PREFACE

and projectile point chronologies commonly used for cross-dating were developed in adjacent areas of theSouthwest and Plains. The chronological data supporting those sequences is often inadequate even in theregions where they were developed, and may not be applicable to southeastern New Mexico. Third, thetemporally-diagnostic artifacts found at a site may not indicate the full range of occupations and may evenbe incidental to the major occupation (e.g., curated Archaic projectile points at Ceramic period sites or potdrops at Archaic sites). Fourth, a significant proportion of sites in the region lack temporal diagnostics,either because they were never discarded at those locations or because the diagnostics have been removedby later groups or modern collectors.Several absolute dating techniques can potentially be employed in southeastern New Mexico, andconsideration should be given to the resolution and limitations of each method. Tree-ring dates have apotential resolution of one year to a few decades, so dendrochronology is clearly the most reliable methodfor determining whether sites or features within a site are contemporaneous. This method has verylimited application, however. Chronologically-sensitive species are present only at higher elevationsalong the western edge of the region, and even in this area, the construction timbers used in Ceramicperiod structures tend to be from trees with complacent growth rings.In the absence of tree-ring dates, archaeomagnetic dating can potentially provide high-resolutionchronological data for the Ceramic and Protohistoric periods. High precision archaeomagnetic dates have95% confidence intervals on the order of ±15 to 50 years, but the intervals for low precision dates areconsiderably larger. Several factors affect precision, but the most important are the iron content of thebaked sediments and the expertise of the technician collecting the sample. Sediment texture plays a lesserrole. Sediments with a higher clay content tend to provide more precise dates, but excellent results havesometimes been obtained from samples consisting of loams and sandy loams.The actual archaeomagnetic date is derived by fitting the orientation of ferrous particles in the sample to amaster curve showing the movement of the magnetic pole over time. Because past movements of thepoles were irregular, these curves have overlapping loops. Archaeomagnetic samples with pole positionsnear those loops will therefore have two or more potential dates, typically about 200 years apart. In thesecases, the archaeologist must employ other chronological indicators (e.g., radiocarbon dates, ceramiccross-dating, etc.) to determine which of the alternative dates is the more probable.Radiocarbon dating will undoubtedly be the principle chronometric method used to date sites insoutheastern New Mexico. Charcoal dates are generally more reliable than dates on bone or soil humates,and should be used whenever possible. Dates on shell are generally unreliable. Nominally, theresolution of radiocarbon dates is about 200 years; that is, a date with a standard error of 50 years is aboutthe best that can be expected. The resolution can be somewhat improved if multiple samples can beprocessed from a single piece of wood or other organic matter. By averaging those dates, the standarderror of the pooled mean can be reduced. There are, however, a number of significant sources of errorthat are not taken into consideration in calculating the standard error, which must be considered wheninterpreting radiocarbon dates. Chief among these are fluctuations in atmospheric carbon isotopes,isotope fractionation and, for dates using wood charcoal, built-in age and cross-section effect.Calibration curves correct for fluctuations in atmospheric C14, so both the radiocarbon and calibratedcalendar dates for a sample should be routinely reported. Isotope fractionation is a problem because someplants do not metabolize carbon isotopes in direct proportion to their availability in the atmosphere. Thusthey can yield erroneous dates. Corn is probably the best known example of this problem, but many aridadaptedspecies also have C4 or CAM metabolic pathways. Isotope fractionation can be corrected readilyif the ratio of stable isotopes is known, so isotope ratios should be requested routinely for all radiocarbonsamples submitted for dating.4-2