Stable silicon isotope signatures reflect the storage and flow paths of fluid draining through both mesoscale hillslopes and natural watersheds.
Authors: Guertin Andrew , Cunningham Charlie , Bouchez Julien , Gelin Marine , Chorover Jon , Troch Peter , Bauser Hannes , Kim Minseok , Derry Louis , Druhan Jennifer 
Published: March 2024

The variety of transit times and pathways water takes from infiltration to discharge through a hillslope determines the dynamic storage of the system, the capacity for water-rock-life reactivity, and ultimately the chemical composition of streamflow. In the laboratory, fluid phase stable Si isotopes (&#948;30Si) enrich through time during secondary silicate mineral formation as Si is removed from solution1. However, despite streams being weighted towards young waters, discharge from individual catchments commonly maintains a stable, often strongly fractionated &#948;30Si signature, reflective of chemically evolved solute signatures2. Furthermore, each individual catchment exhibits its own characteristic &#948;30Si signature in the stream discharge, even for comparable extents of Si depletion from solution. Such intra-site variability was attributed to a combination of multiple fractionation pathways (plant uptake and mineral precipitation) and the unique structure of fluid storage and drainage through each catchment. Here, we use three replicate artificial hillslopes at the Landscape Evolution Observatory (LEO) in Tucson, Arizona as model catchments to test if &#948;30Si of discharge can be described by an isotope-enabled reactive transport model (RTM) constrained by both the characteristic transit time distribution (TTD) and fractionation pathways of LEO. At the LEO hillslopes, the role of vegetation and hence the compounding effects of ecosystem cycling can be omitted, limiting &#948;30Si fractionation solely to the effects of mineral precipitation. We collected samples, with constrained TTDs, and measured &#948;30Si from the discharge at the outlet of each hillslope during three randomized storm events of varying intensity. The &#948;30Si in aqueous discharge reflects a clear and consistent signature of fractionation that is confined to a narrow range of values, much like natural upland watersheds, despite highly variable irrigation scenarios, retaining a signature across the three hillslopes defined by the unique hydrologic flow paths of the replicated system.&#160; We offer a quantitative and process-based framework describing these observations using an isotope-enabled RTM3. Close agreement between this coupled RTM and the discharge measurements from LEO supports our hypothesis that the &#948;30Si of headwater streams is reflective of both characteristic watershed TTDs and fractionation pathways. By applying this new understanding to reexamine upland watershed datasets we can gain insight into fluid flow paths and contributions of various fractionation pathways to water circulation through the shallow subsurface Critical Zone.&#160;1Fernandez, N. M., Zhang, X., &amp; Druhan, J. L. (2019). Silicon isotopic re-equilibration during amorphous silica precipitation and implications for isotopic signatures in geochemical proxies. Geochimica et Cosmochimica Acta, 262, 104-127. https://doi.org/https://doi.org/10.1016/j.gca.2019.07.0292Fernandez, N. M., Bouchez, J., Derry, L. A., Chorover, J., Gaillardet, J., Giesbrecht, I., et al. (2022). Resiliency of silica export signatures when low order streams are subject to storm events. Journal of Geophysical Research: Biogeosciences, 127, e2021JG006660. https://doi.org/10.1029/2021JG0066603Druhan, J. L., &amp; Benettin, P. (2023). Isotope Ratio &#8211; Discharge Relationships of Solutes Derived From Weathering Reactions. American Journal of Science, 323. https://doi.org/10.2475/001c.84469