Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturati... more Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturation state of weathering solutions in column reactors. During the first 750 hours, rapid, nonstoichiometric dissolution was observed. Once steady state had been reached, both the overall dissolution and Sr release became stoichiometric. Under steady state conditions that were far from being in equilibrium with the
Nitrogen isotopes were used to study the source of nitrate and its uptake into the food web of a ... more Nitrogen isotopes were used to study the source of nitrate and its uptake into the food web of a northeastern river in Vermont, USA. In six tributaries of the White River (Vermont) nitrate concentrations were elevated in streams flowing through areas with agricultural land use as compared to streams flowing through pristine forested areas. We observed a strong positive correlation between δ15N values of stream water nitrate and percent of agricultural land-use within a given catchment. Agricultural sites had relatively high δ15N values of nitrate (+7.3‰) compared to forested sites (+2.0‰). These relatively high δ15N values coupled with relatively high concentrations of nitrate in agricultural streams suggest the introduction of 15N-enriched nitrate draining from agricultural areas. Additionally, elevated δ15N values of algae, aquatic insects, and Atlantic salmon (Salmo salar) in agricultural sites compared to lower δ15N values of their counterparts in forested sites suggest the upta...
Recent investigations indicate that wildfires provide a significant flux of mercury (Hg) from ter... more Recent investigations indicate that wildfires provide a significant flux of mercury (Hg) from terrestrial ecosystems to the atmosphere. However, little is known about how geographic location, climate, stand age, and tree species affect Hg accumulation prior to burning and loss during burning. Soil cores collected in sites burned during the summer 2001 Rex Creek Fire in the eastern Cascade Mountains (Washington State, USA) and in adjacent unburned control sites indicate that Hg loss from soils during the Rex Creek Fire averaged 6.7 (± 2.5) g Hg ha − 1 .
A high-resolution age-depth profile is presented for a 16-m deep-sea piston core (EW9709-PC07) us... more A high-resolution age-depth profile is presented for a 16-m deep-sea piston core (EW9709-PC07) using three different methods: magnetostratigraphy, fish-teeth strontium isotope stratigraphy, and radiolarian biostratigraphy. Fish teeth are abundant throughout the core, allowing for precise age determinations by Sr isotope stratigraphy. Magnetostratigraphic ages, though not available for this core, were determined by correlation with the drill core record from adjacent ODP Site 1218. Biostratigraphic ages were independently assigned to the lower 12 m of the core, which contains abundant radiolaria. All three methods define an early Miocene age (~20 Ma) for the core base. A linear sedimentation rate of~2.0 mm/ky was calculated for the lower 10 m of the core, which is dominated by siliceous clays and calcareous ooze. All three methods yield concordant ages over this interval (~20 to 15 Ma). Tectonic migration of the PC-07 site away from the equatorial high productivity zone produced a significant decrease in sedimentation rates after 15 Ma, diminishing to just~0.30 mm/ky in the uppermost 3 m of the core. Correlated magnetic reversal and fish teeth ages are concordant within this upper red clay interval (~10 to 0.0 Ma), which is dominated by eolian dust accumulation; however, within the 15 to 10 Ma interval, fish teeth ages appear to show more scatter, departing from the magnetic ages by as much as 2-3 million years. Age discrepancies in this dominantly siliceous clay interval are most likely due to uncertainties in magnetostratigraphic age correlations. We conclude from this that the eolian dust component in red clay cores can be reliably dated by the fish teeth strontium technique. For otherwise undatable red clay cores from the vast northern Pacific pelagic clay province, this may prove to be the only available method for developing a regional Cenozoic chrono-stratigraphy. D
1] The toxicity of mercury's methylated species and its biomagnification in aquatic food chains a... more 1] The toxicity of mercury's methylated species and its biomagnification in aquatic food chains and global dispersion by the atmosphere are the cause of worldwide health problems. Recent reports have observed natural mass-dependent fractionation in mercury isotopes, and recent theoretical work has demonstrated that isotopic separation in mercury is due primarily to nuclear field shifts (nuclear volume effect), and the magnetic spin effect gives rise to mass-independent fractionation (MIF) of odd neutron number isotopes. Now we present analytical evidence of mass-independent isotopic variations in mercury produced by both nuclear volume and magnetic isotope processes. Even mass number isotopes exhibit a pattern indistinguishable from that produced by mass-dependent fractionation, with both positive and negative 199 Hg and 201 Hg anomalies. MIF is easier to reliably determine in Hg isotopes than mass-dependent fractionation alone, and thus it provides a potentially important key in constraining models of mercury sources and pathways in the environment.
Experimental studies indicate that opaque assemblages rich in refractory siderophile elements wer... more Experimental studies indicate that opaque assemblages rich in refractory siderophile elements were formed within host calcium- and aluminium-rich inclusions (CAIs) by exsolution, oxidation and sulphidization of homogeneous alloys, rather than by aggregation of materials in the solar nebula before the formation of CAIs. These opaque assemblages are thus not the oldest known solid materials, as was once thought, and they
Mechanisms of air-snow exchange of mercury (Hg) during and after atmospheric mercury depletion ev... more Mechanisms of air-snow exchange of mercury (Hg) during and after atmospheric mercury depletion events (AMDEs) remain poorly constrained and this has limited our understanding of the arctic Hg cycle. We measured the Hg concentrations of surface snow through time and carried ...
... Aaron Taylor* Joel D. Blum Department of Earth Sciences, Dartmouth College, 6105 Fairchild, H... more ... Aaron Taylor* Joel D. Blum Department of Earth Sciences, Dartmouth College, 6105 Fairchild, Hanover, New Hampshire 03755 ... Bedrock and glacial deposits in the drainage consist entirely of Archean granite and gneiss typical of the continental shield rocks, and the area has ...
The dissolution kinetics and isotopic composition of Sr released during the weathering of biotite... more The dissolution kinetics and isotopic composition of Sr released during the weathering of biotite and phlogopite were measured under experimental conditions at pH ϳ 3 and 25°C. Although the overall release of Sr was essentially stoichiometric, the 87 Sr/ 86 Sr ratios of weathering solutions were generally higher than the reacting mineral. The initial phase of dissolution was characterized by the simultaneous rapid release of Sr from the sheet silicate and from trace amounts of calcite inclusions. During the initial phase, the 87 Sr/ 86 Sr ratios of the output solutions were lower than either the bulk mineral or the later, steady-state ratios. After the early stages of the experiments, calcite dissolution became limited by the rate at which it was exposed to the weathering solution and the 87 Sr/ 86 Sr of the output solutions increased to values above those of the biotite and phlogopite (after correction for the calcite inclusions). Differences in the location of Sr and Rb within the mica structure may cause the Sr isotopic ratio of the effluent to increase above the mineral ratio. The log values of the steady-state Sr release rate constants (moles per meter per second) were determined to be Ϫ15.1 for biotite and Ϫ15.9 for phlogopite under our experimental conditions. Cation release rates indicate that different sites within the sheet silicate structure reacted at different rates. Interlayer cations were generally released more rapidly than octahedral cations, which reacted slightly faster than the tetrahedral cations. Interlayer potassium release was considered to be controlled by diffusion with a moving boundary condition. Diffusion coefficients calculated from this model were 3.5 ϫ 10 Ϫ19 cm 2 /s for biotite and 1.9 ϫ 10 Ϫ18 cm 2 /s for phlogopite. Iron oxidation appeared to limit the overall biotite dissolution rate. Iron-free phlogopite reacted nearly twice as fast as biotite. The log values of the overall dissolution rates (moles per meter per second) were determined to be Ϫ11.6 for biotite and Ϫ11.2 for phlogopite. Cation release during phlogopite dissolution was also much less stoichiometric than during biotite weathering.
Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturati... more Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturation state of weathering solutions in column reactors. During the first 750 hours, rapid, nonstoichiometric dissolution was observed. Once steady state had been reached, both the overall ...
