Showing papers in "Radiocarbon in 1999"

Reporting C-14 activities and concentrations

Abstract: Three modes of reporting (super 14) C activities are in use, in part analogous to the internationally accepted (IAEA) conventions for stable isotopes: (1) absolute activity, the specific activity of (super 14) C or the activity per gram of carbon; (2) activity ratio, the ratio between the absolute activities of a sample and the standard; and (3) relative activity, the difference between the absolute activities of a sample and standard material, relative to the absolute standard activity. The basic definitions originate from decisions made by the radiocarbon community at its past conferences. Stuiver and Polach (1977) reviewed and sought to specify the definitions and conventions. Several colleagues, however, have experienced inadequacies and pitfalls in the definitions and use of symbols. Furthermore, the latter have to be slightly amended because of the use of modern measuring techniques. This paper is intended to provide a consistent set of reporting symbols and definitions, illustrated by some practical examples.

Radiocarbon dating of old charcoal using a wet oxidation, stepped-combustion procedure

Abstract: We present results that validate a new wet oxidation, stepped-combustion procedure for dating "old" charcoal samples. An acid-base-wet oxidation (ABOX) pretreatment procedure has been developed that is used in place of the conventional acid-base-acid (ABA) pretreatment. Combustions and graphitizations are performed in a vacuum line that is insulated from the atmosphere by a second backing vacuum to eliminate the risk of atmospheric leakage into the line at any stage of the procedure. Combustions are performed at 3 temperatures (330 degrees C, 630 degrees C and 850 degrees C) with a graphite target produced from the CO (sub 2) evolved during each combustion step. In this way, the removal of any contamination can be monitored, and a high degree of confidence can be placed on the final age. The pretreatment, combustion, graphitization, and measurement blank for the procedure, based on the analysis of a "radiocarbon-dead" graphite, is 0.5+ or -0.5 mu g C (1sigma , n = 14), equivalent to 0.04+ or -0.02 pMC or an "age" of approximately 60 ka for a 1 mg graphite target. Analyses of a "radiocarbon-dead" natural charcoal after ABOX pretreatment and stepped combustion suggest that the total blank (including contamination not removed by pretreatment) may be higher than for graphite, ranging up to 0.10+ or -0.02 pMC. Additional experiments confirm good agreement with accepted values for the international low- (super 14) C "New Kauri" standard (0.16-0.25 pMC). They also confirm excellent reproducibility, with 3 separate dates on different aliquots of a charcoal sample from Ngarrabullgan Cave (Queensland, Australia) ranging from 35.2 to 35.5 ka (super 14) C BP. It is also demonstrated that the ABOX pretreatment, in conjunction with the new vacuum line described here, is able to remove contamination not removed by the conventional ABA pretreatment, suggesting that the technique can be used to produce reliable (super 14) C dates on charcoal up to at least 50 ka.

Change of Diet of the Greenland Vikings Determined from Stable Carbon Isotope Analysis and 14C Dating of Their Bones

Abstract: Bone samples from the Greenland Viking colony provide us with a unique opportunity to test and use (super 14) C dating of remains of humans who depended upon food of mixed marine and terrestrial origin. We investigated the skeletons of 27 Greenland Norse people excavated from churchyard burials from the late 10th to the middle 15th century. The stable carbon isotopic composition (delta (super 13) C) of the bone collagen reveals that the diet of the Greenland Norse changed dramatically from predominantly terrestrial food at the time of Eric the Red around AD 1000 to predominantly marine food toward the end of the settlement period around AD 1450. We find that it is possible to (super 14) C-date these bones of mixed marine and terrestrial origin precisely when proper correction for the marine reservoir effect (the (super 14) C age difference between terrestrial and marine organisms) is taken into account. From the dietary information obtained via the delta (super 13) C values of the bones we have calculated individual reservoir age corrections for the measured (super 14) C ages of each skeleton. The reservoir age corrections were calibrated by comparing the (super 14) C dates of 3 highly marine skeletons with the (super 14) C dates of their terrestrial grave clothes. The calibrated ages of all 27 skeletons from different parts of the Norse settlement obtained by this method are found to be consistent with available historical and archaeological chronology. The evidence for a change in subsistence from terrestrial to marine food is an important clue to the old puzzle of the disappearance of the Greenland Norse, obtained here for the first time by measurements on the remains of the people themselves instead of by more indirect methods like kitchen-midden analysis.

Calculation of Past Dead Carbon Proportion and Variability by the Comparison of AMS 14C and Tims U/TH Ages on Two Holocene Stalagmites

Abstract: Twenty-two radiocarbon activity measurements were made by accelerator mass spectrometry (AMS) on 2 Holocene stalagmites from Belgium (Han-stm lb) and from southwest France (Vil-stm lb). Sixteen thermal ionization mass spectrometric (TIMS) U/Th measurements were performed parallel to AMS analyses. The past dead carbon proportion (dcp) due to limestone dissolution and old soil organic matter (SOM) degradation is calculated with U/Th ages, measured calcite (super 14) C activity and atmospheric (super 14) C activity from the dendrochronological calibration curves. Results show that the dcp is different for the 2 stalagmites: between 10,800 and 4780 yr from present dcp = 17.5% (sigma = 2.4; n = 10) for Han-stm lb and dcp = 9.4% (sigma = 1.6; n = 6) between 3070 and 520 yr for Vil-stmlb. Despite a broad stability of the dcp during the time ranges covered by each sample, a slight dcp increase of about 5.0% is observed in the Han-stmlb sample between 8500 and 5200 yr. This change is synchronous with a calcite delta (super 13) C increase, which could be due to variation in limestone dissolution processes possibly linked with a vegetation change. The dcp and delta (super 13) C of the 2 studied samples are compared with 5 other modern stalagmites from Europe. Results show that several factors intervene, among them: the vegetation type, and the soil saturation leading to variable dissolution process systems (open/closed). The good correlation (R (super 2) = 0.98) between the U/Th ages and the calibrated (super 14) C ages corrected with a constant dcp validates the (super 14) C method. However, the dcp error leads to large (super 14) C age errors (i.e. 250-500 yr for the period studied), which is an obstacle for both a high-resolution chronology and the improvement of the (super 14) C calibration curves, at least for the Holocene.

A simple method to separate pollen for AMS radiocarbon dating and its application to lacustrine and marine sediments.

Abstract: We present a simple method for manually separating pollen concentrates for radiocarbon accelerator mass spectrometry (AMS) dating using a mouth pipetting system. The required equipment is readily available from scientific equipment supply houses at minimal cost. Pollen samples from lake sediments required about 4 h of hand picking, whereas samples from marine sediments required about 8 h labor. Pollen dates from marine sediments were much older than expected. We are attempting to resolve whether this is due to contamination of the pollen or the presence of significant quantities of old reworked pollen. Pollen dates from lake sediments associated with Mazama Ash were consistent with other published ages; however, replicate dates on pollen samples from above the ash were consistently older than the surrounding sediment. Our results suggest that caution must be used when interpreting pollen dates if the potential for sediment reworking is present.

Chronology of vegetation and paleoclimatic stages of northwestern Russia during the late glacial and Holocene.

Abstract: We have studied 6 reference sections of bog and lake sediments in the Leningrad and Novgorod provinces to develop a geochronological scale for vegetational and paleoclimatic changes in northwestern Russia during the Late Glacial and Holocene. Every 10-cm layer along the peat and gyttja sections (4-8.5 m thick) was investigated palynologically and the great majority of them were radiocarbon dated. Using the data obtained, standard palynological diagrams were plotted and vegetation history reconstructed. The palynozones indicated on the diagrams were related to the climatic periods and subperiods (phases) of the Blytt-Sernander scheme. On the basis of 230 (super 14) C dates obtained, we derived the geochronology of climatic periods and phases, as well as the chronology for the appearance and areal distribution of forest-forming tree species. The uppermost peat layers were dated by using the "bomb effect". We compared the stages of Holocene vegetation and paleoclimatic changes discovered for the Leningrad and Novgorod provinces with the those obtained for Karelia, which we had studied earlier using the same methodology.

Radiocarbon Age Anomalies in Land Snail Shells from Texas: Ontogenetic, Individual, and Geographic Patterns of Variation

Abstract: Accelerator mass spectrometric (AMS) radiocarbon analyses of live-collected, prebomb samples of shell carbonates of the land snails Rabdotus dealbatus and R. alternatus from Texas were carried out to quantify the characteristic age anomalies of land snails from limestone areas. Age anomalies are similar for the two species; they average +700 yr and vary by + or -180 yr (1sigma ) among samples. Serial analysis of 1 shell reveals a significant ontogenetic trend in (super 14) C age anomalies, with older apparent ages (up to 1200 yr) in the apical part of the shell and younger and uniform ages in the last whorl. No trend in age anomalies was found across a broad range of rainfall conditions (from 300 to 1000 mm mean annual rainfall).

