Browsing Radiocarbon, Volume 50, Number 2 (2008) by Title
Now showing items 11-14 of 14
On the Prospects of AMS 14C with Real-Time Sample Preparation and SeparationThe performance of the CO2-accepting SO-110 ion source at Oxford Radiocarbon Accelerator Unit has been investigated in detail. The purpose was to clarify the possibilities of accelerator mass spectrometry (AMS) radiocarbon measurements with real-time separation, e.g. GC AMS or HPLC AMS. The construction of a gas test injector based on the continuous-flow technique made it possible to characterize the response of the ion source to continuous and pulsed input of CO2 gas. The source exhibited remarkably good linearity over a wide range of CO2-pulse sizes and fast rise time, but the peak shape varied and memory effects were significant. Appropriate tuning of the gas source proved to be critical.
Radiocarbon Chronology of Central Alaska: Technological Continuity and Economic ChangeThis research presents the first comprehensive radiocarbon chronology for central Alaska, encompassing the late Pleistocene and Holocene archaeological record. Dated component distributions, comprised of 274 14C dates from 160 components, indicate changing land-use strategies and subsistence economies, reflecting primarily lowland exploitation of bison, wapiti, and birds prior to 6000 cal BP, followed by increasing caribou and fish exploitation and use of upland areas. Microblade technology is conserved from the earliest components to ~1000 cal BP, and this continuity is not reflected in current cultural history sequences. Using component abundance as a proxy for population, initial colonization is associated with climate amelioration after ~14,000 cal BP, and population declines are associated with the Younger Dryas (13,000-12,000 cal BP) and initial establishment of widespread spruce forests (10,000-9000 cal BP).
Structural Characterization of Charcoal Exposed to High and Low pH: Implications for 14C Sample Preparation and Charcoal PreservationChemical and structural similarities between poorly preserved charcoal and its contaminants, as well as low radiocarbon concentrations in old samples, complicate 14C age determinations. Here, we characterize 4 fossil charcoal samples from the late Middle Paleolithic and early Upper Paleolithic strata of Kebara Cave, Israel, with respect to the structural and chemical changes that occur when they are subjected to the acid-base-acid (ABA) treatment. Differential thermal analysis and TEM show that acid treatment disrupts the structure, whereas alkali treatment results in the reformation of molecular aggregates. The major changes are ascribed to the formation of salt bridges at high pH and the disruption of the graphite-like crystallites at low pH. Weight losses during the treatments are consistently greater for older samples, implying that they are less well preserved. Based on the changes observed in vitro due to pH fluctuations, various methods for removing contamination, as well as a mechanism for preferential preservation of charcoal in nature, are proposed.
Towards a Deeper Understanding of How Carbonate Isotopes (14C, 13C, 18O) Reflect Environmental Changes: A Study with Recent 210Pb-Dated Sediments of the Plitvice Lakes, CroatiaFive short cores (top 40-45 cm of sediment) from 4 lakes of the Plitvice Lakes system (Croatia) were measured for 210Pb, 137Cs, a14C, 13C, and 18O in order to study the influence of environmental changes on the sediment system in small and large lakes. Sediment chronology based on the constant flux (CF) 210Pb model was the most reliable. Lake sediments consisted mainly of autochthonous carbonates with higher sedimentation rates in small lakes. Sediments from 2 large lakes, Proće and Kozjak, showed constant stable isotope profiles for the carbonate fraction and full agreement between the 137Cs and 210Pb chronologies. Sediments from 2 small lakes, Gradinsko and Kaluderovac, showed synchronous increases in 14C and 13C and disturbed 137Cs records. All lakes showed an increase in a14C in the carbonate sediments above the first occurrence of 137Cs, which was interpreted as a damped (~10 pMC increase in a14C) and decades-delayed consequence of the bomb-induced increase in a14C in atmospheric CO2. For the small lakes, increased 13C in the last 2 decades and part of the a14C increase is probably due to an increase in primary productivity, which enhanced biologically induced calcite precipitation with concomitant changes in the carbon isotopic composition of carbonate sediments. 13C values of a near-shore sediment core close to the confluence of one of the tributaries of Lake Kozjak showed that the carbonates in this core are a mixture of autochthonous and eroded allochthonous mineral carbonate. This core had a higher fraction of organic material. The sedimentation rate at this core site was high, but rates could not be quantified by 210Pb, 137Cs, or 14C.