Chemical and radiochemical investigations of surface and deep particles of the Indian Ocean

Abstract

The distribution of “ash” (the non-combustible fraction of marine suspended matter) and concentrations of particulate Al, Ca, Fe, Cr, Ni, Cu, Sr and 234Th in surface waters and of 210Pb, 230Th and 234Th in two vertical profiles (385–4400 m) of the Indian Ocean are reported. The ash concentrations in surface waters follow the primary productivity pattern, with higher abundances in samples south of 40°S and lower concentrations in the equatorial and subtropical regions. Opaline silica and CaCO 3 are the dominant components of the ash in samples from >40°S and from 7°N to 39°S, respectively. Aluminosilicates are only a minor constituent of the surface particulate matter. The metal/Al ratios in the surface particles are significantly higher compared to their corresponding crustal ratios for all the metals analyzed in this work. Comparison of enrichment factors between marine aerosols, plankton and surface oceanic particles, seem to indicate that this high metal/Al ratio in surface particles most likely arises from their involvement in marine biogeochemical cycles. Particulate 234Th activity in surface waters parallels the ash abundance implying that its scavenging efficiency from surface waters depends on the particulate concentration. The particulate 230Th and 210Pb concentration profiles increase monotonously with depth. It is difficult to ascribe this increase to a process other than the in-situ vertical scavenging of 230Th and 210Pb from the water column by settling particles. The mean settling velocities of particles calculated from the particulate 230Th data using a one-dimensional settling model is about 2 × 10 −3 cm/s. The settling velocity computed from the particulate 230Th profiles does not appear to be compatible with the particulate 210Pb depth profiles; one possible explanation to account for the disparity would be that 230Th and 210Pb are scavenged by different size populations of particles. On the whole, the geographic distribution of particulate matter, their composition and settling velocities in the Atlantic, Pacific and Indian Oceans are similar indicating that they are controlled by quite similar processes in the marine hydrosphere.