Isotopic composition of Sedimentary Alkenones
C-uptake in photosynthetic algae
There are 3 naturally occuring isotopes of carbon, 12C, 13C,14C, occuring in the following proportions: 98.9:1.1:0.0001. Variations are expressed in standard delta notation as δ13C.
All plants, including algae, convert CO2 to simple sugars during photosynthesis. In the oceans, the CO2 is sourced from atmospheric CO2 dissolved in the water (CO2(aq)). During the rubisco-mediated conversion 13CO2 reacts more slowly than 12CO2, resulting in an isotopic fractionation (εp) between the organic matter of the algae and the CO2(aq). When CO2(aq) is limited, the isotopic fractionation is reduced as rubisco has to uptake a greater proportion of the CO2(aq).
This results in a clear relationship between εp and the concentration of CO2(aq) in the seawater, which culture experiments have allowed us to define as:
Where εf is the maximum discrimination of the Rubisco enzyme in an organism, µ is the growth rate of the organism, and b’ is a species-dependent coefficient that reflects various physiological factors that govern how CO2 is transported from the environment into the cell and to the site of carbon fixation (adapted from Bidigare et al., 1997)
Reconstructing ancient εp is possible because some of the organic matter, such as the molecules called alkenones (shown above) produced by coccolithophres during photosynthesis, are well-preserved in deep sea sediments. Restricting the source of the organic matter reduces the number of uncertainties and assumptions in calculating CO2 (i.e. limiting the potential for cell size and growth rate variability, see figures below).
The value of εp can be calculated by measuring the carbon isotopic composition of the alkenones, and the carbon isotopic composition of CO2(aq) from foraminiferal shells that lived in the same water. As CO2(aq) is in equilibrium with atmpspheric CO2 in many parts of the surface ocean, atmospheric CO2 can then be calculated using Henry’s law.
There are a number of critical assumptions to calculate CO2 from alkenone isotopes:
• Algal growth rate needs to be assumed to be stable or reconstructed. Typically (e.g. Pagani et al. 1999; Badger et al. 2013; Zhang et al. 2013) low growth rate (e.g. oligotrophic) ocean sites are chosen where growth rates ought to be stable and low.
• Cell geometry can affect the diffusion of CO2 across the cell membrane (Popp et al. 1998). Using alkenones to a large degree reduces variation in cell geometry as the algae that produce them are small and close to spherical, however cell size can change (e.g. Henderiks & Pagani 2007)
• Some organisms actively uptake CO2, especially when CO2 is limited. The above model assumes CO2 is acquired exclusively through diffusion.
• Accurate temperature reconstructions are required.
Three datasets have been rejected from this compilation: (i) Pagani et al., 1999 which uses a temperature record based on oxygen isotopes in poorly preserved foraminifera (see discussion in the SOM of Pagani et al., 2010); (ii) the lith size corrected record of Seki et al., 2010 following acquisition of a new lith size record for the site (see Davis et al., 2013); (iii) the MECO record of Bijl et al. (2010) that although is likely accurate in a relative sense, its absolute CO2 is questionalbe.