Geochemistry and palaeoclimatic reconstruction of a palaeosol sequence at Langebaanweg, South Africa

Neogene Langebaanweg (LBW) palaeosol-sediment-sequences at the West Coast of South Africa are well known for their well-preserved rich palaeontological heritage. The palaeosols developed on estuarine/marine/coastal alluvial deposits stacked on each other, ranging in age from the Mid-Miocene to Quaternary. Elemental mobility and redistribution based on major and minor element geochemistry of 11 horizons of the ~18m sequences were studied to evaluate the degree of chemical weathering, a major process of soil formation; and to reveal important palaeoenvironments in which the palaeosols developed. Elemental geochemistry was carried out on <2mm samples by X-ray fluorescence spectrophotometry. The carbonate horizons (Bk and 2Bkmb2) were dominated by CaO (mean wt.%=32.77), while the other horizons had preponderance of SiO₂ (>50% mean wt.). Two (WI-1 and WI-2) of the four weathering indices applied to evaluate weathering intensity proved unsuitable for use in an alluvial palaeosol-sediment sequences such as LBW. Chemical index of alteration (CIA) and chemical index of weathering (CIW) values were relatively more consistent in distribution pattern across the pedocomplex, but caution should be applied when using them in similar cases. The pedogenic carbonate horizons on average had the lowest values for both CIA and CIW (1.12-2.60) suggesting more advanced weathering. Plots of geochemical ratios (quantifying pedogenic processes) against depth show striking variations among and within the palaeosol and sediment sections. Soil formation was more advanced in the Mid-Miocene and Pliocene than in the Quaternary suggesting a more humid and warmer climate than present. We evaluated selected geochemical climofunctions and conclude that more data are obviously needed for a more robust calibration of the models in Langebaanweg palaeosols.