Radiocarbon Content Research Articles

On the Eve of Urbanization: Bayesian Model Dating for Medieval Turku

The era of early urbanization in Finland coincides with large variations in the atmospheric radiocarbon concentration around the 13th–14th centuries AD. Therefore, the uncertainties of an individual 14C date are too large to support studies of this time period effectively. In this work, we have employ tree-ring wiggle-matching methodology and used Bayesian modeling of dates to account for stratigraphical a priori information within Aboa Vetus Museum, Turku, Finland. The wiggle-match method helps significantly to overcome the calibration-curve-induced challenges within 14C dating. The results of the dating model agree with the typological dates of the archaeological finds and illustrate a gradual evolution of urbanization in one of the oldest cities in Finland.

Optical Detection of Radiocarbon Dioxide: First Results and AMS Intercomparison

The first results of an optical method for measuring radiocarbon concentrations, based on mid-infrared laser spectroscopy of a carbon dioxide gas sample, are presented with the theoretical bases explained in detail. The first measurements on modern and highly enriched samples show the extreme linearity of this technique over more than 5 decades. An intercomparison with accelerator mass spectrometry (AMS) is performed both for modern and 14C-dead samples, assessing the almost perfect agreement of their respectively measured concentration values. The main features of our technique are compared with liquid scintillation counting (LSC) and AMS, and future developments of the current setup are discussed.

Radiocarbon Ages of Annual Rings from Japanese Wood: Evident Age Offset Based on IntCal09

To investigate the radiocarbon concentration of atmospheric CO2 over the past few millennia in Japan, we measured the 14C age of annual rings from 3 Japanese trees with calendar dates ranging from ∼2000 yr old to present, and we compared the tree-ring 14C age with the corresponding 14C age from IntCal09. In some instances, the 14C ages of the annual rings of Japanese trees are not consistent with the IntCal09 data sets. Often, the 14C ages of tree rings are older than those from IntCal09, but younger than those from the SHCal04 data sets. The average shifts in the Nagoya 14C age from IntCal09 data sets and 1σ errors were determined to be +26 ± 36, +24 ± 30, +16 ± 22, +5 ± 21, and +14 ± 22 14C yr for the intervals AD 72– 382, 589–1072, 1413–1615, 1617–1739, and 1790–1860, respectively. The Japanese Archipelago is situated near the boundary of the Intertropical Convergence Zone in summer, and the 14C concentration of atmospheric CO2 over Japan can be influenced by air masses of the Southern Hemisphere with lower 14C concentrations during periods of higher solar activity and heightened East Asian summer monsoons. Our results suggest that the Japanese Archipelago is located in a critical zone where it is difficult to calibrate the 14C age of tree-ring samples using existing calibration data sets. It should be noted that calibration of the 14C dates of Japanese samples with IntCal09 may induce additional systematic shifts of calibrated ages toward older ages by about 30 yr compared with the sample optimum calendar ages.

Optical Detection of Radiocarbon Dioxide: First Results and AMS Intercomparison

The first results of an optical method for measuring radiocarbon concentrations, based on mid-infrared laser spectroscopy of a carbon dioxide gas sample, are presented with the theoretical bases explained in detail. The first measurements on modern and highly enriched samples show the extreme linearity of this technique over more than 5 decades. An intercomparison with accelerator mass spectrometry (AMS) is performed both for modern and 14C-dead samples, assessing the almost perfect agreement of their respectively measured concentration values. The main features of our technique are compared with liquid scintillation counting (LSC) and AMS, and future developments of the current setup are discussed.

Potential Influence of Diet on Bomb-Pulse Dating of Human Plaque Samples

The radiocarbon concentration of different atherosclerotic plaque fragments obtained from 20 patients in Portugal, operated in 2000–2001, has been measured in order to define the year of plaque formation. A difference of 1.8–15 yr was observed, with the mean and median both 9 yr, between the bomb-pulse date estimated with the CALIBomb software and the operation date. Stable isotope (δ13C and δ15N) analysis was also performed and provides insight to the diet of the subjects. The wide range of measured stable isotope values could indicate that the subjects' diet varied, including an abundance of marine foodstuffs. It could also mean a different isotope fractionation process for the different plaque fragments (cap, core, interface to media) and a possible difference in tissues in which the various fragments are formed. Analysis of δ13C and δ15N values of each patient separately revealed subjects considered more influenced by marine foodstuffs consumption.

