Kevin L. Brown

The flux of gold and related metals through a volcanic arc, Taupo Volcanic Zone, New Zealand

We determined the concentrations of gold, silver, arsenic, antimony, and mercury in deep hydrothermal solutions (∼1 km depth, 200 to >300 °C) from six geothermal systems and calculated their fluxes for a 250-km-long segment of volcanic arc in the Taupo Volcanic Zone, New Zealand. The three orders of variation in the concentrations (

Trace metal chemistry and silicification of microorganisms in geothermal sinter, Taupo Volcanic Zone, New Zealand

Abstract As part of a pilot study investigating the role of microorganisms in the immobilisation of As, Sb, B, Tl and Hg, the inorganic geochemistry of seven different active sinter deposits and their contact fluids were characterised. A comprehensive series of sequential extractions for a suite of trace elements was carried out on siliceous sinter and a mixed silica-carbonate sinter. The extractions showed whether metals were loosely exchangeable or bound to carbonate, oxide, organic or crystalline fractions. Hyperthermophilic microbial communities associated with sinters deposited from high temperature (92–94°C) fluids at a variety of geothermal sources were investigated using SEM. The rapidity and style of silicification of the hyperthermophiles can be correlated with the dissolved silica content of the fluid. Although high concentrations of Hg and Tl were found associated with the organic fraction of the sinters, there was no evidence to suggest that any of the heavy metals were associated preferentially with the hyperthermophiles at the high temperature (92–94°C) ends of the terrestrial thermal spring ecosystems studied.

Chemistry and stratification of Antarctic meltwater ponds II: Inland ponds in the McMurdo Dry Valleys, Victoria Land

Meltwater ponds in the Victoria Valley and in the Labyrinth at the head of the Wright Valley of Victoria Land were sampled in January (summer) and October (late winter) of 2004 to establish their geochemistry and stratification, and to compare this with that of coastal meltwater ponds at a similar latitude near Bratina Island. In summer, vertical profiles were measured in 14 ponds; 10 were thermally stratified (maximum ΔT = 11.5°C) and 12 demonstrated a conductivity increase (∼25x) in the lowest 10–20 cm of the water column. When 11 of these ponds were resampled in October, the ice columns were stratified with respect to conductivity and five ponds had highly saline (up to 148 mS cm−1), oxygenated basal brines present under the ice. Basal brines and summer melt waters were Na-Cl dominated, and Victoria Valley pond meltwaters were enriched in Ca relative to the Labyrinth ponds. Early gypsum precipitation directs the chemical evolution of residual brine during freezing. These ponds were enriched in NO3 relative to the coastal ponds at Bratina Island, due to dissolution of nitrate-bearing soil salts, and the reduced influence of marine aerosols and biological productivity on pond chemistry.

PChemistry and stratification of Antarctic meltwater ponds I: Coastal ponds near Bratina Island, McMurdo Ice Shelf

The geochemistry and vertical stratification of shallow meltwater ponds at 78°S near Bratina Island (McMurdo Ice Shelf) have been determined for late winter (October) and summer (January) conditions as part of the Latitudinal Gradient Project. Of the five frozen ponds investigated in October, all were stratified with respect to conductivity, and three had highly saline basal brines beneath the ice at temperatures of −16 to −20°C. In the ice column, inclusions of saline fluid were observed in channels between ice crystals; the abundance increasing with depth and decreasing ice crystal size. In January, seven of the ten ponds investigated (including ponds sampled in October) retained conductivity stratification, whereas significant thermal stratification was observed in only three ponds (maximum ΔT = 5.5°C). Basal brines, ice and meltwaters were Na-Cl or Na-SO4 dominated. FREZCHEM52 modelling, supported by changes in ion ratios, indicated that the precipitation of mirabilite (Na2SO4.10H2O) and gypsum (CaSO4.2H2O) during progressive freezing is an important determinant in chemical evolution of the basal brine. High pH (8.8–11.2) and over-saturation with respect to dissolved oxygen (> 20 mg L−1) in summer, and the presence of sulphide ions in basal brines in winter, occurred in those ponds which experienced high biological productivity during the summer months.

Source and transport of trace metals in the Hatea river catchment and estuary, Whangarei, New Zealand.

Abstract A survey of Cu, Pb, Zn, Cr, and As concentrations in bed sediment, fresh waters, storm waters, and suspended particulate matter (SPM) has been undertaken in the Hatea River catchment, Whangarei, New Zealand. The most recently deposited sediment in the Hatea River estuary has elevated levels of Cu, Pb, and Zn, derived from tributaries draining the more densely‐populated western side of the catchment, city stormwater drains, and Cu‐bear‐ing antifoulants used in the marina. All trace metals were transported in both “dissolved” and particulate form in the freshwater tributaries. However, an estimated loading of metal transported by the lower Hatea River under different flow regimes indicated that trace metals were predominantly conveyed by coarse (>0.45 μm) SPM during the periods of highest river flow. Of the trace metals studied, Pb showed the strongest association with coarse SPM and the greatest potential for accumulation in estuarine sediment, demonstrating little tendency to be leached from sediment under simulated estuarine conditions.

Modelling chloride and CO2 chemistry in the wairakei geothermal reservoir

Abstract The chloride and CO 2 chemistry of the Wairakei geothermal field, New Zealand, has been modelled using an extended version of the geothermal simulator TOUGH2. This version of the simulator solves the equations for the transport of reacting chemical species in multi-phase fluids, and is applied here to a detailed, full-scale geothermal reservoir model for the first time. Reactions involving the speciation of CO 2 to H 2 CO 3 and HCO 3 − are included in the model, as is the “Henrys Law” reaction for exsolution of aqueous CO 2 to the vapour phase. Because CO 2 speciation in water is pH dependent, a reaction involving the most important weak acid buffer at Wairakei (H 4 SiO 4 ) has also been included in the model. The chloride is treated as a conservative, non-reacting species that is present only in the liquid phase. Results from the model compare favourably with measured chloride and CO 2 data from Wairakei covering the period 1959–1987.