Late quaternary carbonate (pteropod) preservation in the Indian Ocean sediments: inferences on the paleoclimate and paleoceanography

To evaluate the preservation pattern of pteropods and their relationship with climatic and oceanographic history in the Laccadive Sea, a temporal variation analysis of pteropod abundance was done. For that, we employed preservation indices from calcite (Globigerina bulloides%, Globorotalia menardii abundance), as well as aragonite (e.g., total pteropod abundance, Limacina Dissolution Index (LDX), fragmentation ratio). To determine if pteropod shells have been preserved over time, we used estimated pteropod abundance. The pteropod preservation record displays excellent preservation during cold stadials, evidenced by the lower values of aragonite dissolution proxies than during the interglacials/interstadials, similar to the preservation records from other northern Indian Ocean cores evidenced by the lower values of aragonite dissolution proxies than during the interglacials/interstadials. The shallow aragonite compensation depth (ACD), weaker oxygen minimum zone (OMZ), and the lower southwest monsoon (SWM)-induced productivity are thought to be the cause of the basin-wide pteropod preservation events during the cold stadials (ACD). Additionally, during an intense northeast monsoon (NEM), the advection of cold, low-saline waters from the Bay of Bengal to the Laccadive Sea, as well as the intrusion of southern-sourced intermediate water ventilation, may have caused a deep vertical mixing of oxygen-rich surface waters, raised the pH of thermocline waters and deepened the ACD. However, the local fluctuations in the water mass properties, such as the increased productivity maxima, the intense OMZ, and shallow ACD, as well as changes in the aragonite, are responsible for the poor pteropod abundance, poor preservation and strong dissolution during the Holocene, Bølling-Allerød (B/A) and interstadial periods.

The calcification proxy indicates that the aragonite undersaturation and reduced calcification occurred during 19-16.5 kyr, preferably due to the depletion in the oceanic alkalinity caused by enhanced upwelling-induced carbonate ion exchange between the intermediate and deep water. In contrast, the preferential dissolution of smaller shells in the sediments (marked by increased average shell size and higher values of Limacina dissolution index (LDX) corresponds to strengthened OMZ and shallower ACD, pointing towards the post-depositional dissolution of aragonite shells. Therefore, the overall decrease of pteropod content of the deposits in the stadial/interstadials suggests a combination of monsoon-associated changes in water column properties, variability in aragonite saturation, intermediate water ventilation and sediment rate.