RADIOCARBON CHRONOLOGY OF CARBONATE DEPOSITS--RELIABLE DATING USING ORGANIC RESIDUES

Organic matter (microbial mats/biofilms, ligneous detritus, pollen/spores, kerogen/degradation products) is often well preserved in travertine, speleothems, pedoconcretions, and skeletal carbonates. Moreover, organic content is generally sufficient for AMS and even conventional radiocarbon assay. Potential concerns include: 1) source and nature of organic materials; 2) contemporaneity with carbonate deposition vs. post-depositional organic contamination; 3) post-depositional isotopic fractionation; and 4) biogenic incorporation of noncontemporaneous environmental carbon ("hard-water effect"). We have satisfactorily addressed these issues in our examples and suggest tests applicable in other contexts. Our assays have yielded reliable ages confirmed independently by archeological associations and other means. In contrast, ¹⁴C assay of mineralic carbon in the carbonate radical typically yields dubious or unconstrainable ages. Concurrent preservation of microfossils (palynomorphs, phytoliths, diatom frustules, arthropod carapaces and eggs), macrofossils (bones, egg shells, plant impressions, roots, and detritus), and conservative stable isotopes (d ¹³C, ¹⁸O, ¹⁵N, ³⁴S, D) in the organic and inorganic fractions of the deposits may provide important, chronologically explicit paleoenvironmental indicators. Many samples are also suitable for uranium decay series dating, providing additional chronological control. Travertine from subaerial spring mounds and similar deposits is a particularly rich source of age and environmental data. Speleothems may incorporate organic material from cavernicoles and exogenous sources such as flood-borne sediment in addition to organic carbon infiltrating through the aquifer and immediate overburden. The age of organic matter in pedoconcretions usually defines the period of pedogenesis, but soil parent materials may be sequestered, as well, indicating the time of sedimentation. Stromatolites, coral bioherms/biostromes, lichen oxalates, bones, and mollusc and egg shells also contain datable organic laminates.