Clumped isotope reordering kinetics in rudist fossils: Impact of burial history and paleoclimatic implications

The low-Mg calcite shells of bivalves can record paleotemperatures over geological time scales. Stable oxygen isotope thermometry (δ¹⁸O) is a widely used paleotemperature proxy for past oceans. However, it relies on an estimate of the δ¹⁸O of the calcification fluid. Carbonate clumped isotope analysis, a paleothermometer independent of the δ¹⁸O of water, overcomes this issue and is a valuable tool for reconstructing Phanerozoic paleotemperatures. In both oxygen and clumped isotope paleothermometry, preservation of the original isotopic signal is essential for paleoenvironmental reconstructions. Apart from recrystallization, clumped isotope resetting at elevated burial temperatures can affect the clumped isotope composition. In this study, we analyzed Late Cenomanian to Santonian rudist bivalves from two regions with different time–temperature evolution (Friuli-Venezia Giulia, in Italy and southern Istria, in Croatia). The reconstructed Δ₄₇-based temperatures (T(Δ₄₇)) differ significantly between the two regions, ranging from 34 to 41 °C in the Istria region and 73 to 101 °C in the Friuli-Venezia Giulia region. Thorough screening excluded any significant contribution of diagenetic alteration of the chemical composition and shell texture, pointing towards clumped isotope resetting in the rudist fossils from Friuli-Venezia Giulia.

Modeling solid-state reordering effects in the Friuli-Venezia Giulia rudists suggests that the rudist T(Δ₄₇) should be interpreted cautiously and demonstrates the need for research on fossil-specific reordering kinetics. The available data do not permit a definite conclusion on rudist reordering kinetics. However, the rudist data correspond most closely to the reordering kinetics similar to that of the belemnites, resulting in resetting at low burial temperatures and at a faster rate. These results further demonstrate the variability in clumped isotope reordering kinetics in calcite fossils.

Excluding the effect of clumped isotope resetting on the Istrian rudists allowed for δ¹⁸O_seawater reconstruction. The resulting values of −0.1 to 1.4 ‰ are within the range observed in modern oceans but are high compared with the assumed Late Cretaceous value of −1 ‰. These results point out the importance of using δ¹⁸O_seawater-independent paleothermometers, especially in the shallow marine environment.