Australia: The Land Where Time Began
Strzelecki and Tirari Deserts - Constraining Soil Formation in Linear Dunes
According to the author1 the formation of desert dunes can occur episodically. Aeolian activity intensity varies with the environmental conditions, including reworking events that are concentrated and periods during which there are prolonged partial activity. Within dune sediments the most unambiguous indicators of past environments are palaeosols, being markers of relative environmental stability as pedogenesis (soil formation) occurs at times when the conditions are not conducive to the formation of dunes. Palaeosols can also be stratigraphic markers. A characteristic of soils in the arid zone is the precipitation of carbonates and gypsum, and cutans, the clay coatings that surround sand grains (Fitzsimmons et al., 2009), that are formed by the downward infiltration of clay in rainwater. In some cases soil horizons may be distinctive enough to isolate different age events, thus allowing the establishment of a chronology (a relative history) of events involved in the formation of a dune (Bowler, 1976). Palaeosols may be more difficult to distinguish or identify in regions that are relatively more arid or where there are low levels of sediment, or reworking by wind of dune sediments may result in palaeosols being preserved incompletely. A critical consideration when reconstructing environments of the past is the potential for stratigraphic preservation being incomplete within dunes.
In this article the author1 reports the results she obtained from a study of the stratigraphy and timing of the formation of soil in desert dunes in the Strzelecki and Tirari Deserts, central Australia. In central Australian dunes it is not possible to distinguish any particular palaeosol from any other in the region as they all display similar characteristics. Many of the dunes in the Strzelecki and Tirari Deserts show several cycles of dune building within their internal stratigraphy, though the same stratigraphy is not present in all dunes. Dune activation occurred over several episodes at times of increased aeolian activity, as indicated by successions of multiple sand units and palaeosols. Palaeosols are not preserved at some sites, and it has been suggested that reworking has removed parts of the stratigraphic record in such sites (Munyikwa, 2005). It has been found that in other parts of the world, such as the Kalahari Desert, southern Africa, no evidence of soil formation has been preserved in desert dunes in comparable landscapes that are relatively more sediment-poor (Stone & Thomas, 2008).
The author1 suggests information regarding the degree of reworking of sediment in desert dunes can be provided by sedimentological analysis, which has implications for dune stratigraphy preservation. An example is provided by the fragmented coatings on grains that indicate abrasion has been caused by aeolian reworking of the underlying soil material, and these coatings are found in dunes across the dunefields of the Strzelecki and Tirari Deserts (Fitzsimmons et al., 2009), in some instances deposits from earlier arid periods may be completely removed by reworking. It has also been found that removal or horizons from individual dunes also occurs, the author1 suggesting that this is possibly the result of limited lateral migration of dune crests. In areas of limited sediment supply dune records may be biased towards the end of an arid period, as suggested by the propensity for the reworking of sediment, though some dunes may preserve prolonged phases of partial aridity and the associated accretion of sediment.
The author1 suggests that in the past linear dunes have been considered to be inefficient as recorders of the aridity of a region as a result of reworking (Munyikwa, 2005), in particular in the assessment of the palaeoenvironmental record of individual dunes or small numbers of dunes. The limitations of sampling bias individual dune studies are prone to are overcome by the use of integrated studies of multiple dunes across a region, which allows the identification of multiple periods in which dune activity and the formation of soil occurred (Fitzsimmons et al., 2007a). Direct dating techniques cannot determine the exact timing of the formation of soil in desert dunes, though the times of dune activity can be dated by OSL techniques, as OSL measures when the sediments were last exposed to sunlight, which occurred at the time the sediments were deposited/buried. As a result palaeosols occur at times when there are no age estimates allowing the identification of dune stabilisation and the development of soils. The OSL chronologies of individual dunes in the Strzelecki and Tirari Deserts (Fitzsimmons et al., 2007a), containing multiple palaeosols, were used to constrain the timing of pedogenesis. The formation of a palaeosol must have taken place at some time between the deposition of the stratigraphic unit in which it formed and the onset of deposition of the unit that overlies it, which results in the identification of 3 periods of pedogenesis. The author1 suggests pedogenesis must have taken place between 106-73 ka, 60-43 ka and 27-24 ka. The overlap of these age ranges inferred the regional formation of soil and associated dune stability. The timing of pedogenesis also corresponds to a lack of ages for dune activity at other sites.
It is suggested that comparing the time of soil formation with that of dune activity on a regional scale is also possible. The author1 suggests that periods that are identified as times of soil formation correspond approximately to times when there are breaks in the accumulation of the dune record across the entire region. It is not possible to constrain the timing of pedogenesis more precisely as palaeosols cannot be dated directly, this being a feature in any area where reworking has a significant role in the formation of dunes. For the approximate timing of geomorphic processes, including pedogenesis, where there are no direct dating techniques, OSL dating can be an effective tool.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading