Elsevier

Chemical Geology

Volume 539, 20 April 2020, 119500
Chemical Geology

Triple oxygen isotope fractionation between CaCO3 and H2O in inorganically precipitated calcite and aragonite

https://doi.org/10.1016/j.chemgeo.2020.119500Get rights and content

Highlights

  • The 17O fractionation between CaCO3 and its parent water is examined experimentally.

  • The triple oxygen isotope fractionation slope, (ln17α/ln18α), is similar in calcite and aragonite.

  • The fractionation slope is independent of temperature at 10, 27 °C but somewhat lower at 35 °C.

Abstract

Carbonate bearing materials, such as foraminifera, mollusks shells, or speleothems, have the potential to preserve geochemical and isotopic signatures reflecting the environmental conditions at the time they formed. Beyond the conventional δ18O, information from triple oxygen isotopes (reported as 17Oexcess, which is defined as (ln (δ17O/1000 + 1) − λref ln (δ18O/1000 + 1)) × 106) in these archives promises to be a valuable tool to reconstruct past hydrological processes. The goal of this study is to determine the triple oxygen isotope fractionation between CaCO3 and H2O under well-constrained laboratory conditions. We performed laboratory experiments to precipitate CaCO3 polymorphs, either calcite or aragonite, at temperatures between 10 and 35 °C. We then evaluated the effect of polymorphism, temperature, and solution concentration on the 17Oexcess of CO2 extracted from these carbonates and the 17O isotopic fractionation (17α) between water and CaCO3. The obtained values of 18α and 17α between CO2 extracted from CaCO3 and parent water allow us to calculate the fractionation slope θ (=ln17α/ln18α). Our observations suggest that θ is indistinguishable at temperatures of 10 and 27 °C, but is slightly lower at 35 °C. The lower value at 35 °C may be related to disequilibrium during these experiments. We found that θ is independent of polymorph and of solution concentration, indicating that 17Oexcess is less sensitive than δ18O to these geochemical parameters and can thus be a robust proxy for reconstructing 17Oexcess of parent water.

Introduction

Calcium carbonate is well-known to have the potential to preserve various geochemical signals, reflecting the environmental conditions at the time in which the carbonate mineral formed. Since the pioneering works of Urey (1947) and Emiliani (1966), the isotopic composition, specifically δ18O, of marine and terrestrial carbonates has been used as a paleoclimate proxy that records a combination of temperature and water isotopic composition. Additional carbonate-based proxies, such as Mg/Ca and clumped isotopes, are often used as geochemical thermometers (e.g., Eiler, 2011; Elderfield et al., 2006), complementing information from δ18O. In terrestrial carbonates, such as speleothems, δ18O is interpreted as a recorder of the weighted mean rainfall δ18O, often reflecting the amount of rainfall in low latitude records (e.g., McDermott, 2004). This, however, does not capture the full hydrological complexity, such as the variability in moisture sources and the effect of local evaporation (although a few studies were able to link speleothem δ18O changes with changes in moisture sources during specific time intervals in the past, such as the 8.2 ka event; e.g., Dominguez-Villar et al., 2009; Voarintsoa et al., 2019). These gaps can be filled by information from 17Oexcess in water that may be archived in a variety of carbonates.

The parameter 17Oexcess is expressed as (ln(δ17O/1000 + 1) − λref ln(δ18O/1000 + 1)) × 106, or simply (δ′17O − λref × δ′18O) × 103, and is reported in per meg (e.g., Barkan and Luz, 2012; Luz and Barkan, 2010). The slope λref is a mass dependent reference, with the value 0.528 used in water, CO2, and CaCO3.

