Soil chronosequence development in dunes on the southeast African coastal plain, Maputaland, South Africa

Abstract

Dunes have accreted on the southeast African coastal plain in coast-parallel patterns of degraded whaleback ridges, sand megaridges and extended parabolic dunes since the Pliocene. In the Maputaland dune field, relative dating is complicated by soil degradation, erosion and vegetation cover. This project assessed alternative relative and numeric dating techniques that can be used to differentiate dune systems and eolian sand bodies. Soil profile characteristics were used to calculate soil development indices (SDI) for soil horizons sampled from hand augered holes and rare exposures. Infrared stimulated luminescence (IRSL) dating of representative dunes and sand units helped define the pedogenesis achieved since deposition. The quartzose dune sands have weathered to form very deep soil profiles. Enhanced SDI values for horizons or sampled intervals down the profile reflect pH decrease, advanced rubification or distinct mottling, clay increase and harder consistency with depth. The youngest Holocene profiles exhibit decreasing horizon index values with depth below the A horizon whereas late Pleistocene profiles display SDI values increasing with depth within the upper 3 m of the profile. Within some Mid- to Late Pleistocene eolian sands greater profile-horizonation and catena complexity manifests as complex soil profiles or distinctly mottled, clay-enriched horizons, at depths of 2–5 m below the surface. The SDI profiles from specific dune systems or stratigraphic units cluster well and effectively characterize soil development in dune sand units of different ages. Depositional age plotted against sampled horizon index values makes it possible to determine generalized rates of soil profile development, and to compare pedogenic processes on the high-rainfall, coastal barrier dunes to those found in the drier interior. Sampled horizon index values distinguish buried soil profiles and highlight possible localized surficial reworking.