Sand coatings in paleosols: Evidence of weathering across the Plio-Pleistocene boundary to modern times on Mt. Kenya

Abstract

A comparison of younger post-Olduvai paleosol horizons with older buried horizons of Plio/Pleistocene age shows that primary and secondary minerals and organic molecules in sand coatings in these paleosols remain intact and provide a record of past weathering events. From IR spectra, concentrations of organic and some inorganic constituents within these coatings reveal supporting evidence of variable weathering trends from Late Pliocene to Holocene time. With some variability, hematite and goethite concentrations in sand coatings parallel similar variable concentrations of secondary Fe-extractable concentrations previously determined for pre- and post-Olduvai paleosols in these Mt. Kenya sections. Negative correlations between metahalloysite and gibbsite, previously determined to result from aggressive leaching of Si from metahalloysite in paleosols, tend to follow similar distributions in the sand coatings reported here. Similarly, additions of gibbsite in selected paleosol horizons are followed by reductions of lesser concentrations of metahalloysite, suggesting that individual horizons within paleosols have independent leaching histories different from the entire paleo-pedon. As with these paleosols analyzed previously, quartz exists in greater proportion within sands recovered from Ah and Ahb horizons, and this mineral has previously been considered to be sourced from the older surrounding African crust rather than from local Neogene volcanic sources. The presence of vermiculite in sand coatings, previously not detected in Mt. Kenya paleosols, may be related to the presence of cedar stands (Juniperus procera) within the timberline zone of the mountain, a forest species-clay mineral correlate that may relate to past forest biome evolution, specifically in the pre-Olduvai paleosols. The occurrence of vermiculite in these older paleosols, a clay species with a known affinity for conifers (Birkeland, 1999), may indicate a higher timberline during pre-Olduvai time. The outcome of this analysis shows that certain horizons act as a singular open weathering system responding to the throughput of either meteoric or groundwater in selective ways, to produce a range of weathering products sourced from primary minerals, and selective allochthonous mineral inputs from near and far. More importantly, sand coatings are shown to provide important paleoenvironmental data parallel to that recovered from macro-paleosol systems. Moreover, the wealth of paleoenvironmental data recovered from sand coatings rivals that in weathering rinds, and the combined data are applicable to other paleoenvironments both on Earth and Mars.