Uncertainties in the Current Global Fluvial Systems Model

As the main pathway for the ultimate preservation of terrigenous production in modern environments, the transfer of organic matter from the land to the oceans via fluvial systems is a key link in the global carbon cycle (Ittekot and Haake 1990; Degens et al. 1991; Hedges et al 1992). Hence, the "role" of rivers in the global carbon cycle is most typically expressed as the fluvial export of total organic and dissolved inorganic carbon from land to the ocean (e.g., Likens et al. 1981). As will be discussed in more detail later in this chapter, the most common literature estimations of the magnitude of these fluxes are 0.4 petagrams of carbon per year (PgC y-1) for total organic carbon (evenly divided between particulate and dissolved organic phases), and 0.4 PgC y-1 for dissolved inorganic carbon. While these bulk fluxes are small components of the global C cycle, they are significant compared to the net oceanic uptake of anthropogenic CO2 (Sarmiento and Sundquist 1992) and to the interhemispheric transport of carbon in the oceans (Aumont et al. 2001).

This chapter examines and expands on several inconsistencies in this conventional model. The first is in the estimates of the flux quantities and in the relative influence of natural and anthropogenic processes in determining these fluxes. While much river research has emphasized concentrations of carbon, sediments, and/or nutrients, with a focus on export to the oceans (Meybeck 1982, 1991; Degens et al 1991; Milliman and Syvitski 1992), considerable uncertainties remain. The second problem is that the role of fluvial systems may not be limited to fluvial exports to the coastal zone. Continental sedimentation may sequester large amounts of carbon in lower depressions and wetlands (Stallard 1998; Smith et al. 2001). More recent estimates indicate that CO2 out-gassing to the atmosphere from river systems may be an important pathway (Cole and Caraco 2001; Richey et al. 2002).

Atmosphere

Soils f

Coastal Zone

Riparian

Floodplains

Figure 17.1. Major reservoirs and pathways of atmospheric CO2 in fluvial systems. Atmospheric CO2 fixed into streams and their riparian (near-stream) zones (as dissolved CO2 or organic matter) is stored as alluvium, released back to the atmosphere, or transported down stream networks to larger river systems. This now-riverine carbon exchanges with floodplains, is outgassed, or is retained behind dams. Finally the "remaining" fluvial carbon is exported to the coastal zone.

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