Constraining climatic controls on hillslope dynamics using a coupled model for the transport of soil and tracers: Application to loess‐mantled hillslopes, South Island, New Zealand

Abstract

Landscapes reflect a legacy of tectonic and climatic forcing as modulated by surface processes. Because the morphologic characteristics of landscapes often do not allow us to uniquely define the relative roles of tectonic deformation and climate, additional constraints are required to interpret and predict landscape dynamics. Here we describe a coupled model for the transport of soil and tracer particles at the hillslope scale. To illustrate the utility of this methodology, we modeled the evolution of two synthetic hillslopes with identical initial and boundary conditions but different histories of climate‐induced changes in the efficiency of soil transport. While one hillslope experienced an initial phase of rapid transport followed by a period of retarded transport, the other hillslope experienced the opposite sequence. Both model hillslopes contain a subsurface layer of tracer particles that becomes exhumed and incorporated into the soil due to transport and mixing processes. Whereas the morphology of the two model hillslopes cannot be easily distinguished at the simulation conclusion, the spatial distribution of tracer particles along the slope is distinctive for the two cases. We applied the coupled model to our study site along the Charwell River, South Island, New Zealand, where tephra deposits within loess‐dominated soils have been exhumed and incorporated into an upper layer of bioturbated and mobile soil. We reconstructed the late Pleistocene hillslope geometry using soil stratigraphic data gathered along the study transect. Because bioturbation appears to be the predominant transport mechanism, the efficiency of soil transport likely varies with time, reflecting the influence of climate‐related changes in the dominant floral or faunal community. The modeled evolution of hillslope form and tephra concentration along the study transect is consistent with observations obtained from topographic surveys and laboratory analysis of tephra concentrations in continuous‐core soil samples. Low transport rates are apparently associated with grass/shrub‐dominated slopes during the late Pleistocene, whereas forest colonization during the Holocene increased flux rates, transforming flat, locally incised slopes into broadly convex ones. Our results attest to the utility of coupled models for tasks such as deciphering landscape history and predicting the downslope flux of soil organic carbon.