Oceanic Inputs

Referring again to Fig. 13-6a, the materials that constitute the oceanic sinks for sulfur are recycled over time by exposure to weathering on the continents. The rates at which these processes occur help to regulate the flux of sulfur into rivers. 

The evaporite source is characterized by covariation of sulfate (from gypsum) and chloride (from halite). That elements can be recycled from the ocean to land by movement of salt-bearing aerosols (so-called cyclic salts ) has confused the interpretation of river flux data somewhat. While this cycling generally follows the ratio of salts in the sea, the S Cl ratio is an exception. Taking the S Cl ratio of the cyclic component to be 2 (based on compositional data for marine rains) and assuming that all chloride in rivers is cyclic, an upper limit for the cyclic influence can be calculated  

However, not all the chloride is cyclic, a fact first appreciated in recent years. An example comes from a detailed study of river geochemistry conducted in the Amazon Basin. In the inland regions, rains typically have a chloride content of 10 juu, whereas major inland tributaries have chloride contents of 20-100 MM. These data suggest that only 25% of the Cl is cyclic, whereas 75% is derived by weathering of evaporites. Indeed, 90% of this 75% can be shown to have its origin in the Andean headwaters, derived 

The other principal source is weathering of sedimentary or igneous sulfides, mainly pyrite, by the oxidation  

What are the relative contributions of these two sources Two approaches have been taken. One is to establish the geology and hydrology of a basin in great detail. This has been carried out for the Amazon (StaUard and Edmond, 1981) with the result that evaporites contribute about twice as much sulfate as sulfide oxidation. The other approach is to apply sulfur isotope geochemistry. As mentioned earlier, there are two relatively abundant stable isotopes of S, and 5. The mean 34/32 ratio is 0.0442. However, different source rocks have different ratios, which arise from slight differences in the reactivities of the isotopes. These deviations are expressed as a difference from a standard in the case of sulfur, the standard being a meteorite found at Canyon Diablo, Arizona. 

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Figure 3 Schematic cartoon depicting oceanic inputs and cycles of select U-Th series isotopes...

A box model fiar the marine silica cycle is presented in Figure 6.11 with respect to the processes that control DSi and BSi. An oceanic budget is provided in Table 16.3 in which site-specific contributions to oceanic outputs are given. This table illustrates that considerable uncertainty still exists in estimating the burial rate of BSi. Regardless, burial of BSi is responsible for most of the removal of the oceanic inputs of DSi, with the latter being predominantly delivered via river runoff. This demonstrates the importance of the biological silica pump in the crustal-ocean-atmosphere factory. 

Ludwig, W., J.-L. Probst, and S. Kempe. 1997. Predicting the oceanic input of organic carbon by continental erosion. Global Biogeochemical Cycles 10 23-41. 

Through the influence of speciation on oceanic input and removal processes it is expected that chemical form should strongly influence not only overall chemical concentrations in the ocean but also chemical distributions. In view of this expectation, assessments of speciation and comparative chemistries in this chapter are made in the context of vertical distributions (concentrations vs depth) of chemical species in the ocean. Since elemental distributions are influenced not... 

Figure 5 A cross-plot of Os/ Os versus Sr/ Sr illustrating the isotopic ratios associated with various oceanic inputs. Solid black arrows schematically illustrate the temporal evolution of seawater during the Cenozoic. Data sources (see Table 1 also) are (-) Cenozoic seawater (o) Loess deposits, Peucker-Ehrenbrink and Jahn (2001) (-I-) Indus paleosols, Chesley et al. (2000) and (x) Ganges paleosols, Chesley et al. (2000).

The input of Sr required to account for the observed enlrancement in Sr/ Sr ratio across the KTB can be determined by mass balance of the isotopes of Sr. Assuming the mid-ocean ridge (MOR) flux and isotopic ratio of Sr were unchanged, the total oceanic input of Sr at tlie KTB is described by the equation... 

Estimates of potential trace metal oceanic input based on atmospheric washout, mining productivity figures, and relative river transport are given in Tables 1 and 2. 

In this budget, both estimates for dissolved inorganic nitrogen (DIN) and silicon fluxes are included. The silicon budget is included as a control. The processes involved in the silicon cycle are fewer than that of nitrogen, and so it might be expected that the budget can be closed more easily. The silicon cycle comprises essentially only river and ocean inputs, removal of dissolved silicon... 

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