Mobility of Mo, U, As, and Sb within modern turbidites

We present and discuss porewaters and sediment data of Mn, Fe, Mo, U, As, and Sb within four recent turbidites from the Capbreton Canyon (SW-France). The deposition of turbidites on continental margins exerts strong influence on the fate of trace-metals, causing trace-metal migration and/or immobilization. The down-core distribution of the elements here depends entirely of ongoing diagenetic processes. Rather than the diffusion of O2, the persistence of Fe/Mn-oxides under anoxic conditions led to anoxic reoxidation of numerous species as NH4+, reduced S compounds, and trace-metals. Metal-oxides thus control the immobilization of Mo and As in the solid phase, and the remobilization of U and Sb to the porewaters. Although the low sulphide production prevents anoxic authigenesis of Mo and As, reducing conditions below each horizon rich in Mn/Fe-oxides resulted in U and Sb accumulation. The exact mechanisms of Sb immobilization are not known, but it is most certainly different from that of As.The profile shapes of Mn and U were used to estimate the age of the turbidite deposits, which cannot be determined by conventional radionuclide methods. Our interpretations, however, depend on calculations of diffusion coefficient, particularly for U. A simple diffusive model applied to Mn suggested that the age of the latest deposit (noted T1) was ~ 22 months. This date is close to the time elapsed between the core sampling and the last known turbiditic event in the Capbreton Canyon (i.e., Martin Storm, ~ 18 months). Using the U data set and depending on diffusion coefficients used, the estimated ages of the deeper layers were 4–7 yr for T2 and 7.5–14 yr for T3. The reliability and precision of this new approach to date sediments at short timescales is limited by the ignorance of a precise value for U diffusion coefficient. Reducing uncertainty on U diffusion coefficient will greatly support application of the proposed approach.