Dinoflagellates — Palaeocean-CO2

Carbon isotopic composition of DinoFlagellates

Biochemical constraints

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).

Figure showing the carbon isotope fractionation in dinoflagellates as a function of CO2 in the growth media.  Taken from Hoins et al. (2015) — Figure showing the carbon isotope fractionation in dinoflagellates as a function of CO2 in the growth media. Taken from Hoins et al. (2015)

Proxy Carrier

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.

High resolution dino-cyst based palaeoclimate records at Bass River New Jersey across the Palaeocene-Eocene Thermal Maximum.  From Sluijs et al. (2007). — High resolution dino-cyst based palaeoclimate records at Bass River New Jersey across the Palaeocene-Eocene Thermal Maximum. From Sluijs et al. (2007).

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.