contemporaneous with weakening of the Indian summer monsoon beginning at ∼5500 cal yr BP which culminated into a major arid phase so-called ca 4.2 ka event in the Indian subcontinent. The EAS experienced low productivity during the early phase of our studied interval (∼7400–6000 cal yr BP) and higher productivity in the middle Holocene (6000–3000 cal yr BP), as revealed by the total organic carbon and benthic assemblages. Furthermore, productivity increased in the warm period and decreased in the Little Ice Age on a decadal scale. The spectral analysis of total organic content and suboxic species indicates a 23-year Hale (solar) cycle during the last millennium, indicating a close relationship between solar activity and monsoon variability. The strengthening of the El Nino-Southern Oscillation perhaps amplified due to increased solar activity in the last millennium, which influenced the climate of the EAS. The study highlights paleo-oxygenation shifts based on benthic foraminiferal assemblages, which are crucial for understanding changes in the oxygen minimum zones (OMZs) and their potential impacts on marine ecosystems.