Source to Sink Workshop Summary

Convenors: Neal Driscoll and Charles Nittrouer

A four-day meeting funded by NSF and JOI was held at Lake Quinault, WA, on Sept. 28-Oct. 1, 1999 to create the science plan for the MARGINS sedimentology and stratigraphy community. The science plan will suggest important directions for future research, recommend strategies for accomplishing this research, and will consider candidate sites for detailed interdisciplinary studies in light of the site criteria accepted at the workshop (and described below). The science plan is expected to provide a blueprint for taking geomorphologic, sedimentary and stratigraphic processes to a substantially higher level of understanding. 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.

Margins constitute material dispersal systems that convey water, sediment and associated chemicals from the continent to the sea via rivers, acquifers, mass movements and turbidity currents. The various parts of this system are typically in a state of flux at geological time scales. Temporal and spatial evolution of margins involve strong interactions between the various zones of the sediment dispersal system. Understanding and predicting these changes requires 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 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 group philosophy, and fully integrating 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.

Criteria for "Focus" Sites
The meeting employed a systems approach to examine coupled land and ocean environments (from mountain tops across shorelines to abyssal plains) and because site selection is an integral part of the MARGINS strategy, substantial discussion was devoted to producing a list of important criteria. The following list of site criteria was agreed upon at the workshop. All of these criteria are deemed important, but not mandatory - that is, failure by a site to meet one or more criteria would not necessarily disqualify it from consideration.

Natural factors for selecting a site:

• Strong forcing that produces strong signals
• Active sedimentation spanning source-to-sink environments
• Active transfer among environments
• Closed system
• High-resolution stratigraphic record
• Presence of carbonate environments
• Significant differences between the two sites

Human considerations for selecting a site:

• Background data and scientific infrastructure
• Manageable logistics
• Definable, preferably small, anthropogenic influence
• Societal relevance
• Potential to leverage resources for research

These criteria are explained below. An overarching goal of the Source to Sink research program is to increase our understanding of the roles that climate, tectonics, and eustasy play in shaping Earth's surface and stratigraphic record. This goal is best achieved by focusing on research areas where these factors exert strong control on evolution of the Earth's surface. Areas experiencing rapid uplift and vigorous atmospheric forcing yield large amounts of sediment, often during catastrophic events. Unraveling the complex interplay of processes and interpreting their history is easier to achieve when various landforms and sedimentary layers can be clearly associated with distinct events arising from strong forcing.

The MARGINS sedimentology/stratigraphy program offers an unparalleled opportunity to blend the skills and expertise of the terrestrial and marine communities. To foster maximum interaction, a site should bridge as many different physiographic environments as possible in going from source to sink. Ideally, a research program will build insight into the dispersal system that transfers particles from mountain tops to abyssal plains.

A guiding principle of this program is that the key to the past lies in the present. Through study of active processes comes understanding of landforms and strata at the Earth's surface. Because this MARGINS program seeks understanding of material transfer from source to sink, sites should, at present, have active transfer among: hill slopes, river valleys, coastal plains, continental shelves, slopes, rises and abyssal plains.

An important metric of understanding source-to-sink transfer in a basin is the accuracy of sediment budgets. Unexplained sources and sinks of sediment confound sediment budgets and thus should be avoided. Closed systems are ones in which all of the major sources and sinks of sediment can be investigated and quantified, and these systems would be best suited for mass-transport studies.

A high-resolution stratigraphic record extending back to glacial stage 5e (125ka), and preferably farther, will provide a history of glacial/interglacial cycles and eustatic sea-level fluctuations. Given the mandate to understand the effects of climate and eustasy on the evolution of the Earth surface, a record of at least this length is vital.

Carbonate components are found mixed with siliciclastic sediments in most marine settings, and in some cases dominate the flux of material to the seabed. The factors that control this mixed siliciclastic/carbonate system need to be understood and quantified. Historically, attention has been drawn to the separation between the two sediment types. This has led to the division between carbonate and siliciclastic specialists, who rarely interact to examine the entire system. This is counter productive, because carbonate material provides unique geochronologic constraints, chemical signals, sea-level information, and other insights.

The selection of sites that are significantly different from each another will provide the most insight into processes affecting the Earth's surface and stratigraphic record. With only two sites to study, the group agreed that environments as different as possible should be selected; e.g., steep versus gentle surface gradients, short versus long dispersal systems, wet versus arid environments, rapid versus slow tectonic uplift.

The ability to understand past, rare, sediment-transporting events is helped markedly by historical data. Quantitative information about rainfall, stream flow, sediment discharge, ocean waves, and winds are all desirable. Aerial photographs, digital models, and remotely sensed data also would aid this MARGINS effort. Going hand-in-hand with historical data is a well maintained data infrastructure. Continuing observations of the above variables as well as easy access to the data would benefit the research.

Manageable logistics focus the attention of scientists on research rather than on the details of completing it. Sites should, for example, be accessible by many scientists and be politically stable. The host country should have trained scientists willing to participate in and augment the proposed research.

Many river systems around the world are so affected by human activity that the transfer of material is anthropogenically dominated. To maintain societal relevance, it would be wrong to eliminate sites with an anthropogenic signal. Rather, the impacts should be quantifiable and should not overwhelm the natural processes.

A large part of the Earth's population lives in river valleys and coastal regions, therefore the proposed research has an enormous potential to strengthen the scientific basis of land-use strategies, as well as those used to protect coastal wetlands and fisheries. Likewise, research into catastrophic events will assist risk assessment and hazard mitigation. The final criterion is the potential for leveraging resources to undertake research. If the work could be conducted in an area hosting other active research programs, both efforts would benefit. "Allied" study sites - One or two of these will be identified because they provide a unique opportunity to investigate a process relevant to, but not accessible in, the "focus" sites.

Site Selection
Focus sites will be chosen based on the consensus of opinions given by e-mail and at the Town Meeting in light of the criteria identified at Lake Quinault. Only two of the seven candidates will be selected as "focus" sites. It is important to note that the candidate sites not selected for the "focus" study areas cannot then be classified as "allied" sites. On the basis of the above site criteria, the workshop participants identified the seven areas listed below to be evaluated by the community as potential "focus" sites. Participants at the workshop either volunteered or nominated the appropriate researchers to provide succinct site descriptions, which illustrate how the sites meet the criteria accepted at the workshop. The objective is to give the community enough information to evaluate the site in light of the criteria. For the two sites ultimately selected, very detailed descriptions will be presented at workshops next year.

Brazos-GoM: Mike Blum, John Anderson

• SW Japan: Gary Parker, Mike Underwood
• New Guinea: Bill Dietrich, Steve Kuehl
• New Zealand: Basil Gomez, Craig Fulthorpe
• Nicaragua: Ray Torres, Sam Bentley
• Santa Monica: Tom Dunne, Bill Normark
• Taiwan: Niels Hovius, Neil Lundberg

Allied site for glacial processes:

• SE Alaska: Bernard Hallet, John Jaeger

MARGINS Web Site and AGU Town Meeting
In November a draft of the conceptual plan, site criteria, and descriptions of the 7 potential study locations will be available on the MARGINS web site and we will solicit e-mail input from the community. A Town Meeting will be held at AGU on Monday evening, December 13 at 5:30-8:00 pm in Room 120 of the Moscone Center. The documents will be discussed and, based on input at the Town Meeting and from e-mail responses, a consensus will be reached regarding focus areas for coordinated research studies.