Dependent Isotope Fractionation Research Articles

Large sulfur isotope fractionation during abiotic consumption of hydrogen sulfide after cessation of bacterial growth in batch culture experiments of sulfate reducing bacteria

Microbial sulfate reduction is a key process in the oceanic sulfur cycle and imparts a large sulfur isotope fractionation. The mechanism of sulfur isotope fractionation during microbial sulfate reduction has been studied from the geochemical and biochemical aspects since the 1950s. Recently, however, the large sulfur isotope fractionation, exceeding 47‰, has been observed in some pure culture experiments of sulfate reducing bacteria and the tentative understanding of microbial sulfur isotope fractionation has not well explained the mechanism. Here we quantified growth phase dependent sulfur isotope fractionation of a type sulfate reducer Desulfovibrio desulfuricans (DSM642) and revealed that the magnitude of isotope fractionation increased from -13.4 ± 3.6‰ in early exponential phase to -65.9 ± 21.0‰ in later exponential phase having clear negative correlation with cell specific sulfate reduction rate. Our results show the importance of cells growth phase and states that control cell specific sulfate reduction rate and sulfur isotope fractionation. Although microbial sulfate reduction likely continued to stationary phase of cells, microbial sulfur isotope fractionation could not be quantified because of the decreasing total dissolved sulfide (ΣH2S: H2S, HS−, and S2−) concentration after stationary phase. A non-microbial ΣH2S-consuming reaction is interpreted to have occurred during the stationary phase and this had large sulfur isotope fractionations, -10.5 ± 1.1‰ and -45.6 ± 12.4‰. Besides, the non-microbial ΣH2S-consuming reaction decreased Δ33S’ value of the ΣH2S. While the end-product of the non-microbial ΣH2S-consuming reaction in the stationary phase remains unidentified, we observed precipitates of some sulfide minerals and organic sulfur in the stationary phase media. Regardless of the end-product, the reaction increases the sulfur isotopic composition of dissolved ΣH2S, which can account for the high sulfur isotopic compositions of ΣH2S compared to pyrite and organic sulfur observed in some modern marine sediments.

Temperature dependent lithium isotope fractionation during glass dissolution

Understanding the mechanisms by which borosilicate glasses corrode in contact with aqueous solutions remains a challenge to the safety case for the geological disposal of vitrified high-level nuclear waste. Here, lithium isotope fingerprinting techniques were applied to the leachates of a simulant Magnox waste glass to probe the mechanisms of aqueous corrosion at both short and long timescales (6 hours to 464 days). Experiments took place at 40 and 90 °C to assess the consistency of the dissolution mechanisms across a range of commonly employed temperatures and the legitimacy of applying higher temperature experimental datasets to understand glass corrosion within a disposal facility at lower temperatures.Two competing release mechanisms were observed for lithium (diffusion and hydrolysis), and the relative proportions of these mechanisms changed through time. Leachates initially had lower δ7Li values than the pristine glass (−2.7‰ at 40 °C and −1.1‰ at 90 °C relative to the pristine glass) at both temperatures due to lithium leaching incongruently through diffusive processes. The greater offset between solution and solid at lower temperatures indicates a larger rate of diffusion (incongruent dissolution) relative to the rate of hydrolysis (congruent dissolution) at lower temperatures. The fraction of lithium released through diffusion relative to the fraction of lithium released through hydrolysis then increased at both temperatures with time up to 126 days, increasing from 0.47 and 0.22 at 6 hours to 0.66 and 0.41 at 126 days at 40 and 90 °C respectively. Subsequently, the fractions of lithium released through diffusion sharply decreased to 0.36 at 40 °C and 0.22 at 90 °C after 464 days, consistent with network hydrolysis coupled with secondary phase precipitation later controlling the long-term release of Li at both temperatures. Throughout the duration of the experiments (464 days) the δ7Li values in solution increased to 9.0‰ at 40 °C and 10.0‰ at 90 °C due to the formation of talc and montmorillonite phases at 40 °C and additional smectite phases at 90 °C. Further, no evidence for the formation of a diffusive barrier to the transport of Li within the alteration layers became apparent during the later stages of dissolution at either temperature. However, the fraction of lithium leached through diffusion was still significant throughout all stages of dissolution. Lithium isotope ratios in solution were correlated with the transition from a system which was increasingly dominated by lithium diffusion as the dissolution rate slowed to one which was controlled by hydrolysis coupled with secondary phase precipitation at long durations. Alongside elemental ratios in solution, these results were consistent with the same set of mechanisms governing dissolution across the temperature range studied.

