A first look at factors affecting aragonite compensation depth in the eastern Arabian Sea

Water column measurements suggest shoaling of aragonite saturation depths (ASD) throughout the world oceans, due to increase in greenhouse gas concentration. Past records of aragonite saturation state under different climatic conditions are required to assess the impact of climatic changes on shoaling/deepening of ASD. The preservation state of organisms having aragonite skeletons, is used to assess the past changes in aragonite saturation depths, with respect to the modern ASD. Here for the first time, we delineate and discuss the factors that affect the modern aragonite compensation depth (ACD) in the eastern Arabian Sea by using pteropod abundance in the surface sediments. A total of 78 spade core-top samples collected along seven latitudinal transects, covering the continental shelf, slope and abyssal region of the eastern Arabian Sea were used. Pteropods were picked from coarse fraction (≥ 63 μm). Based on the pteropod preservation, we report that in the eastern Arabian Sea, ACD lies at a water depth of ≤ 525 m, which matches with the chemically defined aragonite saturation depth. We further report that the ACD shoals from north to south. The zone of high pteropod abundance coincides with low %Corg. The increase in pteropod abundance in the outer shelf region coincides with the drop in dissolved oxygen concentration. The deeper limit of pteropod abundance lies in the center of the oxygen minimum zone with higher %Corg. Therefore, we suggest that the pteropod abundance in the eastern Arabian Sea is not always related with the lower dissolved oxygen, but is strongly influenced by %Corg. This first report of the pteropod based aragonite compensation depth estimates from the eastern Arabian Sea will help in assessing future changes in ACD under the influence of anthropogenic green-house gas emissions.