Internal Water Facilitates Thermal Resetting of Clumped Isotopes in Biogenic Aragonite

Abstract

Primary marine and terrestrial carbonates consist predominantly of two polymorphs of calcium carbonate: calcite and aragonite. The relative abundance of these minerals has varied over 100 myr timescales (Sandberg, 1983;Zhang et al., 2020) due to changes in water chemistry and surface temperature (Adabi, 2004; Balthasar & Cusack, 2014). Despite being a common mineral at the Earth's surface, aragonite is metastable at surface temperatures and pressures and only becomes the more stable crystalline arrangement with significant substitution of ions (Carlson, 1980) and/or at elevated pressure at burial depths (Hacker, 2005). Because of this metastability, aragonite is more susceptible to chemical alteration, and thus its preservation is considered an indicator of pristine geochemistry (Stahl & Jordan, 1969). There is considerable variability in the response of different aragonite materials to alteration processes, which are related to differences in chemical composition and porosity (Pederson et al., 2020). The alteration typically involves the dissolution of aragonite and reprecipitation of the more stable polymorph calcite (Bischoff & Fyfe, 1968), a process termed neomorphism (Folk, 1965).

Analytical Artefacts Preclude Reliable Isotope Ratio Measurement of Internal Water in Coral Skeletons

“…A similar shift was also observed in an experiment on the IRMS system in Amsterdam by Nooitgedacht et al . (2021), which showed an enrichment in heavy isotopes for a coral sample that was heated for 90 min at 175 °C prior to analysis compared with a sample that was not heated prior to analysis.…”

“…2019, Nooitgedacht et al . 2021). However, the temperatures at which these processes were observed (130–175 °C) are somewhat higher than the temperature to which samples are exposed in this study.…”

Fingerprinting Kinetic Isotope Effects and Diagenetic Exchange Reactions Using Fluid Inclusion and Dual‐Clumped Isotope Analysis

“…Each sample's carbonate clumped isotope composition was typically measured with 8–9 replicates, yielding fully error propagated 2SE uncertainties of 0.005–0.010‰ for ∆ 47 and 0.017–0.032‰ for ∆ 48 (Table 1, Figure 3). Fluid inclusion δ 2 H values published by Nooitgedach, van der Lubbe, Ziegler, and Staudigel (2021) are shown in Table 1, these show no significant change during heating for Moroccan aragonite and the bivalve, and a positive change for the belemnite and coral samples. Results for fluid/carbonate δ 18 O measurements and corresponding paleotemperatures are also presented in Table 1.…”

“…The aragonitic samples (all samples excluding belemnites) were treated one of three ways: termed “heated” (175°C for 90 min, then analyzed for fluid inclusions at 110°C), “unheated” (analyzed for fluid inclusions at 110°C), and “uncrushed” (simply powdered in mortar and pestle then analyzed for clumped isotopes). Note that we are using the terminology used by Nooitgedach, van der Lubbe, Ziegler, and Staudigel (2021), however both the “heated” and “unheated” samples were, in fact heated to 110°C, whereas the “uncrushed” samples were never heated to such a degree. The purpose of the “uncrushed” treatment is to determine the suitability of the commonly used crushing cells, (hereafter referred to as a “crusher apparatus”) for fluid inclusion water stable isotope analysis of biogenic carbonates.…”

Fluid inclusions and dual-clumped isotopes: is it checks and balances all the way down?

“…Nevertheless, natural materials often yield discrepant values for primary fluids from these two methods 1 . Experiments in which biogenic minerals are heated in a dry environment have shown that internal water-mineral equilibration can occur, this happens in such a way that these two recorders of temperature/fluid composition may give discrepant values 2 . These alteration processes are problematic for deep-time paleoclimate reconstruction, as they do not necessarily alter mineralogy or microstructure, and are thus difficult to identify in unknown samples 3,4 .…”