Carbon dates

Abstract

The primary goal of the ARPP excavations at the Page-Ladson Site complex was to establish a long nearly continuous stratigraphic record of environmental history during the late Pleistocene and early Holocene. Once the sedimentary sequence and the diversity of organic preservation had been established, it became evident that a refined series of multiple carbon dates was essential to assure sound correlation with virtually any other set of prehistoric events from local to regional and even to a global perspective. In the half century since the radiocarbon method was established it has become the primary standard for communication about chronology within the many interrelated disciplines dealing with the Quaternary. During the past 20 years the application of accelerator mass spectrometry (AMS) to dating radioactive carbon reduced the required sample size from many grams to a few milligrams (Taylor, 1992). Where previously a bone date might require the whole bone of an ungulate, now a tooth of a rodent provides an ample sample. An additional practical advantage of AMS dates is that use of a pinpoint sample goes far toward eliminating potential contaminants that were often incorporated into old-style bulk samples. Throughout this book diverse specialists turn to the carbon-dated framework to integrate and correlate their discussions which range from climatic events and environmental patterns to aquatic and terrestrial flora and fauna and finally to human cultures. Therefore the validity of the carbon dates and their placement within the site profiles become fundamental. In this chapter we describe the field methods by which we acquired carbon samples and the lab procedures by which we submitted them to three major labs. We also discuss various issues regarding sample biases and consider current issues concerning calibration curves and calendar corrections. Finally, we present the essential results by which the Page-Ladson Site complex can claim century-scale chronology for a critical interval of ten millenia. This framework of dates, tied directly to the physical stratigraphy, extends from the time of the last glacial maximum, ca. 18,500 14C BP, to a time just after the final retreat of the Laurentide Ice Sheet, ca. 8200 14C BP.