H. Oeschger

Increase of Atmospheric Methane Recorded in Antarctic Ice Core

Air entrapped in bubbles of cold ice has essentially the same composition as that of the atmosphere at the time of bubble formation. Measurements of the methane concentration in air extracted by two different methods from ice samples from Siple Station in western Antarcitica allow the reconstruction of the history of the increase of the atmospheric methane during the past 200 years.

Information on the CO2 Cycle from Ice Core Studies

Information on the history of the atmospheric CO2 content and the 2SC/12 and 14C/C ratios is recorded in natural ice. Measurements on samples from very cold accumulation regions show that CO2 is occluded not only in air bubbles, but also in the ice lattice. The two CO2 components are of similar size. It is very difficult to measure CO2 in the bubbles and CO2 in the ice lattice separately. By melting the samples and extracting the evolving gases in two fractions, it is possible to estimate CO2 concentration in the bubbles and the ice lattice. Enrichment or depletion of CO2 in the bubbles by exchange with the ice is difficult to estimate. Information about this effect is expected from 13C/12C analysis on the extracted CO2 fractions. To investigate whether atmospheric CO2 content was different during the last glaciation than during the present one, sets of 16 and 20 samples distributed over the last 40,000 years from the two deep ice cores from Camp Century (North Greenland) and Byrd Station (West Antarctica) were measured. The time scales for the two cores -are Tbahsee dd aotna ase rrhieeso lforogmica bl omtho dceol.r eRs esshuolwts sainmdi lcaor ntcrleunsdiosn cso arrreel: ated to a certain degree to - tFhoer 5b7o8t0h pcroorfeilse, s.t he values for the CO2 concentration of the first fraction, considered to best represent the atmospheric composition, show lower values during glaciation - tLhoawn CinO 2t hceo nHcoelnotcraetnieo,n ws iinth tha em fiinrsitm furamct iboenfso r(e2 00thpep men) do fo cf egrltaacinia stiaomnp. les are a strong indication that the atmospheric CO2 concentration during last glaciation was lower than during the postglacial. These low concentrations indicate that, at that time, CO2 concentration in the atmosphere could have been lower than today by a factor of 1.5. Possible explanations for such a change in atmospheric CO., content as well as its influence on atmospheric 14C/C ratio and on the radiation balance is discussed.

super 14) C dating of plant macrofossils in lake sediment.

Macrofossils of terrestrial plants have been picked from a sediment core taken in Lake Lobsigen, a small lake on the Western Swiss Plateau. The sediments were previously analyzed for pollen composition, plant and animal macrofossils, and stable isotopes. Plant macrofossils were selected near pollen zone boundaries in Late Glacial and early Postglacial sediment for C-14 dating by AMS. In the same lake carbonate and gyttja (aquatic plant) samples were dated by decay counting. The dates on terrestrial material are generally younger than those on carbonate and gyttja, ie, material reflecting the C-14/C ratio of dissolved bicarbonate in lake water. This is probably due to a contribution of dissolved limestone carbonate and thus a somewhat reduced C-14/C, ratio in the lakes water (hard water effect).

Radiocarbon measurements on coexisting benthic and planktic foraminifera shells: potential for reconstructing ocean ventilation times over the past 20 000 years

Abstract In this paper the potential of AMS 14C dating of shells handpicked from deep sea sediments is explored. We show that while the age difference between planktonic (surface dwelling) and benthic (bottom dwelling) shells must carry information regarding paleocirculation rates, this message is likely obscured by effects associated with the coupling between bioturbation and dissolution and between bioturbation and abundance change. It is also possible that the 14C/12C ratio in planktonic shells was initially not identical to that in surface water and that the 14C/12C ratio in benthic shells was initially not identical to that in bottom water. These and other biases will plague all attempts to extract the desired information regarding circulation rate changes over the last 20000 years. However in sorting them out, much will be learned about the origin and history of the calcite particles found in deep sea sediments.

10Be in Chinese loess

We report a first series of10Be measurements on Chinese loess covering the last 750,000 years. The10Be concentration shows a tendency to decrease with increasing grain size. Leaching experiments with acid solutions show that10Be is strongly bound to particles and therefore mobility due to dissolution can be excluded. The10Be record in loess shows features similar to those observed inδ180 records from deep-sea sediments which strongly reflect climatic variations. This allows one to correlate the10Be loess records withδ180 deep-sea records and to establish a time scale. Based on this time scale, accumulation rates of loess and10Be fluxes can be calculated. These reveal generally higher values for cold, arid and windy periods than for warm, humid and calmer intervals.

Precision measurements of 14C in AMS — some results and prospects

Abstract Some of the factors affecting the precision in AMS measurements will be discussed and the specific developments undertaken to reduce the errors at the ETH facility are described. Based on a large number of 14C measurements we show the present limitations of our system and consider what improvements might be possible. Further, a comparison is made between high precision measurements of 14C and current achievements relating to Be, Al and Cl.

Secular non-random variations of cosmogenic carbon-14 in the terrestrial atmosphere

Abstract The time dependence of the14C content of bristlecone pine wood samples dated by their tree rings and grown during the last 8000 years was examined. The14C values as measured by the La Jolla Radiocarbon Laboratory were used for the investigation. Two different smoothing techniques were used for constructing values for equal time intervals. In this manner the introduction of regularities, that could have resulted from applied mathematical techniques, could be excluded. There is good evidence for non-random features in the power spectrum, in particular for a 200-year periodicity. The regularities in the power spectrum are further indications supporting the assumption that the14C variations reflect a property of the sun.

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 DELTA-C-14 in the glacial-postglacial transition, 14,000 - 10,000 BP, a range not covered by the tree-ring calibration curve. We measured C-14/C-12 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 C-14 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 requires a reliable, C-14-independent correlation of the cores, obtained through local pollen zone boundaries. C-14 ages were obtained as a function of the depth in the cores. If sedimentation rates are known, ages can be converted into DELTA-C-14 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 DELTA-C-14 curve shows an increase with time during the Allerod and decreases during Preboreal and Bolling periods. Probabilities for these C-14 variations are discussed.

Seasonal variations in the concentration of 10Be, Cl−, NO3−, SO42−, H2O2, 210Pb, 3H, mineral dust, and σ18O in greenland snow

Abstract A detailed snow pit study at Dye 3, South Greenland, has been carried out in order to investigate the seasonal variations of 10Be, Cl−, NO3−, SO42−, H2O2, 210Pb, 3H, Mn and σ18O. Special emphasis was placed on understanding the causes of 10Be variations because this isotope can be used to trace the history of solar activity. A sampling interval of 5 cm was chosen to assure a mean time resolution of about 20 samples per year for the period 1978–1983. Four different categories of seasonal variations were identified: strong summer peaks (σ18O, H2O2, 3H), weak bimodal peaks (NO3−, SO42−, conductivity, 10Be), a fall peak (210Pb) and a winter-spring peak (Cl−, Mn). Although we are still far from a detailed understanding of the mechanisms which control the measured compositions, there are indications that different processes dominate at different seasons. NO3−, SO42−, Mn and Cl− variations suggest that aerosol transport is most important during winter-spring. In summer-fall there seems to exist a second period of long range transport from low latitude continental areas resulting in enhanced concentrations of 210Pb, 10Be and SO42−.

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

Abstract 10 Be 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 14 C 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 14 C concentration which has generally been attributed to a changing geomagnetic dipole moment. This discrepancy implies that the 14 C 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.