Collaborative Research: Lateral weathering gradients typify critical zone architecture in glaciated catchments

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The thin skin of soil on Planet Earth is what makes life possible. The interactions of water and rock, or mineral weathering, are responsible for the development of the soil that sustains life and regulates water quality and flow. As such, mineral weathering is a critical natural service that provides nutrients required for plant growth and controls the cycling of nutrients through the environment and their transport to downstream rivers, lakes, estuaries, and the ocean. In this study, mountainous areas of the northeastern United States provide a natural laboratory to examine processes and rates of mineral weathering where soils are relatively young (less than 10,000 years) and bedrock is often shallow, providing spatial gradients of mineral depletion and accumulation across the landscape. This region with young soils supports forests in some of the most densely forested states in the country and is the source of the major rivers of the northeast, providing major metropolitan areas downstream with abundant, clean water. These forest 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 recharge soil nutrient supply versus the rates at which materials are removed via tree harvest or are transported downstream. Uncertainty in mineral weathering rates, and whether such rates keep pace with losses, is a long-standing question in the sustainability of intensive forest harvest, and in understanding how forests respond to disturbances. This question has redoubled interest due to increasing demands on forests to provide renewable biomass energy sources in addition to more traditional forest products. Many of these same nutrient elements have been depleted by decades of air pollution. Accurate calculation of mineral weathering rates is the centerpiece of critical loads models, increasingly adopted by countries across the northern hemisphere as a tool to guide development of air pollution policy and for the management sustainable ecosystems. This study, taking place at the Hubbard Brook Experimental Forest in New Hampshire, will introduce a new paradigm of mineral weathering at the watershed-scale, describing spatial variation that can be used to reformulate mineral weathering algorithms in critical loads models and address nutrient depletion concerns for forest management. Working at Hubbard Brook with one of the longest records of stream and rainwater chemistry in North America provides a framework by which this research can demonstrate how different portions of watersheds interact to produce material flows transported by forests to downstream rivers and lakes. This will help foresters and watershed managers evaluate how management decisions may be applied to ensure continued productivity of upland forests of the northeastern U.S.Through measurement of hydrologic fluxes, solute fluxes and solid phase characterization at several sites in the northeastern U.S. and at the Hubbard Brook Experimental Forest, mineral weathering and regolith development processes will be examined along transects from exposed bedrock to deep soil, a common landscape gradient in glaciated regions. Data collected at three instrumented transects at Hubbard Brook will be used to test variation in mineral weathering gradients at the hillslope and watershed scale. Single transects at four sites dispersed across the northeastern U.S. will test applicability at the regional scale, with a broader range of climate, bedrock lithology and soil type. Mineral weathering processes, including both primary mineral dissolution and secondary material accumulation in zones hypothesized to represent functionally distinct portions of catchments, will be examined through solid phase studies of regolith (soil, subsoil, and rock fragments) and bedrock using overlapping analytical approaches, including optical petrography, scanning electrode microscopy, electron microprobe mapping, bulk chemical analyses, and secondary material extraction. The regolith studies, indicating long-term weathering progression, will be complemented by measurements of current aqueous flux in weathering derived elements using a combination of passive flow metering and more traditional hydrogeologic monitoring. Area normalized aqueous fluxes at four distinct zones within the catchment will be compared to catchment scale chemical denudation to determine the importance of the study zones to overall catchment mass balance. This will facilitate reinterpretation of a 50+ year record at Hubbard Brook, which will provide essential information for the assessment of forest sustainability by discerning the distinct rates and progression of processes at differing zones within upland catchments.