Chromium-isotope signatures in scleractinian corals from the Rocas Atoll, Tropical South Atlantic

Chromium isotope fractionation during the removal of hexavalent chromium by oak-based biochar

Chromium, especially in its hexavalent form (Cr(VI)), poses significant health risks due to its carcinogenic properties. Emerging research suggests that biochar, a carbon-rich material derived from biomass pyrolysis, holds promise as an effective and sustainable solution for Cr(VI) remediation. Biochar's unique physicochemical properties, such as its high surface area, porous structure, and functional groups, contribute to its exceptional adsorption capacity for metals. In a series of batch experiments with varying durations, an oak-based biochar was exposed to a 48 mg L⁻1 Cr(VI) solution. The results demonstrate that the biochar effectively removed 99% of the Cr(VI) after 100 h, with a removal capacity greater than 5 mg g⁻1. Fourier transform infrared (FTIR) spectroscopy suggests that the removal of Cr(VI) involved aliphatic and aromatic C-H bonds. X-ray photoelectron spectroscopy (XPS) also indicated the role of aliphatic groups in the removal process and suggested the involvement of carbonyl groups. XPS analysis also detected both Cr(III) and Cr(VI) on the surface of the biochar, indicating the occurrence of reduction and sorption processes. The presence of Cr(III) and Cr(VI) was further confirmed at the Canadian Light Source (CLS) synchrotron facility using X-ray absorption near edge structure (XANES) spectroscopy. Cr isotope analysis showed an increase in δ⁵³Cr as the concentration of Cr in solution decreased, indicating Cr(VI) reduction. The isotope data followed a Rayleigh curve with a kinetic fractionation ε53Cr of -1.33‰, highlighting that Cr stable isotopes can effectively be used as an indicator of biochar-Cr reactions.

Structural integrity and skeletal trace elements in intertidal coralline algae across the Northeast Atlantic reveal a distinct separation of the leading and the trailing edge populations

Intertidal macroalgae, such as coralline algae, represent an essential structural element and substrate in rocky coastal zones. They have a high degree of flexibility allowing their survival in environments with severe mechanical stress during tidal cycles. This study characterised the genicula and intergenicula of the calcifying red algae Corallina officinalis across its geographic distribution in the Northeast Atlantic. Poleward populations have constructed more sturdy cell walls compared to equatorward populations, potentially due to greater local adaptations to higher frequency and intensity of environmental factors like storms and wave action. Southern populations showed a lack of local adaptation culminating in survival rather than thriving within their current environment, hence, they are located at the margin of this species' favourable conditions. Results clarify significant differences between latitudes and indicate a north-to-south gradient in this species' skeletal elemental composition. Northern populations were dominated by cadmium, whereas chromium was the major trace element found in southern populations. In the future, these characteristics could lead to a permanent decline and a decrease in the ecosystem functions of C. officinalis in the southern locations in the Northeast Atlantic, which may be accelerated by predicted future climatic changes.

Advances in the application of metallic isotopes to the identification of contaminant sources in environmental geochemistry

The development of the economy and society makes heavy metals (HMs) pollution more and more serious. And, pollution source identification is the primary work of environmental pollution control and land planning. Notably, stable isotope technology has a high ability to distinguish pollution sources, and can better reflect the migration behavior and contribution of HMs from diverse sources, which has become a hot research tool for pollution source identification of HMs. Currently, the rapid development of isotope analysis technology provides a relatively reliable reference for pollution tracking. Based on this background, the fractionation mechanism of stable isotopes and the influence of environmental processes on isotope fractionation are reviewed. Furthermore, the processes and requirements for the measurement of metal stable isotope ratios are summarized, and the calibration methods and detection accuracy of sample measurement are evaluated. Besides, the current commonly used binary model and multi-mixed models in the identification of contaminant sources are also concluded. Moreover, the isotopic changes of different metallic elements under natural and anthropogenic conditions are discussed in detail, and the application prospects of multi-isotope coupling in the traceability of environmental geochemistry are evaluated. This work has some guidance for the application of stable isotopes in the source identification of environmental pollution.