The Rb-Sr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils... more The Rb-Sr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils from a granitic glacial soil chronosequence in the Wind River Mountains, Wyoming, USA, were investigated. Six soil profiles ranging in age from 0.4 to -300 kyr were studied and revealed that the "Sr/*?jr ratio of exchangeable strontium in the B-horizons decreased from 0.7947 to 0.7114 with increasing soil age. Soil digests of the same samples showed much smaller variation in 87Sr/86Sr from 0.7272 to 0.7103 and also generally decreased with increasing soil age. Elevation of the 87Sr/86Sr ratios of Sr released by weathering over the soil digest and bedrock values results from the rapid weathering of biotite to form hydrobiotite and vermiculite in the younger soils. Biotite is estimated to weather at approximately eight times the rate of plagioclase (per gram of mineral) in the youngest soil profile and decreases to a rate of only -20% of that of plagioclase in the oldest soil. 87Rb/86Sr ratios of the soil cation exchange fraction are estimated to be depleted by factors of up to 11 over the "Rb/@Sr ratios released by weathering, due to ion exchange partitioning. This study demonstrates that the 87Sr/86Sr ratio released by weathering of crystalline rocks can deviate significantly from bedrock values, and that in soils less than -20 kyr in age which contain biotite in the soil parent material, weathering-derived "Sr/ 86Sr values can be elevated so dramatically that this factor must be considered in estimations of weathering rates based on strontium isotopes. Copyright 0 1997 Elsevier Science Ltd
Resonance ionization of sputtered atoms followed by time-of-flight mass spectrometry was used for... more Resonance ionization of sputtered atoms followed by time-of-flight mass spectrometry was used for in situ quantitative measurement of OS with a spatial resolution of -70 pm. Osmium concentrations in synthetic metals and iron meteorites were measured to demonstrate the analytical capabilities of the technique. A linear correlation between OS+ signal intensity and the known OS concentration was observed over a range of nearly 1 O4 in OS concentration with an accuracy of -+ 1 O%, a minimum detection limit of 7 parts per billion atomic, and a useful yield of 1%. Resonance ionization of sputtered atoms samples the dominant neutral-fraction of sputtered atoms and utilizes multiphoton resonance ionization to achieve high sensitivity and to eliminate atomic and molecular interferences. Matrix effects should be small compared to secondary ion mass spectrometry because ionization occurs in the gas phase and is largely independent of the physical properties of the matrix material. Resonance ionization of sputtered atoms can be applied to in situ chemical analysis of most high ionization-potential elements (including all of the P&group elements) in a wide range of natural and synthetic materials. The high useful yield and elemental selectivity of this method should eventually allow the in situ measurement of OS isotope ratios in some natural samples and in sample extracts enriched in Pt-group elements by fire assay fusion.
A large ( 130 g), layered tektite was found just south of Riddleville, Georgia, USA, in July 1993... more A large ( 130 g), layered tektite was found just south of Riddleville, Georgia, USA, in July 1993. The specimen has a bulk composition similar to that of splash-form Georgia tektites, but with a much wider range in composition (e.g., the silica content ranges from 69-99 wt%) and it contains numerous white opaque inclusions. Portions of the specimen were studied to determine the petrography, major and minor element, and strontium and neodymium isotopic composition, water content, magnetic properties, ferric/ferrous ratio, and 40Ar/J9Ar age. The white opaque inclusions were found to be zircon crystals, many of which had partly or completely decomposed to baddeleyite plus Si02 glass. The trace element, strontium and neodymium isotopic ratios, water content, age ( -34.5 Ma), and magnetic properties are all similar to normal Georgia tektites. Mijssbauer data indicate Fe'+/Fe*+ ratios between 0.07 and 0.16. This specimen has all the characteristics of a Muong Nong-type tektite, except that it is not as strongly enriched in volatile elements, including water, as are the Australasian Muong Nong-type tektites. The lack of a strong enrichment in volatile elements, plus the fact that many of the included relict zircons have partly or completely broken down to baddeleyite plus SiOZ glass, indicates that this specimen was heated more intensely than most zircon-bearing Australasian Muong Nong-type tektites. The occurrence in the northeastern comer of the Georgia strewn field is consistent with a proposed source crater near Deep Sea Drilling Project Site 612 off the coast of New Jersey.
Mineral phases from opaque assemblages (OAs) in Ca, Al-rich refractory inclusions (CAIs), chondru... more Mineral phases from opaque assemblages (OAs) in Ca, Al-rich refractory inclusions (CAIs), chondrules and matrix in C3V meteorites were chemically analyzed and compared with experimentally determined phase equilibria and partitioning data in the Ni-Fe-Ru, Ni-Fe-S and Ni-Fe-O systems to estimate the temperature, sulfur fugacity (fs2) and oxygen fugaeity (fc~) of OA formation. The kinetics of dissolution and exsolution of metallic phases in the Ni-Fe-Ru system were used to constrain the thermal history 0fOAs that occur in CAIs. Based on this work, we suggest that OAs formed after the crystallization of host CAIs by exsolution, sulfldation and oxidation of precursor alloys at low temperatures (~770 K) and higher than solar gas fs~ and foa. Our model contrasts with previous models that call upon the formation of CAI OAs by aggregation of previously formed phases in the solar nebula prior to the erystaHi~tion of CAIs. Opaque assemblages in CAIs and ehondrules probably originated as homogeneous alloys during melting of the silicate portions of CAIs and chondrules. The compositions of these precursor alloys reflect high-temperature and low-fo~ conditions in the early solar nebula. The similarities in the temperature, fs2 and fo2 of equilibration for OAs that occur in CAIs, chondrules and matrix suggest that these three components of C3V meteorites share a common, late low-temperature history. The mineral phases in OAs do not preserve an independent history prior to CAI and chondrule melting and crystallization, but instead provide important information on the post-accretionary history of C3V meteorites and allow us to quantify the temperature, fs2 and fo~ of cooling planetary environments.
