High-energy tidal beaches are exposed to strong physical forcings. The submarine groundwater discharge (SGD) that occurs in intertidal sandy sediments includes both terrestrial, fresh groundwater flow and seawater recirculation, and plays a significant role in regulating biogeochemical cycles in some coastal zones. In this transition zone between land and sea, complex biogeochemical reactions alter the chemical composition of pore waters that discharge to the coastal ocean. Recent studies highlight that SGD can be a significant source of carbon to the coastal ocean but very few have investigated SGD in high-energy environments. We have characterized the dissolved carbon dynamics in such a high-energy environment (Truc Vert Beach, SW France) through pore-water sampling in key compartments of the SGD system. Dissolved organic carbon (DOC), pH, total alkalinity (TA), and the isotopic composition of dissolved inorganic carbon (δ13C-DIC) were measured in pore waters sampled at regular intervals between 2011 and 2014 in the intertidal zone of the beach, the mixing zone of the subterranean estuary (STE), and the freshwater aquifer upstream from the beach. Results reveal that SGD exports dissolved carbon mostly as DIC to the Aquitaine coast, some of which originates from the aerobic respiration of marine organic matter within the beach aquifer. This is highlighted by the reverse spatial trend of DOC, which is consumed, and DIC, which is produced. Saline pore waters expelled from the beach through tidally-driven recirculation of seawater provide about 4400 tons of carbon per year to the coastal zone of the 240-km long Aquitaine sandy coast. Terrestrial groundwater, characterized by high pCO2 values, is also a significant contributor to the DIC flux to the coastal ocean (16,200 tons per year). This flux is abated by CO2 evasion in the upper beach, at the onset of the salinity gradient in the STE, and within the surficial freshwater aquifer along the forest-beach transect below the coastal foredune. Accordingly, the DIC:TA ratio evolves to below 1, suggesting that this SGD increases the buffer capacity of coastal seawater against acidification. This study demonstrates that high-energy beaches are active vectors of DIC from the land to the coastal ocean as well as significant sources of CO2 to the atmosphere, and must therefore be taken into consideration in SGD carbon budgets.

Abstract. The 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/ 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.

Sandy beaches are places of active organic matter mineralization due to water renewal providing organic matter and electron acceptors in the porous and permeable sands. Recycled biogenic compounds are efficiently transferred to the coastal marine environment via wave and tidal-driven advective flows. The biogeochemical processes in beach aquifers were mainly studied in semi enclosed systems with low tidal amplitude, and with a connection to continental aquifers contributing to solute fluxes to the coast from terrestrial groundwater. We present here the study of a pocket beach isolated from terrestrial aquifers with a high tidal amplitude and a medium energy wave regime. In situ measurements, cross-shore profiles and vertical sampling were conducted during several tidal cycles in spring and autumn. Cross-shore transects, obtained at low tide from holes that represent a mixture of the upper 20 cm of the water saturated zone, showed concentration gradients of redox and recycled compounds. Increase in pCO(2), dissolved phosphate and ammonium concentrations downslope revealed that more products from organic matter mineralization accumulated in the lower beach. The related increase in total alkalinity downslope indicated that the part of anaerobic processes in organic matter oxidation was higher in the lower beach. Concentration and delta C-13 of dissolved inorganic carbon in pore waters suggested that the carbon mineralized in pore waters came from marine plant debris that were mixed with the sand. Continuous probe records of dissolved oxygen saturation and vertical profiles revealed a tidally-driven dynamics of pore water in the first centimetres of the lower beach aquifer. Ventilation of pore waters corresponded to wave pumping and swash-induced infiltration of seawater in the upper 10-20 cm of sediment. Nutrients and reduced compounds produced through organic matter mineralization remained stored in pore water below the layer disturbed by wave. The flux of these components to seawater is possible when this interface is eroded, for example when wave energy increases after a less energetic period. The low extension of the studied aquifer, typical of pocket beaches, limits the connection with continental groundwater. Both tidally-driven and wave-driven recirculation of seawater allows pocket beaches to be efficient bioreactors for marine organic matter mineralization. As such, they provide the coastal environment with recycled nutrients, and not new nutrients.

