In our research, we leverage the capabilities of the Sentinel-3A and Sentinel-3B satellites, launched in February 2016 and April 2018, respectively, to deepen our understanding of the polar regions. These satellites offer a unique blend of high-resolution Ku-band radar altimetry data, synthetic aperture radar (SAR) mode altimetry, and the Ocean and Land Colour Instrument (OLCI) imaging spectrometer. This combination enables the acquisition of both optical imagery and SAR radar altimetry data, extending up to 81 degrees North. Central to our study is the application of deep learning techniques, specifically the Vision Transformers (ViT), which adapt the Transformer algorithm for surface classification in polar environments. This approach is instrumental in distinguishing between sea ice and leads, demonstrating robust performance across various metrics, including accuracy and model roll-out on comprehensive OLCI image datasets. We produce our first lead classification maps at the original OLCI swath level resolution of 300m and a lead fraction prototype mosaic spring pan-Arctic product at gridded level of 1km, 5km and 10km resolution and on daily, weekly and monthly timescales. The use of binned statistics in conjunction with our deep learning classifications provides valuable insights into the spatial distribution and changes of leads within the polar ice. We compare our prototype product with other existing lead products and with auxiliary datasets on thin ice (roughness, thickness). Our work combining different satellite products at pan-Arctic intermediate resolution enhances our capacity to estimate sea ice thickness and aids in forecasting future changes in the Arctic and Antarctic regions, thereby contributing to the field of polar remote sensing with direct applications to the future polar missions CRISTAL and CMIR.

The Sentinel-3A and Sentinel-3B satellites, launched in February 2016 and April 2018 respectively, build on the legacy of CryoSat-2 by providing high-resolution Ku-band radar altimetry data over the polar regions up to 81° North. The combination of synthetic aperture radar (SAR) mode altimetry (SRAL instrument) from Sentinel-3A and Sentinel-3B, and the Ocean and Land Colour Instrument (OLCI) imaging spectrometer, results in the creation of the first satellite platform that offers coincident optical imagery and SAR radar altimetry. We utilise this synergy between altimetry and imagery to demonstrate a novel application of deep learning to distinguish sea ice from leads in spring. We use SRAL classified leads as training input for pan-Arctic lead detection from OLCI imagery. This surface classification is an important step for estimating sea ice thickness and to predict future sea ice changes in the Arctic and Antarctic regions. We propose the use of Vision Transformers (ViT), an approach adapting the popular deep learning algorithm Transformer, for this task. Their effectiveness, in terms of both quantitative metric including accuracy and qualitative metric including model roll-out, on several entire OLCI images is demonstrated and we show improved skill compared to previous machine learning and empirical approaches. We show the potential for this method to provide lead fraction retrievals at improved accuracy and spatial resolution for sunlit periods before melt onset.

The double spike (DS) technique is a highly effective approach for measuring the isotope ratios of many elements. However, it is common for some fraction of the prepared samples to be "overspiked." The usual solution for this problem involves repurifying and reanalyzing the samples to ensure data accuracy. Here, we propose a straightforward mathematical scheme to rectify the isotope ratios of overspiked samples, avoiding repetitive, time-consuming operations. The principle behind this scheme is that adding a standard solution with the certified isotope ratio decreases the overspiked ratio to the normal range. The related theoretical equations and a thorough error propagation model are presented. Taking nickel (Ni) isotopes as an example, we demonstrate how to utilize the spike-to-sample ratios of the overspiked sample and the sample-standard mixture, as well as the spike-subtracted isotope ratios of the mixture (delta 60Nimix), to accurately determine the actual sample isotopes. This method's accuracy and precision (2SD) were evaluated by testing Ni, chromium (Cr), and cadmium (Cd) isotope measurements. Precision consistent with traditional DS measurements can be achieved when the fraction of the added standard solution (fstd) is <= 0.60 (60%) in the mixture or when the overspiked multiple is <= 2.5. The added standard solution is recommended to be the same as the standard used to define the delta scale (e.g., delta 60Ni = 0.000 parts per thousand) to simplify the calculation procedures. This method expands the application of DS from the normal to the overspiked range and can be extended to isotope analyses of many elements where DS is applicable.

