Climate variability, climate change and desert dune mobility

Optical dating of linear sand dunes in the Simpson Desert has recently confirmed the great depth (> half dune height) of sand mobilised in the Holocene in some dunes. These data, and the observation today of locally mobile dunes, contradict the long-held belief that the Australian desert has been inactive since the end of the last glacial interval and force us to reassess how these dunes have became mobile in part, or at times, during the Holocene. Several sites in the Simpson and Strzelecki Desert dune fields were examined to determine the local effect of vegetation on sand mobility on the dune surfaces. The vegetation structure comprises both perennial bushes and ephemeral herbs and forbs which contribute protective cover while growing after rains and for some time after while dead. Cyano-bacterial crusts further enhance the surface area with some protective cover. The responsiveness of the vegetation to drought may see cover vary from >60% to <20% over a drought cycle. As expected there is a positive relationship between sand mobility and the proportion of unvegetated dune surface. There is no strict threshold of sand movement at high cover levels but the exponential relationship between bare area and volume of mobile sand sees effective total mobility at about 50% cover. At this level there is also a morphological shift from hummocky dune surfaces to mobile dunes with avalanche faces. These two states represent contrasting morphodynamic relationships: at high levels of cover the fluctuation of cover with climate variability (especially rainfall modulated by ENSO and PDO) determines the amount of sand transport while at low levels of cover sand transport is free of the influence of vegetation at all times. Extreme, prolonged and rare drought may be required to remove perennial vegetation from dune crests showing this level of activity. Despite these findings, mobile dunes are comparatively rare in Australia today. Most appear to be in areas of localised vegetation disturbance (from grazing, roads etc.) but there is low net lateral transport (it is 'back-and-forth'). The few dated dunes also suggest spatially and temporally variable activity in the Holocene with low volumes of mobile sand. The supply of erodible sand on the dunes themselves is thought to combine with seasonally opposed winds to minimize net sand transport.