Abstract. Carbon dioxide concentrations in the atmosphere play an integral role in many earth system dynamics, including its influence on global temperature. Long-term trends can provide insights into these dynamics though 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 isotopic fractionation associated with CO2-fixation during photosynthesis into organic matter (Ɛp). Previous studies have focused primarily on testing estimates of Ɛp derived from species-specific alkenone compounds in laboratory cultures and mesocosm experiments. Here, we analyze Ɛp derived from general algal compounds from sites at a CO2 seep near Shikine Island (Japan), a natural environment with CO2 concentrations ranging from ambient (ca. 310 µatm) to elevated (ca. 770 µatm). 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, Ɛp positively correlates with PCO2 in agreement with Ɛp theory and previous culture studies. PCO2 reconstructed from the Ɛp 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.
CURRENT CARBON DIOXIDE CONTENT IN THE NEARSURFACE LAYER OF THE EARTH’S ATMOSPHERE: LONG-TERM TRENDS AND INTRA-ANNUAL VARIABILITY