Controlling bioaccumulation of toxic monomethylmercury (MMHg) in aquatic food chains requires dif... more Controlling bioaccumulation of toxic monomethylmercury (MMHg) in aquatic food chains requires differentiation between biotic and abiotic pathways that lead to its production and degradation. Recent mercury (Hg) stable isotope measurements of natural samples suggest that Hg isotope ratios can be a powerful proxy for tracing dominant Hg transforming pathways in aquatic ecosystems. Specifically, it has been shown that photo-degradation of MMHg causes both mass dependent (MDF) and mass independent fractionation (MIF) of Hg isotopes. Because the extent of MDF and MIF observed in natural samples (e.g., fish, soil and sediments) can potentially be used to determine the relative importance of pathways leading to MMHg accumulation, it is important to determine the potential role of microbial pathways in contributing to the fractionation, especially MIF, observed in these samples. This study reports the extent of fractionation of Hg stable isotopes during degradation of MMHg to volatile elemental Hg and methane via the microbial Hg resistance (mer) pathway in Escherichia coli carrying a mercury resistance (mer) genetic system on a multi-copy plasmid. During experimental microbial degradation of MMHg, MMHg remaining in reactors became progressively heavier (increasing d 202 Hg) with time and underwent mass dependent Rayleigh fractionation with a fractionation factor a 202/198 = 1.0004 ± 0.0002 (2SD). However, MIF was not observed in any of the microbial MMHg degradation experiments indicating that the isotopic signature left by mer mediated MMHg degradation is significantly different from fractionation observed during DOC mediated photo-degradation of MMHg. Additionally, a clear suppression of Hg isotope fractionation, both during reduction of Hg(II) and degradation of MMHg, was observed when the cell densities increased, possibly due to a reduction in substrate bioavailability. We propose a multi-step framework for understanding the extent of fractionation seen in our MMHg degradation experiments and, based on estimates of the rates of the various steps involved in this mer mediated pathway, suggest which steps in the process could contribute towards the observed extent of fractionation. This framework suggests that at lower cell densities catalysis by MerB was the rate limiting step while at higher cell densities transport into the cell, which does not cause fractionation, became the rate limiting step. In addition to presenting evidence for absence of MIF during mer mediated Hg transformations, based on the nature of Hg compounds and microbe-Hg interactions, we suggest that the nuclear spin dependent MIF (i.e., the magnetic isotope effect) is also unlikely to occur during other non mer mediated 'dark' microbial Hg transformations (e.g., formation of MMHg and oxidative degradation of MMHg). Because of the important implications of the absence of MIF during biological processes on Hg isotope systematics, we discuss theoretical considerations and experimental strategies that could be used to confirm this suggestion.
We explored changes in the relative importance of carbonate vs. silicate weathering as a function... more We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr Ϸ 0.20 mol/nmol, 87 Sr/ 86 Sr Ϸ 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr Ϸ 0.98 to 5.3 mol/nmol, 87 Sr/ 86 Sr Ϸ 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr Ϸ 1.0 to 2.8 mol/nmol, 87 Sr/ 86 Sr Ϸ 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from ϳ0.5 to ϳ55 kyr revealed that carbonate dissolution provides more than ϳ90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only ϳ1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and 87 Sr/ 86 Sr ratios released by carbonate weathering decrease from ϳ3.6 to ϳ0.20 mol/nmol and ϳ0.84 to ϳ0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment.
Mercury is deposited to the Polar Regions during springtime atmospheric mercury depletion events ... more Mercury is deposited to the Polar Regions during springtime atmospheric mercury depletion events (AMDEs) but the relationship between snow and ice crystal formation and mercury deposition is not well understood. The objective of this investigation was to determine if mercury concentrations were related to the type and formation of snow and ice crystals. On the basis of almost three hundred analyses of samples collected in the Alaskan Arctic, we suggest that kinetic crystals growing from the vapor phase, including surface hoar, frost flowers, and diamond dust, yield mercury concentrations that are typically 2-10 times higher than that reported for snow deposited during AMDEs (∼80 ng/L). Our results show that the crystal type and formation affect the mercury concentration in any given snow sample far more than the AMDE activity prior to snow collection. We present a conceptual model of how snow grain processes including deposition, condensation, reemission, sublimation, and turbulent diffusive uptake influence mercury concentrations in snow and ice. These processes are time dependent and operate collectively to affect the retention and fate of mercury in the cryosphere. The model highlights the importance of the formation and postdeposition crystallographic history of snow or ice crystals in determining the fate and concentration of mercury in the cryosphere.
Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturati... more Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturation state of weathering solutions in column reactors. During the first 750 hours, rapid, nonstoichiometric dissolution was observed. Once steady state had been reached, both the overall dissolution and Sr release became stoichiometric. Under steady state conditions that were far from being in equilibrium with the
Nitrogen isotopes were used to study the source of nitrate and its uptake into the food web of a ... more Nitrogen isotopes were used to study the source of nitrate and its uptake into the food web of a northeastern river in Vermont, USA. In six tributaries of the White River (Vermont) nitrate concentrations were elevated in streams flowing through areas with agricultural land use as compared to streams flowing through pristine forested areas. We observed a strong positive correlation between δ15N values of stream water nitrate and percent of agricultural land-use within a given catchment. Agricultural sites had relatively high δ15N values of nitrate (+7.3‰) compared to forested sites (+2.0‰). These relatively high δ15N values coupled with relatively high concentrations of nitrate in agricultural streams suggest the introduction of 15N-enriched nitrate draining from agricultural areas. Additionally, elevated δ15N values of algae, aquatic insects, and Atlantic salmon (Salmo salar) in agricultural sites compared to lower δ15N values of their counterparts in forested sites suggest the upta...
Recent investigations indicate that wildfires provide a significant flux of mercury (Hg) from ter... more Recent investigations indicate that wildfires provide a significant flux of mercury (Hg) from terrestrial ecosystems to the atmosphere. However, little is known about how geographic location, climate, stand age, and tree species affect Hg accumulation prior to burning and loss during burning. Soil cores collected in sites burned during the summer 2001 Rex Creek Fire in the eastern Cascade Mountains (Washington State, USA) and in adjacent unburned control sites indicate that Hg loss from soils during the Rex Creek Fire averaged 6.7 (± 2.5) g Hg ha − 1 .
A high-resolution age-depth profile is presented for a 16-m deep-sea piston core (EW9709-PC07) us... more A high-resolution age-depth profile is presented for a 16-m deep-sea piston core (EW9709-PC07) using three different methods: magnetostratigraphy, fish-teeth strontium isotope stratigraphy, and radiolarian biostratigraphy. Fish teeth are abundant throughout the core, allowing for precise age determinations by Sr isotope stratigraphy. Magnetostratigraphic ages, though not available for this core, were determined by correlation with the drill core record from adjacent ODP Site 1218. Biostratigraphic ages were independently assigned to the lower 12 m of the core, which contains abundant radiolaria. All three methods define an early Miocene age (~20 Ma) for the core base. A linear sedimentation rate of~2.0 mm/ky was calculated for the lower 10 m of the core, which is dominated by siliceous clays and calcareous ooze. All three methods yield concordant ages over this interval (~20 to 15 Ma). Tectonic migration of the PC-07 site away from the equatorial high productivity zone produced a significant decrease in sedimentation rates after 15 Ma, diminishing to just~0.30 mm/ky in the uppermost 3 m of the core. Correlated magnetic reversal and fish teeth ages are concordant within this upper red clay interval (~10 to 0.0 Ma), which is dominated by eolian dust accumulation; however, within the 15 to 10 Ma interval, fish teeth ages appear to show more scatter, departing from the magnetic ages by as much as 2-3 million years. Age discrepancies in this dominantly siliceous clay interval are most likely due to uncertainties in magnetostratigraphic age correlations. We conclude from this that the eolian dust component in red clay cores can be reliably dated by the fish teeth strontium technique. For otherwise undatable red clay cores from the vast northern Pacific pelagic clay province, this may prove to be the only available method for developing a regional Cenozoic chrono-stratigraphy. D
1] The toxicity of mercury's methylated species and its biomagnification in aquatic food chains a... more 1] The toxicity of mercury's methylated species and its biomagnification in aquatic food chains and global dispersion by the atmosphere are the cause of worldwide health problems. Recent reports have observed natural mass-dependent fractionation in mercury isotopes, and recent theoretical work has demonstrated that isotopic separation in mercury is due primarily to nuclear field shifts (nuclear volume effect), and the magnetic spin effect gives rise to mass-independent fractionation (MIF) of odd neutron number isotopes. Now we present analytical evidence of mass-independent isotopic variations in mercury produced by both nuclear volume and magnetic isotope processes. Even mass number isotopes exhibit a pattern indistinguishable from that produced by mass-dependent fractionation, with both positive and negative 199 Hg and 201 Hg anomalies. MIF is easier to reliably determine in Hg isotopes than mass-dependent fractionation alone, and thus it provides a potentially important key in constraining models of mercury sources and pathways in the environment.
