Chemical weathering and physical erosion are interdependent processes that sculpt landscapes, regulate the development of soils, and deliver solutes and sediment to streams. They also play critical roles in Earth's long-term climatic evolution, since silicate weathering is the ultimate long-term sink for atmospheric CO2. Field studies of climatic controls on chemical weathering have typically measured weathering rates at widely distributed sites (in order to span a wide range of climates), making it difficult to control for variations in bedrock mineralogy and in mineral supply rates from physical erosion. Because the effects of climate, lithology, and erosion have been difficult to isolate from one another, their relative importance in regulating chemical weathering rates remains uncertain.

Intellectual Merit: Recently developed methods now allow long-term rates of physical erosion and chemical weathering to be measured simultaneously in eroding landscapes. Cosmogenic nuclide measurements of long-term denudation rates can be combined with the bulk chemistry and mineralogy of soils and their parent bedrock, yielding measurements of long-term chemical weathering rates, both for individual rock-forming elements and individual minerals. This approach explicitly accounts for the potentially confounding effects of site-to-site differences in erosion rates and bedrock composition.

This proposal employs these new methods to measure rates of physical erosion and chemical weathering across two climate transects in the canyon of the South Fork of the Salmon River. From the summits of adjacent mountains, the canyon of the South Fork is as deep as the Grand Canyon, and on average twice as steep. This dramatic topographic relief generates steep climatic gradients, forming a natural laboratory for studying climatic effects on biogeochemical processes: within distances of less than 6 km, relief varies by 1300-1500 m, mean snow-pack water content varies by 4- to 5-fold, and mean annual temperature varies by 7-9 degrees C (greater than the temperature changes accompanying glacial-to-interglacial transitions in the mid-latitudes).

In 19 study plots along these climate transects we have extensively sampled soils and bedrock, and have installed datalogging thermocouples and soil moisture probes, which are now monitoring the subsurface weathering environment around the clock and throughout the year. Thus variations in weathering rates can be directly linked to the local soil microclimates, as well as above-ground conditions, across these climate transects. The proposed project leverages this investment with geochemical and mineralogical analyses and cosmogenic nuclide measurements, to quantify long-term chemical weathering rates across these climate gradients.

This project will quantify effects of climate on long-term chemical weathering rates in the field, by maximizing the variation in climatic forcing, and accounting for any confounding variations in lithology and erosion rates. These results will contribute to models of long-term climatic evolution, to assessments of sediment and solute delivery from mountainous terrain, and to quantitative understanding of how climate change will affect soil formation and biogeochemical processes in montane ecosystems.

Broader Impacts: This study will provide research training for one Ph.D. candidate. Undergraduates will gain hands-on research experience in fieldwork, sample preparation and laboratory analyses. This work will also further the goals of the Critical Zone Exploration Network (CZEN) initiative, by establishing a 'natural laboratory' in which climatic effects on long-term weathering rates have been rigorously quantified, thus creating opportunities for ourselves and others to study the biogeochemistry, microbiology, hydrology, and geomorphology of weathering processes.

Agency
National Science Foundation (NSF)
Institute
Division of Earth Sciences (EAR)
Type
Standard Grant (Standard)
Application #
0643129
Program Officer
Enriqueta Barrera
Project Start
Project End
Budget Start
2007-03-01
Budget End
2010-02-28
Support Year
Fiscal Year
2006
Total Cost
$130,937
Indirect Cost
Name
University of California Berkeley
Department
Type
DUNS #
City
Berkeley
State
CA
Country
United States
Zip Code
94704