Roberts, Hannah
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Item-typ:Veröffentlichung, Mercury in the hydrothermal fluids and gases in Paleochori Bay, Milos, Greece(Elsevier Science, 2021-06-20); ; ; Seafloor hydrothermal activity may constitute a considerable mercury (Hg) source to the oceans, but the flux from marine shallow-water hydrothermal systems (MSWHS) remains poorly constrained to date. To study the presence of Hg in MSWHS in Paleochori Bay (Milos Island, Greece), sea surface, bottom, pore fluid and hydrothermal gas samples were collected in June of 2017, October of 2018 and July of 2020, and analyzed for Cl, Br, SO4, As, Ca, Fe, K, Mg, Mn, Na, Si, Sr, H2S, unfiltered total Hg (THg), and filtered Hg (Hgdiss). Specific sites were selected for the analysis of volatile elemental Hg (Hgo), dimethylmercury (DMHg), monomethylmercury (MMHg), and Hg in the gas phase (Hggas). Concentrations of THg observed in samples collected from the sea surface were elevated compared to surface samples taken outside Paleochori Bay. The highest surface water concentrations (~10 to 15 pM) were measured in samples collected directly above shallow-water hydrothermal discharge areas. Pore fluids outside Paleochori Bay were significantly lower in THg (0.8 to 8.6 pM) than those taken inside (17.4 to 1511 pM). Porewaters collected from areas with visible gaseous or fluid emission were highly variable but generally elevated in THg concentrations (185 to 5066 pM). Concentrations within gases ranged from 0.7 to 2791 nmol/m3. The vast majority of samples with highly elevated THg (> 100 pM) had low Na/K ratios (< 15), indicative of rapidly rising fluid. Concentrations of Hg0, DMHg, and MMHg were below detection limits in all samples. Bottom substrate type (e.g., rocky vs. sediment covered) likely affected infiltration rates of oxygenated seawater below the sediment-water interface, thereby affecting Hg speciation and removal by precipitation. Flux rates from porewaters compared to those with gaseous emission were high (12.56 to 1088 mol THg/year and 0.37 to 1.85 mol THg/year). Sites with slow gaseous emission rates are hypothesized to have extended subsurface reaction times, resulting in lower Hg concentrations emitted to bottom waters. However, increasing rates of gas emission did not necessarily indicate higher Hg concentrations. The scavenging of Hg in the sediments and advective flux out of Paleochori Bay likely prevent significant accumulations of THg in the water column of Paleochori Bay. The total atmospheric flux from Paleochori Bay using average flux calculations over the entire surface area would contribute 6 mmol Hg/year to the atmosphere. We hypothesize that Hg concentrations within the pore fluids of Paleochori Bay reflect a balance between mixing and precipitation in the subsurface. A three-component mixing system of vapor, brine and seawater determines THg concentrations; however, precipitation due to sulfur cycling, changes in redox conditions and temperature, all play a crucial role in removing Hg from emitted fluids and gases.Wissenschaftlicher ArtikelBand:23366 69 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Marine shallow water systems as natural sources of mercury to local systems(2024-02-09); ; ; Hydrothermal systems transfer heat and mass through circulating water in a permeable geological formation. The circulating water (hydrothermal fluid) undergoes multiple subsurface processes, gaining and losing constituents dependent upon physical, geological, and chemical factors. According to Beaulieu and Szafranski (2020), a total of 184 systems are confirmed active across the globe. Of these, 43 are considered MSWHS (above 200 m depth). Areas of emission are characterized by diffuse fluid or gas emission (diffuse), or distinct localized points of emission (point sources). Both types of emission can be sedimented or unsedimented dependent upon local conditions. The goal of the doctoral project was to provide a fundamental understanding of Hg prevalence in MSWHS from three systems (Milos, Greece; Vulcano, Italy; Panarea, Italy). Each system presented unique environmental conditions where Hg was known to be present. The study area on Milos contained multiple areas of diffuse emission and point sources at depths of less than 5 m. Emission was a mixture of fluids, gases, and brines, with extensive microbial white mat activity. The study area on Vulcano dominantly emitted gases at depths of less than 5 m with limited white mat activity. The study areas on Panarea were at greater depth (5 to 30 m), were a mixture of gas and fluid, with greater biodiversity present than the other sites. White mat activity on Panarea was limited. Primary indicators of Hg emission were evaluated and the effect of MSWHS on local environments were investigated as both direct (e.g., through mercury rich gases to the atmosphere) and indirect (e.g., transport and dissolution of precipitated mercury particles). Each study location represented a unique MSWHS, where environmental conditions greatly affected Hg concentrations in emitted fluids and gases. Additionally, the relationship between Hg and the local environment revealed further information on the subsurface environment of the MSWHS. Samples were collected from diffuse and point sources at approximately 10 cm depth where possible (e.g., sedimented) through PTFE tubes. Gases were collected in Tedlar© bags at the sediment-water interface. Analysis was completed through CV-AFS (THg, Hgdiss, Hg0, DMHg, Hggas, THg in sediments), and gas chromatography (MMHg). Organic species of Hg were not present above detection limits within the hydrothermal fluids sampled at any site. Fluid samples were