The only processes which could have decreased atmospheric CO2 during g
lacial climates without appreciably changing the carbon isotope distri
bution in the sea appear to be (1) dissolved calcium carbonate input t
o the ocean, e.g., coral reef buildup and erosion, (2) the ocean solub
ility pump, due to changes in surface temperature or air-sea exchange,
or (3) decreased biological production of calcium carbonate. It is as
sumed here that one of these mechanisms caused part of the atmospheric
CO2 changes recorded in the 200-kyr-long Vostok ice core. Two residua
l CO2 records are generated by scaling the delta(13)C difference betwe
en planktonic and benthic foraminifera in marine sediment cores to -82
ppm CO2 per 1 parts per thousand increase in Delta delta(13)C and sub
tracting from this the measured CO2 concentration in the ice core. Bot
h residual CO2 records exhibit two broad maximums between about 20 - 5
0 ka and 140 - 200 ka, indicating that during these times, about 40 pp
m of the CO2 decrease from interglacial levels cannot be explained by
the interaction of the ocean's biological and vertical mixing cycles.
The shape of the residual CO2 curve is similar qualitatively to the va
riation of calcium carbonate in central equatorial Pacific sediments d
uring this time period, which would imply that changes in dissolved ca
rbonate input to the ocean contributed the added component of CO2 chan
ge. However, recent models of atmospheric CO2 change in response to ch
anging alkaline input to the ocean exhibit about a 25 to 35 ppm decrea
se per 10(13) mol yr(-1). increase in dissolved CaCO3 input. If compen
sation for the changing input is occurring mostly within an area of ab
out 40 x 10(6) km(2) below the lysocline in the Indo-Pacific, the chan
ge in carbonate accumulation rate corresponding to a -40 ppm CO2 chang
e would be a minimum of 3 mg cm(-2) yr(-1). This can be compared to gl
acial increases of 0.5 to 1.0 mg cm(-2) yr(-1) during the last 200 kyr
in central equatorial Pacific sediments. Thus, the added glacial accu
mulation of carbonate does not seem to match quantitatively with the 4
0 ppm amplitude of the residual CO2 signal, leading one to suspect tha
t solubility plays a greater role than expected on the basis of a 2 de
grees C cooler surface ocean.