River Input

Evidence from 6 C studies (Sackett, 1964 Nissenbaum and Kaplan, 1972) and studies of the offshore decrease of terrestrially derived lignin material (Gardner and Menzel, 1974 Hedges and Parker, 1976) have led investigators to conclude that only a minor fraction of terrestrial humic substances are transported beyond the estuaries. The approach these investigators took was to compare sediment ratios with lignin oxidation 

There are other potential sources of organic matter from the coastal zone to the open ocean. However, we know little about the production of organic carbon from these sources. Coastal macrophyte production such as Spartina 

Interaction of water masses with dissolved and particulate organic species 

Menzel (1974) has reviewed attempts made to relate the concentration of DOC to the movement of water masses. He feels that seasonal or geographical differences in DOC concentrations have failed to correlate with clearly identifiable water mass structure or movement. Transects in the southeast and southwest Atlantic (Menzel and Ryther, 1970) did not show correlations of DOC with the northerly transport of water originating at the subtropical 

It will remain difficult to determine the relative importance of processes controlling the distribution of bulk organic matter in seawater until (1) the disagreement over whether variations of POC and DOC at depth are real or due to sample and analytical variability is resolved, (2) the methods for these analyses become more precise and sensitive, and (3) we can better assess what the terms POC and DOC really mean (Sharp, 1973,1975). Thus, we will turn our attention to individual classes of organic compounds which are more easily definable in terms of their molecular structure. 

---

Now that we have reviewed some basic aspects of the chemical composition of the ocean we can turn to a more fundamental question. What processes determine the composition of the ocean Current evidence suggests that rivers are the most important contributors of dissolved substances to the ocean. Since there is geologic evidence that the concentration and composition of the ocean has been relatively constant over the last 1.5 billion years, we must conclude that the river input must be balanced by removal. 

MacKenzie and Carrels (1966) approached this problem by constructing a model based on a river balance. They first calculated the mass of ions added to the ocean by rivers over 10 years. This time period was chosen because geologic evidence suggests that the chemical composition of seawater has remained constant over that period. They assumed that the river input is balanced only by sediment removal. The results of this balance are shown in Table 10-13. 

Let us define a two-box model for a steady-stafe ocean as shown in Fig. 10-22. The two well mixed reservoirs correspond to the surface ocean and deep oceans. We assume that rivers are the only source and sediments are the only sink. Elements are also removed from the surface box by biogenic particles (B). We also assume there is mixing between the two boxes that can be expressed as a velocity Vmix = 2 m/yr and that rivers input water to the surface box at a rate of Vnv = 0.1 m/yr. The resulting ratio of F mix/V riv is 20. 

There is some debate about what controls the magnesium concentration in seawater. The main input is rivers. The main removal is by hydrothermal processes (the concentration of Mg in hot vent solutions is essentially zero). First, calculate the residence time of water in the ocean due to (1) river input and (2) hydro-thermal circulation. Second, calculate the residence time of magnesium in seawater with respect to these two processes. Third, draw a sketch to show this box model calculation schematically. You can assume that uncertainties in river input and hydrothermal circulation are 5% and 10%, respectively. What does this tell you about controls on the magnesium concentration Do these calculations support the input/removal balance proposed above Do any questions come to mind Volume of ocean = 1.4 x 10 L River input = 3.2 x lO L/yr Hydrothermal circulation = 1.0 x 10 L/yr Mg concentration in river water = 1.7 X 10 M Mg concentration in seawater = 0.053 M. 

Secondly, these quotations emphasize the fact that the same river input that fuels longitudinal heterogeneity in reservoirs also forms a strong link between the reservoir and its watershed (e.g., [6]). This link has been conceptualized mostly in the form of load-response empirical models [7, 8], or mass-balance approaches [9]. Curiously, empirical modelers usually consider reservoirs as stirred reactors, ignoring the longitudinal spatial heterogeneity present in most situations and processes. 

River inputs. The riverine endmember is most often highly variable. Fluctuations of the chemical signature of river water discharging into an estuary are clearly critical to determine the effects of estuarine mixing. The characteristics of U- and Th-series nuclides in rivers are reviewed most recently by Chabaux et al. (2003). Important factors include the major element composition, the characteristics and concentrations of particular constituents that can complex or adsorb U- and Th-series nuclides, such as organic ligands, particles or colloids. River flow rates clearly will also have an effect on the rates and patterns of mixing in the estuary (Ponter et al. 1990 Shiller and Boyle 1991). 

