Atmospheric 14C Concentration Research Articles

Holocene climatic change,14C wiggles and variations in solar irradiance

Evidence from the advances and retreats of alpine glaciers during the Holocene suggests that there were at least 14 century-timescale cool periods similar to the recent Little Ice Age. Here, we examine the hypothesis that these cool periods were caused by reductions in solar irradiance. A statistically significant correlation is found between the global glacial advance and retreat chronology of Röthlisberger and variations in atmospheric14C concentration. A simple energy-balance climate model is used to show that the mean reduction of solar irradiance during times of maximum14C anomaly like the Maunder Minimum would have to have been between 0.22 and 0.55 % to have caused these cool periods. If a similar solar irradiance perturbation began early in the 21st century, the associated climate effects would be noticeable, but still considerably less than those expected to result from future greenhouse gas concentration increases.

Variations in carbon 14 content in tree rings during the last 30 years: application to wood dating

Robiniapseudoacacia stem annual growth rings from 1958 to 1987 were analyzed for 14C content. The radioactivity of annual rings was found to be strongly correlated with 14C concentration in the atmosphere, which showed a very sharp rise until 1963 due to nuclear weapon tests followed by a regular decrease from then on. Taking into account seasonal variations in both atmospheric 14C concentration and CO2 assimilation slightly improved the relation between 14C in wood and the atmosphere. A predictive curve that gives the year of wood formation as a function of radioactivity with a 5% confidence interval is shown. These results can be used to analyze wood for which traditional analysis is impossible, for example, trees with false rings or missing rings, or wood from stumps or roots.

Groundwater 14C activity as affected by initial water-rock interactions in the Judean Mountains, Israel

A data base was formed of field observations, water tables, δ 13C of soil air and of local limestone and dolomite, and 14C, δ 13C and dissolved ions in eight wells spread over a distance of 15 km in a recharge area, along with tritium, dissolved atmospheric noble gases and temperature computations. These data have been applied to formulate a conceptual model of evolution of groundwater composition in the karstified Judean Mountains. Rapid (conduit controlled) passage of recharge water through the aerated zone is indicated: (a) by noble-gas-deduced intake temperatures that are in some of the wells close to the rainy season winter temperature; (b) observed groundwater temperatures that are between winter and average annual temperatures (indicating partial equilibrium); and (c) quick rise of tritium contents in wells following a flood event. The fast passage constrains water-rock interaction to calcite dissolution in the aerated zone, incongruent dolomite dissolution taking over in the saturated zone. Further water interactions downflow in the phreatic aquifer are non-significant. Geochemical-isotopic mass-transfer simulation by the CSOTOP program was used to check the thermodynamic feasibility of alternative combinations of interactions, selecting the flow paths that lead to the observed groundwater compositions. Good agreement was found between the conceptual and simulated models. A major outcome is that the initial (atmospheric) 14C concentration is lowered by a factor Q of 62±3% by water-rock interactions. Applying factor Q to the recharge just before and after the nuclear bomb tests explains the range of 57–86 pmC observed in the wells located along the flow direction in the phreatic aquifer. This factor Q, applied to pre-bomb (∼ 100 pmC) recharge, supplies the value of initial 14C activity to be used for groundwater dating in the confined (tritium devoid) sections that extend from the studied groundwater system.

Simulations of radiocarbon in a coarse‐resolution world ocean model: 2. Distributions of bomb‐produced carbon 14

