Revisiting the Last Glacial-Interglacial Productivity Paradox in the Eastern Arabian Sea

The Southern Ocean enhanced iron fertilization induced primary productivity can account only for ~40 ppm out of the total 80-100 ppm change in atmospheric CO₂ during the last glacial-interglacial transition. Additional mechanism or regions with increased glacial productivity have to be identified to account for the remaining 40-60 ppm change in atmospheric CO₂. The Arabian Sea is one of the most productive tropical regions at present. Even though a majority of the past productivity records from the Arabian Sea suggest reduced primary production during the glacial period, contrasting results have been obtained from the eastern Arabian Sea, suggesting a possible role of this region in controlling glacial-interglacial atmospheric CO₂ changes. Here we reconstruct paleo-productivity changes from the eastern Arabian Sea during the past ~32 kyr, from foraminiferal and geochemical studies on a gravity core (SK 237 GC04).

We infer that the productivity in the southeastern Arabian Sea was high during the MIS3 with well oxygenated waters. An abrupt decrease in productivity is inferred during early deglaciation and it coincides with Heinrich event (HS-1), suggesting a close relationship between monsoon in the eastern Arabian Sea and the climatic change in North Atlantic. A synchronous productivity collapse in the SEAS during global depleted δ¹³C event, suggests that the productivity collapse was partially responsible for the depleted δ¹³C during HS-1. The high glacial productivity as noted in Malabar core as well as a few other cores collected from similar water depths is not noticed in all the cores collected from the southeastern Arabian Sea. It suggests that though the productivity was higher in this region during glacial times but the overall resultant carbon sequestration was confined only to a restricted zone and might not be large enough to substantially alter atmospheric CO₂. The productivity was also high during the early Holocene monsoon optimum. A close correspondence between productivity changes and ice-volume corrected δ¹⁸O (δ¹⁸O_{sw(iv corr.)}), during several intervals in the core, suggests that productivity changes in this region were partially driven by monsoon.