230Th and 232Th concentrations in the water-column and isotope fluxes into sediment traps, located in the North-Eastern Atlantic Ocean (47°N, 20°W, 4550m depth), were measured in order to determine the scavenging regime and particle dynamic in this area. Isotope dilution mass spectrometry (MAT 262 RPQ) allowed us the measurement of 230Th and 232Th in a single run using a sample volume of about 10 liters of seawater. The amounts of 230Th on the filaments were between 10 fg and 100 fg. To obtain dissolved and particulate concentrations collected seawater was passed through 0.45 µm Nucleopore filters. We found a linear increase with depth of dissolved 230Th concentration from 0.06 dpm/1000kg in surface water to about 0.6 dpm/1000kg at a depth of 4500m. Particulate 230Th concentrations also increase with depth and are about 17% of the total concentration. The concentration of dissolved 232Th is relatively uniform with depth ranging from 0.015 dpm/1000kg to 0.025 dpm/1000kg. The higher particulate percentages of 232Th (32%) relative to 230Th (17%) reflects the input of irreversible bound 232Th in terrestial dust particles via atmospheric rain-out. The linear increase in both dissolved and particulate 230Th concentration with depth can be described in terms of a reversible adsorption/desorption model of thorium onto sinking particles (Bacon and Anderson 1982, Nozaki et al., 1987). We estimated a mean particle settling velocity of S = 3 m/d and rate constants of k1 (adsorption) between 0.3 y-1 and 0.5 y-1 and k-1 (desorption) between 1.1 y-1 and 2.5 y-1, respectively. Particulate 234Th data are in agreement with this constants. The overall residence time of total 230Th is 17 years, which is about the half of the residence time in the open pazific ocean (Bacon and Anderson, 1982). Assuming the same residence time for total 232Th a flux into the ocean of 10 dpm/m2year is necessary, which is conform to estimated holocene 232Th fluxes in this region (Thomson et al., 1993). However, our sediment trap data (one year record) reveal a lack in the thorium fluxes of both isotopes between 50% and 70%. Whether this lack is a problem of sediment trap efficiency or of horizontal advection/diffusion of thorium may be solved with additional measurements of thorium isotopes and of 231Pa. Efforts are made to measure 231Pa in dissolved and particulate water samples with isotope dilution mass spectrometry. First measurements yield a Th/Pa ratio of 2-3 in dissolved water samples and between 40 and 70 in sediment trap material, indicating horizontal export fluxes of 231Pa up to 80% and of 230Th up to 20% which is still too small to explain the lack of the isotopic fluxes into the sediment traps.
Bacon, M.P. & Anderson, R.F., J. Geophys. Res. 87, 2045-2056 (1982).
Nozaki, Y. et al., J. Geophys. Res. 92, 772-778 (1987).
Thomson, J. S. et al., Paleoceanography 8(5), 631-650 (1993).