Dennis K. Bird

Arsenic speciation in pyrite and secondary weathering phases, Mother Lode Gold District, Tuolumne County, California

Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of As along the Melones fault zone in the southern Mother Lode Gold District of California. Mine tailings and associated weathering products from partially submerged inactive gold mines at Don Pedro Reservoir, on the Tuolumne River, contain approx. 20-1300 ppm As. The highest concentrations are in weathering crusts from the Clio mine and nearby outcrops which contain goethite or jarosite. As is concentrated up to 2150 ppm in the fine-grained (<63 mu-m) fraction of these Fe-rich weathering products. Individual pyrite grains in albite-chlorite schists of the Clio mine tailings contain an average of 1.2 wt. percent As. Pyrite grains are coarsely zoned, with local As concentrations ranging from approx. 0 to 5 wt. percent. Electron microprobe, transmission electron microscope, and extended X-ray absorption fine-structure spectroscopy (EXAFS) analyses indicate that As substitutes for S in pyrite and is not present as inclusions of arsenopyrite or other As-bearing phases. Comparison with simulated EXAFS spectra demonstrates that As atoms are locally clustered in the pyrite lattice and that the unit cell of arsenian pyrite is expanded by approx. 2.6 percent relative to pure pyrite. During weathering, clustered substitution of As into pyrite may be responsible for accelerating oxidation, hydrolysis, and dissolution of arsenian pyrite relative to pure pyrite in weathered tailings. Arsenic K-edge EXAFS analysis of the fine-grained Fe-rich weathering products are consistent with corner-sharing between As(V) tetrahedra and Fe(III)-octahedra. Determinations of nearest-neighbor distances and atomic identities, generated from least-squares fitting algorithms to spectral data, indicate that arsenate tetrahedra are sorbed on goethite mineral surfaces but substitute for SO4 in jarosite. Erosional transport of As-bearing goethite and jarosite to Don Pedro Reservoir increases the potential for As mobility and bioavailability by desorption or dissolution. Both the substrate minerals and dissolved As species are expected to respond to seasonal changes in lake chemistry caused by thermal stratification and turnover within the monomictic Don Pedro Reservoir. Arsenic is predicted to be most bioavailable and toxic in the reservoirs summer hypolimnion.

Genesis of hexavalent chromium from natural sources in soil and groundwater

Naturally occurring Cr(VI) has recently been reported in ground and surface waters. Rock strata rich in Cr(III)-bearing minerals, in particular chromite, are universally found in these areas that occur near convergent plate margins. Here we report experiments demonstrating accelerated dissolution of chromite and subsequent oxidation of Cr(III) to aqueous Cr(VI) in the presence of birnessite, a common manganese mineral, explaining the generation of Cr(VI) by a Cr(III)-bearing mineral considered geochemically inert. Our results demonstrate that Cr(III) within ultramafic- and serpentinite-derived soils/sediments can be oxidized and dissolved through natural processes, leading to hazardous levels of aqueous Cr(VI) in surface and groundwater.

No climate paradox under the faint early Sun

Environmental niches in which life first emerged and later evolved on the Earth have undergone dramatic changes in response to evolving tectonic/geochemical cycles and to biologic interventions, as well as increases in the Sun’s luminosity of about 25 to 30 per cent over the Earth’s history. It has been inferred that the greenhouse effect of atmospheric CO2 and/or CH4 compensated for the lower solar luminosity and dictated an Archaean climate in which liquid water was stable in the hydrosphere. Here we demonstrate, however, that the mineralogy of Archaean sediments, particularly the ubiquitous presence of mixed-valence Fe(II–III) oxides (magnetite) in banded iron formations is inconsistent with such high concentrations of greenhouse gases and the metabolic constraints of extant methanogens. Prompted by this, and the absence of geologic evidence for very high greenhouse-gas concentrations, we hypothesize that a lower albedo on the Earth, owing to considerably less continental area and to the lack of biologically induced cloud condensation nuclei, made an important contribution to moderating surface temperature in the Archaean eon. Our model calculations suggest that the lower albedo of the early Earth provided environmental conditions above the freezing point of water, thus alleviating the need for extreme greenhouse-gas concentrations to satisfy the faint early Sun paradox.

