Expect the unexpected! In geomorphology, the trinity of lithology, climate and tectonics are oft-cited as controls on landscape form and process. My research delves deeply into the role of biota, climate and lithology (or more specifically rock properties) in determining the rates and styles of geomorphic processes through time.

In July 2015 I began work as an NSF EAR postdoctoral fellow. For more details on my postdoc cross-Critical Zone Observatory research proposal: Cracking the critical zone: Tree roots in fractures and a proposed mechanistic soil production function click here.

For two of my recent projects I explored and quantified linkages between climate, ecosystems, mechanisms of bedrock to soil conversion, and erosion rates.   Working with a 63 m, 50-ky sediment core I collected from a landslide-dammed paleo-lake deposit (Little Lake) in the non-glaciated Oregon Coast Range, I coupled environmental data gleaned from macro-fossils with CRN-derived erosion rates in order to consider: ‘How might erosion rates vary given millennial-scale changes in climate and ecosystem types from a forest to a subalpine setting to the modern temperate Douglas fir forests? 

1. I employed down-scaled paleo-GCMs informed by a bio-climate model derived from macro fossils in the Little Lake core to model the spatial extent and intensity of abiotic (frost cracking)  vs. biotic (tree throw) soil production across the Oregon Coast Range during the Last Glacial Maximum (LGM). The presence of Picea sitchensis (Sitka spruce) and Abies lasiocarpa (subalpine fir) in the core during the LGM imply mean annual temperatures of ~ 1 °C and January mean temperatures of ~ -7 °C. Today these species grow in Southern Alaska and their presence in the core along with other field evidence inspired the region-wide frost cracking modeling. My results suggest that a large percent of the non-glaciated Oregon Coast Range was subject to periglacial process during the last glacial interval (see Marshall et al., Science Advances, 2015).

2. At a much finer temporal and spatial resolution, I have used data from the Little Lake core to couple paleo-ecosystems, inferred -climate and -10Be erosion rates. The Little Lake watershed is ~ 8 km², with direct hillslope to paleo-lake coupling, making it a near-ideal sediment archive.   Be-derived erosion rates increase during the non-glacial interval as temperatures are decreasing and by the glacial interval, 10Be-derived erosion rates are more than 2x modern erosion rates, challenging the notion of a steady-state landscape in the Oregon Coast Range (Marshall et al., in review, GSA Bulletin).

Following in the footsteps of G.K Gilbert and E. Yatsu , I continually come back to the role of rock properties in controlling the rate and style of soil production and erosion.  For example over what scale do minor differences in rock properties in a presumably uniform lithology influence the functional relationship between geomorphic processes and landscape form?’ By using a combination of field work, petrology, rock mechanics and lidar analysis, I  demonstrated how extremely minor diagenetic alterations control soil production, relief and landscape evolution in the Oregon Coast Range. At multiple scales my analyses suggest that even small differences in rock properties, occupying as little as ~ 10 % of a watershed can control geomorphic function to such a degree that morphologic deviations attributed to climate or tectonics may actually derive from variability within these ‘uniform’ lithologies (Marshall et al., 2014).