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Date Permissions Signed
Date of Award
Master of Science (MS)
Schwartz, Maurice L.
Suczek, Christopher A., 1942-2014
Engebretson, David C.
Relative sea-level changes during the past several thousand years were determined at six marshes in three areas of the Puget Basin, Washington, to identify sites of vertical crustal movement, to provide data on regional relative sea-level history, and to collect data that may be used to monitor a proposed acceleration in the rate of eustatic sea-level rise due to global warming. Six salt marshes were cored to obtain radiocarbon ages of basal fossil peat deposits. Uncertainty in relating fossil marsh plants to elevations of former sea-levels reflects both the vertical range in which the plants occur today at the six study sites, and the responses of salt marshes to the variations in tidal range that accompany rising sea-levels.
Stratigraphic analysis reveals a rise in relative sealevel at all sites. Radiocarbon ages of peat samples indicate a relative sea-level rise of approximately 2-3 ± 0.5 m between 5000 and 3000 years ago and approximately 1 _+ 0.5 m between 3000 and 1000 years ago. Relative sea-level in the Puget Basin has probably not risen more than about 1 m in the past 1000 years. These data correlate with those of previous studies from southeastern Vancouver Island and the Fraser Lowland.
No conclusive evidence of either sudden crustal subsidence or differential tilting across the region was found at the six marshes examined, although slow, steady subsidence of the forearc basin of the Cascadia subduction zone may account for some of the apparent eustatic sea-level rise. Differential crustal displacements of more than 1 m have probably not occurred at Padilla Bay in the past 4500 years, at Quilcene Bay in the past 3000 years, and on San Juan Island in the past 800 years.
Should an acceleration in the rate of sea-level rise from global warming be in progress, erosion observed at marshes on San Juan Island may represent an early signalj Coastal areas with the greatest vulnerability to an accelerated sea-level rise, however, are in southern and eastern Puget Sound, where larger tidal ranges would register the greatest increases in amplitude, so that the highest tides would reach even higher elevations than those otherwise expected for the predicted sea-level rise.
Western Washington University
Puget Sound Watershed (Wash.); Pacific Coast
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Beale, Harriet (Harriet Guthrie), "Relative Rise in Sea-level during the Late Holocene at Six Salt Marshes in the Puget Basin, Washington" (1990). WWU Graduate School Collection. 643.