| Reg Beach | Oregon State University | Gail Kineke | University of South Carolina | |
| David Cacchione | U.S. Geological Survey | Paul Komar | Oregon State University | |
| Chip Fletcher | University of Hawaii | Nick McCave | University of Cambridge | |
| Robert Ginsburg | University of Miami | John Milliman | College of William & Mary | |
| Al Hine | University of South Florida | Orrin Pilkey | Duke University | |
| Robert Holman | Oregon State University | William Ryan | Lamont-Doherty Earth Obs. |
The coastal-shelf system of the oceans is a critical environmental interface - a fundamental Earth discontinuity - where terrestrial, marine, and atmospheric processes converge and mutually influence one another across a spectrum of spatial and temporal scales. (Here we define the term "coast-shelf system" in its broadest sense, as the complex of subenvironments from the landward extent of the coastal plain to the geomorphic feature known as the shelf break. This includes formerly inundated regions, tidal streams and estuaries, beaches and inlets, and the shoreface and shelf as they are found on continents and islands around the world.) Society relies upon the coastal system for its rich biological diversity, extensive mineral resources, and its fulfilling scenic and recreational opportunities. The coast satisfies societal needs for waste disposal, transportation venue, and a climate moderated by the heat engine of the oceans. It is these attributes that have led to a massive increase in population along the world's shoreline, a pattern that has stressed available resources and exposed development to marine hazards.
Media reports of storm damage, sea-level rise, coastal erosion, and declining nearshore water quality sound a clarion call from the American constituency for the development of a scientific focus on the nation's shelf and shoreface system. As we dam rivers, armor coastlines, disperse pollutants, and mine the shoreface we are forever altering the flux and partitioning of sediments through a sensitively linked series of littoral and marine ecosystems. Human alteration of the coastal system, in fact, constitutes a series of large-scale experiments that are disturbing the natural variability of the environment. Unfortunately, we take these actions without a full understanding of the fundamental processes that provide for the natural health and viability of the afflicted system.
These fundamental and fascinating questions can only be answered with multidisciplinary and multiscale investigations of sedimentary dynamics, and resulting environmental and stratigraphic imprints, across the land-sea interface of the continental and insular margin.
A number of Federal agencies have identified mission-oriented research programs that address coastal environmental problems. There remains, however, a need to investigate the basic dynamical and historical components of the shelf and shoreface such that the processes governing system behavior, and the role of antecedent controls, are better understood. There are fundamental scientific questions nested in the societally relevant issues. Some of these questions include the following.
Marine Geological and Geophysical (MGG) investigations at the National Science Foundation seek to improve our understanding of the oceans. Hence, it is appropriate that the NSF develop a focus on basic scientific issues of shelf and shoreface systems research in order to improve understanding of how fundamental geophysical processes and histories impact human needs, and how human actions impact natural variability.
A. Focal Points
At the FUMAGES conference in Ashland, Oregon, a group of MGG researchers met and discussed these problems. Collectively, the group represents research activities in sea-level history, coastal morphodynamics, sediment transport and numerical modeling, shelf sedimentation, sequence stratigraphy, slope sedimentation, and basin sedimentation. The group recognized that,
our collective field has been reluctant to develop a coherent program that presents to the rest of the MGG community a clear avenue of future research defined by fundamental questions.
Accordingly, in our discussions we sought to define the creative line of tension between the present state of our science and its future. We had, in addition, the results and recommendations of two earlier planning conferences on the future of siliciclastic and carbonate research (Kuehl and Milliman, 1996), and a pair of "state of the science" papers (Cacchione, 1995; Holman and Beach, 1995). In the course of three days at Ashland, we identified new directions, key problems, and scientific questions that characterize the study of sediments at the ocean margin. We asked questions such as, "What will be in the sedimentology textbooks 20 yrs from now?" and, "What are the exciting areas of research that address important fundamental questions?"
We developed a focus on sedimentary dynamics in the shelf and shoreface environment, and on the history and influence of episodic sea-level movements. We came to realize that improving our understanding of sedimentary dynamics will require interdisciplinary and multiscale investigations of particle transport, numerical modeling, fine-scale facies architecture, large-scale coastal behavior (LSCB) and morphodynamics, extreme events, and shifts in sea-level position. That is, the environments constituting the sedimentary province of the ocean margin are inextricably linked across the spectra of microseconds to millennia and microns to kilometers. Shelf and shoreface environments are integrated through a continuum of biological, geochemical, and physical processes that transcend a single spatial or temporal frame of reference.
To define the problem, we identified a spatial and temporal window of investigation whose outer bounds are defined at one extreme by melt-water pulsed changes in sea-level and the interstadial behavior of sea level. We refer to this extreme as "millennial-scale sea-level change." The other extreme is characterized by the nonlinear dynamical interaction of the mobile bed and water column, including turbulence, incident wind waves, and infragravity-scale oscillations that drive change at the sediment-water interface. We refer to this extreme as "sedimentary particle dynamics." Between these defining spatial and temporal levels exist the intermediate scale dynamics of the evolving coast, LSCB, sediment exchange across the shoreface, extreme events associated with hurricanes, El Nino's, and the nonsteady-state of depositional environments at the ocean margin.
