The Biogeochemistry of Hydrogen Sulfide in the Open Ocean

Hydrogen sulfide is a poorly quantified component of the global sulfur cycle. In seawater, its dissolved species (dissolved sulfide) can react with trace metals to form dissolved complexes and insoluble compounds. Therefore, the study of dissolved° sulfide in the open ocean can yield a better understanding of the biogeochemical cycles of sulfur and trace metals. The biogeochemical cycling of dissolved sulfide was examined using specially developed sampling and analytical methods. In the western North Atlantic and central Equatorial Pacific Oceans, total dissolved sulfide concentrations ranged from /L; they were highest, in the mixed layer and decreased with depth. The depth distribution and the rate of carbonyl sulfide (OCS) hydrolysis to hydrogen sulfide under oceanographic condition were determined because of their importance for the cycling of dissolved sulfide. Depth profiles of OCS show near-surface enrichment up to 150 pmol/1, and depletion at depth. These profiles suggest in situ OCS production, perhaps by organic matter regeneration. A budget was constructed to quantify the sources and sinks of dissolved sulfide in the Sargasso Sea mixed layer. The rate of dissolved sulfide production was 5.43 pmol/L/h (OCS hydrolysis + air/sea exchange), a.ncl the removal rate was 115.2 pmol/1/h (oxidation+ particulate sinking). Similarities in profiles of dissolved sulfide and chlorophyll a or Synechococcus sp., and results from recent phytoplankton culture studies, also suggest its production by phytoplankton; this source is necessary to balance the dissolved sulfide budget. The conditional formation constant of Cu(HS)+ was determined under oceanographic conditions and is 109.1± 0.6. The speciation of dissolved sulfide was computed with this constant and literature thermodynamic data using a multiequilibrium model. The calculation predicts that 80% of dissolved sulfide is uncomplexed in the mixed layer, while over 90% is complexed by zinc and copper below the pycnocline. Because dissolved sulfide is rapidly cycled and interact.s with Irace met.a.ls, future studies should continue to quantify, under oceanographic conditions, its the interactions with trace meta.ls, and its rat.e of product.ion by phytoplankton

Links between size fractionation, chemical speciation of dissolved copper and chemical speciation of dissolved organic matter in the Loire estuary

Environmental context The toxicity of copper to aquatic life is highly dependent on its chemical form. In the vineyards of the Loire valley, mixtures of copper compounds are increasingly being used as fungicides. Our study investigating the origin and behaviour of dissolved copper on the land-sea continuum of the Loire advances our understanding of the chemical forms and fate of copper in estuarine systems. Abstract We present data on the organic speciation of dissolved copper (dCu) in the Loire estuary for the soluble (\textless0.02 µm) and dissolved (\textless0.45 µm) fractions. These results were interpreted according to the distribution of natural organic matter along the estuary. We observed a high concentration of dCu (80 nmol kg−1) upstream of the tidal front (S = 0.1, where S is the salinity), possibly induced by drainage from vineyards located on the watershed of the estuary, and a concentration typical of coastal seawater at the end of the salinity gradient (3–4 nmol kg−1). dCu showed a non-conservative distribution along the estuary with a notably strong decrease in concentration at the very first stage of mixing (S ~ 0.5) that increased again at low salinity (S = 4.7). The latter increase in the concentration of dCu was related to the supply of colloidal copper induced by particle desorption in the maximum turbidity zone. The organic speciation of dCu revealed that the complexing capacity of copper ligands (LCu, up to 147 nanomoles of Cu per kilogram of water) exceeded dCu in both the soluble and dissolved fractions, which kept the activity of cupric ions below toxic levels for most unicellular phytoplankton. Humic substances comprised up to 95 % of LCu in the continental shelf sample, but their complexing capacity did not account for all of the LCu in the inner estuary. We conducted pseudopolarographic experiments and found two other additional ligand classes: thiol-like and amino-acids/proteins. While humic substances are the dominant component of dissolved organic matter (DOM), the study of DOM suggested the release of colloidal DOM from a wastewater treatment plant. This structure could be a major source of proteinaceous LCu in the system that changes the dCu speciation in the middle of the estuary

