Richard G. Cresswell

Cosmogenic chlorine-36 from calcium spallation

Calcium is a major target element for cosmogenic 36Cl production. Consequently 36Cl rapidly reaches detectable levels in minerals such as calcite and calcium feldspar exposed at the Earths surface. Spallation of calcium isotopes typically accounts for 80–90% of 36Cl production in these minerals, with subsidiary contributions from negative muon capture by 40Ca and thermal neutron capture by 35Cl. To provide a basis for surface exposure dating, we have calibrated cosmogenic 36Cl production in calcium feldspar from the 17,300 year old Tabernacle Hill basalt. At an altitude of 1445 m and an effective geomagnetic latitude of 40.9 ° the calcium spallation rate is 152 ± 11 atoms (g Ca)−1 a−1. The corresponding rate at sea level and high latitude is estimated at 48.8 ± 3.4 atoms (g Ca)−1 a−1. The muon capture rate used to derive these values is 8.8 ± 2.2 atoms (g Ca)−1 a−1 at the Tabernacle Hill site, scaled from a value of 4.8 ± 1.2 atoms (g Ca)−1 a−1 at sea level and high latitude. The calcium spallation rate determined in this study is in excellent agreement with previous whole-rock calibration measurements at Tabernacle Hill, when these are recalculated with respect to the absolute timescale. The calibration of 36C1 production from calcium underpins development of an exposure dating technique for calcite. Due to its high calcium content, the 36Cl production rate in calcite is higher than in any other common rock-forming mineral. Measurement of 36Cl in calcite, with an accelerator mass spectrometric detection limit of ~5 × 103 atoms per gram, allows dating of limestone surfaces exposed for periods ranging from ~102–106 years. Alternatively, erosion rates from less than 1 to greater than 1000 μm per year can be determined in the case of eroding karst surfaces. Though the 36Cl production rate is lower in calcium feldspar than in calcite, measurements on this mineral will provide a useful means of dating young basalt lavas.

Cosmogenic Chlorine-36 Production in Calcite by Muons

At depths below a few metres, 36Cl production in calcite is initiated almost entirely by cosmic ray muons. The principal reactions are (1) direct negative muon capture by Ca; 40Ca(μ−,α)36Cl, and (2) capture by 35Cl of secondary neutrons produced in muon capture and muon-induced photodisintegration reactions. We have determined rates for 36Cl and neutron production due to muon capture in calcite from a 20 m (5360 g cm−2) depth profile in limestone. The 36Cl yield from muon capture by Ca in pure calcite is 0.012 ± 0.002 atom per stopped negative muon. The surface production rate of 36Cl by muon capture on Ca in calcite is, therefore, 2.1 ± 0.4 atom g−1a−1 at sea level and high latitude, approximately 11% of the production rate by Ca spallation. If it is assumed that 34% of the negative muons are captured by the Ca atom in calcite, the α-yield from 40Ca following muon capture is 0.043 ± 0.008, somewhat lower than the result of a recent muon irradiation experiment (0.062 ± 0.020), but well within the extremes of existing theoretical predictions (0.0033–0.15). The average neutron yield following muon capture in pure calcite is 0.44 ± 0.15 secondary neutrons per stopped negative muon, in good agreement with existing theoretical predictions. Cosmogenic isotope production by muons must be taken into account when dating young geomorphic surfaces, especially those created by excavation of only a few metres of overlying rock. Attention to isotope production by muons is also crucial to determining surface erosion rates accurately. Due to the deep penetration of muons compared to cosmic ray hadrons, the accumulation of muon-produced 36Cl is less sensitive to erosion than that of spallogenic 36Cl. Although production by muons at the surface is only a small fraction of production by spallation, the fraction of muon-produced 36Cl in rapidly eroding limestone surfaces can approach 50%. In such cases, erosion rates estimated using conventional models which attribute production solely to spallation will be in error by up to 40%. The difference in sensitivity to erosion of spallogenic and muon-produced 36Cl suggests methods for dating deeply eroded surfaces, checking the assumption of steady-state when calculating erosion rates, and unravelling multi-stage exposure and erosion histories.

