Geoscience Reference
In-Depth Information
it allowed a sequence of postglacial climatic stages to be identified for north-west
Europe, including the Older Dryas and the Younger Dryas pollen zones, which were
characterised by colder climatic episodes, or stadials, separated by warmer climatic
intervals, or interstadials. The
Dryas
is a dwarf shrub native to the arctic and alpine
zones of Eurasia and North America and can be seen growing today high in the
Swiss Alps and in the mountains of Scandinavia. The alternation of these warmer
and colder postglacial climatic phases in high northern latitudes has been confirmed
from isotopic analysis of ice cores in Greenland and marine sediments in the North
Atlantic. For example, a relatively short cold phase (the Older Dryas) separates two
longer interstadials, the Allerød and the Bølling, which are sometimes considered
to be a single, complex interstadial, the Bølling-Allerød interstadial (14.6-12.8 ka),
which is followed by the cold Younger Dryas stadial (12.8-11.5 ka).
In certain rare cases, the pollen preserved in sites well away from the deserts and
desert margins can shed light on past climatic fluctuations within those regions. Lake
Suigetsu on the Sea of Japan coast in western Japan offers an excellent example. This
lake preserves an annually laminated fine-resolution pollen sequence extending back
more than 50,000 years and has been used to calibrate the radiocarbon time scale
from 11.2 to 52.8 ka (Bronk Ramsey et al.,
2012
). Because the lake is located north
of the East Asian monsoon front in winter and south of that front in winter, it is
very sensitive to changes in Pacific air mass temperature in summer and Siberian air
mass temperature in winter. The annual pollen record therefore provides a uniquely
detailed history of four separate aspects of climate, namely, winter and summer
monsoon intensity, and the respective temperatures of the Pacific air mass in summer
and the Siberian air mass in winter (Nakagawa et al.,
2006
).TheLakeSuigetsu
winter record shows cooling during the Younger Dryas (YD), which is dated between
12.8 and 11.5 ka in the North Atlantic, but little sign of summer cooling, suggesting
that the YD had a greater impact on the Siberian air mass than it did on the Pacific air
mass. From this Nakagawa et al. (
2006
) concluded that the monsoon front provided
a major paleoclimatic boundary that divided the Northern Hemisphere into several
distinct blocks, each showing different responses to changes in ocean circulation, with
the YD missing or attenuated south of the monsoon front. Nakagawa et al. (
2003
)
had previously observed that climatic changes in the North Atlantic and Japan were
not synchronous, with warming starting earlier in Japan (at 15 ka) and later in the
North Atlantic (at 14.6 ka, the onset of the Bølling-Allerød interstadial) and cooling
in Japan (12.3-11.25 ka) lagging the YD cooling in the North Atlantic (12.8-11.5 ka)
by 250 to 400 years.
Pollen is not only preserved in Quaternary sediments but has also been used to
study much older environments, including those dating back to the Early and Middle
Cenozoic. For example, Alley and Beecroft (
1993
) analysed fossil spore and pol-
len assemblages, as well as foraminifera, in the Eucla Basin of South Australia to
