The late Quaternary evolution of the Apalachicola Barrier Island Complex, North-East Gulf of Mexico, as determined from optical dating.

摘要

Optical stimulated luminescence (OSL) studies of clastic-rich coastal environments have been increasingly the focus of attention, mostly over the past five years, due to the improvement of protocols used to obtained reliable and accurate optical ages on minerals such as quartz. Using 55 quartz-separate samples extracted from at least two different depth intervals on sediment cores (long vertical and short horizontal) retrieved from multiple beach/dune ridges that decorate four Holocene coastal barriers and a Pleistocene lower mainland, the supra-tidal evolution of the western portion of the Apalachicola Barrier Island Complex, on the NE region of the Gulf of Mexico, has been re-evaluated. This study not only provides new reliable OSL ages for the region but also addresses the feasibility of (a) quantifying rates of coastal aggradation and progradation; (b) interpreting the temporal chronology of the coastal geomorphology; and (c) constraining the results with other geochronometric data available at a more precise level (e.g. inter-correlations with accurate [x,y,z] parameters).; OSL results show equivalent doses (DE) ranging between 0.01 +/- 0.00 and 52.28 +/- 1.26 Gy, associated to samples linked to both wind- and water-lain processes, depicting the two principal components of these ridges: a most probable swash-built ridge base and an aeolian cap. The optical ages obtained range from 22 +/- 4 to 154,200 +/- 10,400 years ago (based on 2004 to 2006 datum), representing terminus ante quem ages of formation of the ridges and the ages of the aeolian component at that particular depth. Modern analogues were also optically analyzed to determine the degree of exposure of these types of sediments to sunlight. The results show zero DE, implying zero ages. The sedimentary deposits hosting the cores collected show minimal contents of Uranium and Thorium ( 4 ppm with an average of 0.5 ppm). The Potassium content seems to be more linked to temporal variations (i.e. different stages of evolution of the barrier islands) than to geographical position relative to the Apalachicola River mouth.; DE distribution analyses show that frequency histograms coupled with cumulative frequency (%) curves and radial plots should be used together to better evaluate the overall behaviour of the distribution. For most of the samples OSL-dated for this research, one or two outliers (i.e. aliquots outside +/- 2sigma) were present upon analysis of the DE distribution, but excluded from the final DE calculation used to compute the optical age of each sample. Skewness coefficient ranges were determined and skewness values were calculated to quantify the degree of symmetry of the DE distribution for each sample. This parameter was used to assess the final total error (i.e. +/- lsigma or 1 standard error) to be associated with each final DE value, as well as the heterogeneity or homogeneity of the dose within each sample and see any indications of possible incomplete zeroing or biogeoturbation. Moreover, and associated with the latter, the use of smaller aliquots (e.g. 3 and 1 mm mask sizes) was also implemented to detect if the samples had undergone any significant post- or pre-depositional disturbance. None was found; rather, analyses show that with decreasing mask size, an increasing resolution in the DE distribution was obtained where the values obtained for the larger aliquots (i.e. 8 and 5 mm mask size) were encompassed within those obtained for the smaller aliquots (i.e. 3 and 1 mm mask size).; This dissertation presents the first assessment of supra-tidal coastal evolution using optical ages to determine both the vertical accretion and lateral progradation rates throughout different segments of the coastline. The multi-directionality, patterns and truncations shown by the hundreds of beach and dune ridges and ridge sets demonstrate the morpho- and hydro-dynamic complexity of these coastal barrier systems, located on the apex of the Floridian Panhand