Sandy beaches exposed to tide and waves are characterized by low abundance and diversity of benthic macrofauna, because of high-energy conditions. This is the reason why there are few studies on benthic communities living in such highly dynamic environments. It has been shown recently that tidal sandy beaches may act as biogeochemical reactors. Marine organic matter that is supplied in the sand during each flood tide is efficiently mineralized through aerobic respiration. In order to quantify the role of macrofauna in the whole beach benthic respiration, we studied the macrofauna and the pore water oxygen content of an exposed sandy beach (Truc Vert, SW of France) during four seasons in 2011. The results showed that macrofauna was characterised by a low number of species of specialized organisms such as the crustaceans Eurydice naylori and Gastrosaccus spp. and the polychaetes Ophelia bicornis and Scolelepis squamata. The distribution and abundance of macrofauna were clearly affected by exposure degree and emersion time. The combined monitoring of benthic macrofauna and pore waters chemistry allowed us to estimate (1) the macrofauna oxygen uptake, calculated with a standard allometric relationship using biomass data, and (2) the total benthic oxygen uptake, calculated from the oxygen deficit measured in pore waters. This revealed that benthic macrofauna respiration represented a variable but low (<10%) contribution to the total benthic oxygen consumption. This suggests that oxygen was mainly consumed by microbial respiration.

Inland aquifers are connected to the ocean through permeable coastal sediments. The mixing zone between sea water and fresh-groundwater forms a so-called subterranean estuary. The dynamics and the shape of this interface influence fluxes of fresh water to the coastal zone. On tidal sandy beaches, tide and waves actions lead to the formation of an intertidal saline circulation cell that complements the structure of the subterranean estuary between sea water and fresh water. The upper beach corresponds to the recharge zone, where large volumes sea water penetrate the sediment at high tide, whereas the lower beach corresponds to the discharge zone of older pore water that remains in the sand for several tidal cycles. The objective of our study was to characterize the shape and the evolution of this saline cell in a high-energy macro-tidal beach, the Truc Vert beach (SW France). In order to assess the distribution of saline, brackish and fresh water in this high-energy beach aquifer, we conducted electrical resistivity tomography (ERT) measurements from the lower beach face to the sand dune in winter 2012-2013. We also measured water table elevation and salinity in three piezometers situated at the base of the dune and in pore waters at low tide on tidal cross-shore transects. Results show that the intertidal saline plume can be identified and localized with the ERT method. The fresh-saline water interface is almost vertical in the upper beach. The saline circulation cell occupies the upper part of the tidal beach over a thickness of 5 m deep. Below this saline cell, a brackish water flows to the ocean: about 30% of fresh water was found in pore water in the lower part of the beach. Shifting of the vertical front between saline and fresh waters of about 2-5 m occurred during the spring neap tidal cycle. This migration is small relative to the beach size, suggesting that the transition zone is relatively stable. The variations in salinity of the circulation cell suggest that the position of the saline plume is controlled by the spring tide level. Our results differ slightly from studies based on modelling and piezometric measurement made in less exposed micro-tidal sandy beaches. Our data would allow to better constrain numerical models that would enable to quantify water residence time and solute fluxes in subterranean estuaries of high-energy beaches. (C) 2015 Elsevier Ltd. All rights reserved.

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.

We collected samples from sea water, runnel water, beach pore waters, water from the unconfined surficial aquifer discharging at the beach face, groundwater, and rainwater from the Aquitanian coast in order to determine the flux of dissolved inorganic nitrogen (DIN), phosphorus and silica from terrestrial submarine groundwater discharge (SGD). The flux of fresh groundwater was obtained from a water balance calculation based on precipitation and evapotranspiration and assessment of the coastal watershed from hydrograph separation. Waters with intermediate salinities between sea water and freshwaters are found all along the 240-km-long coast, indicating that SGD is ubiquitous. The estimated fresh water flux is 2.25m3d−1m−1 longshore. Terrestrial SGD provides a DIN flux of 9·106mol each year to the adjacent coastal zone. This flux is about four times lower than the release of DIN due to tidally driven saline SGD. The freshwater DIN flux is low because the upland land use consists almost exclusively of pine forest. Dissolved organic nitrogen represents more than 60% of the total dissolved nitrogen flux. Dissolved iron, phosphorus and silica have much higher concentrations in the anoxic forest aquifer than in the fresh-water end-member of the subterranean estuary sampled in the upper beach aquifer. This suggests that the salinity gradient of the estuary does not correspond to a redox gradient. The redox front between anoxic groundwater and fresh oxic waters occurs below the soil-depleted foredune/yellow dune. Anoxic P- and Si-rich waters seep directly on the beach face only in the north Gironde, where the foredunes are eroded. This study reveals the role of the sandy foredune aquifer in biogeochemical fluxes from SGD, which is to dilute and oxidize waters from the unconfined surficial upland aquifer.

