Altered carbonate shells in the experimental diagenesis and usefulness of Raman spectroscopy as a screening tool

Abstract

In paleoenvironmental reconstructions using fossil calcifiers such as brachiopods, the "high-temperature problem" arises in which unrealistically high paleo sea temperatures are estimated. The burial diagenetic alteration of carbonate fossils is thought to be the key to solving this problem. This study aimed to investigate the behavior of carbonate shells under high temperatures and high pressures using the method of experimental diagenesis by Fujioka et al. (2025) in which modern samples are altered experimentally. The microstructures and chemical compositions (stable isotope ratios and trace element concentrations) of pristine and post-experiment slabs were compared. Additionally, micro-Raman spectroscopic analyses that look promising as a novel screening tool were conducted. In for isotopic compositions, the carbon and oxygen isotope values decreased due to isotope exchange via diagenetic fluid. In Raman spectra, v₁ peak of the carbonate ion showed the higher shift in peak center and narrower peak width at half value, suggesting the usefulness of Raman spectroscopy as a screening tool.