Calcium carbonate composition of surface sediments and influencing factors in the 90°E Ridge of the northeastern Indian Ocean^*

This study addresses the scientific question of the spatial differentiation mechanisms of calcium carbonate (CaCO3) in surface sediments across the 90°E Ridge in the northeastern Indian Ocean, employing a multi-scale analytical approach to elucidate controlling factors and biogeochemical processes. Through bulk and size-fractionated ( > 150 μm, 63-150 μm, 38-63 μm, 25-38 μm, 3 contribution analyses of surface sediments from 10 stations, combined with quantitative statistical analysis of scanning electron microscopy (SEM) microfeatures, the following findings were obtained: (1) The CaCO3 content exhibits significant spatial variability (36.95%-74.76%, mean 56.05%), forming a tripartite gradient pattern of 30%-45%, 45%-60%, and 60%-75%. (2) In regions with water depths above 3000 m, the dominant CaCO3 component is planktonic foraminiferal shells (> 150 μm, contributing > 65%), while stations near or above the lysocline are dominated by the 58%). (3) Quantitative microfeature analysis reveals, for the first time, a co-deposition pattern of calcareous dinoflagellate fossils (relative abundance up to 73.68%) with coccoliths and foraminiferal fragments in the 25-38 μm fraction. Further investigations demonstrate that CaCO3 distribution is governed by a ternary regulatory mechanism involving water depth-dependent dissolution effects, terrigenous clastic input, and siliceous biological dilution. This study innovatively establishes an integrated methodology of “grain-size separation-microscopic statistics-environmental interpretation”, which not only refines theoretical models of CaCO3 distribution in seamount geomorphic units but also expands the understanding of deep-sea inorganic carbon reservoirs by identifying calcareous dinoflagellate fossils as a novel carbon source. The findings provide a critical case study for comparative research on CaCO3 preservation mechanisms in global ridge systems and offer vital scientific insights for parameterizing marine carbon cycle models through improved algorithms for size-specific CaCO3 flux calculations.