Early diagenetic processes in Labrador Sea sediments. Uranium-isotope geochemistry

The concentration and isotopic composition of U dissolved in pore waters from hemipelagic sediments of the Labrador Sea were determined by thermal ionization mass spectrometry in two 30 cm long box cores. The present fluxes of seawater U that diffuses across the sediment-water interface are of the order of 2-4 μg/(cm2.ka). This diffusion imposes decreasing gradients of dissolved U downwards, but the U concentration in pore waters immediately below the surface is much lower than that of free seawater. This cannot be explained by carbonate precipitation due to decompression during core retrieval. More likely, it reflects the presence of a stagnant benthic boundary layer above the sediment, in which molecular diffusion of U is slower than in the overlying turbulently mixed water, or of microzones near the interface where U is bioaccumulated. Uranium is adsorbed at depths onto the the solid phase in response to changes in the redox conditions within the sediments. In the Labrador sea, this occurs at the onset of iron reduction and corresponds to a colour transition from brown to grey. Adsorption of U is sufficiently large to alter the initial content and isotopic composition of U in the detrital component. Accumulation of authigenic U on the solid phase does not proceed at a steady state. This is due to uneven burial rates of organic matter, which is essential to the establishment of redox conditions appropriate for U reduction, and concomitant stepwise displacement of the redox fronts. This indicates that discrete periods of enhanced primary productivity recurred over the last millennium in the Labrador Sea, inducing U fluxes to the sediments greater than they are now. Measured pore water U concentrations are greater than in the overlying seawater at depth in the cores, despite the fact that none of the conditions necessary to release U under reducing conditions are present in the sediments. More likely, U-bearing particles of <0.45 μm were transferred with the solution phase through the filtering device, artificially increasing the pore-water U content. 35 refs., 4 figs., 2 tabs