LONG-TERM SLOPE STABILITY IN COWEETA: A COMPARISON OF DETRITAL CHARCOAL AND MRT CARBON-14 AGES FROM A HUMID TEMPERATE STEEPLAND SETTING

In the Blue Ridge Mountains of North Carolina, little is known about geomorphic stability of colluvial hollows. We investigated this topic by dating soil profiles from two sites in the Coweeta Hydrologic Laboratory. The samples were provided by T.C. Hales as part of a UNC study on vegetation changes and slope stability. Site A is at 1200 m elevation on a side slope in a first-order catchment, approximately 250 m below the local ridge crest; Site B is a colluvial hollow located just below the local ridge crest at 1350 m elevation. Two methods were used to physically isolate material for AMS carbon 14 dating. The first method involved physically picking pieces of charcoal (~0.5 mg) under a microscope, to obtain individual ages of detrital charcoal. The second method is known as mean residence time (MRT) dating and accomplished by wet sieving and rapidly dehydrating to isolate the fine-grained fraction (<58 um) for dating. The MRT samples were chemically pretreated at Lawrence Livermore National Labs to separate the resistant humin fraction from the humic acid fraction, which is thought to contain the decomposition products of lignin and other organic compounds.

At site A, separate charcoal and MRT (humin fraction) samples from the saprolite-soil contact (140 cm bls) are similar (5270±40 and 5060±35 uncal BP, respectively) and are stratigraphically consistent with a charcoal sample from 70 cm bls (2840±40 uncal BP). At site B, results are also stratigraphically consistent, but ages from the saprolite-soil contact show larger disparity: the humic acid and humin fractions of the MRT sample are 5340±40 and 8090±35 uncal BP respectively, and two separate charcoal ages are 23990 and 23540±120 uncal BP. Stratigraphic consistency suggests that the soils aggrade and bioturbation moves up with the ground surface but does not preclude removal of the upper portions of the soil through mass wasting processes. Comparison of the fines and charcoal ages indicate that on steeper slopes, MRT may provide a reasonable estimate of soil age whereas on ridge crests, longer stability of the soil column can result in a condensed MRT age that can reflect mixing of old and young sources of carbon.