Shale Hills Susquehanna Questions/Hypotheses
Question: What processes control the downslope transport and erosion of regolith, and can
these processes be functionally related to regolith thickness in a deterministic fashion?
To understand the feedbacks between regolith transport, thickness, and weathering in the SHO,
we need to i) characterize rates of erosion throughout the watershed and ii) relate these to
distributions of hillslope topography and regolith thickness through a landscape evolution model.
Task 2.1.4. Quantify rates of regolith transport and erosion: Central to the development of a
model to predict regolith evolution in the SHO is the quantitative understanding of the rates and
processes of soil transport on hillslopes. To characterize erosion rates we will exploit a high
resolution data set of hillslope topography and soil depth (H. Lin), densifying these observations
where necessary via a combination of hand-auger, soil pits, and ground penetrating radar (see
2.4). We will measure erosion rates across a range of hillslope gradients and regolith thicknesses
using the concentration of cosmogenic radionuclides from the base of the regolith [22]. These
data will be compared to average sediment yields determined from volumetric surveys of
sediment accumulation in swales [23], and from cosmogenic isotope concentrations in modern
sediment [7]. Together, these data will provide estimates of the dependence of soil production on
regolith thickness, an integral component of our geochemical modeling [9]. Moreover,
understanding how the downslope transport of regolith depends on topographic characteristics
such as hillslope gradient will provide the basis for a more process-based model of sediment
transport (see below, and Task 2.3.1). PIs: Slingerland, Kirby, Lin
Task 2.1.5. Develop and test competing models for soil transport: We propose to develop and
test competing models for soil transport (linear creep, depth-dependent creep, disturbance-driven
(tree-throw) transport) against the present-day distribution of soil depth, hillslope topography and
erosion rate. In particular, observations of pit-and-mound topography in the watershed lead us to
hypothesize that disturbance-driven transport (tree-throw -[24]) is a key component of sediment
transport on hillslopes in the SHO. We will incorporate rules that describe various soil transport
processes into a new landscape evolution model (section 2.3) and test these against the
distributions of topographic gradients, regolith depth, and lowering rates across the watershed.
PIs: Kirby, Slingerland, & Duffy.
Question: What is the role of biologic processes in the shale weathering engine and in
particular, what roles do rooting and tree-throw play?
Our hypothesis is that tree-throw [24] and the physical and chemical activity of roots strongly
influence the evolution of regolith in the SHO by: 1) moving regolith downslope; 2) mixing and
chemically affecting bedrock and subsoil; 3) providing the principal pipes through the regolith
mill for flow of water and solutes; and, perhaps, 4) dictating the microtopography of channelized
flow onthe surface. We will deconvolve these effects with the following tasks.
Task 2.1.6. Quantify the role of tree-throw in regolith transport and landscape evolution: We
will couple measurements of root-length density (see Task 2.1.7) and soil strength as a function
of regolith depth to develop a quantitative model for sediment transport and erosion by tree-throw
that will then be incorporated into a new landscape evolution model (Task 2.3.1). We will link
this model to observations of erosion across the watershed (Task 2.1.4) and with historic records
of climate (primarily, frequency of strong wind storms) to scale individual transport events to
long-term records of denudation. Moreover, we think it is reasonable to assume that tree-throw
swales impart a roughness to the surface that plays an important role in focusing overland flow
and funneling surface water to the regolith-bedrock interface. We will map these surface features
at the meter scale using a total station and terrestrial LiDAR (see letter). We will then conduct a
series of numerical experiments using a new landscape evolution model (Task 2.1.9 and 2.3.1)
subject to varying degrees of roughness based on our surveys. The experiments will test the
hypothesis that regions of simultaneous tree-throw (e.g. during severe storms) determine the
location of first-order channels by providing pathways for flow accumulation and
saturation inthe near subsurface. PIs: Slingerland, Kirby.
Task 2.1.7. Characterize the influence of root distribution on weathering: Trees
also influence regolith formation by direct interactions with the bedrock, including
physical fracturing of the rock along cleavage planes and chemical solubilization. To
investigate these processes, we will dig pits into the bedrock and map root density along
the pit-face (Edwards et al. submitted). In each genetic soil horizon and in cracks in the
bedrock, pore water will be sampled with microlysimeters (Rhizon) in localized zones of
root proliferation and in adjacent zones with few roots. Samples will be taken that include
root + rock + soil for analysis using various microscopies and elemental analysis. PI:
Eisenstatt, Brantley
Question: What are the processes and functional relationships interconnecting
the products and throughput of the regolith mill with topography and local
hydrology, and how have these changed in response to a past climatic
perturbation?
Here we wish to define the numerous interactions and feedbacks between the evolving
three dimensional ground surface, the bedrock-regolith interface, regolith thickness, and
the hydrologic flow field. Furthermore, the SHO landscape has experienced repeated
climate and vegetation changes from glacial to interglacial conditions as well as postcolonial land-use changes. We need to understand the extent to which the present
system is a palimpsest. Our approach to obtaining both objectives is to develop a new
landscape evolution model and conduct numerical experiments with it driven by
fluctuating boundary conditions and calibrated against measurements of both short- and
long-term rates of downslope regolith transport and isotopic measurement of long term
regolith production rates.
Task 2.1.9. Couple sediment transport modules to hydrologic module. The
landscape model will be based on PIHM, the Penn State Integrated Hydrologic Model
(see 2.3). We will add functions describing conservation of bedrock and sediment mass,
multi-grain size sediment routing, channel initiation and growth, hillslope sediment flux
by three-throw, rilling after fires, gravity slides, and regolith-generating functions. This
model will differ from present models such as SIBERIA [25], PRECIPTON [26],
DRAINAL [27], Detachment-limited model [28], GOLEM [29], CASCADE [30], CAESAR
[31], ZSCAPE [32], CHILD [33], and LAPSUS (LandscApe
Process modelling at mUlti-dimensions and scaleS) [34] by containing full treatments of
regolith hydrology and weathering rules, and will describe the evolution of ground and
bedrock surfaces.
PI: Slingerland, Duffy, and Kirby