Addition of aeolian dusts to soils in southeastern Australia: red silty clay trapped in dunes bordering Murrumbidgee River in the Wagga Wagga region

In southeastern Australia, aeolian dust deposits are very common and have a significant influence on soil properties and soil landscapes. However, the characteristics of the pure dust materials and the rates of dust-fall in the past are unclear because of the low overall rate of dust deposition and mixing with locally derived sediments. In the Wagga Wagga region, some dunes have functioned as dust traps. Thin (<1 cm thick) red clayey bands and thick (up to 2.5 m) red clayey layers within the dune sequences are likely to represent illuviated aeolian dust. These dust materials are characterised by a bi-modal particle size distribution, one mode in the clay and another in the coarse-medium silt. The clay minerals are dominated by kaolinite, illite and smectite. Both ¹⁴C and optical dating indicate the most recent period of dune formation was around 3–4 ka. In an example of these young dunes, there is a total of 2 cm equivalent thickness of dust materials, giving a deposition rate of 0.5–0.7 cm ka⁻¹. All three samples from an elevated dune are saturated with respect to environmental radiation dosage, and give minimum optical ages in excess of 80 ka. In this higher dune, the total thickness of dust is 50–80 cm, similar to that (50–100 cm) of the Yarabee Parna, the youngest aeolian dust deposit in the Wagga Wagga region. This may have been deposited unevenly, being more concentrated during the period 25–16 ka, which has been identified as a major dust deposition period in the Tasman Sea. If this variation occurred, the dust deposition rate indicated by the 50–80 cm dust material in the dune could have been as high as 5 cm ka⁻¹ for the period 25–16 ka.