Index Points

A sea-level index point is a sample of material with a known age and elevation that contains information about where sea level was in the past. Material commonly dated includes shells, tree stumps, bulk saltmarsh-peat, plant remains, foraminifera, and many other formerly-living plants and animals. Groups of index points are assembled for a region, and used to plot a sea-level curve. The length of the horizontal line through an index point is an indication of the radiocarbon dating uncertainty. Two examples of sea-level curves are illustrated: (a) a rising sea-level at Halifax, N.S. and (b) a falling and rising sea level at Stephenville, N.L.

Sea level has been rising at Halifax throughout the Holocene which is approximately the last 10 000 years. As more data have become available, the sea-level curve for the area near Halifax, Nova Scotia, has evolved in recent years. This is one interpretation of the data. The depth of the lowest sea level is based on evidence of a submerged delta; the age is based on mussel valve fragments (Mytelis edulis) dated at 11 650 radiocarbon years BP. On the basis of tide gauge records the present rate of sea-level rise at Halifax is 35.6 cm/century.

References

• Edgecombe, R.B., Scott, D.B., and Fader, G.B. 1999. New data from Halifax Harbour: paleoenvironment and a new Holocene sea-level curve for the inner Scotian Shelf. Canadian Journal of Earth Sciences,36, 805-817.
• Forbes, D.L., Boyd, R., and Shaw, J. 1991. Late Quaternary sedimentation and sea-level changes on the inner Scotian Shelf. and Boyd. Continental Shelf Research, 11, 1155-1179.
• Shaw, J., Taylor, R.B., and Forbes, D.L. 1993. Impact of the Holocene transgression on the Atlantic coastline of Nova Scotia. Geographie physique et Quaternaire, 47, 221-238.
• Stea, R.R., Boyd, R., Fader, G.B.J., Courtney, R.C., Scott, D.B., and Pecore, S.S. 1994. Morphology and seismic stratigraphy of the inner continental shelf off Nova Scotia, Canada: Evidence for a -65 m lowstand between 11, 650 and 11,250 C yr. B.P. Marine Geology, 117, 135-154.

Unlike Halifax, late glacial sea level was higher than today at Stephenville, on the west coast of Newfoundland. Sea level dropped rapidly to a lowstand in the early Holocene. The lowstand depth of -25 m is determined on the basis of submerged deltas in the region; the lowstand has just been dated at 9430 radiocarbon years BP (the age of shell contained in the foreset beds of a submerged delta). Tide gauge records from Port aux Basques, just south of Stephenville, indicate that sea level is presently rising at 37.5 cm/century.

References

• Forbes, D.L., Shaw, J., et Eddy, B.G. 1993. Late-Quaternary sedimentation and the postglacial sea-level minimum in Port au Port Bay and vicinity, west Newfoundland. Atlantic Geology 29, 21-26.