Testing algal-based pCO(2) proxies at a modern CO(2) seep (Vulcano, Italy)

Understanding long-term trends in atmospheric concentrations of carbon dioxide (pCO(2)) has become increasingly relevant as modern concentrations surpass recent historic trends. One method for estimating past pCO(2), 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 pCO(2) 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 CO(2) 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 (13)C over the transect from the control site towards the seep, suggesting that CO(2) exerts a strong control on isotopic fractionation in natural phytoplankton communities. The strongest shift in fractionation was seen in phytol, and pCO(2) estimates derived from phytol confirm the utility of this biomarker as a proxy for pCO(2) reconstruction.