Part 1 of this study examined the ability of the Geophysical Fluid Dynamics Laboratory (GFDL) primitive equation ocean general circulation model to simulate the steady state distribution of naturally produced 14C in the ocean prior to the nuclear bomb tests of the 1950s and early 1960s. In part 2 we begin with the steady state distributions of part 1 and subject the model to the pulse of elevated atmospheric 14C concentrations observed since the 1950s. This study focuses on the processes and time scales which govern the transient distributions of bomb 14C in the upper kilometer of the ocean. Model projections through 1990 are compared with observations compiled by the Geochemical Ocean Sections Study (GEOSECS) in 1972, 1974, and 1978; the Transient Tracers in the Ocean (TTO) expedition in 1981, and the French INDIGO expeditions in 1985–1987. In their analysis of the GEOSECS 14C observations, Broecker et al. (1985) noted that much of the bomb 14C which entered the ocean's equatorial belts prior to GEOSECS accumulated in the adjacent subtropical zones. Broecker et al. argued that this displacement of bomb 14C inventories was caused by the wind‐driven upwelling and surface divergence in the tropics combined with convergent flow and downwelling in the subtropics. Similar displacements were invoked to shift bomb 14C from the Antarctic circumpolar region into the southern temperate zone. The GFDL model successfully reproduces the observed GEOSECS inventories, but then predicts a significantly different pattern of bomb 14C uptake in the decade following GEOSECS. The post‐GEOSECS buildup of bomb 14C inventories is largely confined to the subthermocline layers of the North Atlantic, the lower thermocline of the southern hemisphere, and down to 2000 m in the circumpolar region. A great deal of attention is devoted to detailed comparisons between the model and the available radiocarbon data. A number of flaws in the model are highlighted by this analysis. The Subantarctic Mode Waters forming along the northern edge of the circumpolar current are identified as a very important process for carrying bomb 14C into the thermoclines of the southern hemisphere. The model concentrates its mode water formation in a single sector of the circumpolar region and consequently fails to form its mode waters with the correct T‐S properties. The model also moves bomb 14C into the deep North Atlantic and deep circumpolar region much too slowly.

Atmospheric Radiocarbon at the End of the Last Glacial: An Estimate Based on AMS Radiocarbon Dates on Terrestrial Macrofossils From Lake Sediments

The main purpose of this work is to reconstruct the atmospheric Δ 14C in the glacial-postglacial transition, 14,000 – 10,000 BP, a range not covered by the tree-ring calibration curve. We measured 14C/12C ratios on series of terrestrial macrofossils from sediments of two Swiss lakes. We selected exclusively plant remains of recognizable terrestrial origin that are not affected by hard water and thus reflect atmospheric 14C concentration. Due to the scarcity of such material, we used accelerator mass spectroscopy. Cores of two lakes were measured to eliminate local effects and to check the reproducibility of results. This required a reliable, 14C-independent correlation of the cores, obtained through local pollen zone boundaries 14C ages were obtained as a function of the depth in the cores. If sedimentation rates are known ages can be converted into Δ 14C values. We also attempted estimating sedimentation rates; calculations are based on the Swedish varve chronology. Results were combined to form an entire data set. The Δ 14C curve shows an increase with time during the Allerød and decreases during Preboreal and Bølling periods. Probabilities for these 14C variations are discussed.

Variations of atmospheric carbon dioxide concentration at Syowa Station (69°00'S, 39°35'E), Antarctica

Precise measurements of the atmospheric C02 concentration were initiated at Syowa Station, Antarctica in 1983. Preliminary inspection of the data obtained up to the present showed that: (I) a regular diurnal variation is not observable; (2) irregular variations are sometimes observed with extremely small amplitudes of 0.2 ppmv at most; (3) a seasonal variation with the minimum concentration in mid-April and the maximum concentration in mid-October; and peak-to-peak amplitudes of about 1.2 ppmv are detected; (4) annual mean values of the C02 concentration are 341.2 and 342.6 ppmv for 1983 and 1984, respectively.

The Temporal Distribution of ‘Bomb’ 14C in a Forest Soil

Patterns of 14C enrichment in the superficial plant debris and mineral soil horizons of an established woodland have been monitored at regular intervals during the past 15 years. These data are compared with a model evaluation of carbon turnover based on the recorded changes in atmospheric 14C concentration since AD 1900.Leaf litter and decomposing plant debris are characterized by steady-state turnover values of ca 2 and ca 8 years, respectively. A two-component system of ‘fast’ (≤20 yr) and ‘slow’ (ca 350 yr) cycling carbon is indicated for the surface (0–5cm) soil humus; below 10cm, the “fast’ component is rare (<5%).Selective microbal humification of leaf litter, branch, and root debris is proposed to explain a delay of several years in the peak transfer of ‘bomb’ 14C to the soil carbon pool.

