In the last decade, the myth of the pristine tropical forest has been seriously challenged. In central Africa, there is a growing body of evidence for past human settlements along the Atlantic forests, but very little information is available about human activities further inland. Therefore, this study aimed at determining the temporal and spatial patterns of human activities in an archaeologically unexplored area of 110,000 km2 located in the northern Congo Basin and currently covered by dense forest. Fieldwork involving archaeology as well as archaeobotany was undertaken in 36 sites located in southeastern Cameroon and in the northern Republic of Congo. Evidence of past human activities through either artifacts or charred botanical remains was observed in all excavated test pits across the study area. The set of 43 radiocarbon dates extending from 15,000 BP to the present time showed a bimodal distribution in the Late Holocene, which was interpreted as two phases of human expansion with an intermediate phase of depopulation. The 2300–1300 BP phase is correlated with the migrations of supposed farming populations from northwestern Cameroon. Between 1300 and 670 BP, less material could be dated. This is in agreement with the population collapse already reported for central Africa. Following this, the 670–20 BP phase corresponds to a new period of human expansion known as the Late Iron Age. These results bring new and extensive evidence of human activities in the northern Congo Basin and support the established chronology for human history in central Africa.

Since May 1994, a new-generation accelerator mass spectrometer (AMS) has been fully operational at the Centre for Isotope Research in Groningen, The Netherlands. The fully automated and high-throughput accelerator mass spectrometry (AMS) system, manufactured by High Voltage Engineering Europa (HVEE) is dedicated to radiocarbon analysis. The HVEE 4130 14C AMS is able to analyze up to 3000 samples per year. The system is characterized by simultaneous transport of all three isotopes (12C, 13C, 14C) and 14C analysis with a precision below 0.5 pMC and a daily stability below 0.5 pMC. We present here a system description together with stability and performance measurements.

We present a high-resolution multiproxy record (geochemistry, macrofossil, and pollen) from a peatland in the Dajiuhu Basin in eastern central China. The chronology of the 120-cm peat profile was controlled by 6 accelerator mass spectrometry (AMS) 14C dates on plant remains, including 2 post-bomb dates. The age model was based on linear interpolations of calibrated ages. Plant macrofossil results indicate a major transition around 3600 cal BP from Sphagnum section Subsecunda and Drepanocladus sp. to Sphagnum imbricatum dominance, followed by the disappearance of S. imbricatum at 700 cal BP. These changes suggest a general sequence of local environment changes from a wet fen, through a Sphagnum-dominated peatland, to a dry sedge-dominated marsh, which are also reflected by change in peat lithology and composition. The drying trend after 3600 cal BP is in general agreement with the speleothem isotope record from this region and other paleoclimate records from east China, indicating a weakening summer monsoon resulting from a decrease in summer insolation. The shift to a dry environment at 700 cal BP might have been caused by human activities. Appearance of Cerealia pollen at 3600–3200 cal BP suggests the first introduction of crop farming in the region, while its absence at 3200–2000 cal BP could be attributed to abandonment of farmland. The increase of Ti and Si since 1300 cal BP may be related to agricultural activity and landscape erosion. A 2-step increase in Pb concentration at 1600 and 600 cal BP suggests 2 phases of industrial pollution intensity.

Both the 2-L counter, described in GSC I (Radiocarbon, 1962, v. 4, p. 13–26), and the 5-L counter (GSC IV, Radiocarbon, 1965, v. 7, p. 24–46) were operated routinely during the past year. Approximately half the determinations reported were obtained from each counter. The 2-L counter was operated at 2 atm except for August and September, 1968, when it was operated for the first time at 1 atm. This allowed for the counting of most small samples without the necessity of mixing with dead gas. The 5-L counter was operated at 1 atm, except for the same period when it was operated at 4 atm. Carbon dioxide is used as the counting gas, and both counters are of the proportional type.

Rates of CO2 exchange across the air-water interface in oceans and lakes measured to date by the L-DGO group are summarized. They range from 3 to 38 moles/m2/yr. The possible causes for this range include the differences in salinity, mean wind speed, and pH. Wind tunnel studies comparing fresh water and sea water are required before a satisfactory explanation can be found.

