Holocene variability in chemical weathering and ocean redox state: A reconstruction using sediment geochemistry of the Arabian Sea

Abstract

• Geochemistry of a core (0.8–7.2 kyr) from the eastern Arabian Sea was studied. • Provenance and chemical weathering show the minimal change between 7.2 and 2.6 kyr. • Changes in weathering patterns were observed during the Roman Warm Period. • Transition from oxic to reduced bottom water condition was observed since ∼4.2 kyr. Major and trace elemental geochemistry of a sediment core (SSK40/GC10; water depth ∼ 50 m) from the eastern Arabian Sea (17° 59.89123 N, 72° 22.45161 E) have been investigated to reconstruct the continental weathering pattern and ocean redox state between 7.2 and 0.8 kyr BP (kyr, hereafter). The covariations of Fe/Al with K/Al and Ti/Al ratios indicate that the western-flowing Indian rivers (e.g., Narmada, Tapi, and Mahi) and eolian supply from the Thar regions are the major sediment suppliers to this location. Factor analysis of the geochemical dataset identifies four major factors explaining about 80 % of the total variance. Down-core profiles of several detrital proxies (Fe/Al, Ti/Al, V/Al, and Co/Al) show that the sediment sources were nearly uniform until 2.6 kyr. In contrast, changes in provenance indices, K/Al ratios, and CIA* (modified Chemical Index of Alteration) have been observed in the upper part of the core. The period of these weathering changes (2.6–1.6 kyr) mostly overlaps with the Roman Warm Period, indicating a weathering-climate linkage. Variations in the Ba/Al ratios and enrichment factors of U (EF-U) and Mo (EF-Mo), along with TOC/Al, and S/Al trends, indicate a fluctuating oceanic (bottom water) redox state during the studied period. These trends point to a shift from oxic to anoxic bottom water condition since ∼4.2 kyr. The timing of this change is synchronous to the earlier reported drought phase at the beginning of the Meghalayan stage, underscoring climatic control on the bottom water redox state.