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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., Scientific Reports, 11 (1), ISSN:2045-2322

Oxygen is a prerequisite for all large and motile animals. It is a puzzling paradox that fossils of benthic animals are often found in black shales with geochemical evidence for deposition in marine environments with anoxic and sulfidic bottom waters. It is debated whether the geochemical proxies are unreliable, affected by diagenesis, or whether the fossils are transported from afar or perhaps were not benthic. Here, we improved the stratigraphic resolution of marine anoxia records 100–1000 fold using core-scanning X-Ray Fluorescence and established a centennial resolution record of oxygen availability at the seafloor in an epicontinental sea that existed ~501–494 million years ago. The study reveals that anoxic bottom-water conditions, often with toxic hydrogen sulfide present, were interrupted by brief oxygenation events of 600–3000 years duration, corresponding to 1–5 mm stratigraphic thickness. Fossil shells occur in some of these oxygenated intervals suggesting that animals invaded when conditions permitted an aerobic life style at the seafloor. Although the fauna evidently comprised opportunistic species adapted to low oxygen environments, these findings reconcile a long-standing debate between paleontologists and geochemists, and shows the potential of ultra-high resolution analyses for reconstructing redox conditions in past oceans.

Arctic carbon cycling and its regional climate have been observed to deviate from global trends in the Late Jurassic and across the Jurassic–Cretaceous boundary interval, but appear to recouple with global trends in the Early Cretaceous (Galloway et al., 2019; Jelby et al., 2020). We investigate the possible link between these observed trends and volcanism by examining the mercury (Hg) and other element records from Arctic sites in Svalbard (Norway) and the Queen Elizabeth Islands, Canada. We assess whether pulsed phases of the High Arctic Large Igneous Province, or the globally significant emplacement of Paraná-Etendeka or Greater Ontong-Java Plateau, are expressed by stratigraphic Hg trends recorded in the studied sites of Arctic Canada and Svalbard, and how any signals correlate with the regional stable carbon-isotope (δ13C) record. We compare these new data to Hg and δ13C records from other globally distributed sites, focusing on the carbon isotope excursion (CIE) intervals: the Arctic-wide Volgian CIE (“VOICE”), the global Valanginian positive CIE (“Weissert Event”), and the global early Aptian CIE associated with Ocean Anoxic Event 1a (OAE1a).