In seawater, uranium has a long residence time and a constant U concentration and hence a constant production of its daughter radionuclide ²³⁰Th (Chen, et al., 1986, Delanghe, et al., 2002, Robinson, et al., 2004). However, unlike U, ²³⁰Th is particle-reactive and is rapidly removed from seawater with the sediment rain. This results in a short residence time for ²³⁰Th in the ocean water column (10-40 y; Anderson, et al., 1983a and b) and as a result the flux of ²³⁰Th to the sea floor depends mainly on the depth of the overlying water column (Bacon, 1984). An important application is in the detailed assessment of changes in sediment accumulation over time in sediment cores from the open ocean. The ²³⁰Th method has a further application in a situation where a close balance between production of ²³⁰Th in the overlying water column and its inventory in the underlying sediments does not hold. This is on sediments from drifts or ‘contourites’, the constructional deep-sea sedimentary deposits that develop from sustained bottom currents, with the preferential re-deposition or ‘focusing’ of sediment into a localized area. Suman and Bacon (1989) proposed a focussing factor as a measure of the additional amount of sediment focused into the area and deposited on a contourite calculated as the amount of ²³⁰Th actually present in a sediment section divided by the amount that could have been produced in the overlying water column over the same period of time.

Our work (Thomson et al., 2006) tests the validity of the sediment focusing concept by determining the rate of ²³⁰Th deposition in a set of cores from the northeast Atlantic that includes a sub-set from the Feni Drift contourite. Sediment accumulation rates applicable to the Holocene are first calculated from profiles of radiocarbon analyses in the cores. ²³⁰Th measurements were undertaken on the sediments from fifteen box cores recovered between 48-58ºN and 12-22ºW at water depths of 1100-4500 m in the northeast Atlantic. Eight of the cores were from Feni Drift in the Rockall Trough at depths of 1700-2500 m and the remaining seven formed an approximate north/south transect at various water depths from the abyssal plain north on to the Rockall Plateau. Holocene sediment accumulation fluxes were established from profiles of bulk sediment ¹⁴C age against depth to be in the range 3-7 cm ky^-1 off the Drift and 4-23 cm ky^-1 on the Drift. When compared with these ¹⁴C-based sediment accumulation rates, precision measurements of ²³⁰Th reveal that the ²³⁰Th levels present in the sediments match or slightly exceed the potential supply from the vertically overlying water column in a majority of the cores. One core from the open abyssal plain that had the lowest ¹⁴C-based accumulation rate in the entire set also had a close balance between predicted and measured ²³⁰Th values. With the assumption that only ²³⁰Th produced in the overlying water column is supplied to the sediments, the ‘excess’ ²³⁰Th_xs values in this core and the other six containing carbonate ooze sediments all imply a rather constant regionally averaged sedimentation flux of 2.0± 0.2 (1s) g cm^-2 ky^-1 rather than matching the ²³⁰Th fluxes implied by the ¹⁴C data.

This singular value is similar to the mean Holocene flux reported for the northeast Atlantic by previous work that utilized the ²³⁰Th_xs method, although it is somewhat lower than estimates for the Holocene based on oxygen isotope stratigraphy or radiocarbon methods. Consistently higher and more variable regional sediment accumulation fluxes are calculated from the Feni Drift ²³⁰Th_xs data with the constant flux assumption (average 2.8 ± 0.4 g cm^-2 ky^-1), some 40% higher than the constant value measured in the other cores.
It seems likely that the mean sediment accumulation flux at Feni Drift inside Rockall Trough is consistently higher than on the open ocean margin of the basin, so that the regionally averaged sedimentation fluxes indicated for the Drift by the measured ²³⁰Th_xs data are also consistently higher than the singular value measured elsewhere in the northeast Atlantic.