Coeval biochemical and biophysical weathering processes on Quaternary sandstone terraces south of Rabat (Temara), northwest Morocco

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Despite numerous investigations on substrate-inhabiting microflora, especially lichens, very little is known about the colonization of coastal escarpments by lithobiontic microorganisms, inland of a retreating coastline in Africa. Reported herein are the results of a combined field observation and microscopy study focusing on the connection between microrelief of the substrate, colonies of lithobiontic micro-organisms (in particular the lichen Xanthoria parietina) and microstructures of putative bacterial origin. The occurrence of weathering pits in which the early stages of the biotic development occurs, and the subsequent disintegration of the rock indicate that lichens, mosses and fungi act synergistically by alternating chemical and mechanical weathering. Penetration of grains by expansion and contraction of the hyphae depletes the rock matrix and contributes to the mechanical breakdown of the rock. Calcite rhombs on the weathered surfaces of the calcite-cemented sandstones are severely etched with well-developed rhomb-shaped etch pits (‘spiky calcite’), holes, or has one or more of the faces removed, and their cores exposed and leached. Nanofilaments (c. 100-700 nm) and ‘nanomicrobial’ fruiting bodies (c. 250 nm) emanating from micropores appear to be common on affected crystalline structures. Weddellite present immediately below the thallus is a strong indicator of biomineralization.

Quartz responds differently to chemical weathering by producing peeling structures and microbrecciation features. The dissolution of these crystals appears to be a surface reactioncontrolled process mediated by microbial microfilaments and nanofilaments. A model is proposed, firstly indicating early-stage biochemical weathering, followed by biophysical weathering. Disintegration of the rock outcrops in due to a complex interplay of several events, probably beginning at the nanoscale with penetration of sites on crystal faces.

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