A phytol εp-based core-top calibration to reconstruct past changes in atmospheric CO2

Olivia A. Graham; Caitlyn R. Witkowski; Mark A. Stevenson; Francien Peterse; B. David A. Naafs

doi:10.1016/j.gca.2025.04.014

A phytol ε_p-based core-top calibration to reconstruct past changes in atmospheric CO₂

Olivia A. Graham
, Caitlyn R. Witkowski
, Mark A. Stevenson
, Francien Peterse
, B. David A. Naafs^*

^*Corresponding author for this work

Research output: Contribution to journal › Article › Academic › peer-review

Abstract

Proxy-based reconstructions of past changes in atmospheric carbon dioxide concentrations (pCO₂) are essential for understanding climate dynamics. A common method for reconstructing past pCO₂ is based on the carbon isotopic fractionation during photosynthesis by Rubisco (ε_p). This proxy method is based upon the difference (ε_p) between the stable carbon isotopic composition (δ¹³C) of dissolved CO₂ and the δ¹³C of marine photoautotroph biomass, which depends on the concentration of dissolved CO₂, related to pCO₂ through Henry's Law. This method has been applied to the general phytoplankton biomarker chlorophyll (preserved as isoprenoids like phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass δ¹³C. The long-term stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of pCO₂ across the Phanerozoic (∼450 million years). However, the chlorophyll-derived biomarker proxy currently lacks a robust validation within modern settings. Here we investigate the relationship between the δ¹³C of chlorophyll (as phytol) and the concentration of dissolved CO₂ in the modern ocean using a globally distributed set of 30 marine core top sediments and 75 suspended particulate matter samples. Our results demonstrate a positive relationship between the extent of fractionation (higher phytol ε_p) and dissolved CO₂ concentration. This marks the first empirical calibration between phytol ε_p and the concentration of dissolved CO₂ in natural settings. We find that terrestrial input negatively affects this observed relationship, and the exclusion of coastal samples from our dataset improves the correlation. When applied to previously published Pleistocene proxy data, our new calibration provides an improved pCO₂ reconstruction with estimates that are statistically like direct pCO₂ measurements from the Antarctic ice cores. When applied to published data from the entire Phanerozoic, our calibration provides estimates in line with those of other proxy methods, emphasizing the potential of chlorophyll for reconstructions of pCO₂ across geological time.

Original language	English
Pages (from-to)	178-192
Number of pages	15
Journal	Geochimica et Cosmochimica Acta
Volume	398
Early online date	19 Apr 2025
DOIs	https://doi.org/10.1016/j.gca.2025.04.014
Publication status	Published - Jun 2025

Bibliographical note

Publisher Copyright:
© 2025 The Author(s)

Funding

The authors would like to thank the Associate Editor, Aaron Diefendorf, Nemiah Ladd, and Yige Zhang for reviewing this manuscript. The authors wish to thank the NERC for partial funding of the National Environmental Isotope Facility (NEIF; contract no. NE/V003917/1). The authors also thank the NERC (contract no. NE/V003917/1) and funding from the European Research Council under the European Union \u2019s Seventh Framework Programme ( FP/2007-2013 ) and European Research Council Grant Agreement number 340923 for funding GC-MS capabilities as well as the NERC (contract no. NE/V003917/1 ) and the University of Bristol for funding the GC-IRMS capabilities. For providing core top sediment samples, the authors would like to thank the British Ocean Sediment Core Research Facility (BOSCORF), Dr Nicole Bale, Dr Julie Lattaud, and Professor Geert-Jan Brummer. Further, the authors would like to acknowledge the following cruises and the scientists and crew aboard: R/V Pelagia 64PE467 , 64PE393 , 64PE424 , 64PE275 , 64PE300 , R/V Akademik M.A. Lavrentyev 55, RRS James Clark Ross JR16006. O.A.G. acknowledges funding by a NERC GW4 + Doctoral Training Partnership studentship. C.R.W. is supported by the Royal Society Dorothy Hodgkin Fellowship (DHF\\R1\\221014). B.D.A.N was funded through a Royal Society Tata University Research Fellowship. The authors would like to thank the Associate Editor, Aaron Diefendorf, Nemiah Ladd, and Yige Zhang for reviewing this manuscript. The authors wish to thank the NERC for partial funding of the National Environmental Isotope Facility (NEIF; contract no. NE/V003917/1). The authors also thank the NERC (contract no. NE/V003917/1) and funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) and European Research Council Grant Agreement number 340923 for funding GC-MS capabilities as well as the NERC (contract no. NE/V003917/1) and the University of Bristol for funding the GC-IRMS capabilities. For providing core top sediment samples, the authors would like to thank the British Ocean Sediment Core Research Facility (BOSCORF), Dr Nicole Bale, Dr Julie Lattaud, and Professor Geert-Jan Brummer. Further, the authors would like to acknowledge the following cruises and the scientists and crew aboard: R/V Pelagia 64PE467, 64PE393, 64PE424, 64PE275, 64PE300, R/V Akademik M.A. Lavrentyev 55, RRS James Clark Ross JR16006. O.A.G. acknowledges funding by a NERC GW4 + Doctoral Training Partnership studentship. C.R.W. is supported by the Royal Society Dorothy Hodgkin Fellowship (DHF\\R1\\221014). B.D.A.N was funded through a Royal Society Tata University Research Fellowship.

