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Landscapes on the Edge: New Horizons for Research on Earth’s Surface CHAPTER ONE The Importance of Earth Surface Processes 1.1 INTRODUCTION Earth’s surface is the arena for most life and all human activity, yet what lies beneath our feet is as mysterious as it is familiar. Earth scientists or not, we recognize hills, mountains, glaciers, deserts, rivers, wetlands, and shorelines. If a good deal of rain falls, floods may occur; if a storm strikes the coast, the beach may erode; if we are careless with our soil, we may damage or even lose it. These ideas are well known, but with just a few questions we arrive at the edge of our knowledge and face gaps that matter to our safety, our food and water security, the infrastructure of roads and river navigation, and the survival and diversity of ecosystems and services they provide. Any familiar landscape illustrates the point (Figure 1.1). Start with a stream channel and ask a series of simple questions: What controls its size, pattern, and magnitude of flooding? What plants and animals live in and along this stream, and how do biological processes—including human activities—affect the downstream flow of nutrients and water? Next, look about and wonder how this stream relates to its valley and the surrounding hillslopes. How did these landforms arise, and how are they related to one another? Why are hillslopes usually mantled with soil, and why is that soil so much richer and more complex than simple ground bedrock? In addition to landforms and their mantling soil, landscapes host a set of interconnected ecosystems, both visible and microscopic. How have these ecosystems shaped and been shaped by Earth’s surface? How is the flow of nutrients that nourishes ecosystems connected to the landscape? Finally, if we take the longest view, our stream is part of a network that forms a kind of continental circulatory system, carrying water, sediment, nutrients, and biota from high ground to low-lying coastlines. How did this system come to be, how long has it existed, and how is it related to climate (modern and past) or to the tectonic forces that shape continents? How will it behave in the future, and how do human activities influence that behavior?
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 1.1 Landscapes at Earth’s surface host a suite of interconnected landforms and processes that can remain stable for long periods of time and can also respond rapidly to changes in climate or land use. In this view of a recently deglaciated valley in the Juneau Icefield, Alaska, surface features comprise hillslopes, rock falls and slides, glaciers (in the far distance, upper right corner of the image), alluvial fans, streams, wetlands, and biota. Integral processes less visible than the landforms and land cover include weathering, soil formation, climate, surface and groundwater flow, nutrient fluxes, and tectonics. SOURCE: Photograph courtesy of Dorothy Merritts, Franklin and Marshall College, Lancaster, Pennsylvania. Other than a basic goal of explaining the form, composition, and evolution of landscapes, why might questions about Earth surface processes near a stream, or similar types of questions posed along a coastline or in a fragile arctic landscape, matter? At present, we are unable to make confident, process-oriented predictions of how landscapes respond to change. If climate change brings, for example, an increase in rainfall, will soils deliver more or fewer nutrients to groundwater and streams? If humans remove river dams and release the sediment stored behind them, as well as the nutrients and pollutants bound to the sediments, how will downstream fish habitats, estuaries, and coastal marshes be affected? Will the extra sediment stop the retreat of receding beaches, or will the sediment wash out to sea? Because of these and other such critical questions, society has become concerned about landscapes “on the edge” of potentially detrimental and irreversible change and has
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface heightened its demand for scientific guidance in making decisions concerning the future of Earth’s surface in light of these changes. Spurred by growing recognition of the importance and relevance of research in these areas, the National Science Foundation (NSF) requested that the National Research Council (NRC) convene a committee to address challenges and opportunities in Earth surface processes. The committee was asked to address three tasks related to Earth surface processes in the context of both scientific and societal issues: Assess the current state and the fundamental research questions of the field of Earth surface processes; Identify the rate-limiting challenges or opportunities for making significant advances in the field; and Identify the necessary intellectual collaborations and high-priority needs to meet these challenges. In this report, the field of Earth surface processes refers to the study of the form, physical properties, composition, function, and evolution of Earth’s surface, a dynamic interface where physical, chemical, biological, and human processes cause and are affected by forcings in the Earth system, with impact-feedback