generally comprised of Hg bound to colloids and particles greater than 0.45 µm (THg). Concentrations of THg within sampled hydrothermal fluids ranged from below local background seawater values (0.5 to 5 pM) to 249 nM. Bound and unbound Hg in filtered (0.45 µm) samples (Hgdiss) generally represented a small portion of THg (BDL to 273 pM). Additionally, Hg0 was not generally a significant portion of THg (BDL to 5.3 pM). Therefore, the greatest Hg specie in hydrothermal fluid was Hg2+, the vast majority bound to colloids and particles > 0.45 µm. Within hydrothermal gases, total Hg (Hggas) ranged from below detection limits to 2,792 nmol / m3. The greatest indicators of high Hg concentrations within gases and fluids were associated with the rate of flow and the presence of sedimentation. Higher rates of flow, particularly when paired with high temperatures, generally indicated high Hg concentrations within the fluid or gas. However, the presence of sedimentation overlying the hydrothermal source greatly decreased the concentration. Higher rates of flow limited the temporal exposure of hydrothermal fluids and gases to surrounding material, were associated with higher temperatures, and tended to prevent sedimentation at the orifice of the hydrothermal source. However, where sedimentation was present at the orifice, high flow rates were not associated with high Hg concentrations. The effect of MSWHS on local environments was evident through sediment and seawater samples collected at each location. Background sediment samples collected at each site were within normal values. Accumulations of Hg within sediments associated with hydrothermal sources was observed (0.3 to 49.5 nmol / g). However, these accumulations were limited to direct interaction with the point source. Samples taken centimeters away from hydrothermal sources did not show significant Hg accumulation compared to background samples (0.3 to 0.5 nmol / g). The effect of MSWHS on overlying seawater was most dramatically observed in Panarea and Vulcano. In Baia di Levante on the island of Vulcano, the combination of a high density of MSWHS activity coupled with shallow water (< 1 m), resulted in accumulations up to 186 pM THg. Off the coast of Panarea, at La Calcara, a vertical profile directly above the main point source maintained elevated THg (< 1.1 pM) over 20 m of depth. It can be concluded that MSWHS contribute Hg to the local environment. The extent of impact is largely controlled by environmental factors (e.g., flow rate and sedimentation). However, immediate removal to sediments after emission to overlying seawater is not supported. Rather, Hg is transported away from point sources and areas of diffuse emission. Therefore, the impact of MSWHS on Hg cycling may extend beyond the immediate local area.Dissertation242 196 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Hg in the hydrothermal fluids and gases in Baia di Levante, Vulcano, ItalyThe importance of fluid and gaseous mercury (Hg) emissions from hydrothermal systems in the shallow, coastal ocean is poorly constrained. However, there are indicators that they could be a significant natural Hg source. We evaluated the hydrothermal Hg emissions around Vulcano Island, Aeolian Arc, Italy, which is host to a marine shallow-water hydrothermal system (MSWHS) in Baia di Levante. Fluids were collected with porewater probes, and gases were collected into Tedlar© bags. Total Hg (THg) concentrations in the hydrothermal fluids ranged from 2.9 to 2888 pM. The concentrations of volatile Hg were below 8 pM and trended positively with increasing temperature. Monomethyl Hg (MMHg) was not detected. Total Hg in the gases ranged from 0.03 to 1.82 μmol/m3. High concentrations of THg were associated with low Cl-concentrations, low pH-values, and high K/Cl and Mg/Cl ratios. Concentrations of THg in the hydrothermal fluids resulted from mixing between meteoric water, seawater, condensed fumarolic vapor, and a deep hydrothermal fluid. However, not all low-Cl fluids were high THg samples. Samples taken along the coast of the La Fossa crater contained substantially less THg (2.9 to 29.4 pM), despite similar hydrothermal indicators as those in the Baia di Levante samples. These data support subsurface circulation models, which discuss the downslope flow of condensed La Fossa crater gases to the coast. In the hydrothermally active area of Baia di Levante, concentrations of THg were elevated relative to background surface seawater, ranging from 40 to 5110 pM. In comparison, the remainder of the bay ranged from 0.8 to 2.4 pM. The flux of Hg to the atmosphere from surface waters was calculated through dissolved Hg concentrations. The flux at each sampled site ranged from 0 to 19.6 pmol/m2/hr. The highest flux rates were determined for those areas with visible hydrothermal activity, particularly those with high gaseous emission rates. Surface water concentrations declined rapidly away from point sources, indicating atmospheric emission, dilution through mixing, or scavenging and sedimentation. In hydrothermally active areas, Baia di Levante sediments contained THg concentrations of 2.42 nmol/g and 49.52 nmol/g, which were significantly above background (0.03 nmol/g). The largest gaseous point source, Bambino, released >2 L of gas per second. Although, Hg concentrations in the gas were low (113 to 122 nmol/m3) relative to other measured point sources near the beach (1768 to 1817 nmol/m3), due to the volume of discharge, surface water concentrations were elevated (131 pM). The Hg present in the hydrothermal system of Vulcano contributes Hg to the atmosphere and local seawater as a natural source.Wissenschaftlicher ArtikelBand:24460 43