Relative abundance of planktic foraminifera from Eastern Arabian Sea (EAS) deep-sea sediment Core SK291/GC17 suggests surface water palaeoceanographic shifts in the EAS during ~25–3.5 calibrated kilo years before present (cal ky BP). The chronology of the studied core is established based on fifteen AMS 14 C dates derived from a mixed planktic foraminiferal assemblage and using the ‘Bacon R’ statistical package. Between ~21 and 19 ky BP, the mixed-layer oligotrophic planktic foraminifera, such as Globigerinoides ruber and Trilobatus spp. show higher abundance, whereas eutrophic species like Globigerina bulloides and Globigerinita glutinata show a decreasing trend. Pulleniatina obliquiloculata , which thrives in the deeper segment of the thermocline, shows consistently low abundance in the same interval. These changes in planktic foraminiferal abundance imply that upwelling-related productivity declined due to a subdued Indian Summer Monsoon (ISM) during the Last Glacial Maximum. Following the LGM, a significant shift in monsoon-induced productivity occurs at ~11.0 kyr BP, overlapping with the onset of the early Holocene. The low relative abundances of oligotrophic planktic foraminifera and an increase in eutrophic species, including P. obliquiloculata , indicate an increasing influence of the strong ISM in the EAS during the early Holocene. Another significant change in planktic foraminifera is observed from ~6.0 to 3.5 kyr BP, where the abundance of G. bulloides and P. obliquiloculata drastically reduces in this interval, suggesting a weakening of ISM winds and thermocline deepening/or a possible shifting of the upwelling centre due to sea-level rise. A decrease in G. bulloides mean abundance—from ~11.3% in the early Holocene to ~6.9% in the mid-late Holocene—indicates a regional contrast. Correlation with other Arabian Sea datasets suggests that spatial hydrographic variability in the EAS is linked to changes in ISM intensity.
Majumder et al. (Mon,) studied this question.