Rayleigh-based, multi-element coral thermometry: A biomineralization approach to developing climate proxies

This study presents a new approach to coral thermometry that deconvolves the influence of water temperature on skeleton composition from that of “vital effects”, and has the potential to provide estimates of growth temperatures that are accurate to within a few tenths of a degree Celsius from both tropical and cold-water corals. Our results provide support for a physico-chemical model of coral biomineralization, and imply that Mg²⁺ substitutes directly for Ca²⁺ in biogenic aragonite. Recent studies have identified Rayleigh fractionation as an important influence on the elemental composition of coral skeletons. Daily, seasonal and interannual variations in the amount of aragonite precipitated by corals from each “batch” of calcifying fluid can explain why the temperature dependencies of elemental ratios in coral skeleton differ from those of abiogenic aragonites, and are highly variable among individual corals. On the basis of this new insight into the origin of “vital effects” in coral skeleton, we developed a Rayleigh-based, multi-element approach to coral thermometry. Temperature is resolved from the Rayleigh fractionation signal by combining information from multiple element ratios (e.g., Mg/Ca, Sr/Ca, Ba/Ca) to produce a mathematically over-constrained system of Rayleigh equations. Unlike conventional coral thermometers, this approach does not rely on an initial calibration of coral skeletal composition to an instrumental temperature record. Rather, considering coral skeletogenesis as a biologically mediated, physico-chemical process provides a means to extract temperature information from the skeleton composition using the Rayleigh equation and a set of experimentally determined partition coefficients. Because this approach is based on a quantitative understanding of the mechanism that produces the “vital effect” it should be possible to apply it both across scleractinian species and to corals growing in vastly different environments. Where instrumental temperature records are available, a Rayleigh-based framework allows the effects of stress on coral calcification to be identified on the basis of anomalies in the skeletal composition.     

Palaeoenvironmental records from fossil corals: The effects of submarine diagenesis on temperature and climate estimates

The geochemistry of coral skeletons may reflect seawater conditions at the time of deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However the precipitation of cements in the primary coral skeleton during diagenesis may significantly affect bulk skeletal geochemistry. We used secondary ion mass spectrometry (SIMS) to measure Sr, Mg, B, U and Ba concentrations in primary coral aragonite and aragonite and calcite cements in fossil Porites corals from submerged reefs around the Hawaiian Islands. Cement and primary coral geochemistry were significantly different in all corals. We estimate the effects of cement inclusion on climate estimates from drilled coral samples, which combine cements and primary coral aragonite. Secondary 1% calcite or ∼2% aragonite cement contamination significantly affects Sr/Ca SST estimates by +1 °C and −0.4 to −0.9 °C, respectively. Cement inclusion also significantly affects Mg/Ca, B/Ca and U/Ca SST estimates in some corals. X-ray diffraction (XRD) will not detect secondary aragonite cements and significant calcite contamination may be below the limit of detection (∼1%) of the technique. Thorough petrographic examination of fossils is therefore essential to confirm that they are pristine before bulk drilled samples are analysed. To confirm that the geochemistry of the original coral structures is not affected by the precipitation of cements in adjacent pore spaces we analysed the primary coral aragonite in cemented and uncemented areas of the skeleton. Sr/Ca, B/Ca and U/Ca of primary coral aragonite is not affected by the presence of cements in adjacent interskeletal pore spaces i.e. the coral structures maintain their original composition and selective SIMS analysis of these structures offers a route to the reconstruction of accurate SSTs from altered coral skeletons. However, Mg/Ca and Ba/Ca of primary coral aragonite are significantly higher in parts of skeletons infilled with high Mg calcite cement. We hypothesise this reflects cement infilling of intraskeletal pore spaces in the primary coral structure.     

Correlated trace element “vital effects” in tropical corals: A new geochemical tool for probing biomineralization

