ISO-SOITIN · Isotopes in soils: modeling transport under climate interactions

The stable natural isotopes 18O, 2H are instrumental hydrological tracers, widely used to study historical climate change. We explore the opposite approach and study how climate change, which is expected to increase temperatures and climate variability, affects isotope tracer signal quality. Signal quality refers to the extent that water can be traced to distinct infiltration events. As isotope fractionation processes are strongly governed by ambient temperatures, we hypothesize that the signal quality of isotope tracers infiltrating into soils may increase under climate change. However, competing effects that degrade signal quality occur during transport through the vadose zone, yielding uncertain total effects on signal quality. E.g., water may experience more frequent and substantial reversals in flux direction, causing more mixing and information loss. In heterogeneous soils, horizontal mixing, and the spatially separate transport of isotopes and heat (which affect fractionation rates nonlinearly) may further degrade signals across multiple spatial scales of observation.Signal alteration by climate change is especially impactful in cold regions: snowfall has different isotope compositions than rainfall, and freeze-thaw and sublimation-deposition fractionation also occur. Thus, we develop models to simulate isotope transport and fractionation in the vadose zone under spatio-temporally heterogeneous fluxes, validated with field data from diverse vulnerable cold regions. We aim to study how complex interactions arising from climate change, transient fluxes, and soil heterogeneity affect signal quality, and identify implications for isotope tracer research under changing climates. If signal degradation is substantial, paradigm shifts to two-way complex interactions between isotope tracer signals and signal quality is necessary: research must consider how signal quality changes, together with how the signal itself changes, both of which are not easily disentangled.