40Ar39Ar geochronology of Tertiary mafic intrusions along the East Greenland rifted margin: Relation to flood basalts and the Iceland hotspot track

Abstract The East Greeland Tertiary Igneous Province includes the largest exposed continental flood basalt sequence within the North Atlantic borderlands. More than ten layered gabbro complexes, including the ∼55 Ma Skaergaard intrusion, and a large dolerite sill complex are the plutonic equivalents of flood basalts; both lavas and intrusions have been regarded as synchronous with continental breakup at 57-54 Ma. We report ten new ages of the mafic intrusions, determined by40Ar 39Ar incremental heating experiments, demonstrating that the mafic intrusions formed in two distinct time windows. Only Intrusion II of the Imilik Gabbro Complex, the Skaergaard intrusion, and the Sorgenfri Glestcher Sill Complex formed at 57-55 Ma coeval with the eruption of regional flood basalts and continental breakup. Other layered gabbro intrusions at Imilik (Intrusion III), Kruuse Fjord, Igtutarajik, Nordre Aputiteˆq, Kap Edvard Holm, and Lilloise are distinctly younger and formed between 50 and 47 Ma. Plate-kinematic models indicate the axis of the ancestral Iceland mantle plume was located under Central Greenland at ∼60 Ma and subsequently crossed the East Greenland rifted continental margin. We propose that tholeiitic magmatism along the East Greenland rifted margin largely occurred in three distinct pulses at 62-59 Ma (lavas and dykes), 57-54 Ma (lavas, dykes, sills, and some gabbros) and 50-47 Ma (gabbros, dykes and rare lavas), related to discrete mantle melting episodes triggered by plume impact, continental breakup, and passage of the plume axis, respectively. This model implies northwestward continental drift of Greenland relative to the plume axis by ∼3.9-5.0 cm/yr between ∼60 and ∼49 Ma, consistent with estimates from seismic studies of submerged flood basalts.

Chromium Geochemistry of Serpentine Soils

Serpentine soils derived from the weathering of ultramafic rocks, mainly ophiolitic serpentinites, are typically characterized by Cr concentrations in excess of 200 mg kg-1, comparatively higher than non-serpentine soils. We review the chemistry of Cr in serpentine soils and their protoliths, focusing on serpentine soils collected from New Caledonia, Oregon, and California. Overall, serpentine soils are slightly acidic (average pH of ∼6), contain a variety Fe(III) oxides (magnetite and hematite), Fe(III) (oxy)hydroxides, phyllosilicates (serpentine and chlorite), and clays (smectites and vermiculites), and contain concentrations of Cr (> 200 mg kg-1), Ni (> 1,000 mg kg-1), and Mn (> 200 mg kg-1) exceeding values of non-serpentine soils. Although Cr concentrations in serpentine soils have been reported as high as 6 wt% in New Caledonia, Cr values in New Caledonia, Oregon, and California serpentine soils evaluated in this study range from 827 to 9,528 mg kg-1. Chromium(III) is the only valence state observed in the serpentine soil solids; however, Cr(VI) has been identified in New Caledonia and California serpentine soil solutions at concentrations below 30 μM. The enrichment and range of Cr concentrations in serpentine soils are directly related to the presence of Cr-spinels, specifically chromite and Cr-magnetite. These phases are resistant to weathering and are preserved in the soil environment; however, oxidation of Cr(III) from Cr-spinels by high-valent Mn oxides, or other strong oxidants, is a potential source of Cr(VI) identified in serpentine soil solutions. Due to the weathering resistant nature of the Cr-spinels, Cr-bearing silicates including clay minerals, Cr-chlorite, Cr-garnet, Cr-mica, and Cr-epidote are more viable sources of Cr identified in vegetation, soil extractions, soil solutions, and related waters.

