Mechanism Of Isotope Fractionation Research Articles

Iron and Zinc isotope fractionation during magmatism in the continental crust: Evidence from bimodal volcanic rocks from Hailar basin, NE China

This study presents Fe–Zn isotope data for a suite of well–characterized bimodal volcanic rocks from Hailar Basin, northeast China to understand the mechanism of Fe isotope fractionation in highly differentiated igneous rocks. The samples range from basaltic trachyandesites to trachytes–rhyodacites, and rhyolites. The δ56Fe values increase with increasing SiO2 contents with the rhyolites having the highest δ56Fe (up to 0.64±0.02‰) among the previously reported data for igneous rocks at a similar SiO2. The lack of correlation between δ56Fe and Rb/La argues against the effect of fluid exsolution on Fe isotopes. The δ56Fe do not show a clear correlation with δ66Zn and radiogenic isotopes, suggesting that thermal diffusion or crustal contamination cannot produce the high δ56Fe in Hailar volcanic rocks.Fe isotopic variation in Hailar volcanic rocks can be explained by two steps of magmatism. During the first step, partial melting of basaltic trachyandesites with an average δ56Fe of 0.09±0.14‰ produced trachytes–rhyodacites with an average δ56Fe of 0.24±0.27‰. Modelling using rhyolite–MELTS shows that Fe isotopes can be fractionated by preferential partitioning of isotopically different Fe3+ and Fe2+ between the solid residue and partial melt. The second step involves formation of rhyolites with significantly high δ56Fe through partial melting or extensive crystallization of crust materials, during which isotopically heavy Fe preferentially partition into the rhyolitic melt. Therefore, fractionation of Fe isotopes between melts and minerals can result in high δ56Fe in SiO2-rich igneous rocks and apparent Fe isotope heterogeneity within the continental crust.

Calcium isotope fractionation between aqueous compounds relevant to low-temperature geochemistry, biology and medicine

Stable Ca isotopes are fractionated between bones, urine and blood of animals and between soils, roots and leaves of plants by >1000 ppm for the 44Ca/40Ca ratio. These isotopic variations have important implications to understand Ca transport and fluxes in living organisms; however, the mechanisms of isotopic fractionation are unclear. Here we present ab initio calculations for the isotopic fractionation between various aqueous species of Ca and show that this fractionation can be up to 3000 ppm. We show that the Ca isotopic fractionation between soil solutions and plant roots can be explained by the difference of isotopic fractionation between the different first shell hydration degree of Ca2+ and that the isotopic fractionation between roots and leaves is controlled by the precipitation of Ca-oxalates. The isotopic fractionation between blood and urine is due to the complexation of heavy Ca with citrate and oxalates in urine. Calculations are presented for additional Ca species that may be useful to interpret future Ca isotopic measurements.

Δ14CO2 from dark respiration in plants and its impact on the estimation of atmospheric fossil fuel CO2

Radiocarbon (14C) has been widely used for quantification of fossil fuel CO2 (CO2ff) in the atmosphere and for ecosystem source partitioning studies. The strength of the technique lies in the intrinsic differences between the 14C signature of fossil fuels and other sources. In past studies, the 14C content of CO2 derived from plants has been equated with the 14C content of the atmosphere. Carbon isotopic fractionation mechanisms vary among plants however, and experimental study on fractionation associated with dark respiration is lacking. Here we present accelerator mass spectrometry (AMS) radiocarbon results of CO2 respired from 21 plants using a lab-incubation method and associated bulk organic matter. From the respired CO2 we determine Δ14Cres values, and from the bulk organic matter we determine Δ14Cbom values. A significant difference between Δ14Cres and Δ14Cbom (P < 0.01) was observed for all investigated plants, ranging from −42.3‰ to 10.1‰. The results show that Δ14Cres values are in agreement with mean atmospheric Δ14CO2 for several days leading up to the sampling date, but are significantly different from corresponding bulk organic Δ14C values. We find that although dark respiration is unlikely to significantly influence the estimation of CO2ff, an additional bias associated with the respiration rate during a plant's growth period should be considered when using Δ14C in plants to quantify atmospheric CO2ff.

A review of antimony (Sb) isotopes analytical methods and application in environmental systems

Antimony (Sb) is a metalloid element with potential toxicity and carcinogenicity, and is ubiquitous in our living environment. Sb pollution has become a serious global problem, which has been induced by combustion of fossil fuel and Sb-containing waste as well as mining activities. However, Sb geochemical cycle in environment systems and its inherent hazards to human and ecosystem health are poorly known. Sb isotope technology provides a novel method to counter this challenge. Sb isotopic ratio could not be measured precisely until the introduction of the multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) at the beginning of the 20th century. This paper systematically reviewed the analytical methods of Sb isotopes, including pre-concentration and separation methods of thiol cotton fiber (TCF) and ion-exchange chromatography, measurement techniques of MC-ICP-MS with hydride generation (HG) and metal doping. Additionally, the Sb isotope fractionation mechanism was summarized in biological, reduction, adsorption, evaporation, precipitation and mixing processes. The Sb isotope application research status is also reviewed, such as illustrating Sb sources and its characteristics of migration and transformation in hydrologic system. Finally, future prospects for Sb isotopes in environmental systems have been proposed, which will promote the advances in the study of Sb pollution and its prevention significantly.

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 …

Zinc isotope fractionation during adsorption to calcite at high and low ionic strength

The biogeochemical cycle of Zn in the oceans has been of interest for decades, and the newly discovered isotopic variations among major reservoirs of Zn in the marine system have raised new questions and provided some new insights regarding the processes that control the concentration and isotopic distribution of this bioessential element. Fundamental understanding of Zn isotope fractionation mechanisms will be required to optimize the value of data now being generated by efforts such as GEOTRACES. In particular, fractionation driven by reactions taking place at fluid-solid interfaces will be crucial to quantify, since these reactions exert significant control on fluid concentrations and bioavailability of Zn and other trace metals. In this study, we aimed to determine whether interaction between dissolved Zn and Zn bound to calcite surfaces drives significant isotope fractionation. Zinc adsorption experiments were conducted in simple, abiotic systems at low ionic strength (deionized water) and high ionic strength (synthetic seawater) as several series of small batch reactions, at pH8.3, with experimental duration from 0.5 to 120h. We observed that the Zn adsorbed on calcite was isotopically heavier than in the co-existing solutions, with Δ66/64Znadsorbed-solution averaging 0.41±0.18‰ (2SD) and 0.73±0.08‰ (2SD) in low and high ionic strength systems, respectively. One possible mechanistic interpretation of this result is that isotope fractionation between dissolved and adsorbed Zn is an equilibrium effect driven by the coordination number difference between aqueous Zn(H2O)62+ (six-fold) and adsorbed (four-fold) complexes. Alternatively, direct adsorption of isotopically heavy ZnCO30(aq) without further fractionation is possible, since this species has been shown previously with theoretical calculations to be enriched in heavy isotopes relative to other Zn species expected in solution. Our results show that the magnitude of equilibrium fractionation between dissolved and calcite-sorbed Zn is clearly sensitive to aqueous Zn speciation, and a simple mass balance exercise using theoretical predictions in the literature for isotopic fractionation among aqueous species demonstrates that a pool of isotopically light Zn is complexed with chloride ions in high ionic strength solutions, leading to a heavier pool of Zn adsorbed to the calcite surface. Further work will be needed to determine whether the pool of isotopically heavy Zn adsorbing to calcite surfaces can be incorporated during calcite crystal growth to impart an isotopically heavy signature to the resulting calcite.

Revisiting streamside trees that do not use stream water: can the two water worlds hypothesis and snowpack isotopic effects explain a missing water source?

AbstractWe revisit a classic ecohydrological study that showed streamside riparian trees in a semiarid mountain catchment did not use perennial stream water. The original study suggested that mature individuals of Acer negundo, Acer grandidentatum, and other species were dependent on water from “deeper strata,” possibly groundwater. We used a dual stable isotope approach (δ18O and δ2H) to further examine the water sources of these trees. We tested the hypothesis that groundwater was the main tree water source, but found that neither groundwater nor stream water matched the isotope composition of xylem water during two growing seasons. Soil water (0–1 m depth) was closest to and periodically overlapped with xylem water isotope composition, but overall, xylem water was isotopically enriched compared to all measured water sources. The “two water worlds” hypothesis postulates that soil water comprises isotopically distinct mobile and less mobile pools that do not mix, potentially explaining this disparity. We further hypothesized that isotopic effects during snowpack metamorphosis impart a distinct isotope signature to the less mobile soil water that supplies summer transpiration. Depth trends in water isotopes following snowmelt were consistent with the two water worlds hypothesis, but snow metamorphic isotope effects could not explain the highly enriched xylem water. Thus, the dual isotope approach did not unambiguously determine the water source(s) of these riparian trees. Further exploration of physical, geochemical, and biological mechanisms of water isotope fractionation and partitioning is necessary to resolve these data, highlighting critical challenges in the isotopic determination of plant water sources.

Comparative geochemistry of four ferromanganese crusts from the Pacific Ocean and significance for the use of Ni isotopes as paleoceanographic tracers

Ferromanganese (Fe-Mn) crusts are potential archive of the Ni isotope composition of seawater through time. In this study we aim at (1) understanding Ni isotope fractionation mechanisms and metal enrichment processes in Fe-Mn deposits, (2) addressing global vs. local control of Ni isotope composition of these deposits. Two Fe-Mn crusts from the North Pacific Ocean (Apuupuu Seamount, Hawaii) and two Fe-Mn crusts from the South Pacific Ocean (near Rurutu Island, Austral archipelago of French Polynesia) were characterized for their elemental geochemistry and Ni isotope composition. Geochemical analyses were performed at millimeter intervals in order to provide time-resolved record of Ni isotopes. Chronology and growth rates were determined using cosmogenic 10Be isotope abundances. The results show that, despite different growth rates, textures and geochemical patterns, Fe-Mn crusts from both North and South Pacific Oceans have fairly homogenous Ni isotope compositions over the last ∼17Ma, yielding average δ60/58Ni values of 1.79±0.21‰ (2sd, n=31) and 1.73±0.21‰ (2sd, n=21) respectively. In one crust sample, however, layers directly in contact with the altered substrate show anomalously light δ60/58Ni values down to 0.25±0.05‰ (2se) together with rejuvenated 10Be/9Be ratios correlating with elevated Ni/Mn ratios. Such patterns are best explained by protracted fluid–rock interactions leading to alteration of Mn-phases after crust formation. Isotopically light Ni would be the result of Ni isotope fractionation during adsorption rather than the contribution of external Ni sources (e.g. hydrothermal sources) having light Ni isotope compositions. The combination of our results with previously published data on Fe-Mn crusts indicates that the average Ni isotope composition in deep waters has not changed through the Cenozoic (∼70Ma). We propose that Ni isotope variations in Fe-Mn crusts may not only record variations of Ni sources to the oceans, but also post-depositional processes depending on the growth history and geological settings of Fe-Mn crusts.

