Abstract
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; 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 (εp). Previous studies have focused primarily on testing estimates of εp derived from the δ13C of species-specific alkenone compounds in laboratory cultures and mesocosm experiments. Here, we analyze εp 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, εp positively correlates with pCO2, in agreement with εp theory and previous culture studies. pCO2 reconstructed from the εp 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.
Highlights
The current increase in the atmospheric concentration of carbon dioxide plays a leading role in climate change (Forster et al, 2007). pCO2 is significantly higher than it has been in the past 800 ka (Lüthi et al, 2008), and long-term changes in pCO2 are not uncommon over millions of years (Foster et al, 2017), this current spike in pCO2 has occurred within only the past 2 centuries (IPCC, 2013)
Phytol, and cholesterol were targeted for stable carbon isotope analysis as the most abundant general algal biomarkers and with relatively good separation in the GC
We analyzed the δ13C of general algal biomarkers in surface sediments, plankton, benthic diatoms, and macroalgae collected in a transect from a CO2 vent during two seasons
Summary
The current increase in the atmospheric concentration of carbon dioxide (expressed in partial pressure, pCO2) plays a leading role in climate change (Forster et al, 2007). pCO2 is significantly higher than it has been in the past 800 ka (Lüthi et al, 2008), and long-term changes in pCO2 are not uncommon over millions of years (Foster et al, 2017), this current spike in pCO2 has occurred within only the past 2 centuries (IPCC, 2013). Long-term pCO2 trends help us better understand the context for these changes and are reconstructed via indirect means, i.e., environmental proxies. Two proxies can span timescales over 100 Ma (Foster et al, 2017), e.g., the terrestrial paleosol proxies and leaf stomata. The paleosol proxy has large uncertainties due to the difficulties in constraining soil respiration (Breecker et al, 2010; Cotton and Sheldon, 2012) due to carbon isotopic fractionation during microbial decomposition (Bowen and Beerling, 2004), carbonate diagenesis (Quast et al, 2006), and other local and regional influences on carbon cycles in these terrestrial settings. The leaf stomata proxy is often better constrained than
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