The mid-Pleistocene climate transition in the Japan Sea: Insights of a combined palaeoceanographic and monsoon, multi-proxy study using marine sediments of IODP Site U1427 from the shallow, southern Japan Sea

Abstract

In this thesis marine sediments from the Japan Sea (IODP Expedition 346, “Asian monsoon”, Site U1427) are used to investigate the expression of the mid-Pleistocene transition (MPT) in this marginal basin. The thesis establishes a revised geochronology (age model) for the sediments of Site U1427 across the MPT using benthic foraminifera-based trace element and oxygen stable isotope analyses. A multi-proxy approach was applied to decipher the marine versus terrestrial sources of particles in a shallow marine setting with close proximity to land. Combined, these approaches enabled to build a new conceptual model of the palaeoceanography of the Japan Sea across the MPT. Establishing a geochronology for Site U1427 by comparing the benthic foraminiferal oxygen isotope record with a well-dated global reference stack of the same kind, is challenging because the Japan Sea, in contrast to the open ocean, is suggested to have been nearly isolated from the open ocean during glacial sea-level lowstands. This isolation restricted water mass exchange and led to widespread carbonate dissolution, which hampers the creation of continuous oxygen isotope records, and, counterintuitive, reduces glacial oxygen isotope ratios through the effect low saline (and low oxygen isotope ratio) waters accumulating in the basin. Instead, the geochronology is established by combining the benthic foraminiferal oxygen isotope record with trace element and shipboard colour reflectance data, and comparing those with age markers (biostratigraphic, palaeo-magnetic, tephrachronological) and a well-dated, global reference stack (LR04-stack). This approach resulted in identification of marine isotope stages (MIS) 39-17 in the analysed sediments. The Japan Sea sediments, including Site U1427, are influenced by the East Asian monsoons and glacio-eustatic sea-level variabilities, which both control marine productivity in the basin. The applied multi-proxy approach, combining bulk sediment organic matter parameters, i.e. total organic carbon (TOC), total nitrogen contents (TN) and carbon isotope ratios (13Corg), as well as carbonate content (CaCO3) and Rock-Eval pyrolysis, proved indispensable to decipher the signals, as the close proximity to land (35 km to shore) brought some proxies to the limitations of interpretability. In short, CaCO3 is used as a proxy for marine (carbonate) productivity, while TOC-TN (C/N) ratios, 13Corg and Rock-Eval pyrolysis parameters indicate the origin of organic matter (OM), although this is a summary and other processes additionally influence these proxies. The interpretation of C/N ratios is difficult as the proxy is likely influenced by an inorganic component to nitrogen, leading to low C/N ratios and an overestimation of marine-derived organic matter. Combining individual analyses to the above multi-proxy approach enables estimating variations in contributions of marine- and terrigenous-derived organic matter, hence providing clues on changes in monsoon precipitation and sea-level stands across the MPT. The sediments of Site U1427 record monsoon variations on sub-orbital timescales and indicate enhanced precipitation/freshwater input at times during certain glacials, such as MIS 36. A major outcome of this thesis is a revised palaeoceanographic model for the Japan Sea encompassing MPT. Previously, our understanding of the palaeoceanography of the Japan Sea was extrapolated from the most recent glacialinterglacial cycles. The palaeoceanographic model presented in this thesis proposes a better glacial connection between the open ocean and the shallow, southern Japan Sea during the early MPT, MIS 39-25, and a switch to the modern glacial nearisolation of the Japan Sea starting during the later stages of the MPT, MIS 24-17, caused by relatively higher glacial sea-levels, both globally and in the Japan Sea, during the early MPT compared to its later stage.