Holocene sediment deposition on a NE Atlantic transect including Feni Drift quantified by radiocarbon and 230Thexcess methods

Thomson, J., Green, D.R.H., van Calsteren, P., Richter, T.O. and van Weering, T.C.E. (2006) Holocene sediment deposition on a NE Atlantic transect including Feni Drift quantified by radiocarbon and 230Thexcess methods. Earth and Planetary Science Letters, 242 (1-2), 170-185. (doi:10.1016/j.epsl.2005.11.034).

Abstract

Radiocarbon and 230Thexcess measurements were undertaken on the sediments from fifteen box cores recovered between 48–58°N and 12–22°W 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 Rockall Plateau. Mean Holocene sediment accumulation fluxes for all cores were established from profiles of bulk sediment radiocarbon 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 radiocarbon-based sediment accumulation rates, precision measurements of 230Th reveal that the 230Thexcess 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 radiocarbon-based accumulation rate in the entire set also had a close balance between predicted and measured 230Thexcess values. With the assumption that only 230Thexcess produced in the overlying water column is supplied to the sediments, the 230Thexcess 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 (1?, n = 7, min. 1.8, max. 2.3) g cm? 2 ky? 1 rather than matching the 230Thexcess fluxes implied by the radiocarbon data. This singular value is similar to the mean Holocene flux reported for the northeast Atlantic by previous work that utilized the 230Thexcess method, although it is somewhat lower than estimates for the Holocene based on oxygen isotope stratigraphy or radiocarbon methods. The current transport processes that result in the high sediment accumulation rates on Feni Drift have also resulted in a compositional fractionation of the sediments. All sediments from Feni Drift have lower CaCO3 contents than the remainder of the sediments studied, and these are accompanied by lower 230Thexcess specific activities than are found at comparable water column depths elsewhere on the transect. Consistently higher and more variable regional sediment accumulation fluxes are therefore calculated from the Feni Drift 230Thexcess data with the constant flux assumption (average 2.8 ± 0.4 g cm? 2 ky? 1; n = 8, min. 2.3, max. 3.3), 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 230Thexcess data are also consistently higher than the singular value measured elsewhere in the northeast Atlantic.

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