Kansas Geological Survey, Open-file Report 96-35
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Summary and Conclusions

The stratigraphy of loess deposits which have been investigated on Fort Riley exhibits the same sequence of loess units and intercalated buried soils as is found elsewhere in the region, but adds detail unique to the reservation. Composite stratigraphy of the late-Quaternary loess deposits preserved on the reservation consists of the basal Sangamon soil of the Last Interglacial (Illinoian age; c. 120-110 ka), Gilman Canyon Formation (a pedogenically-altered, middle Wisconsinan loess; c. >40-20 ka), Peoria loess (late-Wisconsin; c. 20-10 ka), Brady soil (Pleistocene/Holocene transition; c. 11-10 ka), Bignell loess (Holocene; c. 9-? ka), and modern surface soil.

The record of climatic change during the late Quaternary has been retained in the loess deposits of the central Great Plains, and such a record is extractable using some relatively new approaches. The loess sequences at Fort Riley represent a nearly continuous time series of climatically-forced environmental change for the late Quaternary. Application of two analytical techniques, magnetics and SIRA, is providing proxy data sets that represent a time series of climatically regulated pedogenesis/weathering and botanical composition (Johnson, 1996b). This investigation, however, presents the results of an expanded study of the opal phytolith record contained within sites from various landscape setting on the reservation (cf. Johnson et al., 1994).

Study localities included two upland sites, Sumner Hill and Bala Cemetery; a valley-wall site, Pump House Canyon; and a valley-bottom site, Manhattan Airport. Sumner Hill, a bluff top site on the north side of the Kansas River valley, has the thickest loess accumulation of all sites examined (c. 13m), extending from the Sangamon soil to the surface soil, whereas Bala Cemetery site, in the extreme northwestern part of the reservation, consists of less than 3m of loess above the limestone bedrock. Pump House Canyon site has a relatively thick accumulation of loess, probably in excess (Gilman Canyon and Peoria loesses) of 5m. Sediments at the Manhattan Airport are alluvial, not eolian, and exhibit the alluvial phase of two or more of he soils found in the upland record (Brady and Sangamon and/or Gilman Canyon).

Quality of the opal phytolith record varied appreciably among the sites. Sumner Hill produced only a marginal record for the upper 3 m, i.e., the latest Pleistocene and all of the Holocene. After some abrupt and brief fluctuations in grass composition at the end of the Pleistocene, the xeric warm-season grasses expanded as the Holocene climate and its grassland biome evolved. At the Bala Cemetery site, the Holocene record appears to be absent, with the phytolith assemblages indicating the amelioration of climate at the closure of the Pleistocene, i.e., the mesic, cool-season grasses were gradually yielding their dominance to the more xeric warm-season grasses. The quality of record at this upland site is good. The Pump House Canyon site provided the poorest phytolith record of all sites investigated thus far; phytolith preservation and concentration were good only in the upper 30-40 cm. The limited data indicate a mixed grass environment for the last 2-3,000 years on this strath. The Manhattan Airport site, a high terrace consisting of late-Pleistocene to late-Holocene alluvium, provided a good phytolith record for the upper 2m and offers an interesting mix of regional and local environmental signals. The mesic cool-season grasses slowly yield to the more xeric warm-season grasses as the Pleistocene comes to an end, but the relatively high percentage of C₃ grasses may be exaggerated in the record due to the microclimate of the alluvial bottom (high soil moisture and arboreal shading).

The composite of these data portrays climatic change at the end of the Pleistocene and much of the Holocene: C₃ grasses, such as the bromes, fescues and bluegrasses, gave up their dominance and the C₄ grasses took the advantage, particularly the more xeric chloridoid types, such as the gramas, buffalo grass, and muhly grasses. These data provided no clear indication of major, long-term events during the Holocene, in particular the Altithermal. Notable events, both long- and short-term, are very likely part of the climatic history for the region and the reservation, but the quality of the Holocene phytolith record for the upland loess deposits is limited to the marginal record obtained at Sumner Hill. As a result, possible correlation with the magnetic record is limited.

Despite the limitations of the phytolith data obtained to date, more is known about the late-Quaternary climatic history of Fort Riley, particularly as it relates to landscape evolution and to changing environments and resources of the aboriginal peoples occupying the reservation landscape since the late Pleistocene. Grassland composition and associated gathering potential and game changed, and trees came and went from the landscape. Also, the upland environment was different from the strath and from the alluvial bottoms, and rates and magnitude of change varied among the different landscape elements.

Recommendations

Based on the results of this and past studies at Fort Riley (Johnson et al., 1994; Johnson, 1996b), it is recommended that further opal phytolith analysis be conducted in order to realize the potential of the upland paleoenvironmental record and to generate a geoarchaeological model of sufficient temporal and spatial resolution to provide the basis for development and/or testing of the computer-based geoarchaeological model under development by R. Bras and colleagues at MIT. Specific recommendations include:

1. Selection and analysis of one or more upland sites containing expanded Holocene records in order to improve on the quality of record obtained from the Holocene phytolith record of the Sumner Hill site. After potential sites are identified, a spot check analysis of phytolith isolate quality would be made to locate sites with good records. The Bala Cemetery site indicates that quality phytolith records exist in the upland late..;.Pleistocene (Peoria) loess deposits, but such a quality record needs to be found in Holocene-age (Bignell) loess.
2. Samples should be collected and analyzed from within and adjacent to archaeological sites located on both the uplands and stream terraces. Phytolith analysis has shown itself to be an outstanding techniques for reconstructing paleoenvironments and cultural resources use from archaeologically enriched stratigraphy.