Chromium isotope systematics and the diagenesis of marine carbonates

Stable chromium (Cr) isotopes have emerged as a new tool for tracking broad-scale changes in Earth's surface oxygen levels. Carbonates are one proposed sedimentary Cr isotope archive. In order to contribute to the development of a robust framework for interpreting carbonate Cr isotopic compositions and evaluating their ability to record the global redox state, we explored Cr isotope systematics of modern and Archean carbonate successions—representing end member oxic and anoxic Earth system states. We generated new data from carbonate platform sediments from the Great Bahama Bank with variable post-depositional histories to better understand the effects of diagenetic alteration on carbonate-bound Cr and its isotopic composition in a modern well-oxygenated ocean-atmosphere system. More specifically, we investigated the effects of dolomitization and aragonite-to-calcite neomorphism in marine and meteoric fluids. We also present δ53Cr values from three carbonate successions, the ∼3.0 Ga Chobeni Formation (South Africa), ∼2.8 Ga Mosher Carbonate Formation (Canada), and ∼2.65 Ga Cheshire Formation (Zimbabwe), that were deposited under an anoxic atmosphere. We find that modern Bahamian carbonates have a large range of almost exclusively positive δ53Cr values (from −0.04 to 2.88‰). The δ53Cr values appear to be altered during both meteoric diagenesis and dolomitization but there may also be instances of rock-buffered conditions that may best preserve depositional δ53Cr values. These observations provide a baseline for carbonate Cr isotope behavior in a well-oxygenated ocean-atmosphere system. Our Archean carbonate successions contain both positively fractionated and crustal δ53Cr values, ranging between −0.37 and 0.89‰. The Cr isotope fractionation observed in Archean strata is most plausibly linked to either non-redox dependent Cr isotope fractionation, local Cr redox cycling, late-stage diagenetic alteration, or some combination thereof. Given the significant effects of post-depositional alteration on the isotopic composition of carbonate-bound Cr, we suggest that Cr isotope values from ancient carbonate sediments should be generally interpreted with caution and cannot be straightforwardly linked to changes in atmospheric oxygen abundance.

STABLE MERCURY ISOTOPE FRACTIONATION BEHAVIOURS OF PLANTS

The overarching aim of this study is to define mercury (Hg) isotopic features of plants which have different photosynthetic pathways (C3, C4 and CAM) and to understand if different parts of the plants have different Hg isotopic fractionation behavior. For this, carbon isotopic values of terrestrial plants were analyzed which were used to determine the photosynthetic pathways of plants. Plants were sub-sampled into leaves, stems and roots and their Hg isotopic values were analyzed. 
 Results showed that C3 and C4 plants exhibit mass dependent (even Hg isotopes) and mass independent Hg isotope fractionation (odd Hg isotopes). Both C3 and C4 plants are enriched in light isotopes, but the degree of mass fractionation is approximately three times greater in C3 plants, than in C4 plants. Hg in both C3 and C4 plants exhibit negative MIF isotope effect which reported as depletion “and no clear MIF effect. These findings suggest a connection between the Hg isotopic composition and the photosynthetic pathway.
 In addition, the leaves are slightly more fractionated than the roots. Differences in the degree of MIF between roots and leaves suggest that they obtain Hg from different sources.

Mg isotopic systematics during early diagenetic aragonite-calcite transition: Insights from the Key Largo Limestone