Double spike (DS) method has been extensively used in determining stable isotope ratios of many elements. However, challenges remain in obtaining high-precision isotope data for ultra-trace elements owing to the limitations of instrumental signal-to-noise ratios and the systematics of precision of DS-based measurements. Here, the DS-standard addition (SA) (DSSA) technique is proposed to improve measurements of isotope compositions of ultra-trace elements in natural samples. According to the U-shaped relationship between DS measurement uncertainty and the spike/ sample ratio, theoretical equations and an error propagation model (EPM) were constructed comprehensively. In our method, a spiked secondary standard solution with a high, precisely known spike/sample ratio is mixed with samples such that the mixtures have spike/sample ratios within the optimal range. The abundances of the samples relative to the added standards (sample fraction; fspl) and the samples' isotope ratios can then be obtained exactly using a standard DS data reduction routine and the isotope binary mixing model. The accuracy and precision of the DSSA approach were verified by measurements of cadmium and molybdenum isotopes at as low as 5 ng levels. Compared with traditional DS measurements, the sample size for isotope analysis is reduced to 1/6-1/5 of the original with no loss of measurement precision. The optimal mixing range fspl = 0.15-0.5 is recommended. The DSSA method can be extended to isotope measurement of more than 33 elements where the DS method is applicable, especially for the ultra-trace elements such as platinum group and rare earth element isotopes.

Most researchers assume minimal impact of pretreatment on strontium isotope ratios (87Sr/86Sr) for bones and teeth, and methods vary tremendously. We compared 14 pretreatment methods, including no prep other than powdering enamel, ashing, soaking in water, an oxidizing agent (bleach or hydrogen peroxide) or acetic acid (0.1 M, 1.0 M, and 1.0 M buffered with calcium acetate), and a combination of these steps. We prepared and analyzed aliquots of powdered molar enamel from three proboscideans (one modern captive Indian elephant, Elephas maximus indicus; one Pleistocene mastodon, Mammut americanum; and one Miocene gomphothere, Afrochoerodon kisumuensis). Each pretreatment was performed in triplicate and we measured 87Sr/86Sr, Sr concentration, and uranium (U) concentration, using the same lab space and instrumentation for all samples. Variability in 87Sr/86Sr and Sr and U concentrations was considerable across pretreatments. Mean 87Sr/86Sr across methods ranged from 0.70999 to 0.71029 for the modern tooth, 0.71458 to 0.71502 for the Pleistocene tooth, and 0.70804 to 0.70817 for the Miocene tooth. The modern tooth contained the least Sr and negligible U. The Pleistocene tooth contained slightly more Sr and measurable amounts of U, and the Miocene tooth had approximately 5x more Sr and U than the Pleistocene tooth. For all three teeth, variance in 87Sr/86Sr, Sr concentrations, and U concentrations among replicates was statistically indistinguishable across pretreatments, but there were apparent differences among pretreatments for the modern and Pleistocene teeth. Both contained relatively little Sr, and it is possible that small amounts of exogenous Sr from reagents, building materials or dust affected some replicates for some pretreatments. For the modern tooth, median 87Sr/86Sr varied considerably (but statistically insignificantly) across pretreatments. For the Pleistocene tooth, variability in median 87Sr/86Sr was also considerable; some pretreatments were statistically distinct but there were no obvious patterns among methods. For the Miocene tooth, variability in median 87Sr/86Sr was much smaller, but there were significant differences among pretreatments. Most pretreatments yielded 87Sr/86Sr and Sr concentrations comparable to, or lower than, untreated powder, suggesting selective removal of exogenous material with high 87Sr/86Sr. Further evaluation of the mechanisms driving isotopic variability both within and among pretreatment methods is warranted. Researchers should clearly report their methods and avoid combining data obtained using different methods. Small differences in 87Sr/86Sr could impact data interpretations, especially in areas where isotopic variability is low.

