Oceans composition, Covers

Seawater. A half dozen analyses of Mo isotopes in the Pacific, Atlantic and Indian oceans, covering depths to -3000 m (Barling et al. 2001, Siebert et al. 2003), reveal two important facts. First, there is no detectable 5 Mo variation in the oceans with location or depth. Second, d Mo in the oceans is similar to the heaviest of euxinic sediments, and is heavier than in igneous rocks or ferromanganese sediments by -1.5%o and -2%o, respectively. A uniform isotopic composition in the oceans is consistent with the 10 -1 O " year ocean residence time. The explanation for the heavy isotopic composition is discussed further below. 

Over a considerable fraction of the high-latitude global ocean, sea ice forms a boundary between the atmosphere and the ocean, and considerably influences their interaction. The details and consequences of the role of sea ice in the global climate system are still poorly known. Improved knowledge is needed of the broad-scale time-varying distributions of the physical characteristics of sea ice, particularly ice thickness and the overlying snow-cover thickness, in both hemispheres, and the dominant processes of ice formation, modification, decay and transport which influence and determine ice thickness, composition and distribution. We do not know how accurate present model predictions of the sea ice responses to climate change are, since the representation of much of the physics is incomplete in many models, and it will be necessary to improve coupled models considerably to provide this predictive capability. 

All of the above Europan simulations were done at 1.01 bar (1 atm) total pressure. A 100-km ice-covered ocean on Europa would have a pressure >1200 bars at the base of the ocean. What effect would such a high pressure have on chemical equilibria and the thermodynamic properties of such a Europan ocean How might pressure affect the likely composition of seafloor sedimentary deposits, the chemistry of possible hydrothermal brines,... 

The Earth-atmosphere system consists of the ensemble of the atmosphere, ocean, continents and ice cover. The climate of this system is controlled by the orbit and rotation of the Earth, the physical state and chemical composition of the surface (including liquid water and ice), and by the density and composition of the atmosphere. This last parameter participates mainly in the control of the radiation balance. For this reason our knowledge of the radiation balance of the Earth-atmosphere system will be summarized briefly in this section. The interested reader is referred to Paltridge and Platt (1976) for further details. 

It has been estