Stable Carbon Isotopes in Benthic Foraminifera
231
Actually, if one is able to quantify exactly the
biologically driven influence on the &13C IC02 of a
deep water mass, the subtraction of this biologic
effect would leave the thermodynamic imprint as
a conservative tracer after the water mass is out
of contact with the atmosphere (Lynch-Stieglitz and
Fairbanks 1994). However, although in principle it
is possible to predict a & 13C IC02 value of a given
deep water mass for an ocean without air-sea carbon isotope exchange (Broecker and Maier-Reimer
1992), this kind of quantification of the biologic effect depends on assumptions like a constant photosynthetic isotopic fractionation for all primary
producers and a constant Redfield ratio, both of
which are unlikely on a world-wide scale.
Calibration ofForaminiferal8 13 C versus
813CIC02 of Bottom Water
The benthic foraminifer Anomalina wuellerstorfi
Schwager 1866, more recently assigned to
Planulina, Cibicidoides, or Cibicides and now to
Fontbotia wuellerstorfi (Schwager) (Loeblich and
Tappan 1988), and some other closely related genera, are used by paleoceanographers for
reconstructing the &l3C EC02 of former ocean deep
and bottom-water masses. The &13C values, in turn,
are used as proxies for past nutrient and oxygen
(apparent oxygen utilization) distributions with implications for ocean circulation patterns and atmospheric CO 2 , Thus &13C records of benthic and
planktonic foraminifera are widely used as indicators for changes in paleoproductivity and deep
water circulation during late Quaternary climatic
cycles (e.g. Curry et al. 1988; Duplessy et al. 1988;
Oppo et al. 1990; Raymo et al. 1990; Mackensen
etal. 1994; Sarnthein etal. 1994; Bickert and Wefer
1996). Dramatic changes in oceanic circulation
patterns and carbon system chemistry have occurred during glacial/interglacial global climate cycles (Schnitker 1974; Streeter and Shackleton 1979;
Boyle and Keigwin 1987; Curry et al. 1988;
Duplessy et al. 1988; Oppo et al. 1990; Boyle 1992).
Because photosynthesis in the ocean produces
organic matter depleted in l3C by about 20 %0 relative to seawater &l3C IC02 ' decomposition of organic
matter within the uppermost surface sediment releases l3C-depleted CO 2 to the interstitial water.
Even if dissolution of sedimentary carbonate enriched by roughly 2 %0 to Pee Dee Belemnite
(PDB), which is typical for planktic foraminiferal
ooze, is taken into account, it is evident that the
&13C EC02 of the pore water and the & 13 C gradient
at the sediment/water interface is largely dependent on the flux of organic matter to the seafloor. This
is reflected by the &13C of deep-infaunal taxa like,
for instance, Globobulimina spp. (Grossman 1984;
Mackensen and Douglas 1989; McCorkle et al.
1990). Even Uvigerina peregrina, probably a shallow infaunal, preferentially within the uppermost
sediment centimetre living species, was shown to
be influenced in its carbon isotopic composition by
the organic carbon content of the sediment (Zahn
et al. 1986).
Fontbotia wuellerstorfi, however, and most of
the species of the related genera Cibicides and
Cibicidoides are shown to secrete calcite, not in
isotopic equilibrium with the ambient bottom-water
&13C IC02 ' but in a constant 1: 1 relationship (Woodruff et al. 1980; Belanger et al. 1981; Graham et
al. 1981; Zahn et al. 1986). Moreover, since Lutze
and Thiel (1989) suggested that this species prefers to live in an epibenthic or even elevated position on or above the sediment surface, previous
studies have assumed that species of this generic
group record the &13C IC02 of the overlying bottom
water faithfully.
However, it has been noticed before that some
single late Pleistocene &13C values of Cibicidoides
were much lower relative to what was measured
or expected of the ambient seawater (Sarnthein
et al. 1988). Because these low values were
usually associated with upwelling or coastal
high-productivity areas, it was deduced that these
epibenthic specimens calcified their tests in an environment strongly depleted in l3C at levels more
characteristic of endobenthic living species.
Estimation of Global 8 13 C IC02 Shifts
between Reservoirs
For the global shift of the l3C/I2C ratio in LC0 2
of seawater between the last glacial maximum and
today Duplessy et al. (1988) give a value of
0.32 %0, Curry et al. (1988) a value of 0.46 %0.
