Pteropods reliably record the aragonite compensation depth in the western Bay of Bengal.

Anthropogenic greenhouse gas emissions have a detrimental impact on the carbon sequestration by the oceans. Pteropods, a crucial component of the ocean's planktic community, secrete aragonite shells that are sensitive to increasing atmospheric carbon dioxide levels, making them the first indicators of ocean acidification. Therefore, pteropods are often used to observe the changes in aragonite compensation depth (ACD). Intriguingly, in the major parts of the northern Indian Ocean, the chemically defined ACD is < 800 m, but pteropods have been reported in surface sediments collected from much deeper depths in the same region, which raises questions about the use of pteropods to trace ACD in this area. To address this ambiguity, we conducted a systematic and detailed evaluation of pteropods to trace the changes in ACD in the western Bay of Bengal, which is the first-ever such study. The pteropods population dominated by Heliconoides inflatus was low on the inner shelf, and isolated pockets of high pteropod abundance were restricted to the upper slope. Based on the pteropod abundance in the surface sediments and the ratio of pteropods to planktic foraminifera, we report the baseline ACD in the western Bay of Bengal at ~ 500 m. The aragonite compensation depth based on the pteropod abundance in the surface sediments correlates well with the chemically defined ACD in this region. These findings will help to assess the impact of ocean acidification on aragonite compensation depth in the western Bay of Bengal.