Faunal changes in deep-sea benthic foraminifera in the Equatorial Indian Ocean during the late Quaternary

Abstract

The surface oceanography in the Equatorial Indian Ocean is affected by seasonal alternation of the Indian monsoon through wind mixing. We investigated late Quaternary benthic foraminifera in sediment core GPC1803 (5º20.84’N, 88º50.01’E; 3328 m depth) in the Equatorial Indian Ocean, in order to clarify biotic response of benthic foraminifera with the surface oceanographic changes. This core seems to cover until Marine Isotope Stage (MIS) 13, based on oxygen isotope data of benthic foraminifera. We noted cyclic variations of Epistominella exigua four times. This species has been known as a “phytodetritus species” that can adopt episodic food supply well (e.g., seasonal to multiannual ranges). Although the further refinement is necessary to the current age model, common E. exigua happened in the interglacial periods, MIS 1, 6-7 9 and 13. We note the shift of the fluctuation patters of δ13C of Globigerinoides ruber and Neogloboquadrina dutertrei after MIS 6 in ODP Site 758 (Bolton et al., 2013 [QSR]) adjacent to our core: small offset between the two species in the interglacial periods prior to MIS6 vs. similar values of both the species after MIS 5. Because it is reasonable to suppose that the stratification status of the surface water may affect in the amount and periodicity of food supply to the seafloor in this area, the stratification of the surface ocean may explain the cyclic variations of E. exigua in our study.