Abstract
Understanding the Earth’s climate system during past periods of high atmospheric CO2 is crucial for forecasting climate change under anthropogenically-elevated CO2. The Mesozoic Era is believed to have coincided with a long-term Greenhouse climate, and many of our temperature reconstructions come from stable isotopes of marine biotic calcite, in particular from belemnites, an extinct group of molluscs with carbonate hard-parts. Yet, temperatures reconstructed from the oxygen isotope composition of belemnites are consistently colder than those derived from other temperature proxies, leading to large uncertainties around Mesozoic sea temperatures. Here we apply clumped isotope palaeothermometry to two distinct carbonate phases from exceptionally well-preserved belemnites in order to constrain their living habitat, and improve temperature reconstructions based on stable oxygen isotopes. We show that belemnites precipitated both aragonite and calcite in warm, open ocean surface waters, and demonstrate how previous low estimates of belemnite calcification temperatures has led to widespread underestimation of Mesozoic sea temperatures by ca. 12 °C, raising estimates of some of the lowest temperature estimates for the Jurassic period to values which approach modern mid-latitude sea surface temperatures. Our findings enable accurate recalculation of global Mesozoic belemnite temperatures, and will thus improve our understanding of Greenhouse climate dynamics.
Highlights
Understanding the Earth’s climate system during past periods of high atmospheric CO2 is crucial for forecasting climate change under anthropogenically-elevated CO2
These samples are compared to other aragonites from the same horizon in order to evaluate the putative temperature record based upon the oxygen isotopic composition of well-preserved marine fossils, and thereby to resolve which oxygen thermometry equation is most appropriate for belemnite rostra in Mesozoic paleoclimate studies
Belemnite rostral calcite shows extremely low Mn/Ca and Fe/Ca values (Supplementary Fig. S12), in agreement with Price et al.[42], and electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) electron dispersive spectra (EDS) element maps show that no perceptible diagenetic alteration occurred in the non-apical areas of the belemnite rostra
Summary
Understanding the Earth’s climate system during past periods of high atmospheric CO2 is crucial for forecasting climate change under anthropogenically-elevated CO2. Belemnites, emerging in the earliest Jurassic, were squid-like cephalopods (Mollusca) that built their internal skeleton from calcite and aragonite, and went extinct at the end of the Cretaceous Their ubiquity within Jurassic and Cretaceous seas, and the high preservation potential of their low-Mg calcite skeleton (rostra), make them a favoured target for temperature reconstructions via oxygen isotope thermometry[3,4,5,6,7,8,9,10,11,12]. We present new clumped isotope data derived from co-occurring calcite and aragonite in individual exceptionally well preserved belemnites (Cylindroteuthis)[42] from the Callovian-aged Christian Malford Lagerstätte (Fig. 1) These samples are compared to other (non-belemnite) aragonites from the same horizon in order to evaluate the putative temperature record based upon the oxygen isotopic composition of well-preserved marine fossils, and thereby to resolve which oxygen thermometry equation is most appropriate for belemnite rostra in Mesozoic paleoclimate studies. Our warm temperatures are more in line with the geographical distributions of temperature-sensitive fossil flora and fauna, as well as the results of climate models with increased CO2 levels, and demonstrate that at least several common belemnite genera lived neither deep in the water column nor in hypersaline conditions
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