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Anthropogenic radionuclides, like 236U and 239,240Pu, are present in the environment as a result of global fallout from nuclear weapons tests conducted in the 1950s and 1960s and can potentially be used as tracers in soil erosion and sediment movement studies. Here, we report data on 236U and 239,240Pu in soil samples from the Motueka Valley (New Zealand) and for the first time from two remote islands Rarotonga and Atiu (Cook Islands) in the South Pacific. 236U and 239,240Pu were measured using Accelerator Mass Spectrometry (AMS) at the Australian National University. The 236U and 239Pu isotope concentrations versus soil depth and the 240Pu/239Pu and 236U/239Pu isotope ratios are discussed for each site. The radionuclide depth dependence revealed any soil disturbance, whereas the isotopic signatures indicated the source of the radionuclides' origin.

The 240Pu/239Pu atom ratio is a very effective tool for the identification of the origin of plutonium (Pu) in the soil environment. We examine a dataset of 240Pu/239Pu atom ratios determined from surface and core soils at 240 sites across China. The data were compiled from 18 separate literature sources from the last 20 years. For the first time the spatial distribution (3 latitude bands and 7 natural regions) of the weighted average 240Pu/239Pu atom ratios in Chinese soils is investigated. An area to the West of Xining City, shows a weighted average 240Pu/239Pu atom ratio of 0.167 ± 0.002, lower than that of average global fallout, which likely arises from the addition of local fallout radionuclides from the Chinese nuclear weapon tests at Lop Nor between 1964 and 1980. The Yumen and Jiuquan areas of Northwest China in particular show evidence of very low ratio material from the Chinese nuclear weapon tests. Excluding the impacted area around the test site the weighted average 240Pu/239Pu ratio of 0.182 ± 0.002 suggests that global fallout is the main source of Pu in most Chinese soils.

Three state-of-the-art ionization chambers have recently been designed at the Australian National University (ANU) to further enhance the isotopic and isobaric separation capabilities of the AMS setup at the Heavy Ion Accelerator Facility (HIAF). Two compact split-anode ionization chambers with low noise preamplifiers allow for measurements of actinides and other radioisotopes that do not have atomic isobar interferences. Both assemblies sit inside Dependex-100-Tees and are constructed such that they are retractable from the ion beam under vacuum. A high energy resolution version will be used for Pu, 129I and 55Fe. The second detector serves as the final energy detector in a time-of-flight setup and is employed for 236U and 210Pb detection. The new Flexible-AntiScattering Multi-Anode (FASMA) detector is a 7-anode ionization chamber optimized for the discrimination of 53Mn from 53Cr in the gas-filled magnet setup with the ENGE-split-pole spectrograph. The entire setup was simulated with Raytrace and SRIM. Finally, an existing 8-anode ionization chamber has been upgraded and now provides a 93Zr-93Nb isobar suppression of 5 × 104 resulting in the best limit of detection for 93Zr of any AMS-facility. This article details the various features of these detectors and compares first experimental results to performance values expected from modelling.

Beryllium-10 (10Be) in quartz represents the most common in situ cosmogenic nuclide used for quantifying Earth-surface processes, primarily due to the prevalence of quartz in the Earth's crust. However many landscapes lack quartz-bearing rocks, thus other nuclide-mineral pairs are required for geochronometric and geomorphic applications. Here we describe the successful isolation and measurement of in situ 10Be concentrations in pyroxene from two mafic sample sets: (i) andesite boulders of the Murimotu Formation debris avalanche on Mt. Ruapehu, New Zealand, and (ii) dolerite cobbles deposited in a ∼100 m vertical transect at Mt. Gran by Mackay Glacier, Antarctica. Precise radiocarbon age constraint of the New Zealand site provides further geological constraint of the reference (at sea level and high latitude) 10Be production rate in pyroxene, which we find to be indistinguishable from a previous estimate. Combining our results with previous data yields a reference production rate of 3.2 ± 0.8 at. g−1 yr−1 (n=5; ‘Lm’ scaling). Application of this rate to the glacial cobbles at Mackay Glacier yields a relatively coherent chronology of ice surface lowering between ∼14 and 6 ka, which is broadly consistent with a well-constrained quartz-based 10Be chronology from nearby nunataks. Improving the viability of in situ 10Be for geological applications in mafic domains requires increased analytical precision beyond current levels. This improvement may be best achieved by further modification of the quartz-based methodologies for 10Be purification, in order to better handle the high cationic contaminant loads of ferromagnesian minerals. In addition, further 10Be measurements from suitable mafic sedimentary deposits with independent age control (e.g. existing cosmogenic 3He calibration sites) will help to refine estimates of the reference production rate.

