Origin and decomposition of sinking particulate organic matter in the deep water column inferred from the vertical distributions of its δ15N, δ13 and δ14

Abstract

Sinking particles were analyzed for their nitrogen isotopic ratio δ 15N) of total particulate nitrogen (PN), stable carbon isotopic ratio ( δ 13C) and radioactive isotopic ratio ( δ 14C) of total particulate organic carbon (POC), at three different latitudinal (temperate, subpolar and equatorial) and geomorphological (trench, proximal abyssal plain and distal abyssal plain) sites in the western North Pacific Ocean using year-long time series sediment trap systems, to clarify the common vertical trends of the isotopic signals in deep water columns. Although the δ 15N and δ 13C values of sinking particulate organic matter (POM) were partly affected by the resuspension of sedimentary POM from the sea floor, especially in the trench, the changes in δ 15N and δ 13C values owing to the resuspension could be corrected by calculation of the isotopic mass balance from δ 14C of sinking POC. After this correction, common downward decreasing trends in δ 15N and δ 13C values were obtained in the deep water columns, irrespective of the latitudes and depths. These coincidental isotopic signals between δ 15N and δ 13C values provide new constraints for the decomposition process of sinking POM, such as the preferential degradation of 15N- and 13C-rich compounds and the successive re-formation of the sinking particles by higher trophic level organisms in the deep water column.