Biological remains in ocean sediments document the remarkable history of atmospheric CO2 and its fundamental control on Earth’s climate. Higher resolution studies are needed to better understand the short-term processes that inform imminent anthropogenic climate changes.
Human activities have increased the concentration of carbon dioxide in our atmosphere from 280 ppm before industrialization to 424 parts per millin (ppm) in 2024. Without reductions in emissions, CO2 is projected to rise to >800 ppm by the end of this century, driving warming well in excess of the 1°C already recorded (IPCC 2021). How warm it will get can be projected by complex numerical climate models whose skills are validated using the detailed relationship between atmospheric CO2 and global climate in Earth’s history. Instrumental measurements of CO2 have been collected since 1958 (Lan et al. 2024), and ancient air trapped in Antarctic ice documents Earth’s atmospheric composition over hundreds of thousands of years prior (Bereiter et al. 2015; Yan et al. 2019). However, CO2 during this geologically recent past was generally lower than today, and global temperatures colder. Much warmer intervals occurred in the distant past, but because the atmosphere of that time cannot be sampled directly, paleo-CO2 reconstructions rely on indirect proxies preserved in the sedimentary record.
Reconstructing CO2 from ocean sediments
Deep-sea sediments are key to paleoreconstructions; they are globally distributed and gradually accumulate biogenic and inorganic proxy materials over tens of millions of years, thereby providing excellent age stratigraphy. Uniquely useful in documenting past surface-ocean temperatures and the partial pressure of CO2 (PCO2) are the mineralized and organic remains left behind by organisms that once inhabited the ancient surface ocean. This is because gas exchange at the air-sea interface drives PCO2 in seawater towards equilibrium with PCO2 in the atmosphere. Once absorbed in seawater, CO2 reacts with water (H2O) and forms a suite of carbon species whose abundances are controlled by well-understood chemical equilibrium reactions that also determine seawater acidity (i.e. pH).
Not all oceanic regions are appropriate for paleo-CO2 studies because vigorous photosynthesis can diminish sea-surface CO2 while upwelling of deeper waters delivers respired CO2 to the surface, disturbing the air–sea equilibrium. Therefore, paleo-CO2 studies focus on off-shore regions such as subtropical gyres, where photosynthesis is weak and downwelling of surface waters allows air–sea equilibrium to be established.
There are two main frameworks for marine-based CO2 reconstructions: the stable carbon isotopic composition of organic phytoplankton (δ13Cphytoplankton) remains and the boron isotopic composition (δ11B) of fossilized CaCO3 shells. Briefly, the δ13Cphytoplankton proxy assumes CO2 passively diffuses into an algae cell, and the CO2-fixing enzyme RuBisCo preferentially takes up 12C over 13C during oxygenic photosynthesis. When CO2 is abundant, 12C is preferentially incorporated into organic matter (resulting in relatively lower δ13Cphytoplankton). The opposite occurs at low CO2 (Fig. 1). Although first applied to bulk organic matter (Popp et al. 1989), selective preservation and mixed organic sources imposed problems. These challenges have been resolved by using: (1) specific compounds produced by select algae (e.g. alkenones from Haptophytes); (2) specific compounds produced by the broader phytoplankton community (e.g. chlorophyll), enabling greater spatial and temporal diversity of reconstructions; and (3) organic carbon bound to mineral or organic exteriors of e.g. coccolithophores, diatoms or dinoflagellates. The detailed systematics of these approaches are reviewed in Hollis et al. (2019).
Figure 1: Basic systematics of the two marine CO2 proxies. Fossil organic compounds and CaCO3 shells are preserved in layered ocean sediments that can be extracted by deep-ocean drilling
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Hönisch B., Witkowski C. R., Penman D. E., Harper D. T., Henehan M. J. & Polissar P., 2024. Paleo-atmospheric CO2 reconstructions from deep-ocean sediments. Past Global Changes Magazine 32(2): 86-87. Article.