The reliability of AMS radiocarbon dating of shells from China

Abstract: We tested the feasibility of dating freshwater and terrestrial molluscs from the semiarid and arid zone in China, since these types of shell material deposit only aragonite to form their shell structure, and shell integrity can be easily observed using X-ray diffraction. We also tested the possibility of estimating microenvironmental changes from shell delta (super 13) C values, but variations within shell populations preclude the use of these values as a reliable indicator. Reservoir ages were calculated for living shells of the same species as fossil shells by using their measured (super 14) C ages, which were recalculated using an average value of atmospheric (super 14) C activity for the years spanning their time of collection as the modern standard. The results indicate that freshwater and terrestrial shells are potentially useful as dating material, provided extreme care is taken in their collection and other datable material (in this case wood and pollen) is within the profile to act as a comparison.

A Delta R correction value for Samoa from known-age marine shells.

Abstract: A first-order Delta R correction value for marine samples is presented based on 3 radiocarbon determinations of known-age marine shells from Samoa.

Radiocarbon Analysis of Pinus Lagunae Tree Rings: Implications for Tropical Dendrochronology

Abstract: A promising species for tropical dendrochronology is Pinus lagunae, a pine tree found in Baja California Sur (Mexico) around lat 23.5 degrees N. In 1995, we sampled a total of 27 wood cores from 13 Pinus lagunae trees in Sierra La Victoria (23 degrees 36'N, 109 degrees 56'W), just north of Sierra La Laguna, at an elevation of 1500-1600 m. Selected trees were locally dominant, but their ring-width patterns could not be crossdated. To test the hypothesis that visible growth layers in Pinus lagunae are formed annually, we measured radiocarbon amounts in individual rings by means of accelerator mass spectrometry (AMS). Twenty-three (super 14) C measurements were used to trace the location of the 1963-64 "bomb spike" in 3 wood increment cores. By comparing the location of that Delta (super 14) C extreme with the number of visible radial wood increments, it was possible to conclude that 2 cores had a number of locally absent rings, while the 3rd one included a few years with more than one growth layer. Therefore, ring-width patterns of sampled Pinus lagunae were not consistent from one tree to another, most likely because of climatic regime in combination with microsite features. While the possibility of generating Pinus lagunae tree-ring chronologies cannot entirely be ruled out, the development of dendrochronological proxy records of climate from coniferous species in tropical North America should focus on species and sites that experience a more pronounced seasonality.

AMS 14C Dating of Equipment from the Iceman and of Spruce Logs from the Prehistoric Salt Mines of Hallstatt

Abstract: This paper summarizes radiocarbon measurements of mainly botanical samples from the Iceman ("Otzi") and from his discovery site, an Alpine glacier at the Austrian-Italian border The results were obtained by accelerator mass spectrometry (AMS) at 3 different laboratories (Vienna, Austria; Uppsala, Sweden; Gif-sur-Yvette, France) between 1992 and 1997 All the dates, except 2, are consistent with the time period 3360-3100 BC, as previously determined from bone and tissue samples from the Iceman himself The 2 exceptional dates from wooden artifacts suggest that the site of the Iceman was used as a mountain pass for millennia prior to and after the lifetime of "Otzi" For a 2nd sample complex, we studied logs from the beginning of salt mining in the world's oldest salt mines at Hallstatt in Upper Austria (super 14) C AMS measurements were performed in Vienna on spruce samples found in the prehistoric mines and from a log-house on the surface Data evaluation included "wiggle matching" of different sets of tree rings The results suggest that salt mining in the Hallstatt region took place in the 14th-13th century BC, well before the so-called Hallstatt period We discuss in some detail the chemical pretreatment of the samples and the data evaluation We also present a comprehensive survey of (super 14) C dates available in the literature concerning both botanical remains from the vicinity of the Iceman and from the earliest salt mining in Hallstatt

Reliability of bone gelatin AMS dating : Rattus exulans and marine shell radiocarbon dates from Pauatahanui midden sites in Wellington, New Zealand

Abstract: A suite of 6 bone gelatin accelerator mass spectrometry (AMS) radiocarbon dates for Rattus exulans Peale and associated beta decay (super 14) C dates for Austrovenus stutchburyi shell are presented for 4 middens at Pauatahanui, Wellington, New Zealand. Mean calibrated age ranges of Rattus exulans (520-435 BP and 350-330 BP at 95% confidence level) and shell (465-375 BP at 95% confidence level) from the 4 midden sites overlap. The agreement between Rattus exulans bone gelatin dates and associated shell provides an inter-sample comparison of (super 14) C dating using both gas counting (beta decay) and AMS dating techniques. We examine the adequacy of the standard gelatinization treatment for bone samples, which has been employed consistently at the laboratory since 1995.

Variations of isotopic composition of carbon in the karst environment from southern Poland, present and past.

Abstract: We describe a comprehensive study of carbon isotopes in several karst springs and their environs in a contemporary karst environment in the region of the Cracow-Wielun Upland and Western Tatra Mountains, Southern Poland. We collected samples of water, plants and carbonate deposited on aquatic plants, and obtained (super 13) C values and (super 14) C concentrations. We also investigated a group of the youngest calcium carbonates from caves where deposition is still being observed or ceased no more than a few hundred years ago. The determination of a (super 14) C dilution factor (q) in these carbonates allows us to determine the "true" radiocarbon ages of old speleothems from caves in the area under investigation and enables the use of old speleothems as suitable material for extending the (super 14) C calibration time scale, the "Absolute" age having been determined by U/Th or amino acid racemization (AAR) dating methods. Measurements of delta (super 13) C and (super 14) C concentrations were made on dissolved inorganic carbon (DIC) extracted from water samples. Calculated values of q range from 0.55 to 0.68 and delta (super 13) C values range from -10 per mil to -13 per mil versus VPDB with mean values equal to 0.65 and -12 per mil, respectively. Results indicate that the dissolution process of limestone bedrock is a closed system with the dominating contributor being biogenic carbon dioxide. Isotopic composition of carbon in contemporary plants collected at the karstic springs at 3 localities is highly diverse, with different species distinctly varying in both q and delta (super 13) C values. Extremely light values of (super 13) C (under -40 per mil), observed in Algae and Hyloconium splendens, are correlated with (super 14) C concentrations that are much lower than 100 pMC. Small systematic changes of isotopic composition were found in plants of the same species collected along streams at various distances from the spring. The youngest calcium carbonates from different caves show a relatively high scatter of both delta (super 13) C values and (super 14) C concentration. The lower reservoir effect for (super 14) C is observed in samples with higher value of delta (super 13) C, indicating equilibrium conditions in the sedimentation of carbonate. Pazdur et al. (1995b) presented (super 14) C dating results and paleoclimatic interpretation of 170 (super 14) C analyses of 89 speleothems from 41 caves obtained through 1994. Investigations continued until early 1997, during which time a speleothem, JWi2, was dated by (super 14) C, U/Th and AAR dating methods, and its stable isotope composition (delta (super 13) C and delta (super 18) O) analyzed in detail (reported here). Carbon isotope analyses indicate very large differences among results obtained by U/Th, AAR, and (super 14) C dating methods.

Oceanic Radiocarbon Between Antarctica and South Africa Along Woce Section 16 at 30°E

Abstract: Accelerator mass spectrometry (AMS) radiocarbon measurements were made on 120 samples collected between Antarctica and South Africa along 30°E during the WOCE-France CIVA 1 campaign in February 1993. Our principal objective was to complement the Southern Ocean's sparse existing data set in order to improve the 14C benchmark used for validating ocean carbon-cycle models, which disagree considerably in this region. Measured 14C is consistent with the 0-S characteristics of CIVA1. Antarctic Intermediate Water (AAIW) forming north of the Polar Front (PF) is rich in'4C, whereas surface waters south of the PF are depleted in 14C. A distinct old 14C signal was found for the contribution of the Pacific Deep Water (PDW) to the return flow of Circumpolar Deep Waters (CDW). Comparison to previous measurements shows a 4C decrease in surface waters, consistent with northward displacement of surface waters, replacement by old deep waters upwelled at the Antarctic Divergence, and atmospheric decline in 14C. Conversely, an increase was found in deeper layers, in the AAIW. Large uncertainties, associated with previous methods for separating natural and bomb 14C when in the Southern Ocean south of 45°S, motivated us to develop a new approach that relies on a simple mixing model and on chlorofluorocarbon (CFC) measurements also taken during CIVA1. This approach leads to inventories for CIVA1 that are equal to or higher than those calculated with previous methods. Differences between old and new methods are especially high south of approximately 55°S, where bomb 14C inventories are relatively modest.

