Glen G. Fredlund

Modern phytolith assemblages from the North American Great Plains

The analysis of fifty modem phytolith assemblages demonstrates the practicality of opal-phytolith analysis as a surrogate for pollen analysis in the North American Great Plains. The fifteen native grassland sites used in the study span both the east-west moisture and north-south temperature gradients of the central North American grasslands. Multiple soil samples analysed from each grassland site measure within-site variability and provide evidence of how assemblages are formed. Soil assemblages strongly reflect regional grassland composition rather than local vegetation. Five factors contribute to assemblage formation: decay-in-place, fire, eolian transport, herbivory and fluvial/colluvial deposition. Soil assemblages should be interpreted as the end result of these processes integrated over long time periods. The correspondence of assemblages to local conditions controlled by landscape position (e.g. upland v. lowland) is less important than the regional correspondence but is detectable and predictable. Data presented here will provide a basis for interpretation of fossil assemblages from late Quatemary paleosols in the North American Great

Calibrating grass phytolith assemblages in climatic terms: Application to late Pleistocene assemblages from Kansas and Nebraska

Abstract The use of modern phytolith assemblages for the interpretation of fossil phytolith assemblages demonstrates the utility of phytolith analysis in reconstructing grassland vegetation and climate in the Great Plains of North America. The modern data presented are the typical (average) assemblages for 34 modern localities taken throughout the Great Plains. Modern assemblage variability across the region is substantial. This variability shows a coherent geographical pattern consistent with modern grassland composition. These modern phytolith assemblages are calibrated in terms of modern climate data: specifically July mean temperature. Seven fossil assemblages from three late Pleistocene localities in the central Great Plains are presented. Two alternative phytolith-temperature regression models are applied to these fossil assemblages to estimate paleo-temperature. Resulting estimates are consistent with other efforts to calculate late Pleistocene temperature departures in the region (ca. −8.0 to −7.0°C). The fossil data also show rapid change in grassland composition and climate during the Pleistocene-Holocene transition. These results demonstrate how phytolith analysis can complement, or substitute for, pollen analysis in the Great Plains of North America.

Holocene environments of the central Great Plains: multi-proxy evidence from alluvial sequences, southeastern Nebraska

Abstract Pollen, plant macrofossils, phytoliths, carbon isotopes, and alluvial history from sediments exposed along the South Fork of the Big Nemaha River, southeastern Nebraska, USA, provide an integrated reconstruction of changes in Holocene vegetation, climate, and fluvial activity. From 9000 to 8500 uncalibrated 14 C yr BP, climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly, upland deciduous forest declined, and prairie attained its Holocene dominance. From 8500 to 5800 yr BP. upland forest elements disappeared, and even riparian trees were sparse under dry climatic conditions. Alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr BP. From 5800 to 3100 yr BP, riparian forests returned to prominence, and droughts were intermittent. Alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley. Aggradation on alluvial fans slowed and finally ceased near the end of this period. During a short dry interval from 3100 to 2700 yr BP riparian trees (except elm) disappeared, and prairie and weedy species became more abundant. This interval is represented by the organic Roberts Creek Member, and the alluvial setting was a slightly incised meandering channel belt. Habitats became similar to presettlement conditions during the last 2700 yr BP. Weedy taxa dominate modern samples, reflecting widespread disturbance. Alluvial fans and terrace surfaces were stable during the last 2500 years, but episodes of floodplain aggradation were punctuated by incision of the main channel.

Holocene stratigraphy and chronology of the Casper Dune Field, Casper, Wyoming, USA