Stable isotope ratios of antimony (Sb) in the environment can provide valuable information on sources and processes such as redox transformations. To investigate the fractionation when Sb(V) is chemically reduced by sulfide to Sb(III), experiments with 0.008 to 0.01 mM Sb(V) and 0.009 to 6 mM sulfide at a pH of 1 to 8 were performed. Experiments at pH 1 to 6 precipitated Sb2S3, while at pH 7 to 8 Sb(III) remained in solution. The Sb(III) product was enriched in the lighter isotope. The isotopic fractionation (ε ≈ δinstantaneous product – δreactant) for the pH 1 experiment was -1.42 ± 0.04‰ while the pH 5 to 8 experiments ranged from -0.46 ± 0.04‰ to -0.62 ± 0.04‰. The small magnitude of fractionation observed in experiments at circumneutral pH may decrease the utility of Sb isotope measurement as reduction indicators in natural systems, as adsorption of Sb has been shown to fractionate isotopes in the same direction and similar magnitude (up to 1.14‰) (Wasserman, 2020; Zhou et al., 2023).

Sea-ice surface roughness (SIR) is a crucial parameter in climate and oceanographic studies, constraining momentum transfer between the atmosphere and ocean, providing preconditioning for summer-melt pond extent, and being related to ice age and thickness. High-resolution roughness estimates from airborne laser measurements are limited in spatial and temporal coverage while pan-Arctic satellite roughness does not extend over multi-decadal timescales. Launched on the Terra satellite in 1999, the NASA Multi-angle Imaging SpectroRadiometer (MISR) instrument acquires optical imagery from nine near-simultaneous camera view zenith angles. Extending on previous work to model surface roughness from specular anisotropy, a training dataset of cloud-free angular reflectance signatures and surface roughness, defined as the standard deviation of the within-pixel lidar elevations, from near-coincident operation IceBridge (OIB) airborne laser data is generated and is modelled using support vector regression (SVR) with a radial basis function (RBF) kernel selected. Blocked k-fold cross-validation is implemented to tune hyperparameters using grid optimisation and to assess model performance, with an R2 (coefficient of determination) of 0.43 and MAE (mean absolute error) of 0.041 m. Product performance is assessed through independent validation by comparison with unseen similarly generated surface-roughness characterisations from pre-IceBridge missions (Pearson’s r averaged over six scenes, r = 0.58, p < 0.005), and with AWI CS2-SMOS sea-ice thickness (Spearman’s rank, rs = 0.66, p < 0.001), a known roughness proxy. We present a derived sea-ice roughness product at 1.1 km resolution (2000–2020) over the seasonal period of OIB operation and a corresponding time-series analysis. Both our instantaneous swaths and pan-Arctic monthly mosaics show considerable potential in detecting surface-ice characteristics such as deformed rough ice, thin refrozen leads, and polynyas.

Redox reactions control the mobility and bioavailability of selenium (Se) in biogeochemical systems, both modern and ancient. Se isotope ratio measurements (e.g., 82Se/76Se) have been developed to enhance understanding of biogeochemical transformations and transport of Se. Stable isotope ratios of many elements are known to be powerful indicators of redox reactions, and shifts in 82Se/76Se have been observed for Se reduction reactions. However, Se isotope shifts caused by naturally relevant oxidation reactions have not been published. Here, we report Se isotope fractionation factors for oxidation of Se(IV) by birnessite. Experiments were conducted at pH = 4.0 and 5.5, with two types of birnessite of contrasting composition at two concentrations of suspended birnessite. The results are consistent with a single 82Se/76Se fractionation factor, for all times during all experiments, of 0.99767 (±0.0035 2 s.d.). Expressed as ε, the fractionation is 2.33‰ (±0.08‰).