Geological climate archives from the Holocene Epoch provide baseline information concerning natural climate variability. Temperate mountain glacier extent is sensitive to summer air temperature, thus geological records of past glacier length changes are a useful proxy for this climatic variable. Here we present a new cosmogenic 10Be chronology of glacier length changes at Dart Glacier in the Southern Alps, New Zealand. Prominent moraines deposited 321 ± 44 yr ago (n = 11) and 7.8 ± 0.3 ka (n = 5) show glaciers during the Little Ice Age were less extensive than during the early Holocene. This pattern of net Holocene glacier retreat is consistent with emerging data from other catchments in New Zealand and across the southern mid-latitudes. Using the physical framework of a transient global climate model simulation, we suggest that cool summers in the early Holocene were promoted by the local summer insolation minimum, together with low atmospheric greenhouse gas concentrations, causing an early Holocene austral glacial maximum. An insolation-driven reduction in seasonality at southern mid-latitudes may reconcile differences between early Holocene temperature reconstructions where climate proxies have different seasonal sensitivities. We suggest that rising greenhouse gas concentrations after 7 ka caused regional-scale glacier retreat and appear to be the dominant driver of multi-millennial summer temperature trends in the southern mid-latitudes during the present interglacial.

At present there is a need for the development of new radioisotopes for soil erosion and sediment tracing especially as fallout 137Cs levels become depleted. Recent studies have shown that 239Pu can be a useful new soil erosion and sediment radioisotope tracer. 239Pu was released in the major atmospheric nuclear weapons tests of 1950's and 1960's. However 239Pu has a half-life of 24110 years and more than 99% of this isotope is still present in the environment today. In contrast 137Cs with a half-life of 30.07 year has decayed to In this study an assessment of the 239Pu fallout in Australia was carried out from comparison of measured 239Pu inventories with expected 239Pu inventories from fallout models. 239Pu inventories were also compared with rainfall and measured 240Pu/239Pu ratios across Australia. 239Pu fallout inventories ranged from 430 to 1461 μB/cm2. Central Australia, with fallout 107% in excess of expected values, seems to be strongly impacted by local fallout deposition. In comparison other sites typically show 5–40% variation between expected and measured fallout values. The fallout inventories were found to weakly correlate (using power functions, y = axb) with rainfall with r2 = 0.50 across the southern catchments (25–40°S latitude band). Across the northern catchments (10–25°S latitude band) fallout showed greater variability with rainfall with r2 = 0.24. Central Australia and Alice Springs which seem to be strongly impacted by local fallout are excluded from the rainfall correlation data (with these sites included r2 = 0.08 and r2 240Pu/239Pu atom ratios range from 0.045 to 0.197, with averages of 0.139(0.017), 0.111(0.052) and 0.160(0.027) in the 10–20°S, 20–30°S and 30–40°S latitude bands respectively. The 240Pu/239Pu atom ratios in Central Australia (0.069) likely represent fallout from the Australian tests which also have low 240Pu/239Pu atom ratios i.e., Maralinga (0.113) and Montebello (0.045). The average ratios in the 20–30°S and 30–40° bands are closer to the global average (0.139 and 0.177 respectively when not including the close-in fallout data from the nuclear test sites) if the Australian test sites and Central Australian sites are neglected as they clearly represent the effects of close in fallout.

A slice from a Porites Lutea coral core collected inside the Enewetak Atoll lagoon, within 15 km of all major nuclear tests conducted at the atoll, was analysed for 236U, 239Pu and 240Pu over the time interval 1952-1964 using a higher time resolution than previously reported for a parallel slice from the same core. In addition two sediment samples from the Koa and Oak craters were analysed. The strong peaks in the concentrations of 236U and 239Pu in the testing years are confirmed to be considerably wider than the flushing time of the lagoon. This is likely due to the growth mechanism of the coral. Following the last test in 1958 atom concentrations of both 236U and 239Pu decreased from their peak values by more than 95% and showed a seasonal signal thereafter. Between 1959 and 1964 the weighted average of the 240Pu/239Pu atom ratio is 0.124 ± 0.008 which is similar to that in the lagoon sediments (0.129 ± 0.006) but quite distinct from the global fallout value of ∼0.18. This, and the high 239,240Pu and 236U concentrations in the sediments, provides clear evidence that the post-testing signal in the coral is dominated by remobilisation of the isotopes from the lagoon sediments rather than from global fallout.

A comprehensive series of nuclear tests were carried out by the United States at Enewetak Atoll in the Marshall Islands, especially between 1952 and 1958. A Porites Lutea coral that was growing in the Enewetak lagoon within a few km of all of the high-yield tests contains a continuous record of isotopes, which are of interest (e.g. 14C, 236U, 239,240Pu) through the testing period. Prior to the present work, 14C measurements at ∼2-month resolution had shown pronounced peaks in the Δ14C data that coincided with the times at which tests were conducted. Here we report measurements of 236U and 239,240Pu on the same coral using accelerator mass spectrometry, and again find prominent peaks in the concentrations of these isotopes that closely follow those in 14C. Consistent with the 14C data, the magnitudes of these peaks do not, however, correlate well with the explosive yields of the corresponding tests, indicating that smaller tests probably contributed disproportionately to the debris that fell in the lagoon. Additional information about the different tests can also be obtained from the 236U/239Pu and 240Pu/239Pu ratios, which are found to vary dramatically over the testing period. In particular, the first thermonuclear test, Ivy-Mike, has characteristic 236U/239Pu and 240Pu/239Pu signatures which are diagnostic of the first arrival of nuclear test material in various archives.

