26AL - 10BE ISOCHRON BURIAL DATING OF EARLY-MID PLEISTOCENE RIVER GRAVELS OF THE CHESAPEAKE BAY REGION, EAST–CENTRAL UNITED STATES

We applied a cosmogenic ²⁶Al - ¹⁰Be isochron burial dating method to subsurface paleochannel gravels on the Eastern Shore of the Chesapeake Bay in Maryland, where the proto-Susquehanna River repeatedly incised and back-filled the Chesapeake valley system in response to glacial-interglacial sea-level fluctuations during the Pleistocene. With each cycle, this river system migrated southwest with the growth of the Delmarva Peninsula, helping to preserve and record much of this history. With the last two glacial-interglacial cycles (< 120 ka) spatially and temporally well constrained, our objective was to improve chronologic control of the older, and much more extensive, portion of the sedimentary sequence. We show that ²⁶Al-¹⁰Be burial isochron dating can effectively date the older portion of this record.

We used an isochron method in which 26Al and 10Be concentrations are measured from several (≥3) clasts and/or grain size separates of sand derived from up-gradient settings. As long as clasts were previously subject to differing surface erosion rates, but buried together with identical rates of post-burial nuclide production, the 26Al and 10Be concentrations form a linear isochron in 26Al - 10Be space, whose slope depends only on the duration of burial and not on post-depositional nuclide production.

We produced isochron ages for 9 river gravels of Susquehanna River provenance and more localized paleochannels under western Delmarva. Individual isochrons display a large range in isotopic concentrations; this range presumably stems from the large variability of surface erosion rates and is optimal for the isochron method. Apparent burial ages range from 2.06 ± 0.07 to 0.28 ± 0.05 Ma, spanning the Early-Middle Pleistocene time range, shortly after the onset of Northern Hemisphere continental glaciation at 2.4 Ma. Simple burial dating techniques that do rely on assumptions about post-burial production, and therefore provide independent age estimates, were applied to individual clasts, which helped validate results and justify the rejection of outlier data. These results show promise for unravelling the deep-time history of Chesapeake Bay and demonstrate that major estuaries preserve important histories of upland erosion and lowland river dynamics in response to changing climate during the Pleistocene.