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