14C Variations Research Articles

The 14 C record in bristlecone pine wood of the past 8000 years based on the dendrochronology of the late C. W. Ferguson

When, in 1950, Willard Libby and his coworkers obtained their first radiocarbon ( 14 C) dates, C. W. Ferguson at the University of Arizona Tree Ring Laboratory was working on establishing a continuous tree ring series for the newly discovered bristlecone pine Pinus aristata . Before his untimely death in 1986, he had extended the series nearly 8000 years into the past. From the Ferguson series I obtained for 14 C determinations wood samples grown at various times. Also, two other laboratories obtained such samples. For B.C. times in particular, our measured 14 C-values that deviated consistently from those calculated from tree rings, and the deviations increased with age. This general trend was observed by other laboratories, but the presence of deviations from these trends, of the so-called ‘wiggles’, was questioned by other workers. To me these wiggles indicated the existence of a most interesting geophysical parameter valid for the whole terrestrial atmosphere. Fourier spectra obtained at my request by Kruse in 1972, and by Neftel, demonstrated the consistency of the results, and supported my contention that the secular variations of 14 C in atmospheric CO 2 are related to variations of solar activity.

Annual Reconstruction of the Solar Cycle from Atmospheric 14C Variations

Initially, the rise and fall components of the ll-year solar sunspot cycle are approximated by separate least-squares polynomials for four cycle classifications, which are determined by the magnitude of the average of the annual sunspot numbers per cycle. Following, a method is formulated to generate detailed reconstruction of the annual variation of a solar cycle based on this cycle average, and the results obtained for cycles -4 through to 21 are compared with the annual Zurich values. This procedure is then employed to establish annual sunspot numbers using published average cycle values obtained from atmospheric carbon 14 variations, which have been derived from the chemical analysis of tree ring sections. The reconstructed sequences are correlated with the observed cycle values and with tree ring width index chronologies which exhibit a significant II-year periodicity. It is anticipated that the long carbon 14 records and parallel dendrochronological data could be employed to obtain a more detailed portrayal of previous periods of strong solar activity than that given by current estimates based on historical records.

14C concentration of liberated CO2 in the free fermentation process of Japanese SAKE brewing

The daily variation of14C concentrations of liberated CO2 in the free fermentation process of Japanese SAKE brewing was studied. Each of the concentrations measured in the initial and final stages of the fermentation process correlated with levels of koji rice and steamed rice used for SAKE production, which had been obtained from different areas. This shows that analysis of fermenting CO2 of SAKE could be used to estimate the14C level in a local environment.

High-Resolution 14C Dating of Organic Deposits Using Natural Atmospheric 14C Variations

The occurrence of atmospheric 14C variations complicates calibration, ie, the translation of 14C ages into real calendar ages. The procedure of wiggle matching, however, allows very precise calibration, by matching known 14C variations with wiggles in the floating chronology. In principle, wiggle matching can also be applied to a series of 14C dates from organic (peat) deposits. Where, in general, 14C ages are required at short distances and on small samples, dating by 14C accelerator mass spectrometry (AMS) is required.

Anthropogenic 14C Variations in Atmospheric CO2 and Wines

As an extension of the Bratislava – Tbilisi collaboration, thermonuclear bomb-produced 14C variations in atmospheric CO2 of Bratislava (1967–1987) and in annually produced Georgian wines (1950–1987) are discussed. 14C produced in bomb tests performed in the atmosphere has considerably modified the natural 14C levels in the atmosphere and biosphere. Measurements of 14C in monthly samples of atmospheric CO2 show typical seasonal variations with maxima in summers and deep minima in winters. There is very good agreement between 14C measured in CO2 and in wine samples. Four maxima (1959, 1964, 1970 and 1978) were identified in the wine data. Our results confirm that wines prepared from annually grown grapes without any addition of other substances are good indicators of the 14C content of atmospheric CO2.

