Reassessing Foraminiferal Stable Isotope Geochemistry
497
A spike stock-solution was prepared by dissolving 7 mg of99.1 % Na,l3CO, in 100 ml deionized
water. From this stock-solution 35 ml were added
to I I FSW (2028 f.!Mol LCO,; 013C l :C02 '" 2 %0).
Initial and final samples of the I3C-spiked culture
medium were measured, indicating that the average spike was 77.8 %0 ± 5.2 %0 (n=II). The pulseand chase-solution for each experiment were taken
from the same batch of modified sea water and
therefore had identical carbonate chemistry. The
[COt] varied between 153 and 517 Ilmol/kg at an
average constant alkalinity of2563 ± 3711leq/kg.
Results
Water samples collected at the start and end of
each experiment showed that the carbonate chemistry (as determined by pH, alktand equilibrium
calculations) remained constant throughout the
experiments. The isotope chemistry (O"C SCO ' and
0 18 0 wat') of the culture medium remained fairly
constant as well. Analyses show that 0 18 0 wat" was
constant at -0.23± 0.05 %0 (vs. VSMOW) (n=100).
The ol3C sco , depended on the amount ofNa,CO,
that was added to modify the carbonate chemistry
in each experiment. Analysis of initial and final
water show that o 13 C sco , varied on average 0.16
± 0.10 %0 (n=30).
All foraminiferaW'C data have been standardized to a oI3Cl: C02 = 2.00 %0 (ambient sea water
o l3 C Lco , = 1.90 ± 0.08 %0, n= 18) by adding or subtracting the difference between 2.00 %0 and the
measured experimental water OI'Cl:CO' to each
shell value. Here OISO (and similar for 01lC) =
[C80/160"mpl/ ISO/160,tanda) - I] x 1000. All isotope values are relative to VPDB unless noted. To
convert 0180 w values from the VSMOW to VPDB
scale, the most recent correction of -0.27 %0 (Hut
1987) was used.
Constant Total Alkalinity Experiment
Results demonstrate that 0. universa shell Ol3C
and 0 18 0 values decrease approximately 3.9 %0
and 1.5 %0 respectively as carbonate increases
from 41 to 642 Ilmollkg (pH increases from 7.38
to 8.83) (Fig. 3a, b). Shello ll C values do not increase further when sea water [CO,'·] drops below ca. 100 Ilmol/kg. Comparison of foraminifers
grown under high light (HL = maximum symbiont
photosynthesis) with specimens maintained in the
dark (no symbiont photosynthetic activity) indicates
that HL shells are enriched in 13C by 1.1 %0 and
depleted in 180 by 0.3 %0 relative to dark specimens. These offsets are seen across the full range
of [CO/"]. Although the effect of symbiont
photosynthesis on shello 13 C and 0 18 0 was documented previously(Spero and DeNiro 1987; Spero
and Lea 1996; Spero and Williams 1988), the fact
that the OI3C/[CO,"] and 0ISO/[C0 3 ' -] slopes
from HL and dark experiments are ca. 0.0065 and
0.002 %o/Ilmol kgl respectively (Table 1) indicates
that symbiont photosynthesis does not affect the
stable isotope:carbonate ion relationship.
Six experiments were carried out at a 5°C lower
culture temperature. Although, the temperature
decrease does not seem to significantly affect the
ol3e/[COn nor the oISO/[COn relationship,
additional experiments are needed to further
bolster this conclusion.
Constant Total Inorganic Carbon
Experiment
In a second series of experiments, we manipulated
the carbonate concentration between 75 - 774 Ilmol/
kg (pH increase from 7.87 to 8.97) by varying
alk t and maintaining LCO, at ambient values
(Fig 3c, d). Orbulina universa shello l3 C and 0 18 0
decreased as [CO;] increased, with similar slopes
as under constant alk t : 0.006 and 0.002 %o/Ilmol kg
I respectively (Table I). Again, comparison of
foraminifers grown under HL with specimens
maintained in the dark (no sym biont photosynthetic
activity) indicates that HL shells are enriched in 13C
by 1.1 %0 and depleted in 180 by
0.4 %0 relative to dark specimens and that these
offsets are constant across the full range of
[CO,'"].
We also cultured the non-symbiotic species,
G. bulloides, across a [COn range of 103-436
Ilmollkg at constant LCO,. Because G. bulloides
displays a large chamber-to-chamber ontogenetic
effect for both OI'C and OISO (Spero and Lea
1996), we amputated chambers secreted during the
497
A spike stock-solution was prepared by dissolving 7 mg of99.1 % Na,l3CO, in 100 ml deionized
water. From this stock-solution 35 ml were added
to I I FSW (2028 f.!Mol LCO,; 013C l :C02 '" 2 %0).
Initial and final samples of the I3C-spiked culture
medium were measured, indicating that the average spike was 77.8 %0 ± 5.2 %0 (n=II). The pulseand chase-solution for each experiment were taken
from the same batch of modified sea water and
therefore had identical carbonate chemistry. The
[COt] varied between 153 and 517 Ilmol/kg at an
average constant alkalinity of2563 ± 3711leq/kg.
Results
Water samples collected at the start and end of
each experiment showed that the carbonate chemistry (as determined by pH, alktand equilibrium
calculations) remained constant throughout the
experiments. The isotope chemistry (O"C SCO ' and
0 18 0 wat') of the culture medium remained fairly
constant as well. Analyses show that 0 18 0 wat" was
constant at -0.23± 0.05 %0 (vs. VSMOW) (n=100).
The ol3C sco , depended on the amount ofNa,CO,
that was added to modify the carbonate chemistry
in each experiment. Analysis of initial and final
water show that o 13 C sco , varied on average 0.16
± 0.10 %0 (n=30).
All foraminiferaW'C data have been standardized to a oI3Cl: C02 = 2.00 %0 (ambient sea water
o l3 C Lco , = 1.90 ± 0.08 %0, n= 18) by adding or subtracting the difference between 2.00 %0 and the
measured experimental water OI'Cl:CO' to each
shell value. Here OISO (and similar for 01lC) =
[C80/160"mpl/ ISO/160,tanda) - I] x 1000. All isotope values are relative to VPDB unless noted. To
convert 0180 w values from the VSMOW to VPDB
scale, the most recent correction of -0.27 %0 (Hut
1987) was used.
Constant Total Alkalinity Experiment
Results demonstrate that 0. universa shell Ol3C
and 0 18 0 values decrease approximately 3.9 %0
and 1.5 %0 respectively as carbonate increases
from 41 to 642 Ilmollkg (pH increases from 7.38
to 8.83) (Fig. 3a, b). Shello ll C values do not increase further when sea water [CO,'·] drops below ca. 100 Ilmol/kg. Comparison of foraminifers
grown under high light (HL = maximum symbiont
photosynthesis) with specimens maintained in the
dark (no symbiont photosynthetic activity) indicates
that HL shells are enriched in 13C by 1.1 %0 and
depleted in 180 by 0.3 %0 relative to dark specimens. These offsets are seen across the full range
of [CO/"]. Although the effect of symbiont
photosynthesis on shello 13 C and 0 18 0 was documented previously(Spero and DeNiro 1987; Spero
and Lea 1996; Spero and Williams 1988), the fact
that the OI3C/[CO,"] and 0ISO/[C0 3 ' -] slopes
from HL and dark experiments are ca. 0.0065 and
0.002 %o/Ilmol kgl respectively (Table 1) indicates
that symbiont photosynthesis does not affect the
stable isotope:carbonate ion relationship.
Six experiments were carried out at a 5°C lower
culture temperature. Although, the temperature
decrease does not seem to significantly affect the
ol3e/[COn nor the oISO/[COn relationship,
additional experiments are needed to further
bolster this conclusion.
Constant Total Inorganic Carbon
Experiment
In a second series of experiments, we manipulated
the carbonate concentration between 75 - 774 Ilmol/
kg (pH increase from 7.87 to 8.97) by varying
alk t and maintaining LCO, at ambient values
(Fig 3c, d). Orbulina universa shello l3 C and 0 18 0
decreased as [CO;] increased, with similar slopes
as under constant alk t : 0.006 and 0.002 %o/Ilmol kg
I respectively (Table I). Again, comparison of
foraminifers grown under HL with specimens
maintained in the dark (no sym biont photosynthetic
activity) indicates that HL shells are enriched in 13C
by 1.1 %0 and depleted in 180 by
0.4 %0 relative to dark specimens and that these
offsets are constant across the full range of
[CO,'"].
We also cultured the non-symbiotic species,
G. bulloides, across a [COn range of 103-436
Ilmollkg at constant LCO,. Because G. bulloides
displays a large chamber-to-chamber ontogenetic
effect for both OI'C and OISO (Spero and Lea
1996), we amputated chambers secreted during the