Radiocarbon of sediment trap samples from the Okinawa trough: lateral transport of 14C-poor sediment from the continental slope

Radiocarbon of carbonate (PIC) and of organic carbon (POC) in sediment trap samples from the Okinawa trough was measured by AMS. Concentrations of 14C in PIC and POC (Δ14C-PIC and Δ14C-POC) ranged from approximately +40‰ to −80‰ and average over the entire 2 years was approximately −32‰. These values are much lower than Δ14C values of dissolved inorganic carbon (Δ14C-DIC) in the upper 200 m of the water column (+100‰ on average). Δ14C-PIC and Δ14C-POC showed seasonal variability over 2 years with lower values in winter and higher values in summer. In the 1994–1995 period, Δ14C-PIC was also low in spring. Variations in Δ14C-PIC and Δ14C-POC were positively correlated with concentrations of inorganic and organic carbon, respectively, and negatively correlated with concentration of Al. This suggests that variability in Δ14C-PIC and Δ14C-POC were associated with the input of lithogenic materials. Assuming Δ14C-PIC and Δ14C-POC for two end members (settling particles produced in the overlying water column, and laterally transported materials produced outside of the overlying water column, which originated from the continental slope of the East China Sea), contributions of laterally transported materials to the sediment trap samples were estimated for each collecting period. The contribution of laterally transported materials ranged from approximately 50%–90% and the annual average of flux of old carbon was ca. 5 mg m−2 day−1 in 1993 and 10 mg m−2 day−1 in 1994–1995.