Geochemistry of CO2-Rich Gases Venting From Submarine Volcanism: The Case of Kolumbo (Hellenic Volcanic Arc, Greece)

Studies of submarine hydrothermal systems in Mediterranean Sea are limited to the southern Italian volcanism, while are totally missing in the Aegean. Here, we report on the geochemistry of high-temperature fluids (up to 220°C) venting at 500 m b.s.l. from the floor of Kolumbo submarine volcano (Hellenic Volcanic Arc, Greece), which is located 7 km northeast of Santorini Island. Despite the recent unrest at Santorini, Kolumbo submarine volcano is considered more active due to a higher seismicity. Rizzo et al. (2016) investigated the He-isotope composition of gases collected from seven chimneys and showed that are dominated by CO2 (>97%), with only a small air contamination. Here we provide more-complete chemical data and isotopic compositions of CO2 and CH4, and Hg(0) concentration. We show that the gases emitted from different vents are fractionated by the partial dissolution of CO2 in water. Fractionation is also evident in the C-isotope composition (δ13CCO2), which varies between -0.04 and 1.15‰. We modeled this process to reconstruct the chemistry and δ13CCO2 of intact magmatic gases before fractionation. We argue that the CO2 prior to CO2 dissolution in water had δ13C ∼-0.4‰ and CO2/3He ∼1 × 1010. This model reveals that the gases emitted from Kolumbo originate from a homogeneous mantle contaminated with CO2, probably due to decarbonation of subducting limestone, which is similar to other Mediterranean arc volcanoes (e.g., Stromboli, Italy). The isotopic signature of CH4 (δ13C ∼-18‰ and δD ∼-117‰) is within a range of values typically observed for hydrothermal gases (e.g., Panarea and Campi Flegrei, Italy), which is suggestive of mixing between thermogenic and abiotic CH4. We report that the concentrations of Hg(0) in Kolumbo fluids are particularly high (∼61 to 1300 ng m-3) when compared to land-based fumaroles located on Santorini and worldwide aerial volcanic emissions. This finding may represent further evidence for the high level of magmatic activity at Kolumbo. Based on the geo-indicators of temperature and pressure, we calculate that the magmatic gases equilibrate within the Kolumbo hydrothermal system at about 270°C and at a depth of ∼1 km b.s.l

Bioavailable Mercury Cycling in Polar Snowpacks

International audiencePolar regions are subject to contamination by mercury (Hg) transported from lower latitudes, severely impacting human and animal health. Atmospheric Mercury Depletion Events (AMDEs) are an episodic process by which Hg is transferred from the atmospheric reservoir to arctic snowpacks. The fate of Hg deposited during these events is the subject of numerous studies, but its speciation remains unclear, especially in terms of environmentally relevant forms such as bioavailable mercury (BioHg). Here, using a bacterial mer-lux biosensor, we report the fraction of newly deposited Hg at the surface and at the bottom of the snowpack that is bioavailable. Snow samples were collected over a two-month arctic field campaign in 2008. In surface snow, BioHg is related to atmospheric Hg deposition and snow fall events were shown to contribute to higher proportions of BioHg than AMDEs. Based on our data, AMDEs represent a potential source of 20 ty(-1) of BioHg, while wet and dry deposition pathways may provide 135-225 ty(-1) of BioHg to Arctic surfaces

Special issue of MERMEX project : recent advances in the oceanography of the Mediterranean Sea