North Atlantic reservoir ages linked to high Younger Dryas atmospheric radiocarbon concentrations

North Atlantic reservoir ages linked to high Younger Dryas atmospheric radiocarbon concentrations

Groundwater recharge and evolution in the Dunhuang Basin, northwestern China

Groundwater recharge and evolution in the Quaternary aquifer beneath the Dunhuang Basin was investigated using chemical indicators, stable isotopes, and radiocarbon data to provide guidance for regional water management. The quality of groundwater and surface water is generally good with low salinity and it is unpolluted. The dissolution of halite and sylvite from fine-grained sediments controls concentrations of Na+ and K+ in the groundwater, but Na+/Cl- molar ratios >1 in all samples are also indicative of weathering of feldspar contributing to excess Na+. The dissolution of carbonate minerals yields Ca2+ to the groundwater, thereby exerting a strong influence on groundwater salinity. The delta O-18 and delta H-2 values in unconfined groundwater are enriched along the groundwater flow path from SW to NE. In contrast, confined groundwater was depleted in heavy isotopes, with mean values of -10.4 parts per thousand delta O-18 and -74.4 parts per thousand delta H-2. Compared with the precipitation values, all of the groundwater samples were strongly depleted in heavy isotopes, indicating that modern direct recharge to the groundwater aquifers in the plains area is quite limited. The unconfined water is generally young with radiocarbon values of 64.9-79.6 pmc. In the northern basin, radiocarbon content in the confined groundwater is less than 15 pmc and an uncorrected age of similar to 15 ka, indicates that this groundwater was recharged during a humid climatic phases of the late Pleistocence or early Holocene. The results have important implications for inter-basin water allocation programmes and groundwater management in the Dunhuang Basin. (C) 2012 Elsevier Ltd. All rights reserved.

Towards radiocarbon dating of single foraminifera with a gas ion source

Carbonate shells from foraminifera are often analysed for radiocarbon to determine the age of deep-sea sediments or to assess radiocarbon reservoir ages. However, a single foraminiferal test typically contains only a few micrograms of carbon, while most laboratories require more than 100μg for radiocarbon dating with an accelerator mass spectrometry (AMS) system. The collection of the required amount of foraminifera for a single analyses is therefore time consuming and not always possible. Here, we present a convenient method to measure the radiocarbon content of foraminifera using an AMS system fitted with a gas ion source. CO2 is liberated from 150 to 1150μg of carbonate in septum sealed vials by acid decomposition of the carbonate. The CO2 is collected on a zeolite trap and subsequently transferred to a syringe from where it is delivered to the ion source. A sample of 400μg (50μgC) typically gives a 12C− ion source current of 10–15μA over 20min, yielding a measurement precision of less than 7 per mil for a modern sample. Using this method, we were able to date a single 560μg Cibicides pseudoungerianus test at 14,030±160 radiocarbon years. Only a minor modification to our existing gas handling system was required and the system is fully automatable to further reduce the effort involved for sample preparation.

N14C: A plant–soil nitrogen and carbon cycling model to simulate terrestrial ecosystem responses to atmospheric nitrogen deposition

The dynamic model N14C simulates changes in the plant–soil dynamics of nitrogen and carbon, brought about by the anthropogenic deposition of nitrogen. The model operates with four plant functional types; broadleaved and coniferous trees, herbs and dwarf shrubs. It simulates net primary production (NPP), C and N pools, leaching of dissolved organic carbon and nitrogen (DOC, DON) and inorganic nitrogen, denitrification, and the radiocarbon contents of organic matter, on an annual timestep. Soil organic matter (SOM) comprises three pools, undergoing first-order decomposition reactions with turnover rates ranging from c. 2 to c. 1000 years. Nitrogen immobilisation by SOM occurs if inorganic N remains after plant uptake, and leaching of inorganic N occurs if the immobilisation demand is met. SOM accumulates in the deeper soil by transport and sorption of DOM. Element soil pools accumulate with N inputs by fixation from 12,000 years ago until 1800, when anthropogenic N deposition begins. We describe the parameterisation of N14C with data from 42 published plot studies carried out in northern Europe, plus more general information on N deposition trends, soil radiocarbon, N fixation and denitrification. A general set of 12 parameters describing litter fractionation, N immobilisation, growing season length, DOC and DON leaching, denitrification and NH4 retention was derived by fitting the field data. This provided fair agreements between observations and simulations, which were appreciably improved by moderate (±20%) adjustments of the parameters for specific sites. The parameterised model gives reasonable blind predictions of ecosystem C and N variables from only temperature, precipitation, N deposition, and vegetation type. The results suggest an approximate doubling of NPP due to N deposition, although the majority of the sites remain N-limited. For a given N deposition, leaching rates of inorganic N at conifer and shrub sites exceed those at broadleaf and herb sites.

