Carbon isotopic composition of DinoFlagellates
Dinoflagellates, like the coccolithophorids that produce Alkenones (see here) fix CO2 from seawater via photosynthesis. Recent culturing experiments on dinoflagellates, shown in the figure below indicate there is a marked 13C fractionation relative to dissolved CO2 (εp) that varies as a function of CO2 concentration in the growth medium.
As well as using RuBisCo to fix CO2 they also use carbon concentrating mechanisms (CCMs) that increase internal CO2 concentrations and drive additional isotopic fractionations (see Hoins et al. 2016).
13C fractionation varies substantially between species, and may be suppressed in very large cells (Hoins et al., 2015; Hoins et al., 2016b). This means the best proxy carrier would be dinoflagellate cysts (see picture at top of page) that are widely preserved in deep ocean sediments and used for palaeoclimate reconstructions (see figure below).
Dinocysts can be identified to the species level, and specimens of the same size can be selected for δ13C analysis. This avoids problematic effects of mixing various species and sizes with different fractionation factors and effects. Recently analytical innovations now allow for analyses of single to a few specimens, providing the analytical requirements for such work (Van Roij et al., 2017). In addition, one of the main target species Spiniferites ramosus (the cyst of G. spinifera) is widely distributed in space and time.
Current Proxy Development Status
The dinocyst CO2 barometer is still in its infancy and multiple outstanding issues currently prevent quantitative CO2 reconstructions using this proxy. These include:
- It is still unclear how the δ13C values of motile cells relate to those of cysts.
- The effect of temperature on 13C fractionation is unknown.
- The influence of nutrient concentrations on dinoflagellate δ13C is unknown.
Since the potential of this new proxy are so great there are a number of ongoing efforts to address these issues (see here for who is doing this), with work in the near future including the validation of the approach for Pleistocene glacial-interglacial cycles, exploring the proxy sensitivity for low CO2 concentrations.