Oxygen isotope fractionation during N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O production by soil denitrification

Abstract. The isotopic composition of soil-derived N2O can help differentiate between N2O production pathways and estimate the fraction of N2O reduced to N2. Until now, δ18O of N2O has been rarely used in the interpretation of N2O isotopic signatures because of the rather complex oxygen isotope fractionations during N2O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H2O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N2O production by soil denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the δ18O values of the produced N2O. In our soil incubation experiments Δ17O isotope tracing was applied for the first time to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found that N2O formation in static anoxic incubation experiments was typically associated with oxygen isotope exchange close to 100 % and a stable difference between the 18O ∕ 16O ratio of soil water and the N2O product of δ18O(N2O ∕ H2O) = (17.5 ± 1.2) ‰. However, flow-through experiments gave lower oxygen isotope exchange down to 56 % and a higher δ18O(N2O ∕ H2O) of up to 37 ‰. The extent of isotope exchange and δ18O(N2O ∕ H2O) showed a significant correlation (R2 = 0.70, p &lt; 0.00001). We hypothesize that this observation was due to the contribution of N2O from another production process, most probably fungal denitrification. An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification, isotope exchange occurs prior to isotope branching and that this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0 ± 0.9) ‰, in contrast to fungal denitrification where higher values of up to 30 ‰ have been reported previously. This suggests that δ18O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects.

Isotopic Geochemistry of Cadmium: A Review

AbstractCadmium (Cd) is a scarce, but not an extremely rare element in the Earth's crust (crustal average: 0.2 ppm Cd). Geochemically, Cd exhibits thiophile, lithophile, and volatile behavior in different geologic processes. Biologically, it is a nutrient‐like element that is closely related to P and Zn and is toxic element to organisms. Presently, Cd isotopes have been successfully utilized to trace Cd sources and nutrient cycling in marine systems in addition to unearthing other geochemical processes. Using published studies and our recent work, this survey summarizes the chemical preparation and mass spectrometry of Cd isotopes. It also reviews Cd isotopic compositions and fractionation mechanisms in nature as well as experiments.

Stable Isotope Signatures of Middle Palaeozoic Ahermatypic Rugose Corals - Deciphering Secondary Alteration, Vital Fractionation Effects, and Palaeoecological Implications.

This study investigates stable isotope signatures of five species of Silurian and Devonian deep-water, ahermatypic rugose corals, providing new insights into isotopic fractionation effects exhibited by Palaeozoic rugosans, and possible role of diagenetic processes in modifying their original isotopic signals. To minimize the influence of intraskeletal cements on the observed signatures, the analysed specimens included unusual species either devoid of large intraskeletal open spaces ('button corals': Microcyclus, Palaeocyclus), or typified by particularly thick corallite walls (Calceola). The corals were collected at four localities in the Holy Cross Mountains (Poland), Mader Basin (Morocco) and on Gotland (Sweden), representing distinct diagenetic histories and different styles of diagenetic alteration. To evaluate the resistance of the corallites to diagenesis, we applied various microscopic and trace element preservation tests. Distinct differences between isotopic compositions of the least-altered and most-altered skeleton portions emphasise a critical role of material selection for geochemical studies of Palaeozoic corals. The least-altered parts of the specimens show marine or near-marine stable isotope signals and lack positive correlation between δ13C and δ18O. In terms of isotopic fractionation mechanisms, Palaeozoic rugosans must have differed considerably from modern deep-water scleractinians, typified by significant depletion in both 18O and 13C, and pronounced δ13C-δ18O co-variance. The fractionation effects exhibited by rugosans seem similar rather to the minor isotopic effects typical of modern non-scleractinian corals (octocorals and hydrocorals). The results of the present study add to growing evidence for significant differences between Scleractinia and Rugosa, and agree with recent studies indicating that calcification mechanisms developed independently in these two groups of cnidarians. Consequently, particular caution is needed in using scleractinians as analogues in isotopic studies of extinct coral lineages. Answering some of the pertinent palaeoecological questions, such as that of the possibility of photosymbiosis in Palaeozoic corals, may not be possible based on stable isotope data.

