Ostracod response to monsoon and OMZ variability over the past 1.2 Myr

We present the first continuous middle through late Pleistocene record of fossil ostracods from Maldives in northern Indian Ocean, derived sediment cores taken at Site U1467 by Expedition 359 International Ocean Discovery Program (IODP). lies 487 m water depth Inner Sea archipelago, an ideal place for studying effects South Asian Monsoon (SAM) system on primary productivity, intermediate ocean circulation, and regional oxygen minimum zone (OMZ). The acts as a natural trap that has undergone sedimentation millions years with minor terrestrial influence. Our spans Marine Isotope Stage (MIS) 35 to present, covering mid-Pleistocene transition (1.2–0.6 Ma) Mid-Brunhes Event (MBE, ~480 ka) time when ice age cycles transitioned occurring every 40,000 100,000 years. ostracod data is interpreted alongside existing datasets same site sedimentological (grain-size) XRF-elemental analyses, new organic biomarker also U1467. These support paleoenvironmental interpretation assemblages. Ostracods are abundant diverse, displaying prominent change faunal composition MBE related increase amplitude glacial-interglacial cycles, which deeply affected monsoon thereby past oceanographic conditions Sea. Furthermore, exhibit distinctly different assemblages across particularly after MBE, these changes convincingly correspond variability OMZ. Glacial periods characterized indicators well‑oxygenated bottom due intensification winter contraction Abundant psychrospheric during glacials suggests southern sourced mass, such Antarctic Intermediate Water (AAIW) and/or Subantarctic Mode water, bathed glacial periods. In contrast, interglacial stages species indicate low-oxygen sluggish circulation pointing expansion OMZ strengthening summer monsoon. results highlight sensitivity climate variability.