Insensitivity of alkenone carbon isotopes to CO2 at low to moderate atmospheric CO2 levels

Atmospheric pCO₂ is a critical component of the global carbon system and is considered to be the major control of Earth's past, present and future climate. Accurate and precise reconstructions of its concentration through geological time are, therefore, crucial to our understanding of the Earth system. Ice core records document pCO₂ for the past 800 kyrs, but at no point during this interval were CO₂ levels higher than today. Interpretation of older pCO₂ has been hampered by discrepancies during some time intervals between two of the main ocean-based proxy methods used in its reconstruction: the carbon isotope fractionation that occurs during photosynthesis as recorded by haptophyte biomarkers (alkenones) and the boron isotope composition (δ¹¹B) of foraminifer shells. Here we present alkenone and δ¹¹B-based pCO₂ reconstructions generated from the same samples from the Plio-Pleistocene at ODP Site 999 across a glacial-interglacial cycle. We find a muted response to pCO₂ in the alkenone record compared to contemporaneous ice core and δ¹¹B records, suggesting caution in the interpretation of alkenone-based records at low pCO₂ levels. This is possibly caused by the physiology of CO₂ uptake in the haptophytes. Our new understanding resolves some of the inconsistencies between the proxies and highlights that caution may be required when interpreting alkenone-based reconstructions of pCO₂.