Dynamics of decadally cycling carbon in subsurface soils

Subsurface horizons contain more than half of the global soil carbon (C), yet the dynamics of this C remains poorly understood. We estimated the amount of decadally cycling subsurface C (∼20 to 60 cm depth) from the incorporation of ‘bomb’ radiocarbon (14C) using samples taken over 50 years from grassland and forest soils in the Sierra Nevada Mountains, California. The radiocarbon content of all organic matter fractions (roots, low‐density (LF), high‐density (HF), and non‐oxidizable HF) increased from the pre‐ to post‐bomb samples, indicating ∼1–6 kgC m−2, or about half of the subsoil C, consists of C fixed since 1963. Low‐density (LF‐C) represented <24% (grassland) to 40–55% (forest) of the subsurface C and represented a mixture of post‐bomb C and varying amounts of pre‐1950 charcoal, identified using13C‐NMR spectroscopy. The14C content of HF‐C increased rapidly from 1992 to 2009, indicating a significant time lag (>20 years) for the arrival of ‘bomb’14C to this fraction. A two‐pool (fast‐cycling and passive) model including >20 year time lag showed that 28–73% of the subsoil mineral‐associated C had turnover times of 10–95 years. Microbially respired C was enriched in bomb14C compared to both LF and HF fractions in 2009. Overall, we estimate that C fluxes through decadally cycling pools in the subsurface are equivalent to 1–9% (grassland) to 10–54% (forest) of the surface litterfall at these sites. Our results demonstrate the importance of decadally cycling C for ecosystem C balance, and that a lagged response of the large subsurface C stores to changes in environmental conditions is possible.

Variation of the Schwabe Cycle Length During the Grand Solar Minimum in the 4th Century BC Deduced from Radiocarbon Content in Tree Rings

Solar activity alternates between active and quiet phases with an average period of 11 years, and this is known as the Schwabe cycle. Additionally, solar activity occasionally falls into a prolonged quiet phase (grand solar minimum), as represented by the Maunder Minimum in the 17th century, when sunspots were almost absent for 70 years and the length of the Schwabe cycle increased to 14 years. To examine the consistency of the cycle length characteristics during the grand solar minima, the carbon-14 contents in single-year tree rings were measured using an accelerator mass spectrometer as an index of the solar variability during the grand solar minimum of the 4th century BC. The signal of the Schwabe cycle was detected with a statistical confidence level of higher than 95 % by wavelet analysis. This is the oldest evidence for the Schwabe cycle at the present time, and the cycle length is considered to have increased to approximately 16 years during the grand solar minimum of the 4th century BC. This result confirms the association between the increase of the Schwabe cycle length and the weakening of solar activity, and indicates the possible prolonged absence of sunspots in the 4th century BC as during the Maunder Minimum. Theoretical implications from solar dynamo theory are discussed in order to identify the trigger of prolonged sunspot absence. A possible association between the long-term solar variation around the 4th century BC and terrestrial cooling in this period is also discussed.

