Non-biological fractionation of stable Ca isotopes in soils of the Atacama Desert, Chile

We measured Ca stable isotope ratios (δ^44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m⁻²) is present as sulfates (2.5 kmol m⁻²), and to a lesser extent carbonates (0.4 kmol m⁻²). In aqueous extracts of variably hydrated calcium sulfate minerals, δ^44/40Ca_E values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ^44/40Ca in the top six horizons resembles that of sulfate-δ^44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ^44/40Ca values observed on Earth. Linear correlation among δ^44/40Ca, δ³⁴S and δ¹⁸O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ³⁴S values are positively correlated with sulfate-δ¹⁸O values (R² = 0.78) and negatively correlated with sulfate δ^44/40Ca_E values (R² = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ^44/40Ca fractionation factor of −0.4‰ in CaSO₄. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO₄ is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.     

Calcium isotope ratios in animal and human bone

Calcium isotopes in tissues are thought to be influenced by an individual’s diet, reflecting parameters such as trophic level and dairy consumption, but this has not been carefully assessed. We report the calcium isotope ratios (δ^44/42Ca) of modern and archaeological animal and human bone (n = 216). Modern sheep raised at the same location show 0.14 ± 0.08‰ higher δ^44/42Ca in females than in males, which we attribute to lactation by the ewes. In the archaeological bone samples the calcium isotope ratios of the herbivorous fauna vary by location. At a single site, the archaeological fauna do not show a trophic level effect. Humans have lower δ^44/42Ca than the mean site fauna by 0.22 ± 0.22‰, and the humans have a greater δ^44/42Ca range than the animals. No effect of sex or age on the calcium isotope ratios was found, and intra-individual skeletal δ^44/42Ca variability is negligible. We rule out dairy consumption as the main cause of the lower human δ^44/42Ca, based on results from sites pre-dating animal domestication and dairy availability, and suggest instead that individual physiology and calcium intake may be important in determining bone calcium isotope ratios.     

Characterization of calcium isotopes in natural and synthetic barite

The mineral barite (BaSO₄) accommodates calcium in its crystal lattice, providing an archive of Ca-isotopes in the highly stable sulfate mineral. Holocene marine (pelagic) barite samples from the major ocean basins are isotopically indistinguishable from each other (δ^44/40Ca = −2.01 ± 0.15‰) but are different from hydrothermal and cold seep barite samples (δ^44/40Ca = −4.13 to −2.72‰). Laboratory precipitated (synthetic) barite samples are more depleted in the heavy Ca-isotopes than pelagic marine barite and span a range of Ca-isotope compositions, Δ^44/40Ca = −3.42 to −2.40‰. Temperature, saturation state, , and aCa²⁺/aBa²⁺ each influence the fractionation of Ca-isotopes in synthetic barite; however, the fractionation in marine barite samples is not strongly related to any measured environmental parameter. First-principles lattice dynamical modeling predicts that at equilibrium Ca-substituted barite will have much lower ⁴⁴Ca/⁴⁰Ca than calcite, by −9‰ at 0 °C and −8‰ at 25 °C. Based on this model, none of the measured barite samples appear to be in isotopic equilibrium with their parent solutions, although as predicted they do record lower δ^44/40Ca values than seawater and calcite. Kinetic fractionation processes therefore most likely control the extent of isotopic fractionation exhibited in barite. Potential fractionation mechanisms include factors influencing Ca²⁺ substitution for Ba²⁺ in barite (e.g. ionic strength and trace element concentration of the solution, competing complexation reactions, precipitation or growth rate, temperature, pressure, and saturation state) as well as nucleation and crystal growth rates. These factors should be considered when investigating controls on isotopic fractionation of Ca²⁺ and other elements in inorganic and biogenic minerals.     

