492
Bijmaetal.
in the water column. If foraminifers were fractionating in isotopic equilibrium lightestS l8 0 values should be associated with heaviest Sl3C values. Stable isotope studies on other calcifying organisms such as calcareous algae, hermatypic
corals and echinoderms show similar disequilibria
(McConnaughey 1989b; WeferandBerger 1991)
suggesting that these proxies are also equivocal and
that some environmental or biological parameter(s)
in addition to those already identified must be affecting skeletal stable isotope values. Interestingly,
laboratory and field data show that skeletal
S I8 0 and Sl3C covary (McConnaughey 1989a;
McConnaughey 1989b; Spero et al. 1997; Spero
and Lea 1996) suggesting the mechanism that affects shell 18 0/ 16 0 ratios also has an effect on l3CI
l2C.
Tn this paper we examine the impact of changes
in carbonate chemistry on the stable carbon and
oxygen isotope composition of a symbiont bearing
and a symbiont barren species of planktic
foraminifers and subsequently discuss potential
equilibrium and kinetic fractionation mechanisms.
The implications of the observed isotope effects
for paleo-oceanographic and -climatic reconstructions are discussed in the companion paper by Lea
et al. (this volume).
Our initial hypothesis was that glacial symbiont
bearing foraminifers should be depleted in l3C relative to their Holocene counterparts because dissolved CO 2 availability (the C0 2 aq concentration)
was reduced in the glacial ocean. Since the S l3 C
of algae increases as less C0 2 aq is available
(Farquhar et al. 1982) and [CO aq] was reduced
.
2
111 the glacial ocean, we predicted that symbiont
fractionation and hence l3C enrichment ofthe calcifying environment was reduced during glacial
times. We found that symbiont bearing foraminifers
grown at lower [C0 2 aq] are indeed depleted in l3C
but for a different reason than we initially hypothesised.
Material and Methods
During the summers of 1993 through 1996 culture
experiments were conducted at the Wrigley Environmental Science Center on Santa Catalina Island
(California) to investigate the influence of sea
water carbonate chemistry on the S l3 C and S I8 0
of the shells of the symbiont bearing planktic
foraminifer Orbulina universa (d'Orbigny) and
the symbiont barren Globigerina bulloides
d'Orbigny.
Orbulina Universa
Orbulina universa has a tropical to temperate
distribution in the euphotic zone. Each adult
individual is associated with 3,000 to 7,000
(Spero and Parker 1985) dinoflagellate symbionts
(Gymnodinium beii). The symbionts are distributed in a halo around the calcitic shell (cf. Fig.1 a).
Orbulina universa, like most spinose species,
is carnivorous and feeds primarily on calanoid
copepods at a rate of one to two per day (Spindler
et al. 1984). New chambers are added to the existing trochospire at regular intervals. The transition to the adult stage is marked by the formation
of a spherical chamber, a feature that is unique in
this species. This terminal chamber is secreted
around the thinly calcified trochospiral juvenile
shell. At this stage numerous large pores facilitate
the migration of the symbionts along the spines. The
spherical chamber continues to calcify for a period
of 1-7 days (Spero 1988) before spine resorption
and gametogenetic (GAM) calcification signal impending gametogenesis. GAM calcification produces 13-28 % of the shell mass over a period of
several hours (Be 1980; Be et al. 1983; Spero
1986). After gametogenesis the terminal sphere
constitutes between 90 to 95 % of the calcite by
weight. Frequently the spiral stage is dissolved
completely. Reproduction of this species is tuned
to the lunar cycle (Bijma et al. 1990).