The seawater magnesium (Mg) isotopic composition (δ26MgSW) can be applied to quantify marine Mg cycle in Earth's history. However, reconstruction of δ26MgSW by using marine carbonate has been challenged by the large range of variation in Mg isotopic composition of carbonate (δ26Mgcarb). Such variation, on the one hand, reflects, e.g. the mineralogy, temperature, and precipitation rate dependent isotopic fractionation during Ca‑carbonate precipitation. On the other hand, the pristine seawater record could be modified in carbonate diagenesis, during which less stable carbonate minerals would spontaneously convert to more stable mineral phases. In the modern ocean with high Mg/Ca (molar) ratio (~5), aragonite precipitation is favored. However, aragonite is thermodynamically less stable than low-Mg calcite at ambient surface environment, resulting in the spontaneous transition from aragonite to low-Mg calcite in the earliest stage of diagenesis (i.e. the aragonite-calcite transition). As a common process in the modern ocean, Mg isotopic fractionation in aragonite-calcite transition may provide valuable constraints on the variation of δ26Mgcarb. In this study, we measured Mg isotopic compositions of Pleistocene Key Largo Limestone (Florida), which has been experiencing the aragonite-calcite transition during early diagenesis. The Key Largo Limestone samples consist of a mixture of calcite and aragonite. δ26Mg of aragonite (δ26Mgarag) shows higher and more stable values, while calcite has lower yet more variable Mg isotopes (δ26Mgcal). We suggest a model conceptualizing a water-film attached to the mineral surface, in which the aragonite-calcite transition initiates with aragonite dissolution and the Mg exchange with external fluid can be variable, followed by Mg isotopes fractionate during low-Mg calcite precipitation. In this model, δ26Mgcal is controlled by (1) the chemical composition of water-film, which is dependent on the mineralogical composition of aragonite (i.e. Mg content and δ26Mgarag), the composition of external fluid (i.e. fresh water, seawater, or mixed fresh water and seawater), and the mixing ratio between dissolved aragonite and external fluid, and (2) the isotopic fractionation during calcite precipitation. Because the isotopic fractionation in calcite is sensitive to the rate of calcite precipitation that is dependent on the calcite saturation state of solution. Therefore, δ26Mgcal could be highly heterogeneous depending on the aragonite dissolution and calcite precipitation processes, further complicating the interpretation of carbonate data. Thus, the process-controlled Mg isotopic fractionation should be taken into consideration when reconstructing δ26MgSW by using carbonate sediments/rocks.

The Isotopic Composition of Silver in Ore Minerals

AbstractThe study of the isotopic composition of Ag has applications to numerous fields, including earth and planetary sciences, the environment, and archeology. We present a comprehensive survey of the isotopic composition of Ag in ore deposits globally based on measurement of the 109Ag/107Ag values of 158 samples of native Ag and Ag‐bearing minerals from 65 deposits or districts representative of the main types of Ag mineralization in 20 countries on five continents. The narrow range of δ109Ag values (−0.4‰ to 0.4‰) observed for hypogene native Ag and acanthite/argentite (Ag2S) represents the isotopic composition of Ag in the hydrothermal fluids that are responsible for formation of hypogene (primary) Ag deposits worldwide. Our data provide evidence of mass dependent isotopic fractionation: (a) during low‐temperature remobilization of Ag associated with redox reactions in the supergene (weathering) environment, and (b) among Ag‐bearing mineral phases, including Ag‐halides (bromargyrite, chlorargyrite, iodargyrite, boleite, and pseudoboleite) and As‐bearing sulfosalts (e.g., enargite, tennantite, proustite, and polybasite). We find no evidence of fractionation or isotopic variation related to genetic environment of hypogene mineralization, formation age, or the host rock and geologic terrane of an ore deposit. The mean and range of δ109Ag values from different deposit types largely overlap, and the variation in Ag isotopic composition observed within a mineral district can be as large as that for the respective deposit type. Our initial results suggest limitations to a wider use of Ag isotopes as a geochemical tracer (e.g., in archeometry) and the need to conduct detailed, individual deposit/district studies.

Re-emission of legacy mercury from soil adjacent to closed point sources of Hg emission

