Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 28

Ice Age (AD 1550--1700) and, in particular, the Maunder Minimum when sun spot
activity underwent a prolonged (AD 1640--1710) period of quiescence. Over the
past 1500 years, the aeolian record displays cyclic variations at various scales, the
most obvious of which is a 400 year periodicity centred at 1200--1000, 800--600
and 400--200 years BP. These episodes are interpreted to represent episodes of
windier and dustier conditions compared to the period between 3000 and 1500
year BP. The two other cycles of aeolian activity recognised in the Elk Lake varve
record have periods of 1600 and 84 years. Spectral analysis shows that the former
cycle accounts for 28.3% of variance in the record and the latter 14.1% of the
variance. The 400 year cycle accounts for 21.5% of variance. Dean (1997)wasalso
able to identify the episode of aridity associated with the Dust Bowl decades
of the 1930s--1950s. These peaks in the record represent the greatest aeolian
influx to the lake since AD 1870 and correspond to the increased regional aridity
at that time. Despite the impact of this extended drought on the US economy at
thetime, however, the Elk Lake record shows that this phase of drier conditions
wasnot as severe as the drought episodes during the mid-Holocene.
Holmes et al .(1999)also used dune and lake stratigraphies to determine
past climate changes in northeastern Nigeria. The two chronologies provided
adetailed and accurate picture of climate changes, from the late Glacial to the
early Holocene. They found that a mainly wet phase in the late Glacial was fol-
lowed by a drier period before a wetter climate dominated again between the
early to mid-Holocene. Although the climate was found to be predominantly wet-
ter during this latter period, the combined use of lake and dune stratigraphy
provided sufficiently accurate records to suggest that there were smaller fluctua-
tions in precipitation regimes. A notable environmental deterioration was noted
after 4100 years BP. Since 1500 years BP the climate has been characterised by
severe drought events. These findings suggest that the late 20th Century drought
that has been experienced in the area may not be a unique feature, but rather
areflection of natural climate cycle.
In hydrologically closed basins, changes in lake water levels are mainly a
result of changes in precipitation and evaporation. Changes in these two vari-
ables also commonly result in changes in salinity and solute concentration.
These chemical changes are often recorded in the lake stratigraphy and can be
analysed through sediment, geochemical and palaeontological analysis (Holmes
et al ., 1999). The presence of greigite (Fe 3 S 4 ), an iron sulphide mineral, in lake
sediments can be indicative of past drought conditions. Greigite forms in lake
sediments when lake levels are low and particularly in hydrologically open lakes
during periods of low through flow of water when sulphates in the pore water
of lake sediments are reduced to sulphides by heterotrophic bacteria. Reynolds
et al .(1999)undertook a study of greigite concentrations in sediments in White