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A 40-million-year history of atmospheric CO2

A 40-million-year history of atmospheric CO2
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 ?18O 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 ?18O 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.
Cenozoic carbon dioxide, alkenone–pCO2 method, carbon isotopes, benthic oxygen isotopes, global temperature, cryosphere
1364-503X
20130096
Zhang, Yi Ge
57eb90b7-b182-4203-93a1-0a0be4227c34
Pagani, Mark
fea192f2-422c-4380-a5be-f6e2e75398e6
Liu, Zhonghui
9cd87eaf-0cb6-466c-bdbe-f85afade35d1
Bohaty, Steven M.
af9dbe78-8b9f-44f2-ba1d-20795837d2d1
DeConto, Robert
bffa0d77-3f9f-44c2-bfc3-58a4194c3a9d
Zhang, Yi Ge
57eb90b7-b182-4203-93a1-0a0be4227c34
Pagani, Mark
fea192f2-422c-4380-a5be-f6e2e75398e6
Liu, Zhonghui
9cd87eaf-0cb6-466c-bdbe-f85afade35d1
Bohaty, Steven M.
af9dbe78-8b9f-44f2-ba1d-20795837d2d1
DeConto, Robert
bffa0d77-3f9f-44c2-bfc3-58a4194c3a9d

Zhang, Yi Ge, Pagani, Mark, Liu, Zhonghui, Bohaty, Steven M. and DeConto, Robert (2013) A 40-million-year history of atmospheric CO2. Philosophical Transactions of The Royal Society A, 371 (2001), 20130096. (doi:10.1098/rsta.2013.0096).

Record type: Article

Abstract

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 ?18O 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 ?18O 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.

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More information

Published date: 16 September 2013
Keywords: Cenozoic carbon dioxide, alkenone–pCO2 method, carbon isotopes, benthic oxygen isotopes, global temperature, cryosphere
Organisations: Paleooceanography & Palaeoclimate

Identifiers

Local EPrints ID: 356913
URI: https://eprints.soton.ac.uk/id/eprint/356913
ISSN: 1364-503X
PURE UUID: 90b9eaa1-e99e-4ec6-9287-b4b5ecfaec81
ORCID for Steven M. Bohaty: ORCID iD orcid.org/0000-0002-1193-7398

Catalogue record

Date deposited: 17 Sep 2013 08:50
Last modified: 06 Jun 2018 12:39

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