The influence of temperature, pH, and growth rate on the δ18O composition of inorganically precipitated calcite

Abstract

Editor: T.M. Harrison Keywords: calcite oxygen isotopes carbonic anhydrase paleothermometry kinetic effectsThe oxygen isotope composition of carbonate minerals varies with temperature as well as other environmental variables. For carbonates that precipitate slowly, under conditions that approach thermodynamic equilibrium, the temperature-dependence of 18 O uptake is the dominant signal and the measured 18 O content can be used as a paleotemperature proxy. In the more common case where carbonate minerals grow in a regime where they are not in isotopic equilibrium with their host solution, their oxygen isotope compositions are a convolution of multiple environmental variables. Here we present results from calcite growth experiments demonstrating the occurrence of large (>2h) non-equilibrium oxygen isotope effects under conditions relevant to biogenic calcite growth and many natural inorganic systems. We show that these non-equilibrium effects vary systematically with pH and crystal growth rate. An isotopic ion-by-ion crystal growth model quantifies the competing roles of temperature, pH, and growth rate, and provides a general description of calcite-water oxygen isotope fractionation under nonequilibrium conditions. The crystal growth model results show that (1) there are both equilibrium and kinetic contributions to calcite oxygen isotopes at biogenic growth rates, (2) calcite does not directly inherit the oxygen isotope composition of DIC even at fast growth rates, (3) there is a kinetically controlled variation of about 1h per pH unit between pH = 7.7 and 9.3 at constant growth rate for inorganic calcite as well as biogenic calcite, and (4) extreme light isotope enrichments in calcite in alkaline environments are likely due to disequilibrium among DIC species in aqueous solution. The model can be extended to 13 C uptake into carbonates as well as clumped isotopes but additional data are needed to constrain the kinetic fractionation factors for carbon isotopes. The experimental and model results constitute an important step in separating the relative influence of inorganic and biologic processes on isotopic fractionation and may aid the development of new paleoproxies based on nonequilibrium effects.Published by Elsevier B.V.

Modeling the effects of diagenesis on carbonate clumped-isotope values in deep- and shallow-water settings

“…This assumption is supported by inferred rates of carbonate precipitation in deep-sea sediments (~10 -19 mol/m 2 /sec; Fantle and DePaolo, 2007), which is orders of magnitude lower than previously estimated rates (~10 -12 mol/m 2 /sec) required to form carbonate minerals in carbon, oxygen, and clumped isotopic equilibrium with water regardless of the temperature or pH of the system (Watkins et al, 2013;Watkins et al, 2014;Watkins and Hunt, 2015).…”

“…We note that  carb-H2O values can vary as a function of parameters other than temperature including carbonate growth rates and solution pH (e.g., McCrea, 1950;Kim and O'Neil, 1997;Zeebe, 1999;Dietzel et al, 2009;Watkins et al, 2014) and organism-specific effects (i.e., 'vital effects') if the carbonate is biogenic in origin (e.g., Erez, 1978;Weiner and Dove, 2003). However, in past models, only the temperature-dependent effect has been considered (Schrag et al, 1995).…”

“…The temperature dependence of  carb-H2O is taken from Watkins et al (2013) as given in Figure 6 of that study and equation 19 of Watkins et al (2014). We use this  carb-H2O calibration as it expresses the fractionation factor for carbonate formation at sufficiently slow rates that the oxygen isotopes of carbonate and water are in equilibrium.…”

Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China

“…At present, there is still no clear evidence for any of these mechanisms. Laboratory experiments also reveal that solution pH, alkalinity (Spero et al, 1997), carbonate precipitation rate (Watkins et al, 2014), and salinity (Adkins et al, 2002) could also influence the δ 18 O compositions. However, these effects are modest, only 1-2‰ at most.…”

What do we really know about early diagenesis of non-marine carbonates?

“…In-situ monitoring and data from natural samples demonstrate off-sets as a result of metabolic (Andrews et al, 2004) or kinetic effects during non-equilibrium precipitation in relation to e.g. precipitation rate variations, a process very common in several non-marine carbonate depositional settings such as caves and springs (Coplen, 2007;Dietzel et al, 2009;Kele et al, 2011Kele et al, , 2015Gabitov et al, 2012;Riechelmann et al, 2013;Kluge et al, 2014;Watkins et al, 2014). Carbon isotopic compositions relate to the available Dissolved Inorganic Carbon (DIC) source (Fig.…”