Loess, dust, parna: Unravelling the conceptual continuum in Australasia for improved paleoenvironmental reconstruction in the Quaternary

Abstract

There are a variety of both mechanisms and geomorphic processes which generate, sort and concentrate silt-sized grains: entrainment by wind, then deposition by aeolian processes follow. This can occur in semi-arid to temperate to periglacial geomorphic units. Loess is an accepted term in New Zealand for aeolian, silt-sized deposits of glacial origin. In Australia, a varied terminology ranging from parna, to dust to lithogenic fluxes are all employed to describe aeolian silt sized deposits, generally accepted to be sourced from semi-arid environments. This range of terminology can be a source of confusion and has likely arisen from historical constructs during the development of loess research world-wide. But what we do know is that aeolian sediments and their associated soils in both Australia and New Zealand encompass a wide range of particle size, mineralogy and geochemistry. This reflects differences in the dust source of the contributing regional geomorphic units, dynamics of dust emission from the differing geomorphic units as well as post sedimentary changes to the dust deposits (pedogenesis) driven by gradients of aridity and weathering. We aim to demonstrate that despite these differences, loess, dust, parna also exist on a continuum, influenced by gradients of aridity, weathering and particle size. We will do this by deconstructing some specific examples in Australia and New Zealand. We compared the different provenance, formation pathways, mineralogy and geochemistry for these sediments using a range of techniques including optical micromorphology, granulometric analysis and QEMSCAN® analysis. In Australia (southern NSW), microfabric analysis demonstrated a continuum of pedogenesis of the associated parna soils along gradients of aridity and weathering. In New Zealand, the transport pathway for the loess mantled landscape in the Manawatu district (North Island) is generally accepted, but the morphological and chemical difference between the soils developed on the drier terraces (Tokomaru) compared to those on the moister terraces (Dannevirke) is still not fully understood. On comparing the mineralogy and geochemistry, the dust / parna in Australia is dominated by quartz and clay, and little plagioclase feldspar. While in New Zealand (Manawatu), the grain size distribution of the two soils are very similar. The Dannevirke soil mineralogy is dominated by kaolinite and chlorite while the Tokomaru soils have a greater proportion of weatherable mineral grains. We discuss the results in terms of the conceptual differences in the transport pathways and pedogenesis of loess and how this can inform our ability to reconstruct Quaternary paleoenvironments, and ultimately contribute to improved numerical models of the dust cycle.