The camp century 10Be record: Implications for long-term variations of the geomagnetic dipole moment

10Be concentrations measured in ice samples from Camp Century, Greenland, show short term variations which in general correspond to the 100–200 year “wiggles” in the 14C tree ring record. There is, however, no evidence for a long term variation over the last 5000 years. This constancy is in contrast to the approximately sinusoidal variation of the atmospheric 14C concentration which has generally been attributed to a changing geomagnetic dipole moment. This discrepancy implies that the 14C trend might stem from other causes such as changes of oceanic circulation processes or from higher production rates during the Wisconsin rather than from variation in the geomagnetic field.

Atmospheric Radiocarbon: Implications for the Geomagnetic Dipole Moment

The geomagnetic field is one of the major physical fields of the earth. Because its source is fluid motion in the outer core, it exhibits temporal changes, called secular variation, which are quite rapid compared to most geologic phenomena. The prehistoric secular variation is usually inferred from paleomagnetic data. We will discuss here how changes in the atmospheric 14C content can be used to gain additional insight into the behavior of the dipole moment over the past 8500 years. By rewriting the differential equations representing the 14C geochemical cycle in finite-difference form, we are able to convert the atmospheric 14C activity record into an equivalent radiocarbon dipole moment.

Chilling Damage to Photosynthesis in Young Zea mays

Chilling Damage to Photosynthesis in Young Zea mays

High-Precision 14C Measurement of Irish Oaks to Show the Natural Atmospheric 14C Variations of the AD Time Period

The high-precision 14C measurement of bi-decade and decade samples of Irish Oak for the time period AD 50 to 1830 is presented. The samples were taken from dendrochronologically dated Irish Oak providing an absolute chronology for this period. While the natural atmospheric 14C concentration shows cyclic deviations from a constant value, the amplitudes of such deviations vary considerably. Repeated measurement and interlaboratory checks justify a claim to accuracy in the data given. Comparison of a large number of data sets between Belfast and Seattle show that the different techniques of scintillation counting of 14C6H6 and gas counting of 14CO2 derived from different wood species grown in different continents give no significant bias. Thus, it is justifiable to use this data set as a high-precision calibration curve for the AD period. Over most of this period bi-decade samples were measured giving a slightly reduced resolution of short-term variation when compared to the measurement of decade intervals; however, any difference is not apparent when such curves are compared. It is of much greater importance that a comparison of data sets is without bias if a combined calibration curve is to have sufficient integrity for general use. Averaging the data from Belfast and Seattle would improve the validity of such an AD 14C calibration curve since it would tend to smooth out slight local variations and become internationally more acceptable.

An anomalous Suess effect above Europe

The major impact that man is likely to have on climate comes from the artificial increase of the atmospheric CO2 level, caused by the combustion of fossil fuels1,2. To predict its magnitude, the future rise of the global atmospheric CO2 level must be calculated using carbon reservoir models. Because fossil fuel CO2 lacks 14C and has a 13C/12C ratio which is 18‰ lower than the present average atmospheric 13C/12C ratio, the validity of such geochemical models can be tested using past temporal variations in the atmospheric concentration of these isotopes. However, the past variation of the 13C/12C ratio in global reservoirs is very difficult to infer3. Using Δ14C data of tree rings Suess measured a depletion in the atmospheric 14C concentration over the past century of <10‰ (ref. 4). Some years later more accurate data gave a value for this ‘Suess effect’ of 15–25‰ (ref. 5). As the models have been improved considerably over the past few years6–8 the availability of Δ14C data with an accuracy of better than 1.5‰ was thought to be essential to test these9,10. The present results indicate a good correlation with model predictions taking into account natural 14C variations. However, deviations between American and European tree ring data after AD 1920 suggest a European continental CO2 effect.

Fractionation Corrections in Radiocarbon Dating

Recent experimental work has suggested that the relative fractionation of 14C to 13C may differ from the accepted value of b = 2. In order to explore the implications of this possibility, the standard formulae for correcting radiocarbon dates for fractionation effects are rederived, but without making any of the usual assumptions or approximations. A generalized dating equation is derived (where ASN and AON are normalized sample and standard activities, β is a factor which reflects changes in atmospheric 13C and 14C content, {RST(o)/RST}b accounts for post-depositional changes in sample 13C ratio, and tcal is calendar age in years before ad 1950. The errors in calculated ages which might arise from different b values are estimated and shown to be small relative to other dating uncertainties. The effect of b ≠ 2 may be important in the calibration of radiocarbon dates using tree-ring samples of known age. A theoretical analysis suggests that b ≠ 2 effects may result in a correlation between age anomaly (ie, the difference between radiocarbon age and calendar age) and sample 13C data. However, an analysis of published data reveals no meaningful correlation. This result, while not eliminating the possibility that b ≠ 2, highlights its unimportance even in this high-precision application of radiocarbon dating.

Geomagnetic Strength Over the Last 50,000 Years and Changes in Atmospheric 14C Concentration: Emerging Trends

Palaeomagnetic field strength measurements for the last 50,000 years are summarized. The period before ~12,000 yr bp is characterized by low dipole moments, but high values are associated with the Lake Mungo polarity excursion between ~32,000 and ~28,000 yr bp. The variation since 12,000 yr bp, based on new results from Australia and published data from the Northern Hemisphere has a quasi-cyclic appearance with maxima at ~10,000 and ~3500 yr bp. The geomagnetic record is used to predict variations in atmospheric 14C concentration, and the results are compared with independent comparisons between 14C and other dating methods. Long-term variations in the 14C time-scale are readily explained by known geomagnetic changes.

14C Variations Caused by Changes in the Global Carbon Cycle

A box-diffusion model for the carbon cycle is used to estimate the magnitude of 14C variations caused by changes of reservoir sizes and exchange fluxes in the global carbon system. The influence of changes in atmospheric CO2 concentration, biomass, CO2 exchange rate between atmosphere and ocean, and ocean mixing is considered. Steady-state 14C concentrations as well as the transients are calculated. For changing biomass, atmospheric CO2 levels and 13C/12C ratios are also calculated.Carbon-cycle-induced 14C variations may have been significant in the transition period from Glacial to Postglacial when drastic changes in environmental conditions took place within short time periods, while they were probably less important during the climatically more stable Postglacial.Changes of the oceanic circulation, as supposedly occurred, are considered the most important factor, besides variations of the production rate, affecting the global distribution of 14C. 14C variations due to changes of the atmospheric CO2 level or the air-sea-exchange probably did not exceed one to a few percent. Fluctuations of the forest biomass, which may have occurred between Glacial and Postglacial, hardly affected the 14C concentration over a long term.Responses of the atmospheric 14C concentration are also calculated for variations of the 14C production rate by cosmic radiation. The following cases are considered: a step change, square-wave changes producing “wiggles”, and sinusoidal variations.

Confirmation of the Suess wiggles: 3200–3700 BC

USING dendrochronologically dated tree rings de Vries1 showed that the atmospheric 14C concentration has not been entirely constant during the past few hundred years. By extending the North American bristlecone pine tree-ring chronology2 to nearly 8,000 yr ago, several radiocarbon laboratories succeeded in measuring deviations in the 14C concentration from the mid-nineteenth century natural 14C level to a maximum of about 10% around 4000 BC (refs 3–7). Superimposed on this general trend, Suess drew with ‘cosmic schwung’, medium-term variations through his data. These so-called ‘wiggles’ should correspond with variations in the abundance of radiocarbon in atmospheric carbon dioxide on a time scale between one decade and a few centuries. The resulting irregular shape of Suess' calibration curve has been questioned during the past few years8,9. The overall result, however, is that no generally accepted 14C calibration curve is yet available. It would be most convenient to measure 14C concentrations in single tree rings with much higher precision than the usual 4–6‰. For this, however, much larger samples are required than are available from the bristlecone pine tree. An excellent opportunity was presented by the sub-fossil oak trunks discovered in the river valley sediments of southern Germany9; our measurements of these trunks are reported here.