Researchers investigating the sedimentary province of the ocean margins are confronted with a complex natural system reacting to multiple variables at many temporal and length scales. The coastal zone is unique in that it is strongly affected by terrestrial, atmospheric, and marine processes. Portions of the coastal plain, shoreface, and shelf are alternately exposed and flooded as a result of sea-level movements that may reach tens of mm/yr. Sedimentary environments are spatially and temporally complex. Sedimentary processes are governed by turbulence and the nonlinear interaction of the mobile bed, the dynamic water column, the benthic community, and geochemical fluxes. Coastal processes may operate as a major partitioning factor where shifts in the physical regime associated with extreme events can overprint the work of baseline processes.
Acquiring observational data poses difficulties and certain hazards. Even though our environments of interest are close to land, they are not easily accessible. Many, such as the Surinam mud-dominated coast are extremely remote. Others, such as windward reef margins experience high wave energy and are dangerous places to work.
A. Infrastructure
Within the MGG community there has been an emphasis on developing "blue-water" research vessels. However, these are not effective research platforms in coastal environments. It is not possible to obtain a 100 m core from a ship of 10 m draft in 15 m water depth characterized by high wave energy. Yet, in order for fundamental advances in margin stratigraphy to proceed, such cores are needed. Swath-mapping of sand-banks in depths ranging from 5 to 30 m has not been accomplished. Coring sand remains extremely difficult. Development of appropriate platforms such as high-speed, shallow draft vessels that can access remote areas for certain lengths of time has not been undertaken.
There is a need for shallow-water jack-up drill rigs that are affordable by academic research institutions. Shallow marine environments cannot be drilled by facilities such as the Joides Resolution and as a result, our field has not wide access to these potential data sets. Margin environments are difficult to core because of lithification, compaction, and coarse sediments. Shallow water poses serious problems such as extensive water column reverberations thus limiting resolvable penetration of high resolution seismic reflection profiling and extensive use of swath/multibeam bathymetric profiling. The presence of biogenic gas also limits geophysical remote sensing and potential drilling.
In short, we have major research infrastructure needs.
A. Dynamics
Investigations of sediments at the ocean margin range widely in both the time scales of the processes considered and in the spatial scales of the resulting morphologies or stratigraphic record. One investigator might obtain measurements of orbital velocities under waves in the nearshore, and relate those to the resulting transport rates of sediments or to the dimensions of ripple marks formed on the bed. Another investigator could be considering the processes of tides on the mid-shelf and the formation of huge sand waves. Longer time scales and larger spatial considerations apply to the investigator who relates the cycles of sea-level change to the resulting stratigraphy or architecture of deposits that span the entire ocean margin, crossing the shoreline and extending onto the coastal plain.
This breadth of consideration is illustrated by the accompanying diagram (Fig. 1) that graphs the time scales of processes (dynamics) versus the scales of the sedimentary features (morphology). In the dynamics domain, the shortest time scale is represented by the rapidly-fluctuating turbulent eddies within currents that are important to the entrainment and transport of sediments. Beyond that are the variations due to wind-generated waves that generally range between 5 and 20 seconds, and the hourly changes in water levels due to tides and the associated currents they generate. Important to the nearshore are the occurrences of storms, where "normal" storms generally occur a few times each year at a specific coastal site, while a "major" storm such as a hurricane may occur only once in a decade or longer. Such storms have profound effects on the sediments of the nearshore, and even on the sea-bed sediments across the entire shelf.
Even longer time-scale processes shown in the accompanying diagram are represented by sea-level variations. Tide gauges along our coasts provide a record of relative sea-level change during roughly the past 100 years, the change in global sea level "relative" to the land. Sea-level change can also include punctuated, millennial-scale sea-level events due to shifts in global ice volume (these may influence shelf sediment exchange and sea floor morphology during periods of rapid global change), and transgression/regression cycles that have occurred with glacial-interglacial changes in Earth's climate (Milankovitch cycles). Of critical interest is the knowledge gained from investigations of the Holocene and last interglacial episodes of transgression. These are specifically important since they have been profoundly influential in governing the present configuration of our coastal plains, coasts and shelves. The longer-term processes included in the graph are the tectonics of crustal movement at the coastal interface, processes such as continental margin subsidence, changes in basin configuration, and global tectonics that govern the degree of continental freeboard and the fundamental timing of shelf evolution.