Gomez-Gutierrez AI, Jover E, Bodineau L, Albaiges J, Bayona JM (2006) Organic contaminant loads into the Western Mediterranean Sea estimate of Ebro river inputs. Chemosphere 65 (2) 224-236... 

Broecker and Takahashi, 1978). zs is the depth in km and carbonate concentrations are in mol kg" . Since seawater Ca2+ concentration is allowed to change as a result of calcite precipitation and river input, it is more general to state that the saturation product Ks changes as... 

Meybeck, M. (1979b), "Pathways of Major Elements from Land to Ocean through Rivers", in Review and Workshop on River Inputs to Ocean Systems, FAO, Rome, 26-10. 

Using the rock cycle as an example, we can compute the turnover time of marine sediments with respect to river input of solid particles from (1) the mass of solids in the marine sediment reservoir (1.0 x 10 g) and (2) the annual rate of river input of particles (1.4 X lO g/y). This yields a turnover time of (1.0 x 10 " g)/(14 x lO g/y) = 71 X lo y. On a global basis, riverine input is the major source of solids buried in marine sediments lesser inputs are contributed by atmospheric feUout, glacial ice debris, hydrothermal processes, and in situ production, primarily by marine plankton. As shown in Figure 1.2, sediments are removed from the ocean by deep burial into the seafloor. The resulting sedimentary rock is either uplifted onto land or subducted into the mantle so the ocean basins never fill up with sediment. As discussed in Chapter 21, if all of the fractional residence times of a substance are known, the sum of their reciprocals provides an estimate of the residence time (Equation 21.17). 

The ion proportions in most river water is significantly different from that in seawater. As a result, river runoff can have a local impact on the ion ratios of coastal waters. This effect is most pronounced in marginal seas and estuaries where mixing with the open ocean is restricted and river input is relatively large. The variable composition of river water and its impact on the chemical composition of seawater are discussed further in Chapter 21. 

In this model, the rate of river runoff (uriver) expressed as the depth of a layer of water produced by spreading the annual river-water input across the entire surfece area of the ocean. The annual amount of river water entering the ocean is 47,000 km /y (Figure 2.1). Assuming that the average area of the ocean is equal to that at the sea surfece (3.6 x 10 cm ), the river input represents the annual addition of a layer of water approximately 10 cm deep, making y ver = lOcm/y. 

The overall oceanic recycling efficiency of a biolimiting element is given by the fraction of the river input that is buried in the sediments during one complete mixing cycle. This is calculated as... 

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. 

As shown in Figure 14.8, kaolinite concentrations are highest in tropical and equatorial latitudes, particiflarly off the western coasts of North Africa and Australia (>40%) and the northeastern coasts of Australia and South America (30%). The first two are the result of aeolian transport by the Trade Winds from the Saharan and Australian deserts, respectively. The other two are the result of river input from the eastern Australian continent and the Amazon River. 

Sabkhat also form inland, where river input and saline groundwater seeps contribute salt and water, forming an evaporitic pan. As illustrated in Figure 17.6, these continental sabkhat are fer more isolated from the ocean than a marine sabkha. They also contain far less biogenic detritus. 

The chemical reactions that occur in hydrothermal systems are largely the result of interactions between seawater and relatively yoimg ocean crust. During these reactions, some elements are solubilized and released to seawater as ions or gases. Others are precipitated, forming minerals that end up as a component of new oceanic crust or the metalliferous sediments. For some elements, the resulting elemental fluxes rival those associated with river input, making hydrothermal activity a very important process in the crustal-ocean-atmosphere factory. 

The clay minerals carried by rivers into the ocean represent a net annual addition of 5.2 X 10 mEq of cation exchange capacity. Most of these exchange sites are occupied by calcivun. Within a few weeks to months following introduction into seawater, sodium, potassium, and magnesium displace most of the calcium. As shown in Table 21.7, this uptake removes a significant fraction of the river input of sodium, magnesium, and potassium. 

---