The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 123

however, North and South America had joined by this time, which would have had a
pronounced effect on ocean circulation. What is known, from alkenone analysis and
oxygen isotopes from diatoms, is that at this time late-summer sea-surface temperat-
ures in the sub-Arctic Pacific rose. At first it may seem paradoxical that a sea-surface
temperature rise is associated with glaciation. What seems to have happened is
that at this time, with the re-organisation of ocean circulation, there was increased
water-column stratification (temperature difference), and hence the surface water
failed to sink. Meanwhile, winter sub-Arctic Pacific sea-surface temperatures did
cool. Consequently, the late-summer warm water increased precipitation in the
autumn so encouraging the building up of snow and ice over neighbouring Canada
which, with the cooler winters, was preserved for longer into the following year (Haug
et al., 2005). The northern hemisphere ice sheets grew: ice sheets can only grow if
the preceding year's snow and ice survives the summer into the subsequent winter. In
this way the North American Laurentide ice cap began to expand to complement the
Fennoscandian ice cap over northern Europe and Russia. The Fennoscandian ice sheet
obtained much of its water from prevailing winds running above the North Atlantic
Drift (commonly called the Gulf Stream). The overall result was a step-like progres-
sion in the Earth's cooling that marked the onset of northern hemisphere glaciation.
This is reflected in a number of palaeoclimatic indicators including high-latitude
benthic 18 O, the mass accumulation rate (or MAR) of some northern hemisphere
oceanic biogenic opal (this almost ceased), the magnetic susceptibility record of the
sub-Arctic Pacific (an indicator of ice-rafted debris which increased sharply having
previously been at a low level) and other isotopic mass accumulation rate evidence
of foram species (Ravelo et al., 2004).
With the onset of northern hemisphere glaciation approximately 2.75 mya, which
complemented the already existing Antarctic ice, the current Quaternary (and Plio-
cene) ice age could be said to have begun. Having said this, the key ice deposits
that, according to the International Commission on Stratigraphy, officially mark the
start of the Quaternary (and Pleistocene) date from 2.588 mya. As is often the case,
the trigger for climate change came first and the key geological strata (used for key
dating) came later. Similarly, today we already have had significantly elevated levels
of atmospheric carbon dioxide for the best part of a century (depending on how
you quantify a significant elevation) but only recently have we begun to discern the
climate change fingerprint both biologically and from meteorological records (see
Chapter 6). This biological fingerprint is almost certain to become more pronounced
and then leave behind remains that may be discerned in the future as indicators of
environmental change.
4.4 Thecurrenticeage
Before reviewing the current ice age it is worth noting the nomenclature appearing in
the academic literature as variations in usage can confuse. The depth of the current
Quaternary ice age is defined broadly in the nomenclature of geological timescales in
terms of two parts or epochs. First, there was a series of glacial and interglacial cycles