B. Morphology
The second axis of Figure 1 shows the spatial scales of sedimentary features involved in research investigations. This list is only suggestive of the range of scales and is not an exhaustive account of the many sedimentary bodies found at the ocean margin. At the smallest scale are the sediment particles, obviously important in studies of sediment transport, but also important in the record of grain-size distributions of particles within the resulting deposits that reflect the transport processes. Accumulations of sediment grains form sand ripples or the bars that are an important part of the overall beach morphology, or the large-scale sand waves found in some shelf environments. These morphological features combine to form the entirety of deltas, estuaries, barrier islands and the present-day shelf. Recorded within the sediments of the margins are ancient shelves, stranded sand bodies, fossil reef tracts, and a stratigraphic record of former changes in sea levels.
C. Morphodynamics
To a degree, individual research efforts can be placed within the time-spatial scale of the accompanying graph. These tend to congregate along the 45-degree zone shown. For instance, investigations of sediment-transport processes focus on the time scales of waves and currents and down to the scale of turbulent fluctuations, while considering the movement of sediment grains and the effects of sand ripples on that transport. Other investigators document the response of beach morphology to the occurrence of storms, or the formation and migration on sand waves on the continental shelf where sediment transport is due to tidal currents. Yet another group of investigators is focusing on the effects of sea-level change, with the impacts ranging from the present-day changes in coastlines and estuaries to the long-term record within the stratigraphy of the continental margin. The unique ratio of morphologic and dynamic integration that constitutes research on the shelf and shoreface falls within this "morphodynamical corridor."
D. Collaboration
There is a notable lack of continuity and overlap among separate groups that study discrete morphodynamical ranges. A challenge to our science is to improve the linkage between these research subdisciplines. We must learn to talk to one another more often, and more effectively. Any one investigator tends to be limited to a small range of time scales of processes and resulting spatial scales of sedimentary features. The investigator is familiar with the next lower time-space scale of research, since his/her investigation likely uses the tools of that research (i.e., sediment-transport equations) or relies on its conclusions; the investigator is likely also aware of the implications of his/her research to the next higher time-space scales. While it is seldom that an individual can meaningfully cover an appreciable area of the time-space graph, it is through collaborative research efforts and modeling that connections can be made that lead to a more comprehensive understanding of the processes that are presently important, and were important in the past, to the sediments and sedimentary record of the margin system. A deeper understanding in the future will therefore depend on increased support for such collaborative research efforts.
A. Future Directions
An overriding theme for sediment transport is understanding the interaction between fluid motions and sediment particles based on fundamental physical principles that ultimately depend on the conservation of mass and momentum. Understanding fluid motions requires a consideration of the forcing mechanisms of pressure (waves and tides) and buoyancy (input of fresh water and sediment). These concepts must be applied as tools for linking sedimentation processes across time scales and environments. This includes expanding the domain from the water/seabed interface or bottom boundary layers to the entire water column of all of the shallow marine environments: shelf, coast, and estuary (Fig. 2). The sediment supply to these environments is a critical component, thus rivers cannot be excluded.
To accomplish integration across scales and environments, we must forge strong partnerships between modeling and observations. While this coupling is not new, it must set the course for future study. Commonly, sediment transport models exceed our ability to verify them, however, they have, and must continue to guide the observations before true progress is achieved in understanding the processes governing sediment transport.
B. Key Questions/ Unresolved Issues
1. How do fundamental forces control the development of stratal systems (from bed to sequence) over time scales ranging from seconds to millennia?
Combined modeling and observational studies in progress on continental margins, with relatively simple sedimentological conditions, are exploring processes responsible for the formation of strata on time scales of seconds to a few thousand years, but the evolution of sedimentary environments and processes that operate in more complex dynamical conditions is less clear. The fact that we are presently at a high stand of global sea level requires the search for and use of modern analogues for conditions that were more common in the past (lower sea levels) and thus more relevant in the geologic record. Our present high sea level is somewhat anomalous over the spectrum of the Quaternary. We have limited experience with sedimentary processes operating within submarine canyons and fans, which are most active during conditions associated with Pleistocene low-stands. Hence, it is appropriate to use modeling results based on physical principles of clastic sedimentation to guide studies of sedimentary processes and observations from a range of environments (i.e., convergent or insular margins with narrow shelves where sediment flux is more directly tied to the deep sea). It is equally important to track the evolution and adaptation of reef systems and associated carbonate supply and storage processes with regard to the rapidly fluctuating level of the oceans over the last million years. How do the rapid sea-level changes of the Quaternary control and/or imprint the stratal record?
2. How are river-borne materials partitioned in space and time, and how do natural and anthropogenic changes affect both supply systems and sinks?
Anthropogenic influences are presently occurring that cause perturbations equal to (or exceeding) natural variability in the shallow marine environment. By damming rivers, sediment supply to the coastal region is drastically reduced, high population centers located on coasts experience increased subsidence rates through withdrawal of groundwater, beach sand replenishment projects can emplace enormous volumes of sediment to beaches over a period of months. The processes acting after these perturbations are poorly understood and we are only able to describe them in general, qualitative fashion. However, these are examples of critical "natural" experiments and offer tremendous opportunity for process-response driven investigations. Especially valuable would be the development of a physics based modeling capability for improved understanding of shoreline behavior.