The isotopes (236)U and (239)Pu, both produced during nuclear weapons tests carried out in the 1950s and 1960s, are present in the environment and may be used as tracers for soil erosion studies. Although these radionuclides occur only at ultra-trace levels in nature, they can be readily measured by accelerator mass spectrometry with the 14UD heavy ion accelerator at the Australian National University. We have analysed a series of vegetation samples for their (236)U and (239)Pu concentration and compared the results with those found in the surrounding soil. (236)U could be measured in all collected samples whereas (239)Pu could not be detected in several vegetation samples due to its very low concentration, although it was readily detectable in the soil. We find that, relative to plutonium, (236)U is preferentially taken up by plants with enrichment factors ((236)U/(239)Pu)veg/((236)U/(239)Pu)soil that range between 7 and 52 in the present study.

The Daly River drains a large (52500 km2) and mainly undisturbed catchment in the Australian wet–dry tropics. The basin landscapes are mantled by a thick veneer of kandosol soil which has developed under varying rates of erosion, uplift, bedrock type and climate and has been identified as being suitable for agriculture. Commencement of large scale clearing and cropping since 2002 have raised concerns about the increased loss of top soil from the land clearing and cultivation activities adjacent to the Daly River. This study was undertaken to determine the modern soil loss rates which can be used to develop a sustainable soil conservation strategy for this catchment. 239Pu, released in the 1950s and 1960s by atmospheric nuclear weapons tests, is used to obtain a quantitative assessment of recent rates of soil loss. Soil cores 30–40 cm deep have been collected from fields with various land uses including peanut and hay cropping and cattle grazing. Cores taken from undisturbed and unburnt areas in open eucalypt woodland have been used as reference sites. The soil loss rates have been established by comparing the excess or deficiency of the 239Pu tracer over that of the reference sites. Since land use practices in the catchment are similar, it is likely that the measured soil loss rates are indicative of soil loss rates over the Daly Basin as well. The development of 239Pu as a soil tracer represents a viable alternative to the traditionally used 137Cs tracer. This also represents a new tool in the quantification of catchment soil loss and the adoption of appropriate soil conservation strategies for the tropical regions and regions where increasing settlement and agriculture are encroaching on catchment slopes.

In order to perform higher precision measurements, an upgrade of the ANU accelerator is underway. Fast switching times on the low-energy side, with maximum settling times of 30 ms, are achieved by holding the injector magnet field constant while changing the energy of the different isotopes by changing the pre-acceleration voltage after the ion source. Because ions of the different isotopes then have different energies before injection, it is necessary to adjust the strength and steering of the electrostatic quadrupole lens that focusses the beam before entry into the accelerator. First tests of the low-energy system will be reported. At the high energy end, a larger vacuum box in the analyzing magnet has been designed, manufactured and installed to allow the transport of differences in mass as large as 10% at constant terminal voltage. For the cases where more than one isotope must be transported to the detector an additional refinement is necessary. If the accelerator voltage is to be kept constant, then the trajectories of the different isotopes around both the analyzing and switching magnets must be modified. This will be achieved using bounced electrostatic steerers before and after the magnets. Simulations have been performed with the ion optic code COSY Infinity to determine the optimal positions and sizes of these steerers.

The motivation of this work is to establish a new multi-isotope plutonium standard for isotopic ratio measurements with accelerator mass spectrometry (AMS), since stocks of existing solutions are declining. To this end, certified reference materials (CRMs) of each of the individual isotopes 239Pu, 240Pu, 242Pu and 244Pu were obtained from JRC IRMM (Joint Research Center Institute for Reference Materials and Measurements). These certified reference materials (IRMM-081a, IRMM-083, IRMM-043 and IRMM-042a) were diluted with nitric acid and mixed to obtain a stock standard solution with an isotopic ratio of approximately 1.0:1.0:1.0:0.1 (239Pu:240Pu:242Pu:244Pu). From this stock solution, samples were prepared for measurement of the plutonium isotopic composition by AMS. These samples have been measured in a round-robin exercise between the AMS facilities at CologneAMS, at the ANU Canberra and ETH Zurich to verify the isotopic ratio and to demonstrate the reproducibility of the measurements. The results show good agreement both between the different AMS measurements and with the gravimetrically determined nominal ratios.

In the present work, state of the art isotopic fingerprinting techniques are applied to an Arctic ice core in order to quantify deposition of U and Pu, and to identify possible tropospheric transport of debris from former Soviet Union test sites Semipalatinsk (Central Asia) and Novaya Zemlya (Arctic Ocean). An ice core chronology of 236 U, 239 Pu, and 240 Pu concentrations, and atom ratios, measured by accelerator mass spectrometry in a 28.6 m deep ice core from the Austfonna glacier at Nordaustlandet, Svalbard is presented. The ice core chronology corresponds to the period 1949 to 1999. The main sources of Pu and 236 U contamination in the Arctic were the atmospheric nuclear detonations in the period 1945 to 1980, as global fallout, and tropospheric fallout from the former Soviet Union test sites Novaya Zemlya and Semipalatinsk. Activity concentrations of 239 + 240 Pu ranged from 0.008 to 0.254 mBq cm − 2 and 236 U from 0.0039 to 0.053 μBq cm − 2 . Concentrations varied in concordance with 137 Cs concentrations in the same ice core. In contrast to previous published results, the concentrations of Pu and 236 U were found to be higher at depths corresponding to the pre-moratorium period (1949 to 1959) than to the post-moratorium period (1961 and 1962). The 240 Pu/ 239 Pu ratio ranged from 0.15 to 0.19, and 236 U/ 239 Pu ranged from 0.18 to 1.4. The Pu atom ratios ranged within the limits of global fallout in the most intensive period of nuclear atmospheric testing (1952 to 1962). To the best knowledge of the authors the present work is the first publication on biogeochemical cycles with respect to 236 U concentrations and 236 U/ 239 Pu atom ratios in the Arctic and in ice cores.

A combination of state-of-the-art isotopic fingerprinting techniques and atmospheric transport modelling using real-time historical meteorological data has been used to demonstrate direct tropospheric transport of radioactive debris from specific nuclear detonations at the Semipalatinsk test site in Kazakhstan to Norway via large areas of Europe. A selection of archived air filters collected at ground level at 9 stations in Norway during the most intensive atmospheric nuclear weapon testing periods (1957–1958 and 1961–1962) has been screened for radioactive particles and analysed with respect to the concentrations and atom ratios of plutonium (Pu) and uranium (U) using accelerator mass spectrometry (AMS). Digital autoradiography screening demonstrated the presence of radioactive particles in the filters. Concentrations of 236U (0.17–23 nBq m−3) and 239 + 240Pu (1.3–782 μBq m−3) as well as the atom ratios 240Pu/239Pu (0.0517–0.237) and 236U/239Pu (0.0188–0.7) varied widely indicating several different sources. Filter samples from autumn and winter tended to have lower atom ratios than those sampled in spring and summer, and this likely reflects a tropospheric influence in months with little stratospheric fallout. Very high 236U, 239 + 240Pu and gross beta activity concentrations as well as low 240Pu/239Pu (0.0517–0.077), 241Pu/239Pu (0.00025–0.00062) and 236U/239Pu (0.0188–0.046) atom ratios, characteristic of close-in and tropospheric fallout, were observed in filters collected at all stations in Nov 1962, 7–12 days after three low-yield detonations at Semipalatinsk (Kazakhstan). Atmospheric transport modelling (NOAA HYSPLIT_4) using real-time meteorological data confirmed that long range transport of radionuclides, and possibly radioactive particles, from Semipalatinsk to Norway during this period was plausible. The present work shows that direct tropospheric transport of fallout from atmospheric nuclear detonations periodically may have had much larger influence on radionuclide air concentrations and deposition than previously anticipated. Keywords: Atmospheric dispersion modelling, Source identification, Plutonium, Uranium-236, Atom ratio

On the South Island of New Zealand, large piedmont glaciers descended from an ice cap on the Southern Alps onto the coastal plain of the West Coast during the late Pleistocene. The series of moraine belts and outwash plains left by the Taramakau glacier are used as a type section for interpreting the glacial geology and timing of major climatic events of New Zealand and also as a benchmark for comparison with the wider Southern Hemisphere. In this paper we review the chronology of advances by the Taramakau glacier during the last or Otira Glaciation using a combination of exposure dating using the cosmogenic nuclides 10 Be and 36 Cl, and tephrochronology. We document three distinct glacial maxima, represented by the Loopline, Larrikins and Moana Formations, separated by brief interstadials. We find that the Loopline Formation, originally attributed to Oxygen Isotope Chronozone 4, is much younger than previously thought, with an advance culminating around 24,900 ± 800 yr. The widespread late Pleistocene Kawakawa/Oruanui tephra stratigraphically lies immediately above it. This Formation has the same age previously attributed to the older part of the Larrikins Formation. Dating of the Larrikins Formation demonstrates there is no longer a basis for subdividing it into older and younger phases with an advance lasting about 1000 years between 20,800 ± 500 to 20,000 ± 400 yr. The Moana Formation represents the deposits of the last major advance of ice at 17,300 ± 500 yr and is younger than expected based on limited previous dating. The timing of major piedmont glaciation is restricted to between ∼25,000 and 17,000 yr and this interval corresponds to a time of regionally cold sea surface temperatures, expansion of grasslands at the expense of forest on South Island, and hemisphere wide glaciation.

Some potential applications of the isotopes 36 Cl and 236 U require higher sensitivity than is normally required or achieved. For 36 Cl, one such application is the measurement of the 36 Cl/Cl ratio of seawater. We show that, with appropriate precautions to reduce the effect of ion-source memory, it is possible to reach blank levels below 10 −16 in the 36 Cl/Cl ratio. Measurements of a series of seawater and sea salt samples yield a blank-corrected 36 Cl/Cl ratio of present-day sea water of 0.71 ± 0.08 × 10 −15 . For 236 U, one potential application is as a prospecting tool, where sensitivities in the 236 U/ 238 U ratio at the 10 −13 level are desirable. We demonstrate that this sensitivity can be achieved for 24 MeV uranium ions with time-of-flight detection over a 6.0 m flight path.