Changes in Ocean Ventilation Rates Over the Last 7000 Years Based on 14C Variations in the Atmosphere and Oceans

Changes in the ocean ventilation rate may be one of the causes for a net decrease of 100‰ Δ 14C in atmospheric CO2 over the last 8000 years. Ocean ventilation rates of the past can be derived from the 14C record preserved in planktonic and benthic foraminifera in deep-sea sediments. Results of 14C dating using accelerator mass spectrometry on deep sea sediments from the South China Sea show that the age differences between planktonic (G sacculifer) and benthic foraminifera increase from 1350 yr ca 7000 yr ago to 1590 yr at present. An 11-box geochemical model of global ocean circulation was used for this study. Both tree-ring-determined atmospheric 14C values and foraminifera 14C age differences are used as constraints to place limits on patterns of changes in ocean ventilation rates and in atmospheric 14C production rates. Results indicate: 1) 14C production rates in the atmosphere may have decreased by as much as 30% between 7000 and 3000 yr ago, and may have increased again by ca 15% in the past 2000 yr, and 2) the global ocean ventilation rate may not have been at steady state over the last 7000 yr, but may have slowed by as much as 35%.

Fine and Hyperfine Structure in the Spectrum of Secular Variations of Atmospheric 14C

The coarse structure of the 14C spectrum consists of a secular trend curve that may be closely fit by a sinusoidal curve with period ca 11,000 yr and half amplitude ±51. This long-term trend is the result of changes in the earth's geomagnetic dipole moment. Consequently, it modulates solar components of the 14C spectrum but does not appear to modulate a component of the spectrum of ca 2300-yr period. The ca 2300-yr period is of uncertain origin but may be due to changes in climate because it also appears in the δ18O spectrum of ice cores. This component strongly modulates the well-known ca 200-yr period of the spectrum's fine structure. The hyperfine structure consists of two components that fluctuate with the 11-yr solar cycle. One component results from solar-wind modulation of the galactic cosmic rays and has a half-amplitude of ca ±1.5. The other component is the result of 14C production by solar cosmic rays that arrive more randomly but rise and fall with the 11-yr cycle and appear to dominate the fluctuation of the galactic cosmic-ray-produced component by a factor of two.

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.

Improved Precision 14C Measurements and Natural 14C Variations Around 10,000 Cal BP

Thirty-one samples of 5 tree rings from a single oak trunk with 300 tree rings were measured with improved precision to study natural 14C variations in the period from ca 9750 – 10,050 cal bp. Negative correlation was found between Δ 14C values and tree-ring indices. The pattern of 14C changes in the studied interval closely resembles the changes observed in several periodically spanned intervals within the limits of high-precision calibration data sets.

10Be Concentrations in Antarctic Ice

Measurements of the cosmogenic isotope 10Be (T½ = 1.5 Ma) on Greenland ice cores produced interesting results. Variations in the 10Be concentrations can be interpreted in terms of changes in the production rate and in atmospheric circulation and deposition. During the Holocene, good agreement between short-term variations in 10Be and 14C indicates that the production rate of both isotopes was changing, probably due to solar modulation. During the last ice age, periods with significantly higher 10Be concentrations are observed. The good anti-correlation between 10Be and δ18O suggests that these intervals correspond to periods of low precipitation rates. Work on Antarctic ice cores is in progress, but only relatively few 10Be data have been published yet. 10 Be results from Antarctic ice cores are presented and compared with data from Greenland.

10Be Concentrations in Antarctic Ice

Measurements of the cosmogenic isotope 10Be (T½ = 1.5 Ma) on Greenland ice cores produced interesting results. Variations in the 10Be concentrations can be interpreted in terms of changes in the production rate and in atmospheric circulation and deposition. During the Holocene, good agreement between short-term variations in 10Be and 14C indicates that the production rate of both isotopes was changing, probably due to solar modulation.During the last ice age, periods with significantly higher 10Be concentrations are observed. The good anti-correlation between 10Be and δ18O suggests that these intervals correspond to periods of low precipitation rates.Work on Antarctic ice cores is in progress, but only relatively few 10Be data have been published yet. 10 Be results from Antarctic ice cores are presented and compared with data from Greenland.