Geochemical and mineralogical controls on trace element release from the Penn Mine base-metal slag dump, California

Base-metal slag deposits at the Penn Mine in Calaveras County, California, are a source of environmental contamination through leaching of potentially toxic elements. Historical Cu smelting at Penn Mine (1865–1919) generated approximately 200,000 m3 of slag. The slag deposits, which are flooded annually by a reservoir used for drinking water and irrigation, also may be in contact with acidic ground waters (pH<4) from the adjacent mine area. Slags vary from grey to black, are glassy to crystalline, and range in size from coarse sand to large (0.6×0.7×1.5 m), tub-shaped casts. Metals are hosted by a variety of minerals and two glass phases. On the basis of mineralogy, slags are characterized by 4 main types: fayalite-rich, glassy, willemite-rich, and sulfide-rich. The ranges in metal and metalloid concentrations of 17 slag samples are: As, 0.0004–0.92; Ba, 0.13–2.9; Cd, 0.0014–1.4; Cu, 0.18–6.4; Pb, 0.02–11; and Zn, 3.2–28 wt.%. Leachates from Toxicity Characteristic Leaching Procedure tests (acetic acid buffered at pH 4.93) on two willemite-rich slags contained Cd and Pb concentrations (up to 2.5 and 30 mg/l, respectively) in excess of US Environmental Protection Agency (USEPA) regulatory limits. Analyses of filtered (0.45 μm) water, collected within the flooded slag dump during reservoir drawdown, reveal concentrations of Cd (1.7 μg/l), Cu (35 μg/l), and Zn (250 μg/l) that exceed USEPA chronic toxicity guidelines for the protection of aquatic life. Data from field and laboratory studies were used to develop geochemical models with the program EQ3/6 that simulate irreversible mass-transfer between slag deposits and reservoir waters. These models include kinetic rate laws for abiotic sulfide oxidation and surface-controlled dissolution of silicates, oxides, and glass. Calculations demonstrate that the main processes controlling dissolved metal concentrations are (1) dissolution of fayalite, willemite, and glass; (2) sulfide oxidation; and (3) secondary phase precipitation. Close agreement between model results and measured concentrations of Al, Ba, Cu, Fe, SiO2, and SO4 in the slag dump pore waters suggests that the dissolved concentrations of these elements are controlled by solubility equilibrium with secondary phases. Differences between predicted and measured Cd and Pb concentrations imply that field weathering rates of glass and sulfides are approximately two orders of magnitude lower than laboratory rates. Overprediction of Pb release may also reflect other attenuation processes in the natural system, such as sorption or coprecipitation.

A lower limit for atmospheric carbon dioxide levels 3.2 billion years ago

The quantification of greenhouse gases present in the Archaean atmosphere is critical for understanding the evolution of atmospheric oxygen, surface temperatures and the conditions for life on early Earth. For instance, it has been argued that small changes in the balance between two potential greenhouse gases, carbon dioxide and methane, may have dictated the feedback cycle involving organic haze production and global cooling. Climate models have focused on carbon dioxide as the greenhouse gas responsible for maintaining above-freezing surface temperatures during a time of low solar luminosity. However, the analysis of 2.75-billion-year (Gyr)-old palaeosols—soil samples preserved in the geologic record—have recently provided an upper constraint on atmospheric carbon dioxide levels well below that required in most climate models to prevent the Earths surface from freezing. This finding prompted many to look towards methane as an additional greenhouse gas to satisfy climate models. Here we use model equilibrium reactions for weathering rinds on 3.2-Gyr-old river gravels to show that the presence of iron-rich carbonate relative to common clay minerals requires a minimum partial pressure of carbon dioxide several times higher than present-day values. Unless actual carbon dioxide levels were considerably greater than this, climate models predict that additional greenhouse gases would still need to have a role in maintaining above-freezing surface temperatures.

