Mineral weathering, or the breakdown of rock at the Earth’s surface, is a fundamental process that forms soil and sustains life. Mineral weathering is often considered a “top-down” process because forces that increase weathering, such as root activity, are strongest in the uppermost soil layers and decrease with depth. As this top-down weathering process continues, soils deepen, and the elements liberated from the underlying geologic materials are made available to plants and other organisms.

This project examined a new weathering framework in mountainous regions of the northeastern US that have been glaciated and where bedrock is relatively shallow. In these regions, shallow soils and glacial materials force soil water to move more laterally or downslope as opposed to simply in this top-down vertical fashion. Our hypothesis was that mineral loss of the primary minerals that make up soil, glacial materials, and bedrock and the distribution of weathering products from the breakdown of those minerals would be predictably distributed downslope. In areas of those watersheds where shallow soils force lateral water to move through accumulated organic matter that collects on the forest floor, the groundwater would be more acidic and intensify this process.

We examined this hypothesis in a watershed at the Hubbard Brook Experimental Forest in New Hampshire where a wealth of background data on its bedrock and soil chemical composition, its history in the development of previous studies of weathering, and its similarity to other glaciated mountainous regions of the northeastern US made it an ideal place to carry out our study. Our work showed through multiple methods that weathering of plagioclase, a common soil mineral, varied over short distances (40 m). Plagioclase weathering was highest in thin, upslope soils, and lowest in deeper soils downslope. Upslope soils frequently saturated and drained more acidic groundwater that was high in dissolved organic carbon. Because bedrock was shallow in upslope soils relative to deeper soils downslope, this enhanced mineral weathering in a lateral downslope direction corresponding to water movement. An additional finding was that some minerals important to plants were completely absent from the fine material of upslope soils. These minerals were present within rocks embedded within and underlying the fine mineral, highlighting the potential nutritional significance of rocks.

The systematic variation in groundwater dynamics and its acidic chemistry controls the patterns of soils, mineral depletion, and nutrient availability for ecosystem processes, and it sets the chemical composition for water sources that contribute to headwater streams. This research demonstrates how different portions of landscape interact to produce material flows that are transported to streams and downstream waterways where most people live and use water. These soils also provide other vital services including wildlife habitat and a regional economy driven by sustainable harvest of forest products and recreational opportunities. Many of these services are dependent on the balance between the rate at which rocks break down to supply soil nutrients versus the rates at which materials are removed via tree harvest or are transported downstream. As climate continues to change and these regions recover from acid rain, it is important to understand which parts of the landscape may be critical in driving key services and benefits for people and ecosystem resilience.

During this project, researchers engaged with two cohorts of high school students from the Upward Bound program at Keane State College in New Hampshire, four REU (Research Experience for Undergraduates) students, and an artist. We developed an Upward Bound program for low-income high school students who would be the first in their families to attend college. Our team, which included principal investigators, graduate students and REU students, conducted a hands-on two-day field experience for 28 Upward Bound student to expose them to the scientific process and field-based research while piquing their interest in STEM disciplines. Students participated in basic field characterization of soils and ecosystem characteristics and used their data to develop an understanding of how ecosystems function and support human needs. Our team also convened a seminar for about 100 Upward Bound students on how long-term environmental research can have positive impacts on environmental policy by using the example of how the discovery of acid rain at Hubbard Brook in the 1970s was made possible by the collection of long-term environmental data. The culmination of the seminar was a panel discussion led by the students on our team about pathways to college, studying STEM subjects, and careers in science. As part of an art-science engagement program, our team also worked with an artist who developed exhibits to communicate the broader significance of our work, but also to help non-scientists understand the process of science. Two public exhibits, one in New Hampshire and one near Philadelphia, featured paintings of microphotographs of minerals that were collected during our study for calculations of mineral depletion.

Last Modified: 12/19/2023
Modified by: Donald S Ross