Geomorphic and palaeoclimatic implications of an oxygen‐isotope chronology for Australian deeply weathered profiles

Abstract

A technique for obtaining age estimates for regolith profiles in Australia, based on the oxygen‐isotope composition of the clay mineral assemblage in a profile, is applied to a variety of regolith profiles and kaolinitic sediments from across Australia. Excluding monsoonal regions in the north of the continent, it is possible to distinguish profiles formed in the Late Mesozoic‐Early Tertiary (δ¹⁸O values between +15 and +17.5%δō) from profiles formed in post‐mid‐Tertiary times (>+17.5%ō). In addition it is concluded that there remain widespread remnants of a deep‐weathered regolith which developed in pre‐Late Mesozoic (Early Cretaceous or Jurassic?) times when Australia was at high latitude. The low δ¹⁸O values associated with clays formed in pre‐Late Mesozoic times (+10 to +15%o) suggest that deep weathering took place in a cool to cold and presumably humid climate, contrary to the traditional belief that deep weathering requires tropical to subtropical temperatures. The formation of deep‐weathered profiles at high latitude in a comparatively cold climate may be linked in part to higher past atmospheric CO₂ levels.