This project will implement a new Critical Zone Observatory in the Boulder Creek watershed (BcCZO) in the Front Range of the Rocky Mountains in Colorado. The Earth's critical zone lies between the top of the vegetation and unweathered rock (NRC, 2001); it is in essence our living environment. Design and instrumentation of the BcCZO is driven by a suite of Earth science investigations to understand the transformation of rock into soil, and to quantify how the relevant processes are modulated by and coupled to both external forcings (e.g. climate) and intrinsic properties of the critical zone. The BcCZO extends from the continental divide to the range front, encompassing slightly more than 1000 km2 (~400 square miles) and a range of 2600 m (~8500 ft) in altitude. The relatively large drop in elevation within a single watershed provides an important contrast in climate and ecological regimes, which in turn provides a critical contrast in erosional processes. As a consequence, the properties and structure of the critical zone vary across the watershed, constituting a natural experiment on critical zone evolution and function.
Three subcatchments of the BcCZO representing different states of critical zone development will be densely monitored via a networked infrastructure of geophysical, geochemical, and hydrological instrumentation. Intrinsic properties of the rock-soil column, including microbial biomass and activity, will be measured at multiple localities. The BcCZO will also leverage extant data and instrumentation from partners in the same and neighboring watersheds (e.g., NSF's Long-term Ecological Research (LTER) site, Niwot Ridge), including the unparalleled 50-year Mountain Climate Program. These data, coupled with high-resolution digital topography, cosmogenic nuclide analyses, and numerical hydrochemical models will allow development of the first mechanistic and quantitative description of processes that generate the critical zone. Simulations will be developed to explore the effect of critical zone properties on hydrologic and biogeochemical fluxes.
The BcCZO will exploit intersections with the Niwot Ridge LTER site (which is the National Ecological Observatory Network Core Site, and with the Community Surface Dynamic Modeling System. The observatory will position itself as a resource to the broader scientific community through competitive graduate field fellowships, open science meetings, and maintenance of accessible data and sample archives. Results will be disseminated widely in journal publications, presentations at scientific meetings, and through web pages.
The observatory will support the community-based BASIN.org environmental information network, viewed by more than 150,000 unique visitors per year. Through partnership with Science Discovery at UC Boulder, K-12 classroom, field programs, and curriculum will be developed for elementary-middle schoolers. A graduate level course using BcCZO field sites, and a field trip offered during the Geological Society of America annual meeting will extend the impact of the BcCZO within the scientific community.
The Boulder Creek Critical Zone Observatory (BcCZO) studies how weathering, erosion, hydrology, and biogeochemistry shape critical zone architecture and control its function. The landscape of the Colorado Front Range provide a natural laboratory in which different erosion histories in recent geologic time produced a variety of critical zone architectures including glacially scoured valleys, deep bedrock canyons, low relief soil-mantled forested areas, and broad grassy terraces. BcCZO started in 2008 building infrastructure in Betasso and Gordon Gulch, and augmenting existing instrumentation in Green Lakes valley, Niwot Ridge, and Fourmile Canyon. Data is now collected on streamflow, soil moisture/temperature, groundwater, water chemistry, snow depth, and meteorology, and is stored in a publically accessible database, http://criticalzone.org/boulder. One-time measurements were made of timing of glacial retreat, age of fluvial terraces, soil production rate, microbial ecology, weathered rock strength, soil and weathered rock composition, and depth of weathered rock inferred from geophysical surveys. Broader impacts: BcCZO employs 4 professional research assistants, and supported or collaborated with 6 post-docs, 17 graduate students, and 58 undergraduate students. Completed theses: 2 PhD, 9 MA, and 18 undergraduate. We have published 55 papers. BcCZO faculty offered an undergraduate class on critical zone science, and incorporated CZ in other courses. BcCZO partnered with CU’s Science Discovery program to bring science to K-12 learners and teachers. We worked with underserved elementary classrooms, developed middle school workshops (Earth System Science: Exploring Change in the Critical Zone), and a summer field class. We created videos on the project, accessible on our website. We ran a booth at Operation Water Festival, attended by ~1,000 fifth-grade students and teachers. Findings: 1. Long-term evolution: Boulder Creek watershed brings into relief the strong legacy of past conditions, and the interconnections between different parts of landscapes. Past glaciers in the headwaters carved U-shaped valleys and deposited moraines, then retreated rapidly from 18 -10 thousand years ago. The retreat can be modeled with a 4.5-6°C rise in temperature and no change in precipitation. Downstream, terraces on the Plains indicate erosion as well, although offset in time from the glacial headwaters: terrace ages imply that rivers occupied terraces during glacials and cut down during deep interglacials. A simple interpretation is that climate-driven variations in sediment supply control downstream channel incision. The lithology contrast of the hard crystalline rock (e.g. granodiorite) of the mountains and the soft sedimentary rocks (e.g. shale) of the Plains, accounts for the narrow canyons versus broad terraces, respectively, in an overall pattern of erosion. 2. Slope aspect: At Gordon Gulch the mean annual temperature is near 0°C, hence snow and frost are important, yet vary with slope aspect. We found greater depth to fresh rock and weaker weathered rock on north-facing than south-facing slopes. Although forests vary with aspect, trees cannot explain weathering differences at these depths (>10 m), since forests are post-glacial at this elevation (ca. 2500 m) while the weathering reflects evolution over several hundred-thousand years given erosion rates of 20 m/Myr. Our numerical models demonstrate that weathering and sediment transport by frost processes can reproduce the observed weathering depth asymmetry. The modern snowpack varies with aspect, with south-facing slopes often bare in winter while north-facing slopes accumulate a snowpack. Sustained melt from north-facing slopes in spring flushes water through the soil. In contrast, intermittent melt on south-facing slopes produce lower water fluxes. Models demonstrate that water input in one sustained pulse delivers water more deeply than the same amount delivered in multiple short pulses. Aspect control on water translates into differences in chemical weathering rates. 3. Short-term events: In 2010, the Fourmile Canyon Fire burned 2600 ha. In the following year, high intensity summer convective thunderstorms resulted in flash floods from burned areas, but not non-burned areas. Convective storms transported sediment into the creek and increased DOC, nitrate, and total metals in runoff downstream. The hydrologic response is diagnostic of a shift in runoff generation from subsurface (pre-fire) to surface (post-fire) mechanisms. Much smaller increases in runoff were observed in the second year, despite similar rainfall intensities. Aspect differences in soil temperature/moisture regimes were substantially reduced after the wildfire, which may be important for recovery and ecosystem resilience. 4. Biogeochemistry: Soil microbial communities vary with hillslope position and depth in soil, particularly for taxa associated with carbon and nitrogen dynamics. Microbial diversity variation with depth in the soil was much greater than is seen across biomes. Fluorescence show that soil organic matter (SOM) is more plant-derived with depth. In saprolite, however, microbially-derived SOM becomes dominant, potentially representing a feedback that promotes weathering. Stream dissolved organic material (DOM) originates in the upper soil layers during high runoff. However the DOM in Boulder Creek and Gordon Gulch runoff does not match leachates from the upper soil horizons. Rapid sorption and chemical fractionation, which has been demonstrated in stream sediments, may account for these differences.
