The rate of atmospheric formation of radiocarbon (C-14) was long considered to be relatively constant over time during the Holocene and late Pleistocene epoch. Because living tissue fixes carbon derived from the atmosphere, the result is a fairly constant proportion of radioactive C-14 to stable C-12 in living plants and animals. Using the measured half-life of C-14 (5,730 years), the approximate age of plant and animal tissue containing C-14 extending back as far as 30,000 ybp can be calculated. However, the C-14 dating technique is not as accurate as was once believed because it has been discovered that the ratio of C-14 to C-12 in the atmosphere fluctuates with time.In an attempt to determine the degree of fluctuation in the C-14 to C-12 ratio, scientists have attempted to calibrate C-14 ages with the more reliable dendrochronological ages. Since a dendrochronological time scale has been verified by Becker (1993) for the last 11,000 ybp, the degree of fluctuation in the rate of C-14 production during the past 11,000 ybp is known. This has resulted in the compilation of C-14 calibration curves which reveal the presence of both a long-term and several short-term deviations in the C-14 to C-12 ratio of the atmosphere over the past 11,000 ybp. The difficulty scientists are still currently faced with, however, is how to interpret C-14 ages in the range of 11,000 to 30,000 ybp, beyond the dendrochronological time scale but within the present limit of C-14 dating methods. In an attempt to solve the dilemma of how to interpret C-14 ages in this age range, this paper offers: 1. A detailed summary of the dendrochronological, C-14 dating, and C-14 calibration via dendrochronological analysis methods; and 2. A complete extrapolation of an existing C-14 calibration curve out to 30,000 ybp.Using an extrapolation of both a best-fit straight line trend and a periodic oscillating trend (each with a C-14 deviation of + 100 C-14 years), a method of calibration for C-14 dates greater than 11,000 ybp is created by using both trends in conjunction with one another (Figure 3). This method, when applied to the existing calibration curve in Figure 1, shows a maximum increase in deviation of 335 dendrochronological years in addition to the 265 years produced at a C-14 age of 4,700 yr by by conventional analysis. In the range of extrapolation from 11,000 to 30,000 ybp, a maximum age difference between a best-fit straight line trend and periodic oscillating trend of ± 688 actual cal (calibrated) years is present at an approximate C-14 age of 23,000 yr by (Figure 2). At 30,000 years cal BP the two trends, however, differ by only ± 425 actual cal years which corresponds to a C-14 age of 26,500 yr bp. The 3,500 year difference between C-14 and dendrochronological age scales at 30,000 years cal BP results in part from the long and short-term deviations previously mentioned, and in part from a 3% short fall in using the Libby half-life (5,568 years used by Stuiver and Becker (1993) to make the calibration curve used in Figure 1 of this paper) instead of the more recently determined and more accurate half-life of 5,730 years.This method of extrapolation is in close accordance with the U-Th dating method used by Bard et. al. (1990). The maximum difference of approximately 3,500 years between C-14 and dendrochronological dates at 30,000 ybp equals the approximate 3,500 year maximum difference found between C-14 and U-Th ages at 20,000 ybp by Bard et. al. (1990). In addition, this correlation holds true even though these authors used the more accurate 5,730 year half-life in their extrapolation of C-14 instead of the Libby half-life used in Figure 1 of this paper. Therefore, the method of extrapolation presented here could be used as a reasonable first order check of approximate calendric age ranges for respective C-14 ages. This method of extrapolation would also be useful for checking the reasonableness of other C-14 calibration techniques in the range of 11,000 to 30,000 ybp. This analysis of extrapolating the calibration curve indicates that continued work on measuring the ratio of C-14 to dendrochronologically derived ages in materials older than 11,000 ybp is justified. / Department of Geology

No description available.

No description available.

Thesis (M.S. - Hydrology and Water Resources)--University of Arizona, 1984. / Includes bibliographical references (leaves 64-68).

This project describes the Holocene environmental and pedogenic history of the Hiraethog Moors, particularly in relation to archaeological evidence. Ironpan stagnopodzol, brown podzolic and stagnohumic gley profiles have been studied from Aled Isaf together with cores from Cefn Mawr and Llyn y Foel-frech. Physical, chemical, clay mineralogical, micromorphological and palynological analyses have been undertaken and a time framework has been achieved by radiocarbon dating, including AMS 14C dating of ironpan and charcoal samples. A search for tephra has been undertaken and, although none was located, the presence of a biolith bloom in a core from Llyn Cororion on the Arfon Platform raises the possibility of a geochemical reconstitution of a low volume, fine-grained tephra fall. Parent material was reworked by periglacial processes during the Late-glacial resulting in an oriented fabric, cracked stones and a redistribution of clay and fine siltsized material. Until 6-7,000 years BP soils remained shallow and stony, with a clay mineralogy dominated by hydrous mica and chlorite. Between 6,000 and 4,000 years BP erosion led to deeper soil profiles on the lower slopes, burying flints and charcoal, and the woodland was periodically disturbed by humans. However, man was relatively inactive between 4,500 and 3,500 years BP. At 3,500 years BP woodland cover declined rapidly due to human activity with a subsequent change to a Gramineae- and then a Calluna-dominated vegetation community. In low lying sites the result was increased waterlogging, gleying, structural collapse and the build up of organic matter at the surface i. e. stagnohumic gley. In better drained sites podzolisation occurred to produce the Bs horizon, i.e. brown podzolic soil. In profiles most intensively leached, mor humus and then peat accumulated. This induced surface waterlogging resulting in a mobilisation of iron, structural collapse and the formation of an Eag horizon, within which chlorite was destroyed and hydrous mica weathered to vermiculite, and an ironpan i.e. ironpan stagnopodzol. Through the integration of soil and pollen analysis, 14C dating and archaeological information our understanding of soil development and human activity on Hiraethog has been increased.

No description available.

No description available.

No description available.

No description available.

No description available.