The vertical distribution of the strong ligand in particulate organic matter in the North Pacific

Abstract

Vertical distributions of the strong organic ligand (thorium-complexing capacity: ThCC) in particulate matter (PM), which is calculated from the amount of thorium adsorption onto PM in a 0.1 M solution of HCl, were determined in the North Pacific and the Bering Sea, together with those of particulate organic carbon (POC), particulate nitrogen (PN), proteins and combined amino acids (C.A.A.). The concentrations of the strong organic ligand in PM decreased with increasing depth as did levels of POC, PN, proteins and C.A.A. The surface and vertical distributions of the ThCC in PM strongly suggest that the organic ligand measured by Th is biogenic. Taking into account the coexistence of `refractory' and `fresh' components in suspended PM, we showed that relationships between the levels of chemical constituents in suspended PM, namely, POC, PN, strong organic ligand, proteins and C.A.A. and depth, as well as carbon fluxes are expressed by empirical power functions. The relationships lead to the possibility that the sinking POM is directly related to the `fresh' suspended POM. This possibility allowed us to estimate the export flux of the strong organic ligand (ThCC) at a 100 m depth as 2.0 mmol m−2 y. The values of the exponent in the relationships between POC, PN, ThCC, proteins and C.A.A. and depth may be related to the rates of degradation of the corresponding chemical components in PM. The results suggest that the rates of decomposition of several chemical components in PM decrease in the following order: proteins, PN>strong organic ligand>combined amino acids>POC.