Mercury (Hg) emissions from point sources to air may disperse over long distance depending on Hg speciation in the plume. A significant fraction of Hg, particularly in its divalent forms, deposits locally and causes pollution to surrounding biomes. The objective of this study was to investigate (1) the historic Hg deposition to the immediate vicinity of an industrial complex that had intentional use of Hg (i.e., chlor-alkali and polyvinyl chloride production) for 5 decades until 2011, and (2) the Hg0 re-emission from soil to air soon after the closure of the facility. The spatial distribution of near-ground Hg0 vapor in air, soil Hg concentration and stable isotope ratio, air-soil Hg0 flux and Hg0 concentration in soil pore-gas were measured. It was found that the surrounding soils are severely contaminated with Hg due to the Hg release of the industrial complex, displaying soil Hg content up to 4.8 μg g−1. A spatial trend of Hg mass dependent isotope fractionation signature (δ202Hg = −2.11‰ to 0.72‰) with respect to the distance from the closed facility was identified, representing a mixing between regional background and industrial Hg sources. Hg release from the industrial operation enhanced surface soil Hg content within a 6.5-km radius from the facility. Inside the facility, residual Hg wastes (i.e., electrolysis sludge and consumed HgCl2 catalyst) represent a strong localized emission source of atmospheric Hg0. Near-ground atmospheric Hg0 concentration and soil Hg0 efflux progressively elevated toward the facility with an increase by 2–3 orders of magnitude compared to the values observed in the off-site background. These results suggest that the natural soil surfaces surrounding the closed industrial facility act as a large nonpoint source emitting legacy deposited Hg as much as the release from naturally enriched mines.

Stable isotope composition of mercury forms in flue gases from a typical coal-fired power plant, Inner Mongolia, northern China

Mercury forms emitted from coal combustion via air pollution control devices are speculated to carry different Hg isotope signatures. Yet, their Hg isotope composition is still not reported. Here, we present the first onsite Hg isotope data for gaseous elemental Hg (GEM) and gaseous oxidized Hg (GOM) of flue gases from a typical lignite-fired power plant (CFPP). Significant mass dependent fractionation (MDF) and insignificant mass independent fractionation (MIF) are observed between feed coal and coal combustion products. As compared to feed coal (δ202Hg=−2.04±0.25‰), bottom ash, GEM and GOM in flue gases before and after wet flue gas desulfurization system significantly enrich heavy Hg isotopes by 0.7–2.6‰ in δ202Hg, while fly ash, desulfurization gypsum and waste water show slight but insignificant enrichment of light Hg isotopes. GEM is significantly enriched heavy Hg isotopes compared to GOM and Hg in fly ash. Our observations verify the previous speculation on Hg isotope fractionation mechanism in CFPPs, and suggest a kinetically-controlled mass dependent Hg isotope fractionation during transformation of Hg forms in flue gases. Finally, our data are compared to Hg isotope compositions of atmospheric Hg pools, suggesting that coal combustion Hg emission is likely an important atmospheric Hg contributor.

Recent Developments in Mercury Stable Isotope Analysis

The first Reviews in Mineralogy volume on the Geochemistry of Non-Traditional Stable Isotopes was compiled before it was appropriate to include a chapter on mercury (Hg) stable isotope geochemistry. At that time there were only a few papers on this new topic (Jackson 2001; Lauretta et al. 2001; Hintelmann and Lu 2003), and there were still some important analytical issues that needed to be resolved. But the field has come a long way in a decade. Now we have a different problem; at our last count there were well over 100 publications utilizing mercury stable isotopes and it is becoming very difficult to synthesize this vast amount of exciting and rapidly developing research. Experimental studies have expanded our knowledge of the mechanisms of mercury isotope fractionation and applications of mercury isotope measurements have touched virtually every area of research in mercury biogeochemistry. There have been a number of previous reviews of the mercury stable isotope literature as it has developed (Ridley and Stetson 2006; Bergquist and Blum 2009; Yin et al. 2010; Blum 2011; Hintelmann 2012; Blum et al. 2014). It is our view that the field has become too large to comprehensively review the entire literature on mercury stable isotopes. Ten years ago Hg isotope researchers were just beginning to explore the boundaries of natural Hg isotope variation and the mechanisms that cause this variation in the environment. At that time large and relatively easily measured isotope signals were of great interest and mercury isotope researchers were beginning to develop theories to explain mass dependent isotope fractionation (MDF) and mass independent isotope fractionation of the odd mass-numbered isotopes of mercury (odd-MIF). More recently researchers have discovered a wider range of types of isotopic variability (even-MIF), some of which are subtle and …

