Pascal Lecroart

Along-axis dynamic topography constrained by major-element chemistry

Abstract 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.

Modeling sensitivity of biodiffusion coefficient to seasonal bioturbation

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 L 2 /Db lower than 125, representing the decay constant of the radionuclide. 75 % of the 234 Th-derived Db, and 79 % of the 7 Be-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 234 Th and 7 Be 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 234 Th is detected for marine environments with L 2 /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 ( 210 Pb and 228 Th) 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.

Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid

Abstract The major-element composition of Mid-Ocean Ridge basalts (MORB) shows geochemical variations controlled by the temperature, pressure and source composition produced by convection in the mantle beneath the spreading centers. The long-wavelength signal of the geoid reflects deep mantle processes. In this paper, we characterize the correlations between the major-element chemistry of MORB and the geoid anomalies at different wavelengths to determine the extent to which the major-element composition of the MORB reflects deep mantle circulation. The chemical effects of the low-pressure fractionation have been corrected using a new method based on the FeOt/MgO ratio. The fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the MgO denominator converge to a common point with ∼8% MgO. The separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99% level with the axial depth for both the Mid-Atlantic Ridge (MAR) and the East Pacific Ridge (EPR) and with the geoid only for the MAR. The latter correlation suggests that the major-element composition of the MORB is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle. The lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the K2O/TiO2 ratio and the geoid indicates that either source composition does not influence MORB chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle.

Silicic acid flux to the ocean from tidal permeable sediments: A modeling study

Sandy sediments of tidal beaches are poor in reactive substances because they are regularly flushed by significant flow caused by tidal forcing. This transport process may significantly affect the flux of reactive solutes to the ocean. A two dimensional model coupling the Richards equation that describes the flow in permeable sediments and the conservation equation of the silicic acid was developed to simulate the evolution of the silicic acid concentration into a variably saturated porous media submitted to tidal forcing. A detailed algorithm of drainage zone under tidal forcing and numerical methods needed to solve it are properly presented. Flux to the ocean has been estimated. The silicic acid concentration displays a permanent lens with low silicic acid concentration at the top of the tidal zone. This lens that results from the tidal forcing, presents weak variations of area during the tidal cycle. Silicic outflux to the ocean increases with increasing beach slope, hydraulic conductivity and tidal range. Simulations reveal that the total silicic acid flux to the ocean from the coastal marine sands can be considered as significant compared to the flux supplied by the rivers. These results may alter the previously published global budget of the silicic acid to the ocean.

Experimental Assessment of the Effects of Temperature and Food Availability on Particle Mixing by the Bivalve Abra alba Using New Image Analysis Techniques.

The effects of temperature and food addition on particle mixing in the deposit-feeding bivalve Abra alba were assessed using an experimental approach allowing for the tracking of individual fluorescent particle (luminophore) displacements. This allowed for the computations of vertical profiles of a set of parameters describing particle mixing. The frequency of luminophore displacements (jumps) was assessed through the measurement of both waiting times (i.e., the time lapses between two consecutive jumps of the same luminophore) and normalized numbers of jumps (i.e., the numbers of jumps detected in a given area divided by the number of luminophores in this area). Jump characteristics included the direction, duration and length of each jump. Particle tracking biodiffusion coefficients (Db) were also computed. Data originated from 32 experiments carried out under 4 combinations of 2 temperature (Te) and 2 food addition (Fo) levels. For each of these treatments, parameters were computed for 5 experimental durations (Ed). The effects of Se, Fo and Ed were assessed using PERmutational Multivariate ANalyses Of VAriance (PERMANOVAs) carried out on vertical depth profiles of each particle mixing parameter. Inversed waiting times significantly decreased with Ed whereas the normalized number of jumps did not, thereby suggesting that it constitutes a better proxy of jump frequency when assessing particle mixing based on the measure of individual particle displacements. Particle mixing was low during autumn temperature experiments and not affected by Fo, which was attributed to the dominant effect of low temperature. Conversely, particle mixing was high during summer temperature experiments and transitory inhibited by food addition. This last result is coherent with the functional responses (both in terms of activity and particle mixing) already measured for individual of the closely related clam A. ovata originating from temperate populations. It also partly resulted from a transitory switch between deposit- and suspension-feeding caused by the high concentration of suspended particulate organic matter immediately following food addition.

Cadmium sulfide nanoparticles trigger DNA alterations and modify the bioturbation activity of tubificidae worms exposed through the sediment

Abstract To address the impact of cadmium sulfide nanoparticles (CdS NPs) in freshwater ecosystems, aquatic oligochaete Tubifex tubifex were exposed through the sediment to a low dose (0.52 mg of 8 nm in size of CdS NPs/kg) for 20 days using microcosms. Cadmium (Cd) was released from the CdS NPs-contaminated sediment to the water column, and during this period the average concentrations of Cd in the filtered water fraction were 0.026 ± 0.006 µg/L in presence of oligochaetes. Similar experiments with microparticular CdS and cadmium chloride (CdCl2) were simultaneously performed for comparative purposes. CdS NPs exposure triggered various effects on Tubifex worms compared to control, microsized and ionic reference, including modification of genome composition as assessed using RAPD-PCR genotoxicity tests. Bioaccumulation levels showed that CdS NPs were less bioavailable than CdCl2 to oligochaetes and reached 0.08 ± 0.01 µg Cd/g for CdS NPs exposure versus 0.76 ± 0.3 µg Cd/g for CdCl2 exposure (fresh weight). CdS NPs altered worm’s behavior by decreasing significantly the bioturbation activity as assessed after the exposure period using conservative fluorescent particulate tracers. This study demonstrated the high potential harm of the CdS nanoparticular form despite its lower bioavailability for Tubifex worms.