Uranium relocations and derivation of quasi-isochrons for a turbidite/pelagic sequence in the Northeast Atlantic

Abstract

The systematics of U relocation in the organic-rich distal turbidite sequence of sediments of the Madeira Abyssal Plain are investigated by examination of solid phase [U]/depth and selected pore water [U]profiles. The characteristic U profile developed in these homogeneous turbidites by oxidation fronts can be recognised in examples emplaced up to 750 ky ago. Profiles in individual turbidites can be correlated in cores over 100 km apart. In the surficial turbidite, where an oxidation front is currently active, the pore water profiles from a box core and an in situ sampler show maximum pore water [U]in the vicinity of the front with fluxes both upwards and downwards. The pore water profiles do not, however, support the loss of U to bottom waters implied by solid phase mass balances in this surficial turbidite, and in weakly reducing conditions below the active front, the pore water [U]is as high as that of sea water (3.3 μg · l −1). Examples from two buried turbidites where the oxidation fronts are extinct show excess U in the peaks over that expected from the relocation process, so that these turbidites appear to have gained U, presumably from bottom water, during the time their fronts were active. In one of these examples, the pore water U content now appears proportional to the authigenic sediment [U], although this may be an oxidation artifact. The immobility of the solid phase profiles in reducing conditions is demonstrated by the construction of “isochrons” based on 230Th ingrowth from 238U, where the data from each turbidite conform to a distinctive line. Values of time calculated from the isochrons have poor chronological significance because of violations of the model requirements when the fronts were active, but a 600-ky-old turbidite conforms to the equiline and confirms good chemical closure in the 238U → 230Th decay sequence in the long term.