Reassessing Foraminiferal Stable Isotope Geochemistry: Impact of the
Oceanic Carbonate System (Experimental Results)
J. Bijrna l *, H.J. Spero2 and D.W. Lea 3
1 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40,
D-28334 Bremen, Germany
2Department of Geology, University of California Davis, Davis,
California 95616, US.A.
3 Department of Geological Sciences and the Marine Science Institute, University of
California Santa Barbara, Santa Barbara, California 93106, US.A.
* corresponding author (e-mail):jbijma@uni-bremen.de
Abstract: Laboratory experiments with living planktic foraminifers show that the 8 13 C and 8180
values of shell calcite decrease with increasing sea water pH and/or carbonate ion concentration.
The effect has been quantified in symbiotic (Orbulina universa) and non-symbiotic (Globigerina
bulloides) species and is independent of symbiont activity and temperature. It is concluded that a
kinetic fractionation process affects both the carbon and oxygen isotopic composition of the shell
simultaneously. At present it cannot be determined defmitively whether the relationship is controlled
by the pH dependent balance between hydration and hydroxylation of CO 2 or by [CO,'"] related
variations in the calcification rate. However, independent of which factor ultimately controls the
relationship between the carbonate chemistry and isotopic fractionation, in the real ocean [COn
and pH covary linearly across the relevant pH range. The true relationship between shell isotopic
composition and the bulk carbonate chemistry is masked by the fact that host respiration and
symbiont activity locally modify the carbonate system. Respiration lowers and photosynthesis
increases ambient pH and [CO,2"]. This translates into modified absolute shell values but leaves the
slope between the shell isotopic composition and the bulk carbonate chemistry unaffected. A second level of shell isotopic modification is introduced by the incorporation of respired carbon,
enriched in 12C, which depletes the shell 8 13 C value. In symbiont bearing species this depletion is
partially negated by a she1l8 13 C enrichment in the light. As an alternative to the RUBISCO hypothesis
(enrichment via preferential removal of 12C02), we propose that scavenging of respired CO 2 during
photosynthesis, raises the she1l8 13 C value. Our results have partly been documented before (Spero
et al. 1997) and demonstrate that the carbonate chemistry is undoubtedly a m~or control on temporal
geochemical variability in the fossil record. For instance, the sea water carbonate system ofthe prePhanerozoic world (Berner 1994; Grotzinger and Kasting 1993) or during glacials (Sanyal et al. 1995)
was significantly different from today confounding direct interpretation of foraminiferal stable isotope
data using existing relationships (see companion paper in this volume by Lea et al.).
Introdnction
Spero et al. (1997) describe a series of experiments
demonstrating that planktic foraminiferal8 18 0 and
8 13 e are influenced by sea water carbonate chemistry. In this paper, we describe these experiments
in greater detail, discuss previously unpublished
experiments and speculate on the potential mechanisms controlling the observed responses.
Based on a theoretical study of the thermodynamic properties of isotopic substances, Urey (1947)
proposed that the 18 0 content of calcium carbonFrom FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 489-512
Oceanic Carbonate System (Experimental Results)
J. Bijrna l *, H.J. Spero2 and D.W. Lea 3
1 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40,
D-28334 Bremen, Germany
2Department of Geology, University of California Davis, Davis,
California 95616, US.A.
3 Department of Geological Sciences and the Marine Science Institute, University of
California Santa Barbara, Santa Barbara, California 93106, US.A.
* corresponding author (e-mail):jbijma@uni-bremen.de
Abstract: Laboratory experiments with living planktic foraminifers show that the 8 13 C and 8180
values of shell calcite decrease with increasing sea water pH and/or carbonate ion concentration.
The effect has been quantified in symbiotic (Orbulina universa) and non-symbiotic (Globigerina
bulloides) species and is independent of symbiont activity and temperature. It is concluded that a
kinetic fractionation process affects both the carbon and oxygen isotopic composition of the shell
simultaneously. At present it cannot be determined defmitively whether the relationship is controlled
by the pH dependent balance between hydration and hydroxylation of CO 2 or by [CO,'"] related
variations in the calcification rate. However, independent of which factor ultimately controls the
relationship between the carbonate chemistry and isotopic fractionation, in the real ocean [COn
and pH covary linearly across the relevant pH range. The true relationship between shell isotopic
composition and the bulk carbonate chemistry is masked by the fact that host respiration and
symbiont activity locally modify the carbonate system. Respiration lowers and photosynthesis
increases ambient pH and [CO,2"]. This translates into modified absolute shell values but leaves the
slope between the shell isotopic composition and the bulk carbonate chemistry unaffected. A second level of shell isotopic modification is introduced by the incorporation of respired carbon,
enriched in 12C, which depletes the shell 8 13 C value. In symbiont bearing species this depletion is
partially negated by a she1l8 13 C enrichment in the light. As an alternative to the RUBISCO hypothesis
(enrichment via preferential removal of 12C02), we propose that scavenging of respired CO 2 during
photosynthesis, raises the she1l8 13 C value. Our results have partly been documented before (Spero
et al. 1997) and demonstrate that the carbonate chemistry is undoubtedly a m~or control on temporal
geochemical variability in the fossil record. For instance, the sea water carbonate system ofthe prePhanerozoic world (Berner 1994; Grotzinger and Kasting 1993) or during glacials (Sanyal et al. 1995)
was significantly different from today confounding direct interpretation of foraminiferal stable isotope
data using existing relationships (see companion paper in this volume by Lea et al.).
Introdnction
Spero et al. (1997) describe a series of experiments
demonstrating that planktic foraminiferal8 18 0 and
8 13 e are influenced by sea water carbonate chemistry. In this paper, we describe these experiments
in greater detail, discuss previously unpublished
experiments and speculate on the potential mechanisms controlling the observed responses.
Based on a theoretical study of the thermodynamic properties of isotopic substances, Urey (1947)
proposed that the 18 0 content of calcium carbonFrom FISCHER G, WEFER G (eds), 1999, Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 489-512