Stable isotope and C study of biogenic calcrete in a termite mound, Western Cape, South Africa, and its palaeoenvironmental significance

Late Quaternary terrestrial climate records from the semi-arid zone of the Western Cape of South Africa are rare. However, palaeoenvironmental information may be inferred from ancient termite mounds of the region. Calcrete lenses in these mounds have δ¹³C and δ¹⁸O values that show systematic changes with radiocarbon dates, which range from 33,629–36,709 to 21,676–23,256 cal yr BP. These dates confirm that these heuweltjies had been present in the landscape since the last glacial period. The decrease in δ¹³C and δ¹⁸O from 33,629–36,709 to 21,676–23,256 cal yr BP indicates that climate information is recorded by the calcretes. It is suggested that a progressive decline in air temperature and an increase in moisture availability, and a decline in abundance of C₄ or CAM plants, occurred in the region during the time heuweltjie calcite precipitated.     

Origins of strandline duricrusts around the Makgadikgadi Pans (Botswana Kalahari) as deduced from their chemical and isotope composition

Trace elements together with some O and C isotope analysis were undertaken on duricrust strandline deposits in the palaeo-Makgadikgadi sub-basin (PMSB) to provide insight into palaeo-climatic conditions through the interpretation of calcrete, silcrete–calcrete intergrade and silcrete deposits. Trace element content and relative abundance suggest that the duricrust origins are associated with the long-term weathering of the Karoo Large Igneous Province which underlies the PMSB. This work shows that duricrust origins are related to Ca²⁺ and Si (and associated trace elements) being transported mainly through the groundwater and then subsequently precipitated at different strandline elevations over time. Local groundwater feeding in towards the pan margin and accumulating in near-neutral pan-marginal pools, appears to facilitate Si concentration and permeation of pre-existing calcretes. The silica precipitates as the pH drops when renewed freshwater enters the pools. Hence the inferred palaeo-climatic regime for silcretisation may be similar to that occurring in Botswana at present being dry semi-arid with low seasonal rainfall. In contrast the extensive calcrete precipitation in the strandlines results from abundant Ca²⁺ in adjacent waters which appear to be derived from both local and regional sources. The arrival of Ca²⁺ from regional sources (shown by trace element evidence) infers heavy rainfall in the upper catchment suggesting a major humid event followed by regional drying. Palaeo-climatic inferences suggest the juxtaposition of major humid events interspersed with more normal semi-arid palaeo-climates with an exception obtained from isotope data, of drier and cooler conditions than usual for the region around 80–90 000 years ago. Whereas trace element data can greatly assist in the interpretation of complex deposits such as duricrusts, care should be taken over the use of particular ratios (such as Yb/Gd ratio) which may produce spurious results.     

Mapping an aquitard breach using shear-wave seismic reflection

In multi-layered hydrostratigraphic systems, aquitard breaches caused by faulting or paleo-erosion can allow substantial quantities of water of differing quality to be exchanged between aquifers. Seismic reflection technology was used to map the extent and orientation of an aquitard breach connecting a shallow alluvial aquifer to the deeper semi-confined Memphis aquifer in southwestern Tennessee, USA. Geophysical well logs indicate the presence of the aquitard at borehole locations that define the beginning and end points on two seismic survey lines, which intersect at a borehole where the aquitard is absent. A SE–NW-oriented paleochannel, 350 m wide and approximately 35–40 m deep, is interpreted from the seismic reflection surveys. The paleochannel cuts through the aquitard and into the upper part of the Memphis aquifer, thus creating a hydraulic connection between the shallow unconfined and deeper, semi-confined aquifers. The results indicate the potential of the shear-wave seismic reflection methods to resolve shallow breaches through fine-grained aquitards given availability of sufficient well control.     

Doing interdisciplinarity: motivation and collaboration in research for sustainable agriculture in the UK

This paper studies knowledge production in complex, collaborative research projects that brought together academics from different disciplines, research users and agricultural businesses. It takes a comparative approach, studying the interactions within interdisciplinary research teams from ten case studies, considering the process of collaboration from initial idea through to publication. The research developed a typology of participants in these projects, and identified the motivations and challenges of each. Our results analyse the process of research teams coming together and the relationships that are built up during the research. A particular challenge identified was the building of cooperation and trust. This issue is explored alongside issues of communication, methodology, data analysis and the process of drawing and publicising conclusions.     

Ripple cross-laminated sediments on the East Antarctic Shelf: evidence for episodic bottom water production during the Holocene?

Biosiliceous sediments sampled from a submarine valley system on the continental shelf of East Antarctica contain intervals of ripple cross-lamination interspersed with massively bedded units. Based on radiocarbon dates from one core collected on the Mac.Robertson Shelf, the most intensely cross-laminated sediments were deposited between 6 and 3.5 kyr BP, with isolated cross-laminae deposited at other times in the Holocene. The cross-laminated sediments are interpreted here as a signal of episodic density currents flowing across the outer shelf, which result from the formation of high salinity shelf water (HSSW). This HSSW is formed in winter by brine rejection during sea ice formation and by the exchange and cooling of upwelled saline slope water, and it contributes to the bottom water produced along the continental margin of Antarctica. If this interpretation of the cross-laminae is correct, then bottom water formation and export from the East Antarctic shelf has exhibited temporal, and probably also spatial, variability throughout the Holocene. Such variability would have implications for oceanographers attempting to quantify Antarctic bottom water production rates based only on present day observations.     

