Inferred vegetation during Australia’s dune whorl formation

Dominik Nommensen, Jimmy Daynac, Andrew Gunn 

Australia’s vegetated linear dune crests are oriented in an anticlockwise whorl covering over a third of the continent. The mismatch between its crest orientations and modern sand-flux directions has been interpreted as evidence of a northward wind shift during the Last Glacial Maximum. However, this interpretation implicitly assumes constant vegetation cover via the threshold wind speed for sand transport. Here we show that modern winds can reproduce the whorl across much of the continent if the threshold speed varies spatially. We combine a continental map of crest orientation with a sand-flux model driven by reanalysis winds. The thresholds required to match observed orientations imply lower vegetation cover than the contemporary minimum in many regions, and less spatial contrast across the continent, indicating whorl formation occurred during more arid climates within the glacial cycles. Plausible changes in vegetation alter dune orientation as much as the postulated wind shift. Across-crest observed slope asymmetry and predicted sand flux both agree to imply increasing vegetation as dunes reoriented until they became immobile. Our results do not exclude interglacial wind variability but show that vegetation variability sufficiently reconciles the whorl with modern winds and therefore vegetation should be considered when inferring past climate from dune orientation.