INFERRING AGE AND PALEOCLIMATE FROM A HILLSLOPE PALEOSOL-- A CAUTIONARY TALE FROM THE FRONT RANGE, COLORADO

Physical and chemical properties of a strongly developed buried soil, exposed laterally for 100 m in Betasso Gulch (a Boulder Creek Critical Zone Observatory catchment), illustrate the potential and pitfalls in using paleosols on hillslopes to infer past environmental conditions. The buried soil consists of a >100 cm thick A/Bt/BC profile that developed in granitic colluvium on a 10 degree, forested footslope. Luminescence dating suggests that a 3-m thickness of underlying colluvium accumulated between 18 and 12 ka; younger slope deposits buried the soil by about 6 ka. Organic matter in the buried A gave radiocarbon ages between 8550 and 8995 cal. BP. Hillslopes generally are stable in the dry modern environment and the younger deposits support thin, sandy A/Bw sequences. In contrast, both the A and Bt horizons of the buried soil contain 28 to 35 percent clay, most of it smectite, and substantial amounts of pedogenic Fe, properties consistent with an extended period of soil formation and/or changed climate. However, the ¹⁴C and luminescence ages allow <6 kyr of pedogenesis on the hillslope. NMR spectroscopy also demonstrates that black color of the buried A horizon reflects large amounts of aromatic components (fine charcoal), rather than pedogenic accumulation of organic matter. Thick, smectite-rich soil profiles developed in granitic saprolite crop out along upper slopes and ridgecrests at Betasso. Field and laboratory data suggest that the properties of the Betasso paleosol are consistent with upslope erosion of saprolitic soils, downslope redeposition of older soil material, and considerable eolian input. Soils developed on hillslopes in deeply weathered environments may thus contain only indirect information about time of formation and paleoclimate.