loops that occur over a wide range of temporal and spatial scales. This report identifies nine grand scientific challenges that exemplify compelling directions of research in Earth surface processes (Chapter 2), and proposes four new, high-priority research initiatives designed to transform and strengthen the field in order to support the challenges (Chapter 3). The initiatives represent pathways to meet the demands for scientific information on issues related to planning, mitigation, and response to changes in Earth’s surface now and in the future. Chapter 4 discusses the nature of the national support structure necessary to capitalize fully on these scientific opportunities. The remainder of this chapter highlights some of the key advances and problems that have drawn attention to Earth surface processes research and contributed to its growth in the past several years. These examples focus on how Earth surface processes are interconnected or “coupled” to each other, to the atmosphere, and to the Earth’s interior; on the increasing human impact on Earth’s surface, including climate change; and on new technologies that have spurred recent theoretical advances in Earth surface processes. These topics are elaborated in greater detail in Chapters 2 and 3.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface 1.2 EXAMPLES OF INTERCONNECTED EARTH SURFACE PROCESSES Climate, Tectonics, and Surface Processes Interconnected processes at Earth’s surface are coupled to those of Earth’s interior in various ways that extend to millennial and longer time scales. The height and shape of rising mountains, for example, influence regional weather patterns, which affect rates of erosion via the amount and type of precipitation. As rivers and glaciers fed by topographically controlled precipitation carve deeply into uplifted rock in tectonically active areas, their concentrated erosion draws even more rock upward due to the effect of unloading (Figure 1.2). Spatial variation in erosion across a mountain belt due to climatic differences can affect the pattern of upward and lateral movement of rock toward Earth’s surface. While the volume of rock drawn into a mountain belt is affected by Earth surface processes, the composition of the rock also is altered and this change can affect climate. Chemical weathering of rock freshly exposed in rapidly uplifting mountains affects the chemistry of water draining the mountains and can draw down carbon dioxide in the atmosphere, thereby influencing climate over relatively long periods of geologic time. Even at these geologic time scales, biota are critical to the dynamic processes in mountain belts. Biotic processes mediate rates of rock breakdown (by weathering), soil development, and hillslope erosion and strongly influence the amount, size, and composition of sediment entering rivers. This sediment then influences the rate of bedrock incision, the geometry and dynamics of the channel, and the ecosystems that colonize an area. Human-Landscape Dynamics Largely within the last 3 millennia, humans have removed and replaced land cover, hastened the erosion of upland soils, and increased sediment supply to streams from upland erosion throughout many parts of the world (Figure 1.3). Worldwide damming of rivers has increased sediment trapping and residence times, however, greatly reducing the delivery of sediment to coasts and deltas. Although dams provide substantial societal benefits, including reduced flooding, hydroelectric power, and water for irrigation, their impact on sediment transport has caused the collapse of river ecosystems and starved coasts of sediment, leading to unanticipated delta subsidence, wetland loss, and greater coastal erosion. Nearly every process on Earth’s surface has been changed by human activities, heightening the need for new research on human-landscape dynamics and for a greater capacity to predict process responses to human influence. Earth-surface scientists have a unique and timely opportunity to use new tools and integrative approaches to enhance understanding and to predict future changes. More importantly, they are in position to transfer their
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 1.2 Linked tectonic, climatic, and Earth surface processes shape—and are influenced by— topography. Satellite image of New Zealand (top; February 9, 2002, SeaWiFS image) reveals regional climatic processes that drive spatial variation in rates and processes of erosion that, in turn, influence tectonics. Clouds are banked along the northwestern side of the mountains (Southern Alps) on the South Island; snow covers the northwestern peaks and higher slopes; glaciers occupy valleys draining high topography. Erosion, tectonic deformation, and uplift are focused on the western flank of the mountain range. SOURCE: Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE. The schematic cross section of a convergent plate boundary (bottom) with similarities to the New Zealand example illustrates shortening and thickening of continental crust overlying the flowing mantle. The shape of the mountain range is influenced by climatically controlled, spatially varying erosion and its tectonic response (see also Section 2.3). Large black arrow indicates direction of motion of the converging plate. SOURCE: Modified after Willett (1999) and with permission of the American Geophysical Union.