Climate-induced shift of deep-sea benthic foraminifera at the onset of the mid-Brunhes dissolution interval in the northeast tropical Indian Ocean

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The mid-Brunhes dissolution interval (MBDI; Marine Isotope Stage (MIS) 13 to 7; ~ 533–191 ka) is characterized by various paleoclimatic/paleoceanographic events in the world. We investigated fossil deep-sea benthic foraminifera and sediment geochemistry at the onset of the MBDI (~ 670–440 ka) using Ocean Drilling Program (ODP) Site 758 and core GPC03 in the northeast tropical Indian Ocean (TIO), primarily focusing on the relationship between the paleoceanographic conditions of the surface and deep oceans. Based on multi-dimensional scaling, MDS axis 1 is related to the specific depth habitats of benthic foraminiferal fauna, possibly at the trophic level. In MDS axis 1, the difference between the two core sites was smaller from ~ 610 to 560 ka, whereas it was larger from ~ 560 to 480 ka. In contrast, MDS axis 2 may be related to the low food supply at episodic food pulses/relatively stable and low food fluxes. MDS axis 2 showed generally similar stratigraphic variations between the two cores during ~ 610–560 ka, but was different during ~ 560–480 ka. The proportion of lithogenic matter to biogenic carbonate was relatively low from ~ 610 to 530 ka under the highstand when sediment transport to the study area was reduced. Thus, both the depth gradient in the distribution of benthic foraminiferal fauna and the lithogenic supply between the two cores changed coincidently across the MIS 15/14 (~ 570–540 ka) transition. Such paleoceanographic conditions across MIS 15/14 transition were attributed to the long-term weakening of the wind-driven mixing of surface waters, which might have been caused by the weakening of the Indian summer monsoon in the northeast TIO, possibly with the northward displacement of the InterTropical Convergence Zone in the Northern Hemisphere. In particular, the depth gradient in the distributions of benthic foraminiferal faunas represents the paleoceanographic linkage between the surface and deep oceans through particulate organic matter ballasting by calcareous plankton skeletons in addition to lithogenic matter, which changed transiently and significantly across MIS 15/14 transition close to the onset of the MBDI.