Geoscience Reference
In-Depth Information
('orbital eccentricity'), the tilt of the earth's axis ('obliquity cycle'), which controls
seasonality, and the changing season of the year when the earth is nearest to the sun
('precessional cycle'). The duration of the orbital eccentricity cycle is 96,600 years
and that of the obliquity cycle is 41,000 years. The precessional cycle varies from
16,300 to 25,800 years, and averaged 21,000 years over the past one million years
(Williams et al.,
1998
). In the late Pliocene up until 2.6 Ma ago, the dominant cycles
recognised in the marine record were the 23 ka and 19 ka precessional cycles, with
the 41 ka obliquity cycles dominating from 2.6 Ma until about 0.7 Ma, after which
the 100 ka cycles became dominant (Elderfield et al.,
2012
). The early glacial cycles
were therefore events of relatively low magnitude and high frequency, in contrast to
the high-magnitude, low-frequency glacial cycles of the past 700,000 years.
Attempts to determine the precise timing of postglacial mountain glacier retreat and
its relationship to global and regional temperature changes have proven controversial.
For example, Schaefer et al. (
2006
) compared
10
Be exposure dates for the onset of
major retreat after the LGM in mid-latitude mountain glaciers and obtained a mean
age of 17.3
0.5 ka for the
Northern Hemisphere. From this, they concluded that mid-latitude glacier retreat was
synchronous in both hemispheres immediately after the LGM. They observed that
the onset of glacier retreat coincided with the start of postglacial
warming
in the
Antarctic high-resolution EPICA Dome C ice core record but with a
cooling
trend
in the Greenland GISP 2 ice core, where warming did not begin until the onset of
the Bølling/Allerød (B/A) interstadial event at 14.7 ka (see
Chapter 6
). In seeking
to explain this anomaly, they postulated that a substantial spread of North Atlantic
winter sea ice soon after 17.3 ka would have masked the global summer temperature
increase that was thought to have caused the mid-latitude glacier retreat (Schaefer
et al.,
2006
).
Later work by Clark et al. (
2009
), based on a very large database of 5,704
14
C,
10
Be and
3
He ages showed that the Northern Hemisphere ice sheets began to retreat at
about the same time (20-19 ka) regardless of size, as did most Northern Hemisphere
mountain glaciers. However, they found that the mountain glaciers of Tibet and those
of the Southern Hemisphere started to retreat somewhat later (18-16 ka) and the West
Antarctic Ice Sheet began retreating later still at around 14.5 ka. They also found that
the mountain glaciers in many areas were already at or near their maximum extent
by approximately 30 ka, which was roughly synchronous with the time when global
ice sheets began to reach their maxima. The time of minimum global sea level (26.5-
19 ka) was the interval of 7,500 years in duration when the global ice sheets were in
near equilibrium with climate. They attributed the melting of the northern ice sheets
to three main forcing factors, namely, an increase in high northern latitude insolation,
an increase in atmospheric CO
2
concentration and a rise in tropical Pacific sea-surface
temperatures.
±
0.5 ka for the Southern Hemisphere and of 17.4
±