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 1.3 Humans have transformed nearly all of Earth’s terrestrial surface. These maps illustrate the worldwide extent of human land-use and land-cover change: the geographic distribution of “potential vegetation” that would most likely exist in the absence of human land use (top); and the extent of agricultural land cover (including croplands and pastures) (middle and bottom) across the world during the 1990s. SOURCE: Foley et al. (2005). Reprinted with permission from AAAS.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface knowledge to the greater scientific community, applied practitioners, the public, and policy makers in order to facilitate decision making. As an example of the role of Earth surface science in providing greater understanding of Earth surface processes and in predicting systemic responses to change, consider what happens as aging dams are removed or breached. Tens of thousands of dams of various sizes have slowed the flow of rivers and trapped sediment and nutrients throughout the United States for up to hundreds of years, and dams continue to be built throughout the world. Removal of some of these aging impoundments is desired for reasons that include fish passage, human safety, and improved water quality. Yet pulses of sediment, nutrients, and pollutants are flushed from many breached reservoirs, impacting waterways, water quality, and habitat downstream. The nature and duration of downstream impacts are obvious questions of concern when dam removal is considered, but at present we are not able to predict accurately the changing rate at which sediment and nutrients will be eroded and transported downstream from a breached reservoir (Figure 1.4). What happens upstream of a breached dam is equally uncertain. Can the ecological, hydrological, and geomorphic functions of marshes, streams, and floodplains that lie buried beneath reservoir sediment be recovered after dam breaching? In essence, each dam breach is an experiment in which scientists can investigate interconnected Earth surface processes. Studying such experiments requires the expertise of scientists with diverse backgrounds and an ability to integrate analytical approaches, data, and interpretations across disciplines. The outcome of this kind of integrative research is invaluable to inform engineering practice and policy. Soil Erosion Natural soil erosion by the energy of wind, raindrops, or running water is accelerated by almost every human use of landscapes: agriculture, grazing, and timber harvesting, for example, may expose soil over large areas of continents to greater erosive forces and, in some cases, may degrade lands beyond beneficial use (Box 1.1). Soil eroded in this way may also accumulate in water bodies and create engineering or water quality issues associated with the transport of both nutrients and contaminants. Understanding and quantifying the magnitude of soil erosion and its downstream impacts across the United States are fundamental to inform policy decisions such as whether to subsidize soil conservation, to intensify cultivation for biofuel production, or to regulate land use in order to improve water quality in rivers, lakes, and estuaries. Nevertheless, considerable uncertainty exists about the magnitude of soil erosion and its downstream impacts across the United States. These uncertainties leave the nation in a fairly uninformed state that is exacerbated by inadequacies in the concepts presently used to make policy decisions related to soil conservation and land-use planning and illustrate the critical need for a better understanding of Earth surface processes.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 1.4 What happens when a dam is breached? Within months of dam removal (in upper left of image) in the spring of 2008, more than 180,000 m3 of sediment—far greater than predicted—was scoured from the upper part of the Milltown Reservoir, Clark Fork River, Montana. The Clark Fork River flows from lower right to upper left of image. Colors represent scour amounts determined by calculating the difference between digital elevation models of post-dam removal topography generated from an airborne lidar (light detection and ranging)1 survey and of pre-dam removal topography from photogrammetry and bathymetric surveys. Immediately upstream of the dam site is a Superfund remediation area, where contaminated mining-derived sediments isolated from the river by berms are being mechanically excavated. Eroded sediment, which included arsenic from historic mining, was carried up to 200 kilometers downstream. SOURCE: Data from surveys commissioned by State of Montana; figure produced by and courtesy of Douglas Brinkerhoff and Andrew Wilcox, University of Montana. 1 This report adopts the use of the lowercase form lidar for this acronym in keeping with the definition in Appendix C of the NRC report Elevation Data for Floodplain Mapping (NRC, 2007a). The lowercase word “lidar” is the appropriate form because it is directly analogous to “radar” and to a lesser extent “sonar.”
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Climate Change A relatively recent human impact that has great import to Earth surface processes is climate change. Although climate change research has made significant advances in recent decades, examining the response of the Earth’s surface to this change has just begun. Climate change affects all landscapes, influencing hydrology, flooding, water quality, nutrient loads, ecosystems, soil erosion, and landslide frequency. Retreating glaciers, for example, let loose large chunks of ice and freshwater while freshly eroded rock becomes exposed to weathering in the wake of the glaciers’ retreating termini (Figure 1.5). An understanding of glacial mechanics, especially the basal sliding process, is important in predicting rates of glacial retreat and sea-level rise and is considered a top challenge for reducing uncertainty in climate projection and impact assessments. Another important process in cold regions is thawing of the active layer of permafrost, which can produce nutrient and sediment pulses to coastal zones and increase the flux of carbon to the atmosphere. Permafrost underlies about 25 percent of global land area and is undergoing marked changes associated with recent global warming. Seasonally, the top of permafrost—the active layer—thaws and melts, and this seasonal thaw is increasing. The ecological impacts of warming and increased seasonal thawing of the active layer are complex, but potentially quite profound. Substantial amounts of carbon are stored in boreal soils, with possibly 50 percent or more of soil organic matter stored in high-latitude periglacial environments. Hydrologic conditions and biota play a role in whether thawed organic matter oxidizes or is reduced to methane. Although warming Arctic soils will likely be a source of carbon to the atmosphere, the details of carbon release from these soils are not yet clearly understood. As with the need to address issues of glacial mechanics, understanding these fluxes in permafrost zones is critical to climate models that are sensitive to the changing concentration of greenhouse gases in the atmosphere. Coastal margins in the Arctic illustrate relatively short-term feedbacks between biota and deltaic channel-wetland systems that are linked to global warming and its effects on thawing of the active layer of permafrost. A vast delta (30,000 km2) that formed at the mouth of the Lena River with sediment shed from northern Siberia currently contains extensive tundra wetlands that provide habitat for migratory birds (Figure 1.6). Biota and organic matter in the wetlands stabilize distributary channels and islands. Frozen for more than half of each year, these wetlands store large amounts of carbon that are released during thawing of the active layer in permafrost, which has been hastened by the warming climate. Organic-rich soils collapse as they warm and melt, leading to channel shifting, pulses of nutrient and sediment loading to the Laptev Sea, and the release of carbon dioxide and methane to the atmosphere. Such pulses can affect detrital food chains and benthic productivity along the continental shelf, which in turn affect the rate of erosion of coastal margins. The wetlands and frozen sediments of the
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface BOX 1.1 National Impacts of Soil Erosion and Sedimentation Soil erosion, transport, and deposition are integral parts of the global sedimentary cycle and are important to altering and balancing the water- and nutrient-holding properties of soil profiles and transporting nutrients and sediment in and through rivers, reservoirs, estuaries, or deltas (see figure below). The needs for better methods of assessing and predicting soil erosion and sediment supply at a wide range of landscape scales, and for clarifying concepts that underpin public policy on these matters, suggest an important role for research in Earth surface processes. Major contributions from Earth surface research to assess the quantity and impacts of soil erosion involve computational models and a variety of measurement techniques applied to a range of erosive environments and timescales, and the ability to design and interpret these measurements through rigorous mathematical models of basin sediment budgets. Much of the difficulty in interpreting and predicting the effects of soil erosion on water quality and sedimentation at whole-basin and national scales arises from several factors: (1) the difference between erosion at its source on hillslopes (as sheetwash, gully erosion, and landslides) and the full sediment budget of a basin; (2) the legacy of various natural and anthropogenic perturbations of river basin sediment budgets and the fact that a number of these perturbations have been quantified but largely ignored in assessments of soil erosion, transport, and sedimentation; (3) the general lack of rigor in measurements of soil erosion; and (4) a lack of realistic, quantitative estimates of soil production rates. The legacy perturbations in some regions include vast amounts of sediment that were mobilized by glaciers during the Last Glacial Maximum (~15,000 to 25,000 years ago) and redeposited in conditions and locations from which it is now being chronically remobilized by modern streams, waves of agricultural colonization, dam construction, and timber harvest. These changes in landscape evolution directly affect sediment budgets, and accurate assessments of soil erosion and sedimentation require incorporation of their effects. The actual measurement of soil erosion also requires greater rigor, particularly as it relates to the generalized statistical models generated from these data and subsequently used in land resource management. A need exists to broaden the range of methods used to estimate soil erosion and to organize such measurements through landscape-scale designs and models that will allow realistic interpretation of the rate of soil erosion and the supply of sediment to streams and estuaries. Augmentation or rejuvenation of sediment sampling networks at river gauging stations is also important. The most important part of a strategy, however, would be to ensure that methods for measuring erosion, sediment transport, and accumulation are employed in a coordinated and representative manner with a view to quantifying basin sediment budgets. With more accurate measurements, sediment budgets can then be quantified within the framework of mathematical models that allow the results to be checked for consistency and to be generalized for prediction purposes. Various basin sediment budget models with varying degrees of resolution and complexity are now being developed. A large-scale combined strategy of empiricism and model-based accounting will be required to answer these challenging public policy questions. The skills and tools are available in Earth surface research, which incorporates the whole range of scales of the problem from the production and mobilization of soils at a single point to the catchment- and continent-wide storage and transport of sediment.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Human activities can lead to exceptionally high rates of landscape erosion. The challenge for modern studies of landscape change is to understand and measure the effects of these human activities relative to the background chemical and physical processes that also contribute to erosion and are driven by climate and tectonics. Madagascar is a case in point. Conventional wisdom holds that Madagascar’s modern erosion rates are exceptionally high, and that the lavakas (gullies) which pepper the Malagasy landscape (pictured in the image) are the result of deforestation and local agricultural practices. New research, however, reveals this interpretation to be based on descriptive evaluations and regional extrapolations from few local datapoints. Recent studies, in contrast, show the importance of quantitative investigations at local to regional scales. Analysis of seismic events and mapping of gully distributions from satellite images, for example, show a very strong spatial correlation between earthquake distribution and the locations of dense lavaka clusters, underscoring the importance of tectonics in generating this complicated landscape. Ongoing measurement of sediment generation rates from modern and ancient river sediment will quantify changes in erosion rate due to anthropogenic disturbance. These broad-based studies are typical of modern approaches to understanding and quantifying the causes and effects of soil erosion and are central to the development of effective soil erosion control strategies. SOURCE: Cox et al. (2010); image courtesy of Rónadh Cox, http://drm.williams.edu/u?/lavaka,2141.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 1.5 Global warming, retreating glaciers, and changing landscapes. The Columbia Glacier, flowing into Prince William Sound, Alaska, presently is undergoing rapid retreat that is likely to last another few decades. The glacier produces prodigious quantities of icebergs, which debouch into the sound near the shipping lane from the Valdez terminus of the Alyeska pipeline. The retreat is accompanied by thinning of the ice, revealing up to 400 meters of freshly exposed bedrock that bounds the present glacier. SOURCE: Photo courtesy of Robert S. Anderson. Lena Delta system reveal the interconnectedness among climate change, biota, soils, and landscapes at the centennial to millennial scale and the critical linkages between human activities and Earth surface processes. 1.3 NEW TECHNOLOGIES: MONITORING EARTH SURFACE PROCESSES AT HIGH RESOLUTION IN SPACE AND TIME The evolution and increasing availability of new measurement technologies has enabled many of the advances in Earth surface processes that are discussed throughout this report.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 1.6 Interactions among climate change, biota, and landscapes. The vast Lena River delta, the largest delta in the Arctic region, formed as sediment from the Lena River was deposited where it flows into the Laptev Sea. Tundra wetlands in this delta store large amounts of carbon that potentially could be released by modern global warming. Attributed largely to human activity, warming accelerates permafrost thawing and the erosion of organic-rich delta sediments. Envisat image acquired on June 15, 2006; width of image ~350 kilometers. SOURCE: European Space Agency. Technological advances in remote sensing, geochemistry, geochronology, and computing have fostered great progress in the study of Earth’s surface (Appendix C). For example, recent advances in the areas of digital topography and geochronology enable scientists not just to conduct research faster or more accurately, but to make observations and interpretations that were not possible previously.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Digital Topography Throughout history, the creation of maps has been a means of recording observations that enable us to find and denote paths and patterns and to generate hypotheses about the controls on the spatial relationship of features. Topographic maps, depicting land elevation and displaying landforms, have been crucial to scientific inquiry about the Earth and have been central to land development. In the 1980s, a profound step was taken when line drawings of elevations on topographic maps were digitized and the landscape could be represented via digital elevation models on computers. This innovation launched thousands of scientific studies exploiting this new capability and ultimately gave rise to many new practical applications. In the last decade, technological advances have enabled the first airborne and satellite-mounted surveys of topography using radar (interferometric synthetic aperture radar, InSAR) and laser (light detection and ranging, or lidar) technology, giving unprecedented spatial resolution over large areas. This development has led to a second wave of digital topographic studies that are transforming not just research in Earth surface processes, but also the fields of agriculture, ecology, engineering, and planning. With regard to Earth surface processes, digital elevation data enable us to examine, for the first time, topographic features over broad areas using computer-automated techniques. This ability is leading to new insights and tools that link landscapes to hydrology, geochemistry, tectonics, and