Rapid thinning of Pine Island Glacier in the early Holocene.

Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet, has been undergoing rapid thinning and retreat for the past two decades. We demonstrate, using glacial-geological and geochronological data, that Pine Island Glacier (PIG) also experienced rapid thinning during the early Holocene, around 8000 years ago. Cosmogenic (10)Be concentrations in glacially transported rocks show that this thinning was sustained for decades to centuries at an average rate of more than 100 centimeters per year, which is comparable with contemporary thinning rates. The most likely mechanism was a reduction in ice shelf buttressing. Our findings reveal that PIG has experienced rapid thinning at least once in the past and that, once set in motion, rapid ice sheet changes in this region can persist for centuries.

Ultra-trace analysis of 36Cl by accelerator mass spectrometry: an interlaboratory study.

A first international (36)Cl interlaboratory comparison has been initiated. Evaluation of the final results of the eight participating accelerator mass spectrometry (AMS) laboratories on three synthetic AgCl samples with (36)Cl/Cl ratios at the 10(-11), 10(-12), and 10(-13) level shows no difference in the sense of simple statistical significance. However, more detailed statistical analyses demonstrate certain interlaboratory bias and underestimation of uncertainties by some laboratories. Following subsequent remeasurement and reanalysis of the data from some AMS facilities, the round-robin data indicate that (36)Cl/Cl data from two individual AMS laboratories can differ by up to 17%. Thus, the demand for further work on harmonising the (36)Cl-system on a worldwide scale and enlarging the improvement of measurements is obvious.

Slip-rate measurements on the Karakorum Fault may imply secular variations in fault motion.

Beryllium-10 surface exposure dating of offset moraines on one branch of the Karakorum Fault west of the Gar basin yields a long-term (140- to 20-thousand-year) right-lateral slip rate of approximately 10.7 +/- 0.7 millimeters per year. This rate is 10 times larger than that inferred from recent InSAR analyses ( approximately 1 +/- 3 millimeters per year) that span approximately 8 years and sample all branches of the fault. The difference in slip-rate determinations suggests that large rate fluctuations may exist over centennial or millennial time scales. Such fluctuations would be consistent with mechanical coupling between the seismogenic, brittle-creep, and ductile shear sections of faults that reach deep into the crust.

Changes in deep-water formation during the Younger Dryas event inferred from 10Be and 14C records.

Variations in atmospheric radiocarbon (14C) concentrations can be attributed either to changes in the carbon cycle--through the rate of radiocarbon removal from the atmosphere--or to variations in the production rate of 14C due to changes in solar activity or the Earth's magnetic field. The production rates of 10Be and 14C vary in the same way, but whereas atmospheric radiocarbon concentrations are additionally affected by the carbon cycle, 10Be concentrations reflect production rates more directly. A record of the 10Be production-rate variations can therefore be used to separate the two influences--production rates and the carbon cycle--on radiocarbon concentrations. Here we present such an analysis of the large fluctuations in atmospheric 14C concentrations, of unclear origin, that occurred during the Younger Dryas cold period. We use the 10Be record from the GISP2 ice core to model past production rates of radionuclides, and find that the largest part of the fluctuations in atmospheric radiocarbon concentrations can be attributed to variations in production rate. The residual difference between measured 14C concentrations and those modelled using the 10Be record can be explained with an additional change in the carbon cycle, most probably in the amount of deep-water formation.

Neutron discrepancies in the DS86 Hiroshima dosimetry system.

More than a decade has passed since a complete revision was initiated of the radiation doses received by survivors of the Hiroshima and Nagasaki atomic bombings. The new dosimetry system (DS86) was completed in 1986 and adopted shortly thereafter. Overall, DS86 was noted to be a clear improvement over the old dosimetry system. However, based on limited validation measurements, troublesome inconsistencies were suggested for neutrons. Since 1986, a substantial number of additional neutron activation measurements have been made in mineral and metal samples from Hiroshima. Importantly, a large number of measurements have now been made at distances beyond 1 km. Here, inconsistencies between neutron activation measurements and DS86 calculations for Hiroshima are examined using all available measurement data, including new measurements for 36Cl which extend the measurement range to more than 1.7 km from the epicenter, and Monte Carlo modeling calculations for each sample measured. Results show that thermal neutron activation measured beyond approximately 1 km in Hiroshima (at distances most relevant for radiation-risk evaluation) is two to 10, or more, times higher than that calculated based on DS86. Similar trends observed when comparing results by several independent measurement laboratories, using different analytical methods, suggest that the DS86 calculations for low-energy neutrons are in error. Because of the importance of the Hiroshima data in radiation risk evaluation, this large discrepancy is in need of resolution.

Accelerator mass spectrometry in biomedical dosimetry: relationship between low-level exposure and covalent binding of heterocyclic amine carcinogens to DNA.

Accelerator mass spectrometry (AMS) is used to determine the amount of carcinogen covalently bound to mouse liver DNA (DNA adduct) following very low-level exposure to a 14C-labeled carcinogen. AMS is a highly sensitive method for counting long-lived but rare cosmogenic isotopes. While AMS is a tool of importance in the earth sciences, it has not been applied in biomedical research. The ability of AMS to assay rare isotope concentrations (10Be, 14C, 26Al, 41Ca, and 129I) in microgram amounts suggests that extension to the biomedical sciences is a natural and potentially powerful application of the technology. In this study, the relationship between exposure to low levels of 2-amino-3,8-dimethyl[2-14C]imidazo[4,5-f]quinoxaline and formation of DNA adducts is examined to establish the dynamic range of the technique and the potential sensitivity for biological measurements, as well as to evaluate the relationship between DNA adducts and low-dose carcinogen exposure. Instrument reproducibility in this study is 2%; sensitivity is 1 adduct per 10(11) nucleotides. Formation of adducts is linearly dependent on dose down to an exposure of 500 ng per kg of body weight. With the present measurements, we demonstrate at least 1 order of magnitude improvement over the best adduct detection sensitivity reported to date and 3-5 orders of magnitude improvement over other methods used for adduct measurement. An additional improvement of 2 orders of magnitude in sensitivity is suggested by preliminary experiments to develop bacterial hosts depleted in radiocarbon. Expanded applications involving human subjects, including clinical applications, are now expected because of the great detection sensitivity and small sample size requirements of AMS.

Pattern of bombardment-produced radionuclides in rock 10017 and in lunar soil.

A large number of radionuclides have been measured as a function of depth in lunar rock 10017 and in bulk fines. Data are reported on (10)Be, (22)Na, (26)Al, (36)Cl, (49)V, (53)mn, (54)Mn (55)Fe, (56)Co, (57)Co, and (59)Ni and on upper limits for (46)Sc, (48)V, (51)Cr, and (60)Co. The results for several nuclides show striking evidence of excess surface production attributable to solar flare particles. Data for short-lived species, (56)Co, (57)CO, (54)Mn, (55)Fe, and (22)Na, appear consistent with fluxes from known recent events. Long-lived species demonstrate the existence of solar flare protons and alphas at least for the last 10(5) to 10(6) years, at fluxes comparable to those now observerved.