Tropical cyclones - Extreme Events: A Physical Reconstruction and Risk Assessment - page 95

though to demonstrate that these sand layers are indeed from marine incur-
sions during tropical cyclones and not from other sources such as rivers or
indeed tsunami. Microfossils within the sand layers can help in this regard as
different microfossil assemblages reflect their source location; tsunamis will
tend to transport assemblages from deeper waters compared to tropical cyclone
inundations.
Sand-layer stratigraphies from washover events due to tropical cyclones have
been studied predominantly along the shores of the Gulf of Mexico and the
southeastern and eastern United States. Liu and Fearn (1993, 2000)examined
sand layers in lakes along the Florida and Alabama coasts. Donnelly et al .
(2001a, b)studied similar deposits in New England and New Jersey along the
US Atlantic coast (Fig. 4.4). All of these studies assume that the height and
general nature of the barrier has remained unchanged over the length of the
washover record. Such assumptions seem reasonable when separate sites some
distance apart show the same chronology of events, or at least clusters of events,
suggesting that some regional factor has influenced the behaviour of tropical
cyclones at different times during the past. For example, in Liu and Fearn's
studies of Shelby Lake, Alabama and Western Lake, Florida (Fig. 4.4), which are
approximately 150 km apart, both records show a distinct clustering of appar-
ently high-magnitude cyclones between approximately 3200--1000 14 Cyears BP.
These authors suggest that this clustering was due to a change in the latitu-
dinal position of the jet stream over the mainland US, and as a consequence
thelocation of the Bermuda High which moved further southwest. This caused
Caribbean born tropical cyclones (hurricanes) to track farther westwards before
taking their characteristic curve northward, resulting in more cyclones entering
the Gulf of Mexico and striking US Gulf states. Before and after this time period,
thejet stream was farther north and the Bermuda High farther northeast allow-
ing tropical cyclones to strike the US Atlantic coast. Such assertions appear to
be backed by a number of palaeoclimatic proxy records from various locations
in the US.
Otvos (2002)wascritical of the Liu and Fearn (2000) Shelby Lake washover
record and he argued that most of the sand layers here were deposited, not from
storm washover events, but from a variety of processes including redeposition
from sand dunes that sit adjacent to and near the lake. Liu and Fearn (2002)in
defence argued that the regional synchronicity of the washover record supports
thecyclone surge origin of the sand layers and discounts the localised fluvial
reworking of sand dunes.
While regional patterns in storm washover deposit records are probably the
best indicator of a change in the nature of cyclone activity, and to a certain
extent evidence that the deposits were due to washover events, it can never