Fire, climate, and the persistence of desert varnish near Dampier, Western Australia

Abstract

In the semi-arid region near Dampier, discontinuous black and dark red coatings (desert varnish) have developed mainly on outcrops of granophyre. Coatings which have persisted occur on the most stable surfaces of large, rather than small, boulders. Rock coatings are absent both from fire-affected outcrop margins and around sediment patches within boulder slopes, and from the spray zone on the coastal margin. Sea level rise between about 17,000 and 6000 yr B.P. would have produced a landward-migrating spray zone within which salt weathering probably destroyed varnish before inundation. The remaining rock coatings have variable chemistry, and include compound coatings with superimpositions of thin Fe-rich, thicker SiAlMnFe layers, and an upper coat of SiAlFe, indicating fluctuatinng environments. Within major outcrops, the best-developed coatings occur on large and stable boulders beyond the reach of fire and salt spray, and often in summit or mid-slope positions. Much varnish loss can be attributed to factors other than climatic change, with coatings presently forming at a slower rate than they are being destroyed.