Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 107

during less intense meteorological conditions. Forests, however, can be impacted
substantially during a cyclone through defoliation and tree fall (Boose et al .,
1994). As a consequence, early stage successional plant species will grow in place
of those plants destroyed during the intense cyclone, and new pollen types will
be distributed throughout an area. Pollen in sediments, therefore, may record
episodes of forest destruction and the introduction of different species. Some
of the major challenges facing future research in this area will be to differenti-
ate the impacts of fire and human activities on forest decline from the impacts
of tropical cyclones. Furthermore, the accuracy of such pollen records is also
dependent upon the frequency of cyclone events of sufficient intensity to cause
substantial damage to a forest, and allow pioneer species to grow instead. Oth-
erwise, several events may occur before a forest has time to develop to a mature
state, and the change in the pollen record within a sedimentary sequence will
show only one event (Malmquist, 1997).
Variations in isotope ratios within speleothems (deposits of calcium carbonate
within caves) also hold promise as a new technique to identify the passage of pre-
historic tropical cyclones. Tropical cyclone rain is isotopically lighter than rain
produced by other types of storms. This is because rain within tropical cyclones
experiences little re-evaporation during its descent, whereas considerable re-
evaporation occurs during normal convective showers. As a result, isotopic frac-
tionation of hydrogen and deuterium occurs in normal showers, and little in
cyclone generated rain (Ehhalt and Ostlund, 1970). On the other hand consid-
erable fractionation of oxygen isotopes occurs in tropical cyclones (Lawrence
and Gedzelman, 1996). Changes in these isotopic ratios can be recorded over
time, providing no contamination of the rainwater occurs between it falling
on the ground and its incorporation into speleothems. Similar, isotopic ratios
recording tropical cyclones may also be discernible in tree rings (White et al .,
1994).
Beds of sand and shell within otherwise finer-grained sediments on conti-
nental shelves may also record tropical cyclones. However, as with terrestrial
deposits, it is difficult to distinguish between those sediments deposited by
tsunami and storm waves. Keen and Slingerland (1993) compared model results
of simulated tropical cyclone winds, currents, waves, and sedimentation patterns
to observations of the actual storms. They found that the average thickness and
lateral extent of the simulated storm sediment beds increased non-linearly with
increasing wind speed. The configuration of the coast, however, caused consider-
able variation in the thickness of the beds between locations on the continental
shelf. So far, this technique is useful for determining the frequency of tropi-
cal cyclones over an area, but the relationship to cyclone intensity remains less
certain at this stage.