The Cretaceous Earth, with its greenhouse climate and absence of major ice caps in the polar regions, represents an extreme scenario for modeling future warming. Despite considerable efforts, we are just at the verge of fully understanding the conditions of a warm Earth, and better, more extensive proxy evidence is needed to solve existing discrepancies between the applied temperature proxies. In particular, the Maastrichtian temperature trends are controversial, since data indicate cooling in the South Atlantic and contemporary warming of the North Atlantic. The “heat piracy” hypothesis involves northward heat transport to midlatitudes via oceanic currents and is used to explain the contrasting polar cooling/warming patterns. Here, we present Δ47 and δ18O data from nine coccolith-enriched samples from a shallow core taken from the Danish Basin (Chalk Sea), representing a key location at the northern mid-latitudes. Based on Δ47 data of coccolith-enriched material, sea-surface temperatures for the late Campanian–Maastrichtian ranged from 24 °C to 30 °C, with an average of 25.9 °C ± 2 °C. This is 4–6 °C higher than estimates based on Δ47 of bulk samples and 8–10 °C higher than reported temperatures based on bulk δ18O data from the same core. However, these higher temperature estimates are lower, but overall in line with estimates of Late Cretaceous tropical sea-surface temperatures from TEX86 (tetraether index of 86 carbons), when considering latitudinal differences. The study highlights the potential of clumped isotope paleothermometry on coccoliths as a valid, reliable proxy with which to reconstruct sea-surface temperatures.

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Chalk is usually thought to be a homogeneous sediment with a relatively simple early diagenetic history. Here, clumped isotope analyses of samples from a core of Campanian Maastrichtian chalk are presented, indicating that material smaller than 5 µm has a different origin than the coccolith-dominated coarser fraction. The smallest size fraction (1 to 5 µm) of chalk is dominated by calcite particles without a distinct morphology (micarbs). Clumped isotope data of the micarbs reveals formation temperatures of 14 to 18°C which is 8 to 10°C colder than those derived from coeval coccoliths. The micarbs are interpreted as the product of calcite neoformation, precipitated in the uppermost part of the sediment column (100 metres below sea floor) and linked to early dissolution of aragonitic fossils. These findings prove that early cements can be an abundant component in chalk, and thus challenge the common notion that chalk is always largely composed of calcareous nannofossils, and differs only in terms of minor constituents and degree of lithification.

Carbonate clumped isotope analysis measures the distribution of multiply-substituted isotopologues of CO 2 (e.g., 13 C 18 O 16 O or 12 C 18 O 18 O) liberated from carbonate minerals when acidified (Ghosh et al., 2006). The abundance of the rarer multiply-substituted isotopologues is higher than that expected in a stochastic distribution of isotopes, an effect which diminishes at higher temperatures, making it a useful paleothermometer, typically expressed in "delta notation" as follows.

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The dinosaur–bird transition involved several anatomical, biomechanical, and physiological modifications of the theropod bauplan. Non-avian maniraptoran theropods, such as Troodon , are key to better understand changes in thermophysiology and reproduction occurring during this transition. Here, we applied dual clumped isotope (Δ 47 and Δ 48 ) thermometry, a technique that resolves mineralization temperature and other nonthermal information recorded in carbonates, to eggshells from Troodon , modern reptiles, and modern birds. Troodon eggshells show variable temperatures, namely 42 and 29 ± 2 °C, supporting the hypothesis of an endothermic thermophysiology with a heterothermic strategy for this extinct taxon. Dual clumped isotope data also reveal physiological differences in the reproductive systems between Troodon , reptiles, and birds. Troodon and modern reptiles mineralize their eggshells indistinguishable from dual clumped isotope equilibrium, while birds precipitate eggshells characterized by a positive disequilibrium offset in Δ 48 . Analyses of inorganic calcites suggest that the observed disequilibrium pattern in birds is linked to an amorphous calcium carbonate (ACC) precursor, a carbonate phase known to accelerate eggshell formation in birds. Lack of disequilibrium patterns in reptile and Troodon eggshells implies these vertebrates had not acquired the fast, ACC-based eggshell calcification process characteristic of birds. Observation that Troodon retained a slow reptile-like calcification suggests that it possessed two functional ovaries and was limited in the number of eggs it could produce; thus its large clutches would have been laid by several females. Dual clumped isotope analysis of eggshells of extinct vertebrates sheds light on physiological information otherwise inaccessible in the fossil record.