The 240Pu/239Pu ratio of the global plutonium fallout from atmospheric nuclear weapons testing is typically in the range 0.17–0.19. However, the influence of regional nuclear installations or nearby weapons test sites can lead to local values outside this range. We report 240Pu/239Pu ratios at 14 representative sites across the Australian continent, and find that the weapons tests carried out in Australia appear to have made a significant contribution to the total fallout in the center of the continent, despite their relatively small explosive yield.

The technique of accelerator mass spectroscopy (AMS) has been employed to determine modern soil loss rates through the analysis of 239Pu profiles in soil cores from the Daly basin in Northern Territory, Australia. In areas in which soil conservation banks were not present or were only added recently (

S.Merchel et al., The first international round-robin exercise for the measurement of the long-lived radionuclide 10Be has been conducted. Ten participating accelerator mass spectrometry (AMS) facilities have each measured three samples at the 10−12 to 10−14 10Be/9Be level. All results have been made traceable to the NIST SRM 4325 standard to avoid additional discrepancies that arise when different facilities use different calibration materials. Hence, the data concentrates on pure measurement distinctions. Multivariate statistical investigations have been performed to reveal a bias between facilities, i.e. two distinguished groups could be identified. Maximum discrepancies between two single facilities are in the range of 6–31% depending on the absolute 10Be/9Be value. These findings should be considered when comparing 10Be data produced at one AMS facility with that produced at another facility, which is e.g. often the case for in situ 10Be dating studies. Round-robin exercises are a very helpful tool as part of an overall quality assurance scheme to improve the accuracy, and not only the precision, of AMS data., This work was partially funded within the framework of CRONUS-EU (Marie-Curie Action 6th framework programme; Contract No.: 511927) and by a German–French exchange programme (DAAD/ÉGIDE through Procope FRALAMS; Project Nos.: 500 888 61 and 22077QC).

The construction of high resolution chronologies of sediment profiles corresponding to the last 50-100 years usually entails the measurement of fallout radionuclides (210)Pb and (137)Cs. The anthropogenic radionuclide, (137)Cs, originating from atmospheric nuclear weapons testing can provide an important "first appearance" horizon of known age (1954-1955), providing much-needed validation for the sometimes uncertain interpretations associated with (210)Pb geochronology. However, while (137)Cs usually provides a strong signal in sediment in the northern hemisphere, total fallout of (137)Cs in the southern hemisphere was only 25% that of the north and the low activities of (137)Cs seen in Australian and New Zealand sediments can make its horizon of first appearance somewhat arguable. Low (137)Cs fallout also limited the size of the 1963-1964 fallout peak, a peak that is usually seen in northern hemisphere sediment profiles but is often difficult to discern south of the equator. This paper shows examples of the use of nuclear weapons fallout Pu as a chronomarker in sediment cores from Australia (3 sites) and New Zealand (1 site). The Pu profiles of five cores are examined and compared with the corresponding (137)Cs profiles and (210)Pb geochronologies. We find that Pu has significant advantages over (137)Cs, including greater measurement sensitivity using alpha spectrometry and mass spectrometric techniques compared to (137)Cs measurements by gamma spectrometry. Moreover, Pu provides additional chronomarkers associated with changes in the Pu isotopic composition of fallout during the 1950s and 1960s. In particular, the (238)Pu/(239+240)Pu activity ratio shows distinct shifts in the early 1950s and the mid to late 1960s, providing important known-age horizons in southern hemisphere sediments. For estuarine and near-shore sediments Pu sometimes has another significant advantage over (137)Cs due to its enrichment in bottom sediment relative to (137)Cs resulting from the more efficient scavenging of dissolved Pu in seawater by sediment particles.

A sediment core collected from the sub-aqueous delta of the Yangtze River estuary was subjected to analyses of 137 Cs and plutonium (Pu) isotopes. The 137 Cs was measured using γ-spectrometry at the laboratories at the Nanjing University and Pu isotopes were determined with Accelerator Mass Spectrometry (AMS), measurements made at the Australian National University. The results show considerable structure in the depth concentration profiles of the 137 Cs and 239+240 Pu. The shape of the vertical 137 Cs distribution in the sediment core was similar to that of the Pu. The maximum 137 Cs and 239+240 Pu concentrations were 16.21 ± 0.95 mBq/g and 0.716 ± 0.030 mBq/g, respectively, and appear at same depth. The average 240 Pu/ 239 Pu atom ratio was 0.238 ± 0.007 in the sediment core, slightly higher than the average global fallout value. The changes in the 240 Pu/ 239 Pu atom ratios in the sediment core indicate the presence of at least two different Pu sources, i.e., global fallout and another source, most likely close-in fallout from the Pacific Proving Grounds (PPG) in the Marshall Islands, and suggest the possibility that Pu isotopes are useful as a geochronological tool for coastal sediment studies. The 137 Cs and 239+240 Pu inventories were estimated to be 7100 ± 1200 Bq/m 2 and 407 ± 27 Bq/m 2 , respectively. Approximately 40% of the 239+240 Pu inventory originated from the PPG close-in fallout and about 50% has derived from land-origin global fallout transported to the estuary by the river. This study confirms that AMS is a useful tool to measure 240 Pu/ 239 Pu atom ratio and can provide valuable information on sedimentary processes in the coastal environment.