Micron-scale variations in the trace-element (TE) composition of tropical coral skeletons were measured using laser-ablation ICP-MS (LA-ICP-MS) as part of an investigation into the chemical processes underlying paleoenvrionmental proxy reconstructions. Fluctuations in B, Mg, Sr, Ba and U were measured at high spatial resolution in two Porites corals from the Great Barrier Reef (Australia), and the fine-scale fluctuations (< ∼1.0 mm) were compared with seasonal TE cycles in a third coral. Fine-scale TE variations were found to have a large amplitude over distances corresponding to less than 1 month growth. Variations were quasi-periodic and appeared to have characteristic wavelengths on weekly (6-7 d) and monthly (28 d) scales, although periodicity was not continuous and variations could not be matched either within or between individual corallites. Fine-scale variations between Mg, Sr and U were significantly correlated with each other (Sr and U are positively correlated, but negatively correlated with Mg). This 3D correlation “vector” has the same slope as the seasonal-scale Mg, Sr and U correlations, suggesting that the same chemical/biologic biomineralization process mediates trace element variations at both timescales. Importantly, the fine-scale variations are too large to be caused directly by daily to monthly fluctuations in sea-surface temperature. This means that seasonal variations in these elements cannot reflect purely inorganic temperature-dependent coprecipitation. Models of physicochemical calcification were developed to test whether changes in calcification rate could explain the trace-element correlations. The calculations show that increases in calcification rate will result in correlated decreases in all TE/Ca ratios. The models reproduce the Sr partition coefficient, trace-element correlation slopes, and amplitude of fine-scale variations for an average calcifying pH of 8.5, varying by ±0.2 pH units. The models, however, predict U partition coefficients which are too low, and cannot reproduce the negative correlation between Mg and the other trace elements, which may be caused by crystallographic factors.     

δB, Sr, Mg and B in a modern Porites coral: the relationship between calcification site pH and skeletal chemistry

Sr/Ca, B/Ca, Mg/Ca and δ¹¹B were determined at high spatial resolution across ∼1 year of a modern Hawaiian Porites lobata coral by secondary ion mass spectrometry (SIMS). We observe significant variations in B/Ca, Mg/Ca, Sr/Ca and δ¹¹B over short skeletal distances (nominally equivalent to periods of <20 days). This heterogeneity probably reflects variations in the composition of the extracellular calcifying fluid (ECF) from which the skeleton precipitates. Calcification site pH (total scale) was estimated from skeletal δ¹¹B and ranged from 8.3 to 8.8 (± ∼0.1) with a mean of ∼8.6. Sr/Ca and B/Ca heterogeneity is not simply correlated with calcification site pH, as might be expected if Ca-ATPase activity increases the pH and decreases the Sr/Ca and B(OH)₄⁻/CO₃²⁻ ratios of the ECF. We produced a simple model of the ECF composition and the skeleton deposited from it, over a range of calcium transport and carbonate scenarios, which can account for these observed geochemical variations. The relationship between the pH and Sr/Ca of the ECF is dependent on the concentration of DIC at the calcification site. At higher DIC concentrations the ECF has a high capacity to buffer the [H⁺] changes induced by Ca-ATPase pumping. Conversely, at low DIC concentrations, this buffering capacity is reduced and ECF pH changes more rapidly in response to Ca-ATPase pumping. The absence of a simple correlation between ECF pH and skeletal Sr/Ca implies that calcification occurred under a range of DIC concentrations, reflecting variations in the respiration and photosynthesis of the coral and symbiotic zooxanthellate in the overlying coral tissues. Our observations have important implications for the use of coral skeletons as indicators of palaeo-ocean pH.     

Controls on Sr/Ca and Mg/Ca in scleractinian corals: The effects of Ca-ATPase and transcellular Ca channels on skeletal chemistry

The Sr/Ca of aragonitic coral skeletons is a commonly used palaeothermometer. However skeletal Sr/Ca is typically dominated by weekly-monthly oscillations which do not reflect temperature or seawater composition and the origins of which are currently unknown. To test the impact of transcellular Ca²⁺ transport processes on skeletal Sr/Ca, colonies of the branching coral, Pocillopora damicornis, were cultured in the presence of inhibitors of Ca-ATPase (ruthenium red) and Ca channels (verapamil hydrochloride). The photosynthesis, respiration and calcification rates of the colonies were monitored throughout the experiment. The skeleton deposited in the presence of the inhibitors was identified (by ⁴²Ca spike) and analysed for Sr/Ca and Mg/Ca by secondary ion mass spectrometry. The Sr/Ca of the aragonite deposited in the presence of either of the inhibitors was not significantly different from that of the solvent (dimethyl sulfoxide) control, although the coral calcification rate was reduced by up to 66% and 73% in the ruthenium red and verapamil treatments, respectively. The typical precision (95% confidence limits) of mean Sr/Ca determinations within any treatment was <±1% and differences in skeletal Sr/Ca between treatments were correspondingly small. Either Ca-ATPase and Ca channels transport Sr²⁺ and Ca²⁺ in virtually the same ratio in which they are present in seawater or transcellular processes contribute little Ca²⁺ to the skeleton and most Ca is derived from seawater transported directly to the calcification site. Variations in the activities of Ca-ATPase and Ca-channels are not responsible for the weekly-monthly Sr/Ca oscillations observed in skeletal chronologies, assuming that the specificities of Ca transcellular transport processes are similar between coral genera.     

