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Figure 1. Map showing Atlantic coast of the United
landforms above sea level (<8 m above mean sea level [asl]) near
central Chesapeake Bay (PL, Fig. 1), at the mouth of Chesapeake
Bay (S, Fig. 1), and on the North Carolina coast (MP, Fig. 1) indi-
cate estuarine deposition throughout MIS 3 (67–32 ka). Eustatic
sea level during this time was highly variable but always ~40–80 m
lower than present (Fig. 2) (Siddall et al., 2008). These new data
challenge the long-held implication that locations within the
Chesapeake Bay region, and specifically the Delmarva Peninsula,
Figure 2. (A) Schematic cross section showing relationship of land surface to
relative sea level at specific times in glacial cycles as a function of distance from the
Laurentide ice sheet (LIS). Adapted by permission from D. Krantz and C. Hobbs
(2014, pers. comm.). Location of A–A´ cross section is indicated in Figure 1.
(B) Oxygen isotope and sea-level curves for the past 150 k.y. from Lisiecki and
Raymo (2005) and Thompson and Goldstein (2006), respectively. The
glacioisostatic (land surface) curve (after Scott et al., 2010) is based on ages
produced for shoreline deposits in the mid-Atlantic region and illustrates how
land-surface elevation change induced by glacio-hydro-isostatic adjustment can
account for submergence of the Chesapeake Bay region when eustatic sea level was
much lower than present. VPDB—Vienna Pee Dee Belemnite.
States with population density by county (U.S.
Census Bureau, 2010) placed alongside Late
Holocene and twentieth-century relative sea-level
rise (RSL) rise curves (2 � errors; Engelhart et al.,
2009). RSL rise predicted from glacio-hydro-isostatic
adjustment (GIA) modeling is from the M2 viscosity
model (Peltier, 1996). Yellow shaded region brackets
area of highest RSL rise on the Atlantic coast; dotted
white line indicates maximum extent of the
Laurentide ice sheet (LIS) (Dyke et al., 2002);
magenta lines indicate tracks of major recent storms.
PL, S, MP—Locations of coastal deposits dating to
MIS 3 near central Chesapeake Bay (67–37 ka, n = 8;
Pavich et al., 2006; Litwin et al., 2013), in southern
Virginia (50–33 ka, n = 2; Scott et al., 2010), and
North Carolina (59–28 ka, n = 15; Mallinson et al.,
2008, Parham et al., 2013), respectively. A–A´ shows
location of Figure 2A.
did not experience high-stand deposition after MIS 5 (e.g.,
Ramsey, 2010). The presence of near-shore MIS 3 deposits near
present sea level suggests an alternative sea-level history for the
region, one that implies forebulge uplift of at least 40 m since the
time of deposition. This uplift has been attributed to growth of
the last glacial maximum (LGM; MIS 2) forebulge (Pavich et al.,
2006; Mallinson et al., 2008; Scott et al., 2010; Parham et al., 2013)
that remains uplifted out of isostatic equilibrium (Potter and
Lambeck, 2003).
GSA TODAY | www.geosociety.org/gsatoday
This paper uses multiple methods to date deposits within the
zone of greatest subsidence in the Chesapeake Bay region (Fig. 1)
and place today’s rapid relative sea-level rise into the context of a
several-million-year geologic framework. We used a light detec-
tion and ranging (LiDAR) digital elevation model (DEM) to
analyze low-relief landforms and conducted extensive drilling to
constrain the Pleistocene stratigraphic framework. Our data show
that regional subsidence related to collapse of the MIS 6 glacio-
isostatic forebulge impacted the mid-Atlantic region well into
MIS 3, tens of thousands of years after MIS 5 deglaciation. Long-
lasting subsidence associated with collapse of the MIS 6 forebulge
suggests that present-day subsidence related to the collapse of
the MIS 2 forebulge will continue for the foreseeable future. We
conclude that ongoing subsidence adds to the impacts of sea-level
rise driven by warming climate and melting ice sheets and should
be considered in coastal sea-level risk assessments.
STUDY SITE AND METHODS
To reconstruct the sea-level history in Chesapeake Bay, we
focused on the Blackwater National Wildlife Refuge (~110 km2;
red-bordered rectangle on Fig. 1), which experienced major inun-
dation and transformation of wetlands to open water in the twen-
tieth century (Fig. 3). Sediment from 70 boreholes was described,
analyzed, and sampled. The DEM (Fig. 4) was used to characterize
the geomorphology. We constrained the oldest erosional event
preserved directly above the underlying Miocene strata using
cosmogenic nuclide isochron burial dating (Balco and Rovey,
2008). We dated 28 samples using optically stimulated lumines-
cence (OSL) dating. The OSL ages allow us to develop a
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