Mechanisms of chromium isotope fractionation and the applications in the environment

Chromium (Cr) is a toxic heavy metal that gives rise to environmental pollution and human risk. Chromium stable isotopes have a wide range of applications in both environmental field and earth science field. In this contribution, we focus on the application of the Cr isotope in both tracing pollution sources and monitoring Cr(Ⅵ) pollution. Meanwhile, we also provide a description of the main influencing factors controlling Cr isotope fractionation, chromium isotope analytical methods, and terrestrial Cr release. Chromium isotope tracing of contaminant sources is a new application method, it has a tremendous advantage in searching for the source of Cr pollution, which has not been covered in previous reviews. At the end of the article, the current status of Cr isotope applications in the paleo-environment is explained. Although there are still some uncertainties in practical applications, chromium isotope system shows great promise in the environmental aspects.

Cadmium isotopes in Bahamas platform carbonates: A base for reconstruction of past surface water bioproductivity and their link with chromium isotopes

The distribution of cadmium (Cd) within the oceans strongly suggests that it is used as a nutrient by marine phytoplankton. Biologically induced removal of Cd from modern surface waters is accompanied by an isotopic fractionation leaving surface-waters enriched in isotopically heavy Cd. This first study focusses on tying the Cd isotopic record preserved in modern shallow platform carbonates of the Great Bahama Bank (GBB) to conditions in the upper water column, and provides a base for future studies aiming at reconstructing past bioproductivity levels in ancient ocean/basin surface waters. In addition, we compare δ¹¹⁴Cd values with previously published chromium (Cr) isotope values and link signals of bioproductivity with redox conditions in the surface waters. The GBB core samples yield [Cd] (21-188 μg/kg), which increases with depth alongside changes in carbonate mineralogy related to sediment supply and diagenesis. The δ¹¹⁴Cd values of these carbonates are mainly positively fractionated with an average of 0.11‰ ± 0.17 (2σ; n = 17) relative to the NIST 3108 reference standard. Unlike previously observed for Cr isotopes, there is no control of δ¹¹⁴Cd values by relative abundances of the carbonate polymorphs aragonite and calcite in the studied profile. Likewise, δ¹¹⁴Cd values are not correlated to major and trace element (e.g. Ca, Mg, Mn and Sr) contents. We postulate that the burial and diagenetic processes of carbonate cannot modify the Cd isotope signals. Using the experimental fractionation factor for Cd into calcite (-0.45‰), calculated seawater δ¹¹⁴Cd of +0.56 ± 0.17‰ is in agreement with values for modern North Atlantic Surface Seawater. This study's results suggest that δ¹¹⁴Cd values in carbonates are a reliable tool for reconstruction of bioproductivity levels in past surface seawaters, and open new possibilities in combination with Cr isotopes to link these with past ocean redox.

Chromium Isotope Systematics in Modern and Ancient Microbialites

Changes in stable chromium isotopes (denoted as δ53Cr) in ancient carbonate sediments are increasingly used to reconstruct the oxygenation history in Earth’s atmosphere and oceans through time. As a significant proportion of marine carbonate older than the Cambrian is microbially-mediated, the utility of δ53Cr values in ancient carbonates hinges on whether these sediments accurately capture the isotope composition of their environment. We report Cr concentrations (Cr) and δ53Cr values of modern marginal marine and non-marine microbial carbonates. These data are supported by stable C and O isotope compositions, as well as rare earth elements and yttrium (REY) concentrations. In addition, we present data on ancient analogs from Precambrian strata. Microbial carbonates from Marion Lake (Australia, δ53Cr ≈ 0.99‰) and Mono Lake (USA, ≈0.78‰) display significantly higher δ53Cr values compared with ancient microbialites from the Andrée Land Group in Greenland (720 Ma, ≈0.36‰) and the Bitter Springs Formation in Australia (800 Ma, ≈−0.12‰). The δ53Cr values are homogenous within microbialite specimens and within individual study sites. This indicates that biological parameters, such as vital effects, causing highly variable δ53Cr values in skeletal carbonates, do not induce variability in δ53Cr values in microbialites. Together with stable C and O isotope compositions and REY patterns, δ53Cr values in microbialites seem to be driven by environmental parameters such as background lithology and salinity. In support, our Cr and δ53Cr results of ancient microbial carbonates agree well with data of abiotically precipitated carbonates of the Proterozoic. If detrital contamination is carefully assessed, microbialites have the potential to record the δ53Cr values of the waters from which they precipitated. However, it remains unclear if these δ53Cr values record (paleo-) redox conditions or rather result from other physico-chemical parameters.

