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

Caitlyn R Witkowski,Jaap S Sinninghe Damsté,Nadine T Smit,Stefan Schouten,Marcel T J Van Der Meer

doi:10.1038/s41598-020-67483-8

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.

Highlights

Understanding long-term trends in atmospheric concentrations of carbon dioxide has become increasingly relevant as modern concentrations surpass recent historic trends
The δ13C of dissolved inorganic carbon (DIC) measured in seawater collected in May from the bay does not show notable change over the gradient of CO2 (Table S1), which confirms that lack of change noted in the literature[48]
We tested three general phytoplankton biomarkers in surface sediments in a transect from a naturally occurring CO2 seep located in Levante bay, Vulcano Island, Italy, towards the open Tyrrhenian Sea

Summary

Introduction

Understanding long-term trends in atmospheric concentrations of carbon dioxide (pCO2) has become increasingly relevant as modern concentrations surpass recent historic trends. The concentration of atmospheric carbon dioxide (pCO2, expressed in partial pressure μatm), as directly measured from air trapped in ice cores, has had a major influence on climate over the past 800 thousand years (ka)[1] During this period, pCO2 and temperature oscillated together between stable bounds every 100 k a2. PCO2 has been shown to be one of the dominant physiological control on the δ13C of photoautotrophic biomass[9], studies on Ɛp in algae have shown that other factors may influence this value, primarily growth rate[10] and cell size and s hape[11], as well as minor influences such as light, and temperature[12,13,14,15] These additional influencing factors on Ɛp are considered in pCO2 reconstructions via the catchall term b16, described in the e quation[17] as: CO2[aq] = b/ (ǫf − ǫp). University of Bristol, School of Earth Sciences, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK. *email: Scientific Reports | (2020) 10:10508

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Conclusion