ACCUMULATION RATES OF TWO AUSTRALIAN LOESS DEPOSITS; CORRELATIONS AND CLIMATIC IMPLICATIONS

Basal optical (OSL) ages of 40 to 60 ka in two loessic (parna) mantles on the Central Tablelands of New South Wales indicate significant silt (c. 30 µm mode) deposition commenced well before the onset of the last glacial maximum. Both dated sections show no detectable change in mass accumulation rate around the LGM. The basal ages coincide with both major paleochannel and source-bordering dune activity in the western slopes and plains of NSW and the first dated linear sand dunes of the last glacial cycle, but not strong evidence of dramatic aridity.

The linear age-depth relationship suggests constant accumulation over the last 40-60 ka, rather than a single LGM deposition event. In detail, each mantle consists of > 1m of reddish silty clay loam with an earthy fabric which sits atop manganese and iron/silica hardpans and saprolite. Mixing of saprolitic derived material into the hard pan and also into the silty layer and destruction of internal sedimentary layering suggests slow accumulation with sufficient time for extensive bioturbation to occur. Presently, bioturbation is mostly restricted to the conspicuous topsoil where it has resulted in textural and fabric differentiation not seen deep in the profiles. In other words, there are no buried soils.

The apparent insensitivity of the loess accumulation rates to the sweeping environmental changes of the late Pleistocene is surprising. We cannot rule out post-depositional erosion (by wind or water) as an important factor in producing the measured accumulation rates at these sites and are therefore unable to speculate about changes in the true dust deposition rates.

The accumulation rates of these deposits are the first determined for an Australian loess deposit. The rates of 4-5 g.cm-2.ka-1 are an order of magnitude higher than for dust deposited in the Tasman Sea at the LGM. These figures are in quite good agreement with model predictions (Mahowald et al., 1999, JGR) which include broader source areas in Australia because of more widespread arid conditions. On the other hand, accumulation rates determined for marine cores from the Indian Ocean offshore northwestern Australia are much lower than model predictions. Field evidence suggests that sediment supply restricts dust loads in northwestern Australia and may account for these differences.