Experimental study on stable isotopic fractionation of evaporating water under varying temperature

The variation of stable isotope ratios in natural waters provides valuable information that can be used to trace water movement. Evaporation plays a crucial role in determining the variation of stable isotopes. In this paper, several evaporation experiments were conducted in order to study the stable isotopic fractionation mechanism of water and analyze the influence of different temperatures on evaporation fractionation. Three group experiments of water evaporation under different temperatures and initial isotopic values were carried out. The results show that fractionation factors of hydrogen and oxygen may increase with temperature, and the average enrichment degree of hydrogen isotope D is 3.432 times that of oxygen isotope . The results also show that the isotopic composition of the initial water has little influence on water evaporation fractionation, which is mainly affected by the state variables in the evaporation process, such as temperature. This research provides experimental data for further understanding the evaporation fractionation mechanism. 18O

Magnesium isotope fractionation during shale weathering in the Shale Hills Critical Zone Observatory: Accumulation of light Mg isotopes in soils by clay mineral transformation

Magnesium isotopic ratios have been used as a natural tracer to study weathering processes and biogeochemical pathways in surficial environments, but few have focused on the mechanisms that control Mg isotope fractionation during shale weathering. In this study we focus on understanding Mg isotope fractionation in the Shale Hills catchment in central Pennsylvania. Mg isotope ratios were measured systematically in weathering products, along geochemical pathways of Mg during shale weathering: from bedrock to soils and soil pore water on a planar hillslope, and to sediments, stream water, and groundwater on a valley floor. Significant variations of Mg isotopic values were observed: δ26Mg values (−0.6‰ to −0.1‰) of stream and soil pore waters are about ~0.5‰ to 1‰ lighter than the shale bedrock δ(26Mg values of +0.4‰), consistent with previous observations that lighter Mg isotopes are preferentially released to water during silicate weathering. Dissolution of the carbonate mineral ankerite, depleted in the shallow soils but present in bedrock at greater depths, produced higher Mg2+ concentrations but lower δ26Mg values (−1.1‰) in groundwater, ~1.5‰ lighter than the bedrock. δ26Mg values (+0.2‰ to +0.4‰) of soil samples on the planar hillslope are either similar or up to ~0.2‰ lighter than the bedrock. Hence a heavy Mg isotope reservoir – complementary to the lighter Mg isotopes in soil pore water and stream water – is missing from the residual soils on the hillslope. In addition, soil samples show a slight but systematic decreasing trend in δ26Mg values with increasing weathering duration towards the surface. We suggest that the accumulation of light Mg isotopes in surface soils at Shale Hills is due to a combined effect of i) sequestration of isotopically light Mg from soil water during clay dissolution–precipitation reactions; and ii) loss of isotopically heavy particulate Mg in micron-sized particles from the hillslope as suspended sediments. This latter mechanism is somewhat surprising in that most researchers do not consider physical removal or particles to be a likely mechanism of isotopic fractionation. Stream sediments (δ26Mg values of +0.3‰ to +0.5‰) accumulated on the valley floor are ~0.2‰ heavier than the bedrock, and are thus consistent with that mobile particulates are the heavy Mg isotope reservoir.Our study provides the first field evidence that changes in clay mineralogy lead to accumulation of lighter Mg isotopes in residual bulk soils. This example also demonstrates that transport of isotopically distinct fine particles from clay-rich systems could be a new and important mechanism to drive the Mg isotope compositions of silicate weathering residuals. This mechanism drives fractionation in an opposite direction as might be expected from previous studies, i.e. residual soils are driven to lighter Mg values and sediments become isotopically heavier.

Ultralight Oxygen in Corundum-Bearing Rocks of North Karelia, Russia, as a Result of Isotope Separation by Thermal Diffusion (Soret Effect) in Endogenous Fluid Flow

Helium and argon isotopes in fluid inclusions in minerals of corundum bearing rocks with anomalous light oxygen of the Khitoostrov (North Karelia, Russia) were studied. It was found that atmospheric noble gas component is missing. Therefore, all previously proposed models of participation in the fluid surface ice meltwater are not valid. Proposed and justified endogenous mechanism of isotope fractionation of oxygen and other chemical elements by the mechanism of thermal diffusion. Geologically justified existence of the cascading effect for a number of the separation thermodiffusion cells, which lead to significant isotope effects. Cascading is realized due to the “fluid pump”, in which role are acting amphibolized gabbro in the contact with corundum metasomatites. It is assumed that the mechanism is not specific for the case corundum metasomatic rocks, which are a special case of manifestation of such a scenario in processes involving endogenous fluid flows.