Meteoric water 17Oexcess is largely dependent on the atmospheric conditions, such as relative humidity, in the region where water vapor form (e.g., Angert et al., 2004; Luz and Barkan, 2010). However, there are indications for the influence of additional processes during precipitation, associated with local conditions. The 17Oexcess data of Li et al. (2015) from U.S tap waters suggest a potential influence of mixing of moisture sources and re-evaporation during precipitation. In high latitude regions, 17Oexcess is mainly influenced by kinetic processes related to super-saturation during vapor condensation at cold climate (e.g. Landais et al., 2012; Schoenemann et al., 2014). In mid to low latitude regions, 17Oexcess is mainly affected by evaporation of rain droplets, especially during warm climatic conditions (Landais et al., 2010; Risi et al., 2013; Li et al., 2015; Passey and Ji, 2019).

Whereas fossil water 17Oexcess can be obtained from ice cores, fluid inclusions, or gypsum hydration water (Affolter et al., 2015; Gázquez et al., 2018; Landais et al., 2008), these archives are limited in their geographic distribution. A broader coverage can be obtained by measuring 17Oexcess in carbonates. Recent analytical developments (Passey et al., 2014; Barkan et al., 2015; Mahata et al., 2016) enable high precisions measurements of 17Oexcess in CO2 released from CaCO3, which in turn may serve as a recorder of 17Oexcess of the water in which the mineral grows (Passey et al., 2014; Levin et al., 2014; Passey and Ji, 2019). Despite the recent advancement in measuring triple oxygen isotopes in carbonate materials, the fractionation involved in this proxy has not yet been characterized by laboratory experiments, to provide a framework necessary to quantitatively interpret past records. Here, we examine the triple oxygen isotope fractionation in the CaCO3–H2O system focusing on the effect of mineralogy, using laboratory precipitation experiments of calcite, aragonite, and their mixtures at temperatures of 10, 27, and 35 °C. We also examine the role of initial solution concentrations, as a simple way to assess the role of dissolved inorganic carbon (DIC) concentration on the triple oxygen isotope fractionation. The data allows us to calculate the fractionation slope, θ (=ln17α/ln18α), in order to interpret 17Oexcess in natural carbonates as a geochemical archive for paleo-hydrological processes.

Section snippets

Precipitation experiment

Precipitation experiments were performed by mixing solutions containing NaHCO3 (Baker Analyzed Reagent, J.T. Baker ref. 1-3506) and CaCl2·2H2O (Sigma-Aldrich CAS no. 10035-04-8) to reach an initial DIC concentration of 25 mM (at 10, 27, and 35 °C) and 5 mM (at 27 °C, to test the effect of initial DIC). MgCl2·6H2O (Sigma Aldrich CAS no. 7791-18-6) was added to obtain a total Mg-Ca concentration of 25 mM or 5 mM, respectively. The Mg:Ca ratio for either the 5 mM or 25 mM solution was defined

Results

The results from this study are presented in Fig. 1, Fig. 2, Fig. 3, Fig. 4, and details are given in Table 1, Table 2, Table 3.

Mineralogy and molarity

Calcite and aragonite are two common carbonate minerals that are used in paleoclimate and paleo-environment reconstruction (e.g., in speleothems, foraminifera, mollusk shells, and lake deposits). Often, calcite and aragonite co-occur in one sample (e.g., in speleothems; Holmgren et al., 2003; Voarintsoa et al., 2017). In such cases, the difference in 18α between the two polymorphs (Kim et al., 2007b) has to be accounted for in the aragonite δ18O to conform with the calcite values and to avoid

Conclusions

Results from this experimental study allow us to determine the triple oxygen isotope fractionations in the CaCO3–H2O system. We found that the fractionation slope θCaCO3–H2O is: (1) polymorph independent, i.e., we observe no significant difference between calcite, aragonite, and their mixture; (2) indistinguishable at temperatures of 10 and 27 °C but possibly slightly lower at 35 °C (the lower value may be associated with kinetic effects related to the fast precipitation observed in these

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by ERC (Grant SPADE-724097) to HPA. NRGV is currently supported by the EU-HORIZON Marie SkŁodowska Curie Fellowship H2020-MSCA-IF-2017 no. 796707. We thank the editor and two anonymous reviewers for helpful comments.

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    Current address: Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven, Leuven, Belgium.

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