HETEROGENEOUS PLANKTIC FORAMINIFERAL RESPONSE TO DEGLACIAL INDIAN MONSOON CHANGES

Seasonal rains and winds associated with the Indian Summer Monsoon (ISM) phenomenon profoundly affect the oceanography and productivity of the northern Indian Ocean. Although paleoclimate records indicate strong ISM variability during the last deglaciation, there is a lack of high-resolution records focusing on the response of marine fauna to changes in the monsoon during this period. In this work, we reconstruct a high resolution record (~100 yr/sample) of relative planktic foraminiferal population abundances from the Last Glacial Maximum (LGM; ~20 ka) to present at IODP Site U1446, located offshore the Mahanadi Basin in the Bay of Bengal. We use principal component analysis (PCA) on the relative species abundances and total fluxes to identify shared variance amongst all fauna. We compare our reconstructed PCs with independent geochemical temperature and salinity estimates from the same core site to understand the drivers and phasing of change. We find that PC1 (explaining 48% of overall variance) and PC2 (28%) correlate similarly between independent reconstructions of SST (R = 0.59 and 0.52, respectively) and δ¹⁸O_sw (R = -0.59 and -0.55) from the same core, implying a common driving factor associated with changes in the ISM, potentially tied to nutrient availability in the upper water column. We find that PC1 correlates strongly with G. bulloides and N. dutertrei abundance (R = 0.90, 0.82) whereas PC2 strongly correlates with the relative abundance of G. glutinata (R = 0.90), a species known to strongly prefer nutrient-rich waters. We also explore the coupling of export productivity and genus Globorotalia abundance as these species exclusively feed on organic detritus from the upper ocean. Our results show a significant drop in the abundance of Globorotalia during the Heinrich Event (HS1) and during the early Holocene, suggesting a complete shutdown in productivity during these periods despite opposing monsoonal signatures. Interestingly, we also find that the population abundance of mixed-layer dwelling and symbiont-bearing species G. ruber, hypothesized to prefer warmer waters, shows a strong anti-correlation with temperature. Overall, we conclude that changes in the ISM across the deglaciation strongly influenced marine plankton productivity and ecosystem functionality in the coastal Bay of Bengal.