Uranium isotope composition of waters from South Texas uranium ore deposits

Redox conditions and associated changes in mobility of uranium (U) are tightly linked to a multitude of challenges connected with U mining in sandstone-hosted deposits and new methods that directly measure reduction or oxidation of U can inform on these questions. A novel proxy for understanding U redox chemistry has recently emerged, the volume dependent isotopic fractionation of uranium-238 (238U) from uranium-235 (235U). Novel measurements of 238U/235U isotopic ratio are combined with measurements of the more commonly utilized uranium-234/uranium-238 activity [(234U/238U)] ratio, as both isotopic ratios can be measured simultaneously. However, application of both U isotopic ratios in the contexts of exploration and environmental remediation of U ores requires characterization of these isotopic ratios across a variety of redox settings. Here, 238U/235U and (234U/238U) ratios are examined from eight transects in two U ore bodies (the Rosita and Kingsville Dome deposits) in South Texas; these sites are classic roll front deposits and exhibit a wide variety of both natural and altered redox conditions. Across all transects it is observed that (234U/238U) ratios decrease systematically towards the ore body from both the oxidizing and reducing sides, irrespective of whether the site has been mined or not. This pattern reflects geologically recent and significant U leaching and mobility and is characteristic of active roll fronts. Overall δ238U values in these transects decrease systematically towards the reducing zone. A simple Rayleigh fractionation model, where U ore is deposited from an increasingly isotopically depleted reservoir of dissolved U best explains the overall trend; very negative δ238U values likely reflect multiple cycles of U deposition and dissolution. The South Texas data set indicates that both (234U/238U) ratios and δ238U values can be variable at an individual mine site. However, overall low (234U/238U) ratios and negative δ238U values are characteristic of active roll front deposits. The comprehensive U isotopic composition of both ores and well waters represents a powerful new tool in prospecting of sandstone-hosted U ore and in environmental remediation following extraction of U ore.

Historical (1850–2010) mercury stable isotope inventory from anthropogenic sources to the atmosphere

Abstract Mercury (Hg) stable isotopes provide a new tool to trace the biogeochemical cycle of Hg. An inventory of the isotopic composition of historical anthropogenic Hg emissions is important to understand sources and post-emission transformations of Hg. We build on existing global inventories of anthropogenic Hg emissions to the atmosphere to develop the first corresponding historical Hg isotope inventories for total Hg (THg) and three Hg species: gaseous elemental Hg (GEM), gaseous oxidized Hg (GOM) and particulate-bound Hg (PBM). We compile δ202Hg and Δ199Hg of major Hg emissions source materials. Where possible, δ202Hg and Δ199Hg values in emissions are corrected for the mass dependent Hg isotope fractionation during industrial processing. The framework and Hg isotope inventories can be updated and improved as new data become available. Simulated THg emissions from all sectors between 1850s and 2010s generally show an increasing trend (−1.1‰ to −0.7‰) for δ202Hg, and a stable trend (−0.02‰ to −0.04‰) for Δ199Hg. Δ200Hg are near-zero in source materials and therefore emissions. The δ202Hg trend generally reflects a shift of historically dominant Hg emissions from 19th century Hg mining and liquid Hg0 uses in Au/Ag refining to 20th century coal combustion and non-ferrous metal production. The historical δ202Hg and Δ199Hg curves of GEM closely follow those of THg. The δ202Hg curves of GOM and PBM show no trends. Δ199Hg values for both GOM and PBM decrease from the 1850s to 1950s by ∼0.1‰, and then gradually rebound towards the 2010s. Our updated δ202Hg values (−0.76 ± 0.11 ‰, 1SD, n=9) of bulk emissions from passively degassing volcanoes overlap with δ202Hg of present-day anthropogenic THg emissions.

Controlling factors of Ca isotope fractionation in scleractinian corals evaluated by temperature, pH and light controlled culture experiments

In this study, the 44Ca/40Ca ratios of Porites australiensis grown under three different culture experiments composed of temperature, pH and light controlled culture experiments are measured. The temperature dependent isotope fractionation of 0.02‰/°C deduced from this study is similar to inorganic aragonite, but the degree of isotope fractionation is about +0.4‰ offset in corals. These observations agree with earlier results on different coral species, suggesting Ca isotope fractionation during Ca transmembrane transport in corals.While in cultured corals a significant temperature dependence of δ44Ca is observed, the relationships between calcium isotope fractionation and pH as well as light intensity are negligible. Therefore variation of δ44Ca in Porites corals is mainly controlled by temperature. A combination of δ44Ca and Sr/Ca of corals in temperature controlled experiments cannot be explained by Rayleigh type fractionation directly from a fluid, which is seawater-like in terms of δ44Ca and Sr/Ca. Through coral-specific biomineralization processes, overall mean δ44Ca of scleractinian corals including previous studies are different from biogenic aragonites secreted by sclerosponges and pteropods, but are comparable with those of bivalves as well as calcitic coccolithophores and foraminifers. These findings are important for better understanding biomineralization in corals and in order to constrain the Ca isotopic composition of oceanic Ca sinks in response to climate changes and associated with shifts of calcite and aragonite seas.