The implications for paleodietary and paleoclimatic reconstructions of intrapopulation variability in the oxygen and carbon isotopes of teeth from modern feral horses

We analyzed the isotopic patterns found in the tooth enamel of modern feral horses from Shackleford Banks, North Carolina (USA), which has a temperate climate and supports primarily C₄ grasslands. Enamel δ¹³C values averaged −4.1‰ with a standard deviation (1σ) of 1.7‰, which corresponds to an average diet of 66 ± 12% C₄ plants. Our results differ from dietary reconstructions from 1978 to 1981, which found that horses consumed 91% C₄ plants. This suggests that horses have increased their consumption of C₃ forbs, likely as a result of the removal of cattle, sheep, and goats from the island. Shackleford surface waters had δ¹⁸O values that averaged −3.3 ± 0.5‰ and −1.3 ± 1.8‰ on the western and eastern ends of the island, respectively. Tooth enamel samples averaged 27.3 ± 1.5‰ and displayed the same range of δ¹⁸O values as surface waters. The variability of both δ¹⁸O and the δ¹³C values among individuals within this population demonstrates that horses from relatively homogenous temperate environments can display a wide range of isotopic values. Given the observed range of isotopic values for modern horses, we suggest that researchers use the mean values of multiple (≥9) equids when attempting to reconstruct average paleodiets and/or paleoenvironmental conditions.     

Calcium isotopes in a proglacial weathering environment: Damma glacier, Switzerland

The biogeochemical cycling and isotopic fractionation of calcium during the initial stages of weathering were investigated in an alpine soil chronosequence (Damma glacier, Switzerland). This site has a homogeneous silicate lithology and minimal biological impacts due to sparse vegetation cover. Calcium isotopic compositions, obtained by TIMS using a ⁴³Ca-⁴⁶Ca double spike, were measured in the main Ca pools. During this very early stage of weathering, the young soils which have formed (δ^44/42Ca=+0.44‰) were indistinguishable to the rocks from which they were derived (δ^44/42Ca=+0.44‰) and stream water (δ^44/42Ca=+0.48‰) was also within error of the average rock. This lack of variation indicates that the dissolution of the bulk silicate rock does not strongly fractionate Ca isotopes. The only Ca pool which was strongly fractionated from bulk rock was vegetation, which exhibited an enrichment of light Ca isotopes. Significant Ca isotope fractionation between bulk rock and the dissolved flux of Ca is likely to only occur where the Ca biogeochemical cycle is dominated by secondary processes such as biological cycling, adsorption and secondary mineral precipitation.     

Model for kinetic effects on calcium isotope fractionation (δCa) in inorganic aragonite and cultured planktonic foraminifera

The calcium isotope ratios (δ⁴⁴Ca = [(⁴⁴Ca/⁴⁰Ca)_sample/(⁴⁴Ca/⁴⁰Ca)_standard −1] · 1000) of Orbulina universa and of inorganically precipitated aragonite are positively correlated to temperature. The slopes of 0.019 and 0.015‰ °C⁻¹, respectively, are a factor of 13 and 16 times smaller than the previously determined fractionation from a second foraminifera, Globigerinoides sacculifer, having a slope of about 0.24‰ °C⁻¹. The observation that δ⁴⁴Ca is positively correlated to temperature is opposite in sign to the oxygen isotopic fractionation (δ¹⁸O) in calcium carbonate (CaCO₃). These observations are explained by a model which considers that Ca²⁺-ions forming ionic bonds are affected by kinetic fractionation only, whereas covalently bound atoms like oxygen are affected by kinetic and equilibrium fractionation. From thermodynamic consideration of kinetic isotope fractionation, it can be shown that the slope of the enrichment factor α(T) is mass-dependent. However, for O. universa and the inorganic precipitates, the calculated mass of about 520 ± 60 and 640 ± 70 amu (atomic mass units) is not compatible with the expected ion mass for ⁴⁰Ca and ⁴⁴Ca. To reconcile this discrepancy, we propose that Ca diffusion and δ⁴⁴Ca isotope fractionation at liquid/solid transitions involves Ca²⁺-aquocomplexes (Ca[H₂O]_n²⁺ · mH₂O) rather than pure Ca²⁺-ion diffusion. From our measurements we calculate that such a hypothesized Ca²⁺-aquocomplex correlates to a hydration number of up to 25 water molecules (490 amu). For O. universa we propose that their biologically mediated Ca isotope fractionation resembles fractionation during inorganic precipitation of CaCO₃ in seawater. To explain the different Ca isotope fractionation in O. universa and in G. sacculifer, we suggest that the latter species actively dehydrates the Ca²⁺-aquocomplex before calcification takes place. The very different temperature response of Ca isotopes in the two species suggests that the use of δ⁴⁴Ca as a temperature proxy will require careful study of species effects.     

