Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 22

palaeodroughts

Prehistoric droughts can be identified in a wide variety of environmental
records. These include:

the mobilisation of sand dunes as recorded in the dune stratigraphy;
changes in sediment textures and the geochemical and isotopic signa-
tures of lake and marine sediments;
layers of charcoal in the soils of normally humid forest landscapes;
changes in fossil foraminifera and diatoms in marine and freshwater
sediments;
variations in pollen and palaeobotanical assemblages;

changes in the characteristics of tree rings and annual layers in

speleothems; and
changes in isotope concentrations in fossil teeth.

The variety of signatures of prehistoric droughts is probably more diverse than
any of the other natural records of extreme events. The resolution of drought
palaeorecords, however, is sometimes, but certainly not always, too coarse to dis-
tinctly identify an individual drought episode that occurred hundreds or thou-
sands of years ago. Where this occurs, such records can identify periods of time
when droughts would have predominated. Other records, such as the annual
records of tree rings and speleothems, can very accurately identify individual
drought events that occurred centuries to thousands of years ago.

Sand dunes

Dunes are a type of aeolian or wind generated landform. They can be
composed of sand grains or pellets of clay depending upon the source of the
sediment. Sand dunes typically dominate arid regions of the globe today. Clay
pellet dunes occur in arid regions typically around the shores of former lakes
where the clay has formed into pellets on the lake floor during dry episodes.
Sand dunes form when there is an abundant sediment supply, where winds
are sufficiently strong to move that sediment and where there is a lack of
stabilising vegetation. Their presence in humid regions today, or where they are
now stabilised by vegetation, is an indication that the climate was more arid
in the past (Bleckes et al. , 1997). Stratigraphic and chronologic investigations of
dune fields, therefore, provide information on episodes of drier climates when
droughts would have been more prevalent.
Periods of soil formation on the dunes have been defined as time intervals
when landscape stability allowed for soil forming processes to occur. This stands
to reason as the formation of soils requires the presence of vegetation which