Three modern speleothems were sampled at high resolution for radiocarbon analysis to identify their bomb-pulse signatures and to construct chronologies. Each speleothem exhibited a different 14C response, presumed to be related to site characteristics such as vegetation, temperature, rainfall, depth below the surface, and water pathway through the aquifer. Peak 14C activity for WM4 is 134.1 pMC, the highest cited thus far in the literature and suggestive of a lower inertia at this site. Dead carbon fractions for each stalagmite were calculated and found to be relatively similar for the 3 speleothems and lower than those derived from Northern Hemisphere speleothems. An inverse modeling technique based on the work of Genty and Massault (1999) was used to estimate soil carbon residence times. For each speleothem, mean soil 14C reservoir ages differed greatly between the 3 sites, ranging from 2–6.5 to 32–46 yr.

The largest volcanic crater on Easter Island in the South Pacific contains a lake 1 km in diameter with large floating mats of vegetation, mainly Scirpus californicus. A core taken through a mat near the center produced anomalous dates, with older dates above younger ones. The possibility that the mat had become inverted was considered, but palynological evidence refutes this idea because it shows a progressive upward decline of forest pollen, which is well known from other swamp cores on the island. A new series of radiocarbon dates made directly on pollen concentrates was obtained. These dates also produced inconsistencies, particularly when pollen concentrate ages were compared with 14C ages on plant fragments from the same depth. This series of 14C ages seems to indicate that both old and young organic components in the sediment are deposited contiguously and that the depositional history of these cores is more complex than previously known. Previous age determinations on bulk sediments from Easter Island, which also show anomalous dates, may be too simplistic. This paper provides a warning to other researchers dating sediments from Easter Island. We suggest that sample selection and dating procedures be carefully considered for these sediments.

We present here a model of humus accumulation in recent soils. We have estimated the coefficients of mineralization of humus and humic acid for a typical Chernozem soil. We suggest a technique for calculating the renewal time of soil with specific activity higher than the modern standard and discuss the results for different soils.

Céide Fields is a 12 km2 Neolithic field system in North Mayo in the west of Ireland. The fields, enclosed by an integrated system of stone walls, have been preserved intact by a cover of blanket bog that is >4 m deep in places. At many locations within this blanket bog the stumps of ancient pines (Pinus sylvestris) are found in situ. The pine roots in most cases are either on the surface of the mineral soil under the peat or at an intermediate level in the peat itself. The age of the trees in the bog overlying Céide Fields is therefore of great significance for the dating of the fields, as the trees must be younger than the bog in which they are growing, which in turn must be younger than the field system beneath it. We present here the dates (N = 15) for pine trees from the bogs overlying Céide Fields and the dates (N = 29) of pine trees from other areas of the North Mayo blanket bog. We compare these pine dates with published dates of peat associated with a major pollen analytical study from within the fields and with published dates for bog pine from Scotland. The results of the study suggest that the dates for the construction and period of use of Céide Fields and other Neolithic pre-bog field systems in North Mayo are older than anticipated and that the date for initiation of blanket bog in many parts of North Mayo is also older than previously estimated. Further, the range of dates of the pine stumps indicates a synchronic event contemporary with a similar phenomenon observed in Scotland.

The Late Bronze Age to the Iron Age transition involves profound cultural and political changes in the southern Levant. The transition is dated to the 12th century BC, based on archaeological artifacts and historical documents. A more precise absolute date for this transition for the southern Levant based on radiocarbon is difficult since the 14C calibration curve reduces precision significantly due to wiggles that form an approximately 200-yr-long plateau. This article analyzes 14C samples from the Late Bronze Age to the Iron Age transition at Qubur el-Walaydah. To increase the resolution of 14C dates within the plateau, 14C samples were collected only from well-defined multilayered contexts. 14C dates from 11 contexts were obtained and these were analyzed using a Bayesian model that incorporated the stratigraphic information. Using this integrative approach we date the Late Bronze Age III levels at Qubur el-Walydah, containing the initial phase of locally produced Philistine pottery between 1185–1140 BC, and the Late Bronze to Iron Age transition between 1140–1095 BC.