Compositional and morphological features of aragonite precipitated experimentally from seawater and biogenically by corals

The morphology and composition of abiogenic (synthetic) aragonites precipitated experimentally from seawater and the aragonite accreted by scleractinian corals were characterized at the micron and nano scale. The synthetic aragonites precipitated from supersaturated seawater solutions as spherulites, typically 20-100 μm in diameter, with aggregates of sub-micron granular materials occupying their centers and elongate (fibrous) needles radiating out to the edge. Using Sr isotope spikes, the formation of the central granular material was shown to be associated with high fluid pH and saturation state whereas needle growth occurred at lower pH and saturation state. The granular aggregates have significantly higher Mg/Ca and Ba/Ca ratios than the surrounding fibers.Two types of crystals are identified in the coral skeleton: aggregates of sub-micron granular material and bundles of elongate (fibrous) crystals that radiate out from the aggregates. The granular materials are found in “centers of calcification” and in fine bands that transect the fiber bundles. They have significantly higher Mg/Ca and Ba/Ca ratios than the surrounding fibers.The observed relationship between seawater saturation state and crystal morphology and composition in the synthetic aragonites was used as a framework to interpret observations of the coral skeleton. We propose that coral skeletal growth can be viewed as a cyclical process driven by changes in the saturation state of the coral’s calcifying fluids. When saturation state is high, granular crystals precipitate at the tips of the existing skeletal elements forming the centers of calcification. As the saturation state decreases, aragonitic fibres grow in bundles that radiate out from the centers of calcification.     

Application of calcite Mg partitioning functions to the reconstruction of paleocean Mg/Ca

Calcite Mg/Ca is usually assumed to vary linearly with solution Mg/Ca, that a constant partition coefficient describes the relationship between these two ratios. Numerous published empirical datasets suggests that this relationship is better described by a power function. We provide a compilation of these literature data for biotic and abiotic calcite in the form of Calcite Mg/Ca = F(Solution Mg/Ca)^H, where F and H are empirically determined fitting parameters describing the slope and deviation from linearity, respectively, of the function. This is equivalent to Freundlich sorption behavior controlling Mg incorporation in calcite. Using a power function, instead of a partition coefficient, lowers Phanerozoic seawater Mg/Ca estimates based on echinoderm skeletal material by, on average, 0.5 mol/mol from previous estimates.These functions can also be used to model the primary skeletal calcite Mg/Ca of numerous calcite phases through geologic time. Such modeling suggests that the Mg/Ca of all calcite precipitated from seawater has varied through the Phanerozoic in response to changing seawater Mg/Ca and that the overall range in Mg/Ca measured among various calcite phases would be greatest when seawater Mg/Ca was also high (e.g., “aragonite seas”) and lowest when seawater Mg/Ca was low (e.g., “calcite seas”). It follows that, during times of “calcite seas” when the seawater Mg/Ca is presumed to have been lower, deposition of calcite with low Mg contents would have resulted in a depressed drive for diagenetic stabilization of shelfal carbonate and, in turn, lead to greater preservation of crystal and skeletal microfabrics and primary chemistries in biotic and abiotic calcites.     

Controls on ostracod valve geochemistry, Part 1: Variations of environmental parameters in ostracod (micro-)habitats

The variations of environmental conditions (T°, pH, δ¹³C_DIC, [DIC], δ¹⁸O, Mg/Ca, and Sr/Ca) of ostracod habitats were examined to determine the controls of environmental parameters on the chemical and isotopic composition of ostracod valves. Results of a one-year monitoring of environmental parameters at five sites, with depths of between 2 and 70 m, in Lake Geneva indicate that in littoral to sub-littoral zones (2, 5, and 13 m), the chemical composition of bottom water varies seasonally in concert with changes in temperature and photosynthetic activity. An increase of temperature and photosynthetic activity leads to an increase in δ¹³C values of DIC and to precipitation of authigenic calcite, which results in a concomitant increase of Mg/Ca and Sr/Ca ratios of water. In deeper sites (33 and 70 m), the composition of bottom water remains constant throughout the year and isotopic values and trace element contents are similar to those of deep water within the lake. The chemical composition of interstitial pore water also does not reflect seasonal variations but is controlled by calcite dissolution, aerobic respiration, anaerobic respiration with reduction of sulphate and/or nitrate, and methanogenesis that may occur in the sediment pores. Relative influence of each of these factors on the pore water geochemistry depends on sediment thickness and texture, oxygen content in bottom as well as pore water. Variations of chemical compositions of the ostracod valves of this study vary according to the specific ecology of the ostracod species analysed, that is its life-cycle and its (micro-)habitat. Littoral species have compositions that are related to the seasonal variations of temperature, δ¹³C values of DIC, and of Mg/Ca and Sr/Ca ratios of water. In contrast, the compositions of profundal species are largely controlled by variations of pore fluids along sediment depth profiles according to the specific depth preference of the species. The control on the geochemistry of sub-littoral species is a combination of controls for the littoral and profundal species as well as the specific ecology of the species.     

