Out of the oven and into the fire: Unexpected preservation of the seasonal δ¹⁸O cycle following heating experiments on shell carbonate

Oxygen isotope values of shell carbonate sampled along the growth trajectory of fossil mollusks provide one of the few types of proxy data available for reconstructing seasonal temperature variation in Earth's past. Understanding how diagenesis affects the preservation of seasonal cycles is key to their interpretation in a paleoclimate context. An assumption common in the literature, albeit tenuous, is that the alteration of an aragonite shell to calcite will homogenize intra-shell stable isotope variation, thus attenuating or erasing any seasonal cycle and resetting bulk shell values. Demonstration of regular variation in δ¹⁸O values across growth bands in a still-aragonite mollusk is therefore generally interpreted to reflect retention of the primary environmental signal. However, this implied assumption remains untested, and experimental studies have demonstrated that heating can decrease shell δ¹⁸O values with negligible mineralogical transformation. Here we investigate how dry heat impacts both the geochemistry and mineralogy of shell aragonite by roasting time-equivalent shell sections of a modern bivalve, Spisula solidissima, at 200 °C for intervals ranging from 2 hours to 2 weeks, and comparing seasonal oxygen isotope variations recovered from the same one-year period of shell accretion with those from untreated shell. Oxygen isotope values from heated shell sections decline by as much as 1.5‰ across all parts of the ~2.5‰ annual cycle, and degree of ¹⁸O depletion decreases logarithmically with time heated. Neither Raman spectroscopy nor X-ray diffraction indicate mineralogic change in association with heating. Scanning electron microscopy reveals that heating results in some intra-crystal pitting and subtle smoothing of crystal edges, but heated shell lacks any textural evidence for inversion to calcite, suggesting that fine-scale isotopic exchange may take place with shell-bound water vapor and/or organic matter. The surprising magnitude of this shift in the absence of mineralogic transformation highlights the potential impact of diagenetic processes on the preservation of paleotemperature signals, seasonal or otherwise, in accretionary biogenic carbonates, and suggests a need for more stringent guidelines in assessing the preservation of these materials during the employment of common sclerochronological methodologies.