Previous sediment transport studies have provided insight from a number of marine environments: deep water (HEBBLE), shelves (CODE, STRESS), the shoreline (DUCK series), and the current investigation of strata formation from fluid/sediment interaction (STRATAFORM). However, the mechanisms of how sediment is delivered to the seabed needs to be reexamined. For example, one of the interesting results from AMASSEDS was the dismissal of one paradigm explaining growth of the subaqueous delta. Areas of high accumulation rates were not the result of passive settling through the water column in deep enough water where boundary shear stresses have diminished to the point that accumulation can occur. Rather, sediments are trapped at bottom salinity fronts on the inner shelf in shallow water (~10 m) and migrate off shore to deeper depths (35-50 m) as dense near-bed suspensions, or fluid mud layers. Hyperpycnal flows may be common in many other rivers world wide, especially rivers that experience extreme flow events. Even the well-studied Mississippi may require hyperpycnal flow to explain the regions of high accumulation rates leading to oversteepening and slope failure. This requires consideration of the processes that trap sediments in the estuary (convergence of estuarine flow and fine particle dynamics) and discharge processes during periods of high flow.
3. What fundamental sedimentary processes affect material transport at the shoreline and shelf edge, and hence the development of the continental margin?
The shoreline and shelf edge are gateways through which sediment transits to the sea, and from the shelf to the deep sea, respectively. Delineating sedimentary processes at these gateways, as well as their temporal variation, can enhance our understanding of continental-margin evolution and the nature of the preserved sedimentary record.
4. How do sediments escape from the coastal zone?
We know that sand is supplied to the nearshore zone by rivers, coastal erosion, and benthic productivity. This nearshore zone is only a few kilometers wide and is stirred by waves (and storms) and may also have sand transporting currents driven by wind or tide. We do not know whether, and if so how, sand is transported out of this nearshore zone to the shelf at the present day. Regions with both an active new supply of sand and a shelf contour regime capable of transporting it are quite rare. If we do not understand how, or even whether, sand moves offshore beyond the upper shoreface we are clearly unable to deal with the origin of shelf sand bodies. This is important to not only sedimentologists but also to engineers concerned with beach nourishment, and to stratigraphers interpreting paleoenvironmental patterns. Of course we do know that sand is moved across the shelf by rivers at low stands of sea level and reworked as it rises, but is this the primary means of cross-shelf sand transport? If so, this has profound implications for the interpretation of ancient marine sands and shelf sequences. This is a prime target for large-scale marine sediment dynamics research.
5. How well do we understand sediment processes within the variable character of global shorelines?
Many of the concepts developed to explain sediment transport on sand-rich shores, such as the Outer Banks or the Dutch coast, may not apply to coastal systems with relatively less sediment supply. For instance, many beaches on windward reef flat margins sit upon fossil reef limestone platforms and are apparently sharing little sediment with an offshore region that displays little or no sediment accumulation. The concepts of cross-shore particle flux embodied in depth of closure criteria and volumetric exchanges across the beach profile do not readily explain these systems.
While much progress has been made studying the effects of fluid forcing and sediment response on non-cohesive coasts (sandy beaches), these processes are not transferable to coasts composed of cohesive sediments. Muddy coasts have received almost no study from a dynamic standpoint by comparison to the work on non-cohesive beaches. For example, wave energy (wave height) increases as waves progress shoreward until they break. On muddy coasts, waves may pass over an unconsolidated seabed where wave energy is greatly attenuated as waves move shoreward so that in some cases no wave energy reaches the shoreline. Thus marshes (or mangroves) can be established on an exposed coast and accretion of the shoreline can be governed by the nature of the seabed (sediment type) immediately seaward.
C. New Approaches
If we seek to link larger-scale shelf/marine processes with terrestrial sediment supply we must expand the geographic areas of research to include the nearshore and coast (shelf work has traditionally been seaward of the 10 m isobath) as well as estuaries and rivers. Estuarine research in particular has often fallen into a black hole at the NSF, but it is these sedimentological systems, at the interface of the terrestrial and marine environments that moderate the sediment supply and buffer the signal to the coastal ocean.
High resolution stratigraphic and bathymetric change-detection experiments, small instrument platforms able to resist high wave forces, real-time data delivery to onshore recording stations, rapid response teams able to capture before and after datasets of episodic events such as storms, collaborative studies of 4-dimensional (x, y, z, t) particle production/flux/and storage/fate, and model development and testing with observational data sets of morphodynamical processes - these are approaches that must be implemented with greater frequency and geographic diversity.
A Fundamental Agent of Change
No more fundamental process governs the present and past distribution of coastal sediments and ecosystems, and the underlying topographic blueprint that controls water column circulation on the shelf and shoreface, than sea-level movements in the recent past. The spectrum of climatic oscillations, tectonic convulsions, geoidal shifts, and sediment distribution patterns that drive local relative changes in the position of the sea surface is best recorded over the ~140 kyrs of the late Quaternary (Fig. 3). A focused analysis of sea-level movements, across a spectrum of temporal and spatial scales, would strengthen our framework for understanding the potential for near-term and future patterns of coastal change, for deciphering the role of sediment flux and storage as shorelines translate across the inner shelf and shoreface, and for determining the origin of the present-day bathymetry of the shelf and shoreface.
The mechanisms that drive late Quaternary relative sea-level changes include:
B. Recent Developments
In 1989, Richard Fairbanks published a history of sea level from the submerged coral reefs of Barbados that recorded melt-water pulses in the post-glacial (pre-Holocene) stages of rise. Later papers by E. Bard (1990a, b) improved the chronology of the record, and subsequent workers (Edwards et al., 1993; Bard, et al., 1996; Clark et al., 1996; Montaggioni et al., submitted) confirmed the presence of at least one melt-water pulse but have debated the source, timing, and relative magnitude. Other studies suggest that sea level experienced upward jumps of as much as 11 m during these pulses within a decadal time scale (Blanchon and Shaw, 1995, who also added an additional, third pulse in the early Holocene; MacAyeal, 1993). Present debate centers on the origin of glacial waters with both northern and southern hemisphere sources variously advocated.
Meanwhile, the concept of a last interglacial sea level 6 m above present has been revisited as sites that were once considered stable have been reassessed (Muhs and Szabo, 1994) and the role of geophysical processes has been more fully integrated into the earlier timeframe of Stg. 5e (Lambeck and Nakada, 1992). Simultaneously, examination of marine records of glacial and interstadial sedimentation events have been correlated to the ice core record of interstadial climate oscillations over the period ca. 40 to 12 kyrs B.P. (Bond and Lotti, 1995).
Sea-level researchers and coastal sedimentologists now recognize that the shelf and shoreface has experienced sea-level change reaching sustained rates of tens of mm/yr. To what extent is the archive of sea-level change dominated by these episodic, high-magnitude events? How do galloping sea levels influence shoreline translation histories and sediment distribution patterns? Are the climate oscillations of Bond and Lotti related to global sea-level changes at the same frequency? Is the North Atlantic climate history in the GISP and GRIP archives interpretable in terms of sea-level change? Our new understanding of late Quaternary sea-level history is characterized by rapid and episodic changes of significant magnitude. This character still remains to be rectified with our models of shelf and shoreface sedimentology and strata formation.
C. Key Questions/ Unresolved Issues
1. What are the dominant processes controlling global sea-level change on the time scale of the late Quaternary? What is their timing and magnitude?
An important shift in our understanding of the history and mechanism of sea-level changes has occurred over the last decade. Earlier ideas of global, simultaneous sea-level change, known as eustasy, hinged on a direct linkage between the last great ice sheets and the world ocean level, a concept known as glacio-eustasy. Glacio-eustasy was generally thought to be a relatively gradual process. As researchers from around the world compared geologic archives of sea-level change during the 1970's and 1980's it became apparent that a varied and locally-dominated sea-level history is preserved in Holocene and post-glacial records at different areas. That is, islands in the equatorial Pacific apparently experienced a middle to late Holocene highstand and fall of sea level at the same time that divergent continental margins distal to the former extent of the great ice sheets experienced a shift from a rapidly rising to a slowly rising sea level and recorded no highstand. Convergent continental and insular margins are generally characterized by a co-seismic relative fall in sea level, while large-scale sediment depositional centers underwent rapid, and in cases rate-variable, submergence. By the middle and end of the 1980's the role of glacio-eustasy as the great driving mechanism of global sea levels had been supplanted by an understanding that few coastal margins are truly stable and that the goal of defining a single, detailed eustatic record of the last interglacial cycle is unattainable. Quantifying the history of local relative sea-level behavior has now become the object of researchers working in the Holocene. It is critical, therefore, to accurately characterize the local and global components comprising sea-level archives, including global processes that display local variability (i.e., the shifting geoid; Peltier, 1996).
2. What is the amplitude and timing of sea-level change accompanying fluctuations in glacial volume throughout stages 2-4?
Early- and pre-Holocene episodes of sea-level change, for instance during marine isotope stages 2 through 4, are poorly understood. Every new published record of sea-level change from this period undergoes global scrutiny by practitioners looking for common patterns. Unfortunately, geologic archives covering this period are dominated by emerged shorelines on islands sited upon overriding plates in convergent settings. These records are subject to assumptions based on uplift histories that may not be fully understood. Sea level movements in the period ca. 115 to 6 kyrs B.P. are especially poorly represented along the tectonically stable coasts. A fundamental barrier to research progress on stable shores is the tendency for sea level to reoccupy past positions throughout this period, positions that are submerged at depths below traditional SCUBA access but that are considered too shallow for most submersible work. As a result, researchers are turning to remote mapping and sampling tools that provide data on shelf morphology and paleoshorelines in order to increase the number and diversity of records available from this era.