Accelerator mass spectrometry of plutonium isotopes

The feasibility of measuring plutonium isotope ratios by accelerator mass spectrometry has been demonstrated. Measurements on a test sample of known composition and on a blank showed that isotope ratios could be determined quantitatively, and that the present limit of detection by AMS is ∼ 106 atoms of plutonium. For 239Pu, this limit is at least two orders of magnitude lower than that practicable by alpha-spectrometry. In addition, 240Pu239Pu ratios were measured for four samples for which the combined activity of the two isotopes had been determined previously by alpha-counting. All measurements of plutonium isotope ratios entailed injection of PuO− into the 14UD accelerator operating at 3.5 MV, gas stripping, and analysis of the 7+ charge state after acceleration. Plutonium ions at 28 MeV were detected in a longitudinal-field ionisation chamber with an energy resolution of 3%. Using uranium oxide as a surrogate for plutonium oxide, it was shown that UO− was the predominant negative ion and that the probability for its formation and extraction was 0.3%.

Kinetics of uptake and elimination of silicic acid by a human subject: A novel application of 32Si and accelerator mass spectrometry

Silicon is possibly important in human physiology in protecting against the toxic effects of aluminium, but the kinetics of uptake and excretion of silicic acid, the bioavailable form, are not well characterised. We have used 32Si as a tracer in a human uptake experiment to determine a gastrointestinal uptake factor for silicic acid, and to elucidate the kinetics of renal elimination. Urine collections were made for extending intervals from 2 to 12 h over 2 days following ingestion by a single human subject of a neutral silicic acid solution containing tracer levels of 32Si (t1/2 approximately 150 y). Silicon was isolated as SiO2 and the 32Si content determined by accelerator mass spectrometry (AMS), using a gas-filled magnet technique to eliminate a prolific isobaric interference from 32S. Silicon uptake appears to have been essentially complete within 2 h of ingestion. Elimination occurred by two simultaneous first-order processes with half-lives of 2.7 and 11.3 h, representing around 90% and 10%, respectively, of the total output. The rapidly eliminated 32Si was probably retained in the extracellular fluid volume, whilst the slower component may represent intracellular uptake and release. Elimination of absorbed 32Si was essentially complete after 48 h and was equivalent to 36% of the ingested dose. This establishes only a lower limit for gastrointestinal absorption as, although there was no evidence for longer term retention of additional 32Si, the possibility could not be excluded by these results.

Denudation rates for the southern Drakensberg escarpment, SE Africa, derived from in-situ-produced cosmogenic 36C1: initial results

Cosmogenic 36C1 concentrations in basalt samples from the Drakensberg escarpment on the SE African passive margin give quantitative estimates of denudation and scarp retreat rates. Over the 104–106 year timespan addressed by these data, the calculated escarpment retreat rate has been 50–95 m Ma-1 and the average summit denudation rate 6 m Ma-1. The scarp retreat rate is an order of magnitude less than previously suggested. The rate of summit lowering is sufficient to prevent the long-term intact survival of erosion cycle surfaces formed in the Mesozoic that were previously inferred for this region.

Terrestrial vegetation change inferred from n-alkane σ13C analysis in the marine environment

Abstract We report gas chromatography-isotope ratio monitoring-mass spectrometry (GC-IRM-MS) measurements of the δ13C values of individual biomarker compounds (n-alkanes) extracted from a 3 m marine sediment core taken near the mouth of the Johnstone River, North Queensland, Australia. The technique allows a purely terrestrial isotope signal to be discerned despite mixing of terrestrial and marine-derived carbon. The results indicate that there has been a 2% increase in the δ13C values of terrestrially derived n-alkanes (C29−C33) since clearing of the forested Johnstone River drainage basin for sugarcane and pasture began in the late 19th century. A much slower ∼1% increase in δ13C values after 6,000 years BP and prior to European settlement may be related to a decrease in rainfall in the basin, or to an increase in the abundance of C4 plants as a result of increased aboriginal burning. The results from the sediment core are consistent with data obtained for modern river sediments from forested and cleared subcatchments within the basin, and demonstrate that the δ13C values of terrestrially derived n-alkanes in the marine environment can be used to assess basin-wide vegetation changes in adjacent river catchments on geological timescales.

