A 40-million-year history of atmospheric CO2.

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. [ABSTRACT FROM AUTHOR]