Ocean margins are focal regions in terms of mercury (Hg) exchanges between the continent and the open sea. The aim of this paper is to describe the distribution and partition of Hg between the gaseous, dissolved and particulate phases in the waters of the Northwestern Mediterranean (NWM) margin, in order to assess the Hg sources and exchanges within the continuum between the continental shelf (Gulf of Lions) and the open sea (Northern Gyre). Mean ( standard deviation) of total Hg species (HgT) concentrations in unfiltered water samples were 1.52 +/- 1.00 pmol L-1 (n = 36) in the inner shelf, 1.09 +/- 0.15 pmol L-1 (n = 30) along the slope, and 1.10 +/- 0.13 pmol L-1 (n = 99) in the Northern Gyre. The dissolved phase (<0.45 in) average concentrations were 0.80 +/- 0.47 pmol L-1 (n = 37) in the inner shelf, 0.93 +/- 0.20 pmol L-1 (n = 4) along the slope and 0.84 +/- 0.10 (n = 20) pmol L-1 in the Northern Gyre. The particulate fraction of Hg decreased very strongly seaward from around 60% on the shelf to 10-25% above the Northern Gyre. Very low dissolved HgT concentrations occurred in the inner shelf water, consistent with the results of incubation experiments, which demonstrated that shelf water is very efficient in both production and release of dissolved gaseous Hg into the atmosphere. In the North Gyre waters column HgT presents a distribution pattern inverse to that of dissolved oxygen, and a slight Hg enrichment in the deep layer (Western Mediterranean Deep Water). The Hg from the open sea water is the largest Hg input to the Gulf of Lions (similar to 5.7 kmol yr(-1)), whereas inputs from the riverine source account for similar to 3.4 kmol yr(-1) and atmospheric deposition for less than 0.5 kmol yr(-1). The Hg accumulated in the sediments of the shelf is similar to 4.5 kmol yr(-1), including 0.6-1.7 kmol yr(-1) in the Rhone prodelta sediments. The evasion to the atmosphere represents a Hg flux of similar to 2.6 kmol yr(-1)

Geochemistry of CO2-rich gases venting from submarine volcanism: The case of kolumbo (hellenic volcanic arc, Greece)

Studies of submarine hydrothermal systems in Mediterranean Sea are limited to the southern Italian volcanism, while are totally missing in the Aegean. Here, we report on the geochemistry of high-temperature fluids (up to 220°C) venting at 500 m b.s.l. from the floor of Kolumbo submarine volcano (Hellenic Volcanic Arc, Greece), which is located 7 km northeast of Santorini Island. Despite the recent unrest at Santorini, Kolumbo submarine volcano is considered more active due to a higher seismicity. Rizzo et al. (2016) investigated the He-isotope composition of gases collected from seven chimneys and showed that are dominated by CO2 (&amp;gt;97%), with only a small air contamination. Here we provide more-complete chemical data and isotopic compositions of CO2 and CH4, and Hg(0) concentration. We show that the gases emitted from different vents are fractionated by the partial dissolution of CO2 in water. Fractionation is also evident in the C-isotope composition (δ13CCO2), which varies between -0.04 and 1.15‰. We modeled this process to reconstruct the chemistry and δ13CCO2 of intact magmatic gases before fractionation. We argue that the CO2 prior to CO2 dissolution in water had δ13C ∼-0.4‰ and CO2/3He ∼1 × 1010. This model reveals that the gases emitted from Kolumbo originate from a homogeneous mantle contaminated with CO2, probably due to decarbonation of subducting limestone, which is similar to other Mediterranean arc volcanoes (e.g., Stromboli, Italy). The isotopic signature of CH4 (δ13C ∼-18‰ and δD ∼-117‰) is within a range of values typically observed for hydrothermal gases (e.g., Panarea and Campi Flegrei, Italy), which is suggestive of mixing between thermogenic and abiotic CH4. We report that the concentrations of Hg(0) in Kolumbo fluids are particularly high (∼61 to 1300 ng m-3) when compared to land-based fumaroles located on Santorini and worldwide aerial volcanic emissions. This finding may represent further evidence for the high level of magmatic activity at Kolumbo. Based on the geo-indicators of temperature and pressure, we calculate that the magmatic gases equilibrate within the Kolumbo hydrothermal system at about 270°C and at a depth of ∼1 km b.s.l. © 2019 Rizzo, Caracausi, Chavagnac, Nomikou, Polymenakou, Mandalakis, Kotoulas, Magoulas, Castillo, Lampridou, Marusczak and Sonke

Atmospheric mercury speciation dynamics at the high-altitude Pic du Midi Observatory, southern France