Calcium-isotope fractionation in selected modern and ancient marine carbonates

The calcium-isotope composition (δ^44/42Ca) was analyzed in modern, Cretaceous and Carboniferous marine skeletal carbonates as well as in bioclasts, non-skeletal components, and diagenetic cements of Cretaceous and Carboniferous limestones. In order to gain insight in Ca²⁺_aq-CaCO₃-isotope fractionation mechanisms in marine carbonates, splits of samples were analyzed for Sr, Mg, Fe, and Mn concentrations and for their oxygen and carbon isotopic composition. Biological carbonates generally have lower δ^44/42Ca values than inorganic marine cements, and there appears to be no fractionation between seawater and marine inorganic calcite. A kinetic isotope effect related to precipitation rate is considered to control the overall discrimination against ⁴⁴Ca in biological carbonates when compared to inorganic precipitates. This is supported by a well-defined correlation of the δ^44/42Ca values with Sr concentrations in Cretaceous limestones that contain biological carbonates at various stages of marine diagenetic alteration. No significant temperature dependence of Ca-isotope fractionation was found in shells of Cretaceous rudist bivalves that have recorded large seasonal temperature variations as derived from δ¹⁸O values and Mg concentrations. The reconstruction of secular variations in the δ^44/42Ca value of seawater from well preserved skeletal calcite is compromised by a broad range of variation found in both modern and Cretaceous biological carbonates, independent of chemical composition or mineralogy. Despite these variations that may be due to still unidentified biological fractionation mechanisms, the δ^44/42Ca values of Cretaceous skeletal calcite suggest that the δ^44/42Ca value of Cretaceous seawater was 0.3-0.4‰ lower than that of the modern ocean.     

Controls on the spatial and temporal variability of vadose dripwater geochemistry: Edwards aquifer, central Texas

A 4-yr study of spatial and temporal variability in the geochemistry of vadose groundwaters from caves within the Edwards aquifer region of central Texas offers new insights into controls on vadose groundwater evolution, the relationship between vadose and phreatic groundwaters, and the fundamental influence of soil composition on groundwater geochemistry. Variations in Sr isotopes and trace elements (Mg/Ca and Sr/Ca ratios) of dripwaters and soils from different caves, as well as phreatic groundwaters, provide the potential to distinguish between local variability and regional processes controlling fluid geochemistry, and a framework for understanding the links between climatic and hydrologic processes.The Sr isotope compositions of vadose cave dripwaters (mean ⁸⁷Sr/⁸⁶Sr = 0.7087) and phreatic groundwaters (mean ⁸⁷Sr/⁸⁶Sr = 0.7079) generally fall between values for host carbonates (mean ⁸⁷Sr/⁸⁶Sr = 0.7076) and exchangeable Sr in overlying soils (mean ⁸⁷Sr/⁸⁶Sr = 0.7088). Dripwaters have lower Mg/Ca and Sr/Ca ratios, and higher ⁸⁷Sr/⁸⁶Sr values than phreatic groundwaters. Dripwater ⁸⁷Sr/⁸⁶Sr values also inversely correlate with both Mg/Ca and Sr/Ca ratios. Mass-balance modeling combined with these geochemical relationships suggest that variations in fluid compositions are predominantly controlled by groundwater residence times, and water-rock interaction with overlying soils and host aquifer carbonate rocks. Consistent differences in dripwater geochemistry (i.e., ⁸⁷Sr/⁸⁶Sr, Mg/Ca, and Sr/Ca) between individual caves are similar to compositional differences in soils above the caves. While these differences appear to exert significant control on local fluid evolution, geochemical and isotopic variations suggest that the controlling processes are regionally extensive. Temporal variations in ⁸⁷Sr/⁸⁶Sr values and Mg/Ca ratios of dripwaters from some sites over the 4-yr interval correspond with changes in both aquifer and climatic parameters. These results have important implications for the interpretation of trace element and isotopic variations in speleothems as paleoclimate records, as well as the understanding of controls on water chemistry for both present-day and ancient carbonate aquifers.     