Fe Isotope Fractionation Mechanisms and its Application in Geoscience Field

Fe Isotope Fractionation Mechanisms and its Application in Geoscience Field

Experimental determinations of isotopic fractionation factors associated with N2O production and reduction during denitrification in soils

Quantifying denitrification in arable soils is crucial in predicting nitrogen fertiliser losses and N2O emissions. Stable isotopologue analyses of emitted N2O (δ15N, δ18O and SP=15N site preference within the linear N2O molecule) may help to distinguish production pathways and to quantify N2O reduction to N2. However, such interpretations are often ambiguous due to insufficient knowledge on isotopic fractionation mechanisms. Here we present a complex experimental approach to determine the net fractionation factors (η) associated with denitrification. This determination is based on three laboratory experiments differing in their experimental set-up and soil properties. Static and dynamic incubation techniques were compared. All available methods for independent determination of N2O reduction contribution were used, namely, N2-free atmosphere incubation, acetylene inhibition technique and 15N gas-flux method.For N2O production: (i) the determined difference in δ18O between soil water and produced N2O vary from +18‰ to +42‰ and show very strict negative correlation with soil water saturation; (ii) the determined η15N of N2O production vary from −55‰ to −38‰ and the fractionation decreases with decreasing substrate availability; (iii) the determined SP of produced N2O vary from −3‰ to +9‰. For N2O reduction: (i) the determined η18O and η15N of N2O reduction vary in very wide ranges from −18‰ to +4‰ and from −11‰ to +12‰, respectively, and depend largely on the differences in experimental setups; whereas (ii) the determined ηSP of N2O reduction shows a very consistent value with all previous studies and varies in a rather narrow range from −2‰ to −8‰. It can be concluded that η values of N2O production determined during laboratory incubations yield only roughly estimates for respective values expectable under field study conditions. η18O and η15N associated with N2O reduction may vary largely, probably depending on spatial and temporal coincidence of N2O production and reduction, and are hence not yet predictable for natural conditions. However, the ηSP of N2O reduction appeared to be relatively robust and a most probable value of about −5‰ can be used to constrain N2O reduction based on SP of soil emitted N2O.

What controls silicon isotope fractionation during dissolution of diatom opal?

The silicon isotope composition of opal frustules from photosynthesising diatoms is a promising tool for studying past changes in the marine silicon cycle, and indirectly that of carbon. Dissolution of this opal may be accompanied by silicon isotope fractionation that could disturb the pristine silicon isotope composition of diatom opal acquired in the surface ocean. It has previously been shown that dissolution of fresh and sediment trap diatom opal in seawater does fractionate silicon isotopes. However, as the mechanism of silicon isotope fractionation remained elusive, it is uncertain whether opal dissolution in general is associated with silicon isotope fractionation considering that opal chemistry and surface properties are spatially and temporally (i.e. opal of different age) diverse.In this study we dissolved sediment core diatom opal in 5mM NaOH and found that this process is not associated with significant silicon isotope fractionation. Since no variability of the isotope effect was observed over a wide range of dissolution rates, we can rule out the suggestion that back-reactions had a significant influence on the net isotope effect. Similarly, we did not observe an impact of temperature, specific surface area, or degree of undersaturation on silicon isotope partitioning during dissolution, such that these can most likely also be ruled out as controlling factors. We discuss the potential impacts of the chemical composition of the dissolution medium and age of diatom opal on silicon isotope fractionation during dissolution. It appears most likely that the controlling mechanism of silicon isotope fractionation during dissolution is related to the reactivity, or potentially, aluminium content of the opal. Such a dependency would imply that silicon isotope fractionation during dissolution of diatom opal is spatially and temporally variable. However, since the isotope effects during dissolution are small, the silicon isotope composition of diatom opal appears to be robust against dissolution in the deep sea sedimentary environment.