Experimental studies indicate that opaque assemblages rich in refractory siderophile elements wer... more Experimental studies indicate that opaque assemblages rich in refractory siderophile elements were formed within host calcium- and aluminium-rich inclusions (CAIs) by exsolution, oxidation and sulphidization of homogeneous alloys, rather than by aggregation of materials in the solar nebula before the formation of CAIs. These opaque assemblages are thus not the oldest known solid materials, as was once thought, and they
Mechanisms of air-snow exchange of mercury (Hg) during and after atmospheric mercury depletion ev... more Mechanisms of air-snow exchange of mercury (Hg) during and after atmospheric mercury depletion events (AMDEs) remain poorly constrained and this has limited our understanding of the arctic Hg cycle. We measured the Hg concentrations of surface snow through time and carried ...
... Aaron Taylor* Joel D. Blum Department of Earth Sciences, Dartmouth College, 6105 Fairchild, H... more ... Aaron Taylor* Joel D. Blum Department of Earth Sciences, Dartmouth College, 6105 Fairchild, Hanover, New Hampshire 03755 ... Bedrock and glacial deposits in the drainage consist entirely of Archean granite and gneiss typical of the continental shield rocks, and the area has ...
The dissolution kinetics and isotopic composition of Sr released during the weathering of biotite... more The dissolution kinetics and isotopic composition of Sr released during the weathering of biotite and phlogopite were measured under experimental conditions at pH ϳ 3 and 25°C. Although the overall release of Sr was essentially stoichiometric, the 87 Sr/ 86 Sr ratios of weathering solutions were generally higher than the reacting mineral. The initial phase of dissolution was characterized by the simultaneous rapid release of Sr from the sheet silicate and from trace amounts of calcite inclusions. During the initial phase, the 87 Sr/ 86 Sr ratios of the output solutions were lower than either the bulk mineral or the later, steady-state ratios. After the early stages of the experiments, calcite dissolution became limited by the rate at which it was exposed to the weathering solution and the 87 Sr/ 86 Sr of the output solutions increased to values above those of the biotite and phlogopite (after correction for the calcite inclusions). Differences in the location of Sr and Rb within the mica structure may cause the Sr isotopic ratio of the effluent to increase above the mineral ratio. The log values of the steady-state Sr release rate constants (moles per meter per second) were determined to be Ϫ15.1 for biotite and Ϫ15.9 for phlogopite under our experimental conditions. Cation release rates indicate that different sites within the sheet silicate structure reacted at different rates. Interlayer cations were generally released more rapidly than octahedral cations, which reacted slightly faster than the tetrahedral cations. Interlayer potassium release was considered to be controlled by diffusion with a moving boundary condition. Diffusion coefficients calculated from this model were 3.5 ϫ 10 Ϫ19 cm 2 /s for biotite and 1.9 ϫ 10 Ϫ18 cm 2 /s for phlogopite. Iron oxidation appeared to limit the overall biotite dissolution rate. Iron-free phlogopite reacted nearly twice as fast as biotite. The log values of the overall dissolution rates (moles per meter per second) were determined to be Ϫ11.6 for biotite and Ϫ11.2 for phlogopite. Cation release during phlogopite dissolution was also much less stoichiometric than during biotite weathering.
Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturati... more Labradorite dissolution kinetics and Sr release rates were measured as a function of the saturation state of weathering solutions in column reactors. During the first 750 hours, rapid, nonstoichiometric dissolution was observed. Once steady state had been reached, both the overall ...