Carbon, oxygen, and strontium isotope geochemistry of carbonate rocks of the upper Miocene Kudankulam Formation, southern India: Implications for paleoenvironment and diagenesis

The carbon, oxygen, and strontium isotope compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaeoenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in ¹⁸O and ¹³C compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on δ¹³C and δ¹⁸O values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (δ¹³C from −7.8‰ to −6.0‰ and −9.0‰ to −7.0‰; δ¹⁸O from −9.2‰ to −6.5‰ and −9.4‰ to −2.6‰, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative δ¹⁸O values of various carbonate components (about −9.4‰ to −4.1‰ for whole rocks; about −8.4‰ to −2.6‰ for ooids) suggest that during the late Miocene the paleoclimate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (−7.9‰ to −3.6‰ for whole rocks; −4.9‰ to −1.5‰ for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C₄ type plants, supporting a scenario of expansion of C₄ plants during the late Miocene in the Indian subcontinent as far south as the southern tip of India. The ⁸⁷Sr/⁸⁶Sr ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic ⁸⁷Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene.     

Tropical Atlantic SST history inferred from Ca isotope thermometry over the last 140ka

Exploring the potentials of new methods in palaeothermometry is essential to improve our understanding of past climate change. Here, we present a refinement of the published δ^44/40Ca-temperature calibration investigating modern specimens of planktonic foraminifera Globigerinoides sacculifer and apply this to sea surface temperature (SST) reconstructions over the last two glacial-interglacial cycles. Reproduced measurements of modern G. sacculifer collected from surface waters describe a linear relationship for the investigated temperature range (19.0-28.5 °C): δ^44/40Ca [‰] = 0.22 (±0.05)∗SST [°C] −4.88. Thus a change of δ^44/40Ca[‰] of 0.22 (±0.05) corresponds to a relative change of 1 °C. The refined δ^44/40Ca_modern-calibration allows the determination of both relative temperature changes and absolute temperatures in the past. This δ^44/40Ca_modern-calibration for G. sacculifer has been applied to the tropical East Atlantic sediment core GeoB1112 for which other SST proxy data are available. Comparison of the different data sets gives no indication for significant secondary overprinting of the δ^44/40Ca signal. Long-term trends in reconstructed SST correlate strongly with temperature records derived from oxygen isotopes and Mg/Ca ratios supporting the methods validity. The observed change of SST of approximately 3 °C at the Holocene-last glacial maximum transition reveals additional evidence for the important role of the tropical Atlantic in triggering global climate change, based on a new independent palaeothermometer.     

Calcium isotope (δCa) fractionation along hydrothermal pathways, Logatchev field (Mid-Atlantic Ridge, 14°45′N)

We investigate the Logatchev Hydrothermal Field at the Mid-Atlantic Ridge, 14°45′N to constrain the calcium isotope hydrothermal flux into the ocean. During the transformation of seawater to a hydrothermal solution, the Ca concentration of pristine seawater ([Ca]_SW) increases from about 10 mM to about 32 mM in the hydrothermal fluid endmember ([Ca]_HydEnd) and thereby adopts a δ^44/40Ca_HydEnd of −0.95 ± 0.07‰ relative to seawater (SW) and a ⁸⁷Sr/⁸⁶Sr isotope ratio of 0.7034(4). We demonstrate that δ^44/40Ca_HydEnd is higher than that of the bedrock at the Logatchev field. From mass balance calculations, we deduce a δ^44/40Ca of −1.17 ± 0.04‰ (SW) for the host-rocks in the reaction zone and −1.45 ± 0.05‰ (SW) for the isotopic composition of the entire hydrothermal cell of the Logatchev field. The values are isotopically lighter than the currently assumed δ^44/40Ca for Bulk Earth of −0.92 ± 0.18‰ (SW) [Skulan J., DePaolo D. J. and Owens T. L. (1997) Biological control of calcium isotopic abundances in the global calcium cycle. Geochim. Cosmochim. Acta61,(12) 2505-2510] and challenge previous assumptions of no Ca isotope fractionation between hydrothermal fluid and the oceanic crust [Zhu P. and Macdougall J. D. (1998) Calcium isotopes in the marine environment and the oceanic calcium cycle. Geochim. Cosmochim. Acta62,(10) 1691-1698; Schmitt A. -D., Chabeaux F. and Stille P. (2003) The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance. Earth Planet. Sci. Lett. 6731, 1-16]. Here we propose that Ca isotope fractionation along the fluid flow pathway of the Logatchev field occurs during the precipitation of anhydrite. Two anhydrite samples from the Logatchev Hydrothermal Field show an average fractionation of about Δ^44/40Ca = −0.5‰ relative to their assumed parental solutions. Ca isotope ratios in aragonites from carbonate veins from ODP drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average δ^44/40Ca of −1.54 ± 0.08‰ (SW). The relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine Ca isotope budget, in particular with regard to ocean crust alteration.     

