River water carbon flux

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

Hope, D., M. F. Billett, and M. S. Cresser. 1994. A review of the export of carbon in river water Fluxes and processes. Environmental Pollution 84 301—324. 

Avery G.B. Kieber R.J. Willey J.D. Shank G.C. and Whitehead R.F. (2004). Impact of hurricanes on the flux of rainwater and Cape Fear River water dissolved organic carbon to Long Bay, southeastern United States. Global Biogeochemical Cycles, 18(GB3085), doi 10.1029/2004GB002229. 

In Figure 9.21 all of the carbon eventually used in weathering of minerals by CC>2-charged soil water is shown as entering the atmosphere. The difference between the flux of CO2 owing to precipitation of carbonate minerals in the ocean and the total CO2 released from the ocean is that CO2 used to weather silicate minerals on land, and agrees with the calculations of riverine source materials made earlier in this chapter, in which it was shown that 30% of the HCC>3 in river water comes from weathering of silicate minerals. 

Sea water contains a much lower concentration of dissolved organic matter than river water. More than half of this dissolved organic load is of a humic nature. These dissolved organic acids tend to flocculate as the salinity increases (10). Hair and Bassett (11) have observed an increase in the particulate humic acid load of an estuary as one approaches the sea. Although no studies of the distribution of humic materials throughout an estuarine system have been performed, it would appear that estuaries and their sediments in particular, act as a major sink for the dissolved and particulate humic materials. Nissenbaum and Kaplan (12) have observed that terrestrial humic materials are not deposited at great distances from shore in the marine system. A study of the flux of particulate carbon through the Chesapeake Bay comes to a similar conclusion (13). 

Most natural water systems in contact with calcite (oceans, rivers, lakes, carbonate rock aquifiers) are, however, near equilibrium, and PCO2 dependence cannot be ignored. According to our model, the rate of backward reaction is a significant function of surface pH, and surface pH is determined by calcite equilibrium at the surface PCO2. At the relatively high pH, low PCO2 conditions of most natural waters, the surface pH is least well defined and may depend, in part, on the flux of CO2 between the surface and bulk fluid. 

In addition, the present-day burial rate of organic carbon in the ocean may be about double that of the late Holocene flux, supported by increased fluxes of organic carbon to the ocean via rivers and ground-water flows and increased in situ new primary production supported by increased inputs of inorganic N and P from land and of N deposited from the atmosphere. The organic carbon flux into sediments may constitute a sink of anthropogenic CO2 and a minor negative feedback on accumulation of CO2 in the atmosphere. 

If 0.24 Pg C/a represents riverine DIC delivered to oceans (Meybeck 1993) and if the flux of carbon from rivers/lakes to the atmosphere is 20% (Kling et al. 1991) of the total (i.e., 0.12 Pg C/a), then 0.23 Pg C/a remains in inland lakes and rivers, and in slowly cycled groundwater. Cole et al. (2007) estimated that about 0.2 Pg C/a is buried in inland water sediments. Groundwater may have a greater carbon storage capacity due to its large volume and greater load of carbon than rivers (Kempe 1984). 

Raymond, P.A. Oh, N.H. et al. 2008. Anthropogenically enhanced fluxes of water and carbon from the Mississippi River. Nature, 451, 449-452. 

The rivers play a major role in the transfert of carbon and mineral nutrients from land to the sea and influence significantly the biogeochemical processes operating in coastal waters. Quantification of the material transport, both in the dissolved and particulate forms, has been attempted by several authors in the past (Clarke, 1924 Holeman, 1968 Garrels McKenzie, 1971 Martin et al., 1980 Meybeck, 1982 Milliman Meade, 1983). Depending on the type of sampling techniques and methods of calculations employed there are differences in the reported fluxes. A major problem in such calculations is the paucity of reliable data from some of the major rivers of the world especially of Asia (see e.g. Milliman Meade, 1983). Additionally the difficulty of obtaining representative samples from the rivers will adversely affect flux calculations. Most of the inferences drawn on the nature and transport of riverine materials rest on data collected randomly - at different points in time and space. Seasonal variations in the transport of materials are very common in some of the major world rivers, and in some cases more than 60 % of the material transport occurs within a very short period of time. Furthermore, available data are not always comparable since the analytical techniques used differ from river to river. 

Data collected over the first two years of our study on DOC and POC were used to calculate annual fluxes of carbon from land to sea. The data are presented not for the individual rivers, but for rivers of a particular continent (Fig. 5). The measurement made in the major rivers of a continent were extrapolated to include the total water and sediment discharge from that particular continent. In terms of quantity maximum POC is being transported by the rivers of Asia, followed by those draining North America, Oceania, Africa and Europe, in that order. South American rivers carry maximum amounts of DOC, followed by the rivers of Asia, North America, the Arctic, Africa, Europe and Oceania. 

Inorganic carbon sedimenting and accumulating on the sea floor is mainly as CaCC>3 little dolomite is forming today, and the average Mg content of modem calcareous sediments is about 0.7%, representing a flux of MgC03 to the sea floor of 3 x 1011 moles yl. The Ca2+ for precipitation comes both from rivers and reactions at the sea floor between basalts and sea water, discussed later in this chapter. The overall reaction is ... 

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