Funders	Funder number
Natural Environment Research Council
GC-IRMS
European Research Council
University of Bristol
Seventh Framework Programme	340923, FP/2007-2013
British Ocean Sediment Core Research Facility	64PE424, 64PE467, 64PE300, 64PE393, 64PE275, JR16006
National Environmental Isotope Facility	NE/V003917/1
Royal Society	DHF\R1\221014

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

SDG 13 Climate Action
SDG 14 Life Below Water

Keywords

Biomarker
Calibration
Carbon Isotopes
pCO
Phytol
Phytoplankton

Access to Document

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1-s2.0-S0016703725002054-mainFinal published version, 4.03 MBLicence: CC BY

Cite this

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title = "A phytol εp-based core-top calibration to reconstruct past changes in atmospheric CO2",

abstract = "Proxy-based reconstructions of past changes in atmospheric carbon dioxide concentrations (pCO2) are essential for understanding climate dynamics. A common method for reconstructing past pCO2 is based on the carbon isotopic fractionation during photosynthesis by Rubisco (εp). This proxy method is based upon the difference (εp) between the stable carbon isotopic composition (δ13C) of dissolved CO2 and the δ13C of marine photoautotroph biomass, which depends on the concentration of dissolved CO2, related to pCO2 through Henry's Law. This method has been applied to the general phytoplankton biomarker chlorophyll (preserved as isoprenoids like phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass δ13C. The long-term stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of pCO2 across the Phanerozoic (∼450 million years). However, the chlorophyll-derived biomarker proxy currently lacks a robust validation within modern settings. Here we investigate the relationship between the δ13C of chlorophyll (as phytol) and the concentration of dissolved CO2 in the modern ocean using a globally distributed set of 30 marine core top sediments and 75 suspended particulate matter samples. Our results demonstrate a positive relationship between the extent of fractionation (higher phytol εp) and dissolved CO2 concentration. This marks the first empirical calibration between phytol εp and the concentration of dissolved CO2 in natural settings. We find that terrestrial input negatively affects this observed relationship, and the exclusion of coastal samples from our dataset improves the correlation. When applied to previously published Pleistocene proxy data, our new calibration provides an improved pCO2 reconstruction with estimates that are statistically like direct pCO2 measurements from the Antarctic ice cores. When applied to published data from the entire Phanerozoic, our calibration provides estimates in line with those of other proxy methods, emphasizing the potential of chlorophyll for reconstructions of pCO2 across geological time.",

keywords = "Biomarker, Calibration, Carbon Isotopes, pCO, Phytol, Phytoplankton",

author = "Graham, \{Olivia A.\} and Witkowski, \{Caitlyn R.\} and Stevenson, \{Mark A.\} and Francien Peterse and Naafs, \{B. David A.\}",

note = "Publisher Copyright: {\textcopyright} 2025 The Author(s)",

year = "2025",

month = jun,

doi = "10.1016/j.gca.2025.04.014",

language = "English",

volume = "398",

pages = "178--192",

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TY - JOUR

T1 - A phytol εp-based core-top calibration to reconstruct past changes in atmospheric CO2

AU - Graham, Olivia A.