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 71Lm−2d−1 in winter, spring and summer, respectively) in comparison to uninhabited sediments for which these rates were moderate (averaging 9, 9 and 16Lm−2d−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.

We used a diagenetic model to test the hypothesis that manganese-rich layers in gas hydrate-bearing Arctic Ocean sediments are reliable time markers for interglacial periods. In the model, diagenesis is fuelled by two sources of reactive carbon: particulate organic carbon settling to the sediment surface, and methane diffusing up from deep gas hydrate deposits. The model includes oxidation of organic carbon and soluble reduced manganese by oxygen supplied continuously from an invariant bottom-water oxygen reservoir; reduction of particulate manganese by hydrogen sulfide generated through anaerobic methane oxidation; transport of dissolved oxygen and manganese by diffusion; and advective transport of particulate components by burial. Particulate organic matter and particulate manganese are only supplied to the sediment during interglacials. Sulfate reduction is not modeled explicitly; instead, the effect of anaerobic methane oxidation on Mn reduction is simulated at the lower boundary of the model by prescribing that particulate manganese is reduced there to soluble Mn(II). The soluble reduced Mn then diffuses upward and is oxidatively precipitated to Mn(IV) by downward diffusing oxygen. The upward flux of soluble Mn(II) is thus a function of the rate at which particulate manganese is advected into the Mn-reduction layer at the bottom of the model; it is not synchronous with events at the sediment–water interface. Model runs reveal that, under idealized but realistic conditions for the Arctic Ocean, oxidation of upward-diffusing Mn(II) generates post-depositional manganese enrichments that cannot readily be distinguished from the manganese-rich sediment layers that accumulate during interglacials. This compromises the use of manganese-rich layers as proxies for interglacial periods. In contrast, manganese-rich layers may be used as first-order markers of interglacial periods in sediments where gas hydrates or other forms of reactive carbon are absent.

We conducted six field campaigns to investigate the spatial and temporal evolution of pore water oxygen content on a high-energy sandy beach aquifer during several tidal cycles at different seasons. We buried autonomous probes in intertidal sands to record dissolved oxygen saturation, salinity, temperature and water head at a 2-10 min frequency. Oxygen concentrations display significant changes both with time (tide and seasons) and space (cross-shore and vertical variations). Seawater circulation in tidal sands forms a saline pore water circulation cell with oxygen-saturated pore water in the zone of seawater recharge. Oxygen-depleted pore water in the lower beach is the result of in situ respiration processes that occur during seawater circulation. Oxygen depletion varies throughout the year and anoxic conditions are reached at the end of spring, as planktonic organic matter becomes abundant in seawater and more organic matter is therefore supplied to pore water. On a shorter time scale (weeks to minutes), oxygen variations are driven directly by physical forcing. Tidal amplitude affects the extent of the saltwater circulation cell and the associated location of the recharging and discharging zones of the beach. The evolution of the water table level during the tidal cycle influences the circulatiori of pore water in the sand and ultimately, the timing of oxygen variations during flood and ebb. This first in situ study in a high-energy sandy beach shows that the dynamics of pore water oxygen are governed by biogeochemical processes at the seasonal scale and by physical forcing at the time scale of minutes to a few days. (C) 2015 Elsevier Ltd. All rights reserved.

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.

Precise indices based on n-alkane signatures were developed in order to determine the sources and composition of sedimentary organic matter (SOM) in coastal systems. The Arcachon Bay (France), a well-studied temperate lagoon, was used as an example of a complex coastal system sheltering a wide diversity of OM sources. Three main groups of sources were well discriminated from their n-alkane signatures: seagrass (Zostera sp.) produced mainly n-C-17, n-C-19, n-C-21, n-C-23 and n-C-25 alkanes, algae (Rhodophyta, Chlorophyta) produced n-C-15 and n-C-17 and the terrigenous input [Quercus sp., Spartina sp. and river suspended particulate OM (SPOM)] was characterized by n-C-25, n-C-27, n-C-29, n-C-31 and n-C-33. From the above and literature n-alkane fingerprints, we developed a set of indices (n-alkane ratios) to quantify the contribution of these three major sources of the SOM. At the Arcachon Bay scale, they indicated that SOM was composed mainly of seagrass (ca. 53 +/- 19%) and terrestrial (ca. 41 +/- 17%) material, followed by algae (ca. 6 +/- 9%). Moreover, the new n-alkane indices exhibited more relevant spatial patterns than classical ones - the TAR (C-27 + C-29 + C-31/C-15 + C-17 + C-19; terrestrial to aquatic ratio) and the P-aq (C-23 + C-25/C-23 + C-25 + C-29 + C-31; aquatic plant %) - with a greater contribution from marine sources in the central part of the lagoon where a high density of Zostera seagrass was observed. Therefore, the development of precise indices adapted to the local diversity of OM sources is needed when using n-alkanes for quantifying the source composition of SOM in complex coastal systems. (C) 2015 Elsevier Ltd. All rights reserved.

