Open University Uranium-Series laboratory    
Earth and Environmental Sciences, The Open University, Milton Keynes, UK    

A comparison of predicted and measured 230Thexcess fluxes on Feni Drift (NE Atlantic) during the late Holocene. (Prof. J Thomson, NOC, IP/791/1103)

 

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’ 230Thxs 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.

 
 
 
 
Sediment flux calculations for an area in the NE Atlantic. Circles indicate sediment flux calculated from 230Thxs, filled squares are sediment flux calculated from 14C accumulation rates. Vertical shading indicates cores off the Feni Drift and horizontal shading indicates Feni Drift water depths. Both symbols close together means that the constant flux assumption is justified, 230Thxs, larger than 14C indicates sediment focussing, 230Thxs, smaller than 14C indicates sediment winnowing., from Thomson et al., 2006

 

 

 
   

This singular value is similar to the mean Holocene flux reported for the northeast Atlantic by previous work that utilized the 230Thxs 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 230Thxs 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 230Thxs data are also consistently higher than the singular value measured elsewhere in the northeast Atlantic.

 
   

Anderson et al. 1983a Earth Planet. Sci. Lett. 62 7–23
Anderson et al. 1983b Earth Planet. Sci. Lett. 66 73–90
Bacon 1984 Isotope Geoscience 2 97–111
Chen et al. 1986 Earth Planet. Sci. Lett. 80 241–251
Delanghe et al. 2002 Mar.Chem. 80 79–93
Robinson et al. 2004 Geochim. Cosmochim. Acta 68 1777–1789
Suman and Bacon 1989 Deep-Sea Res. 36 869–878
Thomson et al. 2006 Earth Planet. Sci. Lett. 242 170-185

 
   
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© Peter van Calsteren
Last updated: 23 December, 2011 10:54