LAUSR

Abstract The Mid Pleistocene Transition (MPT, ~1000–700 ka) represents the latest major change in the climate system, when the climate variability changed from ~ 41 to 100 kyr cycles. Changes are reflected in different components of the climate system including the marine biota. In this study we focus on studying the East Equatorial Pacific Ocean (EEP), a relevant area in the present and past climate regulation. We present a high-resolution record of benthic foraminifera from the Ocean Drilling Program Leg 202 Site 1242 (East Equatorial Pacific, 1364 m water depth) from MIS 18 to MIS 30 (716 to 1038 ka). The analysis of benthic foraminiferal assemblages allows to identify palaeoenvironments and to make inferences about the palaeoceanographic processes involved. Bottom waters were deficient in oxygen during mild glacials of Marine Isotopic Stage (MIS) 26 and 28, early MIS 23, glacial MIS 22, and the glacial-interglacial transitions of MIS 20/19 and MIS 18e/d. The overall sustained organic carbon regime suffered a major change during mid MIS 23 (890–900 ka). We interpret this change was caused by an increased influence the open ocean upwelling system of the Costa Rica Dome derivering labile organic carbon to the seafloor. Another substantial change occurred at mid MIS 19 and it is interpreted to involve a shift in the nature of the organic carbon arriving at the seafloor. This shift largely coincides with the disappearance of a group of mainly elongated benthic foraminifera, commonly known as the “Extinction Group”. Our findings suggest the arrival of a particular type of food, for which these foraminifera where not adapted, might have contributed to the final demise of this group of foraminifera in the East Equatorial Pacific.