Experimental calibration of Mg isotope fractionation between dolomite and aqueous solution and its geological implications

Hydrothermal experiments at 220, 160, and 130°C were performed to calibrate the Mg isotope fractionation factor between dolomite and aqueous Mg. Hydrothermal experiments included synthesis of dolomite using different starting materials, as well as exchange experiments that used poorly-ordered proto-dolomite. The morphology of synthesized dolomite was dependent on starting mineralogy, suggesting that dolomite was synthesized by different pathways. Hydrothermally synthesized dolomite was initially fine-grained disordered or poorly-ordered dolomite that, with time, recrystallized to coarser-grained ordered dolomite. Isotopic exchange was monitored using 87Sr/86Sr ratios and 25Mg tracers, and these indicated near-complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments. The Mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology, suggesting attainment of isotopic equilibrium. Combining results from synthesis and exchange experiments, the temperature dependent Mg isotope fractionation factor for ordered dolomite is:Δ26Mgdolo-aq=-0.1554(±0.0096)×106/T2where T is in Kelvin. In contrast, poorly-ordered dolomite has a Δ26Mgdolo-aq fractionation factor that is up to 0.25‰ lower than that of ordered dolomite, and this is attributed to longer Mg–O bonds in imperfectly ordered dolomite. The experimentally calibrated Δ26Mgdolo-aq fractionation factors lie between those calculated by Schauble (2011) and Rustad et al. (2010). The Δ26Mgdolo-aq fractionation factor extrapolated to lower temperatures using the Δ26Mg-T function of this study matches the Δ26Mgdolo-aq fractionation factor obtained by modeling of Mg isotope compositions of ODP drill core samples. This study shows that significant Mg isotope fractionation occurs during dolomite precipitation. These results collectively demonstrate that Mg isotopes in dolomite are a useful tool for studying Mg global cycling and dolomitization.

Neodymium isotope ratio measurements by LC-MC-ICPMS for nuclear applications: investigation of isotopic fractionation and mass bias correction

The causes of isotope ratio drifts observed in LC-MC-ICPMS experiments could be explained by both mass dependent isotopic fractionation on the chromatographic column and distinct time lags between amplifier responses of the Faraday cup configuration.

Stable strontium isotope fractionation in synthetic barite

The mineral barite (BaSO4) accommodates strontium (Sr) in its crystal structure, providing an archive of Sr-isotopes (87Sr/86Sr and δ88/86Sr) in the highly stable sulfate mineral. We investigated mass dependent stable Sr-isotope fractionation (Δ88/86Sr=δ88/86Srsolid−δ88/86Srsolution) during inorganic precipitation of barite from a barium-rich solution by addition of sulfate under controlled conditions and compared this to equilibrium isotopic fractionation calculated using Density Functional Theory modeling. Sr-substituted barite is predicted to have lower 88Sr/86Sr than any other studied species, and at 25°C will be about 0.6–0.7‰ lower than the two modeled Sr(H2O)82+-bearing salts that could approximate aqueous Sr2+. This agrees in direction and order of magnitude with experimental results that estimate equilibrium Sr-isotope fractionation in barite to be 0.3‰ lower than aqueous Sr2+ at ∼20°C. The high ionic strength of some of the precipitating solutions (up to 1M) and potential differences in the average coordination number of aqueous Sr2+ add to uncertainty in a direct comparison of the calculated equilibrium isotopic fractionation values with the experimental results.Stable Sr-isotope fractionation varied along with the distribution coefficient of Sr [Kd(Sr)=[Sr/Ba]barite/[Sr/Ba]solution], which is a function of both temperature and barite saturation state. However the relationship between mass dependent isotopic fractionation and Kd(Sr) is different for conditions of changing temperature versus barite saturation state. With increasing temperature (from 5 to 40°C), the barite phase became isotopically lighter (Δ88/86Sr=−0.29‰ to −0.41‰). Conversely, with increasing saturation state (saturation index of barite=3.0–4.3) the barite phase became isotopically heavier (Δ88/86Sr=−0.25‰ to −0.10‰). These observations suggest chemical kinetic effects control isotopic fractionation rather than equilibrium temperature effects. The relationship with saturation state indicates the potential presence of a diffusive boundary layer. Barite crystal morphology appears to be affected by the diffusion rate of solute (sulfate) to the growing crystal surface relative to the overall growth rate of barite crystals during precipitation.