3. What is the amplitude and timing of sea-level change during the last interglacial?
Various researchers working in the last interglacial have characterized eustatic sea levels over the period ca. 130 to 120 kyrs B.P. to be above present, perhaps as much as 6 m, and perhaps undergoing a mid-interglacial fall ca. 125 kyrs B.P. Hence, a dichotomy exists involving workers in the Holocene who understand that sea-level is merely a reflection of myriad global and local processes (and that no purely global pattern should be expected), and workers in the Pleistocene who understand that sea-level has a consistent global pattern, perhaps consisting of a mid-interglacial fall ca. 125 kyrs.
The shelf and shoreface system of the world has been the target of only modest examination and research. Few areas, even on the heavily populated margins of the continents, have been subjected to the intense light of marine geophysical research. Sediment dispersal pathways through the estuaries and inlets of our coasts are poorly understood and we lack the most rudimentary ability to predict bedload and or suspended sediment flux to the shelf areas. Where are the great sinks of sediment that are shed from our highlands? What is the relative (and quantitative) role of marshes, channels, deltas and mid-shelf accumulation sites in the bypassing of sediments from the hinterlands to the basins? There can be little doubt that our improved understanding of fine-scale sea level oscillations provides new opportunities for determining answers to these questions.
A. Linked Environmental Changes
In light of our evolving appreciation of the myriad process-response relationships connected to sea-levels in the late Quaternary, and the multiple temporal and spatial scales that characterize sea level movements across the ~130 m amplitude of changes, it is apparent that we have entered a new realm of understanding linked environmental processes at the land-ocean interface. Complex relationships between shelf bathymetry and sediment accumulation invite examination using the new understanding of rapid, episodic and multi-scale sea-level history of the last interglacial cycle (Stages 5 to 1).
Carbonate systems provide a wealth of dating opportunities using radiocarbon, Th-230, amino-stratigraphy, and ESR that promise exciting new records of linkages between the evolution of the shelf and shoreface and the driving mechanisms of sea-level change (Fletcher and Sherman, 1995; Locker et al., 1996). High latitude shelf systems can provide limits on the timing and magnitude of ice sheet decoupling events during and at the end of the last ice age that are driving the global response of sea level. Continental margins will preserve drowned shorelines and former sediment depocenters such as estuarine retreat paths and incised valley systems that trace the back-stepping history of the shorelines and associated environments. The world's delta systems are especially sensitive to relative sea-level changes as the major sites of sediment dispersal and accumulation undergo rapid lateral shifts in response to discharge and gradient changes. Little is known of the extent that meltwater pulses have governed the record of accumulated sediments in these environments. It can be said with relative certainty, however, that sedimentary records of sea-level changes on the shelf and shoreface offer a unique opportunity to link deep sea biochemical and sedimentologic records of circulation and climate change to ice core records of climate and atmospheric change over recent geologic history. Linked records of climate, ocean circulation, atmospheric circulation, sediment distribution, and sea level provide a powerful framework for understanding the history of environmental changes that attended the most recent interglacial cycle, as well as the modern environment that is so acutely modified by human activities.
Processes and sediment morphologies are commonly linked through the development of models, which can be either conceptual or numerical. This is illustrated by models developed in application to the nearshore.
Researchers recognize that the morphology of a beach depends on the sediment grain sizes and on the processes of waves, tides and currents. Investigations of a specific beach under a time-varying range of wave energies, and comparisons with other beaches having different grain sizes and perhaps different wave climates, yield a conceptual model for the resulting beach morphology. On a specific beach it is found that morphology changes in a systematic fashion with varying wave energy, influenced also by the range of tides. Much of this morphological response is in the form of the offshore bars, which tend to be linear and parallel to the shoreline when high storm waves reach the beach, becoming regular and crescentic shaped with declining wave energy, and finally breaking up and becoming irregular at still lower wave energies when rip currents exert a stronger effect than the waves. Such a conceptual model is useful as a unifying theme in the study of beaches.
At the same time, numerical models have been developed that in a quantitative fashion link processes and morphology through evaluations of sediment transport rates. These numerical models often represent one aspect of a broader conceptual model, an aspect where our understanding has advanced sufficiently to permit the development of quantitative prediction. For example, cross-shore sediment transport models have been developed that calculate beach response to changing wave conditions, with sand erosion at the shoreline during storms and its transport offshore to bars, and then its return during subsequent low wave conditions. This is a two-dimensional portion of the much broader three-dimensional conceptual model described above. It can be expected that in the future, the present two-dimensional numerical model will be expanded into the third dimension to provide a still better correspondence with what is presently a conceptual understanding.
Conceptual models, and to a degree numerical models, have similarly been developed for other sedimentary environments of the ocean margin, and for the long-term stratigraphic architecture of the sedimentary deposits that are the product of changes in sea level and sediment supply.
