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Seasonal variability in sea surface temperatures plays a fundamental role in climate dynamics and species distribution. Seasonal bias can also severely compromise the accuracy of mean annual temperature reconstructions. It is therefore essential to better understand seasonal variability in climates of the past. Many reconstructions of climate in deep time neglect this issue and rely on controversial assumptions, such as estimates of sea water oxygen isotope composition. Here we present absolute seasonal temperature reconstructions based on clumped isotope measurements in bivalve shells which, critically, do not rely on these assumptions. We reconstruct highly precise monthly sea surface temperatures at around 50 ÂN latitude from individual oyster and rudist shells of the Campanian greenhouse period about 78 million years ago, when the seasonal range at 50 ÂN comprised 15 to 27âÂC. In agreement with fully coupled climate model simulations, we find that greenhouse climates outside the tropics were warmer and more seasonal than previously thought. We conclude that seasonal bias and assumptions about seawater composition can distort temperature reconstructions and our understanding of past greenhouse climates.
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Dinosaurs lived in greenhouse climate with hot summers
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Assessing the composition and origin of the organic matter-rich mudstones, which dominate the early basin fill of the main Karoo Basin of South Africa, are essential for reconstructing the palaeo-environment, palaeo-climate, and potential provenance areas in southern Gondwana during the Early Permian. This study investigates the sedimentary geochemistry and petrography (e.g., major oxides, trace elements, mineral compositions, sedimentary structures, textures) of the lower Ecca Group, which was sampled in newly drilled boreholes. Our results show that the rate of weathering decreased over time as the environment changed from an ice-house to a more hot-house setting (i.e., the Chemical Index Alteration is 72â85 in the Prince Albert Formation and 60â76 in the Collingham Formation). Comparisons of the V/Cr, V(V+Ni), and Ni/Co ratios and Fe-S-TOC ternary plot show oscillations between anoxic and oxic conditions during deposition. Furthermore, there is evidence, albeit somewhat equivocal, for changing palaeo-salinity levels, low sedimentation rates, and high bio-productivity. Trace-element Principal Component Analysis, the Index of Compositional Variability, and Discriminant Function Analysis of the older mudstones (Prince Albert and Whitehill formations) imply different provenance regions with felsic to intermediate signatures predominantly found in the western MKB and mafic signatures in the eastern MKB. The youngest unit, the Collingham Formation, has a more uniform composition across the region, and was mostly sourced from the quartz-arenites and granites most likely associated with the Cape Fold Belt.
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Julia Brugger, Georg Feulner, Matthias Hofmann & Stefan Petri (2021)
A pronounced spike in ocean productivity triggered by the Chicxulub impact.
Geophysical Research Letters (advance online publication)
doi:
https://doi.org/10.1029/2020GL092260https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL092260There is increasing evidence linking the mass-extinction event at the Cretaceous-Paleogene boundary to an asteroid impact near Chicxulub, Mexico. Here we use model simulations to explore the combined effect of sulfate aerosols, carbon dioxide and dust from the impact on the oceans and the marine biosphere in the immediate aftermath of the impact. We find a strong temperature decrease, a brief algal bloom caused by nutrients from both the deep ocean and the projectile, and moderate surface ocean acidification. Comparing the modeled longer-term post-impact warming and changes in carbon isotopes with empirical evidence points to a substantial release of carbon from the terrestrial biosphere. Overall, our results shed light on the decades to centuries after the Chicxulub impact which are difficult to resolve with proxy data.
Plain Language Summary
The sudden disappearance of the dinosaurs and many other species during the end-Cretaceous mass extinction 66 million years ago marks one of the most profound events in the history of life on Earth. The impact of a large asteroid near Chicxulub, Mexico, is increasingly recognised as the trigger of this extinction, causing global darkness and a pronounced cooling. However, the links between the impact and the changes in the biosphere are not fully understood. Here, we investigate how life in the ocean reacts to the perturbations in the decades and centuries after the impact. We find a short-lived algal bloom caused by the upwelling of nutrients from the deep ocean and nutrient input from the impactor.
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