Large-volume in-situ filtration of deep Pacific waters: mineralogical and radioisotope studies

Abstract

The concentrations of CaCO₃ and radioisotopes, ²³⁴Th, ²³⁰Th, ²³⁹Pu, ²²⁶Ra and ²¹⁰Pb have been measured in particles of >1.2 μm collected from ~4000 liters of Pacific waters (33°N-33°S; 124°W-170°E) at depths 700-5500 m. The mean "ash" concentrations are found to be 5 μg/kg seawater, about 3-4 times lower than that in the surface waters. The particulate concentrations (dpm/kg seawater) of ²³⁹Pu shows a peak at 700-1000 m, whereas the particulate concentration of ²³⁰Th and ²¹⁰Pb which are produced in-situ in seawater increases with depth. These results provide an insight into certain aspects of particle dynamics relevant to modification of trace element composition of seawater due to particle transport and sedimentation flux. The salient features of the results are: (1) The mean settling velocity of particles are in the range of (0.3-2) × 10^-3 cm/sec corresponding to a sedimentation rate of 0.1-0.6 cm/10³ yr. (2) The rate constant for the adsorption of Th on particles is deduced to be 8 × 10^-8 sec^-1 assuming a first-order scavenging model.