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Biodiffusion coefficient is the predominant parameter used to constrain biological activity in marine sediments. Bioturbation characterization is important because of the dominant role it plays on the flux determination through the sediment-water interface. Biological mixing is quantified through models of radionuclides diagenesis by both a biodiffusion coefficient (Db) and a mixed depth (L) under the basic steady-state assumption. Based on a new global compilation of radionuclide data in marine sediments and on previously published modeling results, we show that short-live radionuclides are perfectly devoted to quantify biological mixing for sediments associated with λL2/Db lower than 125, λ representing the decay constant of the radionuclide. 75 % of the 234Th-derived Db, and 79 % of the 7Be-derived Db are concerned by this result. However, as transient regimes prevail within marine sediments, especially at a seasonal time scale and within the coastal and shelf environment, it is necessary to model their impacts on Db calculations. A transient model of radionuclide decay and transport is therefore used to perform extensive sensitivity tests of Db calculations in respect to seasonal mixing. Numerical tests of seasonal sensitivity indicate that 234Th and 7Be are the most sensitive tracers to seasonal biological mixing: the steady-state assumption remains valid and applicable for most of natural marine environments. However, systematic tests reveal that incorrect seasonal sensitivity of 234Th is detected for marine environments with λL2/Db lower than 10 and greater than 1000. In these cases, the apparent seasonal variations of the biological activity need to be corrected. The main parameter in selecting the appropriate radionuclide for field analyses is the dimensionless pulse, which defines the relative importance of decay time scale relative to the seasonal time scale. This pulse controls the relative extension of the domain of satisfactory sensitivity. Consequently, long-lived radionuclides (210Pb and 228Th) are not appropriate for predicting seasonal mixing, except for specific environments which display an unexpected sensitivity to seasonal mixing. These marine environments are characterized by a moderate biological mixing and a deep mixed-layer. [ABSTRACT FROM AUTHOR]

Variations in thickness and density of both the crust and the associated uper mantle have been derived from a compilation of zero-age major-element composition along the Mid-Atlantic Ridge, the East Pacific Rise and the Southeast Indian Ridge. Assuming isostatic compensation, the axial depth computed from major-element data correctly agrees with observed axial depth. Discrepancies are essentially located near hotspots such as Iceland and Azores. The residual topography, expressed as the difference between observed and compensated axial depth has a root-mean-square of 426 m along the three spreading axes, which is below the resolution power of the method. This insignificant topography, which is assumed to contain the dynamic surface topography associated with mantle convection, bears an important constraint on the relative variations of the dynamic topography predicted by models of mantle circulation.

Particle mixing induced by the deposit-feeding bivalve Abra alba was assessed using a new experimental approach allowing for the tracking of individual particle displacements. This approach combines the adaptation of existing image acquisition techniques with new image analysis software that tracks the position of individual particles. This led to measurements of particle mixing fingerprints, namely the frequency distributions of particle waiting times, and of the characteristics (i.e. direction and length) of their jumps. The validity of this new approach was assessed by comparing the so-measured frequency distributions of jump characteristics with the current qualitative knowledge regarding particle mixing in the genus Abra. Frequency distributions were complex due to the coexistence of several types of particle displacements and cannot be fitted with the most commonly used procedures when using the Continuous Time Random Walk (CTRW) model. Our approach allowed for the spatial analysis of particle mixing, which showed: 1) longer waiting times; 2) more frequent vertical jumps; and 3) shorter jump lengths deep in the sediment column than close to the sediment-water interface. This resulted in lower DbX and DbY (vertical and horizontal particle mixing bioffusion coefficients) deep in the sediment column. Our results underline the needs for: 1) preliminary checks of the adequacy of selected distributions to the species/communities studied; and 2) an assessment of vertical changes in particle mixing fingerprints when using CTRW. [ABSTRACT FROM AUTHOR]