Geoff S. Humphreys

Soils: A New Global View

Part 1 The processes of soil formation and the resulting soil materials: weathering and leaching new mineral formation and inheritance bioturbation rainwash aeolian processes soil creep. Part 2 The distribution of soil material: the pedological hierarchy the soils of a continental plate centre - Australia, Africa soils of other continental plate centres the soil materials of continental plate margins.

Heating effects on water repellency in Australian eucalypt forest soils and their value in estimating wildfire soil temperatures

Wildfires can induce or enhance soil water repellency under a range of vegetation communities. Accord- ing to mainly USA-based laboratory studies, repellency is eliminated at a maximum soil temperature (T )o f 280-400 ◦ C. Knowledge of T reached during a wildfire is important in evaluating post-fire soil physical prop- erties, fertility and seedbed status. T is, however, notoriously difficult to ascertain retrospectively and often based on indicative observations with a large potential error. Soils under fire-prone Australian eucalypt forests tend to be water repellent when dry or moderately moist even if long unburnt. This study aims to quantify the temperature of water repellency destruction for Australian topsoil material sampled under three sites with contrasting eucalypt cover (Eucalyptus sieberi, E. ovata and E. baxteri). Soil water repellency was present prior to heating in all sam- ples, increased during heating, but was abruptly eliminated at a specific T between 260 and 340 ◦ C. Elimination temperature varied somewhat between samples, but was found to be dependent on heating duration, with longest duration resulting in lowest elimination temperature. Results suggest that post-fire water repellency may be used as an aid in hindcasting soil temperature reached during the passage of a fire within repellency-prone environments.

Tsunami and palaeotsunami depositional signatures and their potential value in understanding the late-Holocene tsunami record

In recent years, much research on modern and palaeotsunami deposits has been published. From these studies, a range of signature types has been identified. Identifying and dating such deposits is an important element in understanding late-Holocene tsunami hazard and risk. However, important questions such as, ‘do modern and palaeotsunami leave similar or dissimilar traces?’ ‘do tsunami leave the same signatures all around the world or are there significant variations?’ and ‘what is the actual record of tsunami in different parts of the world?’ still remain. Answering these questions is not an easy task but examining megatsunami flood deposits should shed some light on these questions because such high-magnitude events should leave very clear and detailed traces within the coastal landscape. The coast of SE Australia is reported to have been affected by numerous palaeomegatsunami in the late Holocene. As such, the coast of New South Wales offers an important natural laboratory to examine in detail deposits associated with such events. Here, we summarize the published characteristics of modern and palaeotsunami deposits globally and within Australia. We briefly outline the tsunami risk to Australia before examining a site called Minnamurra Point on the coastline of SE Australia (south of Sydney) that has previously been described as containing evidence for a palaeomegatsunami of an unknown age. We describe the results of a detailed coastal survey, field stratigraphic investigation and various standard laboratory analyses. Surprisingly, we are unable to replicate the previously reported findings of tsunami deposits. Whilst we prefer the interpretation that the sequence is an in situ soil (the sediment sequence examined contains none of the usually reported lines of evidence to demonstrate tsunami provenance), we recognize and discuss the significance and difficulty of identifying tsunami deposits in the field and consider the implications of our findings to the wider debate about the preservation of tsunami-deposited sediments.

Exploring pedogenesis via nuclide-based soil production rates and OSL-based bioturbation rates

New dating techniques are available for soil scientists to test fundamental pedogenic ideas. Recent developments in applications of terrestrial in situ cosmogenic nuclides (TCN) from bedrock and saprolite allow the derivation of soil production rates, at scales ranging from local (sub-hillslope) to catchment wide, generally averaged over timescales of 104–105 years. Where soil depths are relatively constant over time, soil production rates equal transport rates and are thus essential to establishing sustainable erosion rates. TCN also allow the form of the soil production function to be compared to theoretical models—a difficult task previously. Furthermore, parameterised soil production functions can now be incorporated into numerical surface process models to test landscape evolution ideas. Bedrock and saprolite conversion to soil is demonstrably dependent on the overlying soil depth, and there is general agreement that weathering declines exponentially beyond maximum soil production, consistent with theory. Whether maximum soil production occurs under a finite or non-existent soil cover at particular sites remains unresolved. We suggest that, in general, soil production from saprolite declines exponentially with increasing depth, while production from bedrock follows a humped function. Estimates of the role of flora, fauna and processes such as freeze–thaw that mix soil mantles to depth, have been limited prior to optically stimulated luminescence (OSL) dating techniques. Recently derived OSL mixing rates extend the magnitude of previous partial, short-term bioturbation rates. In fact, bioturbation appears to be the most active pedogenic process operating in many soils, with freeze–thaw environments a noted exception. Although bioturbation far outweighs soil production, it does not always lead to homogenisation as is often reported. We maintain that the above-ground component of bioturbation, i.e. mounding, may alone, or particularly when combined with particle sorting via rainwash processes, lead to horizonisation and texture contrast soils in those materials that can be sorted such as mixtures of sand and clay. Together, TCN- and OSL-based estimates of hillslope soil transport and bioturbation, suggest significant rates of downslope soil mantle movement coupled with rapid mixing, contrary to in situ soil development models.

