Deep Weathering, Groundwater and Climate

Abstract

Weathered rock holds groundwater. The zone of groundwater saturation is topped by a watertable which separates an upper oxidised zone from a lower, reducing zone. Some elementary weathering scenarios envisage weathering in only the acid, oxidizing upper zone, but weathering can proceed at the base of the saturated zone by hydrolysis. Weathering products must be removed if the reaction is to continue: in upper groundwater zones they are washed away; in lower zones chemical diffusion removes weathering products from the weathering front to the zone of mobile groundwater. Since deep weathering profiles may be tens of metres thick, the temperature at the ground surface does not affect the temperature at depth.“Tropical” and “lateritic” are usually undemonstrated and unhelpful adj actives used in describing deep weathering profiles. Independent evidence shows that the climate at the time of formation of some deep weathered profiles was warm and wet, but this relationship is not necessary and some deep weathering occurred under cold conditions. In Australia some deep weathering is as old as Mesozoic, and formed before the breakup of Gondwanaland. The composition of alluvial gravels changed through the Tertiary as deep regolith was progressively stripped. Hydrothermal alteration has seeldom been demonstrated to be the cause of deep kaolin deposits: stable isotope studies indicate they are more likely to be caused by weathering.