The southern African Later Stone Age sequence is widely considered to be well dated based on radiocarbon dates from dozens of archaeological sites, and apparently shows more or less synchronous cultural shifts across an extensive area. Yet, closer examination reveals the inadequacy of many of the decades-old and uncalibrated individual site chronologies that underpin this regional chronology, making robust comparisons of the chronology of technological change across this region impossible. Here, we present 26 new AMS 14C dates and Bayesian modeled chronologies for two important archaeological cave sites in southernmost Africa, Nelson Bay Cave and Byneskranskop 1. The results provide more robust age estimates for these cultural and paleoenvironmental sequences and revise interpretations of these sites in several instances. This project demonstrates the necessity of redating key sites, and the value of currently underutilized methods, including calibration and Bayesian modeling, for southern African archaeology.

Variable 14C ages of paleosol organic matter (OM) cause difficulties in interpreting 14C data. We attempt to determine the reliability of OM 14C dates by examining different carbon-containing materials from soil horizons and paleosol fractions.

Worldwide, dating rock art is difficult to achieve because of the frequent lack of datable material and the difficulty of removing contamination from samples. Our research aimed to select the paints that would be the most likely to be successfully radiocarbon dated and to estimate the quantity of paint needed depending on the nature of the paint and the weathering and alteration products associated with it. To achieve this aim, a two-step sampling strategy, coupled with a multi-instrument characterization (including SEM-EDS, Raman spectroscopy, and FTIR spectroscopy analysis) and a modified acid-base-acid (ABA) pretreatment, was created. In total, 41 samples were dated from 14 sites in three separate regions of southern Africa. These novel protocols ensure that the 14C chronology produced was robust and could also be subsequently applied to different regions with possible variations in paint preparation, geology, weathering conditions, and contaminants.

Radiocarbon ages determined by the AMS method on hand-picked foraminifera shells are reported. The results allow estimates of the ventilation rate of the deep Atlantic and Pacific Oceans during glacial time. They also extend our knowledge of the chronology of events associated with the transition from full glacial conditions ca 15,000 years ago to full interglacial conditions ca 8000 years ago. This and the previous lists (Broecker et al 1988c) contain all the results obtained as part of this program through the fall of 1989.

The cave of Theopetra is located on the northeast side of a limestone rock formation, 3 km south of Kalambaka (21°40′46′′E, 39°40′51′′N), in Thessaly, central Greece. It is a unique prehistoric site for Greece, as the Middle and Upper Paleolithic, Mesolithic, and Neolithic periods are present here, bridging the Pleistocene with the Holocene. Several alternations of the climate during the Pleistocene are recognized in its stratigraphy. Among the most striking finds, two human skeletons, one from the Upper Paleolithic period after the Last Glacial Maximum and one from the Mesolithic period, should be emphasized, while in a deep Middle Paleolithic layer, the oldest human footprints, with remains of fire, were uncovered.

During the 13 years of excavation, evidence of human activity suitable for radiocarbon dating was collected, such as charcoal samples from hearths and bones from the two human skeletons. The use of proportional counters for the measurement of 14C in combination with the recent improvement of the calibration curve has enabled the production of high-precision reliable ages. Sixty 14C-dated samples, originating from 19 pits and from depths ranging from 0.10 m to 4.20 m, have already provided an absolute time framework for the use of the cave. The earliest limit of human presence probably exceeds 48,000 BP and the latest reaches World War II. Within these limits the 14C dating of samples from consecutive layers, in combination with the archaeological data, permits the resolution of successive anthropogenic and environmental events.

