L.K. Fifield’s research while affiliated with Australian National University and other places

Loss of soil organic matter (SOM) from arable land poses a serious threat to soil fertility and crop yields, and it thwarts efforts to conserve soils as carbon sinks to mitigate global warming. Wind erosion can be a major factor in the redistribution of soil fines including SOM, but assessments of its impact have typically been limited by short observation periods of a few years at most. Longer time frames, extending back to the mid-20th century, may however be probed using the concentrations of radionuclides that were globally distributed by nuclear weapon tests conducted during the 1950s and early 1960s. The basic concept is that differences in fallout radionuclide (FRN) activities between undisturbed and arable soils can be used to infer soil particle redistribution. In the present work, we have measured activities of 137Cs and 239+240Pu in soils from three agricultural regions of the plains of the South African Highveld. The three regions represent distinct agroecosystems, and within each region the temporal length of cultivation varies from 0 (i.e. native grassland) to almost 100 years. Wind erosion has previously been shown to play a dominant role in soil particle loss from agricultural sites in the Highveld, and the level plots we investigate here did not show any evidence of fluvial erosion. Hence, we interpret the fate of soil fines, including SOM, to be governed by wind erosion. For the cultivated soils, radionuclide activities are found to be less than in adjacent native grassland, and the magnitude of the reduction is strongly correlated with the duration of cultivation. Specifically, the original inventories of both 137Cs and 239+240Pu are approximately halved after ∼20–40 years of cropping. The initial rate loss relative to the undisturbed soils is, however, considerably higher, with ∼6%yr-1 recorded during the first year after native grassland is converted to arable land. We correlate our radionuclide data with previously published SOM contents from the same sampled material and find that the radionuclides are an excellent indicator of SOM decline at the sites we investigate. We conclude that wind erosion can exert a dominant control on SOM loss in arable land of South Africa.

One of the remaining issues regarding the Anthropocene is the lack of stratigraphic evidence indicating when the cumulative human pressure from the early Holocene began to fundamentally change the Earth system. Herein, we compile anthropogenic fingerprints from various high-precision-dated proxy records for 137 global sites to determine the age of the unprecedented surge in these records over the last 7700 y. The cumulative number of fingerprints revealed an unprecedented surge in diverse anthropogenic fingerprints starting in 1952 ± 3 CE, corresponding to the onset of the Great Acceleration. Notably, the period from 1953 to 1958 CE saw a nearly simultaneous surge in fingerprints across all regions, including Antarctica, the Arctic, East Asia, Europe, North America, and Oceania. This synchronous upsurge reflects the moment when human impacts led to rapid transformations in various natural processes and cycles, with humans becoming a geological force capable of inscribing abundant and diverse anthropogenic fingerprints in global strata. Following this global fingerprint explosion, profound planetary-scale changes, including deviations from the established natural climatic conditions, begin. This unprecedented surge in anthropogenic signals worldwide suggests that human influences started to match many natural forces controlling the processes and cycles and overwhelm some of the functioning of the Earth system around 1952.

Losses of soil organic matter (SOM) from arable land poses a serious threat to soil fertility and crop yields, and thwarts efforts to conserve soils as carbon sinks to mitigate global warming. Wind erosion can be a major factor in the redistribution of soil fines including SOM, but assessments of its impact have typically been limited by short observation periods of a few years at most. Longer timeframes, extending back to the mid 20th century, may however be probed using the concentrations of radionuclides that were globally distributed by nuclear weapon tests conducted 1950s and early 1960s. The basic concept is that differences in fallout radionuclide (FRN) activities between undisturbed and arable soils can be used to infer soil particle redistribution. In the present work, we have measured activities of 137Cs and 239+240Pu in soils from three agricultural regions of the plains of the South African Highveld. The three regions represent distinct agroecosystems and within each region the temporal length of cultivation varies from zero (i.e., native grassland) to almost 100 years. The sampled plots did not show any evidence of fluvial erosion, allowing the contribution of wind erosion to the loss of soil fines, including SOM, to be investigated. For the cultivated soils, radionuclide activities are found to be less than in adjacent native grassland, and the magnitude of the reduction is strongly correlated with the duration of cultivation. Specifically, the original concentrations of both 137Cs and 239+240Pu are approximately halved after ~25–45 years of cropping. The initial rate loss relative to the undisturbed soils is, however, considerably higher, with ~6 % yr-1 recorded during the first year after native grassland is converted to arable land. We correlate our radionuclide data with previously published SOM contents from the same sampled material and find that the radionuclides are an excellent indicator of SOM decline at the sites we investigate. We conclude that wind erosion can exert a dominant control on SOM loss in arable land of South Africa and by implication at comparable settings on Earth.