The Rb-Sr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils... more The Rb-Sr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils from a granitic glacial soil chronosequence in the Wind River Mountains, Wyoming, USA, were investigated. Six soil profiles ranging in age from 0.4 to -300 kyr were studied and revealed that the "Sr/*?jr ratio of exchangeable strontium in the B-horizons decreased from 0.7947 to 0.7114 with increasing soil age. Soil digests of the same samples showed much smaller variation in 87Sr/86Sr from 0.7272 to 0.7103 and also generally decreased with increasing soil age. Elevation of the 87Sr/86Sr ratios of Sr released by weathering over the soil digest and bedrock values results from the rapid weathering of biotite to form hydrobiotite and vermiculite in the younger soils. Biotite is estimated to weather at approximately eight times the rate of plagioclase (per gram of mineral) in the youngest soil profile and decreases to a rate of only -20% of that of plagioclase in the oldest soil. 87Rb/86Sr ratios of the soil cation exchange fraction are estimated to be depleted by factors of up to 11 over the "Rb/@Sr ratios released by weathering, due to ion exchange partitioning. This study demonstrates that the 87Sr/86Sr ratio released by weathering of crystalline rocks can deviate significantly from bedrock values, and that in soils less than -20 kyr in age which contain biotite in the soil parent material, weathering-derived "Sr/ 86Sr values can be elevated so dramatically that this factor must be considered in estimations of weathering rates based on strontium isotopes. Copyright 0 1997 Elsevier Science Ltd
Resonance ionization of sputtered atoms followed by time-of-flight mass spectrometry was used for... more Resonance ionization of sputtered atoms followed by time-of-flight mass spectrometry was used for in situ quantitative measurement of OS with a spatial resolution of -70 pm. Osmium concentrations in synthetic metals and iron meteorites were measured to demonstrate the analytical capabilities of the technique. A linear correlation between OS+ signal intensity and the known OS concentration was observed over a range of nearly 1 O4 in OS concentration with an accuracy of -+ 1 O%, a minimum detection limit of 7 parts per billion atomic, and a useful yield of 1%. Resonance ionization of sputtered atoms samples the dominant neutral-fraction of sputtered atoms and utilizes multiphoton resonance ionization to achieve high sensitivity and to eliminate atomic and molecular interferences. Matrix effects should be small compared to secondary ion mass spectrometry because ionization occurs in the gas phase and is largely independent of the physical properties of the matrix material. Resonance ionization of sputtered atoms can be applied to in situ chemical analysis of most high ionization-potential elements (including all of the P&group elements) in a wide range of natural and synthetic materials. The high useful yield and elemental selectivity of this method should eventually allow the in situ measurement of OS isotope ratios in some natural samples and in sample extracts enriched in Pt-group elements by fire assay fusion.
A large ( 130 g), layered tektite was found just south of Riddleville, Georgia, USA, in July 1993... more A large ( 130 g), layered tektite was found just south of Riddleville, Georgia, USA, in July 1993. The specimen has a bulk composition similar to that of splash-form Georgia tektites, but with a much wider range in composition (e.g., the silica content ranges from 69-99 wt%) and it contains numerous white opaque inclusions. Portions of the specimen were studied to determine the petrography, major and minor element, and strontium and neodymium isotopic composition, water content, magnetic properties, ferric/ferrous ratio, and 40Ar/J9Ar age. The white opaque inclusions were found to be zircon crystals, many of which had partly or completely decomposed to baddeleyite plus Si02 glass. The trace element, strontium and neodymium isotopic ratios, water content, age ( -34.5 Ma), and magnetic properties are all similar to normal Georgia tektites. Mijssbauer data indicate Fe'+/Fe*+ ratios between 0.07 and 0.16. This specimen has all the characteristics of a Muong Nong-type tektite, except that it is not as strongly enriched in volatile elements, including water, as are the Australasian Muong Nong-type tektites. The lack of a strong enrichment in volatile elements, plus the fact that many of the included relict zircons have partly or completely broken down to baddeleyite plus SiOZ glass, indicates that this specimen was heated more intensely than most zircon-bearing Australasian Muong Nong-type tektites. The occurrence in the northeastern comer of the Georgia strewn field is consistent with a proposed source crater near Deep Sea Drilling Project Site 612 off the coast of New Jersey.
Mineral phases from opaque assemblages (OAs) in Ca, Al-rich refractory inclusions (CAIs), chondru... more Mineral phases from opaque assemblages (OAs) in Ca, Al-rich refractory inclusions (CAIs), chondrules and matrix in C3V meteorites were chemically analyzed and compared with experimentally determined phase equilibria and partitioning data in the Ni-Fe-Ru, Ni-Fe-S and Ni-Fe-O systems to estimate the temperature, sulfur fugacity (fs2) and oxygen fugaeity (fc~) of OA formation. The kinetics of dissolution and exsolution of metallic phases in the Ni-Fe-Ru system were used to constrain the thermal history 0fOAs that occur in CAIs. Based on this work, we suggest that OAs formed after the crystallization of host CAIs by exsolution, sulfldation and oxidation of precursor alloys at low temperatures (~770 K) and higher than solar gas fs~ and foa. Our model contrasts with previous models that call upon the formation of CAI OAs by aggregation of previously formed phases in the solar nebula prior to the erystaHi~tion of CAIs. Opaque assemblages in CAIs and ehondrules probably originated as homogeneous alloys during melting of the silicate portions of CAIs and chondrules. The compositions of these precursor alloys reflect high-temperature and low-fo~ conditions in the early solar nebula. The similarities in the temperature, fs2 and fo2 of equilibration for OAs that occur in CAIs, chondrules and matrix suggest that these three components of C3V meteorites share a common, late low-temperature history. The mineral phases in OAs do not preserve an independent history prior to CAI and chondrule melting and crystallization, but instead provide important information on the post-accretionary history of C3V meteorites and allow us to quantify the temperature, fs2 and fo~ of cooling planetary environments.