Serpentinite and the dawn of life.

Submarine hydrothermal vents above serpentinite produce chemical potential gradients of aqueous and ionic hydrogen, thus providing a very attractive venue for the origin of life. This environment was most favourable before Earths massive CO2 atmosphere was subducted into the mantle, which occurred tens to approximately 100 Myr after the moon-forming impact; thermophile to clement conditions persisted for several million years while atmospheric pCO2 dropped from approximately 25 bar to below 1 bar. The ocean was weakly acid (pH ∼ 6), and a large pH gradient existed for nascent life with pH 9–11 fluids venting from serpentinite on the seafloor. Total CO2 in water was significant so the vent environment was not carbon limited. Biologically important phosphate and Fe(II) were somewhat soluble during this period, which occurred well before the earliest record of preserved surface rocks approximately 3.8 billion years ago (Ga) when photosynthetic life teemed on the Earth and the oceanic pH was the modern value of approximately 8. Serpentinite existed by 3.9 Ga, but older rocks that might retain evidence of its presence have not been found. Earths sequesters extensive evidence of Archaean and younger subducted biological material, but has yet to be exploited for the Hadean record.

The Columbian Encounter and the Little Ice Age: Abrupt Land Use Change, Fire, and Greenhouse Forcing

Pre-Columbian farmers of the Neotropical lowlands numbered an estimated 25 million by 1492, with at least 80 percent living within forest biomes. It is now well established that significant areas of Neotropical forests were cleared and burned to facilitate agricultural activities before the arrival of Europeans. Paleoecological and archaeological evidence shows that demographic pressure on forest resources—facilitated by anthropogenic burning—increased steadily throughout the Late Holocene, peaking when Europeans arrived in the late fifteenth century. The introduction of Old World diseases led to recurrent epidemics and resulted in an unprecedented population crash throughout the Neotropics. The rapid demographic collapse was mostly complete by 1650, by which time it is estimated that about 95 percent of all indigenous inhabitants of the region had perished. We review fire history records from throughout the Neotropical lowlands and report new high-resolution charcoal records and demographic estimates that together support the idea that the Neotropical lowlands went from being a net source of CO2 to the atmosphere before Columbus to a net carbon sink for several centuries following the Columbian encounter. We argue that the regrowth of Neotropical forests following the Columbian encounter led to terrestrial biospheric carbon sequestration on the order of 2 to 5 Pg C, thereby contributing to the well-documented decrease in atmospheric CO2 recorded in Antarctic ice cores from about 1500 through 1750, a trend previously attributed exclusively to decreases in solar irradiance and an increase in global volcanic activity. We conclude that the post-Columbian carbon sequestration event was a significant forcing mechanism of Little Ice Age cooling.

High- and Ultrahigh-Pressure Metamorphism in the North Qaidam and South Altyn Terranes, Western China

The North Qaidam and South Altyn terranes extend approximately 1000 km across the northern Tibetan Plateau, and five localities preserve evidence of Early Paleozoic high-pressure (HP) or ultrahigh-pressure (UHP) metamorphism, including the presence of coesite, coesite pseudomorphs, and diamond. A review of the geology, petrology, and geochronology collected over the past 10 years since these localities were discovered supports a correlation of the North Qaidam and South Altyn terranes, offset 350-400 km across the Altyn Tagh fault. Geochronology interpreted to reflect eclogite-facies metamorphism yields ages between 500 and 420 Ma; detailed geochronology from one locality supports a protracted (tens of m.y.) history of HP/UHP metamorphism. Rock associations and geochronology support a passive-margin origin for the protolith of the HP/UHP rocks, which received sediments from a Proterozoic-Late Archean source, and was intruded by Neoproterozoic granites derived from crustal melting.