Oceanic mercury concentrations on both sides of the Strait of Gibraltar decreased between 1989 and 2012

International audienceMercury (Hg) is a toxic metal that threatens the health of aquatic ecosystems and fish consumers. Its natural cycle has been deeply perturbed by anthropogenic Hg emissions, especially since the start of the Industrial Revolution circa 1850 AD. Anthropogenic Hg emissions from North America and Europe have decreased by a factor of two in the last decades following the implementation of strict regulations. The response of North Atlantic Ocean and Mediterranean waters to this decrease remains poorly documented by field observations. A comparison of results obtained between 1989 and 2012 shows a significant decrease of Hg concentrations in waters on both sides of the Strait of Gibraltar. West of Gibraltar, the Hg decrease ranges from ~35% in the upper North East Atlantic Deep Water to ~50% in the North East Atlantic Central Water. East of Gibraltar, the observed decrease is ~30% in the Western Mediterranean Deep Water. No decrease is observed in the deep Atlantic Ocean layer that formed before the industrial era. Our results strongly substantiate the effectiveness of global anti-pollution policies on Hg contamination in oceanic waters. Aconsequent decline of Hg bioaccumulation in Northeastern Atlantic and Western Mediterranean pelagic ecosystems is still to be verified

Effects of short-term environmental disturbances on living benthic foraminifera during the Pacific oyster summer mortality in the Marennes-Oléron Bay (France)

International audienc

Copper, zinc and lead isotope signatures of sediments from a mediterranean coastal bay impacted by naval activities and urban sources

Toulon bay is severely impacted by metal contamination induced by past and recent naval activities. In this work, Cu, Zn and Pb isotope compositions and elemental concentrations of fifty-five surface sediments were determined in order to map the spatial distribution of anthropogenic and natural sources along this land-sea continuum. Two sub-systems of Toulon Bay, the Small and Large bays, showed well-marked patterns on metal contamination levels and isotope signatures for Cu and Pb. The Small bay had the highest metal concentrations, and displayed average Pb and Cu isotope compositions of 1.1664 ± 0.0043 (1s, expressed as 206Pb/207Pb ratios) and −0.17 ± 0.19‰ (1s, expressed as δ65CuNIST values), respectively. It contrasted with the Large bay, with moderate to pristine metal concentrations and average Pb and Cu isotope compositions of 1.1763 ± 0.0079 (1s) and +0.08 ± 0.23‰ (1s), respectively. Lead isotope systematics indicated a binary source mixing process involving industrial and natural sources, while Cu isotope systematics showed a ternary mixing process involving two distinct anthropogenic signatures, interpreted as associated to new diffuse anthropogenic sources and old warfare material. In contrast, Zn isotope compositions in the Small and Large bays were practically the same: +0.06 ± 0.05‰ and +0.06 ± 0.11‰ (1s, expressed δ66ZnJMC values), respectively, denoting an overlap between isotope signatures of natural and anthropogenic sources. This study presents the first detailed spatial distributions of Cu and Zn isotope compositions for an aquatic system, and demonstrates the feasibility to use Cu isotopes as tracers of anthropogenic sources in coastal environments

Mercury released from newly formed volcano influence concentrations in the surrounding ocean

