SIGNIFICANCE OF ERODING MARSHES AS A SEDIMENT SOURCE TO THE PAMLICO SOUND, NORTH CAROLINA

Located in eastern North Carolina, the Pamlico Sound is the largest back-barrier estuary along the Atlantic Coast of the United States.  The physical processes operating within this wind-dominated, microtidal estuary have been well documented, however, little to no geochemical data about sediment sources or sedimentation rates have been published.  One question that remains is the significance of the rapidly eroding marsh peats that surround the sound as a sediment source.

In order to address this, a number of sediment cores from regions of organic rich mud (ORM) accumulation within the sound and numerous grab samples from marshes at sites surrounding the sound were collected.  The particular marsh sites were chosen due to their high erosion rates and extent of peat development.  Grab samples were selected based on changes in lithology at each site, to give an adequate representation of the material being eroded (i.e., surface marsh peat, older underlying peat, submerged peat platforms, etc.).

The sediments are being analyzed for a number of geochemical tracers aimed at quantifying the rates of accumulation and relative contribution of the eroding peats.  The tracers being used include radionuclides (²³⁴Th, ⁷Be, ¹³⁷Cs, and ²¹⁰Pb), carbon (d¹³C) and nitrogen (d¹⁵N) isotopes, C:N ratios, and selected trace metals normalized to aluminum.  The short-term radionuclides, ²³⁴Th and ⁷Be, allow the extent of shallow sediment reworking and mixing over short time scales (days to months) to be determined.  ¹³⁷Cs and ²¹⁰Pb are used to establish a geochronology of sedimentation and examine changes in sedimentation rates over time.  Carbon and nitrogen isotopes and C:N ratios, as well as trace metals, provide a geochemical basis for comparing the ORMs and peats.

Although analysis is ongoing, preliminary results from this study reveal much about the patterns of sedimentation within the Pamlico Sound.  The greatest amount of sediment accumulation occurs along the central axis of the sound, where approximately 40 cm of sediment have accumulated in the last 100-120 years (0.4-0.3 cm/year).  Accumulation rates vary elsewhere in the sound (0.1-0.3 cm/year), but generally decrease with decreasing water depth.  ⁷Be data indicate that reworking is also significant, with activity to depths of 8 cm in some cases.