Validation of carbon isotope fractionation in algal lipids as a <i>p</i>CO<sub>2</sub> proxy using a natural CO<sub>2</sub> seep (Shikine Island, Japan)

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.

Testing algal-based pCO2 proxies at a modern CO2 seep (Vulcano, Italy)

“…2). Consistently, the most abundant among these compounds were loliolide, cholesterol, and phytol, as were also observed at the CO 2 seep site in Japan 50 .…”

“…Several possibilities may explain why the full expression of Ɛ f has not been reached. For one, given the relatively small area of the bay, it is possible that surface sediment has moved around the bay over time due to tidal actions and bottom water currents, which dampens the overall signal by allochthonous organic matter transported from area's outside of the bay, as also inferred for Shikine Island 50 . Furthermore, algae are unlikely to grow and deposit in precisely same location and given that the impact of the CO 2 seep noticeably changes over tens of meters 43 , this likely leads to some mixed signal among sites, resulting in a suppressed signal.…”

“…Studies have suggested an empirical average 170‰ kg µM −1 ± 43 kg µM −1 s.d. for b based on a compilation of δ 13 C values of bulk organic matter in marine surface sediments, as well as some limited phytol studies 39,50 . Furthermore, we use an average Ɛ f for phytoplankton species of 26.5‰ ± 1.5‰ uniform distribution 39 based on the 25 to 28‰ range observed in laboratory cultures 69 .…”

Elevated CO<sub>2</sub>, increased leaf-level productivity, and water-use efficiency during the early Miocene

“…5c). Early Miocene tropical trees appear to have a slightly higher total annual carbon flux (A tot Q 1 -Q 3 = 596-1220 g C m −2 yr −1 ) than today (A tot Q 1 -Q 3 = 329-721 g C m −2 yr −1 ), which, with a yearround growing season in the early Miocene (like today), is likely attributable to a leaf-level fertilization effect, similar to what is observed in modern carbon fertilization experiments (Norby et al, 2003;Bader et al, 2013;Yang et al, 2016). Although this estimate cannot take the number of leaves per unit area into account, these results suggest enhanced leaf-level productivity during higher than modern C a in the early Miocene.…”

“…5b), a somewhat surprising result because G w is expected to be reduced in high C a climates (Franks and Beerling, 2009). Studies on modern forests also suggest the absence of a reduction of G w to enhanced C a (Yang et al, 2016;Gimeno et al, 2018) or even an increase in the G w (Frank et al, 2015). A longer growing season together with increasing VPD was proposed to explain increasing G w in modern European forests (Frank et al, 2015).…”