A.F.M. de Jong

Accelerator mass spectrometry with 14C and 10Be in utrecht

Abstract The Utrecht facility for accelerator mass spectrometry is now in operation for routine measurements of 14C and 10Be in natural samples. Sample preparation techniques have been introduced. A 1% precision for 14 C 12 C ratios is routinely achieved. In the last year, more than 500 samples have been prepared and measured for various applications in archeology, geology and paleontology.

Precision and mass fractionation in 14C analysis with AMS

Modifications of the Utrecht AMS set-up have resulted in 14C analysis with a routine 0.4% analytical precision and a detection limit of 2.0 × 10−15. In analysis with 10Be, 26Al and 36Cl the precision is a few percent and the detection limit 10−14. Measurements on small carbon samples (< 0.4 mg) revealed a sample-mass dependent isotope fractionation effect, which together with the background limits the precision for the smallest samples (0.02 mg) to a few percent.

The late Quaternary sedimentary record of Reykjanes Ridge, North Atlantic

Variability in surface and deep ocean circulation in the North Atlantic is inferred from grain-size characteristics and the composition of terrigenous sediments from a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of grain size data shows that deep-ocean circulation in this area decreased significantly during periods of maximum iceberg discharge. The episodes of reduced circulation correlate with the cold and abrupt warming phases of the Dansgaard-Oeschger cycles as recognized in the Greenland ice cores.

Dry extraction of 14CO2 and 14CO from Antarctic ice

Abstract A dry extraction method was used to obtain trapped CO2 of 2–5 kg ice samples from a blue ice zone in East Antarctica. In situ produced 14C was also extracted in 14CO2 and 14CO concentrations at a ratio of 3.4 ± 0.9. Correction of trapped 14CO2 from in situ resulted in ice dates in the range 5–15 ka. The realistic rates for accumulation and ablation of ice indicate about total efficiency of extraction.

A novel approach for developing high-resolution sub-fossil peat chronologies with 14C dating

Sub-fossil sections from a Florida wetland were accelerator mass spectrometry (AMS) dated and the sedimen- tological conditions were determined. 14C data were calibrated using a combined wiggle-match and 14C bomb-pulse approach. Repeatable results were obtained providing accurate peat chronologies for the last 130 calendar yr. Assessment of the different errors involved led to age models with 3-5 yr precision. This allows direct calibration of paleoenvironmental proxies with meteorological data. The time frame in which 14C dating is commonly applied can possibly be extended to include the 20th century.

In situ produced 14C by cosmic ray muons in ablating Antarctic ice

Abstract Samples of a core (52 m) of ablating Antarctic ice were analysed for 14CO and 14CO2 by accelerator mass spectrometry. The data were compared with a 14C in situ production model that includes muon capture in addition to oxygen spallation by neutrons. The analysis reveals significant in situ 14C at depths below 10 m, which we attribute to 14C production by cosmic ray muons. The age of the ice was determined as 9.3±0.4 14C ka BP.

Kau Bay, Halmahera, a late quaternary palaeoenvironmental record of a poorly ventilated basin

Abstract Kau Bay is a small marine basin in between the two northern arms of the Island of Halmahera (Indonesia), separated from the adjacent Philippine Sea by a sill of 40 m deep. Microfossil and radiocarbon data of piston cores indicate that Kau Bay was a freshwater lake in Weichselian times. At 10 ka BP, the Bay became reconnected with the open ocean. Present-day deep water in Kau Bay is poorly ventilated because the sill limits water exchange with the adjacent ocean. Ventilation occurs through occasional inflow of denser oceanic water, but when this fails oxygen depleted conditions rapidly arise. The permanent presence of oxygen tolerant deep water benthic foraminifers over the past 10 ka indicates that dysoxic conditions prevalied and that anoxic conditions, as observed during the Snellius-I Expedition (May 1930), apparently existed for a short time only and left no trace that could be resolved with our time-resolution of 35 years. Slight and long-term variations in the degree of oxygen deficiency most likely are caused by long-term changes in the density of inflowing oceanic water, possibly in consequence of climatic oscillations. The homothermal surface water of Kau Bay shelters a low-diversity planktonic foraminiferal fauna of primarily shallow-dwelling species. This specific association extends back over the past 8 ka indicating that surface waters remained homothermal over much of the Holocene. Surface water productivity most likely changed with variations in river discharge.

Indications of pleistocene man on Sardinia

Human fossils found in a Pre-Neolithic cave deposit (Corbeddu cave, Sardinia) represent the first human remains associated with an endemic impoverished island fauna. Radiocarbon dating by AMS in Utrecht provided the chronological framework of the cave sediments for better understanding of the time-related human activities. The aberrant morphology of the human fossils and the unique character of worked deer bones discovered, suggest the development of an endemic Pleistocene human culture, adapted to the restricted island conditions and the hunting of ochotonids and deer.

Terrigenous supply of 10Be and dating with 14C and 10Be in sediments of the Angola basin (SE Antlantic)

Abstract 10Be concentrations were measured in 20 (hemi)pelagic core top samples with an age control partly based on AMS 14C datings. The 10Be concentrations do not show any dependence on the bulk mass accumulation rates (MAR), but there is a significant linear correlation between 10Be accumulation rates and MAR. This correlation is best explained by terrigenous 10Be supply modified by biologic scavenging and, of secondary importance, adsorption on particulate matter in the ocean surface water in the Zaire plume. The variable term of the linear regression function indicates a maximum 10Be concentration for the terrigenous component of 5 × 109 at.g−1, the constant suggests an oceanic 10Be precipitation of (100–530) × 106 at.cm−2 ka−1. A 500 ka long record of 10Be concentrations is in agreement with the predicted values, but short-term variations in MAR obscure the expression of a long-term break in MAR about 350 ka ago.

Very little in situ produced radiocarbon retained in accumulating Antarctic ice

Ice samples from Dronning Maud Land, Antarctica, were analyzed for 14 CO2 and 14 CO by accelerator mass spectrometry. Only a small amount (2%) of in situ produced radiocarbon was detected. The calibrated radiocarbon ages, corrected for in situ produced 14 C, are in fair agreement with age estimates obtained from stratigraphical methods added to a gas inclusion model. The ages of the entrapped air range from recent to ca. 1200 AD. ” 2000 Elsevier Science B.V. All rights reserved.