Reservoir models and production rate variations of natural radiocarbon

Distribution and time constants of the exchangeable carbon on the earth's surface are discussed in terms of a two-box model and a three-box model. Emphasis is given to the question of the functional relationship of the 14C level in the atmospheric CO2 to the cosmic ray production rate Q. It is shown in a quantitative way that relatively slow changes (with a time constant of more than 30 years) of the atmospheric 14C content can be described adequately by a two-box model. More rapid changes require more complicated models. The most appropriate definition of ‘residence time’ of 14C (and CO2) in the atmosphere depends on the time scale of the phenomenon under consideration. The observed fluctuations of the 14C content of atmospheric CO2 during the past 7000 years can be explained by assuming production rate variations due to geomagnetic and heliomagnetic modulations of the cosmic ray flux, as long as these fluctuations occur on a time scale longer than ∼100 years. More rapid fluctuations with amplitudes in excess of 1%, if they exist, will require a different explanation.

A Method for the Rapid Measurement of Photosynthesis

This technique was developed to measure the relative photosynthetic capacities of genetically differing tobacco plants for selection in a breeding programme; it should also prove useful for other species. Its most important feature is the speed with which results are obtained, rates of up to 30 per hour being possible. Rapidity is essential if large populations are to be screened and where photosynthetic behaviour may drift during a prolonged screening programme. The procedure involved two phases. In the first the leaf samples were incubated in large numbers at atmospheric C02 concentration and constant temperature and illumination to establish a steady rate of photosynthesis. In the second phase they were transferred one by one to a leaf chamber under the same environmental conditions for photosynthetic rate measurements. Samples of 8 cm squares were cut from the lamina of tobacco leaves so as to exclude the mid vein. Each sample was floated under-surface uppermost on 15 ml of distilled water in a 10-cm-square plastic Petri-dish, large numbers of these dishes then being floated on a water bath at 24 °C under illumination of 23 500 lx from a bank of water-cooled tungsten lamps. The temperature of the water in the Petri-dishes remained within one degree of that of the water bath under these condi tions. After incubation for 1 h the dishes were removed for measurement of photo synthetic rate with an open circuit system and an infra-red C02 analyser. Air with an atmospheric C02 concentration from a compressed-air cylinder was fed to the leaf chamber via a flow meter at a rate controlled at 11 per minute by a 'Elowstat Minor' (G. A. Platon Ltd., Basingstoke, England). A vertical median section of the leaf chamber can be seen in Fig. 1. It was made of 16-mm-thick 'Plexiglas' and was designed to accommodate the leaf material floating in the Petri-dish. It was situated alongside the water bath and was illuminated by the same bank of lights. Cooling was by a water jacket in its base supplied with water drawn in a closed circuit from the main water bath. No water jacket was placed above the chamber because this was found to be unnecessary and would have caused condensation on the chamber roof and interference with light transmission. The Petri-dish containing the leaf sample was inserted and removed at one end of the chamber through a

Tree Ring, Varve and Carbon-14 Chronologies

CARBON-14 dating has become of great importance and it is therefore necessary to determine the possible deviations between radiocarbon and actual ages. A radiocarbon age, given in radiocarbon years elapsed since the origin of the sample, is based on 14C decay and is calculated from the ratio of the present 14C activity of the sample to the 14C activity of the standard (95 per cent of the 14C activity of NBS oxalic acid). The standard 14C activity is assumed to be identical to the atmospheric 14C activity over the whole time interval over which 14C dating is used. Thus the conventional radiocarbon age does not allow for deviations in atmospheric 14C content during the past. As a result, radiocarbon years need not always be equal to solar years.