Late Quaternary Oyster Shells and Sea-Level History, Inner Shelf, Northeast Gulf of Mexico

William W. Schroeder, Albert W. Shultz, Orrin H. Pilkey

Abstract


Oyster (Crassostrea virginica) shells collected at nine sites on the Alabama continental shelf in the northeastern Gulf of Mexico have yielded 27 radiocarbon ages between 8,400 and 36,000 yr BP. Ages and water depths of collection sites of these shells are generally consistent with ages and water depths of shells from the U.S. Atlantic shelf. We have evaluated our data against published interpretations of sea-level change over the past 120,000 yr. The nine youngest shells, from six different sites, have age-depth relations consistent with estuarine origins. Older shells (radiocarbon age > 15,000 yr BP) were collected at various depths, some of which are incompatible with their apparent ages and best estimates of late Pleistocene sea levels. Present distribution would seem to require transport of shells over significant distances on the gently sloping shelf. Although their generally good physical condition makes such transport unlikely, shells may have formed lags during transgressive erosion. Furthermore, accuracy of radiocarbon ages is questionable particularly for the older materials in our set, and the significant likelihood of even greater ages for these shells restores the possibility of local origins.

Notable concentrations of mixed-age shells occur at 40 m and 20 m depths. These concentrations are interpreted as lags that document changes in the rate of sea-level rise, and/or the superposition of Holocene and older paleo-estuarine systems. The -40 m lag resulted from a decreased rate of rise in sea level over the period 9,800 to 9,000 yr BP followed by a brief interval of increased rate of rise in sea level. The - 20m lag represents a reworking of pre-Holocene shells and lithified sediments during a subsequent decrease in the rate of rise in sea level postulated at 8,200 to 7,800 yr BP. Both the - 40 m and - 20 m lags may also be related in depth to interstadial paleo-estuarine deposits dating from 25,000 to 76,000 and 80,000 to 115,000 yr BP, respectively.


Keywords


American oyster (Crassostrea virginica); late Quaternary; sea level; paleoestuaries; cross-shelf transport; radiocarbon dating.

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