A first assessment of organic carbon burial in the West Gironde Mud Patch (Bay of Biscay)

Abstract

On the Bay of Biscay continental shelf, there are several mid-shelf mud patches including La Grande Vasière to the north, the West Gironde Mud Patch (WGMP) off the Gironde estuary and the Basque Mud Patch close to the Spanish border. In general, these deposits are several meters thick and cover coarser substrate. Questions remain about their storage capability for fine particles and carbon. This work investigates the sedimentation of the WGMP in order to develop a first estimate of organic carbon (OC) burial. Interface sediment cores were collected at nine stations along two cross-shelf transects in October -November 2016. X-radiograph imaging and grain-size analyses were used to characterize sedimentary structures. 210 Pbxs depth profiles were established to calculate sediment (SAR) and mass (MAR) accumulation rates.Sedimentary structures indicate episodic sandy inputs overlying older deposits at proximal sites, and relatively continuous sedimentation at seaward locations. On the outer-central portion of the northern transect, a maximum SAR (0.47 cm yr -1 ) was observed, suggesting a depocenter.On the southern transect, excluding two stations where sedimentary inputs appear massive but sporadic, the SARs are lower (<0.3 cm yr -1 ). Quantitative estimates of OC burial rates increase seaward with a maximum of 45 gC m -2 yr -1 . To evaluate carbon loading independent of grainsize variability, OC values were normalized to surface area of sediments (SA). Interestingly, a qualitative comparison of OC burial efficiencies using the OC/SA ratio highlights three groups of sites (low, medium and relatively high OC burial efficiency) which are likely related both to different sedimentary environments and variable deposition conditions linked to local environmental conditions and depth. This work highlights the likely control of hydrodynamic intensity and sedimentary inputs on the amount of OC stored in the WGMP sediments.

First assessment of Rare Earth Element organotropism in Solea solea in a coastal area: The West Gironde Mud Patch (France)

“…The WGMP extends over depth ranging from 30 to 75 m along a main NE-SW orientation (Figure 1; Jouanneau et al, 1989;Lesueur et al, 1991). The WGMP is a temperate coastal system supplied in particles by the Gironde Estuary (Dubosq et al, 2021;Lamarque et al, 2021) and strongly submitted to hydrodynamic influences (Lesueur et al, 2002). Sandy-muddy environments, such as the WGMP, are habitats for several coastal species, including the common sole Solea solea (Koutsikopoulos et al, 1989).…”

Mapping marine ecosystem services in a multifunctional seascape: the case of Grande Vasière (Bay of Biscay)

“…The GV is influenced by sedimentation through winter storms and river flows (Le Loc'h, 2004), also providing nutrient loading and organic matter (Dubosq et al, 2021). We assumed that bioturbation and filter feeding play a role in the trophic chain by recycling these nutrients and biomass through microbial activity stimulation.…”

“…Carbon storage is enhanced by fine grain size sediment (Middelburg, 2019) which traps organic carbon in anoxic conditions (Dubosq et al, 2021). Indeed, mud patches exposed to river flows on continental shelves are known to be carbon sinks due to a high accumulation of fine sediments and low hydrodynamic intensity, resulting in low degradation of organic carbon by bacteria (Yao et al, 2014;Dubosq et al, 2021). Thus, we used sediment distribution as a proxy of the ability of each sediment type to store residual carbon.…”

Spatiotemporal dynamics of surface sediment characteristics and benthic macrofauna compositions in a temperate high-energy River-dominated Ocean Margin

“…30 and 75m depth (Figure 1). This relict paleovalley is the primary depocenter (sedimentation rates between less than 1 and 10 mm.y -1 ; Dubosq et al 2021) of fine particles originating from the Gironde Estuary, which has an annual mean water flow of 944 m 3 .s -1 (Doxaran et al, 2009) with daily flows up to 7,500 m 3 .s -1 during winter floods (Constantin et al, 2018). The WGMP is located in a macro-tidal environment with a tidal range from 1.5 to 5m (Jalón-Rojas et al, 2018).…”