Variations in sedimentary environments in the forearc and backarc regions of the Ryukyu Arc since 25 ka based on CNS analysis of sediment cores

Abstract

In this study, we analyzed 14C dates, grain size, and carbon, nitrogen, and sulfur element contents using sediment cores collected at the eastern edge of the backarc and forearc of the Ryukyu Arc, and demonstrated different temporal variations in CaCO3 contents and ratios of total organic carbon and total nitrogen contents (C/N ratios) compared with previous studies of the Okinawa Trough cores. Increases in CaCO3 contents and decreases in C/N ratios, from the Last Glacial Maximum to the Holocene, in the Okinawa Trough backarc basin correspond with sea-level changes. These results demonstrate that the sedimentary environment in the Okinawa Trough is influenced by sediment supply from mainland China, and reveal temporal variations reflecting changes in the Yangtze River mouth that accompany sea-level changes. In contrast, CaCO3 contents of cores in this study are double those in the Okinawa Trough cores, and show an increasing trend at 7.5–6.0 cal ka BP. Additionally, in the forearc core, the profile of the CaCO3 content decreases at 11.5–8.1 cal ka BP. These variations in CaCO3 contents suggest that marine productivity changed with sea surface trophic conditions. In addition, the lateral transport of terrigenous sediment by the Kuroshio and Ryukyu currents might contribute to an increased CaCO3 content by reducing the influence of dilution. Variations in the C/N ratio after 7.8 cal ka BP suggest that the Kuroshio Countercurrent has varied in strength at 1–2 ka intervals since the early Holocene, influencing the deposition of terrigenous organic matter from the Okinawa Islands in the deeper parts of the east backarc.