EARLY DIAGENETIC PROCESSES IN LABRADOR SEA SEDIMENTS - URANIUM-ISOTOPE GEOCHEMISTRY

The concentration and isotopic composition of U dissolved in pore wate rs from hemipelagic sediments of the Labrador Sea were determined by t hermal ionization mass spectrometry in two 30 cm long box cores. The p resent fluxes of seawater U that diffuses across the sediment-seawater interface are on the order of 2-4 mug/(cm2.ka). This diffusion impose s decreasing gradients of dissolved U downwards, but the U concentrati on in pore waters immediately below the surface is much lower than tha t of open-ocean seawater. This is a primary feature that cannot be exp lained by carbonate precipitation due to decompression during core ret rieval. More likely, it reflects the presence of a stagnant benthic bo undary layer above the sediment - water interface, in which molecular diffusion of U is slower than in the overlying turbulently mixed seawa ter, and (or) of microzones near the interface where U is bioaccumulat ed. Uranium is adsorbed at depths onto 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 c olour transition from brown to grey. Adsorption of U is sufficiently l arge to alter the initial content and the isotopic composition of U in the detrital component. Accumulation of authigenic U on the solid pha se does not proceed at a steady state. This is due to the uneven buria l 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 period s of enhanced primary productivity recurred over the last millenium in the Labrador Sea, inducing U fluxes to the sediments greater than the y are now. Measured pore-water U concentrations are greater than the o verlying seawater at depth in the cores, despite the fact that none of the conditions necessary to release U under reducing conditions are p resent in the sediments. More likely, U-bearing particles < 0.45 Am we re transferred with the solution phase through the filtering device, a rtificially increasing the pore-water U content.