Construction dynamics of a lava channel

We use a kinematic GPS and laser range finder survey of a 200 m-long section of the Muliwai a Pele lava channel (Mauna Ulu, Kilauea) to examine the construction processes and flow dynamics responsible for the channel–levee structure. The levees comprise three packages. The basal package comprises an 80–150 m wide ′a′a flow in which a ∼2 m deep and ∼11 m wide channel became centred. This is capped by a second package of thin (<45 cm thick) sheets of pahoehoe extending no more than 50 m from the channel. The upper-most package comprises localised ′a′a overflows. The channel itself contains two blockages located 130 m apart and composed of levee chunks veneered with overflow lava. The channel was emplaced over 50 h, spanning 30 May–2 June, 1974, with the flow front arriving at our section (4.4 km from the vent) 8 h after the eruption began. The basal ′a′a flow thickness yields effusion rates of 35 m³ s⁻¹ for the opening phase, with the initial flow advancing across the mapped section at ∼10 m/min. Short-lived overflows of fluid pahoehoe then built the levee cap, increasing the apparent channel depth to 4.8 m. There were at least six pulses at 90–420 m³ s⁻¹, causing overflow of limited extent lasting no more than 5 min. Brim-full flow conditions were thus extremely short-lived. During a dominant period of below-bank flow, flow depth was ∼2 m with an effusion rate of ∼35 m³ s⁻¹, consistent with the mean output rate (obtained from the total flow bulk volume) of 23–54 m³ s⁻¹. During pulses, levee chunks were plucked and floated down channel to form blockages. In a final low effusion rate phase, lava ponded behind the lower blockage to form a syn-channel pond that fed ′a′a overflow. After the end of the eruption the roofed-over pond continued to drain through the lower blockage, causing the roof to founder. Drainage emplaced inflated flows on the channel floor below the lower blockage for a further ∼10 h. The complex processes involved in levee–channel construction of this short-lived case show that care must be taken when using channel dimensions to infer flow dynamics. In our case, the full channel depth is not exposed. Instead the channel floor morphology reflects late stage pond filling and drainage rather than true channel-contained flow. Components of the compound levee relate to different flow regimes operating at different times during the eruption and associated with different effusion rates, flow dynamics and time scales. For example, although high effusion rate, brim-full flow was maintained for a small fraction of the channel lifetime, it emplaced a pile of pahoehoe overflow units that account for 60% of the total levee height. We show how time-varying volume flux is an important parameter in controlling channel construction dynamics. Because the complex history of lava delivery to a channel system is recorded by the final channel morphology, time-varying flow dynamics can be determined from the channel morphology. Developing methods for quantifying detailed flux histories for effusive events from the evidence in outcrop is therefore highly valuable. We here achieve this by using high-resolution spatial data for a channel system at Kilauea. This study not only indicates those physical and dynamic characteristics that are typical for basaltic lava flows on Hawaiian volcanoes, but also a methodology that can be widely applied to effusive basaltic eruptions.     

Marine resources,biophysical processes,and environmental management of a tropical shelf seaway: Torres Strait,Australia–Introduction to the special issue

This special issue of Continental Shelf Research contains 20 papers giving research results produced as part of Australia's Torres Strait Co-operative Research Centre (CRC) Program, which was funded over a three-year period during 2003–2006. Marine biophysical, fisheries, socioeconomic-cultural and extension research in the Torres Strait region of northeastern Australia was carried out to meet three aims: 1) support the sustainable development of marine resources and minimize impacts of resource use in Torres Strait; 2) enhance the conservation of the marine environment and the social, cultural and economic well being of all stakeholders, particularly the Torres Strait peoples; and 3) contribute to effective policy formulation and management decision making. Subjects covered, including commercial and traditional fisheries management, impacts of anthropogenic sediment inputs on seagrass meadows and communication of science results to local communities, have broad applications to other similar environments.     

Soft-sediment deformation during thawing of ice-rich frozen soils: results of scaled centrifuge modelling experiments

The origin of periglacial involutions remains uncertain, largely because of the difficulties of field monitoring in modern permafrost regions. This paper describes an alternative approach, in which process studies are based on scaled centrifuge modelling of thawing ice-rich soils. Centrifuge scaling laws allow similitude in self-weight stresses between the model scale and the prototype (field) scale to be achieved. In these experiments, 120- to 130-mm-thick frozen models comprising a sand unit overlying ice-rich kaolinite clay (three models) or ice-rich silt (one model) were thawed under an acceleration of 20 gravities. The models were therefore equivalent to 2·4–2·6 m of frozen sediments (permafrost) at the prototype scale. Temperature profiles and porewater pressures during the thawing of each model are described. Porewater pressures significantly in excess of hydrostatic were not observed in the sand/silt model. In the sand/clay models, however, excess pressures developed rapidly after thawing, and observed fluctuations in pressure were interpreted as water-escape events. After thawing, careful sectioning of the models revealed small-scale deformation structures at the clay–sand interface, resulting from loading of the upper sand layer into very soft fluid-like clay and injection of clay upwards into the base of the sand. It is concluded that these experiments provide analogues for some Pleistocene involutions. Such involutions therefore mark phases of permafrost degradation when high porewater pressures caused loading and injection along sedimentary boundaries.