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  1. The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO2) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO2beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO2record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO2thresholds in biological and cryosphere evolution.

     
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    Free, publicly-accessible full text available December 8, 2024
  2. null (Ed.)
    Throughout Earth's history, CO 2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO 2 reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the Cenozoic—the past 66 million years. We find close coupling between CO 2 and climate throughout the Cenozoic, with peak CO 2 levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼500 ppm in the Miocene, and falling further into the ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO 2 drawdown is explained by an increase in the ratio of ocean alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO 2 record highlights the truly geological scale of anthropogenic CO 2 change: Current CO 2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO 2 levels of the Miocene or Eocene in the coming century. ▪  CO 2 reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. ▪  CO 2 estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO 2 system over the Cenozoic. ▪  CO 2 and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity. ▪  Twenty-first-century carbon emissions have the potential to return CO 2 to levels not seen since the much warmer climates of Earth's distant past. 
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  3. null (Ed.)
    The world-renowned Miocene Clarkia paleolake in northern Idaho (USA) is closely associated with Columbia River Basalt Group volcanism. The flood basalt dammed a local drainage system to form the paleolake, which preserved a plant fossil Lagerstätte in its deposits. However, the precise age and temporal duration of the lake remain unsettled. We present the first unequivocal U-Pb zircon ages from interbedded volcanic ashes at the P-33 type location, constraining the deposition to 15.78 ± 0.039 Ma. Using micro–X-ray fluorescence and petrographic and spectral analyses, we establish the annual characteristics of laminations throughout the stratigraphic profile using the distribution of elemental ratios, mineral assemblages, and grain-size structures, as well as organic and fossil contents. Consequently, the ~7.5-m-thick varved deposit at the type location P-33 represents ~840 yr of deposition, coincident with the end of the main phase of Columbia River Basalt Group eruptions during the Miocene Climate Optimum. The timing and temporal resolution of the deposit offer a unique opportunity to study climate change in unprecedented detail during global warming associated with carbon-cycle perturbations. 
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