The impact of water-rock interaction and vegetation on calcium isotope fractionation in soil- and stream waters of a small, forested catchment (the Strengbach case)

This study aims to constrain the factors controlling the calcium isotopic compositions in surface waters, especially the respective role of vegetation and water-rock interactions on Ca isotope fractionation in a continental forested ecosystem. The approach is to follow changes in space and time of the isotopic composition and concentration of Ca along its pathway through the hydro-geochemical reservoirs from atmospheric deposits to the outlet of the watershed via throughfalls, percolating soil solutions and springs. The study is focused on the Strengbach catchment, a small forested watershed located in the northeast of France in the Vosges mountains. The δ^44/40Ca values of springs, brooks and stream waters from the catchment are comparable to those of continental rivers and fluctuate between 0.17 and 0.87‰. Soil solutions, however, are significantly depleted in lighter isotopes (δ^44/40Ca: 1.00-1.47‰), whereas vegetation is strongly enriched (δ^44/40Ca: −0.48‰ to +0.19‰). These results highlight that vegetation is a major factor controlling the calcium isotopic composition of soil solutions, with depletion in “light” calcium in the soil solutions from deeper parts of the soil compartments due to preferential ⁴⁰Ca uptake by the plants rootsystem. However, mass balance calculations require the contribution of an additional Ca flux into the soil solutions most probably associated with water-rock interactions. The stream waters are marked by a seasonal variation of their δ^44/40Ca, with low δ^44/40Ca in winter and high δ^44/40Ca in spring, summer and autumn. For some springs, nourishing the streamlet, a decrease of the δ^44/40Ca value is observed when the discharge of the spring increases, with, in addition, a clear covariation between the δ^44/40Ca and corresponding H₄SiO₄ concentrations: high δ^44/40Ca values and low H₄SiO₄ concentrations at high discharge; low δ^44/40Ca values and high H₄SiO₄ concentrations at low discharge. These data imply that during dry periods and low water flow rate the source waters carry a Ca isotopic signature from alteration of soil minerals, whereas during wet periods and high flow rates admixture of significant quantities of ⁴⁰Ca depleted waters (vegetation induced signal) from uppermost soil horizons controls the isotopic composition of the source waters. This study clearly emphasizes the potential of Ca isotopes as tracers of biogeochemical processes at the water-rock-vegetation interface in a small forested catchment.     

Controls on calcium isotope fractionation in cultured planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera

Specimens of two species of planktic foraminifera, Globigerinoides ruber and Globigerinella siphonifera, were grown under controlled laboratory conditions at a range of temperatures (18-31 °C), salinities (32-44 psu) and pH levels (7.9-8.4). The shells were examined for their calcium isotope compositions (δ^44/40Ca) and strontium to calcium ratios (Sr/Ca) using Thermal Ionization Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry. Although the total variation in δ^44/40Ca (∼0.3‰) in the studied species is on the same order as the external reproducibility, the data set reveals some apparent trends that are controlled by more than one environmental parameter. There is a well-defined inverse linear relationship between δ^44/40Ca and Sr/Ca in all experiments, suggesting similar controls on these proxies in foraminiferal calcite independent of species. Analogous to recent results from inorganically precipitated calcite, we suggest that Ca isotope fractionation and Sr partitioning in planktic foraminifera are mainly controlled by precipitation kinetics. This postulation provides us with a unique tool to calculate precipitation rates and draws support from the observation that Sr/Ca ratios are positively correlated with average growth rates. At 25 °C water temperature, precipitation rates in G. siphonifera and G. ruber are calculated to be on the order of 2000 and 3000 μmol/m²/h, respectively. The lower δ^44/40Ca observed at ?29 °C in both species is consistent with increased precipitation rates at high water temperatures. Salinity response of δ^44/40Ca (and Sr/Ca) in G. siphonifera implies that this species has the highest precipitation rates at the salinity of its natural habitat, whereas increasing salinities appear to trigger higher precipitation rates in G. ruber. Isotope effects that cannot be explained by precipitation rate in planktic foraminifera can be explained by a biological control, related to a vacuolar pathway for supply of ions during biomineralization and a pH regulation mechanism in these vacuoles. In case of an additional pathway via cross-membrane transport, supplying light Ca for calcification, the δ^44/40Ca of the reservoir is constrained as −0.2‰ relative to seawater. Using a Rayleigh distillation model, we calculate that calcification occurs in a semi-open system, where less than half of the Ca supplied by vacuolization is utilized for calcite precipitation. Our findings are relevant for interpreting paleo-proxy data on δ^44/40Ca and Sr/Ca in foraminifera as well as understanding their biomineralization processes.     

Interpreting the Ca isotope record of marine biogenic carbonates

An 18 million year record of the Ca isotopic composition (δ^44/42Ca) of planktonic foraminiferans from ODP site 925, in the Atlantic, on the Ceara Rise, provides the opportunity for critical analysis of Ca isotope-based reconstructions of the Ca cycle. δ^44/42Ca in this record averages +0.37 ± 0.05 (1σ SD) and ranges from +0.21‰ to +0.52‰. The record is a good match to previously published Neogene Ca isotope records based on foraminiferans, but is not similar to the record based on bulk carbonates, which has values that are as much as 0.25‰ lower. Bulk carbonate and planktonic foraminiferans from core tops differ slightly in their δ^44/42Ca (i.e., by 0.06 ± 0.06‰ (n = 5)), while the difference between bulk carbonate and foraminiferan values further back in time is markedly larger, leaving open the question of the cause of the difference. Modeling the global Ca cycle from downcore variations in δ^44/42Ca by assuming fixed values for the isotopic composition of weathering inputs (δ^44/42Ca_w) and for isotope fractionation associated with the production of carbonate sediments (Δ_sed) results in unrealistically large variations in the total mass of Ca²⁺ in the oceans over the Neogene. Alternatively, variations of ±0.05‰ in the Ca isotope composition of weathering inputs or in the extent of fractionation of Ca isotopes during calcareous sediment formation could entirely account for variations in the Ca isotopic composition of marine carbonates. Ca isotope fractionation during continental weathering, such as has been recently observed, could easily result in variations in δ^44/42Ca_w of a few tenths of permil. Likewise a difference in the fractionation factors associated with aragonite versus calcite formation could drive shifts in Δ_sed of tenths of permil with shifts in the relative output of calcite and aragonite from the ocean. Until better constraints on variations in δ^44/42Ca_w and Δ_sed have been established, modeling the Ca²⁺ content of seawater from Ca isotope curves should be approached cautiously.     

Note on the isotopic geochemistry of fossil-lacustrine tufas in carbonate plateau—A study from Dungul region (SW Egypt)

Lithological, chemical, and stable isotope data are used to characterize lacustrine tufas dating back to pre-late Miocene and later unknown times, capping different surfaces of a Tertiary carbonate (Sinn el-Kedab) plateau in Dungul region in the currently hyperarid south-western Egypt. These deposits are composed mostly of calcium carbonate, some magnesium carbonate and clastic particles plus minor amounts of organic matter. They have a wide range of (Mg/Ca)_molar ratios, from 0.03 to 0.3. The bulk-tufa carbonate has characteristic isotope compositions: (δ¹³C_mean = −2.49 ± 0.99‰; δ¹⁸O_mean = −9.43 ± 1.40‰). The δ¹³C values are consistent with a small input from C4 vegetation or thinner soils in the recharge area of the tufa-depositing systems. The δ¹⁸O values are typical of fresh water carbonates. Covariation between δ¹³C and δ¹⁸O values probably is a reflection of climatic conditions such as aridity. The tufas studied are isotopically similar to the underlying diagenetic marine chalks, marls and limestones (δ¹³C_mean = −2.06 ± 0.84‰; δ¹⁸O_mean = −10.06 ± 1.39‰). The similarity has been attributed to common meteoric water signatures. This raises large uncertainties in using tufas (Mg/Ca)_molar, δ¹³C and δ¹⁸O records as proxies of paleoclimatic change and suggests that intrinsic compositional differences in material sources within the plateau may mask climatic changes in the records.     

