Monographs
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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