Atmospheric cobinf2einf over the past 66 million years from marine archives

Abstract

Throughout Earth's history, CObinf2einf is thought to have exerted a fundamental control on environmental change. Here we review and revise CObinf2einf reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the Cenozoicmdashthe past 66 million years. We find close coupling between CObinf2einf and climate throughout the Cenozoic, with peak CObinf2einf 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 CObinf2einf 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 CObinf2einf record highlights the truly geological scale of anthropogenic CObinf2einf change: Current CObinf2einf levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CObinf2einf levels of the Miocene or Eocene in the coming century. squfensp CObinf2einf reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. squfensp CObinf2einf estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CObinf2einf system over the Cenozoic. squfensp CObinf2einf and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity. squfensp Twenty-first-century carbon emissions have the potential to return CObinf2einf to levels not seen since the much warmer climates of Earth's distant past.