Activation chronologies of dune fields within the North American Great Plains are significant sources of paleoclimate information. Although many regional chronologies exist, several dune fields have been understudied, including the Casper Dune Field of central Wyoming. This study investigated aeolian dune sediment and buried soils of the Casper Dune Field. Complex parabolic and hairpin parabolic dunes dominate the eastern dune field, while simple parabolic and linear dunes dominate the western dune field. Buried soils are found throughout the dune field, though their distribution and degree of development varies. Buried soils in the eastern dune field are weakly developed with typical A-C profiles, whereas soils in the western dune field typically exhibit A-Bt-C profiles. Optically stimulated luminescence (OSL) and radiocarbon ages were used to provide a chronology of dune field activation that spans most of the Holocene. At the onset of the Holocene, alluvium was deposited first, followed by widespread dune activity ~ 10.0—6.2 ka. Following activity, the dune field stabilized until about 4.1 ka. During this stabilization period, however, reactivation occurred in at least one locality within the dune field at 5.1 ka. Subsequent aeolian activity occurred at 4.1 ka and between 1.0 ka and 0.4 ka. The resulting activation chronology is compared with those obtained from elsewhere in Wyoming and from other west-central Great Plains dune fields.

Digital infra-red photography for recording painted rock art

Here is a new application of infra-red photography with a digital camera to record rock art. The need to make full and accurate records of the images, without touching (and thus degrading) the rock, requires a method of remote mapping. Trials with digital IR reported here are very promising and especially useful for painted rock art.

LATE QUATERNARY GEOMORPHIC HISTORY OF LOWER HIGHLAND CREEK, WIND CAVE NATIONAL PARK, SOUTH DAKOTA

Mapping of late Quaternary geomorphic surfaces, and analysis of the soils and sediments buried within them, provides evidence for the history of a small study area within the Red Valley physiographic zone, Black Hills, South Dakota. Geomorphic thresholds for this grassland system are correlated with periods of major climatic change. Well-developed soils dating to the late Pleistocene and early Holocene (14,000 to 9000 yr B.P.) suggest more mesic conditions and geomorphic stability. A mid-Holocene Altithermal (ca. 8000 to 4500 yr B.P.) denudation almost completely stripped the landscape of earlier Holocene sediments and soils. A prolonged, mid-Holocene (ca. 4500 to 3600 yr B.P.) mesic period of landscape stability and soil development followed, but was abruptly terminated around 3600 yr BP. Late Holocene conditions approached stability about 1200 yr BP. After this time, alluvial terrace surfaces remained stable, while alluvial fans experienced periods of stability punctuated by midslope aggradation.

Grass Phytolith Analysis

The vegetative history of eastern North Da kota is broadly known from pollen and multi-proxy investigations of lake basins from across the re gion (Clark et al. 2001; Dean and Schwalb 2000; Wright et al. 2004). These records suggest that parkland or savanna rapidly developed following glacial retreat and subsequent drainage of glacial Lake Agassiz to the south (10,800 to 9400 14C year B.P.) with true grasslands (or prairie) emerging by 9000 14C year B.P. Changes in grassland com position of the region is poorly documented in the fossil record throughout the Holocene. While stable carbon isotopes offer some insight (Clark et al. 2001), this proxy is limited in that it cannot dis tinguish between changes in forest cover from C3 grasslands. Fossil phytolith assemblages are used to infer changes in Holocene grassland vegetation and cli mate at the Rustad site. The evidence presented comes from two stratigraphic columns allowing evaluation of local variability in phytolith assem blages and validation of results. The soils and sedi ments sampled in this study span much of the last 9,000 years. Several major environmental changes are hypothesized to have occurred during this time and should be detectable with opal phytolith analy sis. First, the early Holocene (12,000 to 8000 years B.P.) was a period of major climatic warming. Grassland response to this warming should show up in the phytolith record as a shift from a C3 grass dominated cover to a mixed grass prairie including both tall (mesic adapted) and short (xe ric adapted) C4 grasses along with diverse C3 grass taxa. Second, sometime during the mid-Holocene (8000 to 5000 years B.P.) many, but not all, re gional paleoenvironmental records suggest one or more periods of higher temperatures and greater aridity, broadly labeled the Altithermal. This mid Holocene aridity should be detectable in the phytolith record as a relative increase in phytolith morphotypes from the xeric-adapted, C4 short grasses such as blue grama, side oats grama, and buffalo grass. The goal of this chapter is to report the find ings of phytolith analysis and offer some prelimi nary interpretations. The analysis focuses on the grass phytolith portion of the assemblages, al though there is great potential for additional in formation from the arboreal phytolith component and from other proxy data, including carbon iso topes, terrestrial snails, and vertebrate fauna. METHODS