Comparing carbonate and organic AMS-14C ages in Lake Abiyata sediments (Ethiopia) : Hydrochemistry and paleoenvironmental implications

Abstract: We studied a 12.6-m-long sequence from Lake Abiyata (Central Ethiopia) to establish a reliable and accurate chronology for use in global paleoclimatic reconstructions. The 26 accelerator mass spectrometry radiocarbon (AMS 14C) ages, performed on carbonates and organic matter, define 2 parallel chronologies, representing the complete Holocene period. However, these chronologies show a significant discrepancy from 500 to 900 BP in depth; ages obtained on carbonates were always older than those on organic matter. The hydrogeological and geochemical behavior of the Lake Abiyata basin has shed light on this discrepancy. We found that the carbonate crystallization is due mainly to the mixing of lake waters with groundwaters from the multi-layered aquifer contained in the 600-m-thick basement of the lake. The 14C activity of total dissolved inorganic carbon (TDIC) measured by AMS from bottom and surface lake waters (111.4 and 111.8 pMC, respectively) confirms that the mixing occurs at the water-sediment interface. This evidence of groundwater participation in the carbonate crystallization calls into question the current paleoclimatic reconstructions based on inorganic carbonates in lakes. Specific attention should thus be given to the respective proportions of each end-member in the mixing for the quantitative estimation of the groundwater input. This will help to validate the paleoenvironmental reconstructions and to highlight an eventual diagenetical evolution of inorganic carbonates during burial, via the study of pore waters.

Radiocarbon Distribution in Northwest Belarus Near the Ignalina Nuclear Power Plant

Abstract: Since 1994, the Institute of Geological Sciences has undertaken an environmental monitoring program to measure radiocarbon levels in territory adjacent to active nuclear power plants (NPP). We determined 14 C concentrations in natural objects from areas contiguous to Ignalina NPP as well as 14 C background concentration in areas remote from the NPP. In the environs of the Ignalina station comparatively elevated levels of 14 C were observed in vegetation and waters of Lake Drisvyaty. This appears to be a consequence of release of carbon radioisotope into the atmosphere and probably into waters of the lake during operation of the nuclear reactor.

Dating organic temper of ceramics by AMS; sample preparation and carbon evaluation.

Abstract: We describe a new methodology for separating organic temper from archaeological ceramics from Brazilian Amazonia. These experimental procedures were designed to directly date ceramic samples by accelerator mass spectrometry (AMS). An evaluation of the total carbon indicates the samples' potential for dating.

14C Age of Glaciation in Estrecho de Magallanes–Bahía Inútil, Chile

Abstract: Basal (super 14) C dates from a core of the mire at Puerto del Hambre (53 degrees 36'21"s, 70 degrees 55'53"W), located within the area of glaciation in Estrecho de Magallanes-Bahia Inutil, Chile, are no older than 14,455+ or -115 yr BP. The (super 14) C dates are on samples from which screening isolated autochthonous plant remains. Previous (super 14) C dates of 15,800+ or -200, 16,590+ or -320, and 16,290+ or -140 yr BP are from bulk samples collected from similar basal increments at the site during 3 separate, independent coring operations. The previous suite of (super 14) C dates was suspected to be contaminated by older carbon in the light of chronological evidence, which indicates a (super 14) C age of approximately 14,850 yr BP for glaciation elsewhere in southern Chile. Contamination by "infinitely old" carbon reworked from nearby Tertiary beds and redeposited at Puerto del Hambre is evidently the cause for the older (super 14) C dates.

The Maunder Minimum: An Interlaboratory Comparison of Δ14C from AD 1688 to AD 1710

Abstract: Measurements on same-age tree-ring samples from proximal Ural Mountain trees by the Ioffe Institute research group and at the University of Arizona demonstrate a variance corresponding to a standard deviation of + or -5.1 per mil for Ioffe compared to + or -2.1 per mil for Tucson. There is also a calibration difference of 4.3+ or -1.2 sigma per mil. Comparison of the same years measured in Seattle on wood from the Pacific Northwest shows an offset of 2.2+ or -0.5 sigma per mil. This is not a calibration error, but rather is expected from the well-documented evidence for divergence and upwelling of (super 14) C-depleted CO (sub 2) along the west coast of North America.

The spatial and temporal patterns of stick-nest rat middens in Australia

Abstract: The spatial and temporal distribution of 145 radiocarbon dates on 66 Australian stick-nest rat middens (Muridae: Leporillus spp.) range from modern to 10,900 ± 90 BP. As in American packrat middens, age frequency follows a logarithmic decay, both continentally and at major sites. This is probably a result of natural decay processes. Unlike American middens of similar age, relatively few range changes in plant distribution have been detected in Australia. The distribution of 14C ages and the associated midden materials provide important paleoenvironmental information from the arid interior of Australia. The middens record subtle changes in vegetation and dramatic changes in the fauna unlike those interpreted from sites on the coastal rim or the southeastern periphery of the arid zone.

