Theoretical constraints on the effects of pH, salinity, and temperature on clumped isotope signatures of dissolved inorganic carbon species and precipitating carbonate minerals

Abstract

The use of carbonate 'clumped isotope' thermometry as a geochemical technique to determine temperature of formation of a carbonate mineral is predicated on the assumption that the mineral has attained an internal thermodynamic equilibrium. If true, then the clumped isotope signature is dependent solely upon the temperature of formation of the mineral without the need to know the isotopic or elemental composition of coeval fluids. However, anomalous signatures can arise under disequilibrium conditions that can make the estimation of temperatures uncertain by several degrees Celsius. Here we use ab initio calculations to examine the potential disequilibrium mineral signatures that may arise from the incorporation of dissolved inorganic carbon (DIC) species (predominantly aqueous carbonate and bicarbonate ions) into growing crystals without full equilibration with the crystal lattice.We explore theoretically the nature of clumping in the individual DIC species and the composite DIC pool under varying pH, salinity, temperature, and isotopic composition, and speculate about their effects upon the resultant disequilibrium clumping of the precipitates. We also calculate equilibrium clumped signatures for the carbonate minerals calcite, aragonite, and witherite. Our models indicate that each DIC species has a distinct equilibrium clumped isotope signature such that, (witherite). We define the calcite clumped crossover pH as the pH at which the composite D 47 (DIC pool) = D 47 (equilibrium calcite). If disequilibrium D 47 (calcite) is misinterpreted as equilibrium D 47 (calcite), it is possible to overestimate or underestimate the growth temperature by small but significant amounts. Increases in salinity lower the clumped crossover pH and may cause larger effects. Extreme effects of pH, salinity, and temperature, such as between cold freshwater lakes at high latitudes to hot hypersaline environments, are predicted to have sizeable effects on the clumped isotope composition of aqueous DIC pools.In order to determine the most reliable and efficient modeling methods to represent aqueous dissolved inorganic carbon (DIC) species and carbonate minerals, we performed convergence and sensitivity testing on several different levels of theory. We used 4 different techniques for modeling the hydration of DIC: gas phase, implicit solvation (PCM and SMD), explicit solvation (ion with 3 water molecules) and supermolecular clusters (ion plus 21 to 32 water molecules with geometries generated by molecular dynamics). For each solvation technique, we performed sensitivity testing by combining different levels of theory (up to 8 ab initio/hybrid methods, each with up to 5 different sizes of basis sets) to understand the limits of each technique. We looked at the degree of convergence with the most complex (and accurate) models in order to select the most reliable and efficient modeling methods. The B3LYP method combined with the 6-311++G(2d,2p) basis set with supermolecular clusters worked well.

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

“…The offset of 0.03 ‰ between aragonite and calcite is at the lower end of what we observe. A recent theoretical modeling effort (Hill et al, 2014) has produced similar predicted 47 values for aragonite and calcite although with more depleted (less agreement) values between predicted and observed aragonite 47 and a smaller offset between aragonite and calcite.…”

“…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).…”

Reducing Uncertainties in Carbonate Clumped Isotope Analysis Through Consistent Carbonate‐Based Standardization

“…Additional hypotheses have been advanced, including reaction temperature, acid quality (Zaruur et al, 2013), precipitation method for the carbonates, solution pH, or disequilibrium during precipitation (Hill et al, 2014). Three recent studies, however, have changed this view.…”

Calibration of the dolomite clumped isotope thermometer from 25 to 350 °C, and implications for a universal calibration for all (Ca, Mg, Fe)CO3 carbonates

“…In other words, the dispersion observed in Figure 1 might also partly reflect some true (i.e., not due to measurement and/or data treatment bias) characteristics of the carbonate clumped isotope systematics as for example: i/ isotopic disequilibrium of 13 C-18 O clumping (as observed by Affek et al, 2008;Daëron et al, 2011;Saenger et al, 2012); ii/ ∆ 47 vital effects for some biogenic minerals; iii/ variations in 13 C-18 O clumping distribution with the structure or chemical compositions of the carbonate either inside the mineral lattice (as suggested by Schauble et al, 2006, Hill et al, 2014 or into CO 2 generated by H 3 PO 4 acid digestion (as suggested by Guo et al, 2009)]. Such "material-specific explanations" for the dispersion of ∆ 47 data in Figure 1 for siderite in Fernandez et al, (2014) or carbonate groups in phosphate ; e.g., Eagle et al, 2010).…”