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Vegetated hummocks on scalded clay pan soils – relic or recent deposit?

Zoe Read 1, Adrian Chappell2, Richard Greene1, Brian Murphy1
1Australian National University, Canberra ACT, Australia, 2CSIRO, Canberra ACT, Australia

In the semi-arid rangelands of north-eastern New South Wales on elevated Quaternary meander plains, fertile topsoil has been eroded by wind and water. Soil erosion was accelerated by grazing, drought and subsequent vegetation loss. The landscape has developed swaths of indurated subsoil or scalds with waterponding and isolated vegetated hummocks that protrude approximately 30 cm above the scalded surface. The origin of the hummocks has been attributed to either (i) relics of the former landscape or (ii) accumulated deposits. We aimed to understand the erosion and deposition processes, the arising landforms and the conflicting explanations.

We obtained soil samples from scalds, waterponds and hummocks and measured Caesium-137 (137Cs). This radioactive isotope was released during atmospheric weapons tests which began in the 1950s, peaked in 1963 and fallout ceased in 1984. The 137Cs was scavenged by rainfall nuclei and fixed to the clay fraction of soil typically producing an exponential profile to 20-30 cm in undisturbed soil. Soil redistribution by wind and water causes a change in the soil 137Cs inventory. We modelled a soil 137Cs reference inventory for this site of about 25 mBq cm-2.    137Cs was present in the top 10 cm of the scald profile, but none was detected below that depth (Table 1). At the waterponded location 137Cs was found to 20 cm. There was no 137Cs detected in the top 30 cm of the hummock soil but below 30 cm 137Cs was detected. Relative to the 137Cs reference inventory all sample sites have lost soil.

Table 1. 137Cs inventory data for hummock, scald and waterponded soil (mBq cm-2). Figures in parenthesis show predicted soil erosion loss (t ha-1 yr-1). na = not analysed.

 

Depth (cm) Scald Waterpond Hummock

0-5 0.54 3.63 0

5-10 0.87 2.48 0

10-20 0 1.81 0

20-30 0 0 0

30-50 na na 4.06

Total 1.41 (-30.05) 7.93 (-3.93) 4.06 (-13.13)

There is evidence showing that the scalded surface was created prior to 1984 and therefore was exposed to radioactive fallout. It is reasonable to assume that the 137Cs enriched topsoil has been removed exposing at the new surface the remaining 137Cs found previously at depth. The amount and distribution of 137Cs suggests that waterponds are deposition sites. At the hummock site the presence of 137Cs below 30 cm indicates that the original soil surface has been inundated by an unlabelled soil probably from the subsoil of the banks of a nearby river. The hummock is therefore the likely alluvial remnant of subsequent soil erosion. Since the top 0-30 cm is unlabelled with 137Cs we cannot conclusively establish a relative date. However, given the history of the region it is most likely that the hummocks are recent deposits.