Lake-Sediment record of late Holocene hurricane activities from coastal Alabama
Coastal lake sediments contain a stratigraphically
and chrolologically distinct record of major hurricne strikes during late Holocene
time. Frederic - a category 3 hurricane that struck the alabama coast on the
Gulf of Mexico in 1979 - left a distinct sand layer in the nearshore sediments
of Lake Shelby as a result of storm-tide overwash of beaches and dunes. Sediment
cores taken from the center of Lake Shelby contain multiple sand layers, suggesting
that major hurricanes of category 4 or 5 intensity directly struck the Alabama
coast at ca. 3.2-3.0, 2.6, 2.2, 1.4, and 0.8 ka (14C yr), with an
average recurrence interval of ~600 yr. The Alabama coast is likely to be struck
by a category 4 or 5 hurricane within the next century.
INTRODUCTION
Hurricanes are an important factor in the climate
and coastal evolution of the Gulf Coast region of the United States (Conner et
al., 1989: simpson and Riehl, 1981). Since A.D. 1899, 62 hurricanes have directly
affected the ~1200-km-long coastline between Apalachicola, Florida, and Matagorda
Bay, Texas (Neumann et al., 1987). Of these, 36 were "minor" hurricanes
of category 1 (mazimum sustained wind speed of 119-153 km/h) or 2 (154-177 km/h)
intensity, according to the Saffir-Simpson hurricane intensity scale. Of the
remaining "major" hurricanes, 18 were category 3 (179-209 km/h), seven
were category 4 (211-249 km/h), and only one (Camille in 1969) was category 5
(>249 km/h). Thus, for any particular place along this coast, the probability
of a direct strike by a category 4 or 5 hurricane is quite low. Unfortunately,
the documentary record is too short for estimating the recurrence interval of
these rare but most destructive hurricanes. Documentary records of hurricanes
affecting the eastern United States date back only to A.D. 1871; with far less
confidence, they can be stretched back to the 1700s from written accounts by
early settlers (Neumann et al., 1987; Ludlam, 1963). Therefore, geologic data
offer the only hope of reconstructing a prehistoric record of intense hurricanes
and deciphering any long-term changes in hurricane activities.
Storms can leave distinctive sedimentary structures
in ancient shallow-marine deposits (e.g., Aigner, 1985; Duke, 1985). Recently,
Davis et al. (1989) inferred that hurricanes produced graded or homogeneous facies
of sand, shell gravel, and mud found in predominantly clastic sediments of late
Holocene age in coastallagoonal bays of Florida. We demonstrate here that a stratigraphically
and chronologically distinct record of major hurricane strikes during late Holocene
time can be obtained from coastal lake sediments.
SUMMARY
AND CONCLUSIONS
Intense hurricanes (categories 4 and 5) are historically
rare events. Yet because of their highly destructive forces, they play an important
role as geomorphic agents in causing coastal landform changes, as ecological
agents in ecosystem disturbance and succession, and as natural hazards in causing
catastrophic destruction to life and property (e.g., Conner et al., 1989; Walker
et al., 1991; Simpson and Riehl, 1981). It has been pointed out that intense
hurricane actvity in the western Atlantic and Gulf of Mexico regions is part
of global teleconnections and may be linked to sub-Saharan droughts and El Nino-Southern
Oscillation events (Gray, 1984, 1990; Caviedes, 1991). The frequency and predominant
tracks of intense hurricanes are thus expected to have varied historically as
a function of global climatic changes occurring at various time scales (Barron,
1989; Wendland, 1977; Hobgood and Cerveny, 1988). Therefore, reconstructing the
late Holocene history of these hurricanes and understanding their long-term frequency
and recurrence potential is of great theoretical and practical value. The importance
of this task is underscored by recent predictions of increased frequencies of
intense hurricanes in the western Atlantic correlative with the termination of
the 20-yr drought cycle in sub-Saharan Africa since 1989 (Gray, 1990). Moreover,
climate modeling results based on scenarios of greenhouse warming predict a 40%-50%
increase in hurricane intenities in response to warmer tropical oceans (Emanuel,
1987). These model predictions must be calibrated and tested with long-term climate
data that contain a proxy record of hurricane activities during various climatic
episodes in the past. Lakes along the northern Gulf of Mexico coast have the
potential of yielding multiple proxy records that can be integrated regionally
to document the chronological and spatial variations in hurricane activities
for the past several millennia.
____________________________
Kam-biu
Liu
Miriam
L. Fearn
Dept. of Geography and Anthropology, Louisiana
State University, Baton Rouge, Louisiana 70803