Last 12 ky record of various organic geochemical proxies in the Eastern Arabian Sea

Here we present high-resolution biogeochemical study using nitrogen/carbon isotope ratio measurement and molecular proxies from a sediment core (length = 2.9 m) collected from the center (588 mbsl; Lat: 16⁰49.88’N and Long: 71⁰ 58.55’ E) of the oxygen minimum zone off west coast of India. The core archives the depositional record covering 1.114 to 12.025 ky BP. The concentrations of total organic carbon (TOC) and total nitrogen (TN) range from 0.7 to 4.9 wt% and 0.068 to 0.5 wt % respectively. TOC and TN show parallel trends and the TOC/TN ratio varies within a narrow range of 8 to 11.5. The δ¹³C values range from -20.5‰ to -21.9‰ (V-PDB). The TOC/TN and δ¹³C suggest typical marine organic matter source. This observation is also further supported by the n-alkane distribution pattern where the dominance of nC₂₁ to nC₂₄ and the absence of odd alkane dominance over even suggest predominantly marine organic source. The δ¹⁵N profile shows a steady increase from 5.7‰ at 203 cmbsf (5.5 ky BP) to 7.5‰ at 2 cmbsf (~1ky BP) suggesting gradual increase in denitrification possibly liked to reduced ventilation in the Arabian Sea, whereas, between 5.5 to 12 ky BP, the δ¹⁵N values show marked fluctuations (5.2 to 7.1‰) possibly indicating variable level of oxygenation which in turn controlled the extent of denitrification. Possible influence of diagenesis (microbial degradation of organic matter) on the δ¹⁵N values also need to be investigated for a better understanding of the water column processes.