Complex Biomineralization Pathways of the Belemnite Rostrum Cause Biased Paleotemperature Estimates

Paleotemperatures based on ?18O values derived from belemnites are usually “too cold” compared to other archives and paleoclimate models. This temperature bias represents a significant obstacle in paleoceanographic research. Here we show geochemical evidence that belemnite calcite fibers composed of two distinct low-Mg phases (CP1, CP2). Phase-specific situ measurement revealed systematic offset up 2‰ (~8 °C), showing lead–lag signal between both analyses spaced less than 25 µm apart total fluctuation 3.9‰ (~16 °C) within 2 cm × portion Megateuthis (Middle Jurassic) rostrum. We explain this the for by complex biomineralization The biologically controlled formation CP1 is approximating isotope fractionation conditions with ambient seawater be used calculation. In contrast, CP2 indicates characteristic non-isotope equilibrium due its via an amorphous Ca-Mg carbonate precursor at high solid-to-liquid ratio, i.e., limited amounts water were available during transformation calcite, thus suggesting lower temperatures. occludes syn vivo primary pore space left after CP1. Our findings support paleobiological interpretations as shelf-dwelling, pelagic predators call reassessment reconstructions stable data.