While the latter value is an average value of cores
231
Actually, if one is able to quantify exactly the
biologically driven influence on the &13C IC02 of a
deep water mass, the subtraction of this biologic
effect would leave the thermodynamic imprint as
a conservative tracer after the water mass is out
of contact with the atmosphere (Lynch-Stieglitz and
Fairbanks 1994). However, although in principle it
is possible to predict a & 13C IC02 value of a given
deep water mass for an ocean without air-sea carbon isotope exchange (Broecker and Maier-Reimer
1992), this kind of quantification of the biologic effect depends on assumptions like a constant photosynthetic isotopic fractionation for all primary
producers and a constant Redfield ratio, both of
which are unlikely on a world-wide scale.
Calibration ofForaminiferal8 13 C versus
813CIC02 of Bottom Water
The benthic foraminifer Anomalina wuellerstorfi
Schwager 1866, more recently assigned to
Planulina, Cibicidoides, or Cibicides and now to
Fontbotia wuellerstorfi (Schwager) (Loeblich and
Tappan 1988), and some other closely related genera, are used by paleoceanographers for
reconstructing the &l3C EC02 of former ocean deep
and bottom-water masses. The &13C values, in turn,
are used as proxies for past nutrient and oxygen
(apparent oxygen utilization) distributions with implications for ocean circulation patterns and atmospheric CO 2 , Thus &13C records of benthic and
planktonic foraminifera are widely used as indicators for changes in paleoproductivity and deep
water circulation during late Quaternary climatic
cycles (e.g. Curry et al. 1988; Duplessy et al. 1988;
Oppo et al. 1990; Raymo et al. 1990; Mackensen
etal. 1994; Sarnthein etal. 1994; Bickert and Wefer
1996). Dramatic changes in oceanic circulation
patterns and carbon system chemistry have occurred during glacial/interglacial global climate cycles (Schnitker 1974; Streeter and Shackleton 1979;
Boyle and Keigwin 1987; Curry et al. 1988;
Duplessy et al. 1988; Oppo et al. 1990; Boyle 1992).
Because photosynthesis in the ocean produces
organic matter depleted in l3C by about 20 %0 relative to seawater &l3C IC02 ' decomposition of organic
matter within the uppermost surface sediment releases l3C-depleted CO 2 to the interstitial water.
Even if dissolution of sedimentary carbonate enriched by roughly 2 %0 to Pee Dee Belemnite
(PDB), which is typical for planktic foraminiferal
ooze, is taken into account, it is evident that the
&13C EC02 of the pore water and the & 13 C gradient
at the sediment/water interface is largely dependent on the flux of organic matter to the seafloor. This
is reflected by the &13C of deep-infaunal taxa like,
for instance, Globobulimina spp. (Grossman 1984;
Mackensen and Douglas 1989; McCorkle et al.
1990). Even Uvigerina peregrina, probably a shallow infaunal, preferentially within the uppermost
sediment centimetre living species, was shown to
be influenced in its carbon isotopic composition by
the organic carbon content of the sediment (Zahn
et al. 1986).
Fontbotia wuellerstorfi, however, and most of
the species of the related genera Cibicides and
Cibicidoides are shown to secrete calcite, not in
isotopic equilibrium with the ambient bottom-water
&13C IC02 ' but in a constant 1: 1 relationship (Woodruff et al. 1980; Belanger et al. 1981; Graham et
al. 1981; Zahn et al. 1986). Moreover, since Lutze
and Thiel (1989) suggested that this species prefers to live in an epibenthic or even elevated position on or above the sediment surface, previous
studies have assumed that species of this generic
group record the &13C IC02 of the overlying bottom
water faithfully.
However, it has been noticed before that some
single late Pleistocene &13C values of Cibicidoides
were much lower relative to what was measured
or expected of the ambient seawater (Sarnthein
et al. 1988). Because these low values were
usually associated with upwelling or coastal
high-productivity areas, it was deduced that these
epibenthic specimens calcified their tests in an environment strongly depleted in l3C at levels more
characteristic of endobenthic living species.
Estimation of Global 8 13 C IC02 Shifts
between Reservoirs
For the global shift of the l3C/I2C ratio in LC0 2
of seawater between the last glacial maximum and
today Duplessy et al. (1988) give a value of
0.32 %0, Curry et al. (1988) a value of 0.46 %0.
While the latter value is an average value of cores