Influence of Indian Summer Monsoon on Marine Biological Productivity Across the Late Pliocene and Early Pleistocene

The Indian Summer Monsoon (ISM) significantly impacts the lives of billions of people through rainfall patterns and ocean biological productivity. However, its stability both in the near future and distant past is disputed, particularly its response to changing temperatures and global ice volume. Here, we present a comprehensive multiproxy reconstruction of ISM-induced runoff and marine biological productivity from the Late Pliocene to the Early Pleistocene (3.5–2.3 Ma), using high-resolution deep-sea sediment records from the northern Bay of Bengal (BoB). This critical interval is characterized by high but falling atmospheric CO2 levels and the establishment of Northern Hemisphere glaciation, but with a similar paleogeography to the modern. Our orbitally-tuned records, based on bulk sediment X-Ray fluorescence-elemental data and diatom and planktic foraminiferal assemblages, reveal that periods of diminished marine biological productivity coincide with elevated ISM runoff, reflecting the interplay between freshwater input, turbidity, and nutrient availability off northeastern India. Moreover, our analysis highlights the sensitivity of the ISM strength and BoB ecosystem to orbital forcing. We observe the presence of eccentricity, obliquity, and precession cyclicities in the runoff and productivity records, with multiple transitions in the dominant orbital frequency during the study interval, pointing to unique sensitivities of ISM rainfall/runoff to Earth's orbital forcing. These findings underscore the importance of considering both orbital forcing and internal climate dynamics in understanding ISM variability and its implications for marine ecosystems.