International audienceVolcanic and geothermal areas are important natural sources of mercury, with mercury concentrations in volcanic gases above the atmospheric background. Individual volcanoes exhibit variable degassing features and behavior, leading to considerable uncertainty in global geogenic mercury fluxes estimations. Likewise, studies on mercury emissions from submarine volcanic and hydrothermal sites are scarce. Nevertheless, information on those natural inputs is needed to better estimate the anthropogenic mercury enrichment, and thus for the implementation of the Minamata convention.During Spring 2021, the GEOFLAMME campaign took place at the northern end of the Mozambique channel, where we examined the influence of volcanic inputs from a volcano that had formed less than 2 years ago near Mayotte Island. Water samples were obtained with a trace metal-clean CTD rosette and all-titanium high-pressure samplers using the remotely operated vehicle Victor 6000 on board R/V Pourquoi pas?. Total mercury was measured on board via Cold Vapour Atomic Fluorescence Spectroscopy (CV-AFS) following the EPA method 1631. Exhaled fluid samples from titanium samplers followed the same analytical scheme, but at the shore laboratory.Mercury levels measured from water column showed increased concentrations near the seafloor. Total mercury measured in fluid samples from the different venting sites showed concentrations 3 to 60 times higher than surrounding seawater.Our study provides new insight to the understanding for mercury biogeochemistry, the interactions between magmatism, tectonics and fluids circulation processes, as well as the implications on the physical-chemical properties of the water column. It also improves our knowledge on present-day mercury cycling in the marine environment usingfield-based data. Ongoing work will attempt to quantify seafloor mercury inputs to the vicinity of the Mayotte Island

Can cu isotope composition in oysters improve marine biomonitoring and seafood traceability?

This study provides the first geographic and temporal large-scale analysis of Cu stable isotope compositions in indigenous oysters to biomonitor anthropogenic Cu inputs in aquatic systems. It includes oyster samples from French marine environments with distinct Cu-pollution levels and histories and extends over several decades of biomonitoring. Sample series composed of oysters from the same season and similar size/age intended to avoid biological biases. Oysters in macrotidal estuaries (Loire and Gironde, Western France) display fluctuating Cu concentrations without clear temporal trends, challenging to infer anthropogenic influence. Conversely, the time series of their Cu isotope ratios indicate a constant proportion of natural and anthropogenic Cu sources since the 1980s. In contrast, Cu isotope compositions in oysters from the progressively developing locales (urbanization and recreational boating) of Arcachon and Vilaine bays shift positively over time, along with increasing Cu concentrations. This finding suggests a gradual augmentation in the bioaccumulation of anthropogenic Cu over time. Thus, the time series of Cu isotope compositions in oysters is an unambiguous and robust approach to pinpoint anthropogenic Cu contamination. Furthermore, Cu concentrations combined with isotope compositions yield site-specific fingerprints allowing geographic discrimination among oysters. This two-dimensional Cu signature is promising as a tool for seafood traceability

Reactivity and bioconcentration of stable cesium in a hyperturbid fluvial-estuarine continuum: A combination of field observations and geochemical modeling

International audienceEffective, post-accidental management needs an accurate understanding of the biogeochemical behavior of radionuclides in surface environments at a regional scale. Studies on stable isotopes (element homologs) can improve this knowledge. This work focuses on the biogeochemical behavior of stable cesium (Cs) along a major European fluvial-estuarine system, the Gironde Estuary (SW France). We present results obtained from (i) a long-term monitoring (2014–2017) of dissolved (Csd) and particulate (Csp) Cs concentrations at five sites along the freshwater continuum of the Garonne watershed, (ii) Csd and Csp concentrations during four oceanographic campaigns at contrasting hydrological conditions along longitudinal profiles of the estuarine system, (iii) a 24 h cycle of Csp at the estuary mouth, and (iv) a historical trend of Cs bioconcentration in wild oysters at the estuary mouth (RNO/ROCCH, 1984–2017). In addition, we model the partitioning of Cs within the estuarine environment for clay mineral interactions via PhreeqC. At fluvial sites, we observe a geogenic dependence of the Csp and a seasonal variability of Csd, with a downstream increase of the solid-liquid partitioning (log10 Kd values from 3.64 to 6.75 L kg−1) for suspended particulate matter (SPM) < 200 mg L−1. Along the estuarine salinity gradients, Cs shows a non-conservative behavior where fresh SPM (defined as Cs-depleted particles recently put in contact with Csd) act as a Cs sink during both flood and low discharge (drought) conditions. This sorption behavior was explained by the geochemical model, highlighting the relevance of ionic strength, water and SPM residence times. However, at high salinities, the overall log10 Kd value decreases from 6.02 to 5.20 for SPM ∼300–350 mg L−1 due to the Csd oceanic endmember. Despite wild oysters showing low bioconcentration factors (∼1220 L kg−1) at the estuary mouth, they are sensitive organisms to Cs fluxes

