E12 Surface Geochemistry and Weathering

Topic

Weathering is the physical disintegration and chemical alteration that rocks undergo upon contact with the atmosphere, water, and living organisms; this process generates the material that eventually becomes soil, while releasing nutrients and cations—previously trapped within the rock—into the hydrological system.

Physical weathering—fragmentation caused by freeze-thaw cycles, repeated thermal stress, or salt crystallization within rock crevices—increases the rock's specific surface area; this increase in exposed surface area accelerates chemical weathering, as there is significantly more rock surface available for water to react with.

Chemical weathering involves three distinct types of reactions: hydrolysis (the most significant), which dissolves rock silicates through direct interaction with water; oxidation, which breaks down iron-bearing minerals containing ferrous iron; and carbonation, which dissolves carbonate minerals through the action of carbon dioxide dissolved in water.

A soil production function establishes that the rate at which rock converts to soil decreases exponentially as the thickness of the overlying regolith increases: rocks that are more exposed—lying beneath a thin soil layer—weather much more rapidly because water and soil carbon dioxide have greater access to them. When the system reaches a steady state, the soil production rate equals the erosion rate; this balance between the two rates determines the stable soil thickness maintained on the hillside.

The nutrient cycle at the Earth's surface—involving nitrogen, phosphorus, potassium, and calcium—comprises four successive stages: the release of nutrients through weathering, their assimilation by living organisms, their return to the soil upon the decomposition of organic matter, and their eventual loss via leaching into groundwater. The dissolved load carried by rivers precisely reflects the intensity of chemical weathering occurring within their basins: rivers draining basins with easily weathered silicate lithologies—such as granites or basalts—exhibit higher concentrations of dissolved calcium, sodium, potassium, magnesium, and bicarbonate than those draining carbonate sedimentary basins. In the latter, the carbonates themselves, despite being more soluble, tend to dissolve almost completely upon initial contact with water, leaving little opportunity for further enrichment as the water flows downstream.

The expected behavior of this entire system involves a negative correlation between the rate of physical erosion—which exposes fresh rock—and the concentration of dissolved solutes in the water, which reflects the duration of contact between the water and the rock prior to runoff. Thus, from the initial rock disintegration discussed at the start of this text to the negative correlation described at its conclusion—and encompassing the acceleration of chemical weathering by physical weathering, soil production, and nutrient cycling—surface weathering and geochemistry ensure that a river's chemical composition ultimately serves as an indirect measure of the duration of the ongoing interaction between the water and the rock within its basin.

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