climate. Although many digital elevation data are coarse in scale for studying the features, for example, of mountain belts with long, high hillslopes, the data have been truly revolutionary. The advances in the past decade are akin to those of the 1960s in the fields of seismology and geophysics, when accessibility to global seismic and paleomagnetic data and new tools to process such signals spurred the plate tectonics revolution and greater understanding of Earth’s subsurface processes. One of the most recent transformative phases in the measurement and characterization of landscape topography has been the ongoing development of laser surveying, both from the ground and from airborne instruments. This method is referred to as lidar, or airborne laser swath mapping (ALSM) in the case of aerial surveys. High-resolution swath bathymetry uses sonar for the same types of measurements in marine environments. With lidar, a laser pulse is sent from the instrument, and the time for its return from a reflected surface is detected and used to calculate distance. Current technology permits typical accuracies to about 5 to 10 centimeters vertically and 20 to 30 centimeters horizontally, with data returns every few decimeters. From these returns a point cloud of elevation data is created; various analytical methods are then used to distinguish vegetation from ground (Figure 1.7). For the first time, we now can obtain surveys over broad areas that document topography at the resolution at which transport, erosion, and deposition processes operate. Lidar data also capture important quantitative attributes of vegetation that can be used in studies
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 1.7 Documenting topography at the resolution of transport and erosion processes. Comparison of Google Earth image (bottom, digital air photo) with two lidar-derived images for an area near Flathead Lake, Montana: “bare Earth” topography with vegetation removed (top left) and vegetation color-coded by height above bare Earth (top right). Area in red box on the digital air photo is the area covered by the upper lidar images. Image width of the digital air photo is ~3 kilometers. SOURCE: National Center for Airborne Laser Mapping (NCALM), Bottom Image courtesy of Google Earth.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface of ecohydrology and ecogeomorphology. Landslide scars, channel banks, river terraces, floodplain features, fault traces, and other landforms can be detected, quantified, and used to advance theoretical and practical understanding. Repeat scans allow change detection as never previously possible. These techniques also permit improved understanding of the human impact on types and rates of geomorphic processes. Geochronology To quantify rates of Earth surface processes and ages of landforms, Earth scientists have developed in the past 20 years a wide range of tools that exploit the time-dependent exposure of materials to cosmic rays, heat, and light. The greatest breakthrough came when measurement techniques advanced to the stage that trace concentrations of atoms produced by cosmic rays could be isolated and measured accurately (Box 1.2). Questions posed by early twentieth century Earth scientists about ages of landscapes and their evolutionary sequences, and the underlying mechanisms of erosion and deposition, can now be addressed quantitatively. These rate measurements coupled with new thermochronometers (see Box 2.4) have revealed suspected but previously unmeasurable linkages between erosion and tectonics. Undoubtedly much more will be discovered as these new dating technologies are used to measure the rates of evolution of Earth’s surface. 1.4 STUDY CONSIDERATIONS AND REPORT STRUCTURE Interdisciplinary research2 in Earth surface processes comprises the detailed investigation of contemporary processes that generate and degrade landscapes and change the properties of rocks and soil; the definition of how these processes have functioned over the long periods of time required for the evolution of surface conditions (composition, function, and form); the deep connections among surface processes, climate, tectonics, life, and human activity; and ultimately the prediction of future landscapes and the fluid, solid, and solute fluxes across them. Evidence of the environmental history of landscape development is stored in the geologic and geochemical records of sediments, water, and soils. Building from datasets that extend across space and time and using a growing variety of powerful tools and techniques, scientists are able to measure landforms, probe sediments and 2 This report has adopted the definition of “interdisciplinary research” as provided in the National Research Council (2004) report Facilitating Interdisciplinary Research. The report defines interdisciplinary research as a “mode of research by teams or individuals that integrates information, data, techniques, tools, perspectives, concepts, and/or theories from two or more disciplines or bodies of specialized knowledge to advance fundamental understanding or to solve problems whose solutions are beyond the scope of a single discipline or area of research practice.” The report goes further to indicate that the interactions between fields or disciplines may result in forging a new field or discipline.