Marine shallow water systems as natural sources of mercury to local systems

Abstract

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.