Validation of carbon isotope fractionation in algal lipids as a pCO2 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; pCO(2)) remains a challenge in paleoclimatology. One promising approach for reconstructing past pCO(2) utilizes the isotopic fractionation associated with CO2 fixation during photosynthesis into organic matter (epsilon(p)). Previous studies have focused primarily on testing estimates of epsilon(p) derived from the delta C-13 of species-specific alkenone compounds in laboratory cultures and mesocosm experiments. Here, we analyze epsilon(p) derived from the delta C-13 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 mu atm) to elevated (ca. 770 mu atm) p CO2. We observed strong, consistent delta C-13 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, epsilon(p) positively correlates with pCO(2), in agreement with epsilon(p) theory and previous culture studies. pCO(2) reconstructed from the epsilon(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 pCO(2) sites. Our results show that naturally occurring CO2 seeps may provide useful testing grounds for pCO(2) proxies and that general algal biomarkers show promise for reconstructing past pCO(2).