Vertical distributions of the strong organic ligand in the twilight zone of Southern Hemisphere Ocean particulate matter

Vertical distributions of the strong organic ligand (SOL) in oceanic particulate matter (PM), as determined from the amount of thorium adsorption onto PM in 0.1 M HCl, were measured for the twilight zone (100- to 1000-m depth) of the Southern Hemisphere Ocean (Pacific, Atlantic and Indian) mid-latitude regions. The SOL concentrations in South Pacific PM collected at a depth of 100 m, ranging from 0.56 to 5.4 nM, showed a longitudinal variation with low concentrations occurring from 180° to 150°W in the South Pacific subtropical gyre and high in the Tasman Sea and the eastern South Pacific. The SOL concentrations in 100-m-depth South Atlantic PM, ranging from 1.19 to 2.5 nM, showed a longitudinal gradient with low in the west and high in the east. The SOL concentrations in 100-m-depth Indian Ocean PM, ranging from 1.16 to 5.25 nM, showed a longitudinal variation with a maximum near 69°E and low in the eastern Indian Ocean. The SOL concentrations in PM of the South Pacific and Atlantic twilight zones decreased with increasing depth, whereas the high SOL concentrations in 1000 m depth PM occurred in the central Indian Ocean. Each depth-decreasing vertical profile of the SOL concentration in PM except the central Indian Ocean was expressed by the empirical equation CSOL = CSOL,100 (z/100)−b. The dimensionless scaling factor b in the South Pacific, South Atlantic and Indian Ocean varied from 0.83 to 2.5, from 0.54 to 0.86 and from 0.24 to 0.91, respectively, and was positively correlated with the SOL concentration in PM at 100 m in each sea area. This is a first global measurement of the particulate SOL in the twilight zone. The particulate SOL is a useful tool for better understanding of particle dynamics in the twilight zone, if we have more detail knowledge on spatial and temporal variability of the particulate SOL.