AU - Witkowski, Caitlyn R.

AU - Stevenson, Mark A.

AU - Peterse, Francien

AU - Naafs, B. David A.

PY - 2025/6

Y1 - 2025/6

N2 - Proxy-based reconstructions of past changes in atmospheric carbon dioxide concentrations (pCO2) are essential for understanding climate dynamics. A common method for reconstructing past pCO2 is based on the carbon isotopic fractionation during photosynthesis by Rubisco (εp). This proxy method is based upon the difference (εp) between the stable carbon isotopic composition (δ13C) of dissolved CO2 and the δ13C of marine photoautotroph biomass, which depends on the concentration of dissolved CO2, related to pCO2 through Henry's Law. This method has been applied to the general phytoplankton biomarker chlorophyll (preserved as isoprenoids like phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass δ13C. The long-term stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of pCO2 across the Phanerozoic (∼450 million years). However, the chlorophyll-derived biomarker proxy currently lacks a robust validation within modern settings. Here we investigate the relationship between the δ13C of chlorophyll (as phytol) and the concentration of dissolved CO2 in the modern ocean using a globally distributed set of 30 marine core top sediments and 75 suspended particulate matter samples. Our results demonstrate a positive relationship between the extent of fractionation (higher phytol εp) and dissolved CO2 concentration. This marks the first empirical calibration between phytol εp and the concentration of dissolved CO2 in natural settings. We find that terrestrial input negatively affects this observed relationship, and the exclusion of coastal samples from our dataset improves the correlation. When applied to previously published Pleistocene proxy data, our new calibration provides an improved pCO2 reconstruction with estimates that are statistically like direct pCO2 measurements from the Antarctic ice cores. When applied to published data from the entire Phanerozoic, our calibration provides estimates in line with those of other proxy methods, emphasizing the potential of chlorophyll for reconstructions of pCO2 across geological time.

AB - Proxy-based reconstructions of past changes in atmospheric carbon dioxide concentrations (pCO2) are essential for understanding climate dynamics. A common method for reconstructing past pCO2 is based on the carbon isotopic fractionation during photosynthesis by Rubisco (εp). This proxy method is based upon the difference (εp) between the stable carbon isotopic composition (δ13C) of dissolved CO2 and the δ13C of marine photoautotroph biomass, which depends on the concentration of dissolved CO2, related to pCO2 through Henry's Law. This method has been applied to the general phytoplankton biomarker chlorophyll (preserved as isoprenoids like phytol, phytane, and pristane in the sedimentary record) to reconstruct photoautotroph biomass δ13C. The long-term stability of these chlorophyll-derived biomarkers in the sedimentary record has currently allowed the reconstruction of pCO2 across the Phanerozoic (∼450 million years). However, the chlorophyll-derived biomarker proxy currently lacks a robust validation within modern settings. Here we investigate the relationship between the δ13C of chlorophyll (as phytol) and the concentration of dissolved CO2 in the modern ocean using a globally distributed set of 30 marine core top sediments and 75 suspended particulate matter samples. Our results demonstrate a positive relationship between the extent of fractionation (higher phytol εp) and dissolved CO2 concentration. This marks the first empirical calibration between phytol εp and the concentration of dissolved CO2 in natural settings. We find that terrestrial input negatively affects this observed relationship, and the exclusion of coastal samples from our dataset improves the correlation. When applied to previously published Pleistocene proxy data, our new calibration provides an improved pCO2 reconstruction with estimates that are statistically like direct pCO2 measurements from the Antarctic ice cores. When applied to published data from the entire Phanerozoic, our calibration provides estimates in line with those of other proxy methods, emphasizing the potential of chlorophyll for reconstructions of pCO2 across geological time.

KW - Biomarker

KW - Calibration

KW - Carbon Isotopes

KW - pCO

KW - Phytol

KW - Phytoplankton

UR - https://www.scopus.com/pages/publications/105003828674

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