Biogeochemical cycling of cadmium isotopes in the Southern Ocean along the Zero Meridian

We present depth profiles of Cd isotopes and concentrations from the Southern Ocean at four stations in the Atlantic sector along the Greenwich Meridian (47°S to 68°S) located across the main Antarctic frontal zones and productivity belt. The vertical profiles of Cd concentration typically show low values in surface waters, elevated values at intermediate depths, reflecting remineralization of sinking particulate organic matter, and constant values in deep waters. The surface-to-deep isotopic gradient shows “heavy” Cd isotope signatures in the mixed surface layer, becoming more pronounced northward, with values up to ɛ112/110Cd of around +4.1 in the Subantarctic sector of the Southern Ocean. Deep Antarctic waters display a uniform and “light” ε112/110Cd of +1.18±0.38 and Cd concentrations of 0.761±0.101nmol/kg (n=23, 2SD). Intermediate waters are characterized by ε112/110Cd lying between those of surface and deep waters, with a constant value of about +0.8 in the High Nutrient Low Chlorophyll sector and a notably higher value of +2.3 in the Subantarctic sector.The Cd isotope fractionation in the Southern Ocean closely follows a simple closed-system Rayleigh model, in which biological uptake of Cd imparts the ε112/110Cd signature to the surface layer while that of deep waters is determined by the flux of regenerated isotopically-light Cd from sinking organic matter from the surface ocean and the degree of mixing of distinct water masses.The vertical gradient documented for Cd isotopes and nutrient ratios, along with the meridional gradient in surface waters, highlights the important role played by upwelling in the Southern Ocean in closing the meridional overturning circulation via the export of Antarctic intermediate and mode waters which have a distinctive chemical (low Cd:P) and Cd isotope (“heavy”) signature.The combined Cd–Zn isotope systematics provide evidence for a strong link between the magnitude of biological Cd stable isotope fractionation and Zn availability in the contrasted nutrient and ecological regimes of the Southern Ocean. Substitution of Cd for Zn in the enzyme carbonic anhydrase appears to be the driving mechanism for Cd isotope fractionation in the Antarctic Circumpolar Current, while an “excess-uptake” mechanism seems to predominate in the Weddell Gyre.Our study highlights some of the complexities of the biogeochemical cycling of Cd in the oceans. Nevertheless, systematic variations in Cd isotopic compositions with water mass distribution in the Southern Ocean suggest that Cd isotopes could, with some caveats, be useful tracers of changes in past nutrient utilization and deep water circulation.

Isotope geochemistry of mercury and its relation to earthquake in the Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Based on the Wenchuan Earthquake Fault Scientific Drilling Project (WFSD), we first investigated the distribution of the total mercury (THg) from 600 m to 1035 m of WFSD Hole-1 core (WFSD-1). The concentrations of THg in the fault gouge are significantly higher than in the sandstone, shale, siltstone, and cataclasite in WFSD-1. The THg concentrations in the main seismic fault zone range from 9.9 to 73.5 ng/g while those under the main fault range from 20.5 to 36 ng/g. To determine the source of Hg, typical rocks from different depths were chosen for Hg isotope analysis. The results show that δ202Hg in the main seismic fault zone range from − 0.11‰ to − 2.68‰, and, below the seismic fault zone, δ202Hg are from − 0.64‰ to − 1.33‰. The seismic fault rocks are more enriched in THg and have a larger variation of δ202Hg compared to the other rocks. Mercury has different isotopic compositions and fractionation mechanisms in the fault zone, which are mainly affected by the earthquake and the fluid. The mercury isotope compositions reflect the fluid activities at the main fault zone. We demonstrate that mercury stable isotope ratios could serve as an effective tool for tracing mercury sources and monitoring and predicting earthquakes.