SOURCE TO SINK STUDIES

INTRODUCTION

Continental margins and the sediment dispersal systems that both traverse and shape them are inhabited by a large fraction of the world’s population. In the United States alone some 80% of the population is estimated to live within 100 km of the coastline. Continental margins are subject to environmental hazards such as earthquakes, tsunamis, floods and landslides. In addition such factors as pollution and coastal erosion present threats to sustainable development along margins. These zones also contain important resources such as hydrocarbon fuels, groundwater and agricultural lands as well as coastal wetlands, fisheries and marine algae.

Margins constitute material dispersal systems that convey water, sediment and associated chemicals from the continent to the sea via rivers, mass movements and turbidity currents. The various parts of this system are typically in a state of change on geological time scales. Temporal and spatial evolution of margins involve strong interactions between the various zones of the sediment dispersal system (Figure 1). Understanding and predicting these changes require empirical knowledge of the linkages and feedbacks between the components. At present, we have some understanding of the individual units constituting margins, but little ability to link them interactively in a quantitative and predictive way. The MARGINS Source to Sink program was conceived on the premise that significant improvements in both understanding and predictive ability can be obtained through the pursuit of these linkages. A 10-year program of concerted research based on a holistic philosophy that fully integrates field, experimental and modeling elements should allow for a major breakthrough toward achieving this predictive ability. The intertwining of sediment flux, morphodynamics and stratigraphy offers an unprecedented opportunity for research synergism.

The research goal is to discern the relationships among processes relevant to sediment production, transport, accumulation, and preservation on margins at multiple temporal and space scales, from turbulence to tectonics and from sedimentary fabric to sequence stratigraphy and basin analysis. An expected outgrowth of the program is enhancement of the ability to quantitatively interpret the stratigraphic record of change in both terrestrial and submarine settings. This approach will allow for a notable extension of the record of environmental change affecting human populations and ecosystems along continental margins.

MARGINS SOURCE-TO-SINK STRATEGY

The MARGINS effort encapsulates several conceptual innovations. The first of these involves the recognition of margins as entities extending from sediment source to sediment sink. This idea is best illustrated in terms of the physiographic curve, extending from eroding continental highlands to the portions of the oceans that constitute the ultimate sediment sink (Figure 1).

The second conceptual innovation involves the division of this curve into five discrete units separated by four discrete boundaries that are dynamic and shift in response to perturbations. These are delineated below (Figure 1).

Thus margins, as conceived here, contain two terrestrial units and three submarine units, as well as four boundaries. One of these boundaries, i.e. the plains-shelf boundary contains considerable internal structure. Each of the units may contain subunits, such as bedrock and alluvial subunits within the continental uplands zone. Each unit may produce sediment through erosion and/or act as a sediment sink through deposition, either temporarily or permanently. The zones are interlinked by the flux of sediment through the boundaries.

The third and perhaps most important of the conceptual innovations is recognition that all of the boundaries are in fact dynamic and shift in response to climatic, tectonic, and anthropogenic perturbations. This motion is perhaps most dramatically illustrated in terms of the major changes in shoreline position in response to sea-level change. For example, the shoreline of the East Coast of the United States has migrated over 100 km landward since the end of the last glaciation. It is essential to understand, however, that every other boundary is also a dynamic boundary. For example, the shelf-slope break is migrating outward due to clinoform progradation and the gravel-sand transition is migrating downstream in response to tectonically driven inputs of coarse sediment.

Each unit has a characteristic morphology, which interacts with sediment flux through the laws of morphodynamics. Wherever there is net deposition within a unit, a stratigraphic section records this interaction. The units "talk" to each other by communicating sediment across the moving boundaries. The "solution" for how each unit responds to perturbation is intimately linked with the "solution" of boundary migration. Thus, at the scales of interest to the MARGINS program, no unit can be viewed in isolation, rather the units need to be examined as part of a linked system. Central to the goal of MARGINS is the dynamic quantification of unit interaction in response to the triad of forcing functions, comprising climate change, tectonics, and eustasy.

The linking of the five units allows for even the most distal components to interact with each other, albeit with time lags and filtering of the signal as it propagates through the intermediate units. Thus, rejuvenated tectonism in the continental uplands may ultimately result in increased sedimentation on submarine fans. On the other hand, the increase in sediment supply generated by tectonism may not cause a submarine response if there is sufficient accommodation created by subsidence on the continental plain. Conversely, rapid base-level drop can cause knickpoint migration up bedrock streams on the continental uplands, but only if the effect can propagate through the coastal plain.

The fourth conceptual innovation of the MARGINS program is the recognition of the power of analogy between the subaerial and submarine world. The continental uplands are in many ways loosely analogous with the submarine continental slope. The continental plains are likewise loosely analogous to the submarine fans found at and below the continental rise. More specifically the following analog structures stand out.

