Page 4 - gt1508
P. 4
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
4