The Mt Giluwe shield volcano was the largest area glaciated in Papua New Guinea during the Pleistocene. Despite minimal cooling of the sea surface during the last glacial maximum, glaciers reached elevations as low as 3200 m. To investigate changes in the extent of ice through time we have re-mapped evidence for glaciation on the southwest flank of Mt Giluwe. We find that an ice cap has formed on the flanks of the mountain on at least three, and probably four, separate occasions. To constrain the ages of these glaciations we present 39 new cosmogenic 36 Cl exposure ages complemented by new radiocarbon dates. Direct dating of the moraines identifies that the maximum extent of glaciation on the mountain was not during the last glacial maximum as previously thought. In conjunction with existing potassium/argon and radiocarbon dating, we recognise four distinct glacial periods between 293–306 ka (Gogon Glaciation), 136–158 ka (Mengane Glaciation), centred at 62 ka (Komia Glaciation) and from >20.3–11.5 ka (Tongo Glaciation). The temperature difference relative to the present during the Tongo Glaciation is likely to be of the order of at least 5 °C which is a minimum difference for the previous glaciations. During the Tongo Glaciation, ice was briefly at its maximum for less than 1000 years, but stayed near maximum levels for nearly 4000 years, until about 15.4 ka. Over the next 4000 years there was more rapid retreat with ice free conditions by the early Holocene.

The latter part of the Last Glacial Interval (LGI; 60,000 to 11,700 years ago) experienced a range of climatic and environment extremes. To elucidate the mechanisms of these changes requires records of past variability that are precisely dated and correlated on the same absolute timescale. However, despite decades of research, it is still not possible to align most marine and terrestrial records of past change with ice-core records, largely because of ongoing uncertainties over the conversion of pre-Holocene 14C ages on to a calendar timescale and uncertainties with in ice-core chronologies. As a result, it is equivocal whether climate changes in both hemispheres during the LGI led, lagged or were synchronous with one another. A decadally-resolved radiocarbon calibration is urgently required to test these models of the Earth system. Here we report radiocarbon measurements obtained from subfossil New Zealand kauri (Agathis australis) spanning a collective 3500 years dated between 25,000 and 45,000 years ago. The results are compared to the recently published international calibration curve IntCal09 and to the marine Cariaco Basin datasets. We show that kauri have considerable potential for development of a Southern Hemisphere component of a unified global calibration curve suite, and that tree-ring sequences can be superposed on other radiocarbon records to constrain atmospheric–marine offsets and precisely test the synchronicity of abrupt climate change.

The Yangtze River is the largest single source of sediment to the continental shelf of the East China Sea. The quantity of material exported by the river is expected to decrease substantially as a consequence of an extensive continuing program of dam construction within the river catchment. We report here AMS measurements of plutonium isotope concentrations and ratios for selected depth increments from a sediment core, collected from the sub-aqueous delta of the Yangtze River estuary. The Pu derives from atmospheric nuclear weapons testing in the 1950s and 1960s, and is potentially a useful tracer of sediment deposition times in the marine environment. The results show considerable structure in the depth-concentration profile, and offer an excellent opportunity to compare Pu with the more commonly used 137 Cs isotopic tracer. The AMS data show superior sensitivity and indicate that the 240 Pu/ 239 Pu ratio can provide a check on the deposition dates. The changes in the 240 Pu and 239 Pu concentrations and the 240 Pu/ 239 Pu ratios with sediment depth all indicate the possibility of using Pu as a geochronological tool for coastal sediment studies.

Plutonium fallout from atmospheric nuclear-weapons testing in the 1950s and 1960s constitutes an artificial tracer suitable for the study of recent soil erosion and sediment accumulation rates. Traditionally 137Cs has been the fallout isotope of choice for such studies, but the plutonium isotopes confer a number of advantages, which can be realised using the ultra-sensitive detection technique of Accelerator Mass Spectrometry (AMS). As a first application of plutonium to a whole-of-basin study, Pu has been measured in both soil and sediment across the catchment of the Herbert River, which is one of the major rivers draining into Australia’s Great Barrier Reef Lagoon. Its catchment includes undisturbed areas as well as regions of pasture and sugar cultivation. The Pu measurements allow the relative contributions of surface and gully erosion from the different land use areas to be determined, and permit the discharged material to be apportioned between the relevant sources.

AMS measurement of 32 Si can allow for ice core dating over the last thousand years. Technique developments are reported. Necessary negative-ion yields of 20–30% can now be consistently achieved, and permit an overall efficiency from ice sample to detector of ∼1%. A 30 Si-spike technique has overcome the problem of extremely low intrinsic silicon concentration, with the added benefit of allowing determination of ppb-level silicon via isotope dilution. Improvements have also been made to the ionization detector in the gas-filled magnet that separates the accelerated 32 Si ions from the intense flux of 32 S ions. Preliminary 32 Si AMS results of snow and ice samples from Mt. Cook National Park, New Zealand, are reproducible, and with 32 Si concentrations 1.2–7.2 mBq/m 3 comparable to results from mid-latitude snow samples measured previously via the radiometric technique, demonstrating the feasibility of the method. With these developments, the potential of 32 Si as ice core dating tool is close to being realized, and attempts to determine chronologies for both alpine and Antarctic glaciers are underway.

To study the Pu concentration and isotope ratio distributions present in China, the (239+240)Pu total activities and (240)Pu/(239)Pu atom ratios in core soil samples from Hubei Province in central China were investigated using Accelerator Mass Spectrometry (AMS). The activities ranged from 0.019 to 0.502 mBqg(-1) and the (239+240)Pu inventories of 45 and approximately 55 Bqm(-2) agree well with that expected from global fallout. The (240)Pu/(239)Pu atom ratios in the soil ranged from 0.172 to 0.220. The ratios are similar to typical global fallout values. Hence, any close-in fallout contribution from the Chinese nuclear weapons tests, mainly conducted in the 1970s, must have either been negligible or had a similar (240)Pu/(239)Pu ratio to that of global fallout. The top 10 cm layer of the soil contributes approximately 90% of the total inventory and the maximum concentrations appeared in the 2-4 cm or 4-6 cm layers. It is suggested that climatic conditions and organic content are the two main factors that affect the vertical migration of plutonium in soil.

The nucleogenic isotopes 36 Cl, 236 U and 239 Pu are produced naturally in subsurface environments via neutron capture of thermal and epithermal neutrons. Concentrations are, however, very low and accelerator mass spectrometry (AMS) is required for quantitative measurements. A particular challenge is presented by the measurement of 236 U/ 238 U ratios down to the level of 10 −13 that is expected from rocks with low uranium concentration. Here, we present the AMS methodology that has been developed at the ANU for measuring 236 U/ 238 U ratios at this level. The more established methodologies for 36 Cl and 239 Pu measurements are also summarised. These capabilities are then used to characterize the 36 Cl, 236 U and 239 Pu concentrations in a range of uranium ores. A simple model of the neutron production and capture processes in subsurface environments has been developed and is presented. It is shown that nucleogenic 36 Cl, 236 U and 239 Pu can be used to determine both thermal and epithermal neutron fluxes in subsurface environments. Potential applications include uranium exploration and monitoring of the environmental impact of uranium mining.

Following atmospheric nuclear weapons testing in the 1950s and 1960s significant quantities of 137 Cs and 239+240 Pu were deposited worldwide. In recent decades, 137 Cs has been commonly used as a tracer of soil erosion and sedimentation, particularly in the Northern Hemisphere where atomic deposition was three times as great as in the Southern Hemisphere. The relatively short 30-year half-life of this isotope means that its sensitivity as a tracer is rapidly decreasing. In contrast, with half-lives of 24,110 and 6561 years, the sensitivity of the two plutonium isotopes remains essentially the same as when it was deposited. Here we use the technique of Accelerator Mass Spectrometry to demonstrate the potential of anthropogenic Pu as an alternative to 137 Cs as a tracer of soil transport in Australia. We measure an average 137 Cs/ 239+240 Pu activity ratio of 27.3 ± 1.5 and an average 240 Pu/ 239 Pu atom ratio of 0.149 ± 0.003, both slightly lower than the global average.

The cosmogenic isotope 53Mn is produced by spallation of iron in surface rocks. The long half life of this isotope makes it attractive for use in erosion rate studies in slowly eroding landscapes such as Australia. We describe the development of AMS methods for detection of 53Mn using the 14UD accelerator at the Australian National University. The first step of this development involved the production of 53Mn using a heavy-ion fusion–evaporation reaction to make test standards. Then, the chemistry protocol for isolating 53Mn and reducing the Cr levels, of which 53Cr is a serious interfering isobar, was developed. Lastly we employed a gas-filled magnet which was used to discriminate 53Mn from the intense 53Cr background.

The potential of the 26 Al isotope for exposure and burial dating applications has not been fully realized to date due to the low efficiency for producing the Al − ion in negative ion sources. Here, we show that it is possible to use the AlO − ion instead, which is produced an order of magnitude more efficiently, provided that a gas-filled magnet is employed to reduce the intense flux of 26 Mg ions at the detector. It is then possible to achieve comparable counting statistics for 26 Al as for the much more widely used 10 Be.

In order to detect the long-lived radioisotope 182 Hf ( t 1/2 = 8.9 Ma), a potential supernova isotope signature, we investigated PXAMS as a method of isobar separation for Hf and W. One of the problems of the method is low efficiency. Since there are no reliable quantitative predictions on cross-sections leading to the emission of X-rays from collisions involving fast Hf ions, we determined the energy dependent cross-sections for a number of target elements. A PXAMS system of maximum efficiency was then constructed on the basis of the data obtained. The efficiency of the final setup, defined as the ratio of the number of X-rays detected to the number of incident Hf ions, is 1.2%. The overall efficiency, including negative-ion formation probability and transmission through the accelerator, is 1 × 10 −6 .

