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Archives of environmental history and resources - reflections following CRC LEME Regolith Symposia 2003

John Chappell - Research School of Earth Sciences, ANU

The November Symposia are a showcase where members of CRC LEME present their research achievements, which advance the CRC towards its goals by improving our understanding of regolith composition, origins and processes. The 2003 Symposia, held in Adelaide, Canberra and Perth, ran to a total of seven days and attracted a fine array of scientific papers that included geophysics, geochemistry, geobiology and geochronology. That regolith science now has arrived squarely on the geology scene was reflected by the broad age-range of the participants, and by the generally high standard of presentation.

My first impression of all this, as a relative newcomer to CRC LEME, is that although regolith science inherits something of both sedimentary geology and geomorphology, in reality it is very different from both. For example, marine sediments form thick sequences of bedded deposits, which become younger upwards and are comprised of primary mineral grains that reflect the environmental history of distant sediment sources. Regolith, on the other hand, is relatively thin, lacks large-scale vertical stratigraphy, and includes secondary minerals that reflect the environmental history of the regolith site. In fabric and complexity, accretions of regolith minerals are analogous to metamorphic rocks. Moreover, internal movements cease in sediments after lithification (setting aside tectonic deformation), whereas transport and mixing processes, both physical and chemical, contribute intimately to regolith development.

Before the advent of CRC LEME, the rich environmental history that is subtly preserved in Australian regolith received less than due attention, partly because there was greater interest in the underlying "real" geology. By mustering a large and diverse group of earth scientists, LEME probably has the capacity to bring the systematic interpretation of regolith to a level approaching that of classical geology. But, by my reading of the studies reported at the November 2003 Symposia, we are not there yet. As outlined below, there remains a disjunction between what may be referred to as the mapping and the time dimensions of regolith.

Advances in regolith mapping are impressive. As the Symposia revealed, geophysical methods supported by borehole calibration now can provide 2D profiles and 3D maps of bulk regolith properties such as thickness, horizonation, conductivity and solute content. Precision and spatial resolution virtually are constrained only by budget, which ideally can be tailored to meet task objectives. In contrast with sedimentary basins, however, where basin evolution is explicitly revealed by seismic stratigraphy, the time dimension in regolith is almost invisible to seismic and other geophysical methods. This is because, despite horizonation, age-structures in regolith tend to lack “upness”: at the field-profile scale, specific secondary minerals can become younger downwards, while at the hand-specimen scale, concentric or interlocked age-structures are common.

In their landmark paper on regolith geology of the Yilgarn Craton, Anand and Paine (2002) emphasise that in order to understand this complex material, research needs to be integrated or nested at scales ranging from the microscopic to the regional. Studies presented at the Symposia ranged across all such scales and encompassed a variety of regolith processes, such as geochemical and microbial factors that contribute to in situ genesis or focussing of regolith minerals, including gold. Transport processes, which lead to separation of geochemical anomalies from underlying bedrock sources, generally remain unquantified but the way ahead is indicated by advances in various dating methods. Regolith science still needs to tread the path of the classical petrographers to help understand the processes, but to do this expeditiously with modern micro-analytical tools and new geochronological techniques. Indeed, new advances in microprobe methods offer the prospect that sub-millimetre dating of micro-layered secondary oxide and carbonate structures will soon join the more established field-scale dating methods, by determining the complex, small-scale age structures found in regolith.

We await the deployment of all the new techniques in an integrated study that would aim to richly describe the processes and history within a given regolith complex. However, the Symposia confirmed the integrated nature of CRC LEME, with ample evidence of healthy interaction between the Programs concerned with regolith geoscience, mineral exploration, salinity assessment and other environmental applications. A shared conceptual frame was evident amongst papers in fields as seemingly disparate as salinisation on one hand, and exploration through cover on the other. The same was apparent amongst student papers from all Programs, which reflected both the commonality of the operational frame and the sense of community engendered by CRC LEME. Nonetheless - and this is a personal view - this common frame is geographical rather than geological in nature, in the sense that considerable expertise in 2D and 3D mapping of regolith content and form was evident in the Symposia but a sense of time - that keystone dimension of the geological sciences - often was absent.

That time is of the essence in regolith science emerged in several papers: for example, Holocene radiocarbon ages from precious opal and Early Cretaceous U-Pb ages from anatase in silcrete, both hosted in Great Artesian Basin rocks, demonstrate that regolith processes are both ancient and on-going. Inseparable from the time dimension are past climatic changes, the impacts of which are registered in both the mineralogical structures of regolith and its distribution across the landscape. At the young end of the time scale, differences in apparent rates of soil formation in eastern NSW and southwest WA, determined from in situ cosmogenic nuclides, reflect different impacts of Quaternary climatic changes. Over longer periods, episodes of continent-wide deep weathering are printed palaeomagnetically in the regolith.

Application of regolith science to the management of environmental hazards, such as key problems of salinisation and acid sulphate soils, entails mapping the hazard-forming regolith salts and minerals. Predictive accuracy is enhanced when past fluxes and changes in these factors are known: knowledge of both past climatic impacts and chronology is implicated. The same holds for other environmental applications, which will broaden as we come to manage our regolith resources sustainably. Many other ongoing regolith processes are liable to receive increasing attention, such as in the fields of groundwater or the carbon cycle, all characterised by fluxes that have changed in the past but may be assessed by dating, modelling and other methods.

In conclusion, regolith science is developing rapidly but may not yet have captured the key axis of time. It will be a great triumph if CRC LEME can place regolith science on the same high plane as the older fields of earth science. The potential to reach this goal was displayed at the November 2003 Symposia; it remains to appropriately integrate our very diverse scientific skills in order to realise this potential.

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