Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Restricted accessResearch article

A 40-million-year history of atmospheric CO2

Yi Ge Zhang

Yi Ge Zhang

Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA

[email protected]

Google Scholar

Find this author on PubMed

,
Mark Pagani

Mark Pagani

Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA

Google Scholar

Find this author on PubMed

,
Zhonghui Liu

Zhonghui Liu

Department of Earth Sciences, The University of Hong Kong, Hong Kong, People's Republic of China

Google Scholar

Find this author on PubMed

,
Steven M. Bohaty

Steven M. Bohaty

School of Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK

Google Scholar

Find this author on PubMed

and
Robert DeConto

Robert DeConto

Department of Geosciences, University of Massachusetts-Amherst, Amherst, MA 01003, USA

Google Scholar

Find this author on PubMed

    The alkenone–pCO2 methodology has been used to reconstruct the partial pressure of ancient atmospheric carbon dioxide (pCO2) for the past 45 million years of Earth's history (Middle Eocene to Pleistocene epochs). The present long-term CO2 record is a composite of data from multiple ocean localities that express a wide range of oceanographic and algal growth conditions that potentially bias CO2 results. In this study, we present a pCO2 record spanning the past 40 million years from a single marine locality, Ocean Drilling Program Site 925 located in the western equatorial Atlantic Ocean. The trends and absolute values of our new CO2 record site are broadly consistent with previously published multi-site alkenone–CO2 results. However, new pCO2 estimates for the Middle Miocene are notably higher than published records, with average pCO2 concentrations in the range of 400–500 ppm. Our results are generally consistent with recent pCO2 estimates based on boron isotope-pH data and stomatal index records, and suggest that CO2 levels were highest during a period of global warmth associated with the Middle Miocene Climatic Optimum (17–14 million years ago, Ma), followed by a decline in CO2 during the Middle Miocene Climate Transition (approx. 14 Ma). Several relationships remain contrary to expectations. For example, benthic foraminiferal δ18O records suggest a period of deglaciation and/or high-latitude warming during the latest Oligocene (27–23 Ma) that, based on our results, occurred concurrently with a long-term decrease in CO2 levels. Additionally, a large positive δ18O excursion near the Oligocene–Miocene boundary (the Mi-1 event, approx. 23 Ma), assumed to represent a period of glacial advance and retreat on Antarctica, is difficult to explain by our CO2 record alone given what is known of Antarctic ice sheet history and the strong hysteresis of the East Antarctic Ice Sheet once it has grown to continental dimensions. We also demonstrate that in the Neogene with low CO2 levels, algal carbon concentrating mechanisms and spontaneous biocarbonate–CO2 conversions are likely to play a more important role in algal carbon fixation, which provides a potential bias to the alkenone–pCO2 method.

    Footnotes

    One contribution of 11 to a Discussion Meeting Issue ‘Warm climates of the past—a lesson for the future?’.

    References