Transformation of particulate matter while sinking through the oxygen minimum zone of the Eastern Arabian Sea

The degradation of particulate organic matter in the ocean plays a critical role in regulating the marine carbon cycle and sustaining oxygen minimum zones. In this study, suspended particulate matter from two stations in the eastern Arabian Sea, encompassing both oxic and suboxic water columns, was studied to investigate the transformation of particulate matter during vertical transport. Scanning electron microscopy revealed the presence of diverse planktonic assemblages with a noticeable decline in abundance at greater depths. Particulate organic carbon (POC) and particulate nitrogen (PN) concentrations decreased sharply within the upper 60 m, indicating rapid degradation of labile organic matter in the euphotic zone. The POC/PN ratio increased with depth, suggesting preferential nitrogen loss during remineralisation. δ¹⁵N values ranged from +2.7 ‰ to +6.4 ‰, with enriched signatures in surface and suboxic layers, while δ¹³C values became progressively depleted with depth (up to -25.6 ‰), reflecting the loss of isotopically heavier compounds and the dominance of lighter, more refractory material. Phytoplankton marker pigments were abundant in the upper water column, while degradation products were prevalent in suboxic waters. Fatty acid methyl ester profiles were dominated by palmitic acid, followed by stearic acid, indicating a predominance of diatoms and dinoflagellates. Sterol analysis identified cholesterol, brassicasterol, stigmasterol, and β-sitosterol, with depth-dependent variations in their concentrations. Despite differences in depth and dissolved oxygen profiles between the two stations, the study consistently demonstrated substantial degradation of organic matter within the upper 60 m oxic water column and highlighted the differential preservation efficiencies of various biogeochemical markers.