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

[1] 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 climateinduced 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.INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1625 Global Change: Geomorphology and weathering (1824, 1886); KEYWORDS: landscape evolution, soil transport, tracers, hillslope erosion, bioturbation, numerical model Citation: Roering, J. J., P. Almond, P. Tonkin, and J. McKean (2004), 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, J. Geophys.

Toward process-based modeling of geochemical soil formation across diverse landforms: A new mathematical framework

“…Such mass replacements do not occur for depositional soils or soils on level grounds. In addition, a number of studies have found evidence for transient behavior both in terms of geochemistry (e.g., Birkeland and Berry, 1991) and soil thickness (e.g., Birkeland and Berry, 1991;Roering et al, 2004;Roering and Gerber, 2005) on eroding hillslopes. Indeed, hillslopes' response to changes in climate and tectonics occurs at the timescales that are longer than the period of recent glacial and interglacial cycles (Fernandes and Dietrich, 1997;Roering et al, 2001;Mudd and Furbish, 2007).…”

Topographically controlled grassland soils in the Boreal Cordillera ecozone, northwestern Canada

“…3) resemble those interpreted to be fecal pellets produced by soil invertebrates in other Chernozemic soils (Pawluk and Bal 1985). Physical processes may also be involved in modifying soil microstructure, as in the case of elongated aggregates transitional to platy microstructures similar to those attributed to frost action in southern British Columbia Chernozemic A horizons (Sanborn and Pawluk 1989).…”

“…The parent material complexity exhibited by these pedons, most notably at the Carmacks site, would actually facilitate such research. Dated marker beds with distinctive compositions, such as the White River tephra, can be useful tracers in studies of soil mixing and transport on slopes (Roering et al 2004). In addition, the late Holocene resumption of loess deposition at Kluane Lake provides an unusual opportunity to examine the responses of soils in a grassland-forest mosaic to aeolian processes (Sanborn and Jull 2009).…”

Late Quaternary loess landscape evolution on an active tectonic margin, Charwell Basin, South Island, New Zealand

“…They used the depth variation of a 27 ka tephra within the uppermost loess sheet along a slope transect from interfluve to gully floor. Subsequent simulations (Roering et al, 2004) adequately replicated the evolution of the paleohillslope (as revealed from loess stratigraphy) to the present convex form, and also the profile distribution and concentration of tephra along the transect. This model allows us to make predictions about loess preservation based on topographic variables and compare them with field observations.…”

“…What is that point, and what processes are involved? Roering et al (2002Roering et al ( , 2004 showed that erosion rates on the Dillondale terrace are determined by local curvature. Sediment flux rate, q s (m 3 m − 1 yr − 1 ) varies proportionally with hillslope gradient (in one dimension) as:…”