Sea—sediment interface

The decay of Ra in water generates the noble gas Rn both these are in equilibrium in the water column, except near the air-sea and sea-sediment interfaces. Rn escapes from sea water to the atmosphere near the air-sea boundary, causing it to be deficient relative to Ra, whereas close to the sediment-water interface Rn is in excess over Ra due to its diffusion out of bottom sediments (Figure 11). These disequilibria serve as tracers for mixing rate studies in these boundary layers. In addition, the surface water data have been used to derive Rn emanation rates and parameters pertaining to air-sea gas exchange. 

Joly observed elevated "Ra activities in deep-sea sediments that he attributed to water column scavenging and removal processes. This hypothesis was later challenged with the hrst seawater °Th measurements (parent of "Ra), and these new results conhrmed that radium was instead actively migrating across the marine sediment-water interface. This seabed source stimulated much activity to use radium as a tracer for ocean circulation. Unfortunately, the utility of Ra as a deep ocean circulation tracer never came to full fruition as biological cycling has been repeatedly shown to have a strong and unpredictable effect on the vertical distribution of this isotope. 

Finally, it must be stressed that diffusion of dissolved species in solutions is a key physicochemical process for the sea/sediment interaction and energy exchange at the sediment-water interface. The reader is referred to Cussler (1984) for a more comprehensive presentation of diffusion in fluid systems. 

The advent of new techniques to collect undisturbed sediment cores, with well preserved sediment - water interface has brought into sharper focus the various deep sea sedimentary processes, their rates and their effects on the preserved records. As mentioned earlier, recent studies have shown that the record contained in sediments is not a direct reflection of the delivery pattern of a substance to the ocean floor as has so far been assumed the record is modified as a result of several complex physical, chemical and biological processes. Therefore, information on the temporal variations in the tracer input to oceans, if sought, has to be deciphered from the sediment-residuum. In the following we consider one specific example of retrieval of information from the sediment pile the application of deep sea sediments to obtain historical records of cosmic ray intensity variations. 

Figure 5. Downcore profile of 6 Zn (Marechal et al. 2000) and 6 Cu values (unpublished) in Central Pacific core RC 17-203 (21° 50 S, 132° 53 W, z = 3900 m). The water-sediment interface is located below the carbonate compensation depth and deep-sea clays dominate the mineralogy of the samples.

We have recently developed a microcalorimetric technique for quantifying the energetic changes of microorganisms colonizing a sea-water-sediment interface in experimental microcosms. The inherent low specificity of direct microcalorimetry and, moreover, the high sensitivity and reliability of modern microcalorimeters proved advantageous, since unknown and subtle events, not shown by more specific methods, may be detected. 

Fig. 3 Typical seasonal microcalorimetric and oximetric patterns produced in experimental microcosms, at the sea-water - sediment interface, after an experimental eutrophication A, unimodal summer PTC B, bimodal spring-autumn PTC C, sigmoid winter PTC, with their respective oxygen-time curves (OTCs).

Numerous models have been proposed for the processes occurring near the sediment-water interface in deep sea sediments that lead to a balance between dissolution and retention of calcium carbonate in these sediments. Investigation of these processes is currently one of the most active areas of research in the study of calcium carbonate behavior in the oceans. A major difficulty in studying and modeling these processes is that many of the most important changes take place over distances of only a few millimeters in a highly dynamic environment. 

Emerson et al., 1982) have demonstrated that this approach will not work for studying the carbonate chemistry of deep sea sediment pore waters. The reason is that the solubility of carbonates changes substantially with temperature and pressure, and they are reactive enough to change the pore water chemistry when the cores are recovered. Consequently, most recent studies have relied on extracting pore waters in situ. The major difficulty with this technique is that it usually is not possible to obtain closely spaced samples or samples very near the sediment-water interface. This interface is, unfortunately, the region where most of the chemical changes associated with the carbonate-C02 system take place. 

Not only are accurate data for trace metals in rivers sparse, there are complications that exist at the river-sea interface. The increase in salinity occurring at the river-sea water interface, with its concomitant increase in the concentrations of the major seawater cations, can lead to flocculation and sedimentation of trace metals such as iron (Boyle et al., 1978 Sholkovitz and Copeland, 1983) or to desorption from suspended riverine particles of trace metals such as barium (Edmond et al., 1978). In organic-rich rivers a major fraction of dissolved trace metals can exist in physiochemical association with colloidal humic acids. Sholkovitz and Copeland (1983) used product-mode mixing experiments on filtered Scottish river water, and observed that iron removal was almost complete due to the flocculation of strongly associated iron-humic acid colloids in the presence of the increased... 

Reimers C. E. and Smith K. L. J. (1986) Reconciling measured and predicted fluxes of oxygen across the deep sea sediment-water interface. Limnol. Oceanogr. 31, 305-318. 

With abundant evidence for sulfate reduction in the hydrosphere, the question arises as to the actual site of the sulfate-reducing activity. This depends upon oxygen input to the system, organic matter concentration, and other factors. Suitable conditions are often encountered at, or just below, the water-sediment interface. Here, the population of sulfate-reducers is highest because of the availability of sulfate and organic matter. In some non-eutrophic lakes a secondary population maximum may arise at depths around 3 m in the sediments (Kuznetsov et al., 1963). Sulfate reduction occurs both within the water column and the sediments of the Black Sea and a number of the lakes examined by Ivanov (1964) and Sorokin (1970). 

Benthic flux measurements from bottom chamber devices and porewater flux determinations have been used to estimate the rain rate of organic matter to the sediment-water interface. When compared with global primary production rates and sediment trap particle fluxes, these data indicate that about 1% of the primary production reaches deep-sea sediments and is oxidized there (Table 12.3) Qahnke, 1996). It has also been demonstrated from benthic flux experiments that about 45% of respiration in the ocean below 1000 m occurs within sediments. 

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