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  1. What is the cause, nature, and range of climate and ocean variability at the inter-annual to millennial time scale given that there is no obvious external forcing?

  2. What processes set the sensitivity of the climate system to external (orbital) forcing, and what processes are responsible for the long-term evolution of this sensitivity?

  3. Why are there ice ages in Earth history?

In order to address these questions, integrated model, laboratory, and data studies are essential. The modeling of processes at all levels of complexity—from simple box models to more complex models of mass balance exchange in the ocean-climate system—can provide useful insights into the nature of these processes. Opportunities for model development, including climate, ocean, and biogeochemical models, and accessibility of various types of models for research must be maintained.

Collaboration between researchers across all subdisciplines that study Earth system history must be encouraged and developed. Advances in the field via data-model integration may arise from collaboration of subdisciplines that are not traditionally combined. A critical element of such collaboration will be to have an interdisciplinary peer-review process for interdisciplinary proposals.

GROUP #3: SHELF AND SHOREFACE SEDIMENTS

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. Society relies on the coastal system for its rich biological diversity, extensive mineral resources, and its fulfilling scenic and recreational opportunities. This system satisfies needs for waste disposal, transportation, 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.

  • How do human actions impact natural variability?

  • What are the fundamental processes that unify the multiple temporal and spatial scales constituting the dynamic behavior of the shelf and shoreface?

  • Are there overarching physical/biochemical processes governing natural variability in the spectrum from microseconds to millennia?

These fundamental and fascinating questions can only be answered with multi-disciplinary and multi-scale investigations of sedimentary dynamics, and resulting environmental and stratigraphic imprints, across the land-sea interface of the continental and insular margin.

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 (Figure 4) 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 fides and the associated currents they generate. Also important 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 seabed 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 seafloor 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



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