Change of marine redox environment in the western Okinawa Trough since the mid-Holocene: Evidence based on geochemical records

Abstract

The marine redox environment (MRE) plays a crucial role in the conversion and preservation of seafloor material and is a significant factor in the ocean carbon cycle. The evolution and driving mechanism of MRE are complex and present distinct regional characteristics over different time scales. The Okinawa Trough, situated in the East Asian monsoon realm, is influenced by the Kuroshio Current and flushing water, and its closed geographical pattern and multiple environmental factors make it highly sensitive to climate and marine environment changes. In this study, sediment samples from core GHN056–09 retrieved in the western Okinawa Trough were analyzed to investigate the high-resolution evolution of MRE since the mid-Holocene. The concentration of redox-sensitive elements (V, Co, Ni) and total organic carbon (TOC) were measured. Co/Ni and V/(V+Ni) values in the sediment samples indicated predominantly oxic conditions with matching productivity levels indicated by TOC (in the range of 0.28–1.17%). The oxic condition decreased during 6500–1500 a BP, and a strengthening trend was identified from 1500 a BP to present. By comparing these records with climate and environmental records, we found that weaker El Niño-Southern Oscillation (ENSO) and northward position of Intertropical Convergence Zone (ITCZ) resulted in less monsoon precipitation into the Okinawa Trough. Thinner freshwater lids could enhance seawater mixing at different layers, leading to strong oxic conditions. In contrast, stronger ENSO and southward position of ITCZ controlled by stronger solar insolation supplied more freshwater into the Okinawa Trough. Thicker freshwater barriers blocked the exchange of seawater from bottom to surface, resulting in weak oxic conditions. The trace element records in this study provide robust evidence on the high-resolution variability of MRE driven by the frequency of ENSO, movement of ITCZ, and changes in regional precipitation. Our findings offer new insights into the evolution of MRE in response to global and local climate changes.