Accelerator mass spectrometry and ice core research

AMS has opened up the possibility of measuring 10Be and 36Cl on samples of 1 kg of ice and 14C on CO 2 extracted from 10 kg of ice. The 10Be data of Greenland ice cores show fluctuations which are correlated with the short-term (200 yr) 14C variations observed in tree-rings. Also major climatic changes due to the corresponding precipitation rate changes are reflected in the 10Be concentrations. First 14C results are promising. Precise 14C dating would help to synchronize the climatic events in the two hemispheres during the glacial-postglacial transition. 36Cl/ 10Be ratio data show a strong scatter, probably due to a different geochemical behaviour of Cl and Be.

Seasonal variations of anthropogenic radiocarbon in the atmosphere

Seasonal 14C variations in atmospheric CO 2 caused by nuclear bomb tests and by fossil-fuel combustion are discussed. The investigated time interval is from 1968 to 1980. A comparison of Bratislava and Tbilisi 14C data shows that both are influenced by the Suess effect. Summer-winter 14C variations observed in these data are predominantly due to the fossil-fuel CO 2 increase during winter months. A strong correlation between the 14C concentration and the temperature during a year has been found. In contrast to the 14C variations in CO 2 from urban areas the clean air 14C data have shown that the bomb effect is responsible for the seasonal 14C variations in non-industrialized areas.

14C in the Deep Water of the East Atlantic

The renewal of east Atlantic deep water and its large-scale circulation and mixing have been studied in observed distributions of temperature, silicate, ΣCO2, and 14C. 14C variations in northeast Atlantic deep water below 3500m depth are small. Δ14C values range from − 100‰ to −125‰. 14C bottom water concentrations decrease from Δ14C =−117‰ in the Sierra Leone Basin to Δ14C = − 123‰ in the Iberian Basin and are consistent with a mean northward bottom water flow. The characteristic of the water that flows from the west Atlantic through the Romanche Trench into the east Atlantic was determined by inspection of θ/Δ14C and θ/SiO2 diagrams. A mean potential temperature of θ = 1.50 ± .05°C was found for the inflowing water. A multi-box model including circulation, mixing, and chemical source terms in the deep water has been formulated. Linear programing and least-squares techniques have been used to obtain the transport and source parameters of the model from the observed tracer fields. Model calculations reveal an inflow through the Romanche Trench from the west Atlantic, which predominates over any other inflow, of (5 ± 2) Sv (potential temperature 1.50°C), a convective turnover of (150 ± 50) years and a vertical apparent diffusivity of (4 ± 1) cm2/s. Chemical source terms are in the expected ranges.

Secular Variations of Cosmogenic 14C on Earth: Their Discovery and Interpretation

Measurement of 14C in samples of wood of precisely known age have shown that the cosmogenic 14C content of the CO2 in the atmosphere has not remained constant during the period of time covered by radiocarbon dating. As the terrestrial atmosphere mixes with a time constant of less than 3 years, these variations must be essentially independent of geographic location. The 14C in atmospheric CO2 must be a quantity that, at a given time, pertains to the terrestrial atmosphere as a whole. Not only is its knowledge necessary for deriving accurate radiocarbon dates, but it is also valuable in connection with many geophysical and astrophysical problems. Unfortunately, progress in our knowledge of 14C variations in the terrestrial atmosphere has been delayed by hidden experimental errors in results obtained by many laboratories. By rigorous statistical analysis of the La Jolla results, it is now possible to show that the 14C variations are not simple random fluctuations but show distinct regularities. Similar patterns of variations have been found in the growth rate of trees during the last 5000 years. Measurements of radiogenic 10Be currently being done by European workers promise to conclusively elucidate the 14C findings.