Radiocarbon method in environmental monitoring of CO2 emission

New results of carbon isotopic composition from tree rings have been analyzed. Δ14C and δ13C data, representing the isotopic composition of carbon in “clean air”, were obtained from annual rings of a pine tree (Pinus sylvestris) taken in the Niepołomice area, 25km east Kraków, Poland. All samples were processed to extract α-cellulose, and the radiocarbon concentration in each annual ring was measured using AMS at the University of Nagoya. Stable isotopic composition of carbon was determined using isotope ratio mass spectrometry. The dataset covers the growth period between 1960 and 2003. The average difference between radiocarbon concentrations in Niepołomice and the North Hemisphere zone 1 (NH zone 1) for the period between 1960 and 1999 is 3.5±1.6‰. These data are compared with previously presented results from the city of Kraków, where a local decrease in 14C concentration was observed due to local CO2 emission from fossil fuel use. The differences in observed 14C concentrations were used to estimate a magnitude of the local Suess effect in Kraków. Based on mass balance equations for CO214C concentrations, it was possible to calculate the CO2 concentration associated with fossil fuel emission (Cfoss) into the atmosphere. The highest values of Cfoss were recorded in the years 1986 (11.9±1.4ppmV) and 1983 (8.1±1.3ppmV), while the lowest value of 0.6±1.8ppmV was recorded in 2001.

The radiocarbon composition of tree rings as a tracer of local fossil fuel emissions in the Los Angeles basin: 1980–2008

Quantifying local fossil fuel CO2 emissions in urban areas is challenging due to the heterogeneity in emissions and in atmospheric mixing ratios of CO2. Measurements of the radiocarbon content of urban tree rings are an alternative to large networks of CO2monitoring stations. In this study, we calculated 3‐year averages of CO2 mixing ratios from fossil fuel combustion from 1980 to 2008 using tree rings sampled at six sites within the Los Angeles basin and adjacent mountains. We observed CO2 mixing ratios from fossil fuel combustion of up to 23 μmol·mol−1 in the inland basin and ∼5–10 μmol·mol−1at coastal sites. Although we expected to see increasing trends of fossil fuel‐derived CO2over time, not all sites showed a significant increase. Analysis of correlations between fossil fuel‐derived CO2and socioeconomic variables revealed that fossil fuel‐derived CO2 followed trends in census tract and/or city population or in vehicle statistics at most sites. We also calculated CO/CO2combustion ratios from tree ring radiocarbon and nearby measurements of atmospheric CO mixing ratios. We observed widespread declines in the combustion ratio that support increases in the efficiency of the automobile fleet over the past few decades. This study demonstrates the utility of tree ring radiocarbon measurements for quantifying temporal and spatial patterns in fossil fuel‐derived CO2 emissions in urban areas.

Hydrogeochemical and isotope evidence of groundwater evolution in El Guettar oasis area, Southwest Tunisia

In southern Tunisia, the major aquifer system is characterized by the superposition of three principal levels; the upper unconfined Quaternary alluvial aquifer; the intermediate semi-confined/Confined Complex Terminal (Segui, Beglia and Zebbag Formations) and the Continental Intercalaire (Bou Dinar and Sidi Aïch Formations) which constitutes the deeper confined aquifer. The hydrodynamic functioning of this aquifer system is largely influenced by tectonics (Gafsa and Sehib faults), lithologic variation and recharge conditions. The tectonics has contributed to the discontinuity of the aquifer levels by creating some barriers which play the role of hydraulic sills. A study of the potentiometric maps shows four principal flow directions. These flow directions converge at the Sill Zones (Chott El Guettar and Chott Djerid depressions). The total mineralization evolution shows an increase from the periphery of the basin to the discharge area (1.75 ≤ TDS ≤ 14.4 g/l). Dissolution of halite, gypsum and/or anhydrite-bearing rocks is the principal source of the salinity of the groundwater. The isotopic signatures permit to classify the studied groundwaters into two different groups. Non-evaporated groundwaters that are characterized by depleted δ18O and δ2H contents highlighting the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents reflecting the significance infiltration of return flow irrigation waters and the influence of the water dam. Tritium data in the studied groundwaters lend support to the existence of pre-1950 and post-1960 recharge. According to radiocarbon residence time estimates, the deep groundwater is approximately 40 ka, and was recharged during a period when the climate was wetter and colder. Based on radiocarbon content, the shallow groundwater is older than 0.5 ka, and represents modern waters mixed with a limited quality of palaeo-recharge. Moreover, the insulation of certain compartments of the reservoir and inter-communications between the aquifer levels seem to play a principal role in the degradation of quality of groundwater.