Seasonal changes in sediment particle mixing, surface sediment and seagrass characteristics, and benthic infaunal composition were measured in Arcachon Bay (France), within both a well-developed Zostera noltei meadow and a bare sediment mudflat. Sediment particle mixing intensities (measured by the normal biodiffusion coefficient D-b(N)) were obtained by fitting a continuous time random walk model to in situ measured vertical luminophore profiles. D-b(N) values (mean +/- SD) were between 2.99 +/- 2.75 and 22.45 +/- 43.73 cm(2) yr(-1) within the bare mudflat and between 0.39 +/- 0.30 and 18.07 +/- 18.14 cm(2) yr(-1) within the Zostera meadow. Spatiotemporal changes in infauna and D-b(N) were lower within the Zostera meadow, which supports the buffering effects of seagrass meadows on biological sedimentary processes. Within the Zostera meadow, root biomass declined during the survey, in correlation with increases in (1) the mean value and the variability of D-b(N) and (2) the spatial variability of infaunal composition with a decrease in the dominant polychaete Melinna palmata. At this station, similarity matrices of mean D-b(N) and abundances of a set of 3 infaunal species (including M. palmata) correlated significantly, which further supports the key role of this species in controlling sediment particle mixing through sediment stabilization. When considering the whole data set, the similarity matrices of the coefficients of variation of D-b(N) and of the abundances of a set of 5 species (Abra segmentum, Glycera convoluta, Tubificoides benedii, Heteromastus filiformis, Ruditapes phillipinarum) correlated significantly, which supports the suspected role of these species in controlling sediment particle mixing.

Wave and tide dominated beaches are composed of medium to coarse sandy sediments depleted in organic matter. In these sediments, the availability of organic matter limits benthic heterotrophic respiration processes. This study shows that oxygen consumption linked with nutrient recycling occurs in sands and that the intensity of this process follows a seasonal pattern. Furthermore, substantial seawater exchange occurs through intertidal sandy sediments during each tide. During floods, seawater penetrates sandy sediments, filling the pore spaces with dissolved oxygen and marine organic matter. Water expelled from the sediment during ebb is depleted in oxygen and enriched in nitrate. In order to quantify the extent of respiration processes, we collected pore waters at low tide on tidal cross-shore transects on the Truc Vert beach (France) every two weeks over 20months. Temperature, salinity and oxygen saturation were recorded in situ and nutrient concentrations were determined from collected samples. The seasonal monitoring showed that aerobic respiration occurred in sands. Organic matter supplied by the seawater entering the pore spaces during floods was trapped long enough to be mineralized. An efficient recycling of nutrients was thus found to occur in tidal sandy beaches. The intensity of the respiration processes varied seasonally. The strongest oxygen depletions occurred in spring, when planktonic organic matter was abundant, as well as in summer when we observed occasional denitrification. Nitrate concentration of recycled seeping seawater reached values up to 40μM. According to our calculations, the 240-km-long Aquitanian beach recycles 3500t of carbon and releases 600t of N-nitrate to the adjacent coastal zone each year. Since phosphorus is readily adsorbed on particles, the N:P ratio of recycled nutrients released to the seawater was much higher than the Redfield ratio.

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.

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 D-b(X) and D-b(Y) (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.

The effects of freshwater infaunal invertebrates on sediment geochemical properties were studied through an experimental approach using indoor microcosms during a 56-day experiment. The bioturbating organisms were tubificid worms, which consume sediment at depth and deposit undigested material at the sediment–water interface. Bioturbation intensity was determined using fluorescent tracers, and the distribution of redox-sensitive compounds was studied from replicate experimental units handled 7, 14, 21, 28 and 56 days after tubificid colonization. Worm activity transferred reduced particles and pore water at the sediment surface at a rate of 0.14 cm day−1. Compared to control experimental units, this recycled material represented at the end a several centimetre-thick layer enriched in water content, dissolved nitrate and sulphate, and depleted in oxygen, ammonium and dissolved Mn(II). Tubificids consumed O2 in bottom water, so that the sediment was anoxic, allowing a direct flux of dissolved reduced species into overlying water. Lower ammonium and Mn(II) concentrations and fluxes in anoxic sediment possibly resulted from a decrease in anaerobic microbial metabolism due to competition for labile organic carbon with tubificids. Higher sulphate concentration resulted from burial of surface waters with particle at the sediment surface, but not from bio-irrigation of burrows. Nitrate was produced in anoxic condition, as observed in almost every mixed modern sediments.

