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

Abstract

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; p CO2) remains a challenge in paleoclimatology. One promising approach for reconstructing past p CO2 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 δ 13C of species-specific alkenone compounds in laboratory cultures and mesocosm experiments. Here, we analyze ϵ p derived from the δ 13C 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 μ atm) to elevated (ca. 770 μ atm) p CO2. We observed strong, consistent δ 13C 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 p CO2, in agreement with ϵ p theory and previous culture studies. p CO2 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 p CO2 sites. Our results show that naturally occurring CO2 seeps may provide useful testing grounds for p CO2 proxies and that general algal biomarkers show promise for reconstructing past p CO2.