US MID-ATLANTIC COASTAL PLAIN MOLLUSCAN AMINOCHRONOLOGY AND ITS RELATION TO MIS-3 OSL AGES FOR EMERGENT COASTAL UNITS

The ages and elevations of late Pleistocene coastal and shallow marine units on the US mid-Atlantic coastal plain (~35^o – 40^o N) have significant implications for the history of ice volume, sea level, and isostatic response to the nearby (southern margin at ~41^o N) Laurentide Ice sheet. Multiple studies have assigned ages to these units using radiocarbon, amino acid racemization (AAR), Uranium-series, or luminescence (OSL) methods. OSL ages between ~35 ka and ~65 ka for Virginia and North Carolina coastal units indicate the presence of MIS 3 deposits within ±5 m of present sea level (e.g., Parham et al., 2013: Sedimentology). Mollusk AAR data from >90 offshore and onshore sites between Delaware and North Carolina (Wehmiller et al., 2021: Quat. Geochron.) define three major Pleistocene aminozones in this region. The age of the youngest (lowest D/L values) of these aminozones is well constrained by >50 ka ¹⁴C and ~MIS 5a U-series results. The older (higher D/L) aminozones are stratigraphically consistent with onshore paleochannel stratigraphy and yield age estimates reaching back to the early . Holocene AAR results (calibrated with ¹⁴C) define the youngest of the regional aminozones.

The majority of the sites (>50, both onshore and offshore) is represented by the youngest Pleistocene aminozone (>50 ka), which is found in units at elevations between +5 and -30 m (RSL), always represented by multiple samples from each offshore or onshore collection. This aminozone is also recognized in transported shells found on several beaches in the region, consistently located near local subsurface source units. Collectively, this aminozone represents a late Pleistocene unit found over a broad latitude and elevation range, interpreted as no younger than 50 ka, and most likely MIS 5a based on the available U-series coral ages. The ±10% range of D/L values observed within this aminozone is typical of multiple analyses at a single outcrop or core interval, presumably because of normal diagenetic and taphonomic processes. The available chronologic calibration for this aminozone supports an MIS 5a age assignment, although shells from all of MIS 5 might be represented. The occurrence of this regionally extensive aminozone must be reconciled with the younger OSL ages obtained from non-shell-bearing units at nearby sites in the region.