Abstract: Radiocarbon Dating of Estuarine Carbonate Materials

H. J. Simpson, T. H. Peng, C. R. Olsen, S. C. Williams

Ideal materials for radiocarbon dating are well-preserved wood samples and marine-carbonate materials. The carbon fixed in both is derived from reservoirs about which the time history and homogeneity of radiocarbon concentrations are reasonably well understood. The radiocarbon concentration of dissolved bicarbonate in many freshwater and estuarine environments is substantially lower than atmospheric and surface-ocean inorganic carbon because of the introduction of C-14 free carbon from the weathering of limestone. The magnitude of this effect on the original radiocarbon concentration of fossil carbonate materials from freshwater and estuarine environments cannot be directly established by collection of currently forming samples from the same environments because of the pr sence of bomb radiocarbon. Despite the ambiguities, interpretation of radiocarbon dates from estuarine sedimentary environments is commonly very desirable for paleoclimatic purposes. The most direct procedure for establishing the original radiocarbon concentration of a specific aqueous environment is to analyze the radiocarbon concentration of well-documented carbonate shells which were collected prior to the effects of bomb radiocarbon. We have dated 19th century freshwater carbonate material from the Hudson River and established the inherited age of estuarine carbonate shells in the Hudson estuary to be a maximum of 800 years. The actual correction applied is based on measurements of the carbon-13 content of carbonate shells to estimate the fraction of marine carbonate material present in the sample, and to incorporate the effect of gas exchange between the original aqueous bicarbonate and atmospheric carbon dioxide, both of which reduce the age correction to less than the maximum of 800 years. The magnitude of the corrections for the Hudson estuary should be relatively small compared with other large estuarine systems because the Hudson River has an extremely long tidal freshwater residence time, which allows gas exchange to reduce much of the original radiocarbon anomaly upstream of the saline part of the Hudson estuary.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA