470
Andersen et al.
For reconstruction of paleo-PC0 2 in the ocean,
knowledge of ambient COiaq) concentration in
surface waters and of the sea surface temperature
is needed (see definitions and calculations).
Several studies have demonstrated that Ol3C in
plankton, as well as in suspended and sedimentary
particulate organic matter, varies roughly inversely
as a function of ambient CO 2 (aq) concentration in
surface waters (McCabe 1985; Popp et al. 1989;
Rauetal.l989, 1991, 1992; Jasper and Hayes 1990;
Freeman and Hayes 1992; Fran90is et al. 1993;
Rau 1994; Bentalebetal. 1996; Fischer et al. 1998).
By measuring the Ol3C of bulk organic matter preserved in deep-sea sediments and using the above
relationships, a number of studies have reconstructed oceanic paleo-PC0 2 levels (Pedersen et
al. 1991; F ontugne and Calvert 1992; Thunell et al.
1992; Miilleretal. 1994; Westerhausenetal.1994;
Raymo and Horowitz 1996; Bentaleb et al. 1996).
Several enviromnental variables, other than dissolved CO 2 , have effects on the carbon isotopic
composition of marine phytoplankton including carbon fixation pathway (Descolas-Gros and Fontugne
1985), growth rate (Fry and Wainright 1991;
Hollander and McKenzie 1991; Fran90is etal. 1993;
Goericke et al. 1994; Laws etal. 1995; Bidigare et
al. 1997), cell geometry (popp et al. 1998b), cell
leakiness (Badger et al. 1985), species composition
(Rauetal. 1997), temperature (Degens et al. 1968)
and irradiance (Takahashi et al. 1991; Thompson
and Calvert 1994,1995; Johnston 1996). Most of
these other effects are less important, as it is known
from global data compilations (Rau 1994), that the
correlation between Ol3C of bulk organic matter
and [C0 2 (aq)] in ocean surface waters is strong.
Recent studies suggest that phytoplankton growth
rates (Laws et al. 1995; Bidigare et al. 1997) and
[C0 2 (aq)] in oceanic surface waters are the most
important factors determining the Ol3C of organic
matter (Rau et al. 1997). Therefore, estimates of
ancient concentrations of dissolved molecular carbon dioxide will require, at least, knowledge of both
the stable carbon isotopic fractionation (f) associated with photosynthesis and the phytoplankton
growth rate. Additionally, a large effect from cell
geometry on the carbon isotopic fractionation of
marine phytoplankton has been demonstrated by
Popp et al. (I 998b ) recently. The cell geometry, e.g.,
the ratio of surface area to volume, defines how
effecient CO 2 can be delivered to the inner cell to
compensate for lost CO 2 , which has been used
during photosynthesis.
Empirical and mechanistically-based models
have been used to calibrate Ol3C or Ep as proxy
parameters for reconstructing paleo-PC0 2 ; including qualitative ones (Arthur et al. 1985), linear (Rau
1994; Bentaleb et al. 1996; Fischer et al. 1998), loglinear (Freeman and Hayes 1992), and hyperbolic
models (Farquhar et al. 1982; Rau et al. 1992;
Fran90is etal. 1993, Jasperet al. 1994; Laws et al.
1995; Bidigare et al. 1997). On theoretical grounds
(Farquhar et al. 1982), a hyperbolic relationship is
favored when COiaq) is exclusively utilized for
photosynthesis and also transported only by diffusion across the cell's membrane (see definitions and
calculations). But such a hyperbolic relationship
does not provide a better fit to the field data of bulk
ol3C org than a linear fit. Almost identical linear
equations between [COiaq)] and bulk Ol3C org
were found by Rau (1994), Bentaleb et al. (1996)
and Fischer et al. (1998). Based on concurrent
measurements, Bentaleb et al. (1996) found a reliable relationship between bulk Ol3C of suspended
marine material and [C0 2 (aq)] in the Southern
Ocean only south of the Subtropical Convergence
(STC), where an autotrophic diatom flora dominates. No relationship can be established north of
the STC, because in subtropical and tropical
areas phytoplankton diversity leads to large
differences of carbon metabolisms (auto-, mixo-,
hetereotrophic ).
By using the isotopic record of a biomarker of
marine autotrophic origin, the problem of deriving
paleo-PC0 2 values from sedimentary organic carbon of mixed origins, with partly heterotrophic or
terrigenous contributions, can be avoided. To confirm any trends present in o-values of bulk sedimentary organic matter, Popp et al. (1997) pointed out
that studies about ancient PC0 2 should include
compound-specific isotopic analyses of known
phytoplankton biomarkers.
Only some years ago, a new technique was
developed to routinely measure the isotopic ratio of
individual organic compounds in complex mixtures
Andersen et al.
