Geoscience Reference
In-Depth Information
is the Chinese loess record, which shows an alternation of loess accumulation in central
China during cold, dry and windy climatic interludes and soil development under a re-
established cover of moderately dense vegetation during the warmer, wetter intervals
when the summer monsoon had become stronger once more. The dry intervals were
coeval with glacial or stadial climatic phases and the wet intervals with interglacial
or interstadial phases.
The ice core records from Antarctica and Greenland provide a second, highly
informative, set of terrestrial archives. The Antarctic records from Vostok, near the
centre of the ice cap, and EPICA Dome C now span the past 0.8 million years. They
reveal that temperature over the ice cap fluctuated in parallel with the concentrations
of atmospheric carbon dioxide (pCO
2
) and methane. During glacial maxima and
times of minimum local temperature, the pCO
2
levels were around 180-200 parts per
million by volume (ppmv), rising to 280-300 ppmv during interglacials (Jouzel et al.,
1997
;Luthi et al.,
2008
). The corresponding methane values were around 400 parts
per billion by volume (ppbv) during glacials and 800 ppbv during interglacials (Petit
et al.,
1999
). The dust concentration revealed in the ice cores also shows significant
fluctuations, with peak concentrations of dust blown fromPatagonia and arid Australia
coinciding with glacial maxima.
Finally, the Mediterranean Sea contains a long and detailed record of past climates.
During times of high fluvial discharge into the sea from the Nile and from now inactive
Saharan rivers, dark, organic-rich sediments, or sapropels, were laid down on the
floor of the sea (Rossignol-Strick et al.,
1982
; Larrasoana et al.,
2003
; Ducassou et al.,
2008
; Ducassou et al.,
2009
). The sapropel units are believed to reflect accumulation in
anoxic bottomwaters during times of enhanced freshwater flow into theMediterranean
(Scrivneratal.,
2004
; Tzedakis,
2009
). The sapropels alternate with calcareous muds
that contain a high proportion of Saharan wind-blown dust. The alternating sequence
of sapropels and calcareous muds rich in eolian dust thus reflect times of stronger
and weaker summer monsoons over northern Africa (Freydier et al.,
2001
; Ducassou
et al.,
2008
; Ducassou et al.,
2009
; Tzedakis,
2009
), as well as times of enhanced or
decreased winter rainfall along the north coast of Africa (Rossignol-Strick,
1985
).
3.4.3 Millennial-scale Pleistocene and Holocene climatic fluctuations
The Greenland ice core oxygen isotope records that span the last glacial-interglacial
cycle reveal a pattern of frequent and rapid temperature changes. Dansgaard et al.
(
1993
) found evidence of twenty-four warm
interstadial
events ranging from approx-
imately 2,000 to approximately 500 years in duration. Each showed a rapid temperat-
ure increase of up to 7
°
Cwithin just a few decades, followed by a slower cooling down
to 12-13
C below modern levels (cold
stadial
events). Each of the packets compris-
ing a warm interstadial and a cold stadial is known as a 'Dansgaard-Oeschger' (D-O)
event, lasting 1,000-3,000 years (Williams et al.,
1998
). Heinrich (
1988
) noted peri-
odic influxes of ice-rafted debris into the North Atlantic during the late Quaternary,
°
