Controls on the 13C of primary photosynthate from suspended particulate matter in the equatorial Pacific and in waters along the coastof Peru are evaluated using the 13C of individual lipid biomarkers. In particular, the sterols 24-methyl-cholesta-5(24)-dienol and 4,23,24-trimethylcholest-22E-enol (dinosterol) and the 37:2 alkenone are employed as markers for diatoms, dinoflagellate and hapto-phyte algae, respectively. Controls on the isotopic abundances of the alkenone include [CO2(aq)] and growth rates. This is demonstrated from the observation that a variable which reflects intracellular carbon demand (based on isotopic fractionation factors for haptophytes from alkenone- isotope data) correlates with ambient concentrations of seawater phosphate in both environments. This finding is consistent with the recent compilation of similar data by Bidigare et al. for a wide variety of open-ocean and coastal settings.
In these sampling localities, controls on the 13C abundance in diatom and dinoflagellate sterols are more complex, and in particular, growth rates and the possible activity of alternative mechanisms (besides diffusion) for transporting inorganic
carbon into cells appear to strongly influence the 13C-content of marine diatoms. In addition, diatom cell geometry and size also influence the isotopic compositions of the diatom sterol. However, size-based variations are not strictly a function of nominal diameter (as inferred from filter pore diameter), but vary with the surface area to volume ratios of the typically non-spherical diatom cells.
Isotopic and concentration data for taxon-specific biomarkers are compared to that for more general phytoplankton
molecular markers (i.e., phytol) and total suspended organic matter. Relative abundances and 13C-contents of each taxonomic group can influence the composition of bulk materials. These findings demonstrate that the 13C-content of biomarkers for
haptophytes is a promising proxy for paleo-pCO2, provided paleo-nutrient concentrations are available. In addition, the utility of other materials (including diatom and dinoflagellate biomarkers and bulk organic matter) for purposes of paleo-barometry should be viewed with caution.