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Pleistocene relative sea levels in the Chesapeake Bay
                         region and their implications for the next century

GSA TODAY | AUGUST 2015  Benjamin D. DeJong, U.S. Geological Survey, Reston, Virginia 20192,   INTRODUCTION
                         USA, and Rubenstein School of the Environment and Natural
                         Resources, University of Vermont, Burlington, Vermont 05405, USA;       The sea level for any location at a given point in time represents
                         Paul R. Bierman, Geology Dept. and Rubenstein School of the           a sum of factors, including the volume of ocean water, steric
                         Environment and Natural Resources, University of Vermont,             (thermal) effects, tectonic activity, and crustal deformation in
                         Burlington, Vermont 05405, USA; Wayne L. Newell, U.S. Geological      response to glacio-hydro-isostatic adjustment (GIA) from loading
                         Survey, Reston, Virginia 20192, USA; Tammy M. Rittenour, Utah         and unloading of continental ice and water masses (Church et al.,
                         State University, Logan, Utah 84322, USA; Shannon A. Mahan, U.S.      2010). GIA can be a dominant driver of relative sea level (RSL)
                         Geological Survey, Lakewood, Colorado 80225, USA; Greg Balco,         near ice margins, where the weight of ice displaces the mantle
                         Berkeley Geochronology Center, Berkeley, California 94709, USA;       beneath glaciated regions, uplifting a “forebulge” in the periph-
                         Dylan H. Rood, Dept. of Earth Science and Engineering, Imperial       eral, non-glaciated region (Peltier, 1986). With ice retreat, the
                         College London, South Kensington Campus, London SW7 2AZ, UK,          forebulge progressively subsides at rates dependent on mantle
                         and Scottish Universities Environmental Research Centre, East         rheology and lithosphere thickness (Peltier, 1996).
                         Kilbride G75 0QF, UK
                                                                                                 GIA played a role in RSL near the Chesapeake Bay region of the
                         ABSTRACT                                                              United States (Fig. 1) for many millennia after the ice melted away
                                                                                               (Peltier, 2009). GIA effects were first recognized in the region
                           Today, relative sea-level rise (3.4 mm/yr) is faster in the         when shoreline deposits ~3–5 m above present sea level, long
                         Chesapeake Bay region than any other location on the Atlantic         assumed to be ca. 125 ka (marine isotope stage [MIS] 5e; MIS
                         coast of North America, and twice the global average eustatic rate    designations from Lisiecki and Raymo, 2005), were found to have
                         (1.7 mm/yr). Dated interglacial deposits suggest that relative sea    ca. 80 ka ages (MIS 5a; Cronin, 1981). During this time, global
                         levels in the Chesapeake Bay region deviate from global trends        average sea level was as much as 20 m below its present level
                         over a range of timescales. Glacio-isostatic adjustment of the land   (Fig. 2). While flexural isostatic uplift and subsidence have been
                         surface from loading and unloading of continental ice is likely       documented in the Chesapeake Bay region (i.e., Pazzaglia and
                         responsible for these deviations, but our understanding of the        Gardner, 1993), the rates (~0.006 mm/yr) associated with these
                         scale and timeframe over which isostatic response operates in this    processes are insufficient to account for the age-elevation relation-
                         region remains incomplete because dated sea-level proxies are         ships of MIS 5a shorelines.
                         mostly limited to the Holocene and to deposits 80 ka or older.
                                                                                                 The presence of MIS 5a shorelines 3–5 m above present sea level
                           To better understand glacio-isostatic control over past and         indicates that the land surface within the Chesapeake Bay region
                         present relative sea level, we applied a suite of dating methods to   was significantly lower during the formation of these shorelines
                         the stratigraphy of the Blackwater National Wildlife Refuge, one      due to regional land subsidence from the collapse of the MIS 6
                         of the most rapidly subsiding and lowest-elevation surfaces           forebulge, and that the Chesapeake Bay region experienced
                         bordering Chesapeake Bay. Data indicate that the region was           renewed forebulge uplift during the MIS 2 to raise these shorelines
                         submerged at least for portions of marine isotope stage (MIS) 3       above present sea level (Potter and Lambeck, 2003; Wehmiller et
                         (ca. 60–30 ka), although multiple proxies suggest that global sea     al., 2004). The Holocene stratigraphic record in the Chesapeake
                         level was 40–80 m lower than present. Today MIS 3 deposits are        Bay region helps illuminate forebulge dynamics; differential
                         above sea level because they were raised by the Last Glacial          subsidence from the collapse of the MIS 2 forebulge caused vari-
                         Maximum forebulge, but decay of that same forebulge is causing        able timing and rates of inundation along the eastern seaboard
                         ongoing subsidence. These results suggest that glacio-isostasy        during the Holocene transgression (Peltier, 1996). These differen-
                         controlled relative sea level in the mid-Atlantic region for tens of  tial rates have been exploited to reconstruct the form of the fore­
                         thousands of years following retreat of the Laurentide Ice Sheet      bulge (Engelhart et al., 2009) and to constrain GIA models (Fig. 1)
                         and continues to influence relative sea level in the region. Thus,    (Davis and Mitrovica, 1996; Peltier, 1996).
                         isostatically driven subsidence of the Chesapeake Bay region will
                         continue for millennia, exacerbating the effects of global sea-level    Recent studies employing optically stimulated luminescence
                         rise and impacting the region’s large population centers and valu-    (OSL) dating suggest that the lowest-elevation, emerged estuarine
                         able coastal natural resources.                                       deposits within the mid-Atlantic were deposited during MIS 3,
                                                                                               significantly extending the inferred duration and magnitude of
                                                                                               land subsidence due to collapse of the MIS 6 forebulge. Shoreline

                            GSA Today, v. 25, no. 8, doi: 10.1130/GSATG223A.1.

                            E-mails: DeJong: BDJ@jcomail.com; Bierman: pbierman@uvm.edu; Newell: wnewell@usgs.gov; Rittenour: tammy.rittenour@usu.edu; Mahan: smahan@usgs.gov;
                            Balco: balcs@bgc.org; Rood: d.rood@imperial.ac.uk

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