234U / 238U activity ratios can be considered as a potential tracer of sediment composition and diagenetic transformations. Three types of marine sediments have been studied : low accumulating detritic and terrigenous sediments from the eastern equatorial Pacific (red clay province, Clarion-Clipperton area), calcareous and siliceous biogenic sediments from the north-eastern tropical Atlantic and Indian sector of the southern Ocean respectively, and a hydrogenous polymetallic crust collected in the Romancha trench (eastern equatorial Atlantic). High precision of mass spectrometric measurements allowed a qualitative and quantitative characterisation of the geochemical processes occurring in these different types of sediments.

In the red clays as well as in the polymetallic crust, a 234U diffusion has been observed, leading to a departure of 234U from the sediment to seawater, faster than its production by the 238U decay. The diffusive flux has been calculated using the model of Ku (1965) to represent about 10% of the 234U riverine inputs to the ocean. The existence of this process has been confirmed by Russell et al. (1994) who measured a 234U / 238U activity ratio of 1.535 in interstitial waters of sediments from the Pacific Ocean. With regard to the U oceanic balance, our understanding of the 234U sinks remains incomplete (e.g. hydrothermal circulations, low-temperature alteration of oceanic basalts). Moreover, the precision of the previously determined riverine 234U / 238U activity ratios is insufficient to allow a precise determination of a more precise value for the diffusive flux. Finally, variations of the U riverine inputs to the oceans with glacial / interglacial or orogenic cycles are not excluded. Currently, attempts to determine the precise U oceanic balance remain difficult and may cause erroneous estimates.

Short sediment cores collected with a multitube corer have been analysed to observe the variations of the 234U / 238U activity ratios in the solid phase at depth in the sediment. This ratio appears to be a good proxy to determine the composition of the sediment and to characterise the authigenic origin of the uranium removed at depth when appropriate redox conditions occur. In this latter case, a parallel increase of the total uranium concentrations and of the 234U / 238U activity ratios is observed, confirming the authigenic origin of the uranium trapped in the suboxic sediments of the north-eastern tropical Atlantic.

Radiochemical and isotopic analysis of two long sediment cores collected in a siliceous province of the southern Indian Ocean (Kerguelen plateau) have been completed to test the potential use of the 234U / 238U ratio as a tracer of the variations in surface waters productivity, in response to the quaternary glacial / interglacial cycles. Because of the position of the studied site South of the polar front, these variations have not been observed. Nevertheless, the measurements offered a very distinctive feature : in the sediments associated with high carbonate contents (especially at 123 kyr), the 234U / 238U activity ratios were significantly higher than the seawater typical value, reaching 1.196 (as observed by Henderson and O'Nions (1995) in foraminifera and Bard et al. (1991) in corals). We propose that the large increases in the 234U / 238U activity ratio were caused by a diagenetic secondary 234U exchange between the interstitial waters and the carbonate phase of the sediments.