Additional Issues for Workshop Discussion

Which RiOMar Environments Should be Examined?

The world's ten largest rivers transport approximately 40% of the fresh water and particulate materials entering the ocean (Milliman, 1991). Smaller rivers may quantitatively be very important to global cycles of bioactive elements, yet function very differently in terms of storage in drainage basins and retention / transformations of these elements at the ocean margin interface.

For example, much of our earlier understanding of particulate material transport by rivers was derived from studies of small river systems on passive margins, and many of the relationships and processes from these studies do not apply in larger river systems (Milliman and Meade, 1983). On the other hand, insight from studies of large river systems (typically on passive margins) appear to be inadequate for understanding small coastal mountain rivers (active margins) that may be very important for particulate material input to the oceans.

Rivers also act as natural integrators of surficial processes, including human activities, within their drainage basins. Due to large-scale basin and discharge processes, the material composition of major rivers may also differ from smaller rivers. In addition to differences in terrestrial inputs, large rivers differ from smaller rivers in the way that materials are processed at the land-sea interface. Because of high rates of particulate and water discharge, the estuarine processes associated with major rivers usually take place on the adjacent continental shelf instead of in a physically confined estuary. This may greatly influence the magnitude and selectivity of processes that transform, retain or export terrestrial materials.

For bioactive elements, how do basic cycling processes (and resulting global impact) differ among RiOMar environments as a function of river size (drainage basin area and coastal zone of impact)? -factors such as latitude, lithology, active vs. passive margins, as well as size should be considered.

Approaches for RiOMar Research

At least two possible approaches are possible to address global carbon cycle issues in RiOMar environments. These are: (1) an intensive approach focusing on one river system and (2) an extensive approach that targets representative RiOMar environments globally. The relative merits of both approaches will be discussed at the workshop.

(A) Intensive Approach

As recommended in the CCSP, an initial phase should focus on a continental/regional scale---North America. The Northern Hemisphere (and the United States, specifically) has been implicated as a significant site for terrestrial CO2 uptake and storage (Schimel et al., 2000 and references within). The Mississippi River and its RiOMar environment is one logical candidate for an intensive study examining the connections between the terrestrial and oceanic carbon sinks of North America. The Mississippi River is one of the world's 10 largest rivers (7th in water and sediment discharge and 2nd in drainage basin size). Its drainage basin covers approximately 40% of the conterminous U.S. and parts of Canada. Over 65% of the total suspended matter and dissolved solids transported from the conterminous U.S. to the ocean is carried by the Mississippi. In addition, other rivers in North America (e.g., mountainous coastal rivers), and their associated RiOMar environments should be considered for study. Research on a spectrum of river types will be needed so that insights from specific research sites can be scaled appropriately when extrapolating to a global scale.

(B) Extensive Approach

This approach would examine key carbon cycle processes in a few selected global RiOMar environments. Specific rivers will be identified that cover the spectrum of characteristics (latitude, lithology, tectonics, drainage and receiving basin size) appropriate to answer the outstanding research questions. The selection / prioritization of global rivers will be discussed during the workshop.

A combination of these two approaches promises to yield new and significant insights for an important area of global carbon cycle research. We will discuss the optimal combination of approaches.