Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: The Ca(aq)-calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments

The calcium isotopic compositions (δ⁴⁴Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (α_s−f) between calcite and dissolved Ca²⁺ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of α_s−f at equilibrium in the marine sedimentary section is 1.0000 ± 0.0001, which is significantly different from the value (0.9987 ± 0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ∼14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in δ⁴⁴Ca of +0.15‰ for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.     

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.     

Sr/Ca and Ca/Ca fractionation during inorganic calcite formation: II. Ca isotopes

Ca isotope fractionation during inorganic calcite formation was experimentally studied by spontaneous precipitation at various precipitation rates (1.8 < log R < 4.4 μmol/m²/h) and temperatures (5, 25, and 40 °C) with traces of Sr using the CO₂ diffusion technique.Results show that in analogy to Sr/Ca [see Tang J., Köhler S. J. and Dietzel M. (2008) Sr²⁺/Ca²⁺ and ⁴⁴Ca/⁴⁰Ca fractionation during inorganic calcite formation: I. Sr incorporation. Geochim. Cosmochim. Acta] the ⁴⁴Ca/⁴⁰Ca fractionation during calcite formation can be followed by the Surface Entrapment Model (SEMO). According to the SEMO calculations at isotopic equilibrium no fractionation occurs (i.e., the fractionation coefficient α_calcite-aq = (⁴⁴Ca/⁴⁰Ca)_s/(⁴⁴Ca/⁴⁰Ca)_aq = 1 and Δ^44/40Ca_calcite-aq = 0‰), whereas at disequilibrium ⁴⁴Ca is fractionated in a primary surface layer (i.e., the surface entrapment factor of ⁴⁴Ca, F_44Ca < 1). As a crystal grows at disequilibrium, the surface-depleted ⁴⁴Ca is entrapped into the newly formed crystal lattice. ⁴⁴Ca depletion in calcite can be counteracted by ion diffusion within the surface region. Our experimental results show elevated ⁴⁴Ca fractionation in calcite grown at high precipitation rates due to limited time for Ca isotope re-equilibration by ion diffusion. Elevated temperature results in an increase of ⁴⁴Ca ion diffusion and less ⁴⁴Ca fractionation in the surface region. Thus, it is predicted from the SEMO that an increase in temperature results in less ⁴⁴Ca fractionation and the impact of precipitation rate on ⁴⁴Ca fractionation is reduced.A highly significant positive linear relationship between absolute ⁴⁴Ca/⁴⁰Ca fractionation and the apparent Sr distribution coefficient during calcite formation according to the equation

Δ^44/40Ca_calcite-aq=(−1.90±0.26)·logD_Sr−2.83±0.28     

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.     

An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures

To determine oxygen isotope fractionation between aragonite and water, aragonite was slowly precipitated from Ca(HCO₃)₂ solution at 0 to 50°C in the presence of Mg²⁺ or SO₄²⁻. The phase compositions and morphologies of synthetic minerals were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The effects of aragonite precipitation rate and excess dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution on oxygen isotope fractionation between aragonite and water were investigated. For the CaCO₃ minerals slowly precipitated by the CaCO₃ or NaHCO₃ dissolution method at 0 to 50°C, the XRD and SEM analyses show that the rate of aragonite precipitation increased with temperature. Correspondingly, oxygen isotope fractionations between aragonite and water deviated progressively farther from equilibrium. Additionally, an excess of dissolved CO₂ gas in the initial Ca(HCO₃)₂ solution results in an increase in apparent oxygen isotope fractionations. As a consequence, the experimentally determined oxygen isotope fractionations at 50°C indicate disequilibrium, whereas the relatively lower fractionation values obtained at 0 and 25°C from the solution with less dissolved CO₂ gas and low precipitation rates indicate a closer approach to equilibrium. Combining the lower values at 0 and 25°C with previous data derived from a two-step overgrowth technique at 50 and 70°C, a fractionation equation for the aragonite-water system at 0 to 70°C is obtained as follows:

     

