Ocean riverine transport

More CO2 can actually be absorbed chemically into the ocean than the above reaction sequence suggests. Terrestrial weathering of rocks containing carbonate, such as limestone, and subsequent aerial or riverine transport, means that the ocean is enriched in carbonate. Keeping and Kj constant implies, through eqns. (3) and (4), that enhancing the oceanic [COj ] leads to a greater level of... 

In general, silver concentrations in surface waters of the United States decreased between 1970-74 and 1975-79, although concentrations increased in the north Atlantic, Southeast, and lower Mississippi basins (USPHS 1990). About 30 to 70% of the silver in surface waters may be ascribed to suspended particles (Smith and Carson 1977), depending on water hardness or salinity. For example, sediments added to solutions containing 2 pg Ag/L had 74.9 mg Ag/kg DW sediment after 24 h in freshwater, 14.2 mg/kg DW at 1.5% salinity and 6.9 mg/kg DW at 2.3% salinity (Sanders and Abbe 1987). Riverine transport of silver to the ocean is considerable suspended materials in the Susquehanna River, Pennsylvania — that contained as much as 25 mg silver/kg — resulted in an estimated transport of 4.5 metric tons of silver to the ocean each year (USEPA 1980). The most recent measurements of silver in rivers, lakes, and estuaries using clean techniques show levels of about 0.01 pg/L for pristine, nonpolluted areas and 0.01 to 0.1 pg/L in urban and industrialized areas (Ratte 1999). 

Note that plots, such as Figure 5.1, provide information only on the net outcome of chemical reactions. In the case of iron, a small addition does take place in estuaries as a result of desorption of Fe from the surfaces of riverine particles. As these solids move through the estuarine salinity gradient, the major cation concentrations increase and effectively displace the iron ions from the particle surfeces. Since this release of iron is much smaller than the removal processes, the net effect is a chemical removal of iron. Sedimentation of these iron-enriched particles serves to trap within estuaries most of the riverine transport of reactive iron, thereby preventing its entry into the oceans. 

Some of the clays that enter the ocean are transported by river input, but the vast majority of the riverine particles are too large to travel fer and, hence, settle to the seafloor close to their point of entry on the continental margins. The most abundant clay minerals are illite, kaolinite, montmorillonite, and chlorite. Their formation, geographic source distribution and fete in the oceans is the subject of Chapter 14. In general, these minerals tend to undergo little alteration until they are deeply buried in the sediments and subject to metagenesis. 

At 20 °C, K = 10 - and so water of pH=8.1 in equilibrium with atmospheric O2 (p02 — 0.21 atm) has pe = 12.5. This conforms to surface conditions, but the pe decreases as the O2 content diminishes with depth. The oxygen minimum is particularly well developed beneath the highly productive surface waters of the eastern tropical Pacific Ocean, where there is a large flux of organic material to depth and subsequently considerable oxidation. The O2 becomes sufficiently depleted i.e., hypoxia) that the resulting low redox conditions causes NOs to be reduced to N02 - Aeolian transport of nitrate to Chesapeake Bay can lead to low O2 conditions. Similarly, intermittent hypoxia develops in parts of the Gulf of Mexico due to the riverine transport of nutrients derived from agricultural uses in the Mississippi catchment. 

River transport of clay minerals into the ocean is spatially and temporally variable. The global annual suspended load of river sediment into coastal waters currently averages 12.6 X 10 ton. This flux is approximately 10% less than was delivered before humans began damming rivers. (One notable exception is the Mississippi River, whose sediment load has increased due to very high rates of soil erosion. The riverine sediments deposited in the mouth of the Mississippi River form one of the world s largest deltas.)... 

The abyssal clays are composed primarily of clay-sized clay minerals, quartz, and feldspar transported to the siuface ocean by aeolian transport. Since the winds that pick up these terrigenous particles travel in latitudinal bands (i.e., the Trades, Westerlies, and Polar Easterlies), the clays can be transported out over the ocean. When the winds weaken, the particles fell to the sea siufece and eventually settle to the seafloor. Since the particles are small, they can take thousands of years to reach the seafloor. A minor fraction of the abyssal clays are of riverine origin, carried seaward by geostrophic currents. Despite slow sedimentation rates (millimeters per thousand years), clay minerals, feldspar, and quartz are the dominant particles composing the surface sediments of the abyssal plains that lie below the CCD. Since a sediment must contain at least 70% by mass lithogenous particles to be classified as an abyssal clay, lithogenous particles can still be the major particle type in a biogenous ooze. 

River runoff The transport of water, solutes, and hthogenous particles from the continents to the ocean as a result of riverine input. 

To evaluate the net riverine influx of dissolved species to the ocean, the river load has to be corrected for sea salts transported via the atmosphere from the ocean to the continents and rained out mainly in coastal precipitation. Table 9.7 shows the average concentration of selected dissolved and particulate elements in rivers from Martin and Meybeck (1979), and the corresponding net fluxes corrected for sea-salt cycling from Martin and Meybeck (1979). The corrections of fluxes for cyclic salts and pollution are still debatable estimates (e.g., Holland, 1978 Maynard, 1981), and affect mainly the evaluation of the net flux of Na+ by perhaps as much as 20%. 

Figure 9.20. Schematic diagram of fluxes and processes evaluated for the global cycle of an element. Rj, Rp, Sp, Dp, H

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