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.

Plain Language Summary: The Indian Summer Monsoon (ISM) is a vital weather system that brings significant rainfall to South Asia each summer, affecting billions of people through rain patterns and ocean health, but it is vulnerable to disruption due to anthropogenic climate change. This study investigates ISM runoff and marine biological productivity in the northwestern Bay of Bengal (BoB) from 3.5 to 2.3 million years ago. During this period, Earth experienced major changes with falling atmospheric CO2 concentrations, increasing global ice volume, and falling sea level. By examining marine sediments from the BoB, we found that stronger ISM rainfall led to increased freshwater runoff into the ocean, which reduced marine biological productivity. This is because increased runoff brings more sediments into the ocean, increasing turbidity and limiting light required for phytoplankton in the surface, while also forming a freshwater surface layer that reduces the amount of nutrients upwelling from deeper waters. We found that these changes in ISM strength and marine productivity were influenced by Earth's orbital cycles, which varied in relative importance with time. These findings highlight how closely linked the ISM is to both global climate changes and local marine ecosystems, helping us better understand potential future monsoon behavior.