Stephen A. Hall

DETERIORATED POLLEN GRAINS AND THE INTERPRETATION OF QUATERNARY POLLEN DIAGRAMS

Hall, S.A., 1981. Deteriorated pollen grains and the interpretation of Quaternary pollen diagrams. Rev. Palaeobot. Palynol., 32: 193--206. Pollen assemblages are often altered by deterioration, which results in biased grain counts and possibly erroneous paleoecologic interpretations. Distorted counts are caused by (1) progressive pollen deterioration (a new preservation category), (2) differential pollen preservation, (3) differential recognition of poorly preserved grains, and (4) different kinds of deterioration in different sediment types. The best indicators that pollen assemblages have been altered are high frequencies of deteriorated grains and low total pollen concentrations.

Late Quaternary sedimentation and paleoecologic history of Chaco Canyon, New Mexico

A stratigraphic and pollen analytical study of a thick sequence of alluvium at Chaco Canyon National Monument has provided a detailed history of late Quaternary alluviation, channel trenching, vegetation, and climate for the eastern Colorado Plateau. The alluvium at Chaco Canyon consists of five informal units, each separated by an erosional unconformity: Fajada (late Pleistocene), Gallo (7000 to 2400 B.P.), Chaco (2200 to 850 B.P.), Post-Bonito (600 B.P. to 1860 A.D.), and Historic (1935 to present). The Fajada paleosol occurs throughout northwestern New Mexico and is radiocarbon dated as older than 6,700 yr. Pollen analysis of the four younger alluvial units provides evidence for a mid-Holocene interval of aridity that occurs later than recognized elsewhere in the Southwest. Around 5,800 yr ago both pinyon and ponderosa forests were rapidly diminished by a decrease in precipitation. Maximum reduction of the pine forests and woodlands and the greatest aridity of climate persisted from 5600 to 2400 B.P. Ponderosa forests in the mountains adjacent to Chaco Canyon had begun to expand their range by 2200 B.P., and the major expansion of pinyon woodlands in the Chaco area began about 850 yr ago. The climate at Chaco Canyon when the Basketmaker and Pueblo peoples lived there about 600 to 1150 A.D. was drier and warmer than it is today. The ponderosa pine logs used in pueblo building were obtained from the mountains east, south, or west of Chaco Canyon. Abandonment of the pueblos coincided with the incision of the valley floor by the post-Bonito channel system.

Late Holocene Paleoecology of the Southern Plains

Abstract Analyses of pollen and land snails from rocksheter sites in the Osage Hills of northeastern Oklahoma indicate that the period 2000-1000 yr B.P. was moister than today. During that time, colonies of the prairie vole Microtus ochrogaster were present in the Texas Panhandle. About 1000 yr B.P. the climate changed to dry conditions that have persisted to the present. Disjunct colonies of small mammals in Texas became extinct at the beginning of the dry episode, thereby establishing the composition of the modern fauna. The climatic model for the origin of the Panhandle Aspect (A.D. 1200–1500) is questioned on the grounds that the Southern Plains experienced a long period of dry climate commencing A.D. 950.

Buried trees, water table fluctuations, and 3000 years of changing climate in west-central Oklahoma

Abstract A sequence of radiocarbon-dated buried trees, buried soils, a carbonate zone, and a molluscan fauna from Carnegie Canyon indicate that between 3200 and 2600 yr B.P. the climate of west-central Oklahoma was drier than today. A high water table accompanied a period of moister climate 2000 to 1000 yr B.P. The water table dropped after 1000 yr B.P. due to a change toward dry conditions.

Geochronology of the Bolson sand sheet, New Mexico and Texas, and its archaeological significance

