Cover Image Credits

The figure shows a schematic cross section through the Critical Zone, the zone that extends from unweathered rock to the top of vegetation - the zone “where rock meets life”. The arrows show the principal fluxes of elements (denoted by “X”) within and from this Earth surface reactor: by regolith production (RP), erosion (E), weathering (W) as determined in regolith or in river water, uptake into vegetation (U), return from vegetation into soil by litterfall (L), the recycling of plant-utilized elements (Rec), and the “dissolved export efficiency” (DEE) that quantifies the fraction of element X that appears in the river flux relative to its release from regolith. Also shown are the isotope ratios of stable and radiogenic isotopes commonly used to fingerprint the source and the relative quantities of metal and metalloid elements as they move between Critical Zone compartments.

The schematic is a modified version of figure 2 in von Blanckenburg and others (p. 1111–1163). The authors of this article describe geochemical methods suited to determine these Critical Zone fluxes. The methods were applied to three global sites that differ in erosion rate - the “erodosequence”. The results were used to evaluate whether erosion rate or plant growth dominates as principal control over weathering fluxes. The isotope ratios shown in blue denote the systems explored at the same sites in three companion papers in this issue: Si stable isotopes (Frings and others, p. 1164–1203) and the Ge/Si “pseudoisotope” system (Frings and others, p. 1204–1245), Sr radiogenic and stable isotopes (Bouchez and von Blanckenburg, p. 1246–1248), and Mg stable isotopes that were published previously.