Elemental distribution oceanic profiles

In all three decay series, isotopes of relatively soluble elements like U, Ra, and Rn, decay to isotopes of highly particle-reactive elements (Th, Pa, Po, Pb), and vice versa (Figure 1), resulting in widely different distributions in the water column (Table 2) see Uranium-Thorium Series Isotopes in Ocean Profiles). 

Over the past 60 years refined techniques of analysis have shown the patterns of distribution of the trace elements in ocean water profiles in all the oceans. These results are summarized by the late Yoshiyuki Nozaki in this volume. Similarly the understanding of the stable isotopes of oxygen, carbon and nitrogen in the oceans have played important roles in deciphering both the ancient temperature history of the oceans and the biological pathways of nutrient elements in the marine system. 

The influence of river water inputs on trace metal distributions is illustrated in Figure 11.17c, which shows that the surface-water concentration of dissolved Mn in the Pacific Ocean decreases with increasing distance from the California coast. The vertical profile measured in the coastal zone (Figme 11.17b) exhibits a strong surface enrichment characteristic of scavenged trace elements. A similar vertical gradient is seen in the... 

Group 4 elemental profiles in the ocean are shown in Fig. 12.6. Titanium (Ti), zirconium (Zr) and hafnium (Hf) exhibit substantial covariance in their distributions, and the correspondence between Zr and Hf is particularly close. In the Atlantic Ocean, Zr/Hf concentration ratios (Godfrey et at., 1996) generally range between 170 and 240. 

Figure 12.6 Vertical distributions of Group 4 elements in the North Pacific. Data sources Ti (Orians et at., 1990), Zr (McKelvey and Orians, 1993) and Hf (Godfrey et at., 1996). The Hf distribution (dotted line) was calculated based on the average Atlantic Ocean Zr/Hf ratio of Godfrey et at. (1996) and the Pacific Ocean Zr profile of McKelvey and Orians (1993).

Within the ocean, the exchange of material from the dissolved to the suspended particulate state influences the distribution of several elements. This scavenging process removes dissolved metals from solution and accelerates their deposition. The effectiveness of this process is obvious in the depth profiles of metals, especially those of the surface enrichment type. Furthermore, the removal can be expressed in terms of a deepwater scavenging residence time as indicated in Table 10. 

The elements described here as refractory are not very soluble in water. They have low concentrations in sea water relative to their abundance in the Earth s crust, and short oceanic residence times. They can have extremely large concentration ranges in the oceans. The processes controlling the marine biogeochemistry of these elements are complex, and their distribution types vary considerably. The classic scavenged profile showing a surface maximum is... 

Although such detailed information has been obtained for a few transition metals and heavy metals, initial measurements of the oceanic concentrations and distributions need to be made for elements such as Ti, Ga, Ru, Pd, Ir, Pt, Au, Re Te, Zr, and FIf in many ocean basins before simple vertical and horizontal profiles can be constructed. Using newly developed analytical techniques, researchers have begun to obtain initial data on these metals. For example, the first concentration data on iridium in sea water (North Pacific) have been reported. Iridium concentrations ranged from 0.5 x 10 moll in North Pacific surface waters and increased with depth to a maximum of 0.8 x 10 molD near the bottom. 

The ocean receives Cd mobilized from the crust through riverine and atmospheric input. These fluxes are poorly constrained at present but given an ocean Cd inventory of 10 ° g, the residence time of Cd is similar to biologically utilized elements and approaches lO years [70]. The predominant form of Cd in the ocean is in the dissolved phase with concentrations ranging from 1 to 1000 pmol kg [76-78]. The vertical distribution of Cd in the oceanic water column resembles profiles of phytoplankton nutrients, with minimum concentrations at the surface that increase to maximum values in the main thermocline and remain relatively constant from there to the ocean bottom (Figure 2) [76-78]. Particulate Cd concentrations are significantly lower and fall between 0.04 and 4 pmol kg and are, conversely, maximal in surface waters [79]. This distribution reflects the uptake of Cd by photosynthetic plankton at the surface and the sinking and subsequent decomposition of particulate matter in the water colunrn. 

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