The bioavailability of26Al-labelled aluminium citrate and aluminium hydroxide in volunteers

A study was undertaken to determine the fraction of ingested aluminium taken up by two male volunteers, following their ingestion of either aluminium citrate or aluminium hydroxide. In addition, the effects of simultaneous citrate ingestion on the gastrointestinal absorption of aluminium from its hydroxide was studied. Volunteers received three oral doses of26Al-labelled aluminium compound in water. The doses were administered directly into the stomach using a paediatric feeding tube. Blood samples were collected from the volunteers at 1, 4 and 24 h after administration, and their daily output of urine and faeces was collected for 6 days. These samples were analysed for their26Al content using either coincidence gamma-counting or accelerator mass spectrometry. The uptake of aluminium was greatest following its administration in the citrate form and was least following intake as the aluminium hydroxide suspension. The co-administration of citrate, with the aluminium hydroxide suspension, was found to enhance the levels of26Al uptake in both volunteers. Using a urinary excretion factor based on the results of previous studies, the fractional aluminium uptake from each of the species was calculated: aluminium citrate, 5.23 × 10−3; aluminium hydroxide, 1.04 × 10−4; aluminium hydroxide with citrate, 1.36 × 10−3.

Cosmogenic chlorine-36 production in K-feldspar

The high production rate of in-situ cosmogenic 36Cl from potassium allows K-rich minerals to be dated with high sensitivity and precision. K-rich minerals, such as K-feldspar and biotite are commonly associated with quartz in which 26Al and 10Be are produced, allowing three cosmogenic nuclides, with half-lives ranging from 0.3 Ma to 1.5 Ma, to be measured in a single sample. To calibrate the production rate of 36Cl from potassium we have measured K-feldspar samples from ice-scoured bedrock from three separate sites, covering a range of altitudes, latitudes and exposure ages. The results from each site are internally consistent, with eight glacial pavements from the Sierra Nevada giving a production rate of 1290 ± 220 atom/(gK)/yr at 3000 m and 38°N, three Scottish glacial pavement samples giving 301 ± 27 atom/(gK)/yr at 520 m and 58°N, and two Antarctic bedrock samples giving 1310 ± 50 atom/(gK)/yr at 2000 m and 70°S. Normalising these production rates to sea level and high latitude, results from the Sierra Nevada and Scotland are found to be in good agreement with each other, but the production rate derived from the Antarctic samples is found to be approximately 35% higher. Repeat measurements and stepwise dissolution indicate that the high 36Cl concentration of the Antarctic samples is real and cannot be attributed to meteoric 36Cl. We must therefore conclude that either the 36Cl production rate differ on the 104 and 106 yr time scales, or that the current altitude scaling factor applied to the Antarctic results underestimates the true scaling factor for the Antarctic atmosphere. The production rates derived above are based principally on K-feldspars with low chloride contents. Samples with high chloride contents required corrections for 36Cl produced by neutron capture on 35Cl. The 36ClCl ratio of this component was measured directly in four samples from the Sierra Nevada after crushing the K-feldspar to release Cl-rich fluid from inclusions. The 36ClCl ratios measured appear to be in reasonable agreement with values predicted by recent calibrations of the production by neutron capture on 35Cl, but the final comparison awaits measurements of uranium, thorium and neutron absorber (B, Gd, Sm) concentrations in our samples.

ACCELERATOR MASS SPECTROMETRY OF THE PLANETARY ELEMENTS

Abstract Accelerator mass spectrometry has been applied for the first time to the detection of 237 Np. Sensitivity approaches 10 5 atoms. A first measurement of the mobility of 237 Np in a marine environment is reported, and lends support to the prediction that neptunium should be substantially more mobile than plutonium. Measurements of backgrounds and transmissions for plutonium and neptunium in different charge states are also reported. In addition, the relative negative ion formation probabilities for the monoxide ions of Th, U, Np and Pu have been measured.

A Lateglacial age for the Main Rock Platform, western Scotland

The sea-level record preserved in ancient shorelines forms a basis for studies of tectonic uplift, glacial loading, and the changing volume of the oceans. The existing record is derived largely from depositional features such as beach ridges and coral reefs, which contain material suitable for radiometric dating. Erosional shorelines have proved more difficult to date. Direct age estimates for shore platforms can now be obtained with exposure-dating techniques based on cosmic-ray–produced isotopes. Here we report measurements of cosmogenic 36 Cl on the Main Rock Platform in western Scotland that indicate its formation in a postglacial event spanning less than a few thousand years. Together with isostatic modeling, the 36 Cl results suggest cutting during the Younger Dryas (in Britain, the “Lateglacial” or “Loch Lomond”) Stadial, when stable sea level and severe climatic conditions combined to enhance bedrock erosion.