Use of radiocarbon dating in assessing Christian connections to the Dead Sea Scrolls.

Abstract: We present an analysis of radiocarbon dates on Dead Sea Scrolls that have a bearing on the question of the Scroll documents' relation to Christian origins. We assess details of dating reports, discuss paleographical evidence, and consider the content of the documents. When collated, these findings may be seen as compatible with a view that personalities mentioned in the Scrolls were contemporary with the founders of Christianity.

Comments on "America's oldest basketry".

Abstract: A recent publication on directly dated basketry specimens from the western United States (Berger et al. 1998, Radiocarbon 40(2):615-20) contained some misleading information, and in a few cases discussed radiocarbon ages from unacknowledged sources. We provide the missing original citations along with some clarifications. We focus especially on the age of distinctive Fort Rock and Multiple Warp-style sandals, for which we provide additional previously unreported (super 14) C ages. Direct dates on fibers from Fort Rock sandals from 3 different sites range in age from 10,500 cal BP to about 9200 cal BP. Contextual evidence suggests that Multiple Warp sandals may date as early as 6600 cal BP, but the few directly dated specimens are less than 1000 yr old.

Rudjer Bošković Institute radiocarbon measurements XIV

Abstract: In this report we present dating of several large series of samples mostly collected in Croatia: (i) Iron Age and Medieval samples excavated in Zagreb, (ii) speleothems from a karst site in C Croatia, (iii) shallow lake sediments from the Plitvice Lakes, (iv) tufa from the Krka River, (v) sea sediments from Adriatic sea, and (vi) tree rings from Hungary, Croatia and Slovenia. Sample preparation and proportional counter technique are essentially the same as reported earlier (Srdoc et al. 1971). Processing of data has been computerized (Obelic 1989).

Radiocarbon dates from northern Mongolia.

Abstract: Since 1996, the Mongolian-American Expedition to Northern Mongolia has been excavating in the Egiin Gol Valley. The goal of this research has been to examine the competing hypotheses explaining the emergence of pastoral nomadism and the evolution of nomadic complexity. The chronological placement of burials and sites in the survey area has been a key facet of this research. At present, these investigations have generated 10 radiocarbon dates from archaeological contexts. Presented here are the previously unpublished (super 14) C dates and some comments on their significance.

A 23-year retrospective blind check of accuracy of the Copenhagen radiocarbon dating system.

Abstract: A 23-yr record of the measuring accuracy of the Copenhagen radiocarbon dating laboratory has retrospectively been provided through a true blind test. A total of 92 samples of oak from old tree trunks were dated in the period 1971 to 1993 and their dendrochronological age determined independently. The (super 14) C activity of the dendrochronological samples measured in the Copenhagen radiocarbon laboratory was compared to the activity of the tree rings of the same age measured by Stuiver and Pearson (1993) for calibration purposes. The average difference was found to be 54+ or -72 (super 14) C yr. The results further indicate that the actual standard deviation is only 7% higher than that quoted by the laboratory. The investigation has shown a long-term stability of laboratory accuracy with no systematic laboratory variations either with respect to sample age or to the time of measurement from 1971 to 1993.

Changes in 14C activity over time during vacuum distillation of carbon from rock pore water

Abstract: The radiocarbon activity of carbon collected by vacuum distillation from a single partially saturated tuff began to decline after approximately 60% of the water and carbon had been extracted. Disproportionate changes in (super 14) C activity and delta (super 13) C during distillation rule out simple isotopic fractionation as a causative explanation. Additional phenomena such as matrix diffusion and ion exclusion in micropores may play a role in altering the isotopic value of extracted carbon, but neither can fully account for the observed changes. The most plausible explanation is that distillation recovers carbon from an adsorbed phase that is depleted in (super 14) C relative to DIC in the bulk pore water.

Radiocarbon calibration by the date distribution method.

Abstract: A method is presented for calibrating radiocarbon ages based on statistical analysis of a large number of randomly distributed dates One interesting feature of this method is that it is internal; that is, it allows one to extend a known calibration curve further back in time by using only (super 14) C dates, with no reference to any other dating technique A serious difficulty in implementing this method lies in assembling a sample of dates with the correct statistical properties