Experimental study of the aragonite to calcite transition in aqueous solution

The experimental replacement of aragonite by calcite was studied under hydrothermal conditions at temperatures between 160 and 200 °C using single inorganic aragonite crystals as a starting material. The initial saturation state and the total [Ca²⁺]:[CO₃²⁻] ratio of the experimental solutions was found to have a determining effect on the amount and abundance of calcite overgrowths as well as the extent of replacement observed within the crystals. The replacement process was accompanied by progressive formation of cracks and pores within the calcite, which led to extended fracturing of the initial aragonite. The overall shape and morphology of the parent aragonite crystal were preserved. The replaced regions were identified with scanning electron microscopy and Raman spectroscopy.Experiments using carbonate solutions prepared with water enriched in ¹⁸O (97%) were also performed in order to trace the course of this replacement process. The incorporation of the heavier oxygen isotope in the carbonate molecule within the calcite replacements was monitored with Raman spectroscopy. The heterogeneous distribution of ¹⁸O in the reaction products required a separate study of the kinetics of isotopic equilibration within the fluid to obtain a better understanding of the ¹⁸O distribution in the calcite replacement. An activation energy of 109 kJ/mol was calculated for the exchange of oxygen isotopes between [C¹⁶O₃²⁻]_aq and [H₂¹⁸O] and the time for oxygen isotope exchange in the fluid at 200 °C was estimated at ∼0.9 s. Given the exchange rate, analyses of the run products imply that the oxygen isotope composition in the calcite product is partly inherited from the oxygen isotope composition of the aragonite parent during the replacement process and is dependent on access of the fluid to the reaction interface rather than equilibration time. The aragonite to calcite fluid-mediated transformation is described by a coupled dissolution-reprecipitation mechanism, where aragonite dissolution is coupled to the precipitation of calcite at an inwardly moving reaction interface.     

Oxygen isotope fractionation between synthetic aragonite and water: Influence of temperature and Mg concentration

Aragonite was precipitated in the laboratory at 0, 5, 10, 25, and 40 °C to determine the temperature dependence of the equilibrium oxygen isotope fractionation between aragonite and water. Forced CO₂ degassing, passive CO₂ degassing, and constant addition methods were employed to precipitate aragonite from supersaturated solutions, but the resulting aragonite-water oxygen isotope fractionation was independent of the precipitation method. In addition, under the experimental conditions of this study, the effect of precipitation rate on the oxygen isotope fractionation between aragonite and water was almost within the analytical error of ±∼0.13‰ and thus insignificant. Because the presence of Mg²⁺ ions is required to nucleate and precipitate aragonite from Na-Ca-Cl-HCO₃ solutions under these experimental conditions, the influence of the total Mg²⁺ concentration (up to ∼0.9 molal) on the aragonite-water oxygen isotope fractionation was examined at 25 °C. No significant Mg²⁺ ion effect, or oxygen isotope salt effect, was detected up to 100 mmolal total Mg²⁺ but a noticeable isotope salt effect was observed at ∼0.9 molal total Mg²⁺.On the basis of results of the laboratory synthesis experiments, a new expression for the aragonite-water fractionation is proposed over the temperature range of 0-40 °C:

1000lnα_{aragonite-water}=17.88±0.13(10³/T)-31.14±0.46     

Calibration of the calcite-water oxygen-isotope geothermometer at Devils Hole, Nevada, a natural laboratory

The δ¹⁸O of ground water (−13.54 ± 0.05 ‰) and inorganically precipitated Holocene vein calcite (+14.56 ± 0.03 ‰) from Devils Hole cave #2 in southcentral Nevada yield an oxygen isotopic fractionation factor between calcite and water at 33.7 °C of 1.02849 ± 0.00013 (1000 ln α_{calcite-water} = 28.09 ± 0.13). Using the commonly accepted value of ∂(α_{calcite-water})/∂T of −0.00020 K⁻¹, this corresponds to a 1000 ln α_{calcite-water} value at 25 °C of 29.80, which differs substantially from the current accepted value of 28.3. Use of previously published oxygen isotopic fractionation factors would yield a calcite precipitation temperature in Devils Hole that is 8 °C lower than the measured ground water temperature. Alternatively, previously published fractionation factors would yield a δ¹⁸O of water, from which the calcite precipitated, that is too negative by 1.5 ‰ using a temperature of 33.7 °C. Several lines of evidence indicate that the geochemical environment of Devils Hole has been remarkably constant for at least 10 ka. Accordingly, a re-evaluation of calcite-water oxygen isotopic fractionation factor may be in order.Assuming the Devils Hole oxygen isotopic value of α_{calcite-water} represents thermodynamic equilibrium, many marine carbonates are precipitated with a δ¹⁸O value that is too low, apparently due to a kinetic isotopic fractionation that preferentially enriches ¹⁶O in the solid carbonate over ¹⁸O, feigning oxygen isotopic equilibrium.     

