Micromorphology of a welded paleosol in the Dillondale loess, Charwell Basin, South Island, New Zealand

Abstract

Micromorphological evidence and soil phosphorus chemical data supporting the presence of a welded paleosol from the Dillondale loess are presented. Since both topdown and upbuilding pedogenesis can occur in loess deposits, welding of paleosols will occur when topdown pedogenesis overprints an existing paleosol. Intact soil samples were collected and their micromorphology described, and soil P fractions were determined. The micromorphology and soil P fractions provide strong evidence for a welded soil between 3.0 and 4.0 m depth (L3b basal unit). Throughout this unit the b-fabric is both porostriated and granostriated; amorphous concentration features are moderately to strongly impregnated, postdating clay cutans. We interpret this to reflect greater mechanical stress on the clay domains and hence a longer period of topdown pedogenesis. Low levels of PCa and a high proportion of POcc in the L3b basal unit imply these are highly weathered horizons. This suggests two phases of topdown pedogenesis during a hiatus in loess accretion. We propose that an extended period of illuvial clay deposition in the underlying b2Btg1 and b2Btg2 horizons occurred, reducing macropore continuity in the horizons above. Intense anaerobic conditions would have resulted, with the onset of gleying and the formation of the amorphous concentration pedofeatures.