Prospects for an archaeal lipid pCO2 paleobarometer
Abstract
Understanding the relationship between Earth's temperature and atmospheric carbon dioxide (CO2) concentration is essential to climate research. All estimates of pCO2 from >1 Ma rely on proxy reconstructions, each with different uncertainties and underlying assumptions. Despite efforts to expand the types of geologic samples these proxies are based on, there is not yet consensus about the most accurate history of Cenozoic pCO2 [1]. Recently we proposed a proxy based on the stable carbon isotope signature recorded in archaeal lipids, ɛAr [2, 3]. Like the phytoplankton proxy ɛP, ɛAr is based on lipids of water-column origin recovered from marine sediments and reflects the isotopic difference between seawater HCO3- and crenarchaeol, a product of Thaumarchaeota (ɛAr ≈ δ13CHCO3 - δ13Ccren). The proxy rationale is derived from empirical field observations [2] and from a corroborating theoretical model [3]. In the marine water column, ɛAr correlates with all measures of respiration intensity, including increased CO2 and NO3- concentrations and decreased O2. In contrast to ɛP, values of ɛAr become larger at low CO2(aq). Because Thaumarchaeota use the bicarbonate-requiring 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB) pathway of carbon fixation, we proposed that they are dependent on CO2 diffusion, followed by intracellular catalysis to HCO3-, implying that ɛAr should be a function of both CO2(aq) and growth rate [3]. We will discuss the resulting prospects and limitations for paleobarometry, including its likely useful range (200 - 1500 ppm pCO2) and sources of uncertainty.
[1] Beerling DJ, Royer DL (2011) Convergent Cenozoic CO2 history. Nature Geoscience 4, 418-420. [2] Hurley, S.J., et al. (2019) CO2-dependent carbon isotope fractionation in Archaea, Part II: Natural marine planktonic populations. GCA, doi:10.1016/j.gca.2019.06.043. [3] Pearson A, et al. (2019) CO2-dependent carbon isotope fractionation in Archaea, Part I: Modeling the 3HP/4HB pathway. GCA, doi:10.1016/j.gca.2019.06.042.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMPP42A..07P
- Keywords:
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- 1626 Global climate models;
- GLOBAL CHANGE;
- 4902 Anthropogenic effects;
- PALEOCEANOGRAPHY;
- 4930 Greenhouse gases;
- PALEOCEANOGRAPHY;
- 4932 Ice cores;
- PALEOCEANOGRAPHY