Pyrolysis-combustion 14C dating of soil organic matter

Radiocarbon (C-14) dating of total soil organic matter (SOM) often yields results inconsistent with the stratigraphic sequence. The onerous chemical extractions for SOM fractions do not always produce satisfactory C-14 dates. In an effort to develop an alternative method, the pyrolysis-combustion technique was investigated to partition SOM into pyrolysis volatile (Py-V) and pyrolysis residue (Py-R) fractions. The Py-V fractions obtained from a thick glacigenic loess succession in Illinois yielded C-14 dates much younger but more reasonable than the counterpart Py-R fractions for the soil residence time. Carbon isotopic composition (delta(13)C) was heavier in the Py-V fractions, suggesting a greater abundance of carbohydrate- and protein-related constituents, and delta(13)C was lighter in the Py-R fractions, suggesting more lignin-and lipid-related constituents. The combination of C-14 dates and delta(13)C values indicates that the Py-V fractions are less biodegradation resistant and the Py-R fractions are more biodegradation resistant. The pyrolysis-combustion method provides a less cumbersome approach for C-14 dating of SOM fractions. With further study, this method may become a useful tool for analyzing unlithified terrestrial sediments when macrofossils are absent. (C) 2003 University of Washington. Published by Elsevier Inc. All rights reserved.