Sedimentary processes in the Thau Lagoon (France): From seasonal to century time scales

As a part of the MICROBENT programme, an investigation of the sedimentation framework was carried out at the water-sediment interface in the Thau Lagoon (French Mediterranean coast). Two main sites, C4 in the middle of the lagoon and C5 near oyster farms, were visited six times between December 2001 and May 2003. Interface sediments were studied using classical sedimentology parameters (radiography RX, grain size distribution) and analysis of selected radionuclides (234Th, 7Be, 210Pb, 226Ra). On a century time scale, excess 210Pb (210Pbxs) presents classical profiles with an upper mixed layer, followed by an exponential decrease of activities to undetectable levels below 20 – 30 cm. At the central site, C4, cores seem to register episodic changes in mean grain size, presenting recurrently peaks. The upper 10 cm of 210Pbxs profiles at site C5 exhibit a mixed layer associated with coarser sediments: this could be related to biological activity. Sedimentation rates derived from 210Pbxs varied from 0.15 cm y−1 at the edge of the basin, to 0.25 cm y−1 at the central site. On a seasonal time scale, 234Th and 7Be both show significant variations in activities and in penetration within the sediment. Bioturbation rates derived from both radionuclides agree well and range between 1–10 cm2 y−1 at site C4 and 1–31 cm2 y−1 at site C5. 234Th and 7Be fluxes at the water-sediment interface show too seasonal variations, more pronounced for site C5. This latter site presents especially a higher variability that is well marked with season, probably in relation with its position near oyster farms

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 radionu-clides are perfectly devoted to quantify biological mixing for sediments associated with L2/Db lower than 125, 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 assumptio

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 λL2/Db lower than 125, λ representing the decay constant of the radionuclide. 75 % of the234Th-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

Experimental assessment of particle mixing fingerprints in the deposit-feeding bivalve Abra alba (Wood)

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

Transient States in Diagenesis Following the Deposition of a Gravity Layer: Dynamics of O2, Mn, Fe and N-Species in Experimental Units

International audienceBiogeochemical processes induced by the deposition of gravity layer in marine sediment were studied in a 295-day experiment. Combining voltammetric microelectrode measurements and conventional analytical techniques, the concentrations of C, O2, N-species, Mn and Fe have been determined in porewaters and sediments of experimental units. Dynamics of the major diagenetic species following the sudden sediment deposition of few cm-thick layer was explained by alternative diagenetic pathways whose relative importance in marine sediments is still a matter of debate. Time-series results indicated that the diffusion of O2 from overlying waters to sediments was efficient after the deposition event: anoxic conditions prevailed during the sedimentation. After a few days, a permanent oxic horizon was formed in the top few millimetres. At the same time, the oxidation of Mn2+ and then Fe2+, which diffused from anoxic sediments, contributed to the surficial enrichment of fresh Mn(III/IV)- and Fe(III)-oxides. Vertical diffusive fluxes and mass balance calculations indicated that a steady-state model described the dynamic of Mn despite the transitory nature of the system. This model was not adequate to describe Fe dynamics because of the multiple sources and phases of Fe2+. No significant transfer of Mn and Fe was observed between the underlying sediment and the new deposit: Mn- and Fe-oxides buried at the original interface acted as an oxidative barrier to reduced species that diffused from below. Nitrification processes led to the formation of a NO3−/NO2− rich horizon at the new oxic horizon. Over the experiment period, NO3− concentrations were also measured in the anoxic sediment suggesting anaerobic nitrate production

Ideas and perspectives: Sea-level change, anaerobic methane oxidation, and the glacial-interglacial phosphorus cycle

International audienceThe oceanic phosphorus cycle describes how phosphorus moves through the ocean, accumulates with the sediments on the seafloor, and participates in biogeochemical reactions. We propose a new two-reservoir scenario of the glacial-interglacial phosphorus cycle. It relies on diagenesis in methane hydrate-bearing sediments to mobilize sedimentary phosphorus and transfer it to the oceanic reservoir during times when falling sea level lowers the hydrostatic pressure on the seafloor and destabilizes methane hydrates. The stock of solid phase phosphorus mobilizable by this process is of the same order of magnitude as the dissolved phosphate inventory of the current oceanic reservoir. The potential additional flux of phosphate during the glacial period is of the same order of magnitude as pre-agricultural, riverine dissolved phosphate fluxes to the ocean. Throughout the cycle, primary production assimilates phosphorus and inorganic carbon into biomass, which, upon settling and burial, returns phosphorus to the sedimentary reservoir. Primary production also lowers the partial pressure of CO2 in the surface ocean, potentially drawing down CO2 from the atmosphere. Concurrent with this slow "biological pump", but operating in the opposite direction, a "physical pump" brings metabolic CO2-enriched waters from deep-ocean basins to the upper ocean. The two pumps compete, but the direction of the CO2 flux at the air-sea interface depends on the nutrient content of the deep waters. Because of the transfer of reactive phosphorus to the sedimentary reservoir throughout a glaciation cycle, low-phosphorus and high-CO2 deep waters reign at the beginning of a deglaciation, resulting in rapid transfer of CO2 to the atmosphere. The new scenario provides another element to the suite of processes that may have contributed to the rapid glacial-interglacial climate transitions documented in paleo-records

