Effect of sea-level on organic carbon preservation in the Okinawa Trough over the last 91 kyr

The burial of marine organic carbon, primarily synthesized by marine autotrophic organisms, into sediment removes CO2 from the ocean-atmosphere system. However, linkages between export productivity and organic carbon burial and the controlling factors remained ambiguous, especially for marginal seas. In this study, we measured redox-sensitive trace metal concentrations and planktic foraminifer fauna assemblages for a sediment core collected from the middle Okinawa Trough spanning the past 91 kyr. Through compilation with other published data, we found that the export productivity was decoupled from organic carbon burial. Variability of manganese and total sulfur contents suggested that both bottom water and surface sediment had experienced significant redox change in the last glacial cycle. By using the ratio of marine organic carbon to reactive phosphorus, i.e., OCmarine/Preact., we reconstructed organic carbon burial efficiency which showed relatively higher values during the last glacial and low values during the middle-late Holocene. After synthesizing our new and reported data, we conclude that the middle Okinawa Trough bottom water redox state was controlled by deep water ventilation induced by sea level change and related Kuroshio Current intrusion, instead of by the oxygen consumption owing to local export productivity.