Testing algal-based pCO₂ proxies at a modern CO₂ seep (Vulcano, Italy)

Understanding long-term trends in atmospheric concentrations of carbon dioxide (pCO₂) has become increasingly relevant as modern concentrations surpass recent historic trends. One method for estimating past pCO₂, the stable carbon isotopic fractionation associated with photosynthesis (Ɛ_p) has shown promise over the past several decades, in particular using species-specific biomarker lipids such as alkenones. Recently, the Ɛ_p of more general biomarker lipids, organic compounds derived from a multitude of species, have been applied to generate longer-spanning, more ubiquitous records than those of alkenones but the sensitivity of this proxy to changes in pCO₂ has not been constrained in modern settings. Here, we test Ɛ_p using a variety of general biomarkers along a transect taken from a naturally occurring marine CO₂ seep in Levante Bay of the Aeolian island of Vulcano in Italy. The studied general biomarkers, loliolide, cholesterol, and phytol, all show increasing depletion in ¹³C over the transect from the control site towards the seep, suggesting that CO₂ exerts a strong control on isotopic fractionation in natural phytoplankton communities. The strongest shift in fractionation was seen in phytol, and pCO₂ estimates derived from phytol confirm the utility of this biomarker as a proxy for pCO₂ reconstruction.