Geoscience Reference
In-Depth Information
DOCUMENTING PALAEOCLIMATE
box 13.2
significant
20th-c. advance
Geologists documenting the last Ice Age made the first studies of palaeoclimate 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 the vegetation inferred from
the pollen record. Pollen cores spanning the post-glacial interval, extracted from peatbogs and lake sediments, began to
be widely studied in Europe and North America in the 1950s to 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 (planktonic) and bottom (benthic) foraminifera - began to be investigated. Assemblages of
fauna associated with different water masses (polar, subpolar, mid-latitude, 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 and the record was
extended back in time.
In the southwestern United States, counts of annual tree rings had been used by archaeologists early in the twentieth
century to date timbers in palaeo-Indian structures. In the 1950s to 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, developed under the leadership of H. C. Fritts. Subsequently, F. Schweingruber introduced
the use of ring density variations analysed by X-ray techniques as a seasonal indicator. The 1970s to 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 palaeo-atmosphere over the past 10,000 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
δ
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 sulpfates, 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, but the most important records are from two cores (GISP II and GRIP) from Summit, Greenland, and the
longest from Vostok, East Antarctica. The first two span about 140,000 years and the latter 450,000 years.
Europe. Sea-levels were also lowered by about 130 m
due to the large volume of water locked up in the ice.
Records from tropical lake basins show that these
regions were generally arid at those times. The last such
glacial maximum climaxed about 20,000 years ago,
and 'modern' climatic conditions became established
only during post-glacial (Holocene) time - conven-
tionally dated to 10,000 years
BP
. This timescale is used
whenever dates are based on radiocarbon (carbon-14,
14
C) dating or other radiometric methods involving
isotopic decay processes, such as potassium-argon
(K-Ar).
