Sea Level Rise

Paleotempestology      New England Hurricanes     Sea Level Rise

Late Holocene

Relative sea-level diagrams for North America

Figure 1Spatial distribution of relative sea-
level rise (RSLR) in eastern North America.
Sea level curves and map are color coded to illustrate regions with similar RSLR.
Click to enlarge. (77 KB)

Jeff Donnelly, a former graduate and now a post-doctorate, completed a reconstruction of the history of sea level from several sites in the northeast United States. The best data on late Holocene sea levels in the eastern North America is obtained by the radiocarbon dating of basal salt marsh sediments.  Ideal samples are contain evidence of formation at or near mean high water (MHW), such as vegetation endemic to the high salt marsh or the presence of other proxies such as microfaunal assemblages. Changes in relative sea level recorded in salt marsh deposits integrate the many different components of sea level variations at a variety of spatial and temporal scales.  In previously glaciated regions millennium scale vertical land movements associated with post-glacial isostatic adjustment can dominate sea level changes. A continuum of differential subsidence is evident in eastern North America for the latter half of the Holocene.  A maximum zone of subsidence exists in New Brunswick and Nova Scotia.  Regions of little or no vertical crustal movement exist from approximately Boston, MA northeast along the Maine coast, Bermuda and in the southeastern United States, while an intermediate zone of subsidence exists in southern New England, Long Island and New Jersey. (Figure 1)

Recent

Tide gauge and radiocarbon based sea level records for southern New England indicate an apparent acceleration in the rate of relative sea-level rise (RSLR) from approximately 1 mm/year to over 2.5 mm/year within the last 500 years (Figure 2). Given that the community structure of salt marsh flora and fauna is intrinsically linked to tidal flooding frequency.  Increases in the rate of RSLR can lead to shifts in the distribution of flora and fauna.  In addition, marshes can be lost if the rate of RSLR exceeds the ability of a marsh to accrete vertically maintaining elevation relative to local sea level.

Late Holocene sea level diagram for southern New England

Figure 2. Late Holocene Sea Level Curve
for Southern New England
Click to enlarge. (110 KB)

Macrofossil analysis conducted on sediment cores from numerous southern New England salt marshes reveal changes in community structure from Spartina patens dominated high marsh to communities dominated by stunted Spartina alterniflora occurs within the top 50 cm. Carbon-14, Lead-210 and Cesium-137 methods constrain the timing of these shifts in vegetation structure to the later half of the 19th century.  These results confirm an ongoing regional scale alteration of vegetation structure within New England salt marshes, potentially in response to increased flooding frequency associated with accelerated RSLR.  The Woods Hole Oceanographic Institution Sea Grant program provides funding for this research.


Coastal Studies     Geoarchaeology     Paleoclimates     Vegetation Dynamics    

Paleotempestology      New England Hurricanes     Sea Level Rise