Browsing by Author "Mirecki, June"

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    Amino acid racemization dating of some coastal plain sites, southeastern Virginia and northeastern North Carolina
    (University of Delaware, Newark, DE, 1985) Mirecki, June
    Bivalve mollusks (g. Mercenaria and g. Anadara) have been analyzed from 4 southeastern coastal plain sites using the amino acid racemization (AAR) technique for correlation and age estimate purposes. As seen previously (Wehmiller and Belknap, 1982), a conflict exists between age estimates obtained by AAR and V-series methods at the Norris Bridge, VA locality. Here, V-series dating of coral yields a 187 ±20 KA age, while AAR methods suggest a 400 to 500 KA age. The following methods are employed to understand this discrepancy at the Norris Bridge site: 1). Analysis of standard samples to ensure uniform sample preparation and system operation; 2). Quantification of variation in Mercenari'a and Anadara samples for each field site; 3). Qualitative comparison of AAR behavior between Mercenaria and Anadara to assess suitability of Anadara for dating purposes; 4). Aminostratigraphic correlation of field sites using both genera, and 5). Estimation of AAR ages using the the non-linear model of Wehmiller and Belknap (1982). Standard samples analyzed for this study show relatively good preCISIon. Coefficients of variation (CVs) of D/L leucine values are 5.7% (ILC-A) and 9.1% (ILC-B). Overall, both ILC-A and -B show accuracy within 6% when compared to other University of Delaware analyses. Several valves of each genus were analyzed from all sites to determine field site precision. CVs for multiple Mercenaria samples from each site are as follows: Gomez Pit, VA (5.2%); New Light Pit, VA (7.0%); Moyock Pit, N.C. (8.8%) and Norris Bridge, VA (4.4%). CVs for Anadara samples are slightly higher, but comparable for each field site. No Anadara were analyzed from the New Light Pit locality. On the basis of an identical order of racemization rates (i.e. ASP2:ALA> LEU>VAL) and similar ASP/LEU values in both genera, Mercenaria and Anadara qualitatively show similar racemization behavior. 3 clusters of DjL leucine values have been discerned from these field site analyses. These D/L leucine values are used to correlate fossiliferous strata in the southeastern Virginia and northeastern North Carolina Using the non-linear kinetic model, these Coastal Plain. D/L values suggest the following ages: 0.21 to 0.25 (75 KA) and 0.530 (approximately 400 KA). D/L values ranging between 0.33 and 0.38 are tentatively estimated to be 100 to 200 KA in age.
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    Aminostratigraphy, geochronology and geochemistry of fossils from Late Cenozoic marine units in southeastern Virginia
    (University of Delaware, Newark, 1990) Mirecki, June
    Paired Amino Acid Racemization (AAR) and Electron Spin Resonance (ESR) data have been obtained from individual mollusc valves (g. Mercenaria) collected from stratified nearshore deposits at three sites in southeastern Virginia (Gomez Pit, Norris Bridge and Yadkin Pit). AAR and ESR data are used here to define aminozones in the region, and to infer age relationships among these superposed aminozones. These data are compared to U-Th analyses of corals from both sites (Szabo, 1985). Age differences among aminozones are estimated using models of racemization kinetics. Previous workers have generally concluded that the lithologic section at Gomez Pit represents a single transgression followed by minor regression. However, AAR and ESR data indicate at least one unconformity in this transgressive sequence, representing approximately 100 ka. The uppermost aminozone lla is defined by a mean ALLO/ISO values of 0.141+/-0.026 (n = 55). Aminozone lla directly overlies aminozone llc (ALLO/ISO = 0.335+/-0.028; n = 19). Molluscs representing a third, lowest aminozone (lld; ALLO/ISO = 0.471+/-0.038; n = 9) are recognized, but taphonomic and ESR data suggest that lld molluscs in Gomez Pit may be reworked. At Norris Bridge, only aminozone lld (ALLO/ISO = 0.473+/-0.020; n = 5) is recognized. ESR age estimates for aminozone lla range from 93-125 ka (n = 4), in general agreement with a Stage 5 estimate of 75+/-5 ka from U-Th analyses of corals from the same unit. ESR age estimates of 220 and 262 ka were calculated from 2 shells showing ALLO/ISO = 0.33; these data support a Stage 7 or Stage 9 interpretation for aminozone llc. AAR kinetic models suggest an age $>$400 ka for aminozone lld shells, inconsistent with a 187+/-20 ka age from a solitary coral at Norris Bridge. Attempts to obtain age estimates using ESR have been unsuccessful. Shells representing excellent, good, fair and poor preservation conditions from Gomez Pit aminozone lla were compared to determine the effects of condition on precision of amino acid data. These data were used to test the hypothesis that leaching by groundwater selectively removes the most extensively epimerized (free) amino acids from the mollusc shell, thus lowering the ALLO/ISO value. No statistically significant difference in amino acid concentrations, amino acid fractions or ALLO/ISO values was found among these Gomez Pit shells representing different preservation characteristics.
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    Gomez Pit Statigraphic Sections with Data
    (2014-09-05) Mirecki, June
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    New Light and Gomez Pit Stratigraphic Sections
    (2014-09-05) Mirecki, June