Cosmic-ray constancy and cosmogenic production rates in short-lived chondrites

Five chondrites with short cosmic ray exposure ages (< 3Myr) have been analyzed for ⁵³Mn (t_1.2 = 3.7Myr), ^26Al(t_1.2 = 0.72Myr) and the stable isotopes of He, Ar, and in particular, Ne. In addition, rare gases were determined for five other short-lived chondrites from the same aliquots which had previously been used for ²⁶Al and ⁵³Mn determinations by the Cologne Laboratory. Improved curves of growth were constructed for ²⁶Al and ⁵³Mn to deduce the respective saturation values ²⁶Al₀ and ⁵³Mn₀ and to obtain ²¹Ne production rates (²¹P).²⁶Al and ⁵³Mn saturation values deduced from short-lived chondrites alone agree within 20%, with those deduced from all chondrites. Values of ²¹p based on ⁵³Mn and ²⁶Al are 0.30 ± 0.03 and 0.46 ± 0.05, respectively, in H chondrites. Possible causes for the difference include half-life errors, data selection and a recent (?2.5Myr) increase in the cosmic ray intensity.     

Tinkering at the Limits: agricultural shows,small-scale producers and ecological connections

Agricultural shows in Australia are typically depicted as celebrations of colonisation and scientific and technical modernisation in food production. The historical focus of shows is on competition to maximise perceived quality and yield of goods, from wheat to cattle. Through these frameworks, shows are often understood as supporting industrial-scale agricultural practices that promote an ecologically-blind approach to food production. However, we suggest that competitions in contemporary agricultural shows play a role in contesting the limits of such anthropocentric thinking. We focus on a hitherto unexplored aspect of agricultural shows: small-scale growers who exhibit produce in their annual local show competitions. Through a case study drawing on interviews with exhibitors and judges, combined with participant observation at the 2012 Royal Canberra Show, we highlight the complex relationships between people, place and more-than-humans in this unique cultural site. In so doing, we suggest that exhibiting in the show can intensify urban and peri-urban small-scale producer engagement in practices of ‘tinkering’. This facilitates embodied encounters with the limits of both human mastery and those of the materiality of non-humans involved in food production. Through the ongoing, adaptive processes of tinkering encouraged by such competitions, we suggest that exhibiting in agricultural shows can support the growth of ecologically informed, place-based understandings of the food system and challenge binaries of the rural/city divide.     

Differences in preferential flow with antecedent moisture conditions and soil texture: Implications for subsurface P transport

Preferential flowpaths transport phosphorus (P) to agricultural tile drains. However, if and to what extent this may vary with soil texture, moisture conditions, and P placement is poorly understood. This study investigated (a) interactions between soil texture, antecedent moisture conditions, and the relative contributions of matrix and preferential flow and (b) associated P distributions through the soil profile when fertilizers were applied to the surface or subsurface. Brilliant blue dye was used to stain subsurface flowpaths in clay and silt loam plots during simulated rainfall events under wet and dry conditions. Fertilizer P was applied to the surface or via subsurface placement to plots of different soil texture and moisture condition. Photographs of dye stains were analysed to classify the flow patterns as matrix dominated or macropore dominated, and soils within plots were analysed for their water‐extractable P (WEP) content. Preferential flow occurred under all soil texture and moisture conditions. Dye penetrated deeper into clay soils via macropores and had lower interaction with the soil matrix, compared with silt loam soil. Moisture conditions influenced preferential flowpaths in clay, with dry clay having deeper infiltration (92 ± 7.6 cm) and less dye–matrix interaction than wet clay (77 ± 4.7 cm). Depth of staining did not differ between wet (56 ± 7.2 cm) and dry (50 ± 6.6 cm) silt loam, nor did dominant flowpaths. WEP distribution in the top 10 cm of the soil profile differed with fertilizer placement, but no differences in soil WEP were observed at depth. These results demonstrate that large rainfall events following drought conditions in clay soil may be prone to rapid P transport to tile drains due to increased preferential flow, whereas flow in silt loams is less affected by antecedent moisture. Subsurface placement of fertilizer may minimize the risk of subsurface P transport, particularily in clay.     