Sea ice albedo is a key climate variable that affects the Earth’s radiation budget. Spatio-temporal variation of sea ice albedo can be retrieved from pre existing satellite observation processing chains such as the CLARA2-SAL product. However, currently there is only one albedo product which is derived from instantaneous multi-angle measurements and that is from MISR [1]. The accuracy of surface albedo products is usually affected by error accumulation from atmospheric corrections to the top-of-atmosphere bi-directional reflectance factor (BRF) and the modelling of bottom of the atmosphere BRF and subsequent modelling to bi-directional reflectance distribution function (BRDF) using these BRFs. Sea ice surfaces being both anisotropic and dynamic have satellite product accuracies that also depend on the length of deployed time window, thus requiring sufficient numbers of observations over a short period of time. In this study, we present a data fusion method using the high accuracy near simultaneous sampling of the Multiangle Imaging SpectroRadiometer (MISR) generated at the Langley Research Center applying a Rayleigh atmospheric correction, with the MOD35 cloud mask which is part of the MOD29 Surface Temperature and Ice Extent product derived from the Moderate Imaging Spetroradiometer (MODIS), both onboard the Terra satellite. We assume that the MISR bi-hemispherical reflectance (BHR) albedo is independent of solar angle, a crucial condition for instantaneous albedo products. As the accuracy of MOD29 cloud mask is assessed at >90% [1], this synergistic method can retrieve an improved BHR of the Arctic sea ice between April and September of each year from 2000 to 2019, and of the Antarctic sea ice between September and March of each year from 2000 to 2019. This study is a follow-on from Kharbouche and Muller (2018), that developed this method and focused on the Arctic region for the time span between March and September from 2000 to 2016. For both polar regions, we create four daily sea ice products consisting of different averaging time window (±1 day, ±3 days, ±7 days and ±15 days), each containing the number of samples, mean and standard deviation. For all four MISR cloud-free daily sea ice products, we derive 1km, 5km and 25km spatial resolutions. We perform an assessment of the day-of-year trend of sea ice BHR between 2000 and 2019 for the Arctic, and between 2000 and 2019 for Antarctic, confirming a continuing decline of sea ice shortwave albedo in the Arctic depending on the day of year and length of observed time window, and providing a novel sea ice shortwave albedo product analysis for Antarctica. Acknowledgements. This work was supported by the QA4ECV project www.QA4ECV.eu, of the European Union’s Seventh Framework Programme (FP7/2007–2013) under grant agreement number 607405. We thank our colleagues at JPL and NASA LaRC for processing the MISR data, especially Sebastian Val and Steve Protack and Jeff Walter, respectively and Richard Frey and Steve Ackerman at CIMMS, SSEC, University of Madison, WI for the analysis of the MOD35 cloud mask using CALIPSO shown in [1]. [1] https://doi.org/10.3390/rs11010009

Chapter 10 A Review of the Development of Cr, Se, U, Sb, and Te Isotopes as Indicators of Redox Reactions, Contaminant Fate, and Contaminant Transport in Aqueous Systems Thomas M. Johnson, Thomas M. Johnson Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USASearch for more papers by this authorJennifer L. Druhan, Jennifer L. Druhan Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USASearch for more papers by this authorAnirban Basu, Anirban Basu Department of Earth Sciences, Royal Holloway, University of London, Egham, UKSearch for more papers by this authorNoah E. Jemison, Noah E. Jemison University of New Mexico, Albuquerque, New Mexico, USASearch for more papers by this authorXiangli Wang, Xiangli Wang Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, ChinaSearch for more papers by this authorKathrin Schilling, Kathrin Schilling Department of Environmental Health Sciences, Mailman School of Public Health, Columbia University, New York, USASearch for more papers by this authorNaomi L. Wasserman, Naomi L. Wasserman Nuclear and Chemical Sciences Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, USASearch for more papers by this author Thomas M. Johnson, Thomas M. Johnson Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USASearch for more papers by this authorJennifer L. Druhan, Jennifer L. Druhan Department of Geology, University of Illinois at Urbana-Champaign, Urbana, Illinois, USASearch for more papers by this authorAnirban Basu, Anirban Basu Department of Earth Sciences, Royal Holloway, University of London, Egham, UKSearch for more papers by this authorNoah E. Jemison, Noah E. Jemison University of New Mexico, Albuquerque, New Mexico, USASearch for more papers by this authorXiangli Wang, Xiangli Wang Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, ChinaSearch for more papers by this authorKathrin Schilling, Kathrin Schilling Department of Environmental Health Sciences, Mailman School of Public Health, Columbia University, New York, USASearch for more papers by this authorNaomi L. Wasserman, Naomi L. Wasserman Nuclear and Chemical Sciences Division, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, USASearch for more papers by this author Book Editor(s):Kenneth W. W. Sims, Kenneth W. W. SimsSearch for more papers by this authorKate Maher, Kate MaherSearch for more papers by this authorDaniel P. Schrag, Daniel P. SchragSearch for more papers by this author First published: 15 April 2022 https://doi.org/10.1002/9781119595007.ch10Citations: 1Book Series:Geophysical Monograph Series AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Summary Cr, Se, U, Sb, and Te are toxic, redox-active elements that are more mobile and environmentally problematic in their oxidized forms, and less mobile and bioavailable in their reduced forms. This chapter reviews the development of Cr, Se, U, Sb, and Te isotope ratio measurements as new indicators of redox reactions and contaminant migration. Reliable analytical methods exist, but are still evolving. Understanding of isotopic fractionation induced by various (bio)geochemical processes has been explored in dozens of publications, yet is far from complete: Reduction reactions, the major driver of isotopic variation, have been relatively well studied. However, the magnitude of fractionation is variable and the systematics of that variation are still being explored. Isotopic fractionation induced by oxidation reactions is not well understood. Non-redox reactions, which involve smaller changes in bonding of these elements, tend to induce less isotopic fractionation, but can nonetheless cause significant isotopic shifts. Field applications of Cr, Se, U isotope ratios have demonstrated that they are useful as indicators of reduction in natural systems. A few studies suggest they are also useful as indicators of oxidation and contaminant sources. The physical and chemical complexity of groundwater systems hinders accurate quantitative interpretation of Cr, Se, U isotope data using simple models. Numerical models have been developed that capture the behavior of complex, coupled systems and enable the most effective extraction of information from field data sets. Citing Literature Isotopic Constraints on Earth System Processes RelatedInformation

