Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 25

10 3 years during phases of climate that are in the long term trending toward
more humid conditions.
Rapid departures into megadroughts during the early Holocene also occurred
in northern Australia. Nott et al .(1999)identified episodes of longitudinal sand
dune reactivation near the Gulf of Carpentaria, Australia between 8000 and
6000 years BP. The chronological resolution (thermoluminescence, TL) was not
sufficiently fine to determine whether this consisted of one or two episodes of
drier conditions but the latter may have been possible. The dunes were totally
reworked suggesting that substantial drought episodes must have occurred.
There is little doubt that similar dunes would have formed in the region during
the lastglacial maximum (LGM), approximately 22 000 years BP, but no TL dates
of this age were obtained from the present dunes despite sampling the dune
cores. The early Holocene was also a period of increasingly wetter conditions
in northern tropical Australia. River floods were 5--7 times larger between 8000
and 5000 years BP in the 'Top End' of the Northern Territory (Nott and Price,
1999). But in a similar fashion to tropical Africa, megadroughts in tropical Aus-
tralia still occurred despite the transition towards a generally wetter climate. It
would appear that there may be a global cause for this phenomenon, given its
occurrence on two widely separated continents at the same time. Gillespie et al .
(1983)suggested that the early African megadroughts might have been due to
episodic volcanism, solar variability or cryospheric instability (such as glacial
surges). The rapid change in climatic conditions has also been recorded in ice
cores from Greenland where at approximately 8000 years BP a sudden drop in
methane levels trapped in the ice suggests that the Earth plunged into a sudden
chill or cold phase at this time.
Lake sediments and geochemical signatures
Layers of aeolian sediments in lakes and on the ocean floor can also
be used to reconstruct past episodes of drought. Dean (1997)examined detri-
tal clastic sediments deposited by wind in Elk Lake, northwestern Minnesota,
USA. Sediments in the lake are predominantly millimetre-scale varves which are
annual accumulations of sediment resulting from snow melt in the spring and
lesser amounts of sediment in the winter. The spring and winter components
of each varve differ in colour and sediment texture so, like tree rings, annual
layers are easily identified. Aeolian sediments are also present in the lake and
these are not associated with the normal varve accumulation processes (e.g. from
streams). The aeolian sediments can be distinguished from the stream deposited
sediments (varves) by differences in grain size. The aeolian sediments are also