Overall Fluvial System Atmosphere Exchange Alternative Scenarios

Considerable uncertainty remains in the assessment of the carbon cycle of fluvial systems, including the magnitude of fluxes, how to include processes not previously considered, and delineation of anthropogenic and natural processes, all with an explicit recognition of geography. To assess the implications of each, the earlier discussions are summarized here via five "scenarios" (Figure 17.2).

The bulk transfer of atmospheric C through the land to fluvial systems (assuming a steady-state summation of downstream processes, in a non-steady-state environment) ranges from about 0.6 PgC y-1 (conventional wisdom) to 2.6 PgC y-1 (+ outgassing). Continental sedimentation results in a significant sink, but that sink is reduced with CO2 outgassing (because of the way the sedimentation was computed). The inclusion of continental sedimentation, and then the larger export of OM to the sea (about twice conventional assumptions, under + POC, DOC), yields net sinks of atmospheric CO2 of up to 1.6 PgC y-1. If outgassing is included, however, then the fluvial net sink is reduced to 0.2 PgC y-1. Although partitioning the total fluvial fluxes into natural conditions and anthropogenic transients is problematic at best, substantial evidence suggests that the mobilization of sediments has dramatically increased. While much of this material is captured in reservoirs, it is reasonable to expect that a considerable amount escapes to the sea (especially in non-deltaic regions with steep slopes and few dams).

Summarizing all the components of the riverine carbon cycle, several images emerge. As a global steady-state aggregate, there appears to be a sink (between continental sedimentation and marine sedimentation and dissolution) on the order of 1 — 1.5 PgC y-1,

River _

Sea

Cont Sed

Outgas-►

Atm

DIC -►

Sea

c

DOC -►

Sea

c

POC —►

Sea

c

A Net Atm

+ Cont Sed + POC, DOC + Outgassing Transient

c nz

0.211

Figure 17.2. Scenarios of the fluxes of carbon through fluvial systems relative to atmospheric CO2; including atmosphere to rivers (River ^ Atm, as sum of other fluxes), continental sedimentation (Cont Sed, alluvial, reservoirs), outgassing from rivers to atmosphere (Outgas ^ Atm), export of DIC to the sea (DIC ^ sea, as 50 percent of total DIC, to represent just atmospheric weathering component), DOC export to the sea (DOC ^ sea), export of POC to the sea (POC ^ sea), and by difference the net exchange with the atmosphere (^ Net Atm). For the purposes here, it is assumed that the export fluxes to the sea constitute a sink, with no return to the atmosphere, on an immediate decadal time-scale. Particulate inorganic C is not included (as it is abiotic and does not interact with the other pools). Units are PgC y-1 (scale at bottom lines up with the axis of each graph at 0). Scenarios are CW (conventional wisdom on DOC and POC export, no continental sedimentation or outgassing), + Cont Sed (adding continental sedimentation to CW), + POC, DOC (adding the higher values for POC and DOC fluxes discussed in the text to + Cont Sed), + Outgassing (adding outgassing to + POC, DOC), and Transient (inferring anthropogenic transients, based on the + Outgassing scenario).

with a significant anthropogenically enhanced component. A return flux to the atmosphere, on the order of 1 PgC y-1, reduces the net sink to about 0.2 or 0.3 PgC y-1. There are certainly disjunctures in space and time in this view, however. Because the organic matter in transport appears to be "old," the subsequent mobilization and oxidation/ outgassing would essentially be mining old C. There are significant regional implications in this analysis. With its preponderance of land mass, extensive reservoirs, and agriculture, the bulk of continental sedimentation (and its implications for C sink), is focused in the Northern Hemisphere, between 30° and 50°N. C sequestration in paddy lands would be closer to the equator. The greatest amount of sediment flux to the ocean (and the greatest uncertainty) is in South and Southeast Asia and Oceania. Outgassing is function of both pCO2 concentrations (driven by in situ oxidation) and surface area of water. It is likely most significant in the humid tropics, particularly during the peak of the wet seasons. The highly canalized temperate areas have less area available. The northern latitudes, particularly with warming, are liable to have significant fluxes.

Overall, more carbon is moving through the river system than previously assumed; however, the exact magnitudes remain uncertain. The degree to which fluvial systems constitute a net source/sink relative to the atmosphere is essentially governed by the interplay between mobilization of materials off the landscape and oxidation of those materials back to the atmosphere. Additional data will be necessary to constrain these magnitudes, and it is necessary to partition the model by geographic zone.

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