Stable oxygen and carbon isotope measurements on biogenic calcite and
aragonite have become standard tools for reconstructing past oceanogra
phic and climatic change. In aquatic organisms, O-18/O-16 ratios in th
e shell carbonate are a function of the ratio in the sea water and the
calcification temperature(1). In contrast, C-13/C-12 ratios are contr
olled by the ratio of dissolved inorganic carbon in sea water and phys
iological processes such as respiration and symbiont photosynthesis',
These geochemical proxies have been used with analyses of foraminifera
shells to reconstruct global ice volumes(3), surface and deep ocean t
emperatures(4,5), ocean circulation changes(6) and glacial-interglacia
l exchange between the terrestrial and oceanic carbon pools(7). Here,
we report experimental measurements on living symbiotic and non-symbio
tic plankton foraminifera (Orbulina universa and Globigerina bulloides
respectively) showing that the C-13/C-12 and O-18/O-16 ratios of the
calcite shells decrease with increasing seawater [CO32-]. Because glac
ial-period oceans had higher pH and [CO32-] than today(8), these new r
elationships confound the standard interpretation of glacial foraminif
eral stable-isotope data In particular, the hypothesis that the glacia
l-interglacial shift in the C-13/C-12 ratio was due to a transfer of t
errestrial carbon into the ocean(7) can be explained alternatively by
an increase in ocean alkalinity(25). A carbonate-concentration effect
could also help explain some of the extreme stable-isotope variations
during the Proterozoic and Phanerozoic aeons(9).