Evidence for mass-dependent isotopic fractionation of strontium in a glaciated granitic watershed

The stable isotope composition of strontium (expressed as δ^88/86Sr) may provide important constraints on the global exogenic strontium cycle. Here, we present δ^88/86Sr values and ⁸⁷Sr/⁸⁶Sr ratios for granitoid rocks, a 150 yr soil chronosequence formed from these rocks, surface waters and plants in a small glaciated watershed in the central Swiss Alps. Incipient chemical weathering in this young system, whether of inorganic or biological origin, has no resolvable effect on the ⁸⁷Sr/⁸⁶Sr ratios and δ^88/86Sr values of bulk soils, which remain indistinguishable from bedrock in terms of Sr isotopic composition. Although due in part to the chemical heterogeneity of the forefield, the lack of a resolvable difference between soil and bedrock isotopic composition indicates that these soils have thus far witnessed minimal net loss of Sr; a low degree of chemical weathering is also implied by bulk soil chemistry. The isotopic composition of Sr in streamwater is more radiogenic than median soil, reflecting the preferential weathering of biotite in the catchment; streamwater δ^88/86Sr values, however, are indistinguishable from bulk soil δ^88/86Sr values, implying that no resolvable fractionation of Sr isotopes takes place during release to the weathering flux in the Damma forefield. Analyses of plant tissue reveal that plants (Rhododendron and Vaccinium) preferentially assimilate the lighter isotopes of Sr such that their δ^88/86Sr values are significantly lower than those of the soils in which they grow. Additionally, δ^88/86Sr values of foliar and floral tissues are lower than those of roots, contrary to observations for Ca, for which Sr is often used as an analogue in weathering studies. We suggest that processes that discriminate against Sr in favour of Ca, due to the different nutritional requirement of plants for these two elements, are responsible for the observed contrast.     

Continuous 60-year stable isotopic and earth-alkali element records in a modern laminated tufa (Jaruga,river Krka,Croatia): Implications for climate reconstruction

A continuous 60-year record (1938–1998) of stable isotope compositions of carbon and oxygen, as well as trace metal (Mg, Sr, Ba) concentrations in a laminated calcite crust precipitated in a short artificial tunnel on a non-equilibrium groundwater-fed karstic river is presented. Chemical and isotopic records have been compared to hydrometeorological data, available for the last 48 years. An attempt is made to relate isotopic and geochemical variations in the crust to environmental parameters, such as temperature, precipitation and changes in vegetation cover, as well as to postdepositional recrystallisation of the older crust material. Isotopic composition of the crust is largely influenced by non-equilibrium precipitation, which favours the incorporation of isotopically depleted C and O into the carbonate. Furthermore, because of the complicated hydrological situation, there is no observable correlation between the stable oxygen isotope composition of water and temperature. The result is that the ¹⁸O isotopic thermometers overestimate the measured precipitation temperatures. Temperatures calculated from Mg/Ca ratios of water and the carbonate match the δ¹⁸O palaeotemperatures within ± 2.4 °C in the older part of the crust, precipitated before the onset of industrial pollution of the river. It was demonstrated that the application of Mg palaeothermometry in natural systems, where the Mg/Ca ratio of water is influenced not only by temperature, but also by other environmental parameters such as precipitation, surface runoff, groundwater retention time and anthropogenic influences, is subject to a large uncertainty, up to 10 °C.     

Seawater temperature proxies based on DSr, DMg, and DU from culture experiments using the branching coral Porites cylindrica

In order to investigate the incorporation of Sr, Mg, and U into coral skeletons and its temperature dependency, we performed a culture experiment in which specimens of the branching coral (Porites cylindrica) were grown for 1 month at three seawater temperatures (22, 26, and 30 °C). The results of this study showed that the linear extension rate of P. cylindrica has little effect on the skeletal Sr/Ca, Mg/Ca, and U/Ca ratios. The following temperature equations were derived: Sr/Ca (mmol/mol) = 10.214(±0.229) − 0.0642(±0.00897) × T (°C) (r² = 0.59, p < 0.05); Mg/Ca (mmol/mol) = 1.973(±0.302) + 0.1002(±0.0118) × T (°C) (r² = 0.67, p < 0.05); and U/Ca (μmol/mol) = 1.488(±0.0484) − 0.0212(±0.00189) × T (°C) (r² = 0.78, p < 0.05). We calculated the distribution coefficient (D) of Sr, Mg, and U relative to seawater temperature and compared the results with previous data from massive Porites corals. The seawater temperature proxies based on D calibrations of P. cylindrica established in this study are generally similar to those for massive Porites corals, despite a difference in the slope of D_U calibration. The calibration sensitivity of D_Sr, D_Mg, and D_U to seawater temperature change during the experiment was 0.64%/°C, 1.93%/°C, and 1.97%/°C, respectively. These results suggest that the skeletal Sr/Ca ratio (and possibly the Mg/Ca and/or U/Ca ratio) of the branching coral P. cylindrica can be used as a potential paleothermometer.     

