Widespread occurrence and sources of authigenic carbonates on the western slope of the northern Okinawa Trough

The Okinawa Trough is an active back-arc basin in the western Pacific Ocean located along a convergent continental margin. Although the presence of authigenic carbonates with specific morphological characteristics has been reported at a few sites, their distribution remains poorly understood because of the limited availability of geophysical and geochemical data from the northern Okinawa Trough. This study probes the geochemical characteristics of these authigenic carbonates and investigates their distribution using multibeam bathymetry, revealing a rough seafloor morphology developing on the western slope of the northern Okinawa Trough and extending more than 300 km in a slope-parallel direction. The geochemical analysis of authigenic carbonates recovered from multiple locations across this rough-morphology area reveals 13 C depletion and, thus, indicates that the constituent carbon was primarily sourced through the anaerobic oxidation of methane associated with fluid seepage activity. The Post-Archean Australian Shale–normalized rare-earth element patterns of the examined carbonates show notable enrichment of Y compared with Ho and Dy. The carbonates also exhibit positive correlations between Y/Ho and Y/Dy ratios, which fall between those typical of sediments and seawater. This finding indicates early diagenesis in an open system with substantial contributions from bottom seawater during carbonate precipitation. The wide range of Y/Ho ratios may further imply that the methane-rich fluids originated from a mixture of biogenic and thermogenic sources. The occurrence of methane-derived authigenic carbonates correlates with the distribution of high-backscatter-intensity areas and specific geological units. It thus indicates the widespread presence of carbonates along the western slope of the northern Okinawa Trough. • Widespread authigenic carbonates were recovered from the northern Okinawa Trough. • Carbon and oxygen isotope analyses were used to identify the origin of methane. • The carbonates occur widely and continuously in specific geological units.