Opal phytolith analysis is a relatively new technique and the University of Kansas Palynology Laboratory is one of the few places in the world where it is being done. Application of the technique to the loess deposits of Fort Riley has, therefore, been a learning experience upon which one can build.

Since phytolith analysis was referenced in recommendations included in the review draft of the magnetic and isotope analyses on the reservation (Johnson, 1996b), those recommendations are restated:

1. In that the latest Pleistocene and entire Holocene are of primary interest in development of a cultural resources and landscape evolution model, further emphasis should be placed on this portion of the upland stratigraphic record. The environmental transition at the late-Pleistocene/Holocene boundary needs to be better characterized, as does the detail of the Holocene (e.g., the Altithermal, soil-forming periods). Specifically, high-resolution (e.g., 2 cm contiguous) sampling should be undertaken on two or more new sites with expanded Holocene sequences and on cores from the existing sites of Sumner Hill and Pump House Canyon for the purpose of SIRA of carbon. Further, these sites should be sampled at close interval (c. 25 cm) for ¹⁴C dating; subdividing the samples in order to individually date the total humates and humic acid and humin fractions would be helpful in deciphering the chronostratigraphy (Martin and Johnson, 1995; Johnson and Valastro, 1994). Magnetic analyses (susceptibility and frequency dependence) should also be conducted at the new sites selected.
2. The analysis of trace element content has been used in a number of applications related to the reconstruction of late-Quaternary environmental conditions (e.g., research by Muhs et al., 1990, 1995; Reheis, 1990; and Diekmeyer, 1994). Trace elements (e.g., cerium, strontium, yttrium, zirconium) contained with in the loess and intercalated soils provide a "fingerprint" of the sediment and provide information regarding paleowind direction and loess source region. Data obtainable include not only presence or absence, but also the concentration of individual trace elements. Recent research by Johnson and Muhs (unpub. data; Johnson et al., 1993b) has documented the difference in source regions for loess deposits of eastern Colorado versus those of southern Nebraska and Kansas. For Fort Riley, two levels or orders of information can probably be derived from the loess sequences. First, trace element analysis can indicate the stratigraphic level (and time, with sufficient ¹⁴C control) at which the winds shifted from the north-northwest to the south-southwest, i.e., determine whether the trace element signature is that of the Platte River sediments or of the Kansas River sediments. The second order variations in the data will document the more subtle changes within the trace element time series. These variations will indicate small order changes in wind intensity and direction at a given location, as well as define the distance-decay function along a sampled transect. The stronger the winds, the higher the concentration of the trace elements, since they are not only rare but also represent the heavy mineralogic fraction. Short-term changes in direction will also show up in the signature: Trace element analyses should be conducted on two or more of the sites referred to in the first recommendation, with sampling being conducted at close interval (c. 2cm). Previous research (Johnson et al., 1993b, Diekmeyer, 1994, Muhs and Johnson, 1996) has indicated that a suite of eleven trace elements contains the optimal amount of information for examining Pleistocene and Holocene loess of the central Great Plains. Facilities for preparation of samples for analysis exist within this investigator's laboratory, and those for analysis exist both in the Department of Geology, University of Kansas, and at the U.S. Geological Survey, Denver, Colorado (D.R. Muhs' laboratory).
3. Extensive archaeological surveys currently being conducted on the reservation are yielding new upland prehistoric sites, one or more of which have been found to contain evidence for Clovis (latest Pleistocene/earliest Holocene) occupation. Selected archaeological sites should be analyzed to articulate the specific paleoenvironmental record. As a corollary to the first recommendation, these sites, depending on their location relative to the nonarchaeological sites, should receive high-resolution magnetic, isotope, opal phytolith and perhaps trace-element analyses.
4. Results of D.L. Johnson's survey of the upland loess mantle (D.L. Johnson, 1996) should be used as base line data in developing a detailed map of surface materials in order to provide the precision of information needed to articulate a detailed sequence of landscape evolution on Fort Riley. A combination of shallow coring and ground-penetrating radar surveys can be employed to provide the information for input into a GIS in order to create a detailed map of surficial geology and the spatial variation in thickness of the various map units. Shallow coring can be conducted using the trailer-mounted Giddings soils probes operated by the Department of Geography, University of Kansas. Ground-penetrating radar, conducted by dragging a skid-mounted radar system behind an all-terrain vehicle, and personnel are available through both the Department of Geology, University of Kansas and the Kansas Geological Survey.

Research conducted to date by this investigator (Johnson et al., 1994; Johnson, 1996b; this report) and by D.L. Johnson (D.L. Johnson, 1992, 1994, 1996) has defined the late-Quaternary lithostratigraphy of the reservation and documented the first-order trends in the chrono-, magneto-, and biostratigraphy (SIRA, opal phytoliths). Now that the potential for reconstruction of the late-Quaternary environmental record has been realized and broadly defined, high-resolution study needs to be undertaken at selected existing and new sites in order to extract the interpretable detail.

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