Lead Isotope Dating of Marsh Cores: Preliminary Results

Jessica A. Kastler and Scott E. Robinson, Department of Environmental
Sciences, University of Virginia, Charlottesville, VA 22903


One-meter long sediment cores were collected in July 1992 for the
purposes of lead-210 dating from sites on Phillips Creek Marsh, Fowling
Point Marsh, and Chimney Pole Marsh.  Site locations are indicated by
letters A-G on Figure 1 and Table 1.  Cores were collected 32.5m from the
marsh edge.  Measured compaction for all cores was less than 3% of the core
length.  Cores were sectioned into 2cm intervals.  Subsamples from selected
intervals were dried, ground and passed through a 500um sieve to remove
large pieces of vegetation.  Subsamples were sent to Dr. Jeff Cornwell at
Horn Point Environmental Laboratory for lead isotope analysis.
	
The resulting profiles of lead activity are given in Figure 2 for sites
B-F; site A was a study site of Barr (1989, MS Thesis, Dept. of Env.Sci.,
Univ. of Virginia).  Over the depth range of decreasing lead activity, the
natural log of the activity was regressed against depth.  The slope of the
regression yields the estimated sedimentation rates for sites B-F given in
Table 1 and indicated on Figure 1; the rate for site A is taken from Barr
(1989).  These results are preliminary pending consultation with Jeff
Cornwell concerning details of the method used to compute the sedimentation
rates.



     Table 1. Preliminary sedimentation rates from lead isotope dating
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     Site Description				    Sedimentation rate
							   (mm/yr)
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     Phillips Creek Marsh
      A Barr (1989) site (headwater)			       1.1
      B Kastler site (upstream)				   1.0-1.4
      C Robinson site (near mouth)			   0.2-0.3

     Fowling Point Marsh
      D Robinson site					   1.4-1.7

     Chimney Pole Marsh
      E Kastler site (west edge)			   1.5-2.0
      F Robinson site (on the Straits)			       0.5
      G Ridge site (low marsh on creek west of ridge)	   1.0-1.3
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