Carbon and Oxygen Isotope Effects in Synthesized Carbonates at 25 °C

Abstract

Carbonate minerals have been abundant throughout Earth’s geological history and the carbon and oxygen isotope ratios of carbonates can be used for paleoclimate reconstruction based upon the recognized stable isotopic relationship with the environmental factors. However, their accuracy is obscured by “non-equilibrium isotope effects” caused by physicochemical factors, such as solution chemistry, pH and precipitation rate. This study aimed to better understand these factors to improve the robustness of isotope-based paleotemperature proxy and assist in providing a reference frame for future research. Carbonates were synthesized using passive CO2(g) degassing at two different pH levels (~8.2 and ~11.07) through the dissolution of 5, 15 and 25 mmolal sodium bicarbonate (NaHCO3) or sodium carbonate (NA2CO3) and calcium chloride dihydrate (CaCl2•2H2O) at 25 ± 0.1 oC. The 1000ln18α(CaCO3-H2O) and 1000ln13α(CaCO3-DIC) values were then compared with established isotopic equilibrium values for oxygen and carbon. Samples were also synthesized in the presence of various concentrations of carbonic anhydrase (CA) and it was found that this enzyme may not influence kinetic isotope effects at higher precipitation rates. A positive, concentration based trend was found for the mid-pH solutions between 1000ln13α(CaCO3-DIC) and 1000ln18α(CaCO3-H2O) values which began at the oxygen isotopic equilibrium value proposed by Kim and O’Neil (1997). This trend deviated upwards towards that of Coplen (2007) due to the kinetic influence of precipitation rate and degassing caused by the production of CO2(aq) as a by-product of the aforementioned reaction. The high pH solutions followed an opposite trend, with 1000ln13α(CaCO3-DIC) continuing the enrichment trend but 1000ln18α(CaCO3-H2O) values declining and may have been caused by CO2(aq) not being produced in the high pH reactions. The DIC equilibration time at this pH took 7 days as shown by Kim et al. (2006) and not 45 days as discussed in Beck et al. (2005).