Continuous measurements of atmospheric gaseous elemental mercury (GEM), particulate bound mercury (PBM) and gaseous oxidized mercury (GOM) at the high-altitude Pic du Midi Observatory (PDM Observatory, 2877 m a.s.l.) in southern France were made from November 2011 to November 2012. The mean GEM, PBM and GOM concentrations were 1.86 ng m⁻³, 14 pg m⁻³ and 27 pg m⁻³, respectively and we observed 44 high PBM (peak PBM values of 33–98 pg m⁻³) and 61 high GOM (peak GOM values of 91–295 pg m⁻³) events. The high PBM events occurred mainly in cold seasons (winter and spring) whereas high GOM events were mainly observed in the warm seasons (summer and autumn). In cold seasons the maximum air mass residence times (ARTs) associated with high PBM events were observed in the upper troposphere over North America. The ratios of high PBM ARTs to total ARTs over North America, Europe, the Arctic region and Atlantic Ocean were all elevated in the cold season compared to the warm season, indicating that the middle and upper free troposphere of the Northern Hemisphere may be more enriched in PBM in cold seasons. PBM concentrations and PBM ∕ GOM ratios during the high PBM events were significantly anti-correlated with atmospheric aerosol concentrations, air temperature and solar radiation, suggesting in situ formation of PBM in the middle and upper troposphere. We identified two distinct types of high GOM events with the GOM concentrations positively and negatively correlated with atmospheric ozone concentrations, respectively. High GOM events positively correlated with ozone were mainly related to air masses from the upper troposphere over the Arctic region and middle troposphere over the temperate North Atlantic Ocean, whereas high GOM events anti-correlated with ozone were mainly related to air masses from the lower free troposphere over the subtropical North Atlantic Ocean. The ARTs analysis demonstrates that the lower and middle free troposphere over the North Atlantic Ocean was the largest source region of atmospheric GOM at the PDM Observatory. The ratios of high GOM ARTs to total ARTs over the subtropical North Atlantic Ocean in summer were significantly higher than those over the temperate and sub-arctic North Atlantic Ocean as well as that over the North Atlantic Ocean in other seasons, indicating abundant in situ oxidation of GEM to GOM in the lower free troposphere over the subtropical North Atlantic Ocean in summer

Atmospheric formaldehyde at El Teide and Pic du Midi remote high-altitude sites

International audienceFormaldehyde (CH2O) is a tracer of the photochemical activity of the atmosphere. Linked to air quality, CH2O is an ozone (O3) precursor and serves as a proxy for natural and anthropogenic reactive organic emissions. As a product of the photooxidation of methane (CH4) and other hydrocarbons (e.g., isoprene), CH2O represents an important source of radicals in the remote free troposphere. This work aims at improving the characterization of this part of the troposphere where data are scarce. In particular, this study assesses the presence of CH2O at two high-altitude remote sites: El Teide (TEI, 3570 m a.s.l., Tenerife, Canary Islands, Spain) and Pic du Midi (PDM, 2877 m a.s.l., French Pyrenees). Through ground-based remote sensing measurements performed during two field campaigns in July (TEI) and September (PDM) 2013, this study presents the vertical distribution of CH2O at both locations. Results at PDM show that CH2O mixing ratios follow a decreasing vertical profile with a mean maximum of 0.5 ± 0.2 nmol mol-1 (i.e., ppbv) at the instruments' altitude. At TEI, observations indicate an uplifted layer of CH2O with a mean maximum of 1.3 ± 0.3 nmol mol-1 at 3.8 km a.s.l. (i.e., 300 m above the instrument's altitude). At both remote sites, the observed CH2O levels are higher than expected for background methane oxidation (a threefold increase in the case of TEI). Air mass back trajectory analysis links CH2O observations with abundant natural (e.g. forests) and/or anthropogenic isoprene emissions from the region nearby PDM, while the high CH2O levels detected at TEI indicate in-plume formation of CH2O resulting from its precursors emitted from west-African and Canadian fires. Finally, as a key trace gas for O3 and HOx chemistries, we estimate the upper limit of bromine monoxide (BrO) in the free troposphere at TEI and PDM to be 0.8 and 1.5 pmol mol-1 (i.e., pptv) respectively

Atmospheric mercury concentrations observed at ground-based monitoring sites globally distributed in the framework of the GMOS network

International audienc