Robert A. Zierenberg

Arsenic distribution, speciation and solubility in shallow groundwater of Owens Dry Lake, California

Generation of dust particles from the Owens Lake playa creates a severe air pollution hazard in the western United States. Much of the dust produced from the dry lakebed is derived from salts formed by evaporation of saline groundwater that often contains high concentrations of dissolved arsenic (As). The objectives of this research were to study the spatial distribution of dissolved arsenic in the shallow groundwater, and to examine factors affecting arsenic solubility and speciation. Evapoconcentration, redox potential, pH, and mineral solubility were examined as factors regulating arsenic biogeochemistry. Dissolved arsenic concentrations ranged from 0.1 to 96 mg L 1 and showed a general increase from the shoreline to the center of the lakebed. Arsenic concentrations were strongly correlated to electrical conductivity (EC) and D suggesting that evapoconcentration is an important process regulating total As concentrations. Arsenite (As(III)) was the dominant form of inorganic arsenic at Eh values less than about 170 mV while arsenate (As(V)) was predominant at higher Eh values. Organic arsenic was negligible (0.21%) in all shallow groundwater samples. Dissolved arsenic concentrations do not appear to be strongly regulated by solid-phase reactions. Solid-phase arsenic concentrations generally ranged between 4.0 and 42.6 mg kg 1 and a maximum concentration range (20 to 40 mg kg 1 ) was reached as solution concentration increased up to 80 mg L 1 , indicating minimal sorption and/or precipitation of arsenic. Chemical equilibrium modeling indicated that orpiment (As2S3) was the only solid phase with a positive saturation index (indicating over-saturation), but only at high arsenic and sulfide concentrations. The findings of this research are important for assessing the potential environmental impacts of elevated arsenic concentrations on dust mitigation efforts taking place at Owens Dry Lake. Copyright

Acid-fog deposition at Kilauea volcano: A possible mechanism for the formation of siliceous-sulfate rock coatings on Mars

On the summit of Kilauea volcano, sulfur dioxide, which is continuously emitted from Halemaumau crater and rapidly sequestered into sulfuric-acid–rich aerosol entrained in the prevailing trade winds, is subsequently precipitated as acid fog immediately downwind from Kilauea caldera in the Kau Desert. The characteristic pH of surface tephra deposits is <4.0 in Sand Wash, a region of nearly continuous, acidic aerosol fallout immediately southwest of the caldera. Vertical exposures of unconsolidated tephras of the Keanakakoi Ash found within fissures and small, dry gullies are coated with thin rock coatings of amorphous silica and jarosite. These rock coatings are formed via an evaporative mechanism whereby acidic pore fluids, circulating in the upper few meters within the highly porous tephra, are wicked toward the walls of the gullies. Geochemical modeling of the rock coating formation process implies that the sulfate formation via evaporation occurs subsequent to minimal interaction of acidic pore fluids with the basaltic tephra. This also suggests that the cycle from acid-fog fallout to precipitation of the siliceous-sulfate rock coatings must occur quite rapidly. Acid-fog deposition of sulfate and silica at Kilauea may provide one mechanism for the origin of jarosite-bearing outcrops on Mars.

Origin of a rhyolite that intruded a geothermal well while drilling at the Krafla volcano, Iceland

Magma fl owed into an exploratory geothermal well at 2.1 km depth being drilled in the Krafl a central volcano in Iceland, creating a unique opportunity to study rhyolite magma in situ in a basaltic environment. The quenched magma is a partly vesicular, sparsely phyric, glass containing ~1.8% of dissolved volatiles. Based on calculated H 2 O-CO 2 saturation pressures, it degassed at a pressure intermediate between hydrostatic and lithostatic, and geothermometry indicates that the crystals in the melt formed at ~900 °C. The glass shows no signs of hydrothermal alteration, but its hydrogen and oxygen isotopic ratios are much lower than those of typical mantle-derived magmas, indicating that this rhyolite originated by anhydrous mantle-derived magma assimilating partially melted hydrothermally altered basalts.

