Changes of sediment provenance driven by the sea level and Indian summer monsoon in the northern Ninetyeast Ridge over the past 50 kyr

The Ninetyeast Ridge is situated within the world's largest sediment "source-sink" system, which extends from the Tibetan Plateau to the Bay of Bengal (BoB). Modest and stable sedimentation rate makes it a suitable region for studying long-term land-sea interaction. Recent research has primarily focused on the northern Bengal Fan, leaving limited knowledge about sedimentary processes on the distant Ninetyeast Ridge. This study aims to address this gap by investigating sediment transport pattern and its response to climatic and environmental changes since the last glacial period, using the gravity core (CJ04-50) selected from the northern Ninetyeast Ridge. The provenance discrimination based on delta Eu-(Gd/Yb)N revealed that the Irrawaddy (Irr) River, along with the Ganges-Brahmaputra (G-B) Rivers, represented the primary sources of the study area. Quantitative analysis equations indicated that the contribution of the G-B Rivers decreased from the last glacial period (22%) to the Holocene period (16%), while that of the Irr River increased (from 78% to 84%). Chemical index of alteration (CIA*) values, and terrigenous input proxy indicators (Ti/Ca, Rb/Sr) were used to indicate the changes of weathering and terrigenous input over the past 50 kyr. During the last glacial period, CIA*, Ti/Ca and Rb/Sr revealed significantly millennial-scale periodic changes, including the distinct declining trend during the Last Glacial Maximum (LGM) and Heinrich 2-4 (H2-H4) periods, followed by recovery trend during subsequent warm periods. During these cold periods of LGM and H2-H4, weakened Indian monsoon condition, reduced precipitation, cold climate, and expanding ice sheets, adversely affected physical erosion and chemical weathering in the source area, and the reduced river runoff impeded sediment transport. Conversely, these environmental factors reversed during relatively warm periods. Since the Holocene, CIA*, Ti/Ca and Rb/Sr declined, which was contrast to the climate change, and the sea level changes became the dominant factor. The rapidly rising sea level submerged the main terrigenous input channel (the Swatch of No Ground) of the G-B Rivers, made the depositional center shift from the Bengal Fan to the continental shelf, causing the decline of terrigenous input and lower CIA* values.