[A-OS13] Exploring Variability and Changes in Ocean Biogeochemical Cycles

[AOS13-P02] Sedimentation Dynamics and Redox Conditions in the Ryukyu Accretionary wedge area Over the Last 18.4 ka BP

*Thilina Munasinghe¹, Pierrick Fenies², Ludvig Löwemark³, Nathalie Babonneau⁴, Gueorgui Ratzov⁵, Shu-Kun Hsu⁶, Chih-Chieh Su², Yuan-Pin Chang¹ (1.Department of Oceanography, National Sun Yat-sen University, Taiwan, 2.Institute of Oceanography, National Taiwan University, Taiwan, 3.Department of Geosciences, National Taiwan University, Taiwan, 4.Geo-Ocean, UMR 6538, University of Bretagne Occidental – IFREMER, France, 5.Géoazur, UMR 7329, Côte d'Azur University, France, 6.Department of Earth Sciences, National Taiwan University, Taiwan)

Keywords:Paleo-Redox Proxies, Global Thermohaline Circulation, Philippine Sea, Deglacial period, Pyrite formation, Paleo surface productivity

Understanding paleo-sedimentary processes and geochemical variations in marine basins is crucial for reconstructing past oceanographic and climatic conditions. This study examines the grain size distribution and geochemical characteristics of sediments from core MD18-3532, collected from an intra-slope basin within the Ryukyu accretionary prism near the Yaeyama Ridge. The basin experiences minimal influence from Pacific bottom water and is relatively unaffected by turbidity flows due to the topographical barrier presented by the Yaeyama Ridge. The sediment core spans the last 18.4 kyr BP, as established by an radiocarbon age model, and is primarily composed of dark grey clay. Notable features identified during the deglacial period include multiple black dots, black silty laminations, and prominent black layers. The deglacial interval is characterized by a markedly higher sedimentation rate, likely driven by increased terrigenous input from the East China Sea (ECS) and Taiwan. This influx is attributed to the partial divergence of the Kuroshio Current (KC) and intensified East Asian Winter Monsoon (EAWM) activity. Enhanced sedimentation rates during the deglacial period fostered sediment pore water reducing conditions, as evidenced by elevated total sulfur (TS%) levels, increased Fe/Al and S/Al ratios, and high Fe/magnetic susceptibility, ratios, all indicative of extensive pyrite formation. Following the deglaciation, rising sea levels redirected the KC along with its modern trajectory, diminishing sediment supply from Taiwan and the ECS. This hydrodynamic shift led to reduced surface productivity, decreased sedimentation rates, and a dominance of terrigenous material in Holocene sediments, as confirmed by XRF elemental cluster analysis. Furthermore, Fe/Mn ratios suggest oxic conditions prevailed in bottom waters during the Termination I period. The transition from deglacial to interglacial periods reveals the significant influence of sea level rise on regional sedimentation dynamics. These findings offer valuable insights into the interactions between ocean circulation patterns, sedimentation processes, and redox conditions in the western Pacific over the past 18.4 kyr BP.