Pteropod preservation reveals carbonate dissolution across the Mid-Pleistocene Transition and an Eemian preservation spike in the Andaman Sea

Pteropods are aragonitic holoplanktonic molluscs whose shells preserved in marine sediments provide a sensitive archive of past seawater carbonate saturation state, intermediate-water ventilation, and monsoon-driven oceanographic change. Here, we present a ∼1.2 Myr record of pteropod assemblages and preservation from sediment core NGHP-17 (1344 m water depth) in the Andaman Sea. Sixteen species were quantified across 180 samples, together with the Fragmentation Ratio (FR), Limacina Dissolution Index (LDX), epipelagic/mesopelagic (Epi/Meso) ratio, and selected planktonic and benthic foraminiferal abundances. Pteropod preservation is episodic, with pronounced preservation peaks during several glacial intervals. In contrast, generally poor and intermittent preservation during MIS 25–14 characterizes much of the Mid-Pleistocene Transition (MPT), although preservation windows occur during MIS 22, 18, and 16. This pre-MBDI decline was followed by the Mid-Brunhes Dissolution Interval (MBDI; MIS 13–11), within which a pronounced MIS 12 preservation peak demonstrates that unfavorable conditions were not continuous. An earlier preservation event occurs during MIS 26. The highest pteropod abundance is observed across MIS 6–5e, with a maximum at ∼125 ka and species maxima spanning ∼137–116 ka, indicating a transient Termination II/deglacial-to-early-interglacial preservation event. The significant inverse relationship between total pteropod abundance and FR supports a substantial preservation control on the record. Overall, the results indicate that pteropod preservation at NGHP-17 was strongly influenced by changes in bottom-water aragonite preservation conditions, within a broader framework of large-scale carbonate-system changes and regional hydrographic variability.