Effects of artificial diagenetic alteration on the microstructure, isotopic composition, and metal element concentrations in brachiopod shells

Abstract

In this study, we investigated the effect of burial diagenesis on the microstructure and isotopic (δ¹³C and δ¹⁸O) and chemical compositions of modern brachiopod (Terebratulina crossei) shells through controlled artificial diagenesis experiments. The shells were placed in sediment and artificial seawater mixtures and subjected to experimental conditions of 125 °C and 75 MPa for 720 h. Statistically significant changes were observed in the isotopic and chemical compositions of the shells before and after the artificial experiments. Notable findings include decreases in δ¹⁸O values under all four experimental conditions; increases in Mn concentration in the carbonate powder-artificial seawater mixture, quartz powder-artificial seawater mixture, and artificial seawater; and decreases in the δ¹³C values in the carbonate powder-artificial seawater mixture and sandstone powder-artificial seawater mixture. The observed δ¹⁸O variations were predominantly influenced by temperature rather than by the isotopic and chemical compositions of the ambient sediments and fluids. The Mn concentration increased when the shells were placed in materials relatively poor in Mn (i.e., carbonate, quartz, and artificial seawater). This suggests that Mn originated from organic matter within the shells. The decrease in δ13C values is likely attributable to the thermal degradation of organic matter in the shells. Scanning electron microscopy (SEM) revealed minimal evidence of shell microstructure degradation and destruction due to the experiments. However, transmission electron microscopy (TEM) revealed traces of dissolution that were not discernible using conventional SEM. These findings underscore the importance of nanoscale analysis in future investigations of brachiopod shell-based paleoenvironmental reconstructions.