The importance of all models, both conceptual and numerical, is that they serve to unify the endeavors of various investigators who otherwise focus on different process time scales and spatial scales.
For example, the cross-shore transport model described above relies on sediment-transport equations that are based on small-scale considerations, together with observations of the overall response of the beach morphology to changing wave conditions. While unifying what is known, the level of disagreement between the numerical model and observations serves to illustrate what remains poorly understood and what should be the focus of future research, and can even serve to devise measurement schemes. Returning to the example of the cross-shore sediment transport model, it was found that the inclusion of undertow improves the results, while remaining discrepancies indicate that we need to add evaluations of long-period infragravity motions that are known to be particularly important to the swash of the waves on the beachface. The model also shows that we need to know more about the fundamentals of sediment transport, specifically the relative transport rates of different grain sizes and densities within a natural sediment (most of our sediment-transport equations pretend that the sand is of uniform size and density).
It is apparent that the development of conceptual and numerical models provides linkages between investigators who are otherwise focusing on a limited area within the time/space ranges included in Figure 1. At present, the linkages still remain limited, but one can envision that eventually the entire range of time scales involved in the processes and the full range of sediment features and morphologies will be covered by series of linked models into a unified whole, into a general theory of ocean margin sedimentation.
A. A Diseased Anatomy
As we enter the twenty-first century, seventy percent of the world's population will live and work alongside the sedimentary province of the ocean margins. The deposits of the coastal plain, shoreface and shelf, slope and rise consist of particles and pore fluids arranged in a complex architecture of bedforms, layers, wedges, aprons, etc. that define the anatomy of the province. It contains reserves of oil and gas vital to the global economy, and may exceed 10 km in thickness.
For over a century, human activity has perturbed the coastal sedimentary province by the filling of wetlands and the reclamation of estuaries, through the construction of seawalls and breakwaters, and by starvation of the sediment supply as a consequence of damming rivers for hydroelectricity, irrigation and flood control. Many of the environments of this province are "diseased" from thoughtless use, over-exploitation, and even well-intentioned but ignorant attempts at mitigating the problems that come with human influences. Millions of dollars are spent annually to pump sand back to the shore to replenish beaches, only to have these grains disappear into the sea again following nor'easters, hurricanes or typhoons. Much of this province has been alternately submerged and exposed in the past. Some of it will experience renewed flooding if global warming predictions are correct. Some sectors, such as the extremely populated Nile Delta and portions of the Mississippi Delta, are currently in a state of crisis in the wake of severe coastal erosion. The megalopolis of Bangkok is sinking at an alarming rate of one meter in a human lifetime due to a combination of diminished sediment supply and aquifer depletion.
B. An Integrative Science
True stewardship of the sedimentary resource will require an improved understanding of its anatomy. The piecemeal, ad hoc examination typical of past research will not provide a sufficiently integrated understanding of the character of the system. A new approach must incorporate the next generation of Earth and environmental scientists trained with greater engineering skills and an integrated knowledge of the physical/chemical/biological metabolism of the sedimentary environment. This is necessary preparation for the challenge of quantitative modeling and measurement that lies ahead. A new, integrative science must be the hallmark of future sedimentological research. In order to achieve new goals of understanding the ocean margin, the science of sedimentology must evolve into a systems approach that integrates theories and concepts of biochemistry, geophysics, meteorology, climatology, population statistics, ecology, and hydrodynamics.
C. Actions
The goal of the first quarter of the 21st century is a unifying theory of sedimentation based on the fundamental principles of physics and chemistry, and nourished by a strong linkage to the biospheric sciences. This goal will be reached essentially by assembling the differential equations explaining the energy and mass flux through the system in a fully dynamical sense. Boundary conditions for the equations will come in part from
1) innovative imagery of the anatomy to characterize in detail its 4-D architecture,
2) new sampling tools optimized for shallow-water and the shoreface, and
3) internal remote sensing with logging tools and potential fields to determine the in situ state (thermal, pressure, rheology, permeability, chemical reactivity, etc.).
The equations will be tied to time-series phenomena such as sea-level, wind stress, wave energy, ocean circulation and other climate controls including monsoons and glacial cycles pulsed by solar insolation as deciphered by the paleoceanographers. Space making (accommodation) phenomena (subsidence, lithosphere flexure, stratal compaction, etc.) will come through linkages with the dynamic models of lithosphere evolution derived by the solid Earth community. For example deep-seated thermal and chemical reactions between fluids and rocks both control and result from the alteration of buried strata. This diagenesis can strengthen or weaken formations and thus influence the failure of slopes by slab detachment, slumping or avalanching processes.
Field observations linked to model building must continue where presently successful. New observational efforts should be extended to unstudied geographic locations that represent critical stages in the formation of the sedimentary province of the ocean margins across the spectrum of time and space embodied in the last interglacial cycle.
