Frontiers in Exploration of theWorkshop Organizing Committee Don Sparks, Co-Chair, University of DelawareSue Brantley, Co-Chair, The Pennsylvania State UniversityJon Chorover, The University of ArizonaMary Firestone, University of California, BerkeleyDan Richter, Duke UniversityArt White, USGS, Menlo Park2Frontiers in Exploration of the Critical Zone An NSF-Sponsored Workshop University of Delaware Newark, Delaware Monday October 24 - Wednesday October 26, 2005 The Critical Zone encompasses Earth’s outermost surface defined from the vegetation canopy to the zone of groundwater. This zone, the interface between Earth materials and the biotic world, modulates the transfer of nutrients into terrestrial lifeforms. In this photograph, a tree root anchored onto the Key Largo Limestone, an exposed reef with fossilized hermatypic corals, is shown. The limestone is overlain by thin soil in the Windley Key Fossil Reef Geological State Park, forcing plants to anchor themselves directly to the bedrock for stability and nutrients. To understand processes in such settings, scientists from multiple disciplines must unravel similarly complex inter-relationships within the hydro-, litho-, and biosphere. Photograph by Heather Buss, Penn State.To cite this report: Brantley, S.L., White, T.S. , White, A.F., Sparks, D., Richter, D., Pregitzer, K., Derry, L., Chorover, J., Chadwick, O., April, R., Anderson, S., Amundson, R., 2006, Frontiers in Exploration of the Critical Zone: Report of a workshop sponsored by the National Sci-ence Foundation (NSF), October 24-26, 2005, Newark, DE, 30p.Frontispiece caption:Executive SummaryFigure 1. The coupled chemical, physical, and biological processes that define Earth’s weathering en-gine are driven by climatic, anthro-pogenic, and tectonic forcing that can be investigated at all scales. The characteristic rates and ex-tents of weathering are recorded in the concentrations of atmospheric gases and aerosols in hydrologic responses, and in soil chemistry, and can be inferred from histori-cal data and from the geologic re-cord (ANDERSON ET AL., 2002). Reproduced with permission from the American Geophysical Union.3The surface of the Earth is rapidly changing, largely in response to an-thropogenic perturbation. How will such change unfold, and how will it affect humankind? The Critical Zone is defined as the external ter-restrial layer extending from the outer limits of vegetation down to and including the zone of groundwater. This zone sustains most terrestrial life on the planet. Despite its importance for life, scientific approach-es and funding paradigms have not promoted and emphasized inte-grated research agendas to investigate the coupling between physical, biological, geological, and chemical processes in the Critical Zone. A national initiative is needed that incorporates a systems ap-proach to investigation of Critical Zone processes across a broad array of sciences: geology, soil science, biology, ecology, geo-chemistry, geomorphology, and hydrology. Only with such an approach will we be able to answer the following question: This initiative will enable prediction of complex feedbacks among pro-cesses in the Critical Zone, including changes in fluxes driven by climatic, tectonic, and anthropogenic forcing over a wide range of temporal and spa-tial scales. Of particularly pressing importance is the need to understand how the Critical Zone is being transformed by rapid anthropogenic change. This effort will require a network of observatories and people to quan-tify responses of the Critical Zone to environmental change. The Criti-cal Zone Exploration Network will include short-term deployments of instrumentation at field sites along environmental gradients as well as long-term sites that will be instrumented hierarchically and intensively. Importantly, sites will be chosen by peer review to answer fundamen-tal questions requiring the entire network. By choosing sites with im-portant questions in mind, the Critical Zone Exploration Network will thus tackle questions of great societal and global importance by uniting a multi-disciplinary and diverse community of scientists and their students.How do the physical, chemical, and biological components of Earth’s weathering engine transform mineral and organic matter to nourish and sustain ecosystems, regulate the migration and fate of toxins, sculpt ter-restrial landscapes, and control the exchange of greenhouse gases and dust with the global atmosphere?FORCINGSCL I MATICAN T HROPO G ENICTE C TON I CWEATHERINGIN THECRITICAL ZONEATM OSPHE R ICHY D ROLOG I CSO I LRESPONSESBI O LOGIC A LPH Y SICALCH E MICALIntroduction: What is the Critical Zone?At the Earth’s surface, a complex suite of chemical, biological and physical processes combine to create an engine that transforms bed-rock and biomass into soil (Figure 1). Earth’s weathering engine pro-vides nutrients to nourish ecosystems and human society, controls water runoff and infiltration, mediates the release and transport of toxins to the biosphere, and creates conduits for the water that erodes bedrock. The weathering engine also affects the sequestration and re-lease of greenhouse gases that impact climate change, and generates aerosols and dust that provide nutrients to the land and ocean. All of these processes occur within the Critical Zone, defined by the Na-tional Research Council’s Committee on Basic Research Opportuni-ties in the Earth Sciences (2001) as the external surface of the Earth extending from the outer limits of vegetation down to and including the zone of groundwater. At present, humankind is radically changing the Critical Zone by altering the magnitudes of both the reservoirs and fluxes in ways that we do not understand and cannot predict (Table 1). Despite the fundamental importance of the Critical Zone, our knowl-edge of its central component – soil, the complex biomaterial that pro-motes the growth of terrestrial organisms – is remarkably limited. This limitation in knowledge persists because scientific approaches and funding paradigms have tended to emphasize reductionist approaches 4Table