Strontium heterogeneity and speciation in coral aragonite: implications for the strontium paleothermometer

Sea surface temperatures (SSTs) have been inferred previously from the Sr/Ca ratios of coral aragonite. However, microanalytical studies have indicated that Sr in some coral skeletons is more heterogeneously distributed than expected from SST data. Strontium may exist in two skeletal phases, as Sr substituted for Ca in aragonite and as separate SrCO₃ (strontianite) domains. Variations in the size, quantity, or both of these domains may account for small-scale Sr heterogeneity. Here, we use synchrotron X-ray fluorescence to map Sr/Ca variations in a Porites lobata skeleton at a 5 μm scale. Variations are large and unrelated to changes in local seawater temperature or composition. Selected area extended X-ray absorption fine structure (EXAFS) spectroscopy of low- and high-Sr areas indicates that Sr is present as a substitute ion in aragonite i.e., domains of Sr carbonate (strontianite) are absent or in minor abundance. Variations in strontianite abundance are not responsible for the Sr/Ca fluctuations observed in this sample. The Sr microdistribution is systematic and appears to correlate with the crystalline fabric of the coral skeleton, suggesting Sr heterogeneity may reflect nonequilibrium calcification processes. Nonequilibrium incorporation of Sr complicates the interpretation of Sr/Ca ratios in terms of SST, particularly in attempts to extend the temporal resolution of the technique. The micro-EXAFS technique may prove to be valuable, allowing the selection of coral microvolumes for Sr/Ca measurement where strontium is incorporated in a known structural environment.     

Liquid composition-dependence of calcium isotope fractionation during diffusion in molten silicates

Liquid phase diffusion experiments were carried out to determine whether diffusive isotopic fractionation of a major chemical element (Ca) varies with chemical composition in high-temperature molten silicates. The objective was to determine how differences in silicate liquid structure, such as the ratio of bridging to non-bridging oxygen atoms, as well as bulk transport properties such as viscosity, relate to isotope discrimination during diffusion. This information, in turn, may relate to the lifetimes and sizes of multi-atom structures in the liquid. Diffusion couples consisting of juxtaposed natural mafic and felsic liquids were held at T = 1450 °C and P = 1.0 GPa for durations of 12-24 h in a standard piston-cylinder assembly. Experiments were done using different mafic endmember compositions (two tholeiitic basalts and a ugandite) and a single rhyolite composition. Major-element diffusion profiles and Ca isotope profiles were measured on the recovered quenched glasses. The starting materials were isotopically indistinguishable, but ⁴⁴Ca/⁴⁰Ca variations of ca. 5‰ arose due to a mass dependence of the Ca diffusion coefficients. Results indicate that the mass dependence of Ca diffusion coefficients varies with the magnitude and direction of aluminum gradients and the viscosity of the liquid. Some Ca fractionations result mainly from Al gradients.A simplified multicomponent diffusion model was used to model the experimental results. The model allows for diffusion of Ca in response to gradients in the concentrations of both CaO as well as Al₂O₃, and the model results are consistent with the inferred existence of at least two distinct species of Ca. The magnitude of isotopic discrimination during diffusion also appears to be stronger on the rhyolite versus the basalt/ugandite side of diffusion couples. The results can largely be accounted for by an adaptation of the model of Dingwell (1990), whereby in high silica liquids, Ca diffuses largely by site hopping through a quasi-stationary aluminosilicate matrix, producing strong isotopic effects because the Ca diffusion is not strongly correlated with the movement of the framework atoms. In low-silica liquids, Ca diffusion is correlated with the movement of the other components and there is less mass discrimination. Combining our Ca results with Ca, Mg, and Li data from previous studies, we show that this model can explain most of the cation- and composition-dependence of diffusive isotopic fractionations observed thus far. A key parameter controlling isotopic discrimination is the ratio of the elemental (Ca, Mg, Li) diffusivity to the Eyring (or Si) diffusivity. However, all experiments done so far also exhibit isotopic features that are not yet fully explained; some of these may relate to small temperature gradients in the capsules, or to more complex coupling effects that are not captured in simplified diffusion models.     