Mineralogical and geochemical evolution of a basalt-hosted fossil soil (Late Triassic, Ischigualasto Formation, northwest Argentina): Potential for paleoenvironmental reconstruction

Reconstruction of paleoclimatic conditions in the Ischigualasto basin, northwestern Argentina, has been constrained by fi eld studies coupled with mineralogic, whole-rock, and fi ne-fraction chemical and stable isotope analysis of a Triassic (Carnian) basalt-hosted fossil soil. Field evidence, such as wedgeshaped aggregate structure and slickensides, indicate this was likely a paleo-Vertisol. Whole-rock analysis defi nes down-profi le trends in clay mineralogy and chemical composition that are consistent with modern soils developed upon basalt parent material. X-ray diffraction analysis indicates that the basaltic parent material is dominated by plagioclase with trace amounts of weathered 2:1 phyllosilicate. Overlying weathered horizons show a progressive loss of plagioclase and an increase in phyllosilicates with minor amounts of kandite clays and detrital quartz. X-ray diffraction analysis of the <2 µm fraction shows that the weathered layers are dominated by dioctahedral smectite (montmorillonite) with a minor fraction of kaolinite in the upper layers of the profi le. There is a progressive loss of basic cations in conjunction with an increase in concentration, on a wt% basis, of conservative elements from the basalt upward through the weathering profi le. The combined data indicate that this soil likely formed on a stable landscape in a cool and humid climate. In addition, the presence of quartz in the paleosol profi le suggests an eolian contribution of sediment during pedogenesis. Despite these apparent morphologic and bulk chemical trends indicative of a pedogenic origin, none of the authigenic minerals formed in isotopic equilibrium. However, based on measured oxygen and hydrogen isotope compositions, these minerals apparently formed from meteoric waters with a narrow range of δ 18 O and δD compositions at different temperatures. If this is correct, then amygdaloidal calcites formed at ~60‐100 °C, followed by precipitation of montmorillonites at 49‐57 °C during late-stage hydrothermal alteration. Finally, goethite formed at low temperatures of 6 ± 3 °C in a pedogenic environment. This complex history of hydrothermal alteration and pedogenic overprinting brings to light the need for cautious interpretation of bulk chemical trends in paleosols as a means for paleoclimate reconstruction. Comparison of the calculated Triassic oxygen isotopic compositions of meteoric water and soil temperature with modern environments suggests that this soil formed in a seasonal, humid, and cool climate.

THE LEGACY OF MERCURY CYCLING FROM MINING SOURCES IN AN AQUATIC ECOSYSTEM: FROM ORE TO ORGANISM

Clear Lake is the site of an abandoned mercury (Hg) mine (active intermittently from 1873 to 1957), now a U.S. Environmental Protection Agency Superfund Site. Mining activities, including bulldozing waste rock and tailings into the lake, resulted in approximately 100 Mg of Hg entering the lakes ecosystem. This series of papers represents the culmination of approximately 15 years of Hg-related studies on this ecosystem, following Hg from the ore body to the highest trophic levels. A series of physical, chemical, biological, and limnological studies elucidate how ongoing Hg loading to the lake is influenced by acid mine drainage and how wind-driven currents and baroclinic circulation patterns redistribute Hg throughout the lake. Methylmercury (MeHg) production in this system is controlled by both sulfate-reducing bacteria as well as newly identified iron-reducing bacteria. Sediment cores (dated with dichlorodiphenyldichlorethane [DDD], 210pb, and 14C) to approximately 250 cm depth (representing up to approximately 3000 years before present) elucidate a record of total Hg (TotHg) loading to the lake from natural sources and mining and demonstrate how MeHg remains stable at depth within the sediment column for decades to millenia. Core data also identify other stresses that have influenced the Clear Lake Basin especially over the past 150 years. Although Clear Lake is one of the most Hg-contaminated lakes in the world, biota do not exhibit MeHg concentrations as high as would be predicted based on the gross level of Hg loading. We compare Clear Lakes TotHg and MeHg concentrations with other sites worldwide and suggest several hypotheses to explain why this discrepancy exists. Based on our data, together with state and federal water and sediment quality criteria, we predict potential resulting environmental and human health effects and provide data that can assist remediation efforts.

