Comparisons and Interpretations of Charcoal and Organic Matter Radiocarbon Ages from Buried Soils in North-Central Colorado, USA

Abstract

ABSTRACT. The reliability of radiocarbon ages based on soil organic matter (SOM) from Holocene buried soils in Middle Park, Colorado, is assessed by comparison with ages of charcoal. On average, 14 C ages of SOM from buried surface horizons are 880 ± 230 14 C yr younger than charcoal ages from the same horizon. Humic acid (HA) and low-temperature (400 °C) combustion residue (LT) fractions are 390 ± 230 and 1290 ± 230 14 C yr younger than charcoal ages, respectively, and HA ages are on average 860 ± 140 14 C yr older than LT fractions. We interpret the offsets between 14 C ages of charcoal and SOM fractions and the consistent offsets between the HA and LT fractions to reflect the duration of pedogenesis and different residence times of the SOM fractions examined here. The stratigraphic coherence of charcoal 14 C ages suggests short residence time on the landscape, with little subsequent reworking. 14 C ages of HA and LT fractions are complimentary to charcoal, and HA ages are interpreted to represent minimum ages for the onset of pedogenesis and LT ages are considered maximum ages for burial. The 14 C chronology from buried soils indicates an episode of hillslope erosion in Middle Park during the early Holocene, followed by a long period of land surface stability and soil formation between 9000-4500 BP. Two episodes of late Holocene hillslope erosion between 3500-2500 and 1000-500 BP correspond with warming recognized in the Colorado Front Range, while surface stability and soil formation between 2500-1000 BP is contemporaneous with evidence for cooling at higher elevations.

The geoarchaeological and paleoenvironmental context of Paleoindian sites in western Middle Park, Colorado, USA

“…The Kremmling soil is a relatively well-preserved soil-stratigraphic unit in western Middle Park, as demonstrated by several sites described above and by 14 C ages presented by Mayer, Burr, and Holliday (2008). It represents a stable geomorphic surface in upland and alluvial settings in western Middle Park during much of the early and middle Holocene.…”

“…A previous study determined that charcoal and soil organic matter (SOM) 14 C ages are complementary for understanding landscape evolution in Middle Park (Mayer, Burr, & Holliday, 2008). Charcoal samples received a standard acid-base-acid pretreatment; SOM samples were pretreated using methods presented in Mayer, Burr, and Holliday (2008). Radiocarbon ages were corrected for isotope fractionation and calibrated using Calib version 5.1 (Stuiver & Reimer, 1993).…”

“…Where recovered from a buried and in situ context, diagnostic Early and Late Paleoindian components are associated with a soil-stratigraphic unit informally referred to as the Kremmling soil (Mayer et al, 2008). Horizon sequences observed in Kremmling soil profiles are variable, interpreted to represent facies of the soil (Morrison, 1967(Morrison, , 1978Holliday, 2004) resulting from local variability in soil forming factors (Jenny, 1941;Birkeland, 1999), vegetation, and topography (i.e., aspect and slope position) in particular.…”

The age of large termite mounds—radiocarbon dating of Macrotermes falciger mounds of the Miombo woodland of Katanga, DR Congo

“…In the absence of identifiable macroscopic plant remains in the mounds, dispersed organic matter, assumedly resulting from termite activity, is the only possible material that can be used to date the mounds. Radiocarbon dating of sediments is notoriously difficult, due to common contamination with younger materials as a result of rooting and bioturbation, as well as by percolation of mobile organic substances (Brock et al, 2010;Mayer et al, 2008;Wang et al, 1996).…”

Behavioral Variability in ABA Chemical Pretreatment Close to the 14C Age Limit

“…Literature searches show that chemical solution strengths can vary from 0.1 to 6M, with temperature digestions spanning from room temperature to 90 C. The duration of each step is also highly variable and can run from 20 min to 21 hr (Bird et al 1999;Hatté et al 2001;Kim et al 2001;Santos et al 2001;Hajdas et al 2004;Sakamoto et al 2004;Catanzariti et al 2007;Mayer et al 2008;Brock and Higham 2009). Moreover, several publications do not directly describe the ABA chemical pretreatment used; rather, they refer to it by its epithets (e.g.…”