Hazards in determination and extrapolation of depositional rates of recent sediments
- Univ. of South Alabama, Mobile, AL (United States). Dept. of Geology-Geography
- Volkert Environmental Group, Inc., Mobile, AL (United States)
Calculation of depositional rates for the past 250 years in estuarine sediments at sites in the Gulf of Mexico have been carried out by measuring changes that have taken place on bathymetric charts. Depositional rates during the past 50 to 100 years can similarly be estimated by this method and may be often confirmed by relatively abrupt changes at depth in the content of certain heavy metals in core samples. Analysis of bathymetric charts of Mobile Bay, Alabama, dating back to 1858, disclosed an essentially constant sedimentation rate of 3.9 mm/year. Apalachicola Bay, Florida, similarly, was found to have a rate of 5.4 mm/year. Though, in theory, these rates should provide reliable estimates of the influx of sediment into the estuaries, considerable caution must be used in attempting to extrapolate them to any depth in core samples. The passage of hurricanes in the Gulf of Mexico is a common event and can rapidly, and markedly, alter the bathymetry of an estuary. The passage of Hurricane Elena near Apalachicola Bay in 1985, for example, removed over 84 million tons of sediment from the bay and caused an average deepening of nearly 50 cm. The impact of Hurricane Frederick on Mobile Bay in 1979 was more dramatic. During the approximate 7 hour period when winds from this storm impacted the estuary, nearly 290 million tons of sediment was driven out of the bay and an average deepening of 46 cm was observed. With such weather events common in the Gulf Coast, it is not surprising that when radioactive age dating methods were used to obtain dates of approximately 7,500 years for organic remains in cores from Apalachicola Bay, that the depths at which the dated materials were obtained in the cores corresponded to depositional rates of only 0.4 mm/year, or one-tenth that obtained from historic bathymetric data. Because storm scour effects are a common occurrence in the Gulf, no attempt should be made to extrapolate bathymetric-derived rates to beyond the age of the charts.
- OSTI ID:
- 5687713
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
54 ENVIRONMENTAL SCIENCES
BATHYMETRY
RELIABILITY
COASTAL REGIONS
SEDIMENTATION
ALABAMA
EROSION
ESTUARIES
FLORIDA
GULF OF MEXICO
HURRICANES
SEDIMENTS
ATLANTIC OCEAN
CARIBBEAN SEA
DEVELOPED COUNTRIES
DISASTERS
NORTH AMERICA
SEAS
STORMS
SURFACE WATERS
USA
580000* - Geosciences
540210 - Environment
Terrestrial- Basic Studies- (1990-)