Michael Andree

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.

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.

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.

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.

Can the Greenland climatic jumps be identified in records from ocean and land

Sharp jumps in climate punctuate the records from borings in the Greenland ice cap during the time interval 60,000 to about 20,000 yr ago. Rapid fluctuations are also seen in foraminifera records for cores from the northern Atlantic and in a pollen record from a core from a bog in the Vosges Mountains in France. In this paper we present a new radiocarbon chronology for northern Atlantic deep-sea core V23-81 which permits comparison with the radiocarbon-dated Vosges Mountains pollen record. Because of the lack of a 14C chronology for the Greenland ice record and of distortions peculiar to each of the three records, it is not yet possible to say whether or not the events are genetically related.

14C dating of polar ice

Abstract We have dated ice core samples by 14C AMS to show that this method extends other ice dating methods and enables direct comparison with 14C related climatic events found elsewhere. For the measurement we use the CO2 that was occluded together with the air at the time of ice formation. To extract the CO2 we crush the samples with a milling cutter. For a 14C AMS measurement the CO2 content of about 10 kg of cleaned ice (0.25 cm3) has to be converted to amorphous carbon. First 14C results on the Dye 3 (Greenland) ice core are in good agreement with other age determinations.

14C measurements on foraminifera of deep sea core V28-238 and their preliminary interpretation

Abstract In this paper first results obtained by AMS dating foraminifera are presented. The amount of material used for these studies was 7 to 10 mg calcium carbonate. A preliminary interpretation is given.

Target preparation for milligram sized 14C samples and data evaluation for AMS measurements

Abstract Our preparation technique produces in a glow-discharge an amorphous carbon deposit on a copper substrate. The process starts with 1.6 cm3 CO2 STP (900 μg carbon) which is reduced over hot zinc to CO and subsequently cracked in the discharge. The yield of the process is typically 80%. With these targets in the Zurich ion source ion currents up to 20 μA are obtained. The background of samples prepared with this technique is presently around 30 ka (2.5% MODERN). The precision after half an hour measuring time for a modern sample is 0.7% and 2.7% for a three half-lives old sample, including the errors of the background and the NBS oxalic acid measurement. The method we use to correct for the background of the preparation and the accelerator as well as for the fractionation in the accelerator is presented.

10Be and 14C in the Earth system

In a very short period of time, 10Be data have significantly improved our knowledge in various fields of Earth and planetary sciences. Examples are: (a) solar modulation of isotope production, revealed in 10Be ice-core profiles; (b)geomagnetic m odulation of isotope production, revealed in 10Be ice-core (from the past 10 ka) and ocean-sediment profiles (geomagnetic reversals); (c) climatic effects reflected in 10Be profiles in loess and polar ice cores (10Be behaviour in atmosphere); (d) comparison of 10Be and 14C variations (tree rings) from carbon-cycle models and inform ation on ocean circulation history from 14C m easurements on benthic and planktonic Foram inifera in ocean sediments. An overview on work in collaboration with the Zurich AMS facility (with Professor W. Wolfli) is given.In a very short period of time,