Geomorphic processes that sculpt landscapes and constitute natural hazards vary with climatic fluctuations that have characterized the last several million years of Earth history. While some climate-driven erosional processes, such as glaciers, leave a profound topographic imprint, others are more difficult to decipher. Changes in vegetation, for example, should affect the frequency and magnitude of landslides and the vigor of soil production and transport on hillslopes, yet there is sparse information regarding landscape response to late Quaternary variations in vegetation. By coupling records of paleo-erosion and paleo-ecology from a landslide-dammed lake in the Oregon Coast Range, this research will document how late Quaternary vegetation changes translated into landscape modification. The study will extend and augment a previously published paleoenvironmental record showing that the structure and composition of forests in the study area have changed dramatically over the last 40,000+ years. By analyzing paleo-erosion rates using cosmogenic radionuclides from additional lake sediment cores, the study will determine whether increases in the density and size of trees tend to increase or decrease rates of sediment transport and erosion. Although trees and understory vegetation are important for anchoring soil and suppressing erosion over human timescales, bioturbation is thought to be a major driver of landscape change when integrated through time. As such, the net geomorphic effect of a change from meadow-dominated, parkland forests to closed, coniferous forests is difficult to predict with the current knowledge base. The project results will address first-order linkages between climate and geomorphic processes and test emerging numerical models that assess how landscapes evolve in response to millenial-scale climate and vegetation changes.
The research project seeks to document and quantify how changes in climate and vegetation over the last 40,000 years have affected rates of erosion in a steep, forested landscape. The anticipated outcomes are important for predicting how future climate change will influence landscape dynamics as well as interpreting human-induced changes, such as those associated with timber harvesting. Although numerous numerical models exist to predict landscape adjustment to climate change, the governing equations are poorly understood and in particular, the role of vegetation requires significant quantitative advancement. By exploiting a detailed sedimentary record preserved in a lake from a mountainous environment, the results will reveal how ecosystems and landscapes may have co-evolved to generate the current terrain. The results should also reveal whether 'events' or periods of rapid surface change have occurred in recent geological history in response to climate or vegetation change.
The impact of climate on landscape form and the export of sediment and organic material is commonly studied and quantified in mountainous terrain where iconic U-shaped valleys and glacial moraines reveal past glacial advances. Most of the global population does not live in nor cultivate these areas, however. Rather, unglaciated, soil-mantled terrain is ubiquitous across the Earth’s surface, yet we have a poor understanding of how past climate regimes shaped these regions, if at all. In our Pacific Northwest study area, sedimentary records, climate models, and archives of past vegetation suggest that the landscape experienced widespread increased erosion due to frost processes. In this interdisciplinary research project, we quantified how climate changes related to differences in how the Earth orbits the Sun increased erosion rates about 21,000 years ago when much of Canada was overrun by a ~1 mile thick ice sheet. We accomplished this by combining computer simulations of paleoclimate, a model for how ice and water move through rock to fracture and transport it, and data showing how erosion rates and ecosystems have changed in our study area. The data for this study area comes from a 200 foot deep core of sediment collected from deposits in a 50,000-yr old lake created by a landslide dam. Essentially, the landslide-dammed lake acted as a sediment reservoir, recording how ecosystems, sediments, and erosion have changed in the last 50,000 years. Recent global compilations identify profound climate-driven changes in erosion, particularly in mid-latitude regions, although mechanistic explanations have been lacking. Our results suggest that frost weathering, rather than precipitation, promoted increased erosion in extensive unglaciated terrain during cold periods. Our results are important for interpreting how the modern landscape was shaped by past processes. Interestingly, the legacy of these past cold and dry frost-dominated times appears to have implications for the sustainability of soil (and silviculture) in modern densely forested regions like Western Oregon and Washington.
