Assessing kinetic fractionation in brachiopod calcite using clumped isotopes

Abstract

Brachiopod shells are the most widely used geological archive for the reconstruction of the temperature and the oxygen isotope composition of Phanerozoic seawater. However, it is not conclusive whether brachiopods precipitate their shells in thermodynamic equilibrium. In this study, we investigated the potential impact of kinetic controls on the isotope composition of modern brachiopods by measuring the oxygen and clumped isotope compositions of their shells. Our results show that clumped and oxygen isotope compositions depart from thermodynamic equilibrium due to growth rate-induced kinetic effects. These departures are in line with incomplete hydration and hydroxylation of dissolved CO2. These findings imply that the determination of taxon-specific growth rates alongside clumped and bulk oxygen isotope analyses is essential to ensure accurate estimates of past ocean temperatures and seawater oxygen isotope compositions from brachiopods.

Variation in brachiopod microstructure and isotope geochemistry under low-pH–ocean acidification conditions

“…This suggests that M. venosa present non-equilibrium growth-rate-related isotope effects up to about −2.9 ‰, larger than the approx. −1.5 ‰ previously recorded by Bajnai et al (2018). Provided that this offset can be constrained, brachiopods continue to present robust archives for palaeo-temperature reconstructions.…”

“…It appears that there is about a 2 ‰ difference between the control specimen and samples from the acidification treatments (pH 7.35), with the last one being, strikingly, closer to the equilibrium with seawater DIC. Possibly, this illustrates the variability in kinetic effects (Bajnai et al, 2018), but may also be linked to changes in the source δ 13 C DIC in the control treatment. More studies are needed to fully answer this question.…”

“…Brachiopod shells are commonly used as archives for deeptime paleoenvironmental reconstructions as they potentially record the original geochemical composition of the seawater they lived in (Grossman et al, 1993;Banner and Kaufman, 1994;Mii and Grossman, 1994;Mii et al, 2001;Brand et al, 2003Brand et al, , 2011Brand et al, , 2016Jurikova et al, 2019). Several studies suggest that carbon and oxygen isotope compositions of the secondary layer of brachiopod shells, especially slow-growing species -and particularly the innermost shell parts -tend to be close to equilibrium with the ambient seawater temperature (e.g.…”

“…Popp et al, 1986;Carpenter and Lohmann, 1995;Parkinson et al, 2005;Brand et al, 2013Brand et al, , 2015Brand et al, , 2016Takayanagi et al, 2013;Yamamoto et al, 2013). Recently, Bajnai et al (2018) documented that brachiopods do not incorporate oxygen isotopes in thermodynamic equilibrium with ambient seawater, and appear to be subjected to taxon-specific growth-rate-induced kinetic effects. The documented isotopic offset appears to be relatively constant throughout the range of brachiopod shell production from cold to warm environments.…”

Oxygen Isotopic Exchange Between CO2 and Phosphoric Acid: Implications for the Measurement of Clumped Isotopes in Carbonates

“…The implication of having a different slope between Δ 47 value and temperature at 25 and 90°C means 10.1029/2019GC008209 Geochemistry, Geophysics, Geosystems that the acid fractionation correction would also vary with the fractionation factor decreasing as the absolute Δ 47 value of the carbonate becomes closer to a stochastic value. However, despite the observations of Wacker et al (2013) and the reanalysis of Defliese et al (2015), several workers have produced data that have shown there to be no dependence of the slope as a function of reaction temperature (Bajnai et al, 2018;Came et al, 2014;G. A. Henkes et al, 2013;Kelson et al, 2017).…”