Biotic response of deep-sea benthic foraminifera in the northeast tropical Indian Ocean at the onset of the mid-Brunhes dissolution interval

The mid-Brunhes dissolution interval (~533–191 ka) is characterized by various paleoclimatic/paleoceanographic events in the world that occurred mainly at ~400 ka during Marine Isotope Stage (MIS) 11. The duration of MIS 15–12 is important to understand a succession of various paleoclimatic/paleoceanographic events at the onset of the mid-Brunhes dissolution interval (e.g., Birth et al., 2018). For example, MIS 13 was marked by asymmetrical temperature and precipitation between the hemispheres, northward displacement of InterTropical Convergence Zone (ITCZ), and global carbon cycle changes (Ao et al., 2020). We investigated fossil deep-sea benthic foraminifera and sediment geochemistry (11 major elements by XRF analysis) during ~670–440 ka (MIS 16–12) at ODP Site 758 and core GPC03 (2925 m and 3650 m water depth, respectively) in the northeast tropical Indian Ocean (~5°N, ~90°E).

Two multidimensional scaling (MDS) axes were recognized in the benthic foraminiferal faunas. MDS axis 1 is related to the specific depth habitats of benthic foraminiferal faunas possibly with trophic level, whereas MDS axis 2 may be related to the low food supply with episodic food pulses/relatively stable low food flux. The difference of MDS axis 1 between ODP Site 758 and core GPC03 was smaller during ~610–560 ka (MIS 15), whereas it was larger during ~560–480 ka (MIS 14–13). MDS axis 2 showed generally similar stratigraphic variations between the two cores during ~610–560 ka, whereas it was different during ~560–480 ka. The proportion of lithogenic matter to biogenic carbonate was also relatively low during ~610–530 ka under the highstand. Thus, both the lithogenic supply and the depth gradient of benthic foraminiferal faunas between the two cores were changed across MIS 15/14 (~570–540 ka).

At ODP Site 758, the decrease of Globigerina bulloides across MIS 15/14 implies the long-term weakening of wind-driven mixing of the surface water (Chen and Farrell, 1992). It may be caused by the weakening of the Indian summer monsoon at that time, possibly with the northward displacement of the ITCZ (e.g., Ao et al., 2020). In our study area, the depth gradient of the benthic foraminiferal faunas may be controlled by the linkage between the surface and deep oceans that was altered transiently but significantly across MIS 15/14, possibly through the particulate organic matter ballasting by calcareous plankton skeletons and/or lithogenic matter.