Influence of the mud shrimp Upogebia pusilla (Decapoda: Gebiidea) on solute and porewater exchanges in an intertidal seagrass (Zostera noltei) meadow of Arcachon Bay: An experimental assessment

The aim of the present study was to investigate the influence of the mud shrimp Upogebia pusilla on porewater and solute exchanges in an intertidal Zostera noltei meadow of Arcachon Bay. Laboratory experiments carried out during three seasons showed that U. pusilla strongly enhanced sediment porewater exchange rates (averaging 26, 45 and 71 L m− 2 d− 1 in winter, spring and summer, respectively) in comparison to uninhabited sediments for which these rates were moderate (averaging 9, 9 and 16 L m− 2 d− 1 in winter, spring and summer, respectively). Total Oxygen Uptakes (TOUs) were increased by factors around 2.5 during the three studied seasons whilst oxygen consumption by the shrimp accounted for 64, 28 and 21% of TOU in winter, spring and summer, respectively. U. pusilla also significantly enhanced the uptake of nitrate (up to 13.8-fold in summer) and the release of ammonium (up to 3.0-fold in summer) and silicate (up to 3.8-fold in spring) by the sediment. Overall, our results highlight the key role of U. pusilla in controlling oxygen and nutrient fluxes in Arcachon Bay. Because U. pusilla populations are predominantly found in the sediments of Z. noltei meadows, the full assessment of the functional effects resulting from the decline of this meadow should include the effects associated to the disappearance of U. pusilla as well

The sandy channel-lobe depositional systems in the Gulf of Cadiz: Gravity processes forced by contour current processes

International audienceThe sedimentation in the Gulf of Cadiz (NE Atlantic Ocean) is significantly controlled by the Mediterranean Outflow Water (MOW). Along its pathway onto the continental slope, the MOW is canalized by contourite channels, some of them feeding gravity sandy channel-lobe depositional systems firstly recognized in previous study [Habgood et al., 2003. Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic. Sedimentology 50(3), 483-510.].Using very high resolution acoustic data and cores, a detailed characterization and a new evolution pattern of these channel-lobe depositional systems is established. Complex internal geometry of the lobes shows several depositional units revealing a polyphase evolution of these systems, with a general progradation punctuated by retrogradation and avulsion phases. A gravity origin controlled by contouritic processes and climatic changes is demonstrated for the feeding and the evolution of these sandy channel-lobe depositional systems. Climate oscillations, via the MOW variations, act as a major forcing of the activity of the channel-lobe depositional systems during the Late Quaternary

Bioturbation at the water-sediment interface of the Thau Lagoon: impact of shellfish farming

Quantifying of sediment reworking processes provides new insights into benthic ecosystem functioning of the Thau lagoon, an important European shellfish farming area. In order to evaluate bioturbation rates of surface sediments, profiles of 7Be (half-life: 53 days) and 234Th (half-life: 24.1 days) were measured in cores collected since December 2001 to December 2006. Several sites were selected to sample the diversity of the Thau lagoon: C4, T10, and T11 in the middle of the lagoon, C5, T7 and T8 nearby oyster farming, T2 and T4 in the western edge, T12 in the eastern part closed to industry. 234Th in excess (234Thxs; i.e. supplied to sediment by settling particles) and 7Be both show seasonal variations in activities and in penetration within sediment. Taking into account the moderate sedimentation rates of the Thau lagoon, sites (0.1–0.4 cm per year), the penetration of both short-lived radionuclides to variable depths, from 1 up to 8 cm, indicates efficient biological mixing of upper sediments. Bioturbation rates (Db) to the distribution of excess 234Th (234Pb-Db) and of 7Be (7Be-Db) range between 1 and 35 cm2 y−1, depending on site and season. Surface sediment mixing of the Thau lagoon is primarily controlled by the quality of particle input, i.e. the food supply to the benthic fauna, mainly governed by the hydrological and seasonal conditions. But locally shellfish production is a key parameter that influences bioturbation through biodeposition

The dynamics of phosphorus in turbid estuarine systems: Example of the Gironde estuary (France)

Suspended particles and surface waters were collected in the Gironde estuary (southwestern France) along the salinity gradient. Dissolved inorganic phosphorus (DIP) was analyzed in the filtered surface waters. The suspended particles were sequentially leached to determine five fractions of phosphorus: exchangeable or loosely sorbed P, reactive Fe-bound P, refractory Fe oxide-bound P, P associated with apatite and carbonates, and organic P. Experiments were conducted to determine the effects of temperature, salinity, and DIP concentration on phosphorus sorption/desorption processes. The concentration and distribution of particulate phosphorus was homogeneous along the salinity gradient of the estuary, whereas it was variable in time and space in the freshwater part. DIP showed a rapid desorption of loosely sorbed P at low salinities. Desorption depended on suspended particle concentration, but not on DIP concentration. At high suspended particle concentrations, some exchangeable P remained linked to particles, until suspended particle concentration decreased downstream of the estuary. This delayed desorption of bioavailable P occurs in waters in which the penetration of light is sufficient to support photosynthesis and probably plays a major role in primary production at high salinities. Mineralization of organic phosphorus also releases available phosphorus, but this process is slow relative to the water residence time in the estuary. The budget of DIP, the loss of particulate P from the organic fraction, and the water residence time suggest that the dynamics of P in the Gironde estuary is well explained by the two processes of P release from particles to waters without P uptake