The global scope of episodic millennial sea-level change requires additional definition. The sources of melt-water, the amplitude of resulting eustatic change, and the impact on shelf and shoreface environments all need further research and data collection.
Model development and parameterization should keep pace with, and continue to be tested by observations of sediment dynamics to build a stronger predictive capability. Researchers should set goals and define innovative approaches for broadening our understanding of the dynamical behavior of sediments, the patterns of particle flux, storage, and fate that characterize the modern ocean margin, and their response to shifts in boundary conditions. These should be integrated across relevant temporal scales to better define the environmental system during various stages of its evolution.
Bard, E., Hamelin, B., Fairbanks, R.G., Zindler, A., 1990a. Calibration of the C-14 timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals: Nature, v. 345, p. 405-410.
Bard, E., Hamelin, B., Fairbanks, R.G., 1990b. U-Th ages obtained by mass spectrometry in corals from Barbados: sea-level during the past 130,000 years: Nature, v. 346, p. 456-458.
Bard, E., Hamelin, B., Arnold, M., Montaggioni, L., Cabioch, G., Faure, G., and Rougerie, F., 1996. Deglacial sea level record from Tahiti corals and the timing of global meltwater discharge: Nature, v. 382, p. 241-244.
Blanchon, P. and Shaw, J., 1995. Reef drowning during the last deglaciation: evidence for catastrophic sea-level rise and ice sheet collapse, Geology, v. 23.1, p. 4-8.
Bond, G.C., and Lotti, R., 1995. Iceberg discharges into the North Atlantic on millenial time scales during the last deglaciation: Science, v. 267, p. 1005-1010.
Cacchione, D., A., 1996. A brief report for FUMAGES: Shelf sediment transport -past, present and future: FUMAGES State-of-the-Science white paper, 9p.
Clark, P.U., Alley, R.B., Keigwin, L.D., Licciardi, J.M., Johnsen, S.J., and Wang, H., 1996. Origin of the first global meltwater pulse following the last glacial maximum: Paleoceanography, v. 11.5, p. 563-577.
Edwards, R.L., Beck, J.W., Burr, G.S., Donahue, D.J., Chappell, J.M.A., Bloom, A.L., Druffel, E.R.M. and Taylor, F.W., 1993. A large drop in atmospheric 14C/12C and reduced melting in the younger Dryas, documented with 230Th ages of corals: Science, v. 260, p. 962-968.
Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea-level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation: Nature, v. 342, p. 637-642.
Fletcher, C.H., and Sherman, C.E., 1995. Submerged shorelines on O'ahu, Hawai'i: archive of episodic transgression during the deglaciation? Journal of Coastal Research, S.I. no. 17, p. 141-152.
Grossman, E.G., and Fletcher, C.H., submitted. Sea level 3400 yrs ago in the Northern Main Hawaiian Islands.
Holman, R., and Beach, R., 1995. Nearshore marine geology: FUMAGES State-of-the-Science white paper, 6p.
Kuehl, S.A., and Milliman, J.D., 1996. Siliclastic Margins Record Terrestrial and Oceanographic Conditions. EOS, Transactions, American Geophysical Union, v. 77, no. 37, September 10, p. 356.
Lambeck, K., and Nakada, M., 1992. Constraints on the age and duration of the last interglacial period and on sea-level variations: Nature, v. 357, p. 125-128.
Locker, S.D., Hine, A.C., Tedesco, L.P., and Shinn, E.A., 1996. Magnitude and timing of episodic sea-level rise during the last deglaciation: Geology, v. 24.9, p. 827-830.
MacAyeal, D.R., 1993. Binge/purge oscillations of the Laurentide ice sheet as a cause of the North Altantic's Heinrich Events: Paleoceanography, v. 8.6, p. 775-784.
Montaggioni, L. F., Cabioch, G., Camion, G.F., Bard, E., Ribaud-Laurenti, A., Faure, G., and Dejardin, P., submitted. A 14,000 years continuous record of reef growth in a mid-Pacific island.
Muhs, D.R., and Szabo, B.J., 1994, New uranium-series ages of the Waimanalo Limestone, Oahu, Hawaii: Implications for sea level during the last interglacial period: Marine Geology, v. 118, p. 315-326.
Neumann, A.C., and Hearty, P.J., 1996. Rapid sea-level changes at the close of the last interglacial (substage 5e) recorded in Bahamian island geology: Geology, v. 24, no. 9, p. 775-778.
Nittrourer, C.A., and Wright, L.D., 1994. Transport of particles across continental shelves: Reviews of Geophysics, v. 32, no. 1, p. 85-113.
Peltier, W.R., 1996. Mantle viscosity and ice-age ice sheet topography: Science, v. 273, p. 1359-1364.
Shackleton, N.J., 1987. Oxygen isotopes, ice volume and sea level: Quaternary Science Reviews, v. 6, p. 183-190.
Return to FUMAGES Workshop Page
National Science Foundation
webmaster@joss.ucar.edu Last modified: 25 March 1999