REVISION AND CALIBRATION OF THE LATE PLEISTOCENE/HOLOCENE COASTAL MAINE RELATIVE SEA-LEVEL CURVE

The timing and style of deglaciation and post-glacial sea-level changes affecting Maine have been subjects of study at the University of Maine in order to reconstruct local Quaternary stratigraphy and lithospheric deformation, as well as to better understand global models of rapid sea-level and lithospheric changes at the edges of former ice sheets. Barnhardt et al., (1995) constructed a relative sea-level curve from radiocarbon dates on marine shells, salt-marsh peats, and bulk sediments. Until now, this curve has been presented on the radiocarbon time scale. The acquisition of seven new dates from Penobscot and Muscongus bays prompted us to revise the sea-level curve and calibrate the time scale into calendar years before present.

Dates were calibrated with Calib 4.4 without a reservoir correction. Proposed values of reservoir corrections in the Gulf of Maine range from 600 to 40 years. Studies are ongoing to determine if a reservoir correction is needed and if so, what is an adequate value. Data elevations were corrected for isostatic tilt and tides, where applicable and referenced to present-day mean higher high water. Because the original data come from widely disparate sources published over the past 50 years, a database was created in ArcGIS for spatial and temporal analysis. The database consisted of tables with vital information for each data point. A quality factor was developed based on how well the sample relates to sea level and confidence that the sample was in place.

The new dates and the calibration do not change the overall shape of the curve. They do, however, further constrain the duration of the 60-mbpsl lowstand and the ca. 20-mbpsl still/slow stand. Based on a section of salt-marsh peat recovered from Muscongus Bay, central Maine at 24 mbpsl, the lowstand duration is shorter and the still/slow stand is longer than previously thought. This change has profound influence on the rates of sea-level rise during these times. The new dates also constrain the timing of the final phases of the Penobscot Bay Paleodelta to between 6,000 and 8,000 cal ybp. The calibration of the sea-level curve allows correlation of Maine’s sea-level history with various global climate records and major events, such as the 8.2 ka event. The new GIS database will provide insight into spatial changes in the sea-level history along the Maine coast.