The opportunity to measure the concentrations of ³H and ³⁶Cl released by the Fukushima nuclear accident in 2011 directly in rain was lost in the early stage of the accident. We have, however, been able to reconstruct the deposition record of atmospheric ³H and ³⁶Cl following the accident using a bore hole that was drilled in 2014 at Koriyama at a distance of 60 km from the accident. The contributions of ³H and ³⁶Cl from the accident are 1.4 × 10¹³ and 2.0 × 10¹² atoms m⁻² respectively at this site. Very high concentrations of both ³H (46 Bq L⁻¹) and ³⁶Cl (3.36 × 10¹¹ atoms L⁻¹) were found in the unsaturated soil at depths between 300 and 350 cm. From these, conservative estimates for the ³H and ³⁶Cl concentrations in the precipitation in the ~ 6 weeks following the accident were 607 Bq L⁻¹ and 4.74 × 10¹⁰ atoms L⁻¹, respectively. A second hole drilled in 2016 showed that ³H concentrations in the unsaturated soil and shallow groundwater had returned to close to natural levels, although the ³⁶Cl concentrations were still significantly elevated above natural levels.

Accelerator Mass Spectrometry (AMS) at the Department of Nuclear Physics and Accelerator Applications (Australian National University) is based on a 14UD tandem accelerator. The 14UD has demonstrated exceptional accelerator performance over more than three decades of AMS, e.g. by running regularly above 14 MV. We present the actual performance for the whole range of measured radionuclides and the potential of this specialised high-energy AMS facility - demonstrating the continuing need for such a system. Focus here will be also on the unique setup comprising an Enge split-pole spectrograph used as a gas-filled magnet (GFM). The GFM has seen growing importance at ANU over the years as a means for providing an efficient reduction of isobaric background. We use the Enge now routinely for ¹⁰Be, ²⁶Al, ³²Si, ⁵³Mn and ⁶⁰Fe measurements.

The break-up of molecules in the stripper of an AMS facility induces an energy and angular spread in the beam. This can cause beam losses, thereby reducing the performance of an AMS facility. With an appropriate design, these effects can be minimised and the efficiency enhanced. As an example, the break-up of BeO molecules is discussed here. First the basic physics is described and corresponding models and computer codes are explained. Then experimental data are shown and compared with the models. For gas stripping, beam profiles are available for small and large facilities. All show tails which can be attributed to the dissociation of the molecules. The size of the effect depends on various parameters such as beam energy, selected charge state, stripping gas type and ion optics. Foil stripping leads to energy distributions which are much wider, and show typically a double peak.

Oceanic archives are contemporary witnesses of Earth’s recent astrophysical history by incorporating extraterrestrial radionuclides. VA13/2 - 237KD is one of the most studied ferromanganese crusts and it has been shown that the crust contains live interstellar ⁶⁰Fe. Here, we have characterized a large piece of this crust with a 3D optical scan, a micro-CT scan and 3D modeling, followed by the chemical extraction of highly purified, element-specific fractions for accelerator mass spectrometry. High-accuracy cosmogenic ¹⁰Be dating of two independent drill-holes showed a time-dependent variability in growth rate across the surface of the crust. This well-characterized crust is used to search for interstellar radionuclides, such as supernova-produced ⁶⁰Fe and the r-process nuclide ²⁴⁴Pu. Other extraterrestrial radionuclides including ²⁶Al, ⁵³Mn, ¹²⁹I, ¹⁸²Hf or ²⁴⁷Cm could be investigated in the future.