In no case is the analogy perfect. The degree of similarity, however, is sufficiently close to suggest that they represent different expressions of the same set of physical laws. The pursuit of these analogies allows for a link between the terrestrial and submarine worlds that goes beyond the already powerful link of sediment transport across moving, dynamic boundaries. The MARGINS program is unique in that it views subaerial and submarine processes, morphology and stratigraphy as linked pieces of the same fundamental unit, and views each unit, subaerial or submarine, as one realization of a unified underlying structure.

This unified conceptualization necessitates an interdisciplinary approach to the problem. Field research provides the baseline empirical base and the overall perspective. Experimental research allows for the testing of hypotheses that cannot be directly tested in the field. Theoretical and numerical research builds the basis for predictive capabilities. The framework requires cooperation among geomorphologists, stratigraphers and oceanographers, and requires them to cross the shoreline between the terrestrial and submarine environment. The potential for synergism among communities that have not normally been in close communication is unprecedented. This is why it is essential to involve both terrestrial and marine earth scientists in the MARGINS source-to-sink program.

Ocean Drilling

One of the major objectives of the Source-to-Sink Initiative is to assess the morphologic and stratigraphic response of continental margins to a number of environmental forcing functions acting over a variety of scales. Sampling and imaging strategies spanning entire margins, from the mountain tops to the deep sea, are required for understanding the links between fundamental physical processes and the evolution of landscapes and seascapes. Rapid advances in technology have greatly improved the accuracy and precision with which we can image the earth, both the subaerial and submarine environments. However, marine coring technology available to the research community has not kept pace with geophysical advances, especially in shallow-water environments. Hole stability in, and core recovery of, unconsolidated sediments remains poor, which greatly limits our groundtruthing/correlating capability. Platforms for shallow-water drilling (<100 m) that complement the strengths and capabilities of the JOIDES RESOLUTION are required to address fundamental questions about the development and evolution of dispersal systems on continental margins. New coring and downhole logging technology developed for industry offers exciting prospects for continuously coring and logging unconsolidated sediments in diverse environments (e.g., continental shelf and slope).

Climatic studies as well would benefit from additional shallow-water drilling platforms because most open ocean sediment cores cannot be used to resolve short period climate change, and records from most corals and varved sediment sequences are too short to resolve millennial-scale changes. Every ocean basin is rimmed by continental margins, most of which have exceptionally high rates of sediment accumulation (10 -100's cm/1000 yrs). At these locations, high-resolution paleoclimate studies could complement the records of corals, bivalves, and varved sediments. Continental margins as a whole have been grossly underrepresented in studies of paleoclimate because high terrigenous flux has been viewed as a liability rather than an asset. With shallow-water drilling platforms and chemical analyses using sensitive instruments, it is possible to recover high-resolution series of paleoclimate proxy data. In addition to providing climate data outright, samples recovered from shallow-water drilling will prove essential for correlating marine and terrestrial climatic data. The MARGINS Program views ocean drilling, which optimizes drilling and recovery capabilities and develops additional platforms to drill shallow-water environments, as an essential tool for the success of MARGINS science, especially the Source-to-Sink Initiative.

Fall Education and Planning Workshop

A MARGINS Source to Sink Education and Planning Workshop will be held September 11-15, 2000 at Lake Tahoe. Participants in the workshop will examine the relationships among processes relevant to sediment production, transport, accumulation, and preservation on margins across a large range of temporal and spatial scales, from turbulence to tectonics and from sedimentary fabric to sequence stratigraphy and basin analysis. This research initiative is expected to enhance our ability to interpret in a quantitative way the stratigraphic record of change in both terrestrial and submarine settings. Experts will be invited to speak about various aspects of the selected focus and allied study sites (e.g., New Zealand, New Guinea, and SE Alaska). In addition, plans for coordinated research and an implementation strategy will be developed at the workshop by the participants, and subsequently passed on to NSF. This approach will provide an opportunity for people to submit competitive proposals to the MARGINS 15 January 2001 RFP, even if they were not originally familiar with the selected study sites.

The Workshop will be conducted over four days. It will begin with an overview of the two focus areas in terms of sediment production, transport, accumulation, and preservation on a variety of spatial and temporal scales. Day 2 will be devoted to examining the interaction and feedback between these processes along the path from the eroding continental highlands to the deep sea. The agenda for days 3 and 4 is designed to accelerate progress on the Source to Sink theme by developing interdisciplinary approaches to research in the focus areas, and by implementing a research strategy that maximizes synergy and use of facilities, and minimizes dilution of effort. One of the principal objectives of the workshop is to stimulate cross-disciplinary inquiry into processes on margins, in order to provide a better understanding of landscape and seascape evolution and an intellectual framework in which laboratory, field, and modeling experiments can be posed. Applications to attend the workshop should be submitted to the MARGINS Office by June 15, 2000. Funds to cover lodging and meals, and to defray partial travel costs, of US participants are available. Applicants should send a one-page e-mail message to margins@soest.hawaii.edu containing (1) address and contact information, and (2) a brief description of research interests. Inquiries may be addressed to the MARGINS Office (http://www.soest.hawaii.edu/margins) or directly to the conveners care of Chuck Nittrouer (cnittrouer@ocean.washington.edu) or Neal Driscoll (ndriscoll@whoi.edu).