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.     

Magnesium stable isotope fractionation in marine biogenic calcite and aragonite

This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ²⁶Mg; n = 37), obtained from a coral reference sample (JCp-1).Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ²⁶Mg_{calcite-seawater} = −2.6 ± 0.3‰ that appears to be weakly related to temperature. With one exception (Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ²⁶Mg_{biogenic aragonite-seawater} = −0.9 ± 0.2.Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation.Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO₃ principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO₃ relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO₃.     

Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave - Field study and experiments

The chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings. Despite intensive research, including modelling and novel approaches, speleothem data remain difficult to interpret. A possible way foreword is to apply a multi-proxy approach including non-conventional isotope systems. For the first time, we here present a complete analytical dataset of magnesium isotopes (δ²⁶Mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes δ²⁶Mg values of loess-derived soil above the cave (−1.0 ± 0.5‰), soil water (−1.2 ± 0.5‰), the carbonate hostrock (−3.8 ± 0.5‰), dripwater in the cave (−1.8 ± 0.2‰), speleothem low-Mg calcite (stalactites, stalagmites; −4.3 ± 0.6‰), cave loam (−0.6 ± 0.1‰) and runoff water (−1.8 ± 0.1‰) in the cave, respectively. Magnesium-isotope fractionation processes during weathering and interaction between soil cover, hostrock and solute-bearing soil water are non-trivial and depend on a number of variables including solution residence times, dissolution rates, adsorption effects and potential neo-formation of solids in the regolith and the carbonate aquifer. Apparent Mg-isotope fractionation between dripwater and speleothem low-Mg calcite is about 1000lnα_Mg-cc-Mg(aq) = −2.4‰. A similar Mg-isotope fractionation (1000lnα_Mg-cc-Mg(aq) ≈ −2.1‰) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, ²⁶Mg discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg²⁺ prior to incorporation into calcite, surface entrapment of light isotopes and reaction kinetics. Relevance of kinetics is supported by a significant negative correlation of Mg-isotope fractionation with the precipitation rate for inorganic precipitation experiments.     

Re-evaluating boron speciation in biogenic calcite and aragonite using B MAS NMR

Understanding the partitioning of aqueous boron species into marine carbonates is critical for constraining the boron isotope system for use as a marine pH proxy. Previous studies have assumed that boron was incorporated into carbonate through the preferential uptake of tetrahedral borate B(OH)₄⁻. In this study we revisit this assumption through a detailed solid state ¹¹B magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic study of boron speciation in biogenic and hydrothermal carbonates. Our new results contrast with those of the only previous NMR study of carbonates insofar as we observe both trigonal and tetrahedral coordinated boron in almost equal abundances in our biogenic calcite and aragonite samples. In addition, we observe no strict dependency of boron coordination on carbonate crystal structure. These NMR observations coupled with our earlier re-evaluation of the magnitude of boron isotope fractionation between aqueous species suggest that controls on boron isotope composition in marine carbonates, and hence the pH proxy, are more complex that previously suggested.     

Diagenesis and geochemistry of porites corals from Papua New Guinea: Implications for paleoclimate reconstruction

