Response of Mobile Bay and eastern Mississippi Sound, Alabama, to changes in sediment accommodation and accumulation
Antonio B Rodriguez, D. Lawrence Greene, Jr., John B Anderson, Alexander R Simms, 2008. "Response of Mobile Bay and eastern Mississippi Sound, Alabama, to changes in sediment accommodation and accumulation", Response of Upper Gulf Coast Estuaries to Holocene Climate Change and Sea-Level Rise, John B. Anderson, Antonio B. Rodriguez
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The rate of creation of sediment accommodation and the rate of sediment accumulation are the primary variables that define the evolution of depositional environments, but they are seldom quantified in studies of coastal evolution. From a detailed map of antecedent topography, a sea-level curve, and measured and modeled sedimentation rates, we quantify these variables for Mobile Bay, Alabama, throughout the Holocene. The timing of recorded rapid changes in depositional environments is compared to the calculated changes in sediment accommodation and sediment accumulation to infer causality.
A comparison of cumulative changes in sediment accommodation to cumulative sediment volume changes shows that the estimated volume of sediment accumulation in the estuary kept pace with and slightly exceeded sediment accommodation until around 8.2 ka. Prior to 8.2 ka, Mobile Bay was an intertidal to supratidal marsh, likely part of the delta-plain environment. Remnants of this environment are preserved discontinuously across the bay directly above the exposure surface as peat. At 8.2 ka, parts of the study area were initially submerged, and the central-basin depositional environment was created. Between 8680 and 8200 cal yr B.P., the bay shoreline transgressed up the axis of the study area at a rate of ~100 m/yr, while the extensive delta plain and marsh were eroded and replaced by a central-basin environment. After 8200 cal yr B.P., the locations of the depositional environments of Mobile Bay were relatively static and aggraded throughout the remainder of the Holocene rise in sea level. This record suggests that a threshold was crossed during the early Holocene in which low-gradient antecedent topography was rapidly flooded by sea-level rise. This promoted sediment accretion landward of the study area and facilitated erosion in the study area through inundation and ravinement, which are reflected as a dramatic decrease in the rate of estuarine-basin sediment accumulation between 8680 and 8400 cal yr B.P. Some estuaries and coastal areas are inherently unstable and prone to threshold responses to forcing mechanisms, particularly those characterized by low-gradient and low-elevation topography.