The Bolson sand sheet occurs in the Tularosa Valley, New Mexico, and the Hueco Bolson, Texas, and it consists of two principal eolian sand units. Optically stimulated luminescence (OSL) dating provides a new chronology of the sand sheet that relates as well to the formation, preservation, and visibility of the local archaeological record. The lower sand (unit Q2) (44.8 ± 2.9 ka) and the upper sand (unit Q3) (22.2 ± 1.6–5.2 ± 0.3 ka) have a combined thickness of less than 2 m. The Q2 sand is characterized by a red Bt paleosol, and the Q3 sand has a weak calcic paleosol with stage I carbonate morphology. Elevated amounts of airborne silt were incorporated in the Q3 sand during the period 24–14.5 ka, representing higher amounts of dust in the atmosphere during glacial and late-glacial time. Multiple OSL ages from the Q3 sand indicate a slow net sedimentation rate of 0.06–0.09 mm/yr, similar to other OSL-dated sand sheets in the region; sand deposits in dune fields have higher accumulation rates. The McGregor A horizon soil, radiocarbon-dated to younger than 0.5 ka, occurs at the top of the sand sheet and likely formed with desert grassland vegetation. Thousands of recent mesquite coppice dunes (unit Q4d) mantle the sand sheet, and two are dated to the twentieth century by OSL and 137 Cs. Archaeological sites that postdate 3000 B.C. are concentrated, sometimes together, on the surface of the sand sheet, while sites that predate 3000 B.C., although rare, may be buried within the Q3 eolian sand. The Q2 sand is too old to contain archaeological sites, although site features may intrude into the sand. Previous chronologies of the sand sheet are based on radiocarbon dates of charcoal from archaeological sites, on radiocarbon dates of soil carbonate, and on soil-geomorphology correlations with Rio Grande Valley alluvium. The optical chronology does not support these various correlations. We recommend that the alluvial names Isaacks9 Ranch, Fillmore, and Organ no longer be applied to the Bolson sand sheet.

New correlation of stable carbon isotopes with changing late-Holocene fluvial environments in the Trinity River basin of Texas, USA

A late-Holocene alluvial sequence in north-central Texas has a 1 m thick buried cumulic soil with an A-C profile called the West Fork paleosol. It formed 2300 to 1000 yr BP and is a local equivalent of the Copan paleosol that is present throughout the southern US Great Plains. Stable carbon isotopes indicate that the paleosol and underlying gray clay formed under vegetation dominated by C4 species (mean δ13C: −18.3 ± 0.3‰). Diverse paleoenvironmental studies indicate that the period of paleosol formation was cool and wet and that alluvial water-tables were high, resulting in broad wet meadows across alluvial valleys, characterized by communities of grasses. Present-day wet meadows and bottomlands with Mollisols with A-C profiles along streams in the Great Plains are dominated by C4 tallgrass species and may serve as analogues to wet-meadow environments during the late Holocene. A shift in climate to warm-dry conditions about 1000 yr BP was accompanied by deep channel cutting, low alluvial water-tables, and colonization of abandoned floodplains by trees and other C3 species, as indicated by a change in carbon isotopes to lower values (mean δ13C: −20.8 ± 0.5‰) and correlating with the ‘Medieval Warm Period’. Other stable carbon isotope studies from late-Holocene alluvium in Texas have been mistakenly interpreted as evidence for paleoenvironmental conditions opposite to those presented in this investigation. We conclude that interpretations of stable carbon isotopes from alluvium based on broad patterns of upland C4 grasses and climate can be in error, especially in cases where wet-meadow deposits and soils are present.

Berino Paleosol, Late Pleistocene Argillic Soil Development on the Mescalero Sand Sheet in New Mexico

The Berino paleosol is the first record of a directly dated Aridisol in the American Southwest where paleoclimatic conditions during the time of pedogenesis can be estimated. The noncalcic, argillic paleosol formed in eolian sand during the cool, wet climate of the mid- and late Wisconsin, marine isotope stages 3 and 2, in presently semiarid southeastern New Mexico. Optically stimulated luminescence dating of the Mescalero sand sheet and the Berino indicates that soil formation occurred during the period 50–18 ka. The paleosol is a red 2.5YR hue Bt horizon, 120 cm thick, with 25% clay, 0.36% Fe, and an absence of visible carbonate. It is buried by younger eolian sand, although at the edges of the sand sheet, it is unburied and a relict soil. Red argillic paleosols in other sand sheets in the region may correlate with the Berino. The Berino paleosol is formally named as a pedostratigraphic unit.

Geochronology and stratigraphy of the Bolson sand sheet: Reply

The geology of the Bolson sand sheet has been studied extensively, especially its relationship to prehistoric sites on the Fort Bliss Military Reservation. An early systematic evaluation of the stratigraphy and paleosols of the eolian sand was conducted by [Blair et al. (1990)][1], who named the Q1