Calibration of the Radiocarbon Dates

Reprinted from Oklahoma Archeology, January 2000

Volume 48, Number 1, pages 12-13

Calibration of the Radiocarbon Dates

from 34RM501, the Swift Horse Site

J. Peter Thurmond, Oklahoma Anthropological Society, Inc.: dempseydiv@aol.com

K. C. Kraft, USDA-Natural Resources Conservation Service: kkraft@cpn-net.com,

and Don G. Wyckoff, Oklahoma Museum of Natural History: xtrambler@ou.edu.

The Swift Horse site (34RM501) is a Late Archaic/Woodland campsite on U.S. Forest Service, Black Kettle National Grassland property between the towns of Cheyenne and Reydon in Roger Mills County, Oklahoma. The archaeological deposit is buried in a dark brown (melanized) Late Holocene valley fill remnant on an unnamed left-bank tributary of Croton Creek, in the upper Washita River basin. The site was partially excavated in a contract between the U.S. Forest Service and the Anadarko Basin Museum of Natural History in Elk City, Oklahoma by James Briscoe and Roger Burkhalter in 1986 (Briscoe and Burkhalter 1986, Briscoe 1987a). A mix of corner-notched dart (7) and arrow (1) points and three cord-marked sherds were recovered from the 70 cm deep Late Holocene paleosol, and five woody charcoal samples spanning the depth of this soil were radiocarbon dated (Briscoe 1987a, 1987b).

The second author is presently writing a report of his analysis of the 1988-1991 Oklahoma Anthropological Society excavations at 34RM208, the Beaver Dam site, a Late Archaic/Woodland stratified campsite to the south of Swift Horse, buried in similar valley fill on Brokenleg Creek. We were interested to see how a current calibration of the Swift Horse dates would compare to the eleven radiocarbon dates from Beaver Dam, and the five dates from the Late Archaic/Woodland campsite at 34RM334C, Thurmond Ranch #13C, on Higgins Creek (Thurmond 1989, 1991). The Swift Horse dates were calibrated with the University of Washington Quaternary Isotope Lab Radiocarbon Calibration Program, Revision 4.0 (Stuiver et al. 1993). The results are as follows:

Beta-18429, Sample 1, composite charcoal from 10-40 cm below the preserved paleosol surface; radiocarbon age 1680 ± 80 BP; calibration curve intercept AD 388; 1d range AD 245-435.

Beta-18430, Sample 2, 40-50 cm; 1610 ± 90 BP; intercept AD 428; 1d range AD 343-555.

Beta-18431, Sample 3, 50-60 cm; 1590 ± 70 BP; intercept AD 433; 1d range AD 406-556.

Beta-18432, Sample 4, 60-70 cm; 1650 ± 70 BP; intercept AD 412; 1d range AD 263-527.

Beta-18433, Sample 5, 70-80 cm (base of a pit excavated below the paleosol base); 1820 ± 100; intercept AD 223; 1d range AD 78-339.

The first four dates form a tight group, their calibration curve intercepts ranging AD 388-433, and averaging AD 415. The fifth date appears anomalous at AD 223, and derives from pit fill. The first author participated in the 1986 excavations, and noted that the sandy deposit was riddled with Plains pocket gopher (Geomys bursarius) burrows. Note that the first four samples were not progressively older with depth. We believe the dated charcoal of Samples 1-4 likely derives from a single or few closely spaced occupations of the site during the late fourth to early fifth centuries AD, and that both the artifacts and charcoal were scattered through the paleosol by this bioturbescence. We note that these dates compare well with the middle components at Beaver Dam, AD 390-600 and at Thurmond Ranch #13C, AD 395-625, (Thurmond and Wyckoff 1999), which have produced similar diagnostics. The first four Swift Horse dates also coincide with the AD 400-650 Herring Creek pluvial, a period of enhanced rainfall identified by the first and third authors from their analysis and radiocarbon dating of paleosols in Roger Mills County (op. cit.). The Herring Creek pluvial is one of six identified for the last two millennia in what appears to be a 400 year regional rainfall cycle driven by a coeval cyclicity in solar output (op. cit.).

Briscoe (1987a) related the occupation at Swift Horse to the Lake Creek complex of the Texas panhandle (Hughes 1962). The second author is formulating a social/chronological model for the Late Archaic/Woodland occupation of the western Southern Plains, and assigns the above-noted components at 34RM208-A, 34RM334-C and 34RM501 to his Lake Creek phase of the Certain cluster. All appear to represent Early Woodland occupations of the Ogallala ecotone (Thurmond 1991) during the Herring Creek pluvial.

References Cited

Briscoe, James

1987a Analysis of cultural materials from the Swift Horse site, an Early Plains Woodland site on the Black Kettle National Grasslands, Roger Mills County, Oklahoma. Briscoe Consulting Services, Butler, Oklahoma.

1987b Radiocarbon dates from the Swift Horse site, 34RM501, Roger Mills County. Newsletter of the Oklahoma Anthropological Society 35(5):8-9.

Briscoe, James and Roger Burkhalter

1986 Preliminary report on archaeological and geomorphological investigations at the Swift Horse site, 34RM501, AR-03-03-06-098, Roger Mills County, Oklahoma. Anadarko Basin Museum of Natural History Research Report 1.

Hughes, Jack T.

1962 Lake Creek: a Woodland site in the Texas Panhandle. Bulletin of the Texas Archeological Society 32:65-84.

Stuiver, Minze, Austin Long, Renee S. Kra and J.M. Devine

1993 Calibration - 1993. Radiocarbon 35(1).

Thurmond, J. Peter

1989 An AMS date from a "Late Archaic" component in Roger Mills County, Oklahoma. Newsletter of the Oklahoma Anthropological Society 37(9):3-4.

1991 Archaeology of the Dempsey Divide, a Late Archaic/Woodland hotspot on the Southern Plains. Bulletin of the Oklahoma Anthropological Society 39:103-157.

Thurmond, J. Peter, and Don G. Wyckoff

1999 Paleoclimate and archaeology in far western Oklahoma: intermittent habitation of a climatologically marginal region, and the evidence for a 400 year rainfall cycle operating at least since the Last Glacial Maximum. Paper presented at the 70^th annual meeting of the Texas Archeological Society, Fort Worth, Texas, October 30, 1999.