40Ar/39Ar dating of Late Permian evaporites, southeastern New Mexico, USA

40Ar/39Ar dating of the potassium–magnesium sulfate mineral langbeinite from Permian evaporites of the Salado formation near Carlsbad, New Mexico, provides quantitative evidence that some salts in these deposits have not recrystallized for 251 Myr since deposition. Survival of Permian salts supports the possibility that Bacillus bacteria recovered from nearby halite was isolated in a closed system and represents a sample of uncontaminated Permian life. Local recrystallization of langbeinite and other nearby minerals is also indicated by the dating, suggesting both the need and the opportunity to document closed system behavior more rigorously. The shoaling and desiccation event recorded by the Salado formation began at least 1 Myr before the Permian–Triassic boundary. Temporal correlation of the Salado with the Zechstein evaporites of north-central Europe supports previously inferred regression models for the origin of these deposits. Significant paleoenvironmental change at the Permian–Triassic boundary thus occurred on a time scale more protracted than that implied by geologically instantaneous events such as bolide impacts.

ANTHROPOGENIC STRESSORS AND CHANGES IN THE CLEAR LAKE ECOSYSTEM AS RECORDED IN SEDIMENT CORES

Sediment cores were collected to investigate multiple stresses on Clear Lake, California, USA, through the period of European occupation to the present day. Earlier workers suggested the hypothesis that the use of mechanized earthmoving equipment, starting in the 1920s and 1930s, was responsible for erosion, mercury (Hg) contamination, and habitat loss stresses. Cores (approximately 2.5 m in depth) were collected in 1996 and 2000 from each of the three arms of the lake. Carbon-14 dating suggests that these cores represent as much as 3000 years of the lakes history, beginning long before European settlement. Total mercury (TotHg) and methylmercury (MeHg), dry matter, water, carbon, nitrogen, phosphorus, sulfur, and the stable isotopes 13C and 15N were measured at 5-cm intervals. Nearly all parameters show major changes at depths of 58-135 cm, beginning at ca. 1927 (dated with 210Pb). Accepting this date for concomitant major changes in seven cores yields an estimated 8.6 mm/yr average sedimentation rate after 1927. Pre-1927 sedimentation rates were approximately 1 mm/yr. Total mercury and MeHg, dry matter, phosphorus, and 15N increase significantly, whereas nitrogen, sulfur, carbon, and water content decrease significantly above the 1927 horizon. Both TotHg and MeHg show extremely large increases (roughly 10-fold) above the 1927 horizon. A peak in inorganic deposition rate and minimum values for percentage of water is present at depths corresponding to ca. 1970. Interestingly, the first 75 years of European settlement in the Clear Lake basin (including the most productive years of the Sulphur Bank Mercury Mine) appeared to have had undetectable effects on lake cores. Changes since 1927 were dramatic. The large increase in Hg beginning about 1927 corresponds to the use of heavy equipment to exploit the ore deposit at the mine using open-pit methods. Increases in sediment deposition from increased earthmoving in the basin and sulfate loading from the mine are the most likely explanations for the dramatic changes seen in the post-1927 sections of the cores.

High‐grade contact metamorphism in the Reykjanes geothermal system: Implications for fluid‐rock interactions at mid‐oceanic ridge spreading centers

Granoblastic hornfels identified in cuttings from the Reykjanes seawater-dominated hydrothermal system contains secondary pyroxene, anorthite, and hornblendic amphibole in locally equilibrated assemblages. Granoblastic assemblages containing secondary orthopyroxene, olivine, and, locally, cordierite and spinel occur within groups of cuttings that show dominantly greenschist facies hydrothermal alteration. Granoblastic plagioclase ranges continuously in composition from An54 to An96, in contrast with relict igneous plagioclase that ranges from An42 to An80. Typical hydrothermal clinopyroxene compositions range from Wo49En3Fs48 to Wo53En30Fo17; clinopyroxene from the granoblastic grains is less calcic with an average composition of Wo48En27Fs25. The hornfels is interpreted to form during contact metamorphism in response to dike emplacement, resulting in local recrystallization of previously hydrothermally altered basalts. Temperatures of granoblastic recrystallization estimated from the 2-pyroxene geothermometer range from 927°C to 967°C. Redox estimates based on the 2-oxide oxybarometer range from log fO2 of −13.4 to −15.9. Granoblastic hornfels comprised of clinopyroxene, orthopyroxene, and calcic plagioclase have been described in a number of ancient hydrothermal systems from the conductive boundary layer between the hydrothermal system and the underlying magma source, most notably in Integrated Ocean Drilling Program Hole 1256D, Ocean Drilling Program Hole 504B, and in the Troodos and Oman ophiolites. To our knowledge, this is the first evidence of high-grade contact metamorphism from an active geothermal system and the first description of equilibrated amphibole-absent pyroxene hornfels facies contact metamorphism in any mid-ocean ridge (MOR) hydrothermal system. This contribution describes how these assemblages develop through metamorphic reactions and allows us to predict that higher-temperature assemblages may also be present in MOR systems.

