The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 117

react with carbon dioxide, precipitating it in solid form as calcium carbonate and so
encouraging the further drawdown of atmospheric carbon dioxide (see Chapter 3).
All these factors, together with those discussed in section 3.3.11, helped to switch
the biogeosphere from being more stable in a greenhouse mode (the Eocene and
earlier), to greater stability in a cooler mode (the Oligocene and later: 33.9 mya and
onwards).
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18 O Analysis of forams and ocean sediments to ascertain past calcite compensation
depths (sometimes called CCDs) - the water depth at which the calcium carbonate
rain is balanced by the dissolution rate due to ocean acidity - suggests that the first
indications of the aforementioned shift in stability took place around 42 mya in
the Eocene. A more definite shift then took place somewhere around 34 mya at the
Eocene-Oligocene boundary (Tripati et al., 2005). (This of course is distinct from the
comparatively transient [in geological terms] change in ocean chemistry associated
with the Eocene thermal maximum event; see Chapter 3.)
Earlier, in 2003, US palaeoclimatologists Robert DeConto and David Pollard
demonstrated through a general-circulation computer model coupled with atmo-
sphere and ice-sheet components, and incorporating palaeogeography of the time,
that a combination of declining Cenozoic carbon dioxide (from four times to just
double the pre-industrial level) and the thermal isolation of Antarctica enabled an
ice sheet in the east of the continent to begin to form 34 mya. This is supported by
evidence from geological sediments. The thermal isolation was caused by the open-
ing of oceanic passages between South America and Antarctica (the Drake Passage)
and Australia and Antarctica (the Tasmanian Passage). This enabled the Antarctic
Circumpolar Current (or ACC as it is sometimes abbreviated) to form. However, in
this instance of long-term global cooling, ocean circulation change does not appear
to have been as important early on as the decline in atmospheric carbon dioxide.
This may be because a depth threshold, to allow a critical volume of water to flow,
needs to be reached. But aside from carbon dioxide, it appears that an ice albedo
positive-feedback effect (see Chapter 1) also played a major role. Here the expanding
ice sheet reflected more sunlight and so cooled the continent, allowing the ice sheet
to expand further still, which reflected even more sunlight, and so on.
The first Antarctic ice sheets were sensitive to Milankovitch forcing (see Chapter
1), much as the northern hemisphere ice sheets were during the Quaternary glacial-
interglacial series of waxing and waning ice sheets (which we will examine shortly).
It has been suggested that orbital frequencies of 40 000 years (obliquity) and 125 000
years (eccentricity) dominated the way the ice margins behaved at that time.
This time was one of critical transition as the Earth system crossed a climate
threshold (see section 6.6.8). In addition to the circulation and albedo climate forcing
factors already mentioned, there is strong evidence to suggest that the switch to a
cooler climate was amplified by changes in greenhouse gas concentrations. In 2009
Paul Pearson, Gavin Foster and Bridget Wade used boron ( 11 B/ 10 B) isotope analysis
on Foraminifera shells from the sea surface and the remains of which are found in
marine sediments. The change in the ratio of these isotopes (

11 B) is correlated with
pH of the sea surface and pH in turn is determined by the amount of carbon dioxide
dissolved in the sea (as carbonic acid), and hence by implication the carbon dioxide in
the atmosphere above. Their results suggest a dip in carbon dioxide concentrations at

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