Oxygen isotope composition of waters recorded in carbonates in strong clumped and oxygen isotopic disequilibrium

• Main Authors: C. Thaler, A. Katz, M. Bonifacie, B. Ménez, M. Ader
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-04-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/17/1731/2020/bg-17-1731-2020.pdf
• ISSN: 1726-4170; 1726-4189

<p>Paleoenvironmental reconstructions, which are mainly retrieved from oxygen isotope (<span class="inline-formula"><i>δ</i>¹⁸O</span>) and clumped isotope (<span class="inline-formula">Δ₄₇</span>) 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 <span class="inline-formula"><i>δ</i>¹⁸O</span> and <span class="inline-formula">Δ₄₇</span> offsets from isotopic equilibrium values is largely overlooked. Additionally, in carbonates formed in isotopic equilibrium, the use of the carbonate <span class="inline-formula"><i>δ</i>¹⁸O</span> signature as a paleothermometer relies on our knowledge of the paleowaters' <span class="inline-formula"><i>δ</i>¹⁸O</span> value, which is often assumed. Here, we report the largest <span class="inline-formula">Δ₄₇</span> offsets observed to date (as much as <span class="inline-formula">−0.270</span>&thinsp;‰), measured on microbial carbonates that are strongly linked to carbonate <span class="inline-formula"><i>δ</i>¹⁸O</span> offsets (<span class="inline-formula">−25</span>&thinsp;‰) from equilibrium. These offsets are likely both related to the microorganism metabolic activity and yield identical erroneous temperature reconstructions. Unexpectedly, we show that the <span class="inline-formula"><i>δ</i>¹⁸O</span> value of the water in which carbonates precipitated, as well as the water–carbonate <span class="inline-formula"><i>δ</i>¹⁸O</span> fractionation dependence on temperature at equilibrium, can be retrieved from these paired <span class="inline-formula"><i>δ</i>¹⁸O</span> and <span class="inline-formula">Δ₄₇</span> disequilibrium values measured in carbonates. The possibility to retrieve the <span class="inline-formula"><i>δ</i>¹⁸O</span> 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.</p>