Mining and industrial use over recent decades have released tellurium (Te) into the environment where it potentially contaminates soils, water supplies and food sources. Therefore, it is important to find ways of removing mobile and bioavailable Te from aqueous environments. We report aqueous Te(IV) removal by siderite at varying Te concentrations (2, 6 and 10 mg mL-1 Te(IV)) and pH values (7, 7.9 and 9), together with associated isotope fractionation (epsilon 130Te/125Te). Our results show effective immobilization of Te(IV) that follows pseudo -first order rate kinetics. Formation of magnetite indicates reduction of Te(IV) on siderite surfaces and the for-mation of Te(0). The overall isotope fractionation (epsilon) is small (-0.23 +/- 0.06 parts per thousand) providing evidence that it is primarily controlled by adsorption of Te(IV) occurring simultaneously with reduction of Te(IV). Therefore, Te (IV) removal by siderite under mildly reducing ferruginous conditions may be identifiable by the characteris-tically small isotopic fractionation during both natural attenuation and active remediation. To our knowledge, this is the first study reporting reaction mechanisms of Te(IV) immobilization by an environmentally relevant Fe (II) mineral.

Establishing the nickel (Ni) isotopic composition of the upper continental crust (UCC) is crucial for using the Ni isotope system to trace biogeochemical processes and understand crust-mantle interactions. This study reports the Ni isotopic composition of eighty-four well-characterized upper crustal samples, including granites, granodiorites, tonalite-trondhjemite-gran odiorite (TTG), loess, river sediments and glacial diamictites, to constrain the Ni isotopic composition of the UCC. Significant variations in delta Ni-60 are revealed for I-type (0.02-0.26%), A-type (-0.05-0.08%) and S-type (0.08-0.36%) granites for the first time. These Ni isotopic variations are attributed to magmatic differentiation for I-and A-type granites and source heterogeneity for S-type granites. The delta Ni-60 values of fine-grained clastic sediments (including loess, river sediments and glacial diamictites) range from-0.01% to 0.23%. Such delta Ni-60 variations cannot be explained by Ni isotopic fractionation during chemical weathering because there are no clear correlations between delta Ni-60 and Ni/Al2O3, or the chemical index of alteration (CIA). Instead, the delta Ni-60 variations in fine-grained clastic sediments are likely inherited from source rocks. The delta Ni-60 values of our samples for 3.2-3.5 Ga TTGs (0.00-0.13%), 2.4-2.5 Ga TTGs (0.04-0.13%) and < 0.4 Ga granites (excluding S-type granites) are statistically indistinguishable (P < 0.05, student's t-test), implying limited variation of delta Ni-60 in the felsic igneous UCC since 3.5 Ga. Similarly, the delta Ni-60 values of glacial diamictites suggest insignificant temporal variation in the weathered UCC since 2.4 Ga. The data gathered in this study combined with literature data yields an arithmetic mean delta Ni-60 value of 0.12 +/- 0.15% (2SD) for the UCC (ranging from-0.07% to 0.36%). And the weighted average delta Ni-60 is estimated to be 0.07 +/- 0.10% (2SD) or 0.11 +/- 0.09% (2SD) depending on the assumed delta Ni-60 of the metamorphic rocks. Thus, a lithology-weighted average delta 60Ni needs to be further determined by future studies when the delta 60Ni values of metamorphosed sedimentary rocks in the UCC are constrained. (c) 2022 Elsevier Ltd. All rights reserved.

