Beyond temperature: Clumped isotope signatures in dissolved inorganic carbon species and the influence of solution chemistry on carbonate mineral composition

Abstract

Clumped-isotope" thermometry is an emerging tool to probe the temperature history of surface and subsurface environments based on measurements of the proportion of 13 C and 18 O isotopes bound to each other within carbonate minerals in 13 C 18 O 16 O 2 2À groups (heavy isotope "clumps"). Although most clumped isotope geothermometry implicitly presumes carbonate crystals have attained lattice equilibrium (i.e., thermodynamic equilibrium for a mineral, which is independent of solution chemistry), several factors other than temperature, including dissolved inorganic carbon (DIC) speciation may influence mineral isotopic signatures. Therefore we used a combination of approaches to understand the potential influence of different variables on the clumped isotope (and oxygen isotope) composition of minerals.We conducted witherite precipitation experiments at a single temperature and at varied pH to empirically determine 13 C-18 O bond ordering (D 47 ) and d 18 O of CO 3 2À and HCO 3 À molecules at a 25°C equilibrium. Ab initio cluster models based on density functional theory were used to predict equilibrium 13 C- 18 O bond abundances and d 18 O of different DIC species and minerals as a function of temperature. Experiments and theory indicate D 47 and d 18 O compositions of CO 3 2À and HCO 3 À ions are significantly different from each other. Experiments constrain the D 47 -d 18 O slope for a pH effect (0.011 ± 0.001; 12 P pH P 7). Rapidly-growing temperate corals exhibit disequilibrium mineral isotopic signatures with a D 47 -d 18 O slope of 0.011 ± 0.003, consistent with a pH effect. Our theoretical calculations for carbonate minerals indicate equilibrium lattice calcite values for D 47 and d 18 O are intermediate between HCO 3 À and CO 3 2À. We analyzed synthetic calcites grown at temperatures ranging from 0.5 to 50°C with and without the enzyme carbonic anhydrase present. This enzyme catalyzes oxygen isotopic exchange between DIC species and is present in many natural systems. The two types of experiments yielded statistically indistinguishable results, and these measurements yield a calibration that overlaps with our theoretical predictions for calcite at equilibrium. The slow-growing Devils Hole calcite exhibits D 47 and d 18 O values consistent with lattice equilibrium.Factors influencing DIC speciation (pH, salinity) and the timescale for DIC equilibration, as well as reactions at the mineral-solution interface, have the potential to influence clumped-isotope signatures and the d 18 O of carbonate minerals. In fast-growing carbonate minerals, solution chemistry may be an important factor, particularly over extremes of pH and salinity. If a crystal grows too rapidly to reach an internal equilibrium (i.e., achieve the value for the temperature-dependent mineral lattice equilibrium), it may record the clumped-isotope signature of a DIC species (e.g., the temperature-dependent equilibrium of HCO 3 À ) or a mixture of DIC species, and hence record a disequilibrium mineral composition. For extremely slow-growing cry...

Carbonate “clumped” isotope signatures in aragonitic scleractinian and calcitic gorgonian deep-sea corals

“…Numerous biogenic carbonate materials have been studied to assess their agreement with inorganic and theoretical studies Thiagarajan et al, 2011;Saenger et al, 2012;Eagle et al, 2013;Henkes et al, 2013). The scleractinian corals in this study agree well with the inorganic calibrations of Ghosh et al (2006), Zaarur et al (2013), andTripati et al (2015), while the gorgonian corals show agreement with the Dennis and Schrag (2010), Tang et al (2014), and Kluge et al (2015) inorganic calibrations (e.g., Fig. 4).…”

“…To date, at least seven synthetic calibrations have been published Dennis and Schrag, 2010;Zaarur et al, 2013;Tang et al, 2014;Kluge et al, 2015;Tripati et al, 2015). Numerous biogenic carbonate materials have been studied to assess their agreement with inorganic and theoretical studies Thiagarajan et al, 2011;Saenger et al, 2012;Eagle et al, 2013;Henkes et al, 2013).…”

“…Theoretical modeling of 63 for different carbonate minerals Hill et al, 2014;Tripati et al, 2015) has been combined with theoretical acid digestion fractionations to give predicted 47 values for a variety of carbonate minerals. These predictions generally agree with measured inorganic and biogenic studies; however, they are more depleted in 47 for a given temperature.…”

“…As these corals were recovered from the same environment, analyzed by the same methods and mass spectrometer during the same analytical period, and subject to the same data processing, they represent an opportunity to explore the potential role of mineralogy and biology in governing clumped isotope signatures. We discuss results in the context of recent studies including of acid digestion fractionation and theoretical models and experimental data that have been used to constrain differences in clumped isotope com-position between different dissolved inorganic carbon (DIC) species (Hill et al, 2014;Tripati et al, 2015), as it has been suggested that in cases where a carbonate mineral may inherit an isotopic signature of DIC, then factors effecting DIC speciation such as pH, salinity, and temperature may also influence carbonate clumped isotope signatures (Hill et al, 2014;Tripati et al, 2015).…”

“…Following the initial calibration of synthetic calcites and some coral taxa , several studies have focused on in-depth calibrations of biogenic carbonates that represent potential paleoclimate proxies. Proxy material calibrations thus far have included aragonitic scleractinian zooxanthellate corals Saenger et al, 2012;Tripati et al, 2015), aragonitic scleractinian non-zooxanthellate deep-sea corals Thiagarajan et al, 2011), aragonitic otoliths (Ghosh et al, 2007), calcitic and aragonitic foraminifera , mollusks and brachiopods (Came et al, 2007(Came et al, , 2014Eagle et al, 2013;Henkes et al, 2013) and land snails (Zaarur et al, 2011;Eagle et al, 2013), calcitic speleothems (Affek et al, 2008;Daëron et al, 2011), bioapatite , and calcitic microbialites (Petryshyn et al, 2015). It has been noted that calibration data on different biogenic carbonates generated in the same laboratory at Caltech and using similar analytical methods produce a relationship between temperature and 47 values that was similar to the initial inorganic calcite calibration Eagle et al, 2013).…”