The carbon isotope fractionation in the coccolithophore
Emiliania huxleyi constitutes the basis for the paleo-pCO2
barometry. Under the premise that the carbon isotope
fractionation is dependent on the availability of dissolved
CO2, measurements of the carbon isotope composition of
sedimentary alkenones can potentially produce a record of
ancient atmospheric CO2 levels. Resent studies have
suggested that other factors than CO2 may influence the
carbon isotope fractionation in Emiliania huxleyi.

The effects of irradiance on carbon isotope fractionation
were studied in batch cultures of non-calcifying Emiliania
huxleyi It was found that the biomass became more ^13 C
depleted as the light intensity decreased ? a result
consistent with utilization of CO2 via passive diffusion.
However, previously reported results for a calcifying strain
show the opposite trend with a ^13 C enrichment of the
biomass, suggesting that the carbon utilization of the
calcifying strain of Emiliania huxleyi differ from that of
the non-calcifying strain. This is in agreement with several
studies indicating a connection between the process of
calcification and the supply of carbon for photosynthesis. A
mechanism for the effect of calcification on carbon isotope
fractionation in light limited cells is presented in this
thesis.

To study the effect of these environmental parameters on
carbon isotope fractionation, C37:2-alkenones were extracted
from samples of particulate organic matter. The particulate
organic matter was collected together with information of
the environmental conditions during three cruises in the
North-East Pacific and during a Pacific transect from
Victoria B.C. to Guam. Results from the NE Pacific show a
lower carbon isotope fractionation in samples collected at
the bottom of the euphotic zone compared to samples
collected in the mixed layer.

Apart from light, the availability of nutrients have also
been shown in the literature to affect calcification. The
results from the cruises supported the hypothesis of
nutrient influence on carbon isotope fractionation by the
observation of a correlation between isotope fractionation
and the ratio of phosphate concentration to concentration
of dissolved CO2. Nitrate availability appears to play an
important role in maintaining this relationship as in the
absence of nitrate the carbon isotope fractionation is lower
than could be predicted from the carbon isotope
fractionation -[PO4^3-/[CO2]aq. relationship. In addition,
the C37:2-alkenone based results from the Pacific transect
show a strong correlation between carbon isotope
fractionation and phosphate. This correlation is independent
of the concentration of dissolved CO2 implying a nutrient
dominated control of isotope fractionation.

In conclusion, the results presented in this thesis strongly
suggest that the carbon isotope fractionation in Emiliania
huxleyi is not a direct function of the availability of
dissolved CO2 but rather a result of a complex interaction
between environmental factors such as irradiance and
nutrient availability. In particular, a correlation between
phosphate concentration and carbon isotope fractionation has
been found. This relationship may have been the true
causative factor in the reported cases of Paleo-PCO2 barometry.

The use of carbon isotope composition of alkenones as
Paleo-PCO2 proxies is therefore unadvisable and alternative
biomarkers should be considered.