Validation of carbon isotope fractionation in algal lipids as a <i>p</i>CO₂ proxy using a natural CO₂ seep (Shikine Island, Japan)

Carbon dioxide concentrations in the atmosphere play an integral role in many Earth system dynamics, including its influence on global temperature. The past can provide insights into these dynamics, but unfortunately reconstructing long-term trends of atmospheric carbon dioxide (expressed in partial pressure; pCO₂) remains a challenge in paleoclimatology. One promising approach for reconstructing past pCO₂ utilizes the isotopic fractionation associated with CO₂ fixation during photosynthesis into organic matter (ε_p). Previous studies have focused primarily on testing estimates of ε_p derived from the δ¹³C of species-specific alkenone compounds in laboratory cultures and mesocosm experiments. Here, we analyze ε_p derived from the δ¹³C of more general algal biomarkers, i.e., compounds derived from a multitude of species from sites near a CO₂ seep off the coast of Shikine Island (Japan), a natural environment with CO₂ concentrations ranging from ambient (ca. 310 µatm) to elevated (ca. 770 µatm) pCO₂. We observed strong, consistent δ¹³C shifts in several algal biomarkers from a variety of sample matrices over the steep CO₂ gradient. Of the three general algal biomarkers explored here, namely loliolide, phytol, and cholesterol, ε_p positively correlates with pCO₂, in agreement with ε_p theory and previous culture studies. pCO₂ reconstructed from the ε_p of general algal biomarkers show the same trends throughout, as well as the correct control values, but with lower absolute reconstructed values than the measured values at the elevated pCO₂ sites. Our results show that naturally occurring CO₂ seeps may provide useful testing grounds for pCO₂ proxies and that general algal biomarkers show promise for reconstructing past pCO₂.