Variations in the stable isotope composition of mercury in coal-bearing sequences: Indications for its provenance and geochemical processes

Mercury (Hg) stable isotope variations provide a potential tool to better understand the provenance and geochemical processes that control Hg occurrences in coal deposits. In this study, we explore the variations in Hg isotope compositions in coal benches of a single coal seam (No. 3-1 coal seam, Daizhuang Coal Mine, Jining Coalfield, Shandong Province), and in successive coals seams of a coal-bearing sequence (No. 1 to No. 11-2 coal seams, Zhuji Coal Mine, Huainan Coalfield, Anhui Province) to assess Hg isotopes as a Hg biogeochemical tracer in coal deposits. Large variations in mass dependent Hg isotope fractionation (MDF) were observed in both Daizhuang and Zhuji coal deposits, with δ202Hg values ranging from −2.34 to −0.25±0.12‰ (2SD, n=8), and from −1.62 to 0.44±0.12‰ (2SD, n=18), respectively. Daizhuang coals showed insignificant mass independent Hg isotope fractionation (MIF) in most samples (−0.04 to 0.12±0.08‰ for Δ199Hg, 2SD, n=8), whereas MIF in the younger Nos. 7-11 coal seams of Zhuji coals (0.06 to 0.22±0.08‰ for Δ199Hg, 2SD, n=7) were positive and significant. Increased trends in both Hg concentrations and δ202Hg values going from older to younger Zhuji coal seams were observed, and were possibly related to variations in coal-forming environment of different coal seams. The significant negative correlations of δ202Hg vs. 1/Hg in the Zhuji coals, and of δ202Hg vs. Δ199Hg in the Daizhuang coals suggest that Hg isotopes can potentially be used to trace organic and inorganic Hg end-members in coal deposits. Natural coke, a metamorphosed form of coal, is on average two-fold enriched in Hg compared to coal, indicating that hydrothermal fluids derived from magmatic intrusions bring Hg into coal deposits. In addition, coke has either distinctly higher (0.70 to 0.91±0.12‰, 2SD, n=2) or lower δ202Hg (−4.00 to −3.47±0.12‰, 2SD, n=3) than corresponding coals, demonstrating that significant Hg MDF occurred when coals were thermally contacted by intruded magmas.

Continual in situ monitoring of pore water stable isotopes in the subsurface

Abstract. Stable isotope signatures provide an integral fingerprint of origin, flow paths, transport processes, and residence times of water in the environment. However, the full potential of stable isotopes to quantitatively characterize subsurface water dynamics is yet unfolded due to the difficulty in obtaining extensive, detailed, and repeated measurements of pore water in the unsaturated and saturated zone. This paper presents a functional and cost-efficient system for non-destructive continual in situ monitoring of pore water stable isotope signatures with high resolution. Automatic controllable valve arrays are used to continuously extract diluted water vapor in soil air via a branching network of small microporous probes into a commercial laser-based isotope analyzer. Normalized liquid-phase isotope signatures are then obtained based on a specific on-site calibration approach along with basic corrections for instrument bias and temperature dependent isotopic fractionation. The system was applied to sample depth profiles on three experimental plots with varied vegetation cover in southwest Germany. Two methods (i.e., based on advective versus diffusive vapor extraction) and two modes of sampling (i.e., using multiple permanently installed probes versus a single repeatedly inserted probe) were tested and compared. The results show that the isotope distribution along natural profiles could be resolved with sufficiently high accuracy and precision at sampling intervals of less than four minutes. The presented in situ approaches may thereby be used interchangeably with each other and with concurrent laboratory-based direct equilibration measurements of destructively collected samples. It is thus found that the introduced sampling techniques provide powerful tools towards a detailed quantitative understanding of dynamic and heterogeneous shallow subsurface and vadose zone processes.

