Reassessing Foraminiferal Stable Isotope Geochemistry
491
Mackensen et al. (1993) hypothesised that growth
and reproduction of benthic foraminifers coincides
with the seasonal flux of phytodetritus. Because
Ol3C of freshly accumulated phytodetritus was 3
to 4 0/00 lower than surface sediment organic carbon they argued that ol3C of epibenthic shells,
formed during remineralisation of this fluffy layer
(locally and temporarily depleting 0l3C LC02 )' is depleted compared to 813CLC02 found throughout the
rest of the year.
The main complication associated with the interpretation of Ol3C records from foraminiferal
shells is a prominent vital effect. Already Craig
(Revelle and Fairbridge 1957) had shown that
the carbon isotopes in foraminiferal shells are precipitated out of equilibrium with8 13 C ofkCO z and
potentially reflect utilization of metabolic cq. The
large range of disequilibrium precipitation was confirmed by many studies (for a review see e.g.
Berger 1979; Duplessy 1978). A milestone for
the investigation of vital effects on stable isotopic
fractionation was the establishment of culture techniques for planktic foraminifers by the research
groups of Allen Be, Roger Anderson, Christoph
Hemleben and Michael Spindler (e.g. Be 1979; Be
1980; Be 1982; Be and Anderson 1976; Be et al.
1979; Be et al. 1977; Hemleben and Spindler 1983;
Hemleben et al. 1989; Hemleben et al. 1985;
Hemleben et al. 1987; Spindler and Hemleben
1980). Using laboratory cultures, Erez and Luz
(1983) derived the first empirical paleotemperature
equation for Globigerinoides sacculifer and, for
instance, Bouvier-Soumagnac and Duplessy (1985)
were able to verify their plankton tow based
temperature:o l8 0 relationships. Culture experiments were also imperative to investigate the impact of algal symbiosis on the shello l3 C composition of benthic (Erez 1978b; Williams et al. 1981 b;
Zimmermann et al. 1983) and planktic foraminifers
(Spero and DeNiro 1987). In contrast to benthic
foraminifers and some corals (Erez 1978b; Land
et al. 1977), Spero and Deniro concluded that
increased symbiont photosynthetic activity in
Orbulina universa, resulted in a 13C enrichment
of the shell. Their conclusion was supported
by other studies (Cummings and McCarty 1982;
Goreau 1977; Weber and Woodhead 1970)
and later verified for other species of planktic
foraminifers (Bijma et al. 1998; Spero and Lea
1993; Spero et al. 1991). The enrichment was attributed to the preferential removal of l2 C0 2 by the
CO 2 fixing enzyme ribulose 1,5-biphosphate
carboxylase-oxygenase (RUBISCO) during symbiont photosynthesis, enriching the calcifying environment in l3C. Host respiration was shown to
deplete the shell isotopic composition (e.g. Bijma
et al. 1998; Spero and Lea 1996). This observation
was explained by the contribution of metabolic cq,
enriched in IZC, to the calcifying environment.
During the past four decades, stable oxygen and
carbon isotope measurements on biogenic calcite
and aragonite (for a review see e.g. Wefer and
Berger 1991) have become standard tools for reconstructing paleoceanographic and paleoclimatic
change. By assuming that our understanding of the
major parameters controlling stable isotope incorporation into biogenic calcite is complete, these
geochemical proxies have been used to reconstruct
glacial icevolumes(e.g. Fairbanks 1989; Mix 1987),
sea surface (SST) and deep water temperatures
(e.g. Zahn and Mix 1991), ocean circulation changes
(e.g. Boyle 1990; Charles and Fairbanks 1992;
Labeyrie et al. 1987) and glacial-interglacial shifts
between the terrestrial and oceanic carbon pools
(Shackleton 1977). However, the list off actors controlling the stable isotopic compositions of shells
is still incomplete. Data from isotopic calibration
studies have suggested that an unidentified parameter affects carbon and oxygen isotope ratios in
foraminiferal shells (e.g. Duplessy et al. 1981;
Fairbanks et al. 1982; Williams et al. 1979). Although shell Ol3C values may be affected by vital
effects, 8180 values should be insensitive to physiology. Yet, oxygen isotope data from foraminifers
collected in surface plankton tows yield SST reconstructions that are several degrees warmer than
measured ocean temperatures (Williams et al.
198Ia). Similarly, the covariation of carbon
and oxygen isotopes in planktic foraminifers (e.g.
Berger et al. 1978; Curry and Matthews 1981a;
Curry and Matthews 1981 b; Kahn and Williams
1981) cannot be explained by shifting depth
habitats during ontogeny. In terms of equilibrium
fractionation the trends are opposite to those found
491
Mackensen et al. (1993) hypothesised that growth
and reproduction of benthic foraminifers coincides
with the seasonal flux of phytodetritus. Because
Ol3C of freshly accumulated phytodetritus was 3
to 4 0/00 lower than surface sediment organic carbon they argued that ol3C of epibenthic shells,
formed during remineralisation of this fluffy layer
(locally and temporarily depleting 0l3C LC02 )' is depleted compared to 813CLC02 found throughout the
rest of the year.
The main complication associated with the interpretation of Ol3C records from foraminiferal
shells is a prominent vital effect. Already Craig
(Revelle and Fairbridge 1957) had shown that
the carbon isotopes in foraminiferal shells are precipitated out of equilibrium with8 13 C ofkCO z and
potentially reflect utilization of metabolic cq. The
large range of disequilibrium precipitation was confirmed by many studies (for a review see e.g.
Berger 1979; Duplessy 1978). A milestone for
the investigation of vital effects on stable isotopic
fractionation was the establishment of culture techniques for planktic foraminifers by the research
groups of Allen Be, Roger Anderson, Christoph
Hemleben and Michael Spindler (e.g. Be 1979; Be
1980; Be 1982; Be and Anderson 1976; Be et al.
1979; Be et al. 1977; Hemleben and Spindler 1983;
Hemleben et al. 1989; Hemleben et al. 1985;
Hemleben et al. 1987; Spindler and Hemleben
1980). Using laboratory cultures, Erez and Luz
(1983) derived the first empirical paleotemperature
equation for Globigerinoides sacculifer and, for
instance, Bouvier-Soumagnac and Duplessy (1985)
were able to verify their plankton tow based
temperature:o l8 0 relationships. Culture experiments were also imperative to investigate the impact of algal symbiosis on the shello l3 C composition of benthic (Erez 1978b; Williams et al. 1981 b;
Zimmermann et al. 1983) and planktic foraminifers
(Spero and DeNiro 1987). In contrast to benthic
foraminifers and some corals (Erez 1978b; Land
et al. 1977), Spero and Deniro concluded that
increased symbiont photosynthetic activity in
Orbulina universa, resulted in a 13C enrichment
of the shell. Their conclusion was supported
by other studies (Cummings and McCarty 1982;
Goreau 1977; Weber and Woodhead 1970)
and later verified for other species of planktic
foraminifers (Bijma et al. 1998; Spero and Lea
1993; Spero et al. 1991). The enrichment was attributed to the preferential removal of l2 C0 2 by the
CO 2 fixing enzyme ribulose 1,5-biphosphate
carboxylase-oxygenase (RUBISCO) during symbiont photosynthesis, enriching the calcifying environment in l3C. Host respiration was shown to
deplete the shell isotopic composition (e.g. Bijma
et al. 1998; Spero and Lea 1996). This observation
was explained by the contribution of metabolic cq,
enriched in IZC, to the calcifying environment.
During the past four decades, stable oxygen and
carbon isotope measurements on biogenic calcite
and aragonite (for a review see e.g. Wefer and
Berger 1991) have become standard tools for reconstructing paleoceanographic and paleoclimatic
change. By assuming that our understanding of the
major parameters controlling stable isotope incorporation into biogenic calcite is complete, these
geochemical proxies have been used to reconstruct
glacial icevolumes(e.g. Fairbanks 1989; Mix 1987),
sea surface (SST) and deep water temperatures
(e.g. Zahn and Mix 1991), ocean circulation changes
(e.g. Boyle 1990; Charles and Fairbanks 1992;
Labeyrie et al. 1987) and glacial-interglacial shifts
between the terrestrial and oceanic carbon pools
(Shackleton 1977). However, the list off actors controlling the stable isotopic compositions of shells
is still incomplete. Data from isotopic calibration
studies have suggested that an unidentified parameter affects carbon and oxygen isotope ratios in
foraminiferal shells (e.g. Duplessy et al. 1981;
Fairbanks et al. 1982; Williams et al. 1979). Although shell Ol3C values may be affected by vital
effects, 8180 values should be insensitive to physiology. Yet, oxygen isotope data from foraminifers
collected in surface plankton tows yield SST reconstructions that are several degrees warmer than
measured ocean temperatures (Williams et al.
198Ia). Similarly, the covariation of carbon
and oxygen isotopes in planktic foraminifers (e.g.
Berger et al. 1978; Curry and Matthews 1981a;
Curry and Matthews 1981 b; Kahn and Williams
1981) cannot be explained by shifting depth
habitats during ontogeny. In terms of equilibrium
fractionation the trends are opposite to those found