Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 57

2.3.6 Initiation of Antarctic glaciation 34-33 million years ago

Liu et al. (2009) said:

''About 34 million years ago, Earth's climate shifted from a relatively ice-free
world to one with glacial conditions on Antarctica characterized by substantial
ice sheets ... . The abrupt shift to glacial conditions ... 33.7 million years ago

) change in oxygen isotopic ( d 18 O)
values of benthic foraminifera (1-3) in 300,000 years, which is indicative of
continental ice accumulation and high-latitude cooling

(Ma) is characterized by a +1.5 per mil (

%

. Proposed causes for
this fundamental change in Earth's climate state include changes in ocean circula-
tion due to the opening of Southern Ocean gateways, a decrease in atmospheric
CO 2 , and a minimum in solar insolation.''

...

Liu et al. (2009) reported sea surface temperature (SST) changes, which were
determined from the alkenone unsaturation index and the tetrather index from 11
globally dispersed ocean localities. They estimated benthic cooling of 3 to 5 C
during the transition at 33.7Myr.
Pearson et al. (2009) pointed out that the ''principal geochemical fingerprint of
the Eocene-Oligocene transition (EOT) is an approximately 11.5% 'shift' towards
more positive values of the oxygen isotope ratio of deep-sea carbonates between
34.0 and 33.5 million years ago, the last part of which is a prominent 'step' of
about 10.5% at about 33.5Myr ago.'' They used ''boron isotope ( d 11 B) analysis
of the carbonate shells of upper-ocean planktonic foraminifera to establish palaeo-
surface ocean pH'' from which they inferred the dissolved CO 2 concentration,
[CO 2 ] aq which they assumed was in approximate equilibrium with atmospheric
pCO 2 . The main uncertainties were stated to be ''the value for the boron isotope
ratio of seawater ( d 11 B sw ), sea surface temperature, and the requirement to

estimate one other parameter of the carbonate system (for example, total

alkalinity).'' Their results are shown in Figure 2.18 .
Pearson et al. (2009) interpreted their results to:

strongly suggest that the primary cause [for the transition to Antarctic
glaciation] was a diminishing greenhouse effect. Although greenhouse gases other
than CO 2 (for which there are no proxies) may have contributed, changing pCO 2
atm is likely to have had the greatest forcing. Ours is the first proxy-based study
to confirm a substantial pCO 2 decline during the climate transition. We also find
a sharp pCO 2 increase after maximum ice growth as the global carbon cycle
adjusted to the presence of a large ice cap and there was a nonlinear hysteresis
effect as the ice cap withstood this transient pCO 2 rise. This study rearms the
links between cryosphere development and atmospheric carbon dioxide levels at
the largest and most important climatic tipping point of the last 65 million
years.''

'' ...