Salinity dependent hydrogen isotope fractionation in alkenones produced by coastal and open ocean haptophyte algae

The hydrogen isotope fractionation in alkenones produced by haptophyte algae is a promising new proxy for paleosalinity reconstructions. To constrain and further develop this proxy the coastal haptophyte Isochrysis galbana and the open ocean haptophyte alga Emiliania huxleyi were cultured at different salinities. The fractionation factor, αalkenones–water, ranged between 0.853 and 0.902 for I. galbana and 0.789 and 0.822 for E. huxleyi. The results show a strong linear correlation between the fractionation factor α and salinity for E. huxleyi, in agreement with earlier studies, but also for I. galbana. Both haptophytes show the same response to changes in salinity, represented by the slopes of the α–salinity relationship (∼0.002 per salinity unit). This suggests that the same process, in both coastal as well as open ocean haptophytes, is responsible for reducing fractionation with increasing salinity. However, there is a significant difference in absolute isotope fractionation between E. huxleyi and I. galbana, i.e. E. huxleyi produces alkenones which are 90‰ more depleted in D under the same culturing conditions than I. galbana. Our data suggest that the δD of alkenones can be used to reconstruct relative shifts in paleosalinity in coastal as well as open ocean environments with careful consideration of species composition and other complicating factors especially in coastal regions.

Concentration dependent nitrogen isotope fractionation during ammonium uptake by phytoplankton under an algal bloom condition in the Danshuei estuary, northern Taiwan

In July 2009 an intense algal bloom with maximum Chl-a concentration reaching 166μgL−1 occurred in the highly eutrophic Danshuei River estuary, which receives waste discharges from the densely populated Taipei metropolitan area in northern Taiwan. The estuary is often burdened with very high concentration of ammonium (up to ~550μM), which dominates the dissolved inorganic nitrogen species in the estuary. The observed δ15N values of particulate nitrogen ranged from −8.6‰ to 0.2‰, and the δ15N values of coexisting ammonium ranged from 4.6 to 11.9‰. Notably the offset between δ15NPN and δ15NNH4 (Δδ15N) showed significant correlation with ammonium concentration. The ε-values were calculated to be between −4.7 and −16.4‰. The range overlaps with that of previous estimates (−6.5 to −18.1‰) based on field observations. We plotted all field observed ε-values vs. corresponding ammonium concentrations and found a trend similar to that previously observed for marine bacterium, Vibrio harveyi, in laboratory cultures. Thus, we constructed a concentration dependent curve of the ε-value for ammonium uptake by phytoplankton in natural waters. The curve shows the maximum magnitude of ε-value (−20‰) at ammonium concentration around 100μM with decreasing isotope effect on both sides; at lower concentrations, the ε-value diminishes to zero; at higher concentrations, it slopes gradually towards an asymptotic value around −2‰. More than half of the ε-values derived from laboratory cultures of diatoms also fall on this curve. However, a few culture-based ε-values fall on another curve with similar pattern but considerably larger maximum magnitude. The maximum isotope effect is probably attributed to the cumulative isotope effects from ammonium deprotonation and the subsequent membrane diffusion of ammonia. This study provides the first field observed evidence of concentration dependent nitrogen isotope fractionation during ammonium uptake by phytoplankton and reconciles partially the disparity between estimates from field observations and from laboratory cultures.

Li+ speciation and the use of 7Li/6Li isotope ratios for ancient climate monitoring

We have carried out a theoretical study of aqueous Li+ speciation including effects of ligand coordination, temperature and solution pH. We have calculated the isotope exchange equilibrium constant, KOH−H2O7/6Li, associated with the Li acid/base equilibrium and can constrain it to positive values. The consequences of this species dependent isotope fractionation are then studied using a model for Li-Carbonate coordination. We define an effective isotope fractionation, effΔ7/6LiCO32−−H2O, to model temperature and pH induced changes in Li isotope fractionation associated with the formation of Li-Carbonate species. We predict that, under normal oceanic conditions, effΔ7/6LiCO32−−H2O is not sensitive to pH indeed but may be significantly influenced (on the order of 12‰) by temperature. Since the isotope ratios found in Li containing calcium carbonate shells show no indications of variation in isotopic composition, we conclude that the mechanism by which Li is incorporated to this material is not dependent on aqueous Li speciation.