For reconstruction of paleo-PC0 2 in the ocean,
knowledge of ambient COiaq) concentration in
surface waters and of the sea surface temperature
is needed (see definitions and calculations).
Several studies have demonstrated that Ol3C in
plankton, as well as in suspended and sedimentary
particulate organic matter, varies roughly inversely
as a function of ambient CO 2 (aq) concentration in
surface waters (McCabe 1985; Popp et al. 1989;
Rauetal.l989, 1991, 1992; Jasper and Hayes 1990;
Freeman and Hayes 1992; Fran90is et al. 1993;
Rau 1994; Bentalebetal. 1996; Fischer et al. 1998).
By measuring the Ol3C of bulk organic matter preserved in deep-sea sediments and using the above
relationships, a number of studies have reconstructed oceanic paleo-PC0 2 levels (Pedersen et
al. 1991; F ontugne and Calvert 1992; Thunell et al.
1992; Miilleretal. 1994; Westerhausenetal.1994;
Raymo and Horowitz 1996; Bentaleb et al. 1996).
Several enviromnental variables, other than dissolved CO 2 , have effects on the carbon isotopic
composition of marine phytoplankton including carbon fixation pathway (Descolas-Gros and Fontugne
1985), growth rate (Fry and Wainright 1991;
Hollander and McKenzie 1991; Fran90is etal. 1993;
Goericke et al. 1994; Laws etal. 1995; Bidigare et
al. 1997), cell geometry (popp et al. 1998b), cell
leakiness (Badger et al. 1985), species composition
(Rauetal. 1997), temperature (Degens et al. 1968)
and irradiance (Takahashi et al. 1991; Thompson
and Calvert 1994,1995; Johnston 1996). Most of
these other effects are less important, as it is known
from global data compilations (Rau 1994), that the
correlation between Ol3C of bulk organic matter
and [C0 2 (aq)] in ocean surface waters is strong.
Recent studies suggest that phytoplankton growth
rates (Laws et al. 1995; Bidigare et al. 1997) and
[C0 2 (aq)] in oceanic surface waters are the most
important factors determining the Ol3C of organic
matter (Rau et al. 1997). Therefore, estimates of
ancient concentrations of dissolved molecular carbon dioxide will require, at least, knowledge of both
the stable carbon isotopic fractionation (f) associated with photosynthesis and the phytoplankton
growth rate. Additionally, a large effect from cell
geometry on the carbon isotopic fractionation of
marine phytoplankton has been demonstrated by
Popp et al. (I 998b ) recently. The cell geometry, e.g.,
the ratio of surface area to volume, defines how
effecient CO 2 can be delivered to the inner cell to
compensate for lost CO 2 , which has been used
during photosynthesis.
Empirical and mechanistically-based models
have been used to calibrate Ol3C or Ep as proxy
parameters for reconstructing paleo-PC0 2 ; including qualitative ones (Arthur et al. 1985), linear (Rau
1994; Bentaleb et al. 1996; Fischer et al. 1998), loglinear (Freeman and Hayes 1992), and hyperbolic
models (Farquhar et al. 1982; Rau et al. 1992;
Fran90is etal. 1993, Jasperet al. 1994; Laws et al.
1995; Bidigare et al. 1997). On theoretical grounds
(Farquhar et al. 1982), a hyperbolic relationship is
favored when COiaq) is exclusively utilized for
photosynthesis and also transported only by diffusion across the cell's membrane (see definitions and
calculations). But such a hyperbolic relationship
does not provide a better fit to the field data of bulk
ol3C org than a linear fit. Almost identical linear
equations between [COiaq)] and bulk Ol3C org
were found by Rau (1994), Bentaleb et al. (1996)
and Fischer et al. (1998). Based on concurrent
measurements, Bentaleb et al. (1996) found a reliable relationship between bulk Ol3C of suspended
marine material and [C0 2 (aq)] in the Southern
Ocean only south of the Subtropical Convergence
(STC), where an autotrophic diatom flora dominates. No relationship can be established north of
the STC, because in subtropical and tropical
areas phytoplankton diversity leads to large
differences of carbon metabolisms (auto-, mixo-,
hetereotrophic ).
By using the isotopic record of a biomarker of
marine autotrophic origin, the problem of deriving
paleo-PC0 2 values from sedimentary organic carbon of mixed origins, with partly heterotrophic or
terrigenous contributions, can be avoided. To confirm any trends present in o-values of bulk sedimentary organic matter, Popp et al. (1997) pointed out
that studies about ancient PC0 2 should include
compound-specific isotopic analyses of known
phytoplankton biomarkers.
Only some years ago, a new technique was
developed to routinely measure the isotopic ratio of
individual organic compounds in complex mixtures