Alkenone Ol3C as a Proxy for Past PC0 2 in Surface Waters
471
(GC-IRMS method; Hayes et al. 1990; Freeman
et al. 1990). To estimate the concentration of CO 2
dissolved in ancient oceans, the carbon-isotopic
composition of phytoplankton biomarkers like geoporphyrins (with conventional mass spectrometry
- Popp et al. 1989; Freeman and Hayes 1992) and
the C37:2 alkenone (Jasper and Hayes 1990;
Jasper and Hayes 1994; Jasper et al. 1995) were
measured in marine sediments. The latter compound
is an exclusive product ofhaptophytes, e.g. the marine coccolithophoridEmiliania huxleyi. This alga
species is generally regarded as the predominant
present-day source of alkenones in open ocean
sediments (e.g. Brassell 1993). By analysing the
carbon-isotopic composition of sedimentary C 3702
alkenones, Jasper and Hayes (1990) were able to
estimate ancientPC0 2 from sediments ofthe northern Gulf of Mexico, where a large contribution of
terrigenous organic carbon significantly affected the
total organic carbon Ol3C.
There are several reasons why compoundspecific isotopic analyses of alkenones seem to be
especially suitable for PC0 2 reconstructions:
(1) from both culture experiments (Bidigare et
al. 1997; Popp et al. 1998a) and natural studies
(Bidigare et al. 1997), there are good indications that
alkenone Ep depends on Co"
(2) In contrast to other biomarkers, the
fractionation between aiken ones and the total
biomass of alkenone producers (Ealkenone) is well
known and constant. No effect of growth rate on
Ealkenone was found for chemostat cultures of
E. huxleyi (Popp et al. 1998a). This is an important requirement for paleo-PC0 2 reconstructions
using compound-specific isotopic analyses (Popp et
al. 1997, 1998a).
(3) In contrast to other biomarkers (e.g. from
diatoms, Pancost et al. 1997), the cell geometry has
no important effect to be considered for PC0 2 -reconstructions using alkenones (Popp et al. 1998b).
The alkenone source in open-marine waters
is highly specific causing the cell geometry of
alkenone producers to be nearly constant (Popp et
al. 1998b).
(4) In contrast to other algae (Burns and
Bearda1l1987; Raven and Johnston 1991), there are
strong indications that, with normal sea water conditions, CO 2 is reaching the photosynthetic site in
important marine alkenone producers only by passive diffusion (Paasche 1964; Bidigare et al. 1997;
NimerandMerrett 1996; Nimeretal. 1996, 1997).
Therefore Ep can be described by a simple equation (Bidigare et al. 1997) and most likely be used
as a paleo-barometer. The use of other biomarkers
may be more difficult. In contrast to the alkenone
Ep from the Peru upwelling region, the isotopic
fractionation of a diatom-derived biomarker exhibits a large scatter and no correlation with local
PO/c e (Pancost et al. 1997). This may indicate
active transport mechanisms during diatom photosynthesis or variable cell geometry.
(5) the paleo-water temperature, also needed
for PC0 2 calculations, can be estimated from the
unsaturation index of alkenones (e.g. Miiller et al.
1998). Since both factors are derived from the C 37
alkenones, their signals have been recorded under
identical conditions (e.g. same water depth and life
cycle).
(6) as shown by feeding experiments (Grice et
al. 1998), the stable carbon isotopic composition of
alkenones remains constant during zooplankton
herbivory and can therefore still be used as a proxy
parameter. This is important, because most of the
sedimentary organic material reaches the ocean
floor by the large particles and faecal pellets produced by zooplankton herbivory are often a major
contributor to this flux (e.g. Volkman et al. 1980).
In the present study we analysed the Ol3C of
C 3702 alkenones for reconstructing paleo-PC0 2 •
The sediment core studied (GeoB 1016-3) is from
the Angola Current (AC), which presently is a
source region for atmospheric CO 2 throughout the
year (Takahashi et al. 1993). Miiller et al. (1994)
have previously estimated paleo-PC0 2 for the same
sediment core as investigated in this paper.
They used the isotopic composition of bulk organic
carbon and sea-surface temperatures derived
from alkenone data to calculate PC0 2 according
to models of Popp et al. (1989) and Rau et al.
(1989,1991). According to their results the Angola
Current was a permanent CO 2 source during
glacial periods, but weaker compared to interglacial periods.
