SEA LEVEL HISTORY OF CHESAPEAKE BAY SINCE 117ka BASED ON OSL DATING OF ESTUARINE SEDIMENTS

Late Quaternary estuarine sediment units of the Kent Island, Maryland Point and Tabb formations flank Chesapeake Bay and its tributaries. Wave bedforms, burrows, and ravinement surfaces identify these as transgressive units deposited during sea level highstands. Optically stimulated luminescence (OSL) ages of estuarine sands from six shoreline exposures and one core indicate depositional intervals from early OIS5 through OIS3 (OIS5e, 117ka; OIS5a , 86ka and 77ka; OIS3, 55ka, 37ka and 32ka) that are consistent with finite and infinite radiocarbon ages from the same locations. An age vs. elevation plot for dated units from -7 to +12m shows OSL age decreasing with increasing elevation (r²=0.7), an apparent rising sea level trend. This age/elevation relation is opposite the global eustatic trend of falling sea level between 117ka and 32ka (Lambeck and Chappell, 2001). This contrast between the sediment age/elevation trend in the Chesapeake basin and the global eustatic trend is explained by the subsidence of Chesapeake Bay area as a result of post-OIS6 glacial forebulge collapse. Subsidence controlled the net rate of sediment accumulation below erosive wave base and allowed for stacking of transgressive units amenable to OSL dating. A subsidence rate of 0.9mm/yr and net sediment accumulation rate of 0.2mm/yr fits the independently dated eustatic curve. Between 32ka and the mid-Holocene, regional uplift of about 70m elevated these estuarine deposits to their present positions in response to the growing OIS2 glacial forebulge. The Chesapeake Bay area presently is subsiding as the OIS2 glacial forebulge decays. Irrespective of eustatic trend estimates (i.e. a rise of between 20-80cm/century) subsidence alone will contribute ~20cm/century to ongoing relative sea level rise.