Algal biomarkers as a proxy for pCO2: Constraints from late quaternary sapropels in the eastern Mediterranean
Witkowski, C.R.; van der Meer, M.T.J.; Blais, B.; Sinninghe Damsté, J.S.; Schouten, S. (2020). Algal biomarkers as a proxy for pCO2: Constraints from late quaternary sapropels in the eastern Mediterranean. Org. Geochem. 150: 104123. https://doi.org/10.1016/j.orggeochem.2020.104123 In: Organic Geochemistry. Elsevier: Oxford; New York. ISSN 0146-6380; e-ISSN 1873-5290, more |   |
| Author keywords | pCO2; organic geochemical proxies; sapropels; phytol; alkenones |
| Authors | | Top | - Witkowski, C.R., more
- van der Meer, M.T.J., more
- Blais, B.
| - Sinninghe Damsté, J.S., more
- Schouten, S., more
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| Abstract | Records of carbon dioxide concentrations (partial pressure expressed as pCO2) over Earth’s history provide trends that arecritical to understand our changing world. To better constrain pCO 2 estimations, here we test organic pCO2 proxies against the direct measurements of pCO2 recorded in ice cores. Based on the concept of stable carbon isotopic fractionation due to photosynthetic CO2 fixation (Ɛp), we use the stable carbon isotopic composition (δ13C) of the recently proposed biomarker phytol (from all photoautotrophs), as well as the conventionally used alkenone biomarkers (from specific species) for comparison, to reconstruct pCO2 over several Quaternary sapropel formation periods (S1, S3, S4, and S5) in the eastern Mediterranean Sea. The reconstructed pCO2 values are within error of the ice core values but consistently exceed the ice core values by ca. 100 µatm. This offset corresponds with atmospheric disequilibrium of present day CO2[aq] concentrations in the Mediterranean Sea from global pCO2, equivalent to ca. 100 µatm, although pCO2 estimates derived from individual horizons within each sapropel do not covary with the ice core values. This may possibly be due to greater variability in local CO2[aq] concentration changes in the Mediterranean, as compared with the global average pCO2, or possibly due to biases in the proxy, such as variable growth rate or carbon-concentrating mechanisms. Thus, the offset is likely a combination of physiological or environmental factors.Nevertheless, our results demonstrate that alkenone- and phytol-basedpCO2 proxies yield statistically similar estimations ( P-value = 0.02, Pearson’s r-value = 0.56), and yield reasonable absolute estimations although with relatively large uncertainties (±100 µatm). |
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