Persistence of soil organic matter in eroding versus depositional landform positions

Soil organic matter (SOM) processes in dynamic landscapes are strongly influenced by soil erosion and sedimentation. We determined the contribution of physical isolation of organic matter (OM) inside aggregates, chemical interaction of OM with soil minerals, and molecular structure of SOM in controlling storage and persistence of SOM in different types of eroding and depositional landform positions. By combining density fractionation with elemental and spectroscopic analyses, we showed that SOM in depositional settings is less transformed and better preserved than SOM in eroding landform positions. However, which environmental factors exert primary control on storage and persistence of SOM depended on the nature of the landform position considered. In an annual grassland watershed, protection of SOM by physical isolation inside aggregates and chemical association of organic matter (complexation) with soil minerals, as assessed by correlation with radiocarbon concentration, were more effective in the poorly drained, lowest‐lying depositional landform positions, compared to well‐drained landform positions in the upper parts of the watershed. Results of this study demonstrated that processes of soil erosion and deposition are important mechanisms of long‐term OM stabilization.

Possible source of ancient carbon in phytolith concentrates from harvested grasses

Abstract. Plants absorb and transport silicon (Si) from soil, and precipitation of Si within the living plants results in micrometric amorphous biosilica particles known as phytoliths. During phytolith formation, a small amount of carbon (<2%) can become occluded in the silica structure (phytC) and therefore protected from degradation by the environment after plant tissue decomposition. Since the major C source within plants is from atmospheric carbon dioxide (CO2) via photosynthesis, the current understanding is that the radiocarbon (14C) content of phytC should reflect the 14C content of atmospheric CO2 at the time the plant is growing. This assumption was recently challenged by 14C data from phytoliths extracted from living grasses that yielded ages of several thousand years (2–8 kyr BP; in radiocarbon years "Before Present" (BP), "Present" being defined as 1950). Because plants can take up small amounts of C of varying ages from soils (e.g., during nutrient acquisition), we hypothesized that this transported C within the plant tissue could be attached to or even embedded in phytoliths. In this work, we explore this hypothesis by reviewing previously published data on biosilica mineralization and plant nutrient acquisition as well as by evaluating the efficiency of phytolith extraction protocols from scanning electron microscope (SEM) images and energy dispersive spectrometer (EDS) analyses from harvested grasses phytolith concentrates. We show that current extraction protocols are inefficient since they do not entirely remove recalcitrant forms of C from plant tissue. Consequently, material previously measured as "phytC" may contain at least some fraction of soil-derived C (likely radiocarbon-old) taken up by roots. We also suggest a novel interpretation for at least some of the phytC – which enters via the root pathway during nutrient acquisition – that may help to explain the old ages previously obtained from phytolith concentrates.

Deciphering groundwater flow between the Complex Terminal and Plio-Quaternary aquifers in Chott Gharsa plain (southwestern Tunisia) using isotopic and chemical tools

The Complex Terminal (CT) and Plio-Quaternary (P-Q) aquifers in the Chott Gharsa plain in southwestern Tunisia have been investigated with the aid of chemical and isotopic tools. It has been demonstrated that groundwater from the CT is mainly of palaeo-origin, especially in the western and central parts of the plain where the most negative values of δ18O and δ2H were observed (between −8.1 and −7.6‰ for δ18O, and −60 to −57‰ for δ2H), combined with low concentrations of radiocarbon (6.8–7.5 pmc) and absence of tritium. Modern recharge of the aquifer occurs only in the eastern part of the system where younger waters were observed, as indicated by their stable isotope composition, relatively high radiocarbon content and presence of tritium. Groundwater from the P-Q multi-layer aquifer represents mixtures of ascending deep CT waters and modern water recharging the P-Q aquifer system. Isotope mass balance was used to quantify mixing proportions. The calculations showed that the contribution of deep CT groundwater to the P-Q aquifer system reaches about 75% in the western and central parts of the plain where the CT aquifer remains strongly artesian. This contribution decreases to about 15% towards the eastern part of the plain, as a consequence of significant reduction of artesian pressure in this area of the CT aquifer. Chemical data suggest that mineralization of the studied groundwater systems is controlled mainly by dissolution of evaporative minerals (halite, anhydrite and gypsum) and cation exchange reactions with the matrix, possibly enhanced by recent anthropogenic disturbance of the system caused by lowering of the water table due to heavy exploitation and return flow of saline irrigation water into the P-Q aquifer. Editor D. Koutsoyiannis; Associate editor E. Custodio Citation Yangui, H., Abidi, I., Zouari, K., and Rozanski, K., 2012. Deciphering groundwater flow between the Complex Terminal and Plio-Quaternary aquifers in Chott Gharsa plain (southwestern Tunisia) using isotopic and chemical tools. Hydrological Sciences Journal, 57 (5), 967–984.

