Geoscience Reference
In-Depth Information
13.2 Documenting Paleoclimates
Geologists documenting the last Ice Age made the first studies of paleoclimate in the late nineteenth
century. Early progress was hampered by uncertainty as to the age of the earth and the length of
the geological record. However, by 1902 it was accepted that there had been at least four or five
glacial episodes in the Alps and in North America during the Pleistocene epoch. Explanations were
sought in variations of the astronomical periods affecting the earth's orbit, notably by J. Croll (1875)
and M. Milankovitch (1920, 1945), and in variations of the solar constant (G. C. Simpson, 1934, 1957).
Confirmation that astronomical periodicities act as a 'pacemaker' of the Ice Ages was not forthcoming
until the timing of major changes in planktonic foraminiferae in ocean sediment records could be
accurately deciphered and dated in the 1970s (J. Hays, J. Imbrie and N. Shackleton, 1976)
The use of proxy evidence to investigate past climate began almost a century ago. In 1910 the
Swedish scientist Baron G. de Geer used the annual deposits of sediments (varves) in glacial lakes
to date changes in vegetation inferred from the pollen record. Pollen cores spanning the post-glacial
interval, extracted from peat bogs and lake sediments, began to be widely studied in Europe and
North America in the 1950s-1960s following the development of radiocarbon dating of organic
materials by W. Libby in 1951. At the same time, the ocean sedimentary record of changes in marine
microfauna - both surface-dwelling (planktonic) and bottom-dwelling (benthic) foraminifera -
began to be investigated. Assemblages of fauna associated with different water masses (polar,
subpolar, midlatitude, tropical) enabled wide latitudinal shifts in ocean temperatures during the
Quaternary epoch to be traced. The use of oxygen isotopic ratios (O
18
/O
16
) by C. Emiliani and S.
Epstein provided independent estimates of ocean temperature and particularly changes in global
ice volume. These records showed that there had been eight glacial/interglacial cycles during the
past 800,000 years.
In the southwestern United States, counts of annual tree rings had been used by archeologists
early in the twentieth century to date timbers in paleo-Indian structures. In the 1950s-1960s, ring
width was investigated as a signal of summer drought in the desert margins and summer
temperature at high elevations. The field of dendroclimatology, employing statistical methods, was
developed under the leadership of H. C. Fritts. Subsequently, F. Schweingruber introduced the use
of ring density variations analyzed by X-ray techniques as a seasonal indicator. The 1970s-1980s
saw numerous sophisticated biological indicators in use. These included insects, particularly
beetles, diatoms, ostracods, pack rat middens containing plant macrofossils, and corals.
The most comprehensive information on the paleo-atmosphere over the past 1000 to 100,000
years has been retrieved from deep ice cores in Greenland, Antarctica, and plateau ice caps in low
latitudes. The principal types of proxy data are: atmospheric temperatures from
O
18
(developed
for glacier ice by W. Dansgaard), accumulation from the annual layer thickness, carbon dioxide and
methane concentrations from air bubbles trapped in the ice, volcanic activity from electrical
conductivity variations caused by the sulfates, aerosol load and sources (continental, marine and
volcanic). The earliest deep cores were collected at Camp Century in northwest Greenland and Byrd
station in West Antarctica in the 1960s. Subsequently many cores have been recovered and
analyzed. Those of particular note are the GISP II and GRIP cores from Summit, Greenland, spanning
about 140,000 years, and the Vostok and EPICA cores from Antarctica, spanning about 450,00 years
and 720,000 years, respectively.
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