Implications of a Carbonate Ion Effect on Shell Carbon and Oxygen
Isotopes for Glacial Ocean Conditions
D. W. Leal', J. Bijma 2 , H. J. Spero3 and D. Archer
I Department of Geological Sciences and the Marine Science Institute, University of
California, Santa Barbara, California 93106, USA.
2Universitdt Bremen, Fachbereich Geowissenschaften, Postfach 330440,
D-28334 Bremen, Germany
3 Department of Geology, University of California, Davis, California 95616, USA.
4Department of Geophysical Sciences, University of Chicago, 5734 S Ellis Ave, Chicago,
fL, 60615, USA.
*corresponding author (e-mail): lea@magic.geol.ucsb.edu
Abstract: Experimental work demonstrates that the carbon and oxygen isotopic composition of
planktic foraminifera shells is directly influenced by the carbonate equilibrium state of seawater
(Spero et al. 1997). Because the pC0 2 of the sea surface and the atmosphere must be in approximate
equilibrium, the glacial drop in atmospheric carbon dioxide recorded in ice cores must have been
accompanied by an increase in surface water carbonate ion concentration. Calculations indicate that
increased carbonate ion during glacial episodes can account for much or all of the observed negative Ol3C shift in planktic shells. This discovery provides a novel explanation for the large, negative
swings in planktic Ol3C observed in the glacial sections of Southern Ocean cores. It also presents an
alternative hypothesis to the terrestrial biosphere-to-ocean transfer of carbon that is generally
accepted as an explanation for lower glacial shello l3 C values (Shackleton 1977). Our analysis suggests
that shell isotopic composition can provide a constraint on the extent of oceanic carbonate ion
changes, potentially narrowing explanations for the glacial pC0 2 drawdown. Correcting for the
influence of the carbonate ion effect on shell 0180 lowers glacial tropical sea surface temperature
(SST) estimates by up to 1°C, which brings oxygen isotope paleotemperatures closer to those marine
SST proxies (e.g. coral Sr/Ca) and those terrestrial indicators (e.g. snow-line and ice core ( 18 0) that
suggest more intense tropical cooling atthe last glacial maximum (Guilderson et a!. 1994; Thompson
eta!' 1995).
Introduction
Shackleton first pointed out that benthic shells from
glacial sections of deep-sea cores are characterized by lower 13CfI 2 C values (Shackleton 1977). He
interpreted this change as a consequence of the
transfer during glacial episodes of organic carbon
depleted in 13C from the terrestrial biosphere to the
oceanic dissolved inorganic carbon (DIC) pool.
Subsequent studies revealed that downcore
changes in benthic 013C can also be ascribed to
changes in other factors, including shifts in the
abyssal circulation (Boyle and Keigwin 1982; Curry
and Lohmann 1982; Shackleton et al. 1983 b), airsea exchange (Charles et al. 1993) and physiological and/or environmental artifacts (McCorkle et a!.
1990; Mackensen et al. 1993). Although the combined influence of these factors complicates the
interpretation of benthic 013C records, comparison
of records from different basins makes it possible
to extract the mean ocean shift in 013C (Boyle and
Keigwin 1985,1986).
Recent compilations suggest that mean ocean
013C was 0.32%0 more negative during the Last
Glacial Maximum (LGM), 20 ka (Curry et a!. 1988;
Duplessy et al. 1988; Boyle 1992). If this shift is
entirely attributed to a transfer of carbon from the
From FISCHER G. WEFER G (eds). 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 513-522
Isotopes for Glacial Ocean Conditions
D. W. Leal', J. Bijma 2 , H. J. Spero3 and D. Archer
I Department of Geological Sciences and the Marine Science Institute, University of
California, Santa Barbara, California 93106, USA.
2Universitdt Bremen, Fachbereich Geowissenschaften, Postfach 330440,
D-28334 Bremen, Germany
3 Department of Geology, University of California, Davis, California 95616, USA.
4Department of Geophysical Sciences, University of Chicago, 5734 S Ellis Ave, Chicago,
fL, 60615, USA.
*corresponding author (e-mail): lea@magic.geol.ucsb.edu
Abstract: Experimental work demonstrates that the carbon and oxygen isotopic composition of
planktic foraminifera shells is directly influenced by the carbonate equilibrium state of seawater
(Spero et al. 1997). Because the pC0 2 of the sea surface and the atmosphere must be in approximate
equilibrium, the glacial drop in atmospheric carbon dioxide recorded in ice cores must have been
accompanied by an increase in surface water carbonate ion concentration. Calculations indicate that
increased carbonate ion during glacial episodes can account for much or all of the observed negative Ol3C shift in planktic shells. This discovery provides a novel explanation for the large, negative
swings in planktic Ol3C observed in the glacial sections of Southern Ocean cores. It also presents an
alternative hypothesis to the terrestrial biosphere-to-ocean transfer of carbon that is generally
accepted as an explanation for lower glacial shello l3 C values (Shackleton 1977). Our analysis suggests
that shell isotopic composition can provide a constraint on the extent of oceanic carbonate ion
changes, potentially narrowing explanations for the glacial pC0 2 drawdown. Correcting for the
influence of the carbonate ion effect on shell 0180 lowers glacial tropical sea surface temperature
(SST) estimates by up to 1°C, which brings oxygen isotope paleotemperatures closer to those marine
SST proxies (e.g. coral Sr/Ca) and those terrestrial indicators (e.g. snow-line and ice core ( 18 0) that
suggest more intense tropical cooling atthe last glacial maximum (Guilderson et a!. 1994; Thompson
eta!' 1995).
Introduction
Shackleton first pointed out that benthic shells from
glacial sections of deep-sea cores are characterized by lower 13CfI 2 C values (Shackleton 1977). He
interpreted this change as a consequence of the
transfer during glacial episodes of organic carbon
depleted in 13C from the terrestrial biosphere to the
oceanic dissolved inorganic carbon (DIC) pool.
Subsequent studies revealed that downcore
changes in benthic 013C can also be ascribed to
changes in other factors, including shifts in the
abyssal circulation (Boyle and Keigwin 1982; Curry
and Lohmann 1982; Shackleton et al. 1983 b), airsea exchange (Charles et al. 1993) and physiological and/or environmental artifacts (McCorkle et a!.
1990; Mackensen et al. 1993). Although the combined influence of these factors complicates the
interpretation of benthic 013C records, comparison
of records from different basins makes it possible
to extract the mean ocean shift in 013C (Boyle and
Keigwin 1985,1986).
Recent compilations suggest that mean ocean
013C was 0.32%0 more negative during the Last
Glacial Maximum (LGM), 20 ka (Curry et a!. 1988;
Duplessy et al. 1988; Boyle 1992). If this shift is
entirely attributed to a transfer of carbon from the
From FISCHER G. WEFER G (eds). 1999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 513-522