The properties of the surface and discharged water measured at the time of the dye experiment are the basis for the information on mixing given in Figure 3 which shows the degree of warming needed to bring different mixtures of discharged and surface water to the same density as the surface water. Sinking of a mixture containing 20 percent deep- and 80 percent surface-water would be limited by its density of 23.1 to the upper mixed layer (about 5 to 20 m at this season; below these depths the density gradient increases rapidly). 31-659 74 16 T/S diagram showing discharged water of T= 22°C; S= 34.923% (point B) and surface water of T=29°C; S-35.898% (point A). Percentages of discharged water are given along the bottom of the line of mixing joining points A and B. The italicized numbers above this line give the degree of warming re-g quired (°C) to bring the mixtures to the same density as the surface water (hatched curve). Numbers along the 29°C abscissa are the densities (0) of the various mixtures should they come into temperature equilibrium with the surface water without further mixing. Solid curves are lines of Continued mixing with surface water, spreading, heating by solar radiation and by conduction from the surrounding water, generation of convection cells within the discharged water mass, wind and wave action, are some factors complicating this simplistic model. There would be less heat conduction in a larger diameter suction-dredge pipe, resulting in a denser discharge which would be more likely to sink unless it were sprayed over the sea surface to ensure rapid mixing and warming. This technique could be used in other parts of the world's oceans where manganese nodules are found (Horn et al., 1972) and where the vertical temperature, salinity and density profiles differ considerably from those on the Blake Plateau. We concluded that under the experimental conditions of the Blake Plateau mining test, the discharged water will remain at or near the surface during August, when the sea-surface density is at its minimum, and will have an even greater tendency to do so in other seasons. Because the most likely areas of manganese nodule mining in the deep sea have oceanographic characteristics different from those prevailing in the region of the Blake Plateau where the pilot-scale experiment was conducted, we undertook model studies to predict the amount of warming that would be required in different seasons to bring bottom water to the same density as surface water for the North Atlantic. The same model can, of course, also be applied to other oceanic areas. To do this, we profiled all data filed at the National Oceanographic Data Center for North Atlantic hydrographic stations with water column data to within 300 m of the bottom. This pointed out the extreme lack of data in the Atlantic, from which only 1600 stations meeting our criterion (sampling within 300 m of the bottom) were available. FIGURE 5 The amount of warming (°C) that would be required in winter to bring North Atlantic bottom water to the same density as the surface water. We calculated the amount of warming that would be required in summer (Fig. 4) and in winter (Fig. 5) to bring such bottom water to the same density as the surface water. A large body of the ocean would have to be warmed by 20° or more, assuming no mixing with the surface and no heating in the airlift pumping process. Since bottom conditions do not vary much seasonally, the differences are due to seasonal variations in surface conditions. In some areas of the ocean, however, bottom water is more saline than surface water; therefore its temperature would have to be raised higher than that of the surface water to keep it there. Figures 6 (July-Aug-Sept) and 7 (Jan-Feb-Mar) chart the differences between bottom and surface waters in σ units. Where this difference is negative, the water would sink unless there was warming and mixing with surface water. In summer, large areas of very low salinity water, originating from the Amazon discharge, sometimes become isolated within the equatorial flow. These layers of low salinity water largely disappear in winter. FIGURE 6 water in σ units in the North Atlantic in sum mer. If mining operations were undertaken in areas of low surface salinity, particularly in summer, techniques such as spraying the effluent over the surface to ensure rapid mixing would have to be utilized. OXYGEN CONCENTRATION OF THE EFFLUENT DEEP WATER A nine-day experiment was done to establish the effects of finely-suspended organic material from the sediments on oxygen concentration in the discharged deep water. The results (Fig. 8 and Table 2) indicated that most of the available nutrients were utilized by phytoplankton growth in the light bottle, whereas these nutrients remained high in the dark bottle. Eighty percent of the available oxygen in the dark bottle was used up by the organic fraction from the water or its sediment during the first 5 days and the oxygen concentration remained unchanged thereafter. The low level of oxygen of the remaining water observed under these experimental conditions might never be reached in the open sea since mixing and gas exchange would counteract this. In contrast, in the light bottle, oxygen was depleted during the first 5 days and then regenerated to saturation by the eighth day as a result of photosynthesis by a green coccoid alga which developed rapidly from the sixth day TABLE 2. NUTRIENT CONCENTRA- Deposition of sediments is slow and the bottom environment is an oxidizing one in areas where manganese nodules are found; therefore, the ease and rapidity with which the organic material oxidized indicated that the rigorous airlift pumping of the sediment may have rendered the organic fraction more labile as a result of oxygen supersaturation and vigorous mixing. It is, therefore, very unlikely that anoxic conditions will develop under conditions like those observed on the Blake Plateau because the discharged water remained at the surface where it can support plankton growth, producing oxygen. Moreover, low levels of oxygen are unlikely to be reached in the open sea because mixing and free exchange of oxygen from the atmosphere would preclude the formation of an anoxic layer. DAY 8 OF THE OXYGEN Light Dark Nitrate (μg-at/1) 0.95 20.73 (ug-at/1) 0.13 0.17 Ammonia (μg-at/1) 0.16 0.94 Phosphate (ug-at/1) 0.46 2.72 |