Pollen stratigraphic correlation and dating of barrier-beach peat sections

Coastal researches use data from core sections to reconstruct past sedimentary environments and barrier-beach dynamics over the last 350 years. Disjunct organic and silt strata separated by sand deposits represent former salt marshes and lagoons that were buried in the normal course of barrier migration processes. Interpretations are invariably limited by a lack of close time control needed to correlate sections and to tie stratigraphic evidence to documented changes in sea level, storms, inlet activity, overwash, and human disturbances. Regional pollen spectra from organic and silt strata in a large number of barrier-beach cores have been matched with spectra from a 210Pb and pollen-dated reference profile from the centre of the study area on Great South Beach, Long Island, New York, U.S.A. The dramatic vegetation changes following European settlement allowed for a high degree of vertical (and thus, temporal) resulution in profiles of regional pollen. For visual matching of pollen samples, percentages of agricultural-indicator types, the percentages of all regional types, stratigraphic relationships, and sediment concentrations of industrially-derived opaque spherules were considered. Chord-distance comparisons that measure the degree of dissimilarity between pairs of pollen spectra, produced the same correlations as visual estimates for 80-87% of the comparisons between spectra of post-settlement age (i.e. spectra containing agricultural indicators). These correlations were used to establish isochrones through a transect of cores across the barrier beach. 210Pb provided an absolute chronology for a reconstruction of geomorphic events that included inlet changes overwash, and salt-marsh formation. The advantages of the present methods for both absolute dating and chronostratigraphic correlation were evident when the age estimates derived from pollen data were compared to 29 14C dates. © 1986.

Duke Scholars

Author James S. Clark Environmental Sciences and Policy