Archaeological Investigations at Yourhaney Plantation (38GE18)

ARCHAEOLOGICAL INVESTIGATIONS AT YOURHANEY PLANTATION123V. RESULTS OF OXIDIZED CARBONRATIO DATINGINTRODUCTIONThe original purpose for taking OCR samples at Yauhannah Bluff was to assist in documenting anyuse of the site as a trading post. Secondary to that purpose was to experiment with the method,since many features would have datable historic ceramics. OCR dates obtained for the siteprovided no clear-cut evidence of an early 18 th century occupation. A discussion of how OCRsamples were taken, how OCR works, as well as critiques and defenses of its use are presentedhere.BACKGROUNDAccording to Douglas Frink, the effect of biochemical degradation of charcoal and soil humicmaterial can be measured by the ratio of the total carbon to the readily oxidizable carbon in thesample, or Oxidizable Carbon Ratio (OCR). The rate of biochemical degradation of the relativelystable forms of organic matter varies within the specific physical and environmental contexts of thesample. To determine an age for the carbon sample, a systems formula was designed to accountfor the biological influences of oxygen, moisture, temperature, and the soil’s reactivity. Thesevariables are measured by soil texture, depth below the soil surface, the site specific mean annualtemperature and rainfall, and the soil pH. Residual influences on this system are included througha statistically derived constant (Frink 2004).Rainfall and temperature affect soil development as soil pH decreases with increased rainfall,indicating that the extent of leaching and organic decomposition decreases. At the same time, thedepth to leached carbonates in the soil increases. Also, nitrogen content increases which indicatesthe degree of organic decomposition in the soil. Clay content increases, reflecting the leachingand mineral decomposition in the soil. Also, for every 10° centigrade increase in temperature, therate of chemical reactions increases by a factor of 2 to 3 (Frink 1997).Soil depth and texture affect the rate of oxygen diffusion into the soil and therefore the growth rateand depth of root development. Coarse-textured soils have a higher rate of oxygen diffusion, witha corresponding increase in the rate and depth of root growth. Nutrients available to plants andsoil micro-organisms are dependent on the parent material, but also influenced by the biologicalcommunity. Soil pH affects both chemical and biological processes in the soil. Also, the factor oftime affects the rate and duration of biochemical processes (Frink 1997).Control samples at 38GE18 were obtained from soil columns in shovel tests prior to mechanicalstripping near the features dated using OCR. A sample was obtained every 0.3 feet in the columnsto a depth of just above subsoil. Samples from features were taken from a sealed portion of thefeature, near the top. Each sample submitted for dating was at least 100 grams after air-drying.