14C Budget of Sinking Particulate Organic Matter in the Japan Trench: A New Approach to Estimate the Contribution from Resuspended Particles in Deep Water Column

Abstract

The 14C contents of organic carbon (δ 14C) were measured in sinking particles collected by year-long time-series sediment trap systems moored at four different depths on the deepest axis of the Japan Trench. The annual mean δ 14C values in sinking particles decreased with depth consistently, from +39‰ at 1171 m to −39‰ at 8681 m, while there were also some seasonal changes in δ 14C of sinking particles at each depth. Because the δ 14C value of sinking particulate organic carbon (POC) originated from surface water cannot change significantly during its settling process in deep water column due to its relatively long half life (5720 yrs), this nearly 80‰ downward decrease in δ 14C value inevitably indicated that some “old” carbon is entrained into sinking particles in the deep water column. Considering that the δ 14C values of sinking POC in the central regions of oceans do not show such large vertical changes, this “old” carbon must come from surface sediments on the continental slope area, adjacent of the trap site, through resuspension of sedimentary particles. The δ 14C value of sinking POC at each trap depth was compared with the δ 14C values of surface sediments and the hypothesized δ 14C values of sinking POC exported from surface water, in order to calculate the proportion of the resuspended sediment fraction in each sinking POC sample. While the results showed that 24–56% of sinking organic carbon at 8681 m depth may come from the surface sediment beneath the trench, this flux of resuspended “old” surface sediment could not solely explain the increase in sinking POC flux near bottom of water. Consequently, it is also suggested that there is not only the resuspension of “old” sedimentary particles but also the “rebound” of fresh sinking particles on the bottom which induces the anomalous increase in sinking particle flux near sediment surface.