Sedimentation in the Southern Okinawa Trough - Rates, turbidites and a sediment budget

摘要

Forty-seven box cores collected from the westernmost part of the Southern Okinawa Trough were analyzed for fallout radionuclides (~(210)Pb and ~(137)Cs) and sedimentary properties to study sedimentation rates and dynamics. The results indicate steady hemipelagic sedimentation in the lower slope and deep basin at water depths below ca. 1000 m, with sedimentation rates generally decreasing with increasing water depth or distance offshore barring a localized depression below the 1400 m isobath where sediment deposition is enhanced by episodic inputs of turbidites. The time of deposition of various turbidite layers can be dated circa 2002, 1986, 1966, 1959, 1947 and 1922 AD, which can be correlated not only spatially among sites but also temporally with the history of major submarine earthquakes in and around the region. Based on the distribution of sedimentation rates at 52 sites, including five with sediment trap-measured rates, a finite element approach was used to calculate the budget of sediment. Although the study area in the Southern Okinawa Trough occupies less than 10 percent of the Okinawa Trough's area, it receives more sediment than the rest of the Okinawa Trough. However, sediment deposited in the study area, approximately 14 MT annually, accounts for only 5 percent of the sediment discharged from Taiwanese rivers and barely matches the annual load discharged from the nearby Lanyang River. Thus, the Southern Okinawa Trough as a sink for sediment in the continental margin accretion wedge is not as important as generally speculated.