CLEAR LAKE SEDIMENTS: ANTHROPOGENIC CHANGES IN PHYSICAL SEDIMENTOLOGY AND MAGNETIC RESPONSE

We analyzed the sedimentological characteristics and magnetic properties of cores from the three basins of Clear Lake, California, USA, to assess the depositional response to a series of land use changes that occurred in the watershed over the 20th century. Results indicate that distinct and abrupt shifts in particle size, magnetic concentration/mineralogy, and redox conditions occur concurrently with a variety of ecological and chemical changes in lake bed sediments. This coincidence of events occurred around 1927, a datum determined by an abrupt increase in total mercury (Hg) in Clear Lake cores and the known initiation of open-pit Hg mining at the Sulphur Bank Mercury Mine, confirmed by 210Pb dating. Ages below the 1927 horizon were determined by accelerator mass spectrometry on 14C of coarse organic debris. Calculated sedimentation rates below the 1927 datum are approximately 1 mm/yr, whereas rates from 1927 to 2000 are up to an order of magnitude higher, with averages of approximately 3.5-19 mm/yr. In both the Oaks and Upper Arms, the post-1927 co-occurrence of abrupt shifts in magnetic signatures with color differences indicative of changing redox conditions is interpreted to reflect a more oxygenated diagenetic regime and rapid burial of sediment below the depth of sulfate diffusion. Post-1927 in the Oaks Arm, grain size exhibits a gradual coarsening-upward pattern that we attribute to the input of mechanically deposited waste rock related to open-pit mining activities at the mine. In contrast, grain size in the Upper Arm exhibits a gradational fining-upward after 1927 that we interpret as human-induced erosion of fine-grained soils and chemically weathered rocks of the Franciscan Assemblage by heavy earthmoving equipment associated with a road- and home-building boom, exacerbated by stream channel mining and wetlands destruction. The flux of fine-grained sediment into the Upper Arm increased the nutrient load to the lake, and that in turn catalyzed profuse cyanobacterial blooms through the 20th century. The resulting organic biomass, in combination with the increased inorganic sediment supply, contributed to the abrupt increase in sedimentation rate after 1927.

Chemistry of vent fluids and its implications for subsurface conditions at Sea Cliff hydrothermal field, Gorda Ridge

We report here the first compositional data for fluids from the Sea Cliff hydrothermal field, northern Gorda Ridge, collected in 2000, 2002, and 2004. An unusual aspect of this site is its location ∼2.6 km east of the axis of spreading, leading to speculation since its discovery in 1988 that this may be an “older” hydrothermal field, as it occurs on crust that the spreading rate would predict to be ∼100,000 years old. Our results suggest this hydrothermal system is being driven by subsurface magma, as evidenced by (1) elevated 3He/heat ratios, (2) relatively high concentrations of He, and (3) chloride contents less than seawater in the hydrothermal fluids. The measured fluid temperatures were ≤308°C, but we infer they were >400°C at depth. In spite of these elevated temperatures, the fluids exiting from these vents are clear, a consequence of extremely low transition metal concentrations. We attribute the low transition metal contents to loss of these metals below the seafloor, most likely as a result of the slightly elevated pH of the fluids. Neither the fluid compositions nor the setting provides evidence that buried sediments and/or organic matter are responsible for raising the fluid pH. Our favored explanation for the elevated pH is that calcite, deposited as a vein-filling mineral at this site, perhaps when it was closer to the axis and a hydrothermal downflow zone, is currently being dissolved by the hydrothermal fluids. This hypothesis is supported by our geochemical modeling results that suggest the fluids are close to saturation with calcite at in situ conditions. Elevated fluid pH is observed at a number of hydrothermal sites on the global mid-ocean ridge system, and the reason for this has not been well understood. Dissolution of previously deposited calcite may be a heretofore unrecognized mechanism that can explain these observations. Finally, our data suggest the compositions of these fluids are unchanged from 2002 to 2004 and are consistent with water column observations first made at this site in 1985. We therefore interpret the Sea Cliff site to be another hydrothermal area marked by long-term stability in fluid compositions and temperatures.