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface water, quantify process rates, and model the changing face of Earth. As interdisciplinary approaches increase the power of landscape research, a complex picture is beginning to emerge of landscape functioning, evolution, and interactions with life and human activity. Research in this area is integrative because it involves linkages to many related fields and because the core of the research lies at discovering the interactions and feedbacks involving physical, chemical, biological, and human processes. The field of Earth surface processes overlaps with studies of the Critical Zone as defined by the NRC report to NSF on Basic Research Opportunities in Earth Science (BROES) (NRC, 2001a). The BROES report first developed the concept of the “Critical Zone” as the “heterogeneous, near-surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life-sustaining resources.” In addition to these surface interactions, the investigation of Earth surface processes as developed in this report involves features and transfer processes that place greater emphasis on geological history and on interactions and feedbacks both with humans and with deep-Earth processes (e.g., tectonics) than those initially conceived in the definition of Critical Zone studies. Notably, the NSF-supported Critical Zone Observatories, which have grown out of this interest in the Critical Zone, are an integral part of the effort to advance the understanding of processes operating at the surface of the Earth (see Box 2.5). In this report, the committee did not develop static definitions of Critical Zone science or Earth surface processes, but considered them part of the same urgent effort to understand processes operating at or near Earth’s surface. To identify most effectively the greatest challenges and most promising opportunities in Earth surface processes, the committee sought input in its public meetings from panelists whose expertise augmented that of committee members (Appendixes A and B) and remained abreast of the meeting activities of a parallel, but separate, NRC study Strategic Directions in the Geographical Sciences. The committee also sought input from a broad section of the international scientific community relevant to Earth surface processes through an online questionnaire (see Appendix B). Responses to the questionnaire emphasized a number of recurring themes including (1) the interconnectedness of diverse processes acting on Earth’s surface; (2) the importance of incorporating human dynamics in research on Earth surface processes; (3) the value of new technologies to advance understanding of Earth surface processes (see also Appendix C); and (4) the scientific and practical challenges that face the community in its effort to advance this field. Although this report focuses on the terrestrial surface, the committee emphasizes that research on the submarine surface and on marine processes is as active and exciting as terrestrial research. However, without any marine scientists on the committee, we did not have the resources to do justice to topics addressed by this important, allied community. Numerous boxes and figures throughout the report are designed to highlight specific concepts, tools, and examples related to Earth surface processes research that otherwise are
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface BOX 1.2 The Cosmogenic Radionuclide Revolution Earth is bombarded constantly by high-energy cosmic rays such as protons and neutrons. Cosmogenic isotopes are rare isotopes that form in Earth’s atmosphere and near surface when cosmic rays hit the nucleus of target atoms such as 16O, 14N, 40Ca, and 28Si. The target atoms are abundant in rock, soil, and the atmosphere. The rates of production and decay of many cosmogenic nuclides are known sufficiently well that measured nuclide concentrations in samples can, among other things, be used as an absolute dating technique. The production of cosmogenic nuclides in a wide range of Earth materials and knowledge of the production rates have led to a revolution in our ability to quantify the timing and rates of Earth surface processes from thousands to millions of years with radiogenic nuclides (7Be, 10Be, 14C, 26Al, 36Cl), or even longer with stable nuclides (3He, 21Ne), in a diverse range of environments and over a wide range of time scales (see figure below). Most Earth surface applications of cosmogenic nuclides include either dating the exposure time (or age) of a surface to cosmic rays and/or quantifying landscape denudation rates. Examples of exposure age dating include dating glacial moraines to quantify the timing and magnitude of ice ages, dating of groundwater to understand long-term migration velocities and the effects of climate changes on recharge, and dating of fluvial terraces, alluvial fan surfaces, and landslide deposits to investigate the effects of tectonic processes on crustal deformation, faulting, and erosion processes (see figure below). Denudation rates or magnitudes can be quantified by measuring nuclide concentration in modern or ancient fluvial sediments for catchment average rates or from bedrock samples at points across a landscape. The ages of sediments can now be documented to within a few years over the last two centuries, and sediments from individual floods can be identified during the following season. More recently, cosmogenic nuclide concentrations in soils have been used to understand the rates and magnitudes of soil mixing due to biologic and periglacial processes and soil production rates from bedrock. Accelerator mass spectrometry (AMS) is required for measurement of most radionuclides. NSF, and other science programs globally, have committed funds to the development and maintenance of AMS facilities. Subsidized facilities such as the Purdue Rare Isotope Measurement (PRIME) laboratory at Purdue University keep the costs of data collection reasonable for the benefit of all. The future of cosmogenic isotope geochemistry is promising. Examples of frontiers in this field include (1) refinement of nuclide production rates through the United States and European Cosmic-Ray produced NUclide Systematics on Earth (CRONUS) and CRONUS-EU initiatives to reduce uncertainties in exposure age and denudation rate calculations, (2) development of significantly smaller and more affordable AMS facilities to reduce costs and increase sample throughput, and (3) continued development and application of cosmogenic noble gas techniques to a wider range of minerals. Of particular interest is recent progress in measuring cosmogenic 3He in common mineral phases that are present in many rocks and are more retentive of helium than quartz.