471
(GC-IRMS method; Hayes et al. 1990; Freeman
et al. 1990). To estimate the concentration of CO 2
dissolved in ancient oceans, the carbon-isotopic
composition of phytoplankton biomarkers like geoporphyrins (with conventional mass spectrometry
- Popp et al. 1989; Freeman and Hayes 1992) and
the C37:2 alkenone (Jasper and Hayes 1990;
Jasper and Hayes 1994; Jasper et al. 1995) were
measured in marine sediments. The latter compound
is an exclusive product ofhaptophytes, e.g. the marine coccolithophoridEmiliania huxleyi. This alga
species is generally regarded as the predominant
present-day source of alkenones in open ocean
sediments (e.g. Brassell 1993). By analysing the
carbon-isotopic composition of sedimentary C 3702
alkenones, Jasper and Hayes (1990) were able to
estimate ancientPC0 2 from sediments ofthe northern Gulf of Mexico, where a large contribution of
terrigenous organic carbon significantly affected the
total organic carbon Ol3C.
There are several reasons why compoundspecific isotopic analyses of alkenones seem to be
especially suitable for PC0 2 reconstructions:
(1) from both culture experiments (Bidigare et
al. 1997; Popp et al. 1998a) and natural studies
(Bidigare et al. 1997), there are good indications that
alkenone Ep depends on Co"
(2) In contrast to other biomarkers, the
fractionation between aiken ones and the total
biomass of alkenone producers (Ealkenone) is well
known and constant. No effect of growth rate on
Ealkenone was found for chemostat cultures of
E. huxleyi (Popp et al. 1998a). This is an important requirement for paleo-PC0 2 reconstructions
using compound-specific isotopic analyses (Popp et
al. 1997, 1998a).
(3) In contrast to other biomarkers (e.g. from
diatoms, Pancost et al. 1997), the cell geometry has
no important effect to be considered for PC0 2 -reconstructions using alkenones (Popp et al. 1998b).
The alkenone source in open-marine waters
is highly specific causing the cell geometry of
alkenone producers to be nearly constant (Popp et
al. 1998b).
(4) In contrast to other algae (Burns and
Bearda1l1987; Raven and Johnston 1991), there are
strong indications that, with normal sea water conditions, CO 2 is reaching the photosynthetic site in
important marine alkenone producers only by passive diffusion (Paasche 1964; Bidigare et al. 1997;
NimerandMerrett 1996; Nimeretal. 1996, 1997).
Therefore Ep can be described by a simple equation (Bidigare et al. 1997) and most likely be used
as a paleo-barometer. The use of other biomarkers
may be more difficult. In contrast to the alkenone
Ep from the Peru upwelling region, the isotopic
fractionation of a diatom-derived biomarker exhibits a large scatter and no correlation with local
PO/c e (Pancost et al. 1997). This may indicate
active transport mechanisms during diatom photosynthesis or variable cell geometry.
(5) the paleo-water temperature, also needed
for PC0 2 calculations, can be estimated from the
unsaturation index of alkenones (e.g. Miiller et al.
1998). Since both factors are derived from the C 37
alkenones, their signals have been recorded under
identical conditions (e.g. same water depth and life
cycle).
(6) as shown by feeding experiments (Grice et
al. 1998), the stable carbon isotopic composition of
alkenones remains constant during zooplankton
herbivory and can therefore still be used as a proxy
parameter. This is important, because most of the
sedimentary organic material reaches the ocean
floor by the large particles and faecal pellets produced by zooplankton herbivory are often a major
contributor to this flux (e.g. Volkman et al. 1980).
In the present study we analysed the Ol3C of
C 3702 alkenones for reconstructing paleo-PC0 2 •
The sediment core studied (GeoB 1016-3) is from
the Angola Current (AC), which presently is a
source region for atmospheric CO 2 throughout the
year (Takahashi et al. 1993). Miiller et al. (1994)
have previously estimated paleo-PC0 2 for the same
sediment core as investigated in this paper.
They used the isotopic composition of bulk organic
carbon and sea-surface temperatures derived
from alkenone data to calculate PC0 2 according
to models of Popp et al. (1989) and Rau et al.
(1989,1991). According to their results the Angola
Current was a permanent CO 2 source during
glacial periods, but weaker compared to interglacial periods.