Heavy Mineral Compositions of Sediments in the Southern Okinawa Trough and Their Provenance-Tracing Implication

Heavy mineral assemblages have been widely used to effectively trace sediment sources. Heavy mineral assemblages are rarely used in research to trace sediment sources in the southern Okinawa Trough compared with geochemical proxies. In this study, the TESCAN Integrated Mineral Analyzer (TIMA) revealed the full-size heavy mineral assemblages in the five layers of the core sediment H4-S2 in the southern Okinawa Trough. During the past 700 years, the heavy mineral assemblages in the sediments of the southern Okinawa Trough were very similar to the East China Sea shelf/Yangtze River, mainly composed of mica and chlorite; dolomite; actinolite; and hematite/magnetite. The grain size distribution of heavy minerals is in the clay–sand range and mainly in silt. Actinolite and hornblende can indicate the supply of sediments from the East China Sea shelf/Yangtze River to the southern Okinawa Trough. Due to their complex sources, pyrite, epidote, and hematite/magnetite are not adequate indicators for distinguishing between the different provenance areas. Because previous studies have used a variety of analytical methods, especially using heavy liquids with different densities, dolomite cannot be used as a marker for sediments on the Yangtze River/East China Sea shelf. Therefore, the East China Sea shelf/Yangtze River is a vital provenance of sediments from the southern Okinawa Trough since the late Holocene period.

Atypical Diagenesis and Geochemistry of Redox-Sensitive Elements in Hydrothermal Sediments of the Southern Okinawa Trough

Early diagenesis processes and the geochemistry of redox-sensitive elements (RSEs) in four sediment cores in an active hydrothermal field in the southern Okinawa Trough (OT) was investigated. Dissolved Fe, Mn, ΣHS–, and several other RSEs (Mo, U, and V) in pore water were measured. We also studied metal distribution in sediments using sequential extraction methods. Very high dissolved Fe concentrations (∼140 μmol L–1) but insignificant dissolved Mn were observed in surficial pore waters in the station adjacent to the hydrothermal vent, where highly reactive Fe, Mo, U in the sediments were also measured. Such an atypical diagenetic sequence found in those cores could be driven by the overwhelmingly high reactive Fe fraction (mostly Fe oxides) delivered from the vents. Consequently, significant upward benthic fluxes of Fe and Mo were estimated for the studied stations. In addition, we performed a principal component analysis (PCA), together with relative ratios of carbonate-related elements (Sr, Ca, Mg), to identify particles’ origins in the hydrothermal field; two endmembers being the hydrothermal source and hydrogenous processes. This comprehensive study on a unique set of samples collected by advanced technology provided valuable data to demonstrate distinctive geochemical features that occur in hydrothermal sediments.

Characteristics of seismicity in the southern Okinawa Trough and their relation to back-arc rifting processes

The southern part of the Okinawa Trough forms a narrow back-arc rift basin where evidence for submarine volcanoes and active hydrothermal venting is observed. The region is also known to cause large crustal earthquakes frequently which often accompany a rapid increase in seismicity rate. Although such swarm-like activities are common in active volcanic regions and are considered to be primarily induced by crustal fluid flows, potential interactions between tectonic, magmatic and hydrologic processes have been poorly examined in the southern Okinawa Trough despite these processes happening in the proximity. Here, I report the spatial and statistical characteristics of seismic activity in the southern Okinawa Trough and discuss their relation to other rifting-related phenomena. Most of the earthquakes with magnitude greater than 5 are localized around the rift axes (the Yaeyama Rift and the Yonaguni Rift) where seismic reflection data indicated the presence of solidified magmatic intrusions into the shallow sedimentary layers. I found the areas around the rift axes show low b values of < 0.8 and suggest that stress changes directly induced by dike intrusions beneath the rift axes control the occurrence of swarm activities. On the contrary, regions with high b values (> 1.2) are found around the Ishigaki Knoll and the Hatoma Knoll. These two areas are located between the rift axes and the Ryukyu Islands and correspond to potential submarine volcanoes proposed by seafloor bathymetry and seismic reflection images. This result may constrain the location of the volcanic front in the region.