Our group’s research focuses on two themes: surface processes and sea-level change. This work is motivated by two overarching questions: How do climate and tectonics influence the topographic and geochemical evolution of mountains? How do sea-level changes drive topographic changes and respond to them? We address these topics with a quantitatively rigorous and novel blend of numerical modeling, lab measurements, field monitoring, and analogue experiments.

Landscape evolution

Hanalei River, Kaua‘i
Hanalei River, Kaua‘i

How does climate affect how topography evolves?

Earth’s topography evolves as rock and sediment move from one place to another. The rate at which this happens is hugely variable in time and space, with some landscapes eroding hundreds of thousands of times faster than others. We use a variety of tools to investigate why this is, including measurements of cosmogenic isotopes, which yield erosion rates over millennial timescales, as well as analysis of high-resolution topographic data, satellite imagery, and numerical modeling of landscape evolution.

Our work involves studying so-called natural laboratories—places with large variations in climate and minimal variations in other factors that can affect rates of landscape evolution. For instance, the Hawaiian island of Kaua‘i is home to some of the world’s largest and steepest rainfall gradients, while containing relatively small variations in rock type and rock uplift rates. Deep canyons incised into the underlying basalt have left sharp, steep ridges high above the rivers, and provide an exceptional natural laboratory for studying how climate steers the evolution of topography. Our work suggests that mean annual precipitation rates influence basin-averaged erosion rates as well as the efficiency of bedrock channel incision, and provides rare empirical support for theories about the size and asymmetry of mountain ranges.

Sea level dynamics

Modeled rates of sea level change
Modeled rates of sea level change

How does sediment affect sea level?

On decadal timescales, changes in sea level pose a serious hazard by modulating flood frequency, and on geologic timescales they steer the evolution of coastlines, marine sedimentary deposits, and continental topography.  We are investigating how sea level responds to the massive redistribution of sediment during continental erosion and marine sedimentation around rivers with the world’s largest sediment fluxes.  Our model simulations suggest that sediment erosion and deposition can significantly perturb rates and patterns of sea level change, and that these effects can persist for tens of thousands of years.  Our investigations add a new piece of physics to our understanding of sea-level change, and imply that modern rates of sea level change and patterns of paleo-sea level change must be interpreted in light of sediment fluxes over the past tens of thousands of years.

Chemical weathering

Modeled responses of chemical erosion rate (W) to a step increase in rock uplift. Red indicates an increase in W, blue a decrease.

What sets the rate of mineral weathering in soils?

Measuring chemical erosion rates in natural environments is important for a number of reasons: chemical weathering accelerates landscape evolution, releases solutes that provide the nutritional foundation for life, neutralizes acidic precipitation, and stabilizes Earth’s climate over long timescales.

San Jacinto Peak, California

Steep mountains provide an exceptional environment in which to measure chemical erosion rates over a range of climates. Our measurements in granitic mountain ranges show that chemical erosion rates are fastest at the highest elevations, where it is coldest and where snow cover is most persistent, suggesting that soil water content exerts a stronger control on dissolution than does temperature. Our measurements also show that dust can play an important role in setting soil chemistry, which led us to propose a novel method for inferring rates of dust incorporation into soils.  Our modeling suggests that soil chemical erosion should be fastest at intermediate physical erosion rates, a prediction with implications for the long-term stability of Earth’s climate. This work also shows that transient responses of chemical erosion rates should be dominantly controlled by the size of first-order catchments, contrary to expectations that they should be controlled by dissolution kinetics. This in turn suggests that climate and biota may mainly affect transient chemical weathering signals by influencing rates of river incision and soil transport (which control the scale of first-order catchments), rather than by affecting rates of mineral dissolution.