Naturally occurring Cr(VI) contamination of surface and groundwater occurs globally, and is common in the Circum-Pacific region. Cr in mafic and ultramafic rocks occurs as Cr(III), which is sparingly soluble and non-toxic. In the presence of Mn-oxides, Cr(III) oxidizes to Cr(VI), which is soluble, mobile, and highly toxic. Geogenic Cr(VI) tends to have elevated δ 53 Cr values ranging from 0.7 to 5.1 ‰, significantly greater than δ 53 Cr values of the source rocks (Bulk silicate earth ~ -0.14 ‰). The cause of this offset is not understood. Even though Mn oxides

We examined Hg stable isotope fractionation after the partial reduction of Hg(II) to Hg(0) by the siderite and green rust of ferrous iron minerals. The fractionation of Hg isotopes in closed-system experiments followed an equilibrium fractionation model, with Hg(II) enriched in heavier isotopes. The results indicated isotopic fractionation (delta(20)2Hg(II)-delta Hg-202(0)) of 2.43 +/- 0.38 and 2.28 +/- 0.40% for the siderite and green rust experiments, respectively. Experiments were also performed to determine if the rapid attainment of isotopic equilibrium was attributed to isotopic exchange between Hg(II) and Hg(0). In the absence of other redox-active species, we observed that the d202Hg values of both Hg(0) and Hg(II) shifted substantially toward equilibrium within minutes and evolved to constant delta Hg-202 differences between the Hg(II) and Hg(0) pools. Mixing experiments conducted in water and 10 mM NaCl yielded delta Hg-202(II)-delta Hg-202(0) differences of 2.63 +/- 0.37 and 2.77 +/- 0.70%, respectively. The Hg-199/Hg-198 and Hg-201/Hg-198 results were consistent with previously published experimental and computational studies indicating the involvement of nuclear volume effects in the observed fractionations between the mercury species. Together, these findings suggest that rapid Hg isotopic exchange can facilitate Hg stable isotope fractionation in Hg(II)-Hg(0) redox systems and overprint isotopic fractionation caused by kinetic processes.

The Copernicus operational Sentinel-3A since February 2016 and Sentinel-3B since April 2018 build on the CryoSat-2 legacy in terms of their synthetic aperture radar (SAR) mode altimetry providing high-resolution radar freeboard elevation data over the polar regions up to 81N. This technology combined with the Ocean and Land Colour Instrument (OLCI) imaging spectrometer offers the first space-time collocated optical imagery and radar altimetry dataset. We use these joint datasets for validation of several existing surface classification algorithms based on Sentinel-3 altimeter echo shapes. We also explore the potential for novel AI techniques such as convolutional neural networks (CNN) for winter and summer sea ice surface classification (i.e. melt pond fraction, lead fraction, sea ice roughness). For lead surface classification we analyse the winters of 2018/19 and 2019/20 and for summer sea ice feature classification we focus on the Sentinel-3A &3B tandem phase of the summer 2018. We compare our CNN models with other existing surface classification algorithms.