Absence of hexavalent chromium in marine carbonates: implications for chromium isotopes as paleoenvironment proxy

The oxygenation of Earth's atmosphere is widely regarded to have played an important role in early-life evolution. Chromium (Cr) isotopes recorded in sedimentary rocks have been used to constrain the atmospheric oxygen level (AOL) over geological times based on the fact that a positive Cr isotopic signature is linked to the presence of Cr(VI) as a result of oxidative continental weathering. However, there is no direct evidence of the presence of Cr(VI) in sedimentary rocks yet. Carbonates are most widely distributed over geological times and were thought to have incorporated Cr(VI) directly from seawater. Here, we present results of Cr valence states in carbonates which show Cr(III) is the dominant species in all samples spanning a wide range of geological times. These findings indicate that Cr(VI) in seawater was reduced either before or after carbonate precipitation, which might have caused Cr isotopic fractionation between seawater and carbonates, or marine carbonates preferentially uptake Cr(III) from seawater. As Cr(III) can come from non-redox Cr cycling, which also can cause isotopic fractionation, we suggest that positively fractionated Cr isotopic values do not necessarily correspond to the rise in AOL.

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean

While chromium stable isotopes (δ53Cr) have received significant attention for their utility as a tracer of oxygen availability in the distant geological past, a mechanistic understanding of modern oceanic controls on Cr and δ53Cr is still lacking. Here we present total dissolved δ53Cr, concentrations of Cr (III) and total dissolved Cr, and net community productivity (NCP) from the North Pacific. Chromium concentrations show surface depletions in waters with elevated NCP, but not in lower productivity waters. Observed Cr deficits correspond well with calculated Cr export derived from NCP and Cr:C ratios of natural phytoplankton and marine particulates. Chromium (III) concentrations are stable over the diel cycle yet correlate with NCP, with maxima found in highly productive surface waters but not in lower productivity waters, indicating biological control on Cr (III). The relationship between Cr (III) and δ53Cr suggests that δ53Cr distributions may be controlled by the removal of isotopically light Cr (III) at an isotopic enrichment factor (∆53Cr) of -1.08‰ ± 0.25 relative to total dissolved δ53Cr, in agreement with the global δ53Cr-Cr fractionation factor (-0.82‰ ± 0.05). No perturbation to δ53Cr, Cr, or Cr (III) is observed in oxygen-depleted waters (~10 μmol/kg), suggesting no strong control by O2 availability, in agreement with other recent studies. Therefore, we propose that biological productivity is the primary control on Cr and δ53Cr in the modern ocean. Consequently, δ53Cr records in marine sediments may not faithfully record oxygen availability in the Late Quaternary. Instead, our data demonstrate that δ53Cr records may be a useful tracer for biological productivity.

Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol, Northwest Territories, Canada

