Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 48

4

0

Lost colony
drought
1587-1589

Jamestown drought
1606-1612

− 4

1560 1600 1640 1680

Year

Figure 2.12. Tree-ring reconstruction of droughts using the July Palmer hydrological
drought index (PHDI) showing major droughts that affected the first colonists in the
USA (from Stahle et al ., 1998).

and only 3400 people survived of the original 6000 who settled there between
AD 1608 and 1624. Most people died of malnutrition.

Speleothem records

Speleothems are deposits of calcium carbonate often found in lime-
stone caves. They can house information of past climates over a greater tem-
poral span than that often achieved accurately through tree-ring analysis (Ming
et al. , 1997). Like trees, speleothems can produce annual banding. The banding is
usually best developed in speleothems in temperate climates with strong season-
ality. Banding can be expressed by a number of characteristics such as alternat-
ing fluorescent and non-fluorescent calcite layers, alternating fibrous and less
dense fibrous layers, and alternating aragonite/calcite layers. Fluorescent (lumi-
nescent) banding develops where strong seasonal changes in temperature result
in marked decreases in soil microbial activity and organic acid solubility during
thewinter. Porosity/density banding is due to marked seasonality in precipita-
tion where growth rates and fluid saturation are controlled by the precipita-
tion regime. Mineralogically defined banding (alternating aragonite and calcite
layers) also reflect marked seasonal changes in precipitation patterns (Liu et al .
1997).
Liu et al .(1997)examined a stalagmite (speleothem extending upwards from
acave floor) from Shihua Cave, Beijing and identified three kinds of annual
banding; a luminescent banding that is only visible under a fluoromicroscope; a
lamina layer which is visible to the naked eye; and a transparent microband that
can be observed through a polarising microscope. The transparent microbanding