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Dating landforms and determining fault slip rates. Different ages of alluvial fan deposits form surfaces with varying degrees of surface roughness, desert varnish, and other indicators of relative age. Cosmogenic dating, in this case 10Be, provides chronologic control for different ages of alluvial fan surfaces offset along the northern Death Valley fault (fault strikes northwest to southeast along the western part of a lidar-derived bare-Earth topographic image). The development of these fans has been linked to changes in climate. The age estimates of these features are used to reconstruct the offset fan surfaces along the fault in order to calculate a slip rate of 4.5 millimeters per year. SOURCE: Frankel et al. (2007) and with permission of the American Geophysical Union.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface mentioned only briefly in the text. Neither these boxes nor the examples in the body of the text can represent the entire range of research covered by the field, but they are intended to draw attention to contributions from various disciplines to further research on Earth surface processes. Similarly, we do not cite many specific research publications from among the vast number in this area of research except to provide proper attribution for a point of fact, a figure, a direct quotation, or an explicit concept. The committee also notes that the study charge is focused on research and, for this reason, has deliberately not included education and human resource issues in its discussion. Although clearly of importance to all areas of science and engineering, capturing education and workforce topics in adequate detail was beyond the study scope. Appendix D provides some background to the growth in this field at universities and in the international professional community. In addition to the collaborative and integrative approaches emphasized in this report, the committee recognizes that the emerging science of Earth surface processes often has relied on fairly simple, descriptive approaches. Empirical methods have underlain much of the theory development for understanding landscapes, and observations and data collection will remain important components of studies of the Earth’s surface. Nevertheless, significant advances will require developing quantitative predictive capability for how landscapes form, evolve, and respond to change. Such capability is especially important as Earth surface processes are increasingly altered by human activity and climate change. For the foreseeable future and for most landscape processes, predictive models will of necessity continue to be partially empirical, even as improvements in understanding underlying processes gradually are achieved. A useful analogy here is one of weather forecasting, which combines sophisticated numerical solutions of the governing equations of atmospheric dynamics with empirical relations for incompletely understood processes (e.g., cloud formation) to make forecasts that are quantitative and, at the same time, stochastic. Although the underlying equations for landscape evolution are not known at present and may be quite diverse, the general approach to landscape prediction is likely to be similar. 1.5 CLOSING REMARKS Intended for use by NSF, decision makers, and research communities in academia, the private sector, and federal agencies, this report identifies high-priority areas for research in Earth surface processes. Many of the research areas address critical societal needs and issues. The report also suggests means to coordinate and support the necessary research. Basic research in Earth surface processes is one part of the research portfolio encompassed by the Section of Surface Earth Processes at NSF. Many of the exciting research activities and intellectual advances in the study of Earth surface processes, however, have gone beyond traditional disciplinary boundaries of Earth science, reaching into research domains that fall within other areas at NSF, such as climate, ecosystem sciences, tectonic processes, and
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Earth’s interior. The field of Earth surface processes lies at the intersection and integration of diverse natural science disciplines—Earth, life, atmospheric, and ocean sciences—that address explicitly the function, composition, form, and evolution of Earth’s surface and near-surface environment. Increasingly, as the human impact strengthens, the field of Earth surface processes requires integration with the social and behavioral sciences as well. Earth’s surface is the only habitat available to the human race. Understanding the processes by which that habitat has been created and continually altered is important to determine the causes of environmental degradation, to restore what is degraded, and to guide policy decisions toward a sustainable Earth surface. The agencies and individuals responsible for natural resources and public welfare widely acknowledge that environmental change occurs on a scale and with an intensity that is important for society’s long-term plans and investments. This acknowledgment now puts a responsibility on the shoulders of natural scientists as well as professionals in economics, social science, engineering, and other fields to interpret the record of ongoing environmental change and to anticipate and, in some cases, make quantitative predictions of future events or conditions. Earth-surface scientists who study such records have distinctive insights, methods, and skills to understand the form, composition, properties, function, and evolution of Earth’s surface and to contribute to resolving modern environmental challenges. We have the technological ability to monitor closely human impacts on the environment. The need to observe, measure, and model human-landscape interactions in an integrated, predictive fashion is clear. To develop this capability, fundamental research is needed to understand and quantify the impact and feedback relationships between human activity and Earth surface processes. In many cases, the socioeconomic will and capacity exist to attempt to alter the impacts and responses initiated in human-influenced landscape systems. With new scientific questions about various components of the Earth system, opportunities and tools for research, rapid growth of the human population, and unprecedented changes in biota, land cover, process rates, and global climate, an appraisal of the study of Earth surface processes is both timely and crucial.
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