The ca. 1.9 Ga Beaverlodge Lake paleosol was studied using redox-sensitive Cr isotopes in order to determine the isotopic response to paleoweathering of a rhyodacite parent rock 500 million years after the Great Oxidation Event. Redox reactions occurring in modern weathering environments produce Cr(VI) that is enriched in heavy Cr isotopes compared to the igneous inventory. Cr(VI) species are soluble and easily leached from soils into streams and rivers, thus, leaving particle-reactive and isotopically light Cr(III) species to build up in soils. The Beaverlodge Lake paleosol and two other published weathering profiles of similar age, the Flin Flon and Schreiber Beach paleosols, are not as isotopically light as modern soils, indicating that rivers were not as isotopically heavy at that time. Considering that the global average δ⁵³ Cr value for the oxidative weathering flux of Cr to the oceans today is just 0.27 ± 0.30‰ (1σ) based on a steady-state analysis of the modern ocean Cr cycle, the oxidative weathering flux of Cr to the oceans at ca. 1.9 Ga would have likely been shifted to lower δ⁵³ Cr values, and possibly lower than the igneous inventory (-0.12 ± 0.10‰, 2σ). Mn oxides are the main oxidant of Cr(III) in modern soils, but there is no evidence that they formed in the studied paleosols. Cr(VI) may have formed by direct oxidation of Cr(III) using molecular oxygen or H₂ O₂ , but neither pathway is as efficient as Mn oxides for producing Cr(VI). The picture that emerges from this and other studies of Cr isotope variation in ca. 1.9 Ga paleosols is of atmospheric oxygen concentrations that are high enough to oxidize iron, but too low to oxidize Mn, resulting in low Cr(VI) inventories in Earth surface environments.

Heterogeneity and incorporation of chromium isotopes in recent marine molluscs (Mytilus)

The mollusc genus Mytilus is abundant in various modern marine environments and is an important substrate for palaeo‐proxy work. The redox‐sensitive chromium (Cr) isotope system is emerging as a proxy for changes in the oxidation state of the Earth's atmosphere and oceans. However, potential isotopic offsets between ambient sea water and modern biogenic carbonates have yet to be constrained. We measured Cr concentrations ([Cr]) and isotope variations (δ⁵³Cr) in recent mollusc shells (Mytilus) from open and restricted marine environments and compared these to ambient sea water δ⁵³Cr values. We found a large range in mollusc [Cr] (12–309 ppb) and δ⁵³Cr values (−0.30 to +1.25‰) and in the offset between δ⁵³Cr values of mollusc shells and ambient sea water (Δ53CrseawaterbulkMytilus, −0.17 to −0.91‰). Step digestions of cultivated Mytilus edulis specimens indicate that Cr is mainly concentrated in organic components of the shell (periostracum: 407 ppb, n = 2), whereas the mollusc carbonate minerals contain ≤3 ppb Cr. Analyses of individual Cr‐hosting phases (i.e., carbonate minerals and organic matrix) did not reveal significant differences in δ⁵³Cr values, and thus, we suggest that Cr isotope fractionation may likely take place prior to rather than during biomineralisation of Mytilus shells. Heterogeneity of δ⁵³Cr values in mollusc shells depends on sea water chemistry (e.g., salinity, food availability, faeces). The main control for δ⁵³Cr values incorporated into shells, however, is likely vital effects (in particular shell valve closure time) since Cr can be partially or quantitatively reduced in sea water trapped between closed shell valves. The δ⁵³Cr values recorded in Mytilus shells may thus be de‐coupled from the redox conditions of ambient sea water, introducing additional heterogeneity that needs to be better constrained before using δ⁵³Cr values in mollusc shells for palaeo‐reconstructions.

Mid-Proterozoic redox evolution and the possibility of transient oxygenation events

It is often assumed that rising environmental oxygen concentrations played a significant role in the timing of the first appearance of animals and the trajectory of their early proliferation and diversification. The inherent large size and complexity of animals come with large energy requirements — levels of energy that can best, if not only, be acquired through aerobic respiration. There is also abundant geochemical evidence for an increase in ocean–atmosphere O2 concentrations in temporal proximity with the emergence of the group. To adequately test this hypothesis, however, a thorough understanding of the history of environmental oxygenation in the time between the first appearance of eukaryotes and the eventual appearance of animals is necessary. In this review, we summarize the evidence for the prevailing long-term conditions of the Proterozoic Eon prior to the emergence of Metazoa and go on to highlight multiple independent geochemical proxy records that suggest at least two transient oxygenation events — at ca. 1.4 and ca. 1.1 billion years ago (Ga) — during this time. These emerging datasets open the door to an important possibility: while prevailing conditions during much of this time would likely have presented challenges for early animals, there were intervals when oxygenated conditions were more widespread and could have favored yet undetermined advances in eukaryotic innovation, including critical early steps toward animal evolution.