Stable silicon isotope fractionation reflects the routing of water through a mesoscale hillslope

Abstract

Concentration - isotope Ratio - Discharge (C-R-Q) relationships offer a promising means of disentangling the underlying hydrologic, geochemical, and ecological factors that produce variations in stream solute chemistry across a variety of critical zone systems. However, natural environments are both temporally and spatially complex, and prevailing interpretations of these C-R-Q patterns remain difficult to validate. Here we employ three replicate artificially constructed hillslopes at the Landscape Evolution Observatory (LEO) in Tucson, Arizona as simplified analogs to natural catchments. LEO allows us to record silicon stable isotope (δ30Si) signatures of fluid discharge under a controlled irrigation schedule in a system devoid of vegetation. This unique meso‑scale experiment enables, for the first time, evaluation of metalloid stable isotope signatures at the scale of natural hillslopes constrained to known fluid transit time distributions (TTDs) and limited to fractionation in association with secondary mineral formation. We report δ30Si in discharge samples collected over three randomized storm events of varying intensity. The data reflect consistent enrichment in the fluid phase between +1.00 and +2.07 ‰ across the three hillslopes, despite highly variable irrigation scenarios, reflecting substantial loss of SiO2 from solution due to secondary mineral precipitation. We compare results from an isotope-enabled Reactive Transport Model (RTM) and the discharge measurements from LEO to constrain the contributions of both characteristic watershed TTDs and fractionation pathways in emergent δ30Si signatures. Our study confirms and expands upon recent work in natural systems attributing intra-site variability in silicon stable isotope signatures to the routing of fluid through catchments.