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

Abstract

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 (Ɛp) has shown promise over the past several decades, in particular using species-specific biomarker lipids such as alkenones. Recently, the Ɛp 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 Ɛp 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.