Geography Reference
In-Depth Information
on pollen analysis, which was later supplemented by information
from a wider range of 'natural archives' including, for example,
lake sediments, loess (wind-deposited silt), ice cores, speleothems
(precipitated cave sediments), tree rings, and corals, which
together provide data on past changes in all the component
spheres of the geo-ecosphere.
With continuing development of new techniques for accurate
dating and precise measurement of 'proxy' environmental data
from such archives, it has been possible to reconstruct past
environmental changes in great detail. Ice cores, for example,
obtained from drilling through the Greenland and Antarctic
ice sheets, have revealed changes in atmospheric composition
through the last several glacial-interglacial cycles, have
demonstrated the existence of abrupt environmental changes
over relatively short timescales (temperature increases of
10 degrees Centigrade have occurred within decades), and have
shed light on the complexities of the interactions within the
land-ocean-atmosphere system.
With increasing realization of the importance of past and
present human impacts on the geo-ecosphere, there has also
been a new emphasis on the short-term climatic changes of
the Late Glacial and the Present Interglacial or Holocene (the
last 11,500 years) and their effects. For example, decadal- to
millennial-scale climatic variations underlie the Holocene glacier
variations illustrated in Figure 8. Episodes of glacier expansion
(neoglacial events) have been reconstructed from the record
of suspended sediment deposition in stream-bank mires and
lakes downstream of glaciers in the mountains of Jotunheimen,
southern Norway. Glacier expansion and contraction resulted
in variations in the production of fi ne sediment (especially silt)
by glacier erosion. These variations are in turn refl ected in the
thickness and composition of the sediment layers retrieved by
excavating the mires and coring the lake sediments. Radiocarbon
dating of organic material provides the detailed timescale.
