Within-river transport processes carry these eroded materials downstream through the river network. Transport is not passive; significant transformations occur along the way. Rivers exchange with their floodplains (depending on how canalized and diked a river is). The movement of POC is, of course, directly linked to the movement of suspended sediments. Sediments are deposited and remobilized multiple times and over long timescales. In the Amazon, for example, Dunne et al. (1998) computed that as much sediment was being recycled within a reach as was leaving it. Presumably, a significant amount of the erosion-excess sediment discussed in the previous section makes it some distance downstream but is then slowed and retained within the alluvial flood-plains.
An additional process within flowing water significantly affects organic matter (OM)—the mineralization to pCO2. Most river and floodplain environments maintain pCO2 levels that are supersaturated with respect to the atmosphere. High partial pressures of CO2 translate to large gas evasion fluxes from water to atmosphere. Early measurements in the Amazon suggested that global CO2 efflux (fluvial export plus respiration) from the world's rivers could be on the order of 1.0 PgC y-1. Recent measurements of temperate rivers lead to estimates of global river-to-atmosphere (out-gassing) fluxes of ~0.3 PgC y-1, which is nearly equivalent to riverine total organic carbon (TOC) or dissolved inorganic carbon (DIC) export (Cole and Caraco 2001). Richey et al (2002) computed that outgassing from the Amazon alone was about 0.5 PgC y-1. Assuming that the fluxes computed for the Amazon are representative of fluvial environments of lowland humid tropical forests in general, surface water CO2 evasion in the tropics would be on the order of roughly 0.9 PgC y-1 (three times larger than previous estimates of global evasion). Factoring in the recent Amazon results, a global flux of at least 1 PgC y-1 directly from river systems to the atmosphere is likely.
What is the source of the organic matter being respired? Is it labile contemporary organic matter, recently fixed in the water by plankton or nearshore vegetation, or is it some fraction of the allochthonous (terrestrial) matter in transport? The prevailing wisdom is that riverborne organic matter is already very refractory and not subject to oxidation (after centuries on land). The "age" of riverine organic matter yields some important insights.
Measurements of the 14C ages of organic matter and CO2 in river water are very few, but the results are intriguing (Hedges et al 1986). Cole and Caraco (2001) found that the POC (and to a lesser degree dissolved organic carbon [DOC]) entering the Hudson is greatly depleted in suggesting that the particulate material was originally formed on average ~5,000 years ago. Their analysis of 813C suggests that this material is of terrestrial origin and unlikely to be ancient marine sedimentary rocks. Furthermore, they found that organic matter (OM) pools became selectively enriched in 14C downstream. Based on an inverse modeling approach, they hypothesized that this enrichment is due to utilization of old organic carbon, with dilution by recent primary production. That is, organic C that had resided in soils for centuries to millennia, without decomposing, is then decomposed in a matter of a few weeks in the riverine environment (how this happens is an intriguing question in its own right). Their results are not unique to the Hudson. Raymond and Bauer (2001) found that four rivers draining into the Atlantic are sources of old (14C-depleted) and young (14C-enriched) terrestrial dissolved organic carbon and of predominantly old terrestrial particulate organic carbon. Much of the younger (relatively speaking) DOC can be selectively degraded over the res idence times of river and coastal waters, leaving an even older and more refractory component for oceanic export.
Thus, pre-aging and degradation may alter significantly the structure, distribution, and quantity of terrestrial organic matter before its delivery to the oceans. As noted by Ludwig (2001), the OM that runs from rivers into the sea is not necessarily identical to the OM upstream in river catchments. Cole and Caraco (2001) observe that the apparent high rate of decomposition of terrestrial organic matter in rivers may resolve the enigma of why OM that leaves the land does not accumulate in the ocean (sensu Hedges et al. 1997). Overall, this sequence of processes suggests that the OM that is being respired is translocated in space and time from its points of origin, such that, over long times and large spatial scales, the modern aquatic environment may be connected with terrestrial conditions of another time.
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