The rare isotopes 236 U and 239 Pu are produced naturally by neutron capture in uranium ores. Here we measure 236 U and 239 Pu by accelerator mass spectrometry (AMS) in the same ore samples for the first time. To ensure efficient extraction of both elements and isotopic equilibrium between the 239 Pu in the ore and a 242 Pu spike, we developed a new sample preparation protocol. AMS has clear advantages over previous methods because it achieves better discrimination against molecular interferences with higher sensitivity and shorter counting times. Measurements of 236 U and 239 Pu hold considerable promise as proxy indicators of neutron flux and uranium concentration.

A nearby supernova would deposit radionuclides on earth. The long-lived radionuclide 182Hf (t1/2 = 8.9 Ma) is one of a number of candidates for an isotopic signature of such an event. Together with 60Fe, observation of 182Hf would be direct evidence for a supernova site of the r-process. The most suitable site for searching for such a signature would be a deep-sea sediment of slow deposition rate. Measurement of 182Hf at the anticipated level requires very effective suppression of the interfering stable isobar 182W. Chemical separation and the injection of HfF 5 - allow for suppression by several orders of magnitude, but more is needed for detection of 182Hf as a supernova isotope signature. We are currently developing AMS methods for measuring 182Hf/180Hf isotope ratios at the required level using a 15 MV tandem accelerator . Both projectile X-ray emission and the use of a solid passive absorber with a subsequent measurement of the residual energy are being explored. The former suffers from low efficiency, but the latter looks promising.

Sample preparation and AMS methodology for 99Tc have been refined with the aim of improving sensitivity to permit measurements on as little as 0.25 l of seawater. Sample preparation improvements have been directed towards minimizing 99Ru and maximizing the yield of the 99TcO− ion from the ion source. A range of chemical extraction techniques and the purity of various reagents have been explored. In order to monitor the chemical yield, the γ-emitting isotope 95mTc (T1/2=61 days), produced by the 95Mo(p,n)95Tc reaction on a natural Mo foil, was added as a tracer. On the AMS side, a new gas-filled ionization detector has been developed. This differs from that used previously in having an anode plane optimised for 99Tc, which makes eight measurements of the energy loss along the ion track. A new algorithm for maximizing the separation of 99Tc and 99Ru from the resulting multiparameter data has been developed, and allows rejection of >90% of the 99Ru ions while retaining >90% of the 99Tc counts.

We present results of the atmospheric mercury deposition rate in the Amazon region during the last 43 000 years. Lake sediment samples were collected from the Lagoa da Pata, a small and remote lake in northern Brazilian Amazon. The samples were divided in sub-samples, for C, Hg, N and 14C-AMS analyses. Three main paleoclimatic events could be identified. The mercury accumulation rates were found to be larger during the periods of the Holocene and Pleistocene associated with high temperatures and frequency of forest fires.

Plutonium fallout from atmospheric nuclear weapons testing in the 1950s and 1960s constitutes an artificial tracer suitable for the study of recent soil erosion and sediment accumulation rates. Long-lived Pu isotopes provide an alternative tracer to the more widely used 137Cs (t1/2=30 a), the concentration of which is decaying at a rate that will limit its long-term application to these studies. For 239,240Pu, the sensitivity of AMS is more than an order of magnitude better than that afforded by α-spectroscopy. Furthermore, AMS can provide a simple, direct measure of the 240Pu/239Pu ratio. Sample profiles from two sites along eastern Australia have been determined with both AMS and α-spectroscopy to provide comparative measurements of the sediment accumulation rate in water bodies and of the soil erosion rate. The two methods are in good agreement. The 228Ra/226Ra ratio potentially provides a probe for tracing the dispersion of uranium mining residues into the neighboring environment. Soil depth profiles of the ratio may provide information on the rate at which mining-derived radioactivity is spread by surface waters, and could be used to assess the effectiveness of remediation and rehabilitation technologies. AMS offers several advantages over the more usual α- and γ-spectroscopy techniques in that it can directly and quickly measure both isotopes in a sample of small size and with simple sample preparation. We show that AMS can be used to measure these isotopes of radium at the sensitivity required for environmental samples using RaC2− as the injected beam species.

Natural cosmogenic 36Cl found in seawater originates from spallation of atmospheric 40Ar, capture of secondary cosmic-ray neutrons by dissolved 35Cl, and river runoff which contains 36Cl produced in situ over the surface of the continents. The long residence time of chloride in the ocean and long half-life of 36Cl compared to the oceanic mixing time should result in a homogenous 36Cl/Cl ratio throughout the ocean. Production by neutron capture in the course of nuclear weapons testing should be insignificant averaged over the oceans as a whole, but may have led to regions of elevated 36Cl concentration. Previous attempts to measure the 36Cl/Cl ratio of seawater have been hindered by interferences, contamination, or insufficient analytic sensitivity. Here we report preliminary measurements on seawater samples, which demonstrate that the 36Cl/Cl ratio is 0.5 ± 0.3 × 10−15, in reasonable agreement with calculated contributions from the sources listed above.