CZ-tope at Susquehanna Shale Hills CZO: Synthesizing multiple isotope proxies to elucidate Critical Zone processes across timescales in a temperate forested landscape

The application of multiple isotope proxies on the same location within a Critical Zone (CZ), which we term “CZ-tope”, elucidates the interactions of geochemical, geomorphological, hydrological and biological processes together with anthropogenic influences in the CZ across widely disparate timescales. We exemplify the CZ-tope approach by summarizing the emerging hypotheses developed from isotopic measurements at the Susquehanna Shale Hills CZ Observatory (SSHCZO), Pennsylvania (U.S.A). At SSHCZO, measurements of U-series isotopes and meteoric 10Be in regolith provide evidence that the catchment is approaching a steady state at the ridgetops where regolith production is balanced by erosive loss. Isotopic measurements of δ13C, 87Sr/86Sr, and δ34S in the regolith, bedrock and water, together with 3H, δ2H and δ18O in various water reservoirs (precipitation, soil water, stream water and groundwater) support the hypothesis that nested reaction fronts have developed in the subsurface over timescales of millennia. Combinations of U-series and meteoric 10Be in bedrock and regolith and measurements of soil water δ18O led to the hypothesis that freeze-thaw is the dominant soil creep mechanism controlling regolith fluxes and hillslopes. Utilizing the CZ-tope approach of measuring δ26Mg, δ56Fe and δ11B isotopes on identical samples, we also developed a working hypothesis that particle transport in the subsurface represents a significant weathering loss from the catchment. Likewise, the use of δ18O, Ge/Si and 87Sr/86Sr ratios in xylem source waters (precipitation, soil, stream- and ground-), along with 87Sr/86Sr ratios from soils, led to the hypothesis that O isotope fractionation occurs near clay surfaces. Finally, analyses of 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and 137Cs in soil from hillslope profiles have also revealed the imprint of widespread human activity. CZ-tope — the interpretation of multiple elements' isotopic ratios quantified for identical samples in one landscape — thus paints an emerging picture of SSHCZO as a relatively fast-eroding but slow-weathering landscape in which: i) nutrients are tightly cycled by vegetation, ii) soils move downslope largely by freeze-thaw, iii) subsurface particle transport is an important flux for mass loss, iv) mobile and immobile reservoirs act to fractionate water and cations into trees and stream water, and v) the imprint of humans is manifested in the metal contents of the topsoil.