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

• Main Authors: C. R. Witkowski, S. Agostini, B. P. Harvey, M. T. J. van der Meer, J. S. Sinninghe Damsté, S. Schouten
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-11-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/4451/2019/bg-16-4451-2019.pdf
• ISSN: 1726-4170; 1726-4189

<p>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; <span class="inline-formula"><i>p</i>CO₂</span>) remains a challenge in paleoclimatology. One promising approach for reconstructing past <span class="inline-formula"><i>p</i>CO₂</span> utilizes the isotopic fractionation associated with <span class="inline-formula">CO₂</span> fixation during photosynthesis into organic matter (<span class="inline-formula"><i>ε</i>_p</span>). Previous studies have focused primarily on testing estimates of <span class="inline-formula"><i>ε</i>_p</span> derived from the <span class="inline-formula"><i>δ</i>¹³C</span> of species-specific alkenone compounds in laboratory cultures and mesocosm experiments. Here, we analyze <span class="inline-formula"><i>ε</i>_p</span> derived from the <span class="inline-formula"><i>δ</i>¹³C</span> of more general algal biomarkers, i.e., compounds derived from a multitude of species from sites near a <span class="inline-formula">CO₂</span> seep off the coast of Shikine Island (Japan), a natural environment with <span class="inline-formula">CO₂</span> concentrations ranging from ambient (ca. 310&thinsp;<span class="inline-formula">µ</span>atm) to elevated (ca. 770&thinsp;<span class="inline-formula">µ</span>atm) <span class="inline-formula"><i>p</i>CO₂</span>. We observed strong, consistent <span class="inline-formula"><i>δ</i>¹³C</span> shifts in several algal biomarkers from a variety of sample matrices over the steep <span class="inline-formula">CO₂</span> gradient. Of the three general algal biomarkers explored here, namely loliolide, phytol, and cholesterol, <span class="inline-formula"><i>ε</i>_p</span> positively correlates with <span class="inline-formula"><i>p</i>CO₂</span>, in agreement with <span class="inline-formula"><i>ε</i>_p</span> theory and previous culture studies. <span class="inline-formula"><i>p</i>CO₂</span> reconstructed from the <span class="inline-formula"><i>ε</i>_p</span> 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 <span class="inline-formula"><i>p</i>CO₂</span> sites. Our results show that naturally occurring <span class="inline-formula">CO₂</span> seeps may provide useful testing grounds for <span class="inline-formula"><i>p</i>CO₂</span> proxies and that general algal biomarkers show promise for reconstructing past <span class="inline-formula"><i>p</i>CO₂</span>.</p>