Carbon Isotopes in Atmospheric CO2 of the Krakow Region: A Two-Year Record

We have been measuring concentration of atmospheric CO2 and its carbon isotope composition in the Kraków region for about two years. Samples are continuously collected at two-week intervals at ca 20m above ground level, close to the center of the industrialized urban area. Sampled atmospheric CO2 is sorbed in a molecular sieve and, after recovery by heating, is converted to benzene. 14C is measured in a liquid scintillation spectrometer, and δ13C of the CO2 is determined in a mass spectrometer. The annual record shows winter-summer variation of 14C, 13C, and CO2 concentration. A long-term trend for 1983 and 1984 indicates a slight decrease of 14C activity (1 22.0 in January 1983; − 1.2% per year), a permanent decrease of δ13CPDB (−9.3‰ in January 1983; −0.3% per year) and an increase of CO2 concentration (344 ppm in January 1983; 1.4ppm per year).

Extension of the Holocene Dendrochronology by the Preboreal Pine Series, 8800 to 10,100 BP

Holocene tree-ring chronologies have been established for south-central Europe covering the past 11,000 years. The Hohenheim absolute oak chronology extends to 4089 bc. The C-calibrated mid-Holocene floating oak master covers a 3181-year period from ca 4045 to 7225 bc. The earliest well-replicated floating oak master (estimated calendar age 7215 to 7825 bc) extends the European oak dendrochronology back to Boreal times.Further extension of the Holocene dendrochronology has been achieved by subfossil oak and pine trees from the Rhine, Main, and Danube Rivers. A 774-year floating series of Preboreal pine has been established. 14C ages range (from younger to older end) from 9200 to 9800 BP. Within this series a major atmospheric 14C variation is indicated, resulting in nearly constant 14C ages (9600 BP) over a period of 370 tree-rings.The European oak and pine tree-ring chronologies cover without major gaps the entire Holocene epoch. Based on the length of the dendro-records, an approximate solar year age of 11,280 years is calculated for the Holocene/Pleistocene boundary.The Preboreal pine forests along the rivers were replaced by mixed oak forests between 9200 and 8800 BP. By linking the earliest oak masters and the Preboreal pine series, the European dendrochronology can be extended up to the end of Late Glacial times.

Annual Variations of the 14C Content of Soil CO2

A 6-year and a 2-year record of 14C measurements of soil CO2 in two soils are presented and discussed. The annual 14C variation of soil CO2 is controlled by the seasonally varying contribution of root respiration and of microbial decomposition of organic matter producing soil CO2. The Δ14C soil CO2 difference between summer and winter is ca 50‰ in a soil where turnover of organic matter is fast (τ = 2.5a) and ca 100‰ in a soil of slow turnover (τ = 60a). A simple model describing the movement and turnover of organic matter is derived, giving the depth distributions of organic carbon and of 14C. The model needs a subdivision of the carbon reservoir into at least two reservoirs with residence times of τ1 = la and τ2 = 100a, respectively, and with a vertical transfer velocity in the order of 0.6mm/a.

Magnetic studies of erosion in a Scottish lake catchment. 1. Core chronology and correlation1

Magnetic susceptibility, 210Pb, 137Cs, 14C, and paleomagnetic secular variation are used to establish the chronology of sedimentation in a set of cores from Loch Frisa in western Scotland. The 14C dates obtained are not compatible with the chronology derived from all the other techniques and this is ascribed to inwash of “old” particulate carbon from the watershed. Central cores show little evidence of changes in sedimentation rate over the last 150 years whereas marginal cores contain evidence of major increases arising from ploughing and from drainage associated with catchment afforestation since 1935. The results illustrate the value of a multiple core approach to sedimentation and erosion studies even where complex lake morphometry precludes calculation of sediment budgets.

Bomb 14C in the Ocean Surface 1966–1981

Bomb 14C has been used as a tracer for CO2 in ocean surface water to study CO2 exchange between atmosphere and ocean. Using ordinary cargo ships for sampling, we have been able to cover some parts of the Atlantic, Pacific, and Indian Oceans for certain periods. A total number of 520 samples from 89 locations were measured during the last 15 years. The data are presented both in tables and graphs. A maximum 14C concentration (Δ14C) of ca 20% was observed in temperate northern latitudes, and a few per cent lower at southern latitudes. A seasonal trend in the 14C variation, with summer maximum and winter minimum, was observed both in downwelling and upwelling areas.