The relationship between particulate organic carbon (POC) concentrations measured in modern sediment and fluxes of exported POC to the sediment surface needs to be understood in order to use POC content as a proxy of paleo-environmental conditions. The objective of our study was to compare POC concentrations, POC mineralization rates calculated from O2 consumption and POC burial rates. Benthic O2 distributions were determined in 58 fine-grained sediment cores collected at different periods at 14 stations in the southeastern part of the Bay of Biscay with depths ranging from 140 to 2800m. Depth-dependent volume-specific oxygen consumption rates were used to assess rates of aerobic oxidation of organic matter (OM), assuming that O2 consumption solely was related to heterotrophic activity at the sediment–water interface. Heterogeneity of benthic O2 fluxes denoted changes in time and space of fresh organic material sedimentation. The most labile fraction of exported POC engendered a steep decrease in concentration in the upper 5mm of vertical O2 profiles. The rupture in the gradient of O2 microprofile may be related to the bioturbation-induced mixing depth of fast-decaying carbon. Average diffusive O2 fluxes showed that this fast-decaying OM flux was much higher than buried POC, although diffusive O2 fluxes underestimated the total sediment oxygen demand, and thus the fast-decaying OM flux to the sediment surface. Sedimentary POC burial was calculated from sediment mass accumulation rate and the organic carbon content measured at the top of the sediment. The proportion of buried POC relative to total exported POC ranged at the most between 50% and 10%, depending on station location. Therefore, for a narrow geographic area like the Bay of Biscay, burial efficiency of POC was variable. A fraction of buried POC consisted of slow-decaying OM that was mineralized within the upper decimetres of sediment through oxic and anoxic processes. This fraction was deduced from the decrease with depth in POC concentration. At sites located below 500m water depth, where the fast-decaying carbon did not reach the anoxic sediment, the slow-decaying pool may control the O2 penetration depth. Only refractory organic material was fossilized in sedimentary records at locations where labile OM did not reach the anoxic portion of the sediment.

Bioturbation refers to the mixing of sediment particles resulting from benthic faunal activity. It is the dominant particle mixing process in most marine sediments and exerts an important control on diagenetic processes. In models, bioturbation is usually treated as a diffusive process where the biodiffusion coefficient (Db) characterizes the biological mixing intensity. Biodiffusion coefficients are classically computed by fitting a diffusive model to vertical profiles of particle-bound radioisotopes. One peculiar observation is tracer-dependence: Db values from short-lived tracers tend to be larger than those obtained from long-lived tracers from the same site. Recent theoretical work, based on random walk theory and Lattice Automaton Bioturbation Simulations (LABS), has suggested that this tracer-dependence is simply a model artifact and has concluded that the biodiffusion model is not applicable to the short observational time scales associated with short-lived radioisotopes. Here we have compiled a global dataset of Db values obtained from different radiotracers to assess tracer-dependence from a data perspective. Tracer-dependence is significant in low-mixing environments like slope and deep-sea sediments, but is not present in intensely mixed coastal areas. Tracer-dependence is absent when the number of mixing events is larger than 20, or the potential length scale is greater than 0.5cm. Roughly this comes down to tracer-derived Db values greater than 2cm2yr−1. This condition is met for 68%, 50%, and 8% of published Db values obtained from coastal, continental slope, and abyssal environments, respectively. These results show that short-lived radioisotopes are suitable to quantify biodiffusion mixing in sedimentary environments featuring intense bioturbation.

Several types of sediment failures in the Gulf of Cadiz were observed using multibeam bathymetry, acoustic imagery and high-resolution seismic. These instabilities are mainly sediment failures and flows. Their width and length vary from 1 to more than 10 km. The failures are mainly related to high sedimentation rates, particularly in places where the Mediterranean Outflow Water (MOW) spills over, such as channel bends and the outer side of the giant contourite levee. Steep slopes are also a trigger for failure at the continental shelf-slope transition, on valley sides, on canyon flanks, and on the sides of bathymetric highs. Other mass movements are related to fluid escape (mud volcanoes) and earthquakes. In areas where the MOW flows along the seafloor, the constant shearing and related erosion can add to the overall stresses. The frequency of failures can be estimated using the deposits resulting of their distal transformations into turbidites.