Assessing the belemnite archive for Mesozoic seawater temperature reconstruction by clumped isotope thermometry

The reconstruction of Mesozoic seawater temperature is valuable to further understand the link between atmospheric carbon dioxide concentration and earth surface temperature variation. Belemnites are an effective archive for this purpose, owing to their distribution over an extensive latitudinal range in Jurassic and Cretaceous seas. However, uncertainty remains as to whether belemnites precipitate rostrum calcite in oxygen and clumped isotope equilibrium with surrounding seawater. Furthermore, recent study indicates that belemnite calcite may be more susceptible to thermal resetting of ∆₄₇ values through oxygen isotope exchange with internal water than other calcites (Looser et al., 2023).

Here, we demonstrate identification of thermal resetting of ∆₄₇ values in belemnite calcite using a combination of measured δ¹⁸O of fluid inclusions, δ¹⁸O and ∆₄₇ values of belemnite calcite. We then apply dual carbonate clumped isotope thermometry (i.e. the simultaneous measurement of ∆₄₇ and ∆₄₈), to assess the potential importance of kinetic limitations during belemnite biomineralization (Bajnai et al., 2020; Guo, 2020). We demonstrate that Maastrichtian agebelemnites sampled at 4 sites with a range of paleolatitudes from 34 to 45 ˚N, yield ∆₄₇ and ∆₄₈ values that fall on the experimentally derived equilibrium calibration of Fiebig et al., (2021) indicating that rostrum calcite precipitated in clumped isotope equilibrium. ∆₄₇-derived temperatures are combined with other proxy-based reconstructions of Maastrichtian seawater temperature to examine its latitudinal variability.