Some of the most valuable paleoclimate archives yet recovered are the multi-proxy records from the Greenland GISP2 and GRIP ice cores. The crucial importance of these data arises in part from the strong correlations that exist between the Greenland δ18O records and isotopic or other proxies in numerous other Northern Hemisphere paleoclimate sequences. These correlations could, in principle, allow layer-counted ice-core chronologies to be transferred to radiocarbon-dated paleoclimate archives, thus providing a 14C calibration for the Last Glacial Maximum and Isotope Stage 3, back to the instrumental limits of the 14C technique. However, this possibility is confounded by the existence of numerous different chronologies, as opposed to a single (or even a “best”) ice-core time scale. This paper reviews how the various chronologies were developed, summarizes the differences between them, and examines ways in which further research may allow a 14C calibration to be established.

Bomb 14C from nuclear tests in the atmosphere has proved to be a particularly useful tool in the study of the carbon cycle. We provide here a ca. 30-yr time series of 14C concentrations in the atmosphere between 28°N and 71°N and in the ocean surface between 45°S and 45°N. More recently (since 1990), a north-south profile also has been obtained for 14C in the surface waters of the Atlantic Ocean. The measurements were performed using the conventional technique of beta counting or large samples (4 to 5 liter CO2) in CO2 proportional counters. These data show that the 14C concentration in the atmosphere is leveling off with a time constant of 0.055 yr-1, and is now approaching that of the ocean surface at lower latitudes.

Additional tracer studies have been concerned especially with the penetration of bomb 14C into the deep ocean. The Norwegian and Greenland seas are of interest as a sink for atmospheric CO2 and also a source of water for the deep Atlantic Ocean. During the last five years, several 14C depth profiles have been measured from the Fram Strait (79°N) to south of Iceland (62°N), using the AMS technique available at the University of Arizona AMS Facility. We considered it important to repeat and compare a few of the profiles with those produced by the GEOSECS expedition in 1972 and the TTO expedition in 1981. The profiles show that water descending to the deep Atlantic Ocean is originating mainly from intermediate and surface depths in the Nordic Seas. However, the ventilation rate of the Norwegian Sea deepwater is too slow to be an important component in the transfer of water over the Greenland-Scotland Ridge.

During 1963 and 1964 the La Jolla Radiocarbon Laboratory continued to follow essentially the same techniques as in previous years. The three counters described in La Jolla III have continued to yield measurements in virtually complete agreement. As before, we add, except for the measurements of apparent age on organisms collected alive (see below), ca. 100 yr to the one-sigma statistical counting error. Dates are still computed on the basis of the half-life estimate of 5570 yr.

This study investigates the accuracy of the radiocarbon-based calculation of the biogenic carbon fraction for different biogas and biofossil gas mixtures. The focus is on the uncertainty in the 14C reference values for 100% biogenic carbon and on the 13C-based isotope fractionation correction of the measured 14C values. The separately (AMS) measured CO2 and CH4 fractions of 8 different biogas samples showed 14C values between 102‰ and 116% (pMC). The δ13C values of these samples varied between –6‰ and +31‰ for the CO2 fraction and between –28‰ and –62‰ for the CH4 fraction. The uncertainty in calculated biogenic carbon fractions due to uncertainty in the 14C reference values depends on the available information about the origin of the used biogenic materials. It varies between ±0.5% and ±3.5% (absolute) depending on the type of biogas. A method is proposed to minimize this uncertainty for different groups of biogases. The calculated biogenic carbon fraction deviates up to ±2.5% for biofossil gas mixtures, if the applied isotope fractionation correction is based on the δ13C value of the mixed biofossil sample instead of the biogenic δ13C value. Combination of both error sources shows that the uncertainty in the calculated biogenic carbon fraction varies between ±0.7% and ±4.5%, depending on the type of biogas in the sample.

A new 14C detector system containing nine, independently working, CO2 proportional counters is described. The system is designed for a sufficient measuring capacity at a precision level better than σ = ± 2‰, which requires a counting time of about one week per sample. The size of the installation requires a simple and economic design of counters and electronics. A single anticoincidence shield for all counters consists of five newly developed flat counters. The modern counting rate (52cpm) is sensitively checked by running Heidelberg sodium carbonate standard samples wth a counting rate of about 10 times modern. A microcomputer (DEC PDP-11 /03) is used for data acquisition. Recent developments in laboratory techniques (preparation and gaschromatographic purification of samples) are also reported.