Controlling bioaccumulation of toxic monomethylmercury (MMHg) in aquatic food chains requires dif... more Controlling bioaccumulation of toxic monomethylmercury (MMHg) in aquatic food chains requires differentiation between biotic and abiotic pathways that lead to its production and degradation. Recent mercury (Hg) stable isotope measurements of natural samples suggest that Hg isotope ratios can be a powerful proxy for tracing dominant Hg transforming pathways in aquatic ecosystems. Specifically, it has been shown that photo-degradation of MMHg causes both mass dependent (MDF) and mass independent fractionation (MIF) of Hg isotopes. Because the extent of MDF and MIF observed in natural samples (e.g., fish, soil and sediments) can potentially be used to determine the relative importance of pathways leading to MMHg accumulation, it is important to determine the potential role of microbial pathways in contributing to the fractionation, especially MIF, observed in these samples. This study reports the extent of fractionation of Hg stable isotopes during degradation of MMHg to volatile elemental Hg and methane via the microbial Hg resistance (mer) pathway in Escherichia coli carrying a mercury resistance (mer) genetic system on a multi-copy plasmid. During experimental microbial degradation of MMHg, MMHg remaining in reactors became progressively heavier (increasing d 202 Hg) with time and underwent mass dependent Rayleigh fractionation with a fractionation factor a 202/198 = 1.0004 ± 0.0002 (2SD). However, MIF was not observed in any of the microbial MMHg degradation experiments indicating that the isotopic signature left by mer mediated MMHg degradation is significantly different from fractionation observed during DOC mediated photo-degradation of MMHg. Additionally, a clear suppression of Hg isotope fractionation, both during reduction of Hg(II) and degradation of MMHg, was observed when the cell densities increased, possibly due to a reduction in substrate bioavailability. We propose a multi-step framework for understanding the extent of fractionation seen in our MMHg degradation experiments and, based on estimates of the rates of the various steps involved in this mer mediated pathway, suggest which steps in the process could contribute towards the observed extent of fractionation. This framework suggests that at lower cell densities catalysis by MerB was the rate limiting step while at higher cell densities transport into the cell, which does not cause fractionation, became the rate limiting step. In addition to presenting evidence for absence of MIF during mer mediated Hg transformations, based on the nature of Hg compounds and microbe-Hg interactions, we suggest that the nuclear spin dependent MIF (i.e., the magnetic isotope effect) is also unlikely to occur during other non mer mediated 'dark' microbial Hg transformations (e.g., formation of MMHg and oxidative degradation of MMHg). Because of the important implications of the absence of MIF during biological processes on Hg isotope systematics, we discuss theoretical considerations and experimental strategies that could be used to confirm this suggestion.
We explored changes in the relative importance of carbonate vs. silicate weathering as a function... more We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr Ϸ 0.20 mol/nmol, 87 Sr/ 86 Sr Ϸ 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr Ϸ 0.98 to 5.3 mol/nmol, 87 Sr/ 86 Sr Ϸ 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr Ϸ 1.0 to 2.8 mol/nmol, 87 Sr/ 86 Sr Ϸ 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from ϳ0.5 to ϳ55 kyr revealed that carbonate dissolution provides more than ϳ90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only ϳ1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and 87 Sr/ 86 Sr ratios released by carbonate weathering decrease from ϳ3.6 to ϳ0.20 mol/nmol and ϳ0.84 to ϳ0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment.
Mercury is deposited to the Polar Regions during springtime atmospheric mercury depletion events ... more Mercury is deposited to the Polar Regions during springtime atmospheric mercury depletion events (AMDEs) but the relationship between snow and ice crystal formation and mercury deposition is not well understood. The objective of this investigation was to determine if mercury concentrations were related to the type and formation of snow and ice crystals. On the basis of almost three hundred analyses of samples collected in the Alaskan Arctic, we suggest that kinetic crystals growing from the vapor phase, including surface hoar, frost flowers, and diamond dust, yield mercury concentrations that are typically 2-10 times higher than that reported for snow deposited during AMDEs (∼80 ng/L). Our results show that the crystal type and formation affect the mercury concentration in any given snow sample far more than the AMDE activity prior to snow collection. We present a conceptual model of how snow grain processes including deposition, condensation, reemission, sublimation, and turbulent diffusive uptake influence mercury concentrations in snow and ice. These processes are time dependent and operate collectively to affect the retention and fate of mercury in the cryosphere. The model highlights the importance of the formation and postdeposition crystallographic history of snow or ice crystals in determining the fate and concentration of mercury in the cryosphere.
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Papers by Joel Blum