Distribution of thorium isotopes between dissolved and particulate forms in the deep sea

Abstract

The distribution of 234Th, 23øTh, and 228Th between dissolved and particulate forms was determined in 17 seawater samples from the Guatemala and Panama basins. Sampling was carried out in situ with battery-powered, submersible pumping systems in which the seawater first passed through a Nuclepore filter (1.0-/am pore size) and then through a cartridge packed with Nitex netting that was impregnated with MnO2 to scavenge the dissolved Th isotopes. Natural 234Th was used as the tracer for monitoring the efficiency of scavenging. For all three isotopes, most of the activity was found in the dissolved form. On the average 4% of the 234Th, 15% of the 228Th, and 17% of the 23øTh occurred in the particulate form, though the percentages were found to be strongly dependent on particle concentration. These distributions are not consistent with Chemical scavenging models that assume irreversible uptake of Th on particle surfaces. The results can be explained, however, if continuous exchange of Th isotopes between seawater and the particle surfaces is assumed. Vertical profiles of both particulate and dissolved 23øTh show increasing concentrations with depth, as required by the assumption of reversible exchange. Some of the dissolved 23øTh profiles, however, show a reversal of this trend near the bottom, indicating accelerated scavenging near the water/sediment interface. Kinetics of both adsorption and desorption can be examined if at least two Th isotopes are measured in the same samples. Results show that reaction times are short (a few months) compared to the residence time of suspended matter in the deep ocean (several years), indicating that particles suspended in the deep sea are close to equilibrium with respect to exchange of metals at their surfaces.

A two-tracer (210Po–234Th) approach to distinguish organic carbon and biogenic silica export flux in the Antarctic Circumpolar Current

“…We now develop the non-steady-state solution for 210 Pb and 210 Po of the classical radionuclidescavenging model of Bacon and Anderson (1982), and disregard advection and diffusion, for reasons discussed later…”

“…Direct measurements of particle fluxes with sediment traps (Fischer et al, 1988;Wefer and Fischer, 1991) and indirect estimates from particle reactive radionuclide export rates are well known. Since these radionuclides are produced and decay at known rates, they provide a good tool to quantify particle export (Cherry et al, 1975;Kharkar et al, 1976;Bacon and Anderson, 1982;Fisher et al, 1988). With knowledge of the fluxes and the adsorption behavior of the radionuclides, it is possible to make good estimates of carbon export (Moore and Dymond, 1988;Buesseler et al, 1992).…”

Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes

“…The observed near-linear increases in concentration with depth ( Fig. 2) reflect the effects of reversible scavenging (Bacon and Anderson, 1982) whereby 230 Th, produced uniformly in the water column, is concentrated at depth through cycles of adsorption and desorption with sinking particles. This behavior, especially in the upper 1000 m, has been observed in almost all other 230 Th profiles from the region (Nozaki et al, 1981(Nozaki et al, , 1987Nozaki and Nakanishi, 1985;Roy-Barman et al, 1996).…”

U-Th dating of deep-sea corals

“…Since both 230 Th concentrations and 230 Th/ 232 Th ratios increase with depth (Anderson et al, 1983;Bacon and Anderson, 1982;Cochran, 1992;Cochran et al, 1987;Guo et al, 1995;Huh and Beasley, 1987;Huh et al, 1989;Moran et al, 1995;Nozaki and Horibe, 1983;Nozaki et al, 1987;RoyBarman et al, 1996), we expect much higher initial 230 …”