Compound specific radiocarbon content of lignin oxidation products from the Altamaha river and Coastal Georgia

Compound-specific isotope analysis (CSIA) is a powerful tool in organic geochemistry by providing detailed information about an individual organic compound’s history with regard to its source and process of formation. Most CSIA involves measurement of the stable isotope ratio of carbon (13C/12C) and hydrogen (D/H) following separation by gas or liquid chromatography. New applications are being developed using compound-specific radiocarbon (14C) content for delineating age of materials, rates of decomposition and residence time in various environments. This paper details the isotopic work on specific lignin monomers derived from terrestrial plants transported and deposited within the Altamaha River, estuary and off-shore Georgia in the Atlantic Ocean. By using gas chromatographic separation and identification of selected lignin oxidation products (LOP), the harvesting of these compounds using preparative fraction collection, and measurement of their 14C content using accelerator mass spectrometry, details of the age and presence of specific biomarkers unique to a given terrestrial source are revealed. Radiocarbon ages determined from water-column particulate organic carbon and sediment LOPs indicate a range of ages from modern to well over 5,000years for the former and latter respectively. Transport mechanisms and particle size associations on mineral grains may play a significant role in 14C distribution in estuary and near-shore coastal environments. This data indicates higher than modern 14C activities in large particle-size sediment fractions in contrast to older LOP 14C ages found associated with the same coarse grain sediments. Individual LOP ages substantiate older terrestrial materials persist in the off-shore environment even though in the presence of modern marine 14C sources.

Hydrological control of the dead carbon fraction in a Holocene tropical speleothem

Over the past decade, a number of speleothem studies have used radiocarbon (14C) to address a range of palaeoclimate problems. These have included the use of the bomb pulse 14C to anchor chronologies over the last 60 years, the combination of U-Th and 14C measurements to improve the radiocarbon age-calibration curve, and linking atmospheric 14C variations with climate change. An issue with a number of these studies is how to constrain, or interpret, variations in the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that reduces radiocarbon concentrations in speleothems. In this study, we use 14C, stable-isotopes, and trace-elements in a U-Th dated speleothem from Flores, Indonesia, to examine DCF variations and their relationship with above-cave climate over the late Holocene and modern era. A strong association between the DCF and hydrologically-controlled proxy data suggests that more dead carbon was being delivered to the speleothem during periods of higher cave recharge (i.e. lower δ18O, δ13C and Mg/Ca values), and hence stronger summer monsoon. To explore this relationship, we used a geochemical soil-karst model coupled with 14C measurements through the bomb pulse to disentangle the dominant components governing DCF variability in the speleothem. We find that the DCF is primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than kinetic fractionation and/or variations in the age spectrum of soil organic matter above the cave. Therefore, we infer that periods of higher rainfall resulted in a higher DCF because the system was in a more closed state, which inhibited carbon isotope exchange between the karst water dissolved inorganic carbon and soil-gas CO2, and ultimately led to a greater contribution of dead carbon from the bedrock.

Radiocarbon analysis of methane emitted from the surface of a raised peat bog

We developed a method to determine the radiocarbon (14C) concentration of methane (CH4) emitted from the surface of peatlands. The method involves the collection of ∼9 L of air from a static gas sampling chamber which is returned to the laboratory in a foil gas bag. Carbon dioxide is completely removed by passing the sample gas firstly through soda lime and then molecular sieve. Sample methane is then combusted to CO2, cryogenically purified and subsequently processed using routine radiocarbon methods. We verified the reliability of the method using laboratory isotope standards, and successfully trialled it at a temperate raised peat bog, where we found that CH4 emitted from the surface dated to 195–1399 years BP. The new method provides both a reliable and portable way to 14C date methane even at the low concentrations typically associated with peatland surface emissions.