Litter dams and microterraces formed on hillslopes subject to rainwash in the Sydney Basin, Australia

Abstract Observations of the results of erosion on hillslopes destabilized by fire were made over various periods up to 4.5 years in the Sydney Basin, Australia. On many sites a microrelief form of litter dams and microterraces were noted and the conditions favouring their formation identified. Litter dams formed during the first runoff event after the fire. In normal conditions they stabilized rapidly then slowly deteriorated over 3 or 4 years. A general model of litter dam and microterrace morphology and sedimentology is presented and their significance in the genesis of hillslope soils is discussed.

Age of loess deposits in the Central Tablelands of New South Wales

Basal optically stimulated luminescence (OSL) ages of more than 50 000 years in loessic (parna) mantles on the Central Tablelands of New South Wales indicate significant aeolian silt (c. 30 μm mode) deposition commenced well before the onset of the last glacial maximum. Each mantle consists of >1 m of reddish silty clay loam–silty clay with an earthy fabric which sits atop manganese and iron pans and saprolite. Mixing of saprolite-derived material into the pans and also into the silty layer indicates a site history of steady accumulation of aeolian loess and continual pedogenesis. No palaeosols are found. The OSL chronology of both sites, while low resolution, indicates an almost constant mass accumulation rate from 50 000 years ago through the last glacial maximum and into the Holocene. Local factors affecting retention of deposited dust may be responsible for the apparent passivity of the sites.

Changes in soil water repellence and its distribution in relation to surface microtopographic units after a low severity fire in eucalypt woodland, Sydney, Australia

The distribution and persistence of water repellence was altered by low severity fire and subsequent rain in soil supporting eucalypt woodland on sandstone terrain in Sydney, south-eastern Australia. Water drop penetration times were recorded to depths of 0.08 m in situ 9 months before the experimental fire, immediately after fire, 4 weeks later when rainfall had modified the soil surface but the soil was dry, and 5 weeks after the fire when the soil was moist. Spatial variation in water repellence was high in all cases except immediately after the fire, when soil was almost uniformly strongly repellent to 0.03 m depth, and less uniformly repellent at 0.04-0.06 m depth. Heavy rain moved litter, ash, and mineral soil, modifying the soil surface into microtopographic units including litter dams, microterraces, and other areas with differing proportions of litter, bare soil, and gravel. Post-rain water drop penetration measurements were taken on and beneath these different surface conditions, in order to investigate the possible contribution of water repellence to the formation and maintenance of litter dams and microterraces characteristic of this terrain after fire. Water repellence did not appear to be directly correlated with the presence or absence of surface litter. Results suggest the long-term and spatially variable water repellence found in soils associated with vegetation dominated by eucalypts in Australia is maintained by factors different from those causing extreme and uniform water repellence after fire. Variable water repellence at the soil surface and within the profile may contribute to differential survival of seedlings after fire. The possible breakdown of water repellent compounds formed after fire and the distribution and development of hydrophobic biotic structures including fungal hyphae and proteoid roots need to be investigated.

Wood-ash stone in Angophora costata (Gaertn.) J.Britt. following Sydney bushfires

Summary Wood-ash stone, consisting of lumps of carbonate several cubic centimetres in size, were found in the burnt portion of a stump and a trunk (burnt on the ground) of a common Sydney sandstone tree, Angophora costata, following the 2002 new-year fires in the Lane Cove valley. It appears to result from the fusion and crystallisation of the ash at temperatures <1000°C. The study material is composed mostly of calcite, as determined by X-ray diffraction and petrological analysis, and in comparison to other examples it contains higher concentrations of Ca and Mg but much lower levels of K, Na and P. This composition probably accounts for the absence of rare potassium carbonates that are reported in other studies. This is the second known occurrence of wood-ash stone in Australia and outside North America.

Use of a large world soil database for modeling the global soil distribution

The ISRIC WISE Global soil database is used to demonstrate the important role that large, properly established soil databases can play in the understanding and modeling of the global distribution of soils and their properties. This database, which contains 4382 georeferenced soil profiles, was systematically examined with respect to 128 environmental regimes, which exhibited combinations of different parent material, climate, and topography. There was generally good correlation of the WRB soil types and key soil properties with these environmental parameters. While most of the results are in accord with the standard theories of soil formation, there are several unexpected findings and subtle complexities that deserve further examination. The results allow for a reflection on soil genesis relationships and provide a useful base for the development of global soil predictive models.

Inheritance and formation of smectite in a texture contrast soil in the Pilliga State Forests, New South Wales.

Smectite genesis is generally considered to require an alkaline environment, for in acid environments it is reportedly unstable. This study shows that smectite is forming in an acid, texture-contrast soil in the Pilliga State Forests in north-western New South Wales. Three modes of smectite genesis in the study soil are presented. The first mode involves direct inheritance from the underlying parent rock. The second and third modes involve precipitation of smectite from solution and its deposition from suspension, respectively. While the bulk of the smectite in the study soil is inherited from the transformation of labile primary minerals and rock fragments in the parent rock, restricted drainage coupled with a parent material capable of supplying the elemental constituents of smectite are also important factors in its genesis.