Paper No. 13-5
Presentation Time: 9:40 AM
SOUTHERN DELMARVA BARRIER ISLAND BEACHES: LINKING OFFSHORE AND ONSHORE UNITS USING RACEMIZATION GEOCHRONOLOGY TO INFER SEDIMENT SOURCES DURING SHORELINE MIGRATION
Smith, Wreck, and Parramore islands (southern Delmarva peninsula) have experienced significant shoreline migration over recent decades. Amino acid racemization (AAR) data for beach shell samples from North Parramore (1992-1993), South Wreck (2015), and North Smith (2016) quantify the relative distribution of Holocene vs. Pleistocene-age shell material at these sites. 14C paired analysis was conducted on 12 of the AAR samples and in all instances confirmed the initial AAR age estimates. Whole valves of Mercenaria or Spisula were used for all analyses (20 to 35 per site) in order to avoid issues related to fragmentation. The Parramore collection has a mixture of Holocene and Pleistocene Mercenaria but only Pleistocene Spisula; the Wreck and Smith collections have only Holocene Mercenaria and only Pleistocene Spisula. This side-by-side positioning of Holocene and Pleistocene shells indicates distinct sedimentary units either being transported to the present-day beaches (e.g., through overwash), or being excavated on the beaches through erosion. The D/L values for the Pleistocene Spisula from all three islands are also seen in nearby onshore and offshore sites, demonstrating a link to regional Quaternary units. The D/L values are associated with a range of 14C ages between ~30 and >44 ka, and independent stratigraphic evidence for an age of at least MIS 5a (~75-85 ka), suggesting that all the “finite” 14C ages (those < lab detection limit) have modern carbon addition. The range of D/L values from beach and shelf samples (water depths below 15 m) could include early MIS 3 (~60 ka) units. Recently collected offshore and nearshore sub-bottom data in southern Delmarva identify multiple shallow (< 60 m) seismic stratigraphic units. Based on stratigraphic relationships and dating results from earlier borehole and vibracore studies these units are interpreted to range in age from Late Pleistocene to Holocene and likely source the beach shells. Although limited in number and therefore not statistically representative of each sites’ collection, the AAR results, combined with the offshore seismic stratigraphy, identify multiple shell-bearing source units, help with a synthesis of the Quaternary stratigraphy of the regional coastal system and give insight into mechanisms controlling barrier island evolution.