C. Alderliesten

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.

Aridity in equatorial Africa during the last 225,000 years; a record of opal phytoliths/ freshwater diatoms from the Zaire (Congo) deep-sea fan (Northeast Angola Basin).

Maps of accumulation rates of freshwater diatoms and opal phytoliths in the surface sediments of the Zaire fan show that both types of microfossils were supplied to the ocean floor by the Zaire River, and that opal phytoliths also have a southern source, probably the region of the Namibian desert. The PhFD ratio, of opal phytoliths to freshwater diatoms, can be regarded as an aridity index for equatorial Africa, and probably for large parts of the central and southern Atlantic. In two cores, the record of the PhFD ratio indicates humidity ca 225-190 ka BP, aridity 190-135 ka BP with maxima ca 170 and 140 ka BP and a humid excursion 150 ka BP, an increase in humidity 115 ka BP, a less humid period 90-30 ka BP, more humidity ca 30-17 ka BP with possibly more arid intervals ca 22.5 and 20 ka BP. In general, glacial (sub) stages were more arid and interglacial (sub) stages more humid. For the last 20 ka, the PhFD ratio corresponds closely with the known climatic events in tropical Africa.

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.

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.

AMS measurements of 10Be and 26Al for studying shielding effects in meteorites

Abstract The 10 Be activites in 21 diogenites vary from 17 to 27 dpm/kg. These variations can primarily be ascribed to shielding effects, as was deduced from their correlation with the 22 Ne/ 21 Ne ratio. The 10 Be production rates in small diogenites are 10–30% higher than was predicted by a semi-empirical model based on the Knyahinya chondrite. This implies that 10 Be is less shielding dependent than expected. Our first AMS measurements of 26 Al in chondrites agree with earlier 26 Al γ-ray results.

A CCBA description of the (p, t) reaction to low-lying O+ states in the Ge isotopes

Differential cross sections for the (p, t) reaction to 0+ states in the even Ge isotopes have been measured at an incident energy (Ep = 13 MeV) where the triton energy is below the Coulomb barrier. The data are described by a coupled-channels calculation with wave functions consisting of neutron configurations only. Sequential transfer processes can explain the anomalies, which have led previously to the introduction of shape isomerism or proton excitations.

In-situ radiocarbon production by neutrons and muons in an antarctic blue ice field at Scharffenbergbotnen: a status report

In the radiocarbon accelerator mass spectrometry ( (super 14) C AMS) analysis of gases obtained in a dry extraction from a 52-m Antarctic ice core, we observed (super 14) CO (sub 2) and (super 14) CO concentrations decreasing with depth. The concentrations are explained in terms of in-situ production by neutrons and captured muons in ablating ice. The ratio of the (super 14) CO (sub 2) concentration to that of (super 14) CO has been found to be constant at 1.9+ or -0.3. The ablation rates obtained of 42+ or -18 cm.yr (super -1) and 40+ or -13 cm.yr (super -1) for the neutron and muon components, respectively, are about three times higher than observed from stake readings. The discrepancy may point to an incomplete extraction of the dry extraction method. Using the constant ratio in (super 14) CO (sub 2) and (super 14) CO concentrations we correct for the in-situ component in the trapped (super 14) CO (sub 2) and deduce an age of 10,300+ or -900 BP for the ice core.