Near Equilibrium ¹³C¹⁸O Bonding During Inorganic Calcite Precipitation Under Chemo‐Stat Conditions

Abstract

We report results of 13C/12C, 18O/16O, and 13C18O “clumped” isotope analyses from a series of calcite precipitation experiments from aqueous solutions under laboratory conditions. Chemo‐stat precipitation experiments were performed to synthetically form calcite from aqueous solution onto 43Ca‐labeled calcite seed crystals. Formation rate was controlled during the experiments to investigate the effect of precipitation rate and temperature on 13C18O bonding in calcite, where rates ranged from 10−6.88 to 10−8.20 mol m−2 s−1 at three temperatures (10, 20, and 30°C). No relation was observed between precipitation rate and 13C18O bonding proportion over the range of precipitation rates used. The relation between Δ47 and temperature produced was comparable to calibration studies which report a relatively high sensitivity of 13C18O bonding to temperature over the range investigated. Comparing solution conditions across multiple experimental data sets indicates an inverse relation between saturation state and 13C18O bonding, where high super‐saturation conditions are likely to be furthest from equilibrium 13C18O partitioning. Carbon fractionation between calcite and HCO3(aq)− was found to be a temperature independent value of +1.6‰. The temperature‐dependent calcite‐water 18O/16O fractionation relation determined in this study is slightly different (larger αcalcite‐H2normalO value) than those measured in several previous investigations. Significantly, we observe a dependence of the 18O/16O isotope fractionation factor on growth rate. Taken together, these findings suggest carbonate growth in our experiments approached equilibrium more closely than previous experiments of this type, yet did not achieve full O isotope equilibrium.

Stable and clumped isotope characterization of authigenic carbonates in methane cold seep environments

“…The Caltech clumped isotope lab consistently produces calibration lines with steeper slopes (Ghosh et al, 2006;Tripati et al, 2010;Thiagarajan et al, 2011;Eagle et al, 2013;Hines et al, 2019). Additionally, the slowest growth inorganic calcite in an inorganic calcite experiment show that  47 values are consistent with the steep slope  47 -temperature calibration of Ghosh et al 2006 (see Figure 6 and 7 in (Levitt et al, 2018)). We use two "steep-slope" calibrations (Ghosh et al, 2006;Thiagarajan et al, 2011) updated into the absolute reference frame to correct our data.…”

“…Previous work has shown that the measured oxygen isotopic fractionation of carbonates may vary depending on the rate of precipitation (McConnaughey, 1989;Watkins and Hunt, 2015;Staudigel and Swart, 2018) or may also be independent of precipitation rate (Tarutani et al, 1969;Kim and O'Neil, 1997). Similarly for clumped isotopes, an ion-by-ion growth model predicts there to be a rate dependent kinetic isotope effect on clumped isotopes (Watkins and Hunt, 2015), while observations of inorganic laboratory calcite indicate that  47 is independent of precipitation rate (Tang et al, 2014;Levitt et al, 2018). Disentangling the effects of carbonate precipitation rate on oxygen and clumped isotopes is beyond the scope of our study but is a promising line of future research.…”

Kinetic fractionation of the isotope composition of 18O, 13C, and of clumped isotope 18O13C in calcite deposited to speleothems. Implications to the reliability of the 18O and Δ47 paleothermometers

“…It is important to note that most of the data points and calibration lines are below the Daeron-Coplen equilibrium calibration. This is also the case for the data of feng et al (2014) and also valid for experiments with synthetic calcite with growth rates of several 10 -9 mmol cm -2 s -1 (Levitt et al 2018) as can be seen from fig. 6 in Johnston et al (2013).…”

“…The values decrease with increasing precipitation rate. Recently Levitt et al (2018) reported of synthetical calcite precipitated close to equilibrium with rates between 10 -9 and 10 -8 mmol cm -2 s -1 . Watkins et al (2014) proposed a model that rep- Fig.…”

Seafloor weathering and the oxygen isotope ratio in seawater: Insight from whole-rock δ18O and carbonate δ18O and Δ47 from the Troodos ophiolite

“…those of Kim and O'Neil, 1997) kinetic effects lead to nonequilibrium calcite  18 O (e.g. Coplen, 2007;Watkins et al, 2013;Levitt et al, 2018).…”