Natural Experiments in Landscape Dynamics and Carbon Cycling

Author: Geyman, Emily C.

Year: 2027

Degree: Dissertation (Ph.D.)

Advisor: Lamb, Michael P.

Committee Members: Fischer, Woodward W.; Frankenberg, Christian; West, A. Joshua; Lamb, Michael P.

Option: Geology

Abstract

Many changes to Earth's landscapes evade direct observation because they unfold either too slowly or too episodically for modern instrumental records. This thesis develops two complementary strategies that extend the observational record in opposite directions: natural experiments that translate long periods of time into space, and sensitive remote-sensing methods that resolve the small increments from which long-term change is built.

The first strategy is to use landscapes in which time is expressed across space. Along Arctic rivers, channel migration continually creates new floodplain terrain, producing chronosequences within a common environment that reveal how soils, permafrost, vegetation, and terrestrial carbon storage evolve over centuries to millennia. An analogous timeline occurs along the San Andreas Fault, where lateral plate motion creates a small mountain range preserving an approximately 100,000-year record of uplift and erosion.

The complementary strategy is to make short-term landscape change visible. I develop a Fourier-based image-matching method that resolves lateral river migration at length scales 5-10x smaller than the pixel size of satellite imagery, enabling measurements of river mobility over individual seasons and flood events. I also develop a mass-conserving method for co-registering repeat lidar surveys that resolves millimeter-scale catchment-averaged erosion. Applied to recently burned landscapes, this method identifies the sediment sources of post-wildfire debris flows.

I use these observations to calibrate and test reduced-complexity models. Topography-based sediment-routing models predict how and where post-wildfire debris flows occur. Simple scaling laws connect river mobility to the ages and elevations of the floodplains that rivers construct. Floodplain carbon budgets quantify how greater plant inputs compete with accelerated soil carbon decomposition following permafrost thaw. A heterogeneous-rate soil-carbon model explains how rapidly cycling carbon can coexist with carbon that persists for millennia, helping to explain the anomalously old age of carbon in global soils. Together, these approaches use observable landscapes to constrain geomorphic and carbon-cycle processes extending beyond the duration of modern records.