Alkenone 8 13 C as a Proxy for Past PC0 2 in Surface Waters:
Results from the Late Quaternary Angola Current
N. AndersenY* P. J. MUller 2 , G. Kirstl and R. R. Schneider
1 Geolofisches Institut, ETH-Zentrum, CH-8092 Zuerich, Switzerland
Fachbereich Geowissenschaften, Universitdt Bremen,
Postfach 33 0440, D-28334 Bremen, Germany
* corresponding author (email):andersen@erdw.ethz.ch
Abstract: Carbon dioxide is one ofthe most important greenhouse gases which are increasing in
atmospheric concentration due to human activities. For using natural CO, dynamics as a key to
understanding the climatic consequences of anthropogenic pCO, rise, the ocean plays an important
role due to its much larger carbon pool compared to the atmosphere. By studying the ratio of stable
carbon isotopes in organic matter from marine sediments, it is possible to estimate the partial pressure
of CO, in surface waters during ancient times. The organic compound C 37 , alkenone, whose sole
origin is from autotrophic marine algae, was chosen for Ol3C analysis and its isotopic composition
used to reconstruct past PCO, levels in the surface layer of the eastern Angola Basin for the last
200,000 years. In addition to the variation of ancient concentrations of dissolved CO, ([CO,(aq)] =
cJ, the effect of carbon demand which depends on algal growth rate was considered. Here to, carbon
isotopic fractionation ofC m alkenones (Bp) in core-top sediments from the equatorial and the South
Atlantic was calibrated against pre-industrial [CO,(aq)] and phosphate concentrations in surface
waters. From these data, a variable b = (25 %0 - e p ) * c, which reflects intracellular carbon demand was
calculated. This variable b correlates with the ambient concentration of seawater phosphate and
depends on growth rates. The bulk sediment ol5N was used as a proxy parameter for calculating
ancient b-values, taking into account that ol5N in core-top sediments is correlated to phosphate
concentration in modern surface waters. On this basis, the alkenone Ol3C record of GeoB 1016-3
documents a permanent oceanic source for atmospheric carbon dioxide during the last 200,000
years. As a consequence of using olIN derived b-values instead ofb = constant, the Angola Basin
appears to have been an even stronger CO, source during glacial periods than at present. Qualitatively
similar results were reported by Jasper et a!. (1994) for the central Equatorial Pacific. These
observations suggest that enhanced productivity of low-latitude upwelling areas during glacial
periods is not responsible for the lower CO, content of the glacial atmosphere.
Introduction
The variations in atmospheric CO, levels for the
last 200,000 years are well known from ice-core
records (Barnola et al. 1987; Jouzel et al. 1993).
Glacial levels were about 80-100 ppmv lower than
average interglacial values. In contrast, little information exists about the paleoceanic sources and
sinks of carbon dioxide. The ocean represents the
largest fast recycling carbon pool in direct
exchange with the atmosphere and contains 60
times more carbon than the atmosphere (e.g.
Siegenthaler and Sarmiento 1993). Therefore, it has
been proposed that the variations in atmospheric
paleo-pCO, are the result of changes in oceanic circulation, biological productivity, and/or chemistry
(Broecker 1982; Knox and McElroy 1984;
Sanniento and Toggweiler 1984; Boyle 1988; Keir
1988, 1990, 1995; Sarnthein et al. 1988; Lyle and
Pisias 1990; Martin 1990; Heinze et al. 1991;
Pedersen et al. 1991; Paillard et al. 1993; Ganeshram
et al. 1995; Franyois et a!. 1997).
From FISCHER G. WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 469-488
Results from the Late Quaternary Angola Current
N. AndersenY* P. J. MUller 2 , G. Kirstl and R. R. Schneider
1 Geolofisches Institut, ETH-Zentrum, CH-8092 Zuerich, Switzerland
Fachbereich Geowissenschaften, Universitdt Bremen,
Postfach 33 0440, D-28334 Bremen, Germany
* corresponding author (email):andersen@erdw.ethz.ch
Abstract: Carbon dioxide is one ofthe most important greenhouse gases which are increasing in
atmospheric concentration due to human activities. For using natural CO, dynamics as a key to
understanding the climatic consequences of anthropogenic pCO, rise, the ocean plays an important
role due to its much larger carbon pool compared to the atmosphere. By studying the ratio of stable
carbon isotopes in organic matter from marine sediments, it is possible to estimate the partial pressure
of CO, in surface waters during ancient times. The organic compound C 37 , alkenone, whose sole
origin is from autotrophic marine algae, was chosen for Ol3C analysis and its isotopic composition
used to reconstruct past PCO, levels in the surface layer of the eastern Angola Basin for the last
200,000 years. In addition to the variation of ancient concentrations of dissolved CO, ([CO,(aq)] =
cJ, the effect of carbon demand which depends on algal growth rate was considered. Here to, carbon
isotopic fractionation ofC m alkenones (Bp) in core-top sediments from the equatorial and the South
Atlantic was calibrated against pre-industrial [CO,(aq)] and phosphate concentrations in surface
waters. From these data, a variable b = (25 %0 - e p ) * c, which reflects intracellular carbon demand was
calculated. This variable b correlates with the ambient concentration of seawater phosphate and
depends on growth rates. The bulk sediment ol5N was used as a proxy parameter for calculating
ancient b-values, taking into account that ol5N in core-top sediments is correlated to phosphate
concentration in modern surface waters. On this basis, the alkenone Ol3C record of GeoB 1016-3
documents a permanent oceanic source for atmospheric carbon dioxide during the last 200,000
years. As a consequence of using olIN derived b-values instead ofb = constant, the Angola Basin
appears to have been an even stronger CO, source during glacial periods than at present. Qualitatively
similar results were reported by Jasper et a!. (1994) for the central Equatorial Pacific. These
observations suggest that enhanced productivity of low-latitude upwelling areas during glacial
periods is not responsible for the lower CO, content of the glacial atmosphere.
Introduction
The variations in atmospheric CO, levels for the
last 200,000 years are well known from ice-core
records (Barnola et al. 1987; Jouzel et al. 1993).
Glacial levels were about 80-100 ppmv lower than
average interglacial values. In contrast, little information exists about the paleoceanic sources and
sinks of carbon dioxide. The ocean represents the
largest fast recycling carbon pool in direct
exchange with the atmosphere and contains 60
times more carbon than the atmosphere (e.g.
Siegenthaler and Sarmiento 1993). Therefore, it has
been proposed that the variations in atmospheric
paleo-pCO, are the result of changes in oceanic circulation, biological productivity, and/or chemistry
(Broecker 1982; Knox and McElroy 1984;
Sanniento and Toggweiler 1984; Boyle 1988; Keir
1988, 1990, 1995; Sarnthein et al. 1988; Lyle and
Pisias 1990; Martin 1990; Heinze et al. 1991;
Pedersen et al. 1991; Paillard et al. 1993; Ganeshram
et al. 1995; Franyois et a!. 1997).
From FISCHER G. WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 469-488