The speciation of mercury in hydrothermal systems,with applications to ore deposition

Hg in hydrothermal systems is generally thought to be transported as Hg-S complexes. However, the abundance of Hg⁰_vap, in geothermal emissions suggests that Hg⁰_eq, is present in the liquid phase of geothermal systems. Calculations for reducing fluids (HS^? dominant over SO⁼₄) in equilibrium with cinnabar indicate that Hg⁰_eq, can be quite abundant relative to other species at temperatures above 200°C. Increasing pH and temperature, and decreasing total S, ionic strength, and pO₂ all promote the abundance of Hg⁰_eq. When a vapor phase develops from a geothermal liquid, Hg partitions strongly into the vapor as Hg⁰_vap. Vapor transport at shallow level then results in the formation of Hg halos around shallow aquifers as well as in a flux of Hg to the atmosphere. Hg deposition may occur in response to mixing with oxidizing or acidic water, turning Hg⁰_eq, into Hg⁺⁺, with subsequent cinnabar precipitation. When pyrite is the stable Fe-sulfide, cinnabar solubility is at its lowest, so cinnabar + pyrite assemblages are common. Cinnabar + hematite ± pyrite can precipitate from more oxidized or S-poor water. Hg⁰_liq, can occur as a primary mineral, in coexistence with all common Fe-sulfides and oxides. Cinnabar ± Hg⁰_liq cannot coexist with pyrrhotite or magnetite at temperatures between 100° and 250°C. Evidence from Hg deposits indicates that many formed from dilute hydrothermal fluids in which Hg probably occurred as Hg⁰_eq. In S-rich systems, Hg may occur as Hg-S complexes, and in saline waters it can occur as Hg-Cl complexes.     

Tide circulation patterns in a coastal lagoon under sea-level rise

This study evaluates the patterns and effects of relative sea-level rise on the tidal circulation of the basin of the Ria Formosa coastal lagoon using a process-based model that is solved on an unstructured mesh. To predict the changes in the lagoon tidal circulation in the year 2100, the model is forced by tides and a static sea level. The bathymetry and the basin geometry are updated in response to sea-level rise for three morphological response scenarios: no bed updating, barrier island rollover, and basin infilling. Model results indicate that sea-level rise (SLR) will change the baseline current velocity patterns inside the lagoon over the ~100-year study period, due to a strong reduction in the area of the intertidal basin. The basin infilling scenario is associated with the most important adjustments of the tidal circulation (i.e., increases in the flood velocities and delays in the ebb tide), together with an increase in the cumulative discharges of the tidal inlets. Under sea-level rise and in the basin infilling scenario, the salt marshes and tidal flats experience increases in the tidal range and current asymmetry. Basin infilling changes the sediment flushing capacity of the lagoon, leading to the attenuation of the flood dominance in the main inlet and the strengthening of the flood dominance in the two secondary inlets. The predictions resulting from these scenarios provide very useful information on the long-term evolution of similar coastal lagoons that experience varying degrees of SLR. This study highlights the need for research focusing on the quantification of the physical and socio-economic impacts of SLR on lagoon systems, thus enabling the development of effective adaptation strategies.