The role of seasonal hypoxia and benthic boundary layer exchange on iron redox cycling on the Oregon shelf

Widespread hypoxia occurs seasonally across the Oregon continental shelf, and the duration, intensity, and frequency of hypoxic events have increased in recent years. In hypoxic regions, iron reduction can liberate dissolved Fe(II) from continental shelf sediments. Fe(II) was measured in the water column across the continental shelf and slope on the Oregon coast during summer 2022 using both a trace metal clean rosette and a high‐resolution benthic gradient sampler. In the summer, Fe(II) concentrations were exceptionally high (40–60 nM) within bottom waters and ubiquitous across the Oregon shelf, reflecting the low oxygen condition (40–70 μM) at that time. The observed inverse correlation between Fe(II) and bottom water oxygen concentrations is in agreement with expectations based on previous work that demonstrates oxygen is a major determinant of benthic Fe fluxes. Rapid attenuation of Fe(II) from the benthic boundary layer (within 1 m of the seafloor) probably reflects efficient cross‐shelf advection. One region, centered around Heceta Bank (~ 44°N) acts a hotspot for Fe release on the Oregon continental shelf, likely due to its semi‐retentive nature and high percent mud content in sediment. The results suggest that hypoxia is an important determinant of the inventory of iron is Oregon shelf waters and thus ultimately controls the importance of continental margin‐derived iron to the interior of the North Pacific Basin

Reactivity and bioconcentration of stable cesium in a hyperturbid fluvial-estuarine continuum: A combination of field observations and geochemical modeling

Effective, post-accidental management needs an accurate understanding of the biogeochemical behavior of radionuclides in surface environments at a regional scale. Studies on stable isotopes (element homologs) can improve this knowledge. This work focuses on the biogeochemical behavior of stable cesium (Cs) along a major European fluvial-estuarine system, the Gironde Estuary (SW France). We present results obtained from (i) a long-term monitoring (2014–2017) of dissolved (Csd) and particulate (Csp) Cs concentrations at five sites along the freshwater continuum of the Garonne watershed, (ii) Csd and Csp concentrations during four oceanographic campaigns at contrasting hydrological conditions along longitudinal profiles of the estuarine system, (iii) a 24 h cycle of Csp at the estuary mouth, and (iv) a historical trend of Cs bioconcentration in wild oysters at the estuary mouth (RNO/ROCCH, 1984–2017). In addition, we model the partitioning of Cs within the estuarine environment for clay mineral interactions via PhreeqC. At fluvial sites, we observe a geogenic dependence of the Csp and a seasonal variability of Csd, with a downstream increase of the solid-liquid partitioning (log10 Kd values from 3.64 to 6.75 L kg−1) for suspended particulate matter (SPM) < 200 mg L−1. Along the estuarine salinity gradients, Cs shows a non-conservative behavior where fresh SPM (defined as Cs-depleted particles recently put in contact with Csd) act as a Cs sink during both flood and low discharge (drought) conditions. This sorption behavior was explained by the geochemical model, highlighting the relevance of ionic strength, water and SPM residence times. However, at high salinities, the overall log10 Kd value decreases from 6.02 to 5.20 for SPM ∼300–350 mg L−1 due to the Csd oceanic endmember. Despite wild oysters showing low bioconcentration factors (∼1220 L kg−1) at the estuary mouth, they are sensitive organisms to Cs fluxes