Globigerina Bulloides
Globigerina bulloides (Fig. 1 b) is a non-symbiotic species that is typically associated with temperate to sub-polar water masses but is also characteristic in lower latitudes upwelling environments
(Be and Hutson 1977; Be and Tolderlund 1971·
Naidu and Malmgren 1996a; 1996b).In these re~
gions, G. bulloides often dominates the flux to the
ocean floor (Sautter and Thunelll989; Sautter and
Sancetta 1992) and is therefore an important source
Bijmaetal.
in the water column. If foraminifers were fractionating in isotopic equilibrium lightestS l8 0 values should be associated with heaviest Sl3C values. Stable isotope studies on other calcifying organisms such as calcareous algae, hermatypic
corals and echinoderms show similar disequilibria
(McConnaughey 1989b; WeferandBerger 1991)
suggesting that these proxies are also equivocal and
that some environmental or biological parameter(s)
in addition to those already identified must be affecting skeletal stable isotope values. Interestingly,
laboratory and field data show that skeletal
S I8 0 and Sl3C covary (McConnaughey 1989a;
McConnaughey 1989b; Spero et al. 1997; Spero
and Lea 1996) suggesting the mechanism that affects shell 18 0/ 16 0 ratios also has an effect on l3CI
l2C.
Tn this paper we examine the impact of changes
in carbonate chemistry on the stable carbon and
oxygen isotope composition of a symbiont bearing
and a symbiont barren species of planktic
foraminifers and subsequently discuss potential
equilibrium and kinetic fractionation mechanisms.
The implications of the observed isotope effects
for paleo-oceanographic and -climatic reconstructions are discussed in the companion paper by Lea
et al. (this volume).
Our initial hypothesis was that glacial symbiont
bearing foraminifers should be depleted in l3C relative to their Holocene counterparts because dissolved CO 2 availability (the C0 2 aq concentration)
was reduced in the glacial ocean. Since the S l3 C
of algae increases as less C0 2 aq is available
(Farquhar et al. 1982) and [CO aq] was reduced
.
2
111 the glacial ocean, we predicted that symbiont
fractionation and hence l3C enrichment ofthe calcifying environment was reduced during glacial
times. We found that symbiont bearing foraminifers
grown at lower [C0 2 aq] are indeed depleted in l3C
but for a different reason than we initially hypothesised.
Material and Methods
During the summers of 1993 through 1996 culture
experiments were conducted at the Wrigley Environmental Science Center on Santa Catalina Island
(California) to investigate the influence of sea
water carbonate chemistry on the S l3 C and S I8 0
of the shells of the symbiont bearing planktic
foraminifer Orbulina universa (d'Orbigny) and
the symbiont barren Globigerina bulloides
d'Orbigny.
Orbulina Universa
Orbulina universa has a tropical to temperate
distribution in the euphotic zone. Each adult
individual is associated with 3,000 to 7,000
(Spero and Parker 1985) dinoflagellate symbionts
(Gymnodinium beii). The symbionts are distributed in a halo around the calcitic shell (cf. Fig.1 a).
Orbulina universa, like most spinose species,
is carnivorous and feeds primarily on calanoid
copepods at a rate of one to two per day (Spindler
et al. 1984). New chambers are added to the existing trochospire at regular intervals. The transition to the adult stage is marked by the formation
of a spherical chamber, a feature that is unique in
this species. This terminal chamber is secreted
around the thinly calcified trochospiral juvenile
shell. At this stage numerous large pores facilitate
the migration of the symbionts along the spines. The
spherical chamber continues to calcify for a period
of 1-7 days (Spero 1988) before spine resorption
and gametogenetic (GAM) calcification signal impending gametogenesis. GAM calcification produces 13-28 % of the shell mass over a period of
several hours (Be 1980; Be et al. 1983; Spero
1986). After gametogenesis the terminal sphere
constitutes between 90 to 95 % of the calcite by
weight. Frequently the spiral stage is dissolved
completely. Reproduction of this species is tuned
to the lunar cycle (Bijma et al. 1990).
Globigerina Bulloides
Globigerina bulloides (Fig. 1 b) is a non-symbiotic species that is typically associated with temperate to sub-polar water masses but is also characteristic in lower latitudes upwelling environments
(Be and Hutson 1977; Be and Tolderlund 1971·
Naidu and Malmgren 1996a; 1996b).In these re~
gions, G. bulloides often dominates the flux to the
ocean floor (Sautter and Thunelll989; Sautter and
Sancetta 1992) and is therefore an important source