Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 31

much of the last glacial cycle caused the Gulf to exist as a lake because it was
separated from neighbouring seas by shallow sills extending from the Australian
mainland to Papua New Guinea.
Hesse (1994)reconstructed dust records from sediment cores from the
Tasman Sea between southeast Australia and New Zealand. The resolution of the
record was not sufficiently fine to identify drought episodes but it nonetheless
highlights the tremendous potential for this kind of research in the future. Dust
from the Australian continent is blown into the Tasman Sea by zonal westerly
winds. Dust concentrations increase markedly during glacial phases of climate
and highlight the onset of aridity on the Australian continent during Oxygen Iso-
tope Stage 10 (around 400 000 years BP). Hesse and McTainsh (1999) also demon-
strated from the particle grain size of the dust in the Tasman Sea cores that
phases of aridity and dust mobilisation in arid and semi-arid Australia were
due to a decrease in vegetation cover as opposed to increased wind strength.
Foraminifera
Foraminifera are small animals with a hard calcium carbonate body
that live in marine waters. These animals have very specific environmental tol-
erance ranges. Species will only be found in various environmental conditions
such as specific water depths, temperature ranges and salinity levels. The species
Rotalidium annectens usually inhabits reasonably high-salinity waters and is rarely
found near river mouths where salinity levels are diluted by freshwater influxes.
The presence of this species in sea floor sediments near river mouths suggests
that these animals were living at this location when freshwater influxes were
low or absent for some period of time and salinity levels remained relatively
high. Nigram et al .(1995)examined a core taken from sea floor sediments at
20 m water depth offshore from the Kali River in western India. They examined
three aspects of the foraminifera extracted from the cored sediments; the angu-
lar asymmetry of the external test morphologies, which has been shown to be
an excellent indicator of palaeo-precipitation, the percentage abundance of R.
annectens ,and the diameter of the proloculus, or the mean proloculus size (MPS).
The proloculus is the initial chamber of the animal body. Figure 2.5 shows each
of these parameters plotted against time ( ∼ 450 years).
Nigram et al .(1995)found that the indicator variations in the foraminifera
varied approximately every 77 years suggesting that droughts, as shown by a
lack of river discharge, have been occurring with this cyclicity in this region
over the past 4--5 centuries. This cycle is very similar in scale to the 80 year
Gleissberg cycle of sunspot variations (Fairbridge and Fougee, 1984). Nigram et al .
(1995)suggestthat these cyclic variations in solar energy reaching the Earth