Late Quaternary landscape evolution, environmental change, and paleoindian geoarchaeology in Middle Park, Colorado

Stratigraphic records in western Middle Park (∼2350-2800 m) in north-central Colorado provide evidence for the late Quaternary geomorphic and environmental history of a non-glaciated Southern Rocky Mountain basin. Episodes of geomorphic instability apparent in the stratigraphic record appear to coincide with changes in paleoenvironmental records from above 2750 m in north-central Colorado, suggesting that the western Middle Park landscape was sensitive to environmental changes affecting the region over the last ∼14,000 cal years. Tributaries were incised prior to 14.0 ka, but deposits older than 12.0 ka are rare. Upland erosion and incision followed by rapid aggradation in alluvial settings between 12.0 and 11.0 ka coincide with evidence for regional temperatures at or above present, and is interpreted to signal the onset of Holocene summer-wet precipitation. A widespread soil-stratigraphic marker (the Kremmling soil) represents a long period of landscape stability between <11.0 and 6.0 ka in upland and alluvial settings. Pedologic evidence from upland settings indicates the expansion of grass and forest cover to lower elevations that today are characterized by sagebrush steppe, probably during a period of increased summer precipitation relative to present. During the late Holocene, episodes of aggradation in alluvial valleys at 6.0-1.0 ka and 0.6-0.2 ka and soil formation in uplands at 5.0-3.5 ka and 2.5-1.0 ka overlap with evidence for cooling at higher elevations. Incision of valley floors documented at 1.0-0.6 ka and during the last few centuries and episodes of erosion in uplands at 3.5-2.5 ka, after 1.0 ka, and within the last few centuries, are roughly synchronous with evidence for warming. Upland and alluvial stratigraphic records are interpreted to indicate that during cool intervals summer precipitation was diminished, resulting in relative hillslope stability and gradual valley bottom aggradation, while pulses in summer precipitation accompanying warmer episodes caused basin-wide geomorphic instability. The recent increasing frequency of geomorphic instability appears to correspond with an increase in sagebrush steppe at the expense of forest and grass cover, interpreted to represent progressive drying during the late Holocene. It stands to reason that future warming, if accompanied by similar patterns in precipitation, will result in continued erosion on a landscape already at a threshold of geomorphic instability.;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 based on charcoal. On average, 14C ages of SOM from buried surface horizons are 880+/-230 14C 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 14C yr younger than charcoal ages, respectively, and HA ages are on average 860+/-140 14C yr older than LT fractions. We interpret the offsets between 14C 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 14C ages suggests short residence time on the landscape, with little subsequent reworking. Radiocarbon 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.;Geoarchaeological investigations in western Middle Park produced information important for understanding the soil-stratigraphic context of Paleoindian components (∼12.9-8.9 ka), which are associated with sheetwash mantles on upland hillslopes and alluvial valley fills. In situ components occur in a moderately developed buried soil (the Kremmling soil) formed over thin (≤1m) hillslope mantles uplands. The Kremmling soil reflects early and middle Holocene geomorphic stability in upland and alluvial settings, and represents a buried landscape with the potential to produce additional Paleoindian assemblages, though elsewhere in western Middle Park early Archaic sites occur in morphologically similar soils. While portions of all excavated sites have been lost to post-Kremmling soil erosion, several untested Paleoindian localities are likely to contain buried and in situ components. Paleoindian-age alluvium is relatively well preserved along drainage axes, but to date only a single Paleoindian-age component is recorded in alluvial settings. Whether or not this results from inadequate survey, an absence of sites, or both, is unknown, but alluvial settings deserve further investigation.