Ice core data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 160

sinking of brines resulting in reduced upward diffusion of dissolved CO 2 . They
claimed that these factors were able to generate the increase of CO 2 content in the
atmosphere from 190 ppm at the LGM to 260 ppm at the start of the Holocene.
However, their model does not seem to be unique and its parameters seem to be
rather arbitrarily chosen. Brovkin et al. (2011) also modeled the changes in CO 2
concentration during glacial cycles.
Chikamoto et al. (2011) found that enhanced solubility of CO 2 during the
LGM only accounted for a 20-23 ppm decrease in CO 2 concentration. They also
found that ''neither a weakening of North Atlantic Deep Water formation nor
an increase of Antarctic Bottom Water (AABW) formation causes a large
atmospheric pCO 2 change. A marked enhancement in AABW formation is
required to represent the reconstructed vertical gradient of dissolved inorganic
carbon during LGM conditions.''
Marchitto et al. (2007) made radiocarbon measurements of corals and
planktonic foraminifera which indicated that the radiocarbon activity ( d 14 C) of
the atmosphere during the latter part of the last glacial period ( 30-40 kybp ) was
very high, and around 30 kybp it began to steadily decrease toward present day
low values. Models indicate that the rate of production of 14 C in the atmosphere
by cosmic rays was too small to account for the observed high levels of 14 C during
the period prior to 30 kybp unless there was a significant decrease in the uptake of
14 C by the deep ocean. According to Marchitto et al. (2007), ''This requires a con-
comitant 14 C depletion in a deep-ocean dissolved inorganic carbon reservoir that
was relatively well isolated from the atmosphere. Renewed ventilation of this
reservoir could theoretically explain the drop in atmospheric 14 C and the rise in
atmospheric CO 2 across the last deglaciation'' (the past 15,000 years). This is a
very appealing picture; however, it has some di culties that were glossed over by
Marchitto et al. (2007). Figure 4.22 shows that d 14 C began decreasing around
30 kybp , well before the maximum ice buildup at the LGM (about 20 kybp ).
From 30 to 20 kybp , ice sheets continued to expand and CO 2 concentration
remained low. So, why did d 14 C decrease from 700 to 400

during this
period? On the other hand, it is noteworthy that during the period shown in gray
in this figure (about 17 to 14 kybp ) the CO 2 concentration in the atmosphere
jumped up from about 190 ppm to almost 240 ppm while d 14 C decreased from
about 380 to about 180

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over this same period, which would be in line with the
conclusions of Marchitto et al. (2007).
As we have seen, the dramatic rise and fall of CO 2 concentration in glacial-
interglacial cycles remains dicult to explain although several aspects of the
process have been discussed in the literature. In particular, the results of
Marchitto et al. (2007) are very important in pointing to oceanic processes that
store and release CO 2 .
At about the same time that Marchitto et al. (2007) published their paper,
Stott et al. (2007) published relevant results. They ''used radiocarbon ( 14 C) dating
to establish the timing of the deep-sea and tropical-surface ocean temperature
changes during the last glacial termination and compared this history with the
timing of CO 2 change and deglacial warming in the southern high latitudes during

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