Anomalous unradiogenic 87Sr//86Sr ratios in ultrahigh‐pressure crustal carbonates – evidence for fluid infiltration during deep subduction?

Marbles from Changpu (Dabie Shan, eastern China), subducted to 4.4 GPa, have ⁸⁷Sr/⁸⁶Sr values < 0.7040. These low ⁸⁷Sr/⁸⁶Sr values, which would imply a sedimentation age > 2 Ga if considered as primary signature, reflect fluid–rock interaction with a fluid from a low‐⁸⁷Sr/⁸⁶Sr source. The introduction of low‐⁸⁷Sr/⁸⁶Sr was paralleled by introduction of Mg and loss of Si, K and Na in such a way that carbonates from the purest marbles have the least evolved Sr isotopic composition. Introduction of Mg is also indicated by the distribution of calcite and dolomite. Calcite forms inclusions in garnet, whereas dolomite is restricted to the matrix. These chemical changes, inferred from the mineralogy, in combination with textural evidence require a mobile metamorphic fluid. P–T–X constraints for fluid generation and for permeability increase related to mineral reactions and phase transitions suggest that the marbles acquired their anomalous Sr‐isotopic composition during subduction below 60 km. The marbles with the least radiogenic Sr isotopic composition demonstrate that crustal rocks may lose their isotopic fingerprint during deep subduction.     

Interaction between shallow and deep aquifers in the Tivoli Plain (Central Italy) enhanced by groundwater extraction: A multi-isotope approach and geochemical modeling

In the Tivoli Plain (Rome, Central Italy) the interaction between shallow and deep groundwater flow systems enhanced by groundwater extraction has been investigated using isotopic and chemical tracers. A conceptual model of the groundwater flowpaths has been developed and verified by geochemical modeling. A combined hydrogeochemical and isotopic investigation using ion relationships such as DIC/Cl⁻, Ca/(Ca + Mg)/SO₄/(SO₄ + HCO₃), and environmental isotopes (δ¹⁸O, δ²H, ⁸⁷Sr/⁸⁶Sr, δ³⁴S and δ¹³C) was carried out in order to determine the sources of recharge of the aquifer, the origin of solutes and the mixing processes in groundwater of Tivoli Plain. Multivariate statistical methods such as principal component analysis and Cluster analyses have confirmed the existence of different geochemical facies and the role of mixing in the chemical composition of the groundwater.     

The geographic distribution of strontium isotopes in Danish surface waters - A base for provenance studies in archaeology, hydrology and agriculture

In this paper Sr isotope signatures are reported for 192 surface water (lakes/ponds and rivers/creeks) samples from within Denmark and an isotope distribution map is presented that may serve as a base for provenance applications, including archaeological migration studies, ground water - surface water - seawater interaction/contamination monitoring, and potentially for agricultural applications, including cases of authenticity proof for particular food products. The Sr isotopic compositions of surface waters range from ⁸⁷Sr/⁸⁶Sr = 0.7078 to 0.7125 (average 0.7096 ± 0.0016; 2σ). This average value lies above the range of ⁸⁷Sr/⁸⁶Sr values between 0.7078 and 0.7082 expected from Late Cretaceous to Early Tertiary (Oligocene) limestones which form the dominant bedrock type in a NW-SE trending belt in Denmark. The elevated ⁸⁷Sr/⁸⁶Sr signatures >∼0.7095 are explained by additions to the surface waters of radiogenic Sr predominantly derived from the near-surface weathering and wash-out of Quarternary glaciogenic tills and soils deposited and formed during and after the last two ice age stages (Saale and Weichsel). The Sr isotopic compositions and concentrations of the surface waters can, therefore, best be modeled by a two-component mixing involving carbonaceous bedrock and glaciogenic cover sediments as the two predominant Sr sources. A feasibility study for using Sr isotopic compositions of surface waters as a proxy for bio-available Sr signatures was conducted in a representative test area on Zealand (Land of Legends, Lejre) where there is no use and application of commercial fertilizers. It is demonstrated that the Sr isotopic signatures of lake waters from within this area are slightly higher (but statistically still indistinguishable) from the average value defined by snail shells and soil leachates considered to characterize the true bio-available Sr. In combination with results from other studies, this is interpreted to reflect the wash-out of Sr with a higher ⁸⁷Sr/⁸⁶Sr signature released by weathering in the topsoils into the saturated water tables, a component which is consequently not readily transferred into the shallow-rooting plants and into small herbivores feeding on them. Since drinking water is a likely important source of Sr uptake of humans and larger animals, the contention is that a surface water isotopic composition can potentially characterize the bio-available component relevant for human and carnivorous large animals. Spline functions and ordinary linear kriging were used for modeling the geographic distribution of bio-available Sr isotopes over territorial Denmark. As expected, based on the two-source mixing scenario, the water ⁸⁷Sr/⁸⁶Sr ratio contour maps (with some notable exceptions) neither mirror the pre-Quarternary bedrock geology nor a (Pleistocene) soil-type distribution map of Denmark.As a conservative suggestion, the use of the average ⁸⁷Sr/⁸⁶Sr ratio of 0.7096 ± 0.0015 (2σ) is recommended as an average for bio-available Sr from within Denmark (Bornholm excluded) for human and larger animals and as a confidence band for distinguishing “local” from “non-local” signatures in archaeological provenance studies. The study also reveals that the average ⁸⁷Sr/⁸⁶Sr ratio of ∼0.7088 defined from soil extracts and small herbivores lies approximately 0.15% lower than that defined by the surface waters. The authors recommend using this lower value as a “local”-“non-local” discriminator for food and plant authenticity control in agricultural applications.     