Diet and habitat of toxodont megaherbivores (Mammalia, Notoungulata) from the late Quaternary of South and Central America

The toxodont megaherbivores Toxodon and Mixotoxodon were endemic to South and Central America during the late Quaternary. Isotopic signatures of 47 toxodont teeth were analyzed to reconstruct diet and ancient habitat. Tooth enamel carbon isotope data from six regions of South and Central America indicate significant differences in toxodont diet and local vegetation during the late Quaternary. Toxodonts ranged ecologically from C₃ forest browsers in the Amazon (mean δ¹³C = −13.4‰), to mixed C₃ grazers and/or browsers living either in C₃ grasslands, or mixed C₃ forested and grassland habitats in Honduras (mean δ¹³C = −9.3‰), Buenos Aires province, Argentina (δ¹³C = −8.7‰), and Bahia, Brazil (mean δ¹³C = −8.6‰), to predominantly C₄ grazers in northern Argentina (δ¹³C = −4.4‰), to specialized C₄ grazers in the Chaco of Bolivia (δ¹³C = −0.1‰). Although these toxodonts had very high-crowned teeth classically interpreted for grazing, the isotopic data indicate that these megaherbivores had the evolutionary capacity to feed on a variety of dominant local vegetation. In the ancient Amazon region, carbon isotope data for the toxodonts indicate a C₃-based tropical rainforest habitat with no evidence for grasslands as would be predicted from the Neotropical forest refugia hypothesis.     

Rate-controlled calcium isotope fractionation in synthetic calcite

The isotopic composition of Ca (Δ⁴⁴Ca/⁴⁰Ca) in calcite crystals has been determined relative to that in the parent solutions by TIMS using a double spike. Solutions were exposed to an atmosphere of NH₃ and CO₂, provided by the decomposition of (NH₄)₂CO₃, following the procedure developed by previous workers. Alkalinity, pH and concentrations of CO₃²⁻, HCO₃⁻, and CO₂ in solution were determined. The procedures permitted us to determine Δ(⁴⁴Ca/⁴⁰Ca) over a range of pH conditions, with the associated ranges of alkalinity. Two solutions with greatly different Ca concentrations were used, but, in all cases, the condition [Ca²⁺]>>[CO₃²⁻] was met. A wide range in Δ(⁴⁴Ca/⁴⁰Ca) was found for the calcite crystals, extending from 0.04 ± 0.13‰ to −1.34 ± 0.15‰, generally anti-correlating with the amount of Ca removed from the solution. The results show that Δ(⁴⁴Ca/⁴⁰Ca) is a linear function of the saturation state of the solution with respect to calcite (Ω). The two parameters are very well correlated over a wide range in Ω for each solution with a given [Ca]. The linear correlation extended from Δ(⁴⁴Ca/⁴⁰Ca) = −1.34 ± 0.15‰ to 0.04 ± 0.13‰, with the slopes directly dependent on [Ca]. Solutions, which were vigorously stirred, showed a much smaller range in Δ(⁴⁴Ca/⁴⁰Ca) and gave values of −0.42 ± 0.14‰, with the largest effect at low Ω. It is concluded that the diffusive flow of CO₃²⁻ into the immediate neighborhood of the crystal-solution interface is the rate-controlling mechanism and that diffusive transport of Ca²⁺ is not a significant factor. The data are simply explained by the assumptions that: a) the immediate interface of the crystal and the solution is at equilibrium with Δ(⁴⁴Ca/⁴⁰Ca) ∼ −1.5 ± 0.25‰; and b) diffusive inflow of CO₃²⁻ causes supersaturation, thus precipitating Ca from the regions exterior to the narrow zone of equilibrium. The result is that Δ(⁴⁴Ca/⁴⁰Ca) is a monotonically increasing (from negative values to zero) function of Ω. We consider this model to be a plausible explanation of most of the available data reported in the literature. The well-resolved but small and regular isotope fractionation shifts in Ca are thus not related to the diffusion of very large hydrated Ca complexes, but rather due to the ready availability of Ca in the general neighborhood of the crystal-solution interface. The largest isotopic shift which occurs as a small equilibrium effect is then subdued by supersaturation precipitation for solutions where [Ca²⁺]>>[CO₃²⁻] + [HCO₃⁻]. It is shown that there is a clear temperature dependence of the net isotopic shifts that is simply due to changes in Ω due to the equilibrium “constants” dependence on temperature, which changes the degree of saturation and hence the amount of isotopically unequilibrated Ca precipitated. The effects that are found in natural samples, therefore, will be dependent on the degree of diffusive inflow of carbonate species at or around the crystal-liquid interface in the particular precipitating system, thus limiting the equilibrium effect.     

Isotopic constraints on ice age fluids in active geothermal systems: Reykjanes, Iceland

The Reykjanes geothermal system is located on the landward extension of the Mid-Atlantic Ridge in southwest Iceland, and provides an on-land proxy to high-temperature hydrothermal systems of oceanic spreading centers. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, δD values of these fluids are as low as −23‰, which is indicative of a meteoric water component. Here we constrain the origin of Reykjanes hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of hydrothermal epidote from geothermal drillholes at depths between 1 and 3 km. δD_EPIDOTE values from wells RN-8, -9, -10 and -17 collectively range from −60 to −78‰, and δ¹⁸O_EPIDOTE in these wells are between −3.0 and 2.3‰. The δD values of epidote generally increase along a NE trend through the geothermal field, whereas δ¹⁸O values generally decrease, suggesting a southwest to northeast migration of the geothermal upflow zone with time that is consistent with present-day temperatures and observed hydrothermal mineral zones. For comparative analysis, the meteoric-water dominated Nesjavellir and Krafla geothermal systems, which have a δD_FLUID of ∼ −79‰ and −89‰, respectively, show δD_EPIDOTE values of −115‰ and −125‰. In contrast, δD_EPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is −68‰; comparable to δD_EPIDOTE from well RN-10 at Reykjanes.Stable isotope compositions of geothermal fluids in isotopic equilibrium with the epidotes at Reykjanes are computed using published temperature dependent hydrogen and oxygen isotope fractionation curves for epidote-water, measured isotope composition of the epidotes and temperatures approximated from the boiling point curve with depth. Calculated δD and δ¹⁸O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric or glacial origin are a significant component of the geothermal solutions. Additionally, δD_FLUID values in equilibrium with geothermal epidote are lower than those of modern-day fluids, whereas calculated δ¹⁸O_FLUID values are within range of the observed fluid isotope composition. We propose that modern δD_EPIDOTE and δD_FLUID values are the result of diffusional exchange between hydrous alteration minerals that precipitated from glacially-derived fluids early in the evolution of the Reykjanes system and modern seawater-derived geothermal fluids. A simplified model of isotope exchange in the Reykjanes geothermal system, in which the average starting δD_ROCK value is −125‰ and the water to rock mass ratio is 0.25, predicts a δD_FLUID composition within 1‰ of average measured values. This model resolves the discrepancy between fluid salinity and isotope composition of Reykjanes geothermal fluids, explains the observed disequilibrium between modern fluids and hydrothermal epidote, and suggests that rock-fluid interaction is the dominant control over the evolution of fluid isotope composition in the hydrothermal system.     

Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?

In rivers draining the Himalaya-Tibetan-Plateau region, the ²⁶Mg/²⁴Mg ratio has a range of 2‰ and the ⁴⁴Ca/⁴²Ca ratio has a range of 0.6‰. The average δ²⁶Mg values of tributaries from each of the main lithotectonic units (Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Series (HHCS) and Lesser Himalayan Series (LHS)) are within 2 standard deviation analytical uncertainty (0.14‰). The consistency of average riverine δ²⁶Mg values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average δ²⁶Mg values by more than 2‰. Tributaries draining the dolostones of the LHS differ in their values compared to tributaries from the TSS and HHCS. The chemistry of these river waters is strongly influenced by dolostone (solute Mg/Ca close to unity) and both δ²⁶Mg (−1.31‰) and (0.64‰) values are within analytical uncertainty of the LHS dolostone. These are the most elevated values in rivers and rock reported so far demonstrating that both riverine and bedrock values may show greater variability than previously thought.Although rivers draining TSS limestone have the lowest values at −1.41 and 0.42‰, respectively, both are offset to higher values compared to bedrock TSS limestone. The average δ²⁶Mg value of rivers draining mainly silicate rock of the HHCS is −1.25‰, lower by 0.63‰ than the average silicate rock. These differences are consistent with a fractionation of δ²⁶Mg values during silicate weathering. Given that the proportion of Mg exported from the Himalaya as solute Mg is small, the difference in ²⁶Mg/²⁴Mg ratios between silicate rock and solute Mg reflects the ²⁶Mg/²⁴Mg isotopic fractionation factor () between silicate and dissolved Mg during incongruent silicate weathering. The value of of 0.99937 implies that in the TSS, solute Mg is primarily derived from silicate weathering, whereas the source of Ca is overwhelmingly derived from carbonate weathering. The average value in HHCS rivers is within uncertainty of silicate rock at 0.39‰. The widespread hot springs of the High Himalaya have an average δ²⁶Mg value of −0.46‰ and an average value of 0.5‰, distinct from riverine values for δ²⁶Mg but similar to riverine values. Although rivers draining each major rock type have and δ²⁶Mg values in part inherited from bedrock, there is no correlation with proxies for carbonate or silicate lithology such as Na/Ca ratios, suggesting that Ca and Mg are in part recycled. However, in spite of the vast contrast in vegetation density between the arid Tibetan Plateau and the tropical Lesser Himalaya, the isotopic fractionation factor for Ca and Mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of Ca and Mg in these catchments.The discrepancy between solute and solid Ca and Mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of Ca and Mg, bound as minerals in rock, and released as cations in rivers. The fractionation of Ca and Mg isotope ratios may prove useful for tracing mechanisms of chemical alteration. Ca isotope ratios of solute riverine Ca show a greater variability than previously acknowledged. The variability of Ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global Ca cycling, if past variations in oceanic Ca isotope ratios are to be of use in constraining the past carbon cycle.     

Oxygen isotopic composition of mammalian skeletal phosphate from the Natufian Period,Hayonim Cave,Israel: Diagenesis and paleoclimate

Recent and fossil (prehistoric, Natufian) gazelle bones, dentin and enamel were analyzed for their oxygen isotopic composition (δ¹⁸O) of the phosphate and carbonate, as well as their crystallinity. Fossil bones and dentin have better crystallinity than recent specimens, indicating diagenetic change. Fossil enamel, on the other hand, is identical in crystallinity to recent enamel, indicating the lack of diagenetic alteration. Comparison of δ¹⁸O of carbonate and phosphate of the skeletal elements suggests that the coexisting phosphate and carbonate of both the recent and fossil specimens are close to isotopic equilibrium. This might suggest that both phases were affected by the same degree of diagenetic alteration, and that comparison of their δ¹⁸O is not useful for the selection of pristine material for paleoclimatic reconstruction purposes. Oxygen isotope analysis of gazelle enamel from the Natufian period from Hayonim Cave, Israel, show depletion in δ¹⁸O in comparison with recent enamel. This depletion is interpreted to represent a colder and/or wetter climate in the Natufian of northern Israel. © 1999 John Wiley & Sons, Inc.     

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.     

Calcium isotope fractionation in modern scleractinian corals

The ⁴⁴Ca/⁴⁰Ca ratios of cultured (Acropora sp.) and open ocean (Pavona clavus, Porites sp.) tropical reef corals are positively correlated with growth temperature. The slope of the temperature-fractionation relation is similar to inorganic aragonite precipitates. However, δ^44/40Ca of the coral aragonite is offset from inorganic and sclerosponge aragonite by about +0.5‰. This offset can neither be explained by the very fast, biologically controlled calcification of scleractinian corals, nor as a consequence of calcification from a partly closed volume of fluid. As corals actively transport calcium through several cell layers to the site of calcification, the most likely explanation for the offset is a biologically induced fractionation. Our results indicate a limited use of Ca isotopes in scleractinian corals as temperature proxy.