Abstract
Abstract. Paleoenvironmental reconstructions, which are mainly retrieved from oxygen isotope (δ18O) and clumped isotope (Δ47) compositions of carbonate minerals, are compromised when carbonate precipitation occurs in isotopic disequilibrium. To date, knowledge of these common isotopic disequilibria, known as vital effects in biogenic carbonates, remains limited, and the potential information recorded by δ18O and Δ47 offsets from isotopic equilibrium values is largely overlooked. Additionally, in carbonates formed in isotopic equilibrium, the use of the carbonate δ18O signature as a paleothermometer relies on our knowledge of the paleowaters' δ18O value, which is often assumed. Here, we report the largest Δ47 offsets observed to date (as much as −0.270 ‰), measured on microbial carbonates that are strongly linked to carbonate δ18O offsets (−25 ‰) from equilibrium. These offsets are likely both related to the microorganism metabolic activity and yield identical erroneous temperature reconstructions. Unexpectedly, we show that the δ18O value of the water in which carbonates precipitated, as well as the water–carbonate δ18O fractionation dependence on temperature at equilibrium, can be retrieved from these paired δ18O and Δ47 disequilibrium values measured in carbonates. The possibility to retrieve the δ18O value of paleowaters, sediments' interstitial waters or organisms' body water at the carbonate precipitation loci, even from carbonates formed in isotopic disequilibrium, opens long-awaited research avenues for both paleoenvironmental reconstructions and biomineralization studies.
Highlights
Oxygen isotope composition (δ18O) paired with clumped isotope composition ( 47) of carbonate minerals is increasingly used for reconstructing paleoenvironmental or diagenetic conditions (Ghosh et al, 2006; Henkes et al, 2018; Mangenot et al, 2018a, b)
Thaler et al.: δ18O signature of waters recorded in disequilibrium carbonates erful paleothermometers as empirical calibrations taking vital effects into account allow temperature reconstructions, it has become crucial to determine if the δ18O and 47 disequilibria observed in carbonates as diverse as those found in coral reefs (Saenger et al, 2012), brachiopods (Bajnai et al, 2018), microbialites and methane seep carbonates (Loyd et al, 2016) along with speleothems (Affek et al, 2014) could be explained by oxygen isotope disequilibria occurring in dissolved inorganic carbon (DIC) involved in carbonate precipitation
A paired 47 and δ18Ocarbonate disequilibrium indicates that carbonates have precipitated in a dynamic environment where DIC and water did not reach isotopic equilibrium
Summary
Oxygen isotope composition (δ18O) paired with clumped isotope composition ( 47) of carbonate minerals is increasingly used for reconstructing paleoenvironmental or diagenetic conditions (Ghosh et al, 2006; Henkes et al, 2018; Mangenot et al, 2018a, b). Thaler et al.: δ18O signature of waters recorded in disequilibrium carbonates erful paleothermometers as empirical calibrations taking vital effects into account allow temperature reconstructions, it has become crucial to determine if the δ18O and 47 disequilibria observed in carbonates as diverse as those found in coral reefs (Saenger et al, 2012), brachiopods (Bajnai et al, 2018), microbialites and methane seep carbonates (Loyd et al, 2016) along with speleothems (Affek et al, 2014) could be explained by oxygen isotope disequilibria occurring in dissolved inorganic carbon (DIC) involved in carbonate precipitation In this case, δ18O and 47 disequilibria in biogenic carbonates would record information, unavailable yet, on the physiological characteristics of carbonate-forming organisms. We later show how and to what extent this can be applied to previously published cases of oxygen isotopic offsets from equilibrium values in both biogenic and abiotic carbonates
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