Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 29

Rock Lake, Dallas, USA. They related peaks in greigite concentrations to episodes
of drought identified in historical records. Authigenic greigite here has its high-
est peak around AD 1950, which corresponds to one of the most severe historical
droughts in the region.
Calcium carbonate (CaCO 3 ) concentrations in lake sediments have also been
recognised as a proxy for past climatic conditions. Calcium carbonate is regarded
as a first-order proxy of climatic change because bioinduced calcite precipita-
tion responds to changes in the timing of lake stratification and the length of
thesummer photosynthetic period (Mullins, 1998). Higher levels of CaCO 3 are
deposited on the lake floor as flora and fauna respond to warmer temperatures.
The reverse happens during colder and drier periods. Mullins (1998)examined
thestratigraphy of Cayuga Lake in New York and found five distinct phases of
cooler and drier conditions, based upon CaCO 3 levels, over the past 10 000 years.
These dry episodes corresponded to drops in the calcium carbonate content
of lake sediments. The five excursions into drier conditions can be interpreted
as episodes of severe drought that lasted several centuries. Mullins (1998) sug-
geststhat the occurrence of these drier episodes followed a cycle with a period
of 1800--2200 years. These oscillations are likely to reflect hemispheric and/or
global climate trends, rather than local drainage-basin effects, which also corre-
spond well with the cool and dry climate intervals recognised in the Greenland
ice cores.
The strontium content of ostracods in lakes varies directly with the salinity
levels of lake water. Chivas et al .(1985)demonstrated that ostracods buried within
lake floor sediments record palaeosalinities of prehistoric lake waters and hence
record changes in climate based upon the volume of water in the lake. They
examined sediment cores from Lake Keilambete, Victoria, Australia which, as
avolcanic maar lake, is a simple closed water basin whose water level varies
directly with precipitation and evaporation. The overall amount of salt in the
lake is assumed to remain constant but its concentration varies directly with
theextent of evaporation of water from the system. The strontium/calcium ratio
(Sr/Ca) of fossil ostracod shells over the past 10 000 years showed variations in
salinity levels which corresponded well with independent assessments of past
salinity levels based upon lake sediment texture. As with many other prehistoric
records of lake volumes, both in Australia and globally, the Sr/Ca ratios showed
reduced salinity levels and higher lake levels around 6000 years BP and a sharp
increase in salinity after 4000 years BP. Salinity levels peak around 2500 years BP.
Figure 2.4 shows the results of the strontium palaeosalinity measurements from
ostracods in Lake Keilambete sediment cores. They clearly show fluctuations in
thesalinity levels after approximately 4200 years BP which could be interpreted
as episodes of severe drought.