1. Sea Level Change and
Post-Glacial Rebound in the Northeastern U.S.
When glaciers dominated North
America in the last Ice Age, the crust and mantle
beneath the ice suffered severe compression and
bulging. Though the the ice is gone, the mantle
and crust are still reshaping from the immense
load in a process known as postglacial rebound
(PGR). In the Northeastern U.S., a significant
portion of the relative sea level rise is
believed to be due to subsidence (crustal
downward motion) caused by postglacial effects.
The region lies in the dynamically supported
"peripheral bulge" geologic structure
surrounding the main post-glacial rebound area in
Canada. As the glaciers retreated, the mechanism
supporting the peripheral bulge vanished and the
collapse of the bulge, which began roughly 10,000
years ago, continues today.
2. Correcting Tide Gauges
by Vertical Positioning Using the Global
Positioning System (GPS)
All recent estimates of global
sea level rise derived from tide gauge data have
depended on the use of a model to correct for the
effects of post glacial rebound [Tushingham
and Peltier, 1989]. These models, which
depend on values of the lower mantle viscosity
and the historical ice load, are subject to large
uncertatinties [Davis and Mitrovica, 1996].
In addition, tide gauge records do not account
for other sources of ground movement such as
subsidence caused by fluid withdrawal (water,
oil, gas, etc.) or other tectonic motion. In
essence, even the best models are no substitute
for monitoring the ground motion using precise
geodetic techniques. The Global Positioning
System (GPS) has emerged as a cheap alternative
to laser ranging for providing accurate point
positioning. It is now possible to continuously
model the precise position of a significant
number of tide gauges at relatively modest costs.
3. The Beginning: The
Chesapeake Bay GPS Network - Project BAYONET
Fragile wetland ecosystems, which
support an abundance of wildlife, are being lost
around the Chesapeake Bay at an alarming rate due
to an increase in sea level. The Blackwater
National Wildlife Refuge in the Bay has suffered
wetlands losses estimated at 25 percent in this
century alone. The 1993-98 BAYONET study
investigated the tide gauge data which recorded
the sea level rise. The tide gauge data was
corrected for land subsidence using a small
network of GPS recievers placed near the gauges.
This corrected data still shows sea level rising
trends around 1.7-1.8 mm/yr [Schenewerk, et
al., 1999] . The Chesapeake Bay, however, is
only one piece of a global puzzle of tide gauge
observations involving climate-induced sea level
change convolved with vertical crustal motion.
Eventually, a global network of GPS corrected
tide gauge measurements may provide boundary
conditions for the development of improved
postglacial rebound models and foundational data
for global warming investigations.
Davis, J. L., and J. X. Mitrovica, Glacial
isostatic adjustment and the anomalous tide gauge
record of Eastern North America, Nature,
379, 331-333, 1996.
EOS, Transactions, American
Geophysical Union, Vol. 79, No. 12, March 24,
1998. pp 149, 156-157.
Schenewerk, M., vanDam, T., and
Nerem, R.S., Monitoring Subsidence around
Chesapeake Bay, GPS World, Vol. 10, No.
5, pp.34-41, May 1999.
Tushingham, A. M., and W. R.
Peltier, ICE 3-G: A New Global Model of Late
Pleistocene Deglaciation Based Upon Geophysical
Predictions of Post Glacial Relative Sea Level
Change, J. Geophys. Res., Vol. 96, No.
B3, pp. 4497-4523, 1991.