Geoscience Reference
In-Depth Information
the precipitation/evaporation ratio results in greater net moisture available for water bodies and
organisms than is available in the same area today or in the preceding regimen. Conversely,
we shall take
nonpluvial
to mean a climatic regimen in which the precipitation/evaporation
ratio is less than that of today, or distinctly less than that of a preceding or following pluvial
regimen.
It is clear from Flint's definition that we are dealing with relative changes in pre-
cipitation and evaporation that are specific in time and place rather than changes in
some arbitrary amount. The distinction is important. An increase (or decrease) in
mean annual precipitation of, say, 250 mm (10 inches) will have minimal effect on
the vegetation of a tropical rainforest but will have a very significant impact in dry
regions if prolonged for decades or centuries.
12.3 Pluvial lakes in North America
Flint (
1971
, p. 442) mentions that in 1776, Velez de Escalante discovered shells near
Salt Lake in Utah and inferred that a much larger lake once covered the entire area.
It was probably the first recognition of what later became known as Lake Bonneville.
We have already seen in
Chapter 5
that nineteenth-century geologists working in the
semi-arid, inter-montane basins in the United States had identified and mapped the
shorelines of a series of formerly very large lakes, of which Lake Bonneville (Gilbert,
1890
), Lake Lahontan (Russell,
1885
) and Searles Lake in California are perhaps the
best-studied (Flint,
1971
, pp. 446-451; Smith and Street-Perrott,
1983
; Lemons et
al.,
1996
; Madsen et al.,
2001
). The close association between glacial moraines and
high-level strandlines recognised by Gilbert (
1890
) seemed to show that the lakes
were high during glacial episodes. Flint (
1971
, p. 19) noted that a few decades before
then, Jamieson (
1863
) in Edinburgh, Lartet (
1865
) in Paris and Whitney (
1865
)in
California had all independently arrived at the conclusion that climatic conditions
conducive to glaciation in temperate latitudes would have lowered evaporation and
lessened aridity to produce higher lake levels in now arid areas, such as the Dead Sea,
the Aral Sea, the Caspian Sea and Lake Balkhash in central Asia, and Lop Nor in
western China - all of which are now vast saline or brackish lakes. According to Flint
(op. cit., p. 20), the term
pluvial
was probably first used by Hull in his 1865 report on
the geology of the Dead Sea region.
Flint (
1971
) summarised what was then known of the Quaternary fluctuations in
pluvial lake levels in the present-day deserts of North America. Smith and Street-
Perrott (
1983
) added significant detail to the record twelve years later, while voicing
concern over the reliability of many of the radiocarbon ages obtained on samples that
had either undergone geochemical change since initial deposition or had absorbed
unknown quantities of older carbon.We have discussed some of the pitfalls of radiocar-
bon dating in
Chapter 6
, so it will suffice to say here that many of the inconsistencies
between lake level histories obtained by different workers and illustrated by Smith and
Street-Perrott (
1983
) arise from problematic age control. What they did show in their