Stable isotope ratios are widely used to solve environmental, geological, medical, and forensic problems. The double spike technique is considered to be one of the most robust and efficient methods to correct for instrumental mass bias and isotopic fractionation that may occur during sample preparation. However, various hidden errors can arise from data processing and have been largely overlooked in previous studies. Several of these hidden errors were investigated in this work using measurement and synthetic data. Double spike inversion of chromium isotope raw data from 1116 natural samples demonstrated that averaging raw isotope ratios before double spike inversion can add significant errors to inverted isotope values, and such errors can be 1.5 times larger than the true analytical precision. Synthetic data were used to investigate the errors on inverted Cr isotope data caused by spike:analyte ratio and Fe-Ti-V interferences, and the following threshold values are recommended to minimize such errors: 54Crspike/52Crsample ratio greater than 0.5, 56Fe/52Cr less than 0.2, 49Ti/52Cr less than 0.04, and 51V/52Cr less than 1. Sample preparation can potentially lead to large errors in inverted Cr isotope data if preparation-induced isotope fractionation deviates from the exponential law used in the double spike inversion, but such errors can be minimized by achieving >70% Cr yield. Our findings provide important insights for the double spike inversion procedure and assessing the reliability of inverted isotope data for not only the chromium isotope system but also other elements commonly analyzed using the double spike technique.

Adsorption is an important geochemical process constraining the global cycling of selenium (Se) in the environment. However, Se isotope fractionation during adsorption onto clay minerals has been rarely reported. In this study, Se isotope fractionation during adsorption onto montmorillonite and kaolinite was investigated by a combination of adsorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. Results showed that a small adsorption and negligible isotope fractionation were observed during Se(VI) adsorption on montmorillonite and kaolinite at pH 4.5. By contrast, Se isotope fractionations (Delta 82/76Sedissolvedadsorbed) during the adsorption of Se(IV) on montmorillonite and kaolinite were < 0.23%0, suggesting that lighter Se(IV) isotopes are preferentially adsorbed onto clay minerals. In addition, different effect of ionic strength on Se (IV) isotope fractionation was found between kaolinite and montmorillonite, which is likely ascribed to distinct surface charge and structure between the two clay minerals. These little or no Se isotope fractionations could be related to the fact that Se oxyanions are mainly adsorbed on montmorillonite and kaolinite via the outer-sphere complexation, as revealed by Se K-edge EXAFS analysis. The findings from this study would help further identify and constrain the key geochemical processes causing Se isotope variations, providing a foundation to develop Se isotope as a proxy for biogeochemical cycling of Se in the natural environment.

Oxidation of selenium (Se) largely drives the mobilization of soluble selenium oxyanions from rock and soil. Isotopic fractionation of selenium (Se-82/Se-76) has been used to track attenuating processes like reduction of Se(IV) and Se(VI). Isotopic shifts associated with oxidative dissolution of selenide-bearing minerals are poorly understood, despite their potential importance in determining Se-82/Se-76 of selenium sources and causing isotopic variation in contaminated systems. We examined Se-82/Se-76 of dissolved Se(IV) and Se(VI) during the oxidation of ferroselite (FeSe2) and berzelianite (Cu2Se) with low and high initial hydrogen peroxide (H2O2) concentrations in solution (0.007-1 mM) and under atmospheric oxygen levels. The Se-82/Se-76 of Se(VI) produced by oxidation in the experiments ranged from 1.5 to 14 parts per thousand greater than the initial minerals. These isotopic shifts arise from oxidation of Se(IV), reduction of Se(IV) or Se(VI) by mineral phases, and/or isotopic exchange between Se(IV) and Se(VI). At low concentrations of H2O2, isotopic fractionation associated with the reduction of Se(IV) to Se(0) on ferroselite is apparent. At high concentrations of H2O2, this process for ferroselite and berzelianite oxidation is either absent or diluted by a larger flux of Se(IV) created by rapid mineral oxidation. As Se(VI) is more mobile than Se(IV), our results suggest that oxidative weathering of selenium-bearing minerals, previously thought to induce minimal isotopic fractionation, tends to produce an isotopically heavy selenium weathering flux.