“CZ-topes”: Metal stable isotopes in the Critical Zone

The “Critical Zone” is the thin layer between the upper surface of unweathered rock and the upper boundary of the vegetation. It includes soil, ground water, and plants. In the Critical Zone rock meets life. Models predict that degree soil weathering and nutrient availability depend on landscape rejuvenation through erosion. We test this prediction using innovative isotope models in mountain field sites that differ in erosion rate.

The Critical Zone. The “Critical Zone” is the thin layer between the upper surface of unweathered rock and the upper boundary of the vegetation. It is a giant reactor in which rivers and wind continuously move sediment eroded from soils. Yet soils have mantled most of this surface persistently in times before human land use. What is preventing this relatively thin layer from being completely stripped away? What are the processes that are continuously replenishing it? How can the chemical mineral dissolution and pre-anthropogenic erosion processes, governing soil production and removal, balance each other over the long-term, regardless of tectonic or climatic forcings? Despite being key to understanding the foundation of our livelihood, the processes that maintain soils in balance are yet to be identified and understood.

The use of metal stable isotopes (left) and meteoric cosmogenic 10Be/9Be ratios (right) to derive material fluxes in the Critical Zone.

We focus our research in the Critical Zone on the methodical development of new techniques and observations in field laboratories.

With methodological developments we explore and develop new tools that serve to investigate the processes in the Critical Zone, its underground structure, and the fluxes of matter. We are developing chemical and mass spectrometric procedures that enable us to precisely measure a whole array of these novel isotope ratios (7Li/6Li, 26Mg/24Mg, 30Si/28Si, 56Fe/54Fe, 88Sr/86Sr) in the compartments of the Critical Zone: parent rock, saprolite, soil, soil water, higher plants, river water, river sediment. Each of the corresponding chemical elements displays a distinct behaviour at the Earth Surface, allowing to disclose the relevant processes such as dissolution of primary minerals, precipitation of secondary minerals, or uptake by plants.

In Field laboratories our focus is on bridging time scales and looking into the past of the Critical Zone. Our focus is further the deep (groundwater and deep weathering) processes taking place at the base of the zone. We explore the Critical Zone from the soil profile to the watershed scale. Models predict that degree soil weathering and nutrient availability depend on landscape rejuvenation through erosion. We test this prediction in mountain field sites that differ in erosion rate and that all feature granitoid rock type: the slowly eroding Highlands of Sri Lanka, a moderately eroding mountain upland in Sierra Nevada, California, and a rapidly uplifting alpine mountain belt in the Swiss Central Alps. Together these landscapes provide a wide range of denudation rates ranging from 5 to 5600 tons /km2 /y1 with which we can effectively study the patterns of erosion–weathering coupling. At these sites, we selected soil profiles along recently exposed roadcuts, along which we sampled surface soils, through saprolite, to bedrock. We also extracted pore waters from soils and saprolites, main types of vegetation, and collect stream water.