A low-order model for the response of the Atlantic thermohaline circulation to climate change

Concern has been expressed that anthropogenic climate change may lead to a slowdown or even collapse of the Atlantic thermohaline circulation (THC). Because of the possibly severe consequences that such an event could have on the northern North Atlantic and northwestern Europe, integrated assessment models (IAMs) are needed to explore the associated political and socioeconomic implications. State-of-the-art climate models representing the THC are, however, often too complex to be incorporated into an integrated assessment framework. In this paper we present a low-order model of the Atlantic THC which meets the main requirements of IAMs: it (1) is physically based, (2) is computationally highly efficient, (3) allows for comprehensive uncertainty analysis and (4) can be linked to globally aggregated climate models that are mostly used in IAMs. The model is an interhemispheric extension of the seminal Stommel model. Its parameters are determined by a least-squares fit to the output of a coupled climate model of intermediate complexity. Results of a number of transient global warming simulations indicate that the model is able to reproduce many features of the behaviour of coupled ocean–atmosphere circulation models such as the sensitivity of the THC to the amount, regional distribution and rate of climate change.Responsible Editor: Richard Greatbatch     

Thermotectonic evolution of a rifted continental margin: fission track evidence from the Kangerlussuaq area, SE Greenland

Fission track analyses of apatites from sediments, Precambrian gneisses and Caledonian to Tertiary intrusive rocks from the Kangerlussuaq region reveal its post-Caledonian thermotectonic history. Inland the history involves cooling to temperatures within the high temperature part of the apatite annealing interval and slow cooling (or reheating) continued in Cretaceous times through this interval. Apatites from coastal areas between Kangerlussuaq and Tasiilaq reveal only Tertiary cooling. In Tertiary times cooling accelerated after the main intrusive phase in the Tertiary. The evolution is taken as evidence for a general uplift/erosion since Caledonian times probably disturbed by basin formation and sedimentation and reheating due to magmatic activity. Thermal subsidence of the rift shoulders following the opening of the adjacent oceanic basin is not indicated. Annealing patterns inland of the plume centre in the Kangerlussuaq provide no evidence for the earlier movement of the plume from a westerly direction.     

Reconstructing past eutrophication trends from diatoms and biogenic silica in the sediment and the pelagic zone of Lake Constance, Germany

In Lake Constance, phosphorus concentrations and the seasonal development of phytoplankton communities in water samples from the pelagic zone were regularly recorded since the 1950's. Before the 1950's, there were occasional investigations of plankton communities since 1896. We compared these data with the sedimentary record in two sediment cores. Then, the eutrophication history of Lake Constance was inferred from diatoms. The record of biogenic silica in the cores is discussed with respect to diatom biomass increase.Diatom assemblages in the sediment cores precisely reflected the pelagic diatom development for the period 1971--1992. Both sediment cores and the water samples have a high interannual variability of diatom assemblages. Below a sediment depth of 27 cm (AD 1920), more than 50% of the diatoms were partly corroded, and we limited the reconstruction of trophic state changes to the interval of 1920--1993. Oligotrophic conditions of Lake Constance were indicated by the dominance of various Cyclotella taxa from 1920 to 1940. Since 1939/1940, increasing abundance of it Tabellaria fenestrata showed oligotrophic to mesotrophic conditions. Between 1953 and 1956, increasing Stephanodiscus hantzschii and disappearing Cyclotella indicated advanced eutrophication and total phosphorus values ranged between 8--10 mg m-3 during turnover in late winter. Further eutrophication was shown by disappearing T. fenestrata and increasing S. minutulus in 1963. Maximum TP concentrations of 87 mg m-3 occurred in 1979/80 and was accompanied by increasing abundances of Aulacoseira granulata. From 1986 to 1992, reoccurrence of Tabellaria fenestrata and Cyclotella indicate some recovery of Lake Constance.Biogenic silica and diatom abundances were similar among cores but indicate a 3--4 fold increase of diatom biomass only. This was far below the estimate of biomass increase from sedimentary pigment data (25 fold) and the estimate of phytoplankton data from the literature (70 fold).