Light and temperature effects on Sr/Ca and Mg/Ca ratios in the scleractinian coral Acropora sp.

This study was designed to investigate the effect of light and temperature on Sr/Ca and Mg/Ca ratios in the skeleton of the coral Acropora sp. for the purpose of evaluating temperature proxies for paleoceanographic applications. In the first experiment, corals were cultivated under three light levels (100, 200, 400 μmol photons m⁻² s⁻¹) and constant temperature (27 °C). In the second experiment, corals were cultivated at five temperatures (21, 23, 25, 27, 29 °C) and constant light (400 μmol photons m⁻² s⁻¹). Increasing the water temperature from 21 to 29 °C, induced a 5.7-fold increase in the rate of calcification, which induced a 30% increase in the Mg/Ca ratio. In contrast, by increasing the light level by a factor of 4, the rate of calcification was increased only by a factor of 1.7, with a corresponding 9% increase in the Mg/Ca ratio. Thus, the relative change in the calcification rate in the two experiments (5.7 vs. 1.7) scales with the corresponding relative change in Mg/Ca ratio (30% vs. 9%). We conclude that there is a strong biological control on the incorporation of Mg.For Sr/Ca, good correlations were also observed with water temperature and the calcification rate induced by temperature changes. However, in sharp contrast with the Mg/Ca ratio, a temperature-induced 5.7-fold increase in the calcification rate only induced a 4.5% change (decrease) in the Sr/Ca ratio. An important finding for paleoceanographic applications is that the Sr/Ca ratio did not appear to be sensitive to changes in the light level, or to changes in calcification rate induced by changes in the light level. Thus, in this study, water temperature was found to be the dominant parameter controlling the skeletal Sr/Ca ratio.     

Compositional variations at ultra-structure length scales in coral skeleton

Distributions of Mg and Sr in the skeletons of a deep-sea coral (Caryophyllia ambrosia) and a shallow-water, reef-building coral (Pavona clavus) have been obtained with a spatial resolution of 150 nm, using the NanoSIMS ion microprobe at the Muséum National d’Histoire Naturelle in Paris. These trace element analyses focus on the two primary ultra-structural components in the skeleton: centers of calcification (COC) and fibrous aragonite. In fibrous aragonite, the trace element variations are typically on the order of 10% or more, on length scales on the order of 1-10 μm. Sr/Ca and Mg/Ca variations are not correlated. However, Mg/Ca variations in Pavona are strongly correlated with the layered organization of the skeleton.These data allow for a direct comparison of trace element variations in zooxanthellate and non-zooxanthellate corals. In both corals, all trace elements show variations far beyond what can be attributed to variations in the marine environment. Furthermore, the observed trace element variations in the fibrous (bulk) part of the skeletons are not related to the activity of zooxanthellae, but result from other biological activity in the coral organism. To a large degree, this biological forcing is independent of the ambient marine environment, which is essentially constant on the growth timescales considered here.Finally, we discuss the possible detection of a new high-Mg calcium carbonate phase, which appears to be present in both deep-sea and reef-building corals and is neither aragonite nor calcite.