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.
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.
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.
Miriam L. Fearn
Dept. of Geography and Anthropology, Louisiana State University, Baton Rouge, Louisiana 70803