Coral proxy records of sea surface temperature (SST) and hydrological balance have become important tools in the field of tropical paleoclimatology. However, coral aragonite is subject to post-depositional diagenetic alteration in both the marine and vadose environments. To understand the impact of diagenesis on coral climate proxies, two mid-Holocene Porites corals from raised reefs on Muschu Island, Papua New Guinea, were analysed for Sr/Ca, δ¹⁸O, and δ¹³C along transects from 100% aragonite to 100% calcite. Thin-section analysis showed a characteristic vadose zone diagenetic sequence, beginning with leaching of primary aragonite and fine calcite overgrowths, transitional to calcite void filling and neomorphic, fabric selective replacement of the coral skeleton. Average calcite Sr/Ca and δ¹⁸O values were lower than those for coral aragonite, decreasing from 0.0088 to 0.0021 and −5.2 to −8.1‰, respectively. The relatively low Sr/Ca of the secondary calcite reflects the Sr/Ca of dissolving phases and the large difference between aragonite and calcite Sr/Ca partition coefficients. The decrease in δ¹⁸O of calcite relative to coral aragonite is a function of the δ¹⁸O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though generally lower relative to aragonite, ranging from −2.5 to −10.4%. The variability of δ¹³C in secondary calcite reflects the amount of soil CO₂ contributing ¹³C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on δ¹⁸O; the calcite compositions reported here convert to SST anomalies of 115°C and 14°C, respectively. Based on calcite Sr/Ca compositions in this study and in the literature, the sensitivity of coral Sr/Ca-SST to vadose-zone calcite diagenesis is 1.1 to 1.5°C per percent calcite. In contrast, the rate of change in coral δ¹⁸O-SST is relatively small (−0.2 to 0.2°C per percent calcite). We show that large shifts in δ¹⁸O, reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca-SSTs, cannot be caused by calcite diagenesis. Low-level calcite diagenesis can be detected through X-ray diffraction techniques, thin section analysis, and high spatial resolution sampling of the coral skeleton and thus should not impede the production of accurate coral paleoclimate reconstructions.     

Surface kinetic model for isotopic and trace element fractionation during precipitation of calcite from aqueous solutions

A surface reaction kinetic model is developed for predicting Ca isotope fractionation and metal/Ca ratios of calcite as a function of rate of precipitation from aqueous solution. The model is based on the requirements for dynamic equilibrium; i.e. proximity to equilibrium conditions is determined by the ratio of the net precipitation rate (R_p) to the gross forward precipitation rate (R_f), for conditions where ionic transport to the growing crystal surface is not rate-limiting. The value of R_p has been experimentally measured under varying conditions, but the magnitude of R_f is not generally known, and may depend on several factors. It is posited that, for systems with no trace constituents that alter the surface chemistry, R_f can be estimated from the bulk far-from-equilibrium dissolution rate of calcite (R_b or k_b), since at equilibrium R_f = R_b, and R_p = 0. Hence it can be inferred that R_f ≈ R_p + R_b. The dissolution rate of pure calcite is measureable and is known to be a function of temperature and pH. At given temperature and pH, equilibrium precipitation is approached when R_p (=R_f − R_b) ? R_b. For precipitation rates high enough that R_p ? R_b, both isotopic and trace element partitioning are controlled by the kinetics of ion attachment to the mineral surface, which tend to favor more rapid incorporation of the light isotopes of Ca and discriminate weakly between trace metals and Ca. With varying precipitation rate, a transition region between equilibrium and kinetic control occurs near R_p ≈ R_b for Ca isotopic fractionation. According to this model, Ca isotopic data can be used to estimate R_f for calcite precipitation. Mechanistic models for calcite precipitation indicate that the molecular exchange rate is not constant at constant T and pH, but rather is dependent also on solution saturation state and hence R_p. Allowing R_b to vary as , consistent with available precipitation rate studies, produces a better fit to some trace element and isotopic data than a model where R_b is constant. This model can account for most of the experimental data in the literature on the dependence of ⁴⁴Ca/⁴⁰Ca and metal/Ca fractionation in calcite as a function of precipitation rate and temperature, and also accounts for ¹⁸O/¹⁶O variations with some assumptions. The apparent temperature dependence of Ca isotope fractionation in calcite may stem from the dependence of R_b on temperature; there should be analogous pH dependence at pH < 6. The proposed model may be valuable for predicting the behavior of isotopic and trace element fractionation for a range of elements of interest in low-temperature aqueous geochemistry. The theory presented is based on measureable thermo-kinetic parameters in contrast to models that require hyper-fast diffusivity in near-surface layers of the solid.     

A critical evaluation of calcium isotope ratios in tests of planktonic foraminifers

We critically evaluate the applicability of Ca-isotope ratios in planktonic foraminifers as proxy for past sea surface temperatures (SST) and isotope composition of paleo-seawater (δ⁴⁴Ca_sw) reconstructions. Previous studies have shown discrepancies regarding the temperature sensitivity of Ca isotope fractionation in foraminifers of more than one order of magnitude. We present new data from the planktonic foraminifer species Orbulina universa, Globigerinoides sacculifer and Neogloboquadrina pachyderma (sinistral) from culture experiments, multinet deployments and coretop samples. Specimens of G. sacculifer cultured at low salinities (33-34.5) show predominantly no major temperature dependent Ca isotope fractionation, in contrast to previous individuals cultured at higher salinities of 34.5-36. The new data of O. universa are consistent with previously published results, revealing a small but significant temperature sensitivity. Calcium isotope fractionation in tests of N. pachyderma shows a significant variation with temperature, which is not uniform over the total investigated temperature range (−1.6 °C to +10 °C), possibly reflecting the influence of additional controlling factors besides temperature. Controlled dissolution experiments in the laboratory indicate that the Ca-isotope composition of G. sacculifer and N. pachyderma is relatively insensitive to partial dissolution of their tests.Calcium isotope ratios in the planktonic foraminifers G. sacculifer and N. pachyderma (s) reveal a complex Ca isotope fractionation behaviour, which is not yet fully understood. Additional validation studies are crucial to enhance the basic understanding of the calcium isotope systematics in planktic foraminifer shells, and the potential for applying Ca-isotope ratios as proxies for seawater temperature and the oceanic Ca budget.     

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.     

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.     

On the Direction and Magnitude of Oxygen Isotope Fractionation Between Calcite and Aragonite at Thermodynamic Equilibrium

Oxygen isotope fractionation factors between calcium carbonates and water have been applied to ancient marine geochemistry principally for the purpose of geothermometry. The problem was encountered, however, with respect to the direction and magnitude of oxygen isotope fractionation between calcite and aragonite at thermodynamic equilibrium. This basically involves sound understanding of both thermodynamics and kinetics of oxygen isotope fractionation between inorganically precipitated carbonate and water at low temperatures. Thus the crucial issues are to acknowledge the processes of chemical reaction and isotopic exchange during precipitation of CaCO₃ minerals in solution, the kinetic mechanism of isotope equilibrium or disequilibrium, the effect of polymorphic transition from metastable aragonite to stable calcite under hydrous or anhydrous conditions, and the presence or absence of isotope salt effect on oxygen isotope exchange between carbonate and water in response to the hydrous or anhydrous conditions at thermodynamic equilibrium. Because good agreements exist in carbonate–water oxygen isotope fractionation factors between theoretical calculations and experimental determinations, it is encouraging to applying the thermodynamic and kinetic data to isotopic paleothermometry and geochemical tracing.     

Effects of diagenesis on paleoclimate reconstructions from modern and young fossil corals

The integrity of coral-based reconstructions of past climate variability depends on a comprehensive knowledge of the effects of post-depositional alteration on coral skeletal geochemistry. Here we combine millimeter-scale and micro-scale coral Sr/Ca data, scanning electron microscopy (SEM) images, and X-ray diffraction with previously published δ¹⁸O records to investigate the effects of submarine and subaerial diagenesis on paleoclimate reconstructions in modern and young sub-fossil corals from the central tropical Pacific. In a 40-year-old modern coral, we find secondary aragonite is associated with relatively high coral δ¹⁸O and Sr/Ca, equivalent to sea-surface temperature (SST) artifacts as large as −3 and −5 °C, respectively. Secondary aragonite observed in a 350-year-old fossil coral is associated with relatively high δ¹⁸O and Sr/Ca, resulting in apparent paleo-SST offsets of up to −2 and −4 °C, respectively. Secondary Ion Mass Spectrometry (SIMS) analyses of secondary aragonite yield Sr/Ca ratios ranging from 10.78 to 12.39 mmol/mol, significantly higher compared to 9.15 ± 0.37 mmol/mol measured in more pristine sections of the same fossil coral. Widespread dissolution and secondary calcite observed in a 750-year-old fossil coral is associated with relatively low δ¹⁸O and Sr/Ca. SIMS Sr/Ca measurements of the secondary calcite (1.96-9.74 mmol/mol) are significantly lower and more variable than Sr/Ca values from more pristine portions of the same fossil coral (8.22 ± 0.13 mmol/mol). Our results indicate that while diagenesis has a much larger impact on Sr/Ca-based paleoclimate reconstructions than δ¹⁸O-based reconstructions at our site, SIMS analyses of relatively pristine skeletal elements in an altered coral may provide robust estimates of Sr/Ca which can be used to derive paleo-SSTs.     

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.