Gérard Blanc

The distribution of Mo, U, and Cd in relation to major redox species in muddy sediments of the Bay of Biscay

In order to understand the relationship between authigenic precipitation of U, Mo, Cd, and the redox properties of the marine environment, we have studied the vertical distribution of these metals and of the major redox species (oxygen, nitrate, manganese, reactive iron, sulfate, carbon, and sulfur) in modern sediment. We have sampled four sites at 150 to 2800 m depth in the Bay of Biscay. At the shallowest station, where sediment is highly bioturbated, organic carbon levels are above 2% and particulate sulfides are abundant. At the deepest stations, sediment is much less bioturbated, and organic carbon levels are lower. At all sites, early diagenesis follows a well-established depth sequence of redox reactions, based on the bacterially mediated oxidation of organic matter. Manganese-oxides and authigenic U and Mo were extracted by an ascorbate solution. Molybdenum is associated with these oxides in the oxic part of cores. In the anoxic layer of the sediment, Mo precipitates as a detectable authigenic phase only when sulfide minerals are present, i.e. when sulfate reduction becomes important. In anoxic sediments, Cd enrichment is a good indicator of sulfide production from sulfate reduction even if the production is weak. Uranium precipitates at the depth of reactive iron (III) reduction. No relationship was observed between U and S. The U concentration at the depth of precipitation agrees with values calculated using estimated accumulation rates and a downward diffusive flux of dissolved U from the bottom seawater into the sediment. However, authigenic U concentrations continue to increase in the anoxic part of the cores. Therefore, it is difficult to establish a direct relationship between the process of U accumulation and the flux of Corg to the sediment. D 2002 Elsevier Science B.V. All rights reserved.

Ten-year observation of the Gironde tributary fluvial system: fluxes of suspended matter, particulate organic carbon and cadmium

A program of long-term observation of suspended solids (TSS), particulate organic carbon (POC) and cadmium transported into the Gironde estuary (France) by its major tributaries has been carried out between 1990 and 1999. This decade included contrasting hydrologic cycles and appears representative of a much longer period (1959-1999). The Garonne and the Dordogne river systems are the main tributaries of the Gironde estuary and derive their waters from drainage basins with different geological, industrial and agricultural features. To better understand their respective contributions, they have been observed separately and compared. Water and TSS fluxes of the Garonne River show greater temporal variations and discharge is more related to the hydrology of the drainage basin (e.g. wet/dry years, local flood events etc.). As POC and particulate Cd concentrations in suspended matter are much less variable than turbidity, their fluxes are mainly controlled by the TSS transport. A major part of annual fluxes of TSS and associated pollutants may occur within few flood days (depending on various parameters, e.g. intensity, duration, season, etc.), and also the succession of dry and wet years has an important influence on annual fluxes. The presented data allow calculating fluvial inputs into the Gironde as the sum of fluxes transported by its major tributaries, the Garonne and the Dordogne river systems. Mean annual fluxes into the Gironde observed in 1990-1999 are about 34 x 10 9 m 3 year -1 for river water, 3.24 x 10 6 t year -1 for suspended solids (TSS) and 9.88 × 10 9 mol year -1 for particulate organic carbon (POC). Generally, these fluxes are dominated by the contributions of the Garonne River. However, in dry years, the mean contribution of the Dordogne river system (including Dronne and Isle rivers) to the POC input into the estuary exceeded that of the Garonne. This reflects significant differences in vegetation and soil due to natural properties and land management of the basins. Mean Cd fluxes into the estuary are about 110 x 10 3 mol year -1 of which 19.6 × 10 3 mol year -1 are transported in the dissolved and 90.8 x 10 3 mol year -1 in the particulate phases, respectively. In 1991 (dry year), the net (dissolved) Cd flux towards the ocean exceeded the gross fluvial input of total Cd, suggesting the release of Cd from an important stock in the maximum turbidity zone (MTZ) or the fluid mud of the Gironde estuary.

A cadmium budget for the Lot-Garonne fluvial system (France)

Routine measurements of river discharge and total suspended sediment concentration (TSS) are combined with regular analyses for particulate and dissolved cadmium to produce a box model that allows us to propose a cadmium mass balance for the Lot-Garonne man-influenced river system (8400 km2). Nearly half the cadmium in the Garonne river is supplied by the tributary Lot river. Cadmium input onto the Lot river comprises wet deposition from the atmosphere, molecular diffusion at the sediment-water interface, surface-water runoff and discharge from the leaching of waste at a zinc refining plant. Approximately 85% of the cadmium in the Lot river is derived from anthropogenic origin. Cadmium in the industrial discharge is 80% dissolved and 20% in the particulate phase (4.2 and 1.1 t yr−1, respectively). Total inputs are estimated at 4.81 t yr−1 and 1.54 t yr−1 for the dissolved cadmium and for the particulate phase, respectively. Budgeting estimates an output onto the Garonne river of 0.54 t yr−1 for the dissolved cadmium (about 8%) and 6.13 t yr−1 for the particulate cadmium (about 92%) indicating that downstream sediment-associated cadmium fluxes are enhanced by the 4.27 t yr−1 removed from solution and the 0.32 t yr−1 remobilized by the erosion of sediment blanketing the Lot river bed. These figures are found to be comparable with those generated by a dilution model which suggests that 97% of dissolved cadmium is taken up by the particulate phase over 0.5 km downstream from the primary anthropogenic source.

The behaviour of arsenic in muddy sediments of the Bay of Biscay (France)

Abstract We have studied particulate and dissolved arsenic species in sediment and porewaters at sites in the Bay of Biscay, France, ranging in depths from 150 to 2,800 m. At all stations, major redox species (oxygen, nitrate, ammonia, total and reactive iron and manganese, sulphate and sulphur) reflect early diagenetic depth sequences of redox reactions comparable to other marine environments. Vertical distributions of dissolved and particulate As species and major redox species are related to changes in redox conditions and their major carrier phases, such as Fe and Mn-oxides. Arsenic diagenesis appears strongly dependent on Fe cycling. A subsurface maximum of dissolved As and surface enrichment of particulate As correspond to dissolution and precipitation of Fe (III) phases. Except for the shallowest and most bioturbated site, flux calculations show three different vertical diffusive As fluxes: two upwards and one downwards. Phase changes of recycled As result in local accumulations of reactive As at different redox fronts. Mass-balance calculations indicate that the upward As flux toward the oxidized layer can explain the enrichment of HCl extractable particulate As in this layer. A portion of the upward diffusing As can escape the sediment and may be fixed onto settling Fe-oxides by adsorption or co-precipitation and contribute to reactive particulate As input (i.e., As is recycled across the water sediment interface).

Long-term monitoring (1960–2008) of the river-sediment transport in the Red River Watershed (Vietnam): Temporal variability and dam-reservoir impact

The Red River (China/Vietnam, A=155,000 km(2)) is a typical humid tropics river originating from the mountainous area of Yunnan Province in China. Based on information on daily discharge (Q) and suspended particulate matter (SPM) concentration between 1960-2008 for the SonTay gauging station (outlet of the River and entry to the Delta) provided by the National Institute IMHE-MONRE, the mean annual SPM flux was estimated at 90 Mt/yr, corresponding to a sediment yield of 600 t/km(2)/yr. The temporal variability of annual SPM fluxes (ranging from 24 to 200 Mt/yr) is strongly related to the interannual hydrological conditions. However, some years of high water flow were not associated with high sediment fluxes, especially after 1989 when the HoaBinh dam came into operation. Therefore, the median discharge pre- (3389 m(3)/s) and post 1989 (3495 m(3)/s) are similar indicating there was little or no change between both periods. Sediment rating curves (power law-type; SPM=aQ(b)) were fitted for both periods (1960-1989; 1990-2008). The analysis of the pre- and post-1989 sediment rating parameters (a, b) suggests a downshift of b-parameter values after 1989, attributed to a decrease of the sediment supply due to the commissioning of the HoaBinh dam. A single sediment rating curve derived from 1960-1989 data was used to simulate the annual variability of former sediment delivery, generating excellent cumulative flux estimates (error ~1%). In contrast, applying the same rating curve to the 1990-2008 data resulted in systematic and substantial (up to 109%) overestimation. This suggests that the HoaBinh dam reduces annual SPM delivery to the delta by half, implying important metal/metalloid storage behind the HoaBinh dam.

Metal mobilization in the Gironde Estuary (France): the role of the soft mud layer in the maximum turbidity zone

Vertical profiles of heavy metals (Cd, Cu, Cr, Fe, Mn, Ni and Pb) in the particulate and dissolved phases and redox sensitive parameters (dissolved oxygen, nitrate, nitrite and ammonium) were analyzed for the first time in the continuum Maximum Turbidity Zone (MTZ)–Fluid Mud (FM; 50 g l 1 <suspended particulate matter (SPM)<500 g l 1 )-consolidated sediment of a macrotidal, highly turbid estuary: the Gironde. The results show that the fluid mud is a layer of intense metal mobilization due to redox induced dissolution, releasing trace metals into the water column. This newly identified phenomenon creates a transient situation characterized by the onset of diagenetic sequences in the FM, i.e. in the lower part of the water column. These sequences overlie permanent diagenetic sequences in the consolidated sediment, similar to those typically observed in marine and estuarine sediments. The discontinuity of dissolved Cd, Ni and Pb concentrations at the sediment surface indicates that Mn reduction is faster in the FM than in the upper sediment, isolated from the oxic water column by the suboxic FM. Two separate diagenetic signals are preserved, as the installation of the diagenetic sequence in the FM is faster than the molecular diffusion of dissolved compounds through the FM–sediment interface. The diagenetic signal of trace metals (e.g. dissolved Cr maximum) in the upper sediment layer near the FM–sediment interface is interpreted as a transient record of past hydrologic situations, during which the absence of the FM layer permitted the installation of an oxic/suboxic front in the upper sediment. The resuspension of the FM during the spring tide probably results in a new distribution of these elements in the estuary, where they may be stabilized in the dissolved phase (e.g. by chloride complexes or dissolved organic compounds) or be adsorbed onto reactive particles (e.g. freshly precipitated Mn oxyhydroxides). D 2004 Elsevier B.V. All rights reserved.

Heat and salt fluxes in the Atlantis II Deep (Red Sea)

The Atlantis II Deep is located on the Red Sea axial rift. It is a topographic depression enclosing a volume of about 17 km3 between 1,900 m and 2,200 m depth that contains layered brines of temperatures up to 66°C and salinities up to 270‰. Previous geochemical investigations showed that the hot brines result from discharge of hydrothermal solutions that have exchanged heat and chemical components with the basaltic substratum. The last investigation of the Deep in 1992 showed that the brines occurred in four well mixed layers with the shallowest at a depth of 2,000 m. The temperature and salinity profiles describe a transition zone from 2,000 m to 1,900 m, above which Red Sea water occurs. The distribution of temperature and salt in this transition zone appears to be controlled by the topography of the Deep. The hydrographic structure of the brine column has been documented in the literature for seven time intervals between 1966 and 1992. Examination of these data shows that the system changed with time. The evolution of the entire package of brines that fill the Deep shows the following changes: (1) the temperature of the brines increased; (2) the salinity of the solutions increased; (3) the two brine layers described in 1966 still existed in 1992, but new layers appeared above them; (4) for 26 years, almost all additional heat and salt supplied in the Deep were confined to the depression and were not dispersed into the overlying seawater. The fourth point indicates that a heat and salt balance for the Deep can be calculated. The calculation of the heat that entered into the system was divided into two components: the temperature increase of the brines and the heat loss at the wall-rock; the latter component was negligible. The rate of heat input to the Deep was constant during the period considered, and amounted to 0.54 × 109 W. The salt input was also constant, and equalled 250–350 kg/s. During the period 1966–1992, heat and salt were most probably supplied by a hydrothermal solution with an average range of flow rate, temperature, and salinity of 670–1000 kg/s, 195–310°C, and 270–370‰, respectively.

The particulate 7Be/210Pbxs and 234Th/210Pbxs activity ratios as tracers for tidal-to-seasonal particle dynamics in the Gironde estuary (France): Implications for the budget of particle-associated contaminants

The short-lived natural radionuclides (7)Be (T(1/2)=53 days), (234)Th(xs) (T(1/2)=24.1 days) and (210)Pb(xs) (T(1/2)=22.3 years), i.e. (234)Th and (210)Pb in excesses of that supported within particles by the decay of their parent isotopes, were analysed in suspended particulate matter (SPM) to study the particle dynamics in the Gironde fluvial estuarine system (France), strongly impacted by heavy metal pollution. From surveys of this land-ocean interface in 2006 and 2007, we established a times series of these radioisotopes and of their activity ratios ((7)Be/(210)Pb(xs) and (234)Th/(210)Pb(xs) ARs) in particles sampled under different hydrological conditions. The particulate (7)Be/(210)Pb(xs) AR varies along the fluvial estuarine system mainly due to variations in (7)Be activities, controlled by riverine, oceanic and atmospheric inputs and by resuspension of old (7)Be-deficient sediments. These processes vary with river discharge, tidal cycle and season. Therefore, seasonal particle transport processes can be described using variations of the SPM (7)Be/(210)Pb(xs) ARs. During high river discharge, the SPM (7)Be/(210)Pb(x) ARs decrease from river to the ocean. The turbidity maximum zone (TMZ) is dispersed and the particles, and the associated contaminants, are rapidly transported from river to coastal waters, without significant retention within the TMZ. During low river discharge, the TMZ intrudes into the fluvial estuary, and the lowest (7)Be/(210)Pb(x) ARs are observed there due to resuspension of (7)Be-deficient sediments. Away from the TMZ, from the middle to lower estuary, SPM (7)Be/(210)Pb(x) ARs increase, indicating that the particles have been recently tagged with (7)Be. We explain this trend as being caused by marine input of dissolved radionuclides, as traced by SPM (234)Th/(210)Pb(xs) ARs, followed by scavenging in the estuary. This result indicates that particle transport models based on (7)Be and trace-metal budgets must consider oceanic dissolved inputs as an additional source of (7)Be and, possibly, of contaminants to estuaries.

New stratification in the hydrothermal brine system of the Atlantis II Deep, Red Sea

Temperature and salinity records from the REDSED cruise (September 1992) reveal a new stratification in the hydrothermal brine system of the Atlantis II Deep. This stratification consists of the preexisting lower and upper convective layers (LCL and UCL1), with temperatures of about 66 ° C and 55 ° C, respectively, and two additional upper convective layers (UCL2 and UCL3) that formed more recently and have distinct temperature, salinity, and thickness. In accordance with the physical process that leads to the formation of brine layers (i.e., double-diffusive convection), a flow rate of about 200 L/s has been estimated for the hydrothermal discharge.

Long-term records of cadmium and silver contamination in sediments and oysters from the Gironde fluvial-estuarine continuum - Evidence of changing silver sources

The Gironde fluvial estuarine system is impacted by historic metal pollution (e.g. Cd, Zn, Hg) and oysters (Crassostrea gigas) from the estuary mouth have shown extremely high Cd concentrations for decades. Based on recent work (Chiffoleau et al., 2005) revealing anomalously high Ag concentrations (up to 65 mg kg(-1); dry weight) in Gironde oysters, we compared long-term (~1955-2001) records of Ag and Cd concentrations in reservoir sediment with the respective concentrations in oysters collected between 1979 and 2010 to identify the origin and historical trend of the recently discovered Ag anomaly. Sediment cores from two reservoirs upstream and downstream from the main metal pollution source provided information on (i) geochemical background (upstream; Ag: ~0.3 mg kg(-1); Cd: ~0.8 mg kg(-1)) and (ii) historical trends in Ag and Cd pollution. The results showed parallel concentration-depth profiles of Ag and Cd supporting a common source and transport. Decreasing concentrations since 1986 (Cd: from 300 to 11 mg kg(-1); Ag: from 6.7 to 0.43 mg kg(-1)) reflected the termination of Zn ore treatment in the Decazeville basin followed by remediation actions. Accordingly, Cd concentrations in oysters decreased after 1988 (from 109 to 26 mg kg(-1), dry weight (dw)), while Ag bioaccumulation increased from 38 up to 116 mg kg(-1), dw after 1993. Based on the Cd/Ag ratio (Cd/Ag~2) in oysters sampled before the termination of zinc ore treatment (1981-1985) and assuming that nearly all Cd in oysters originated from the metal point source, we estimated the respective contribution of Ag from this source to Ag concentrations in oysters. The evolution over the past 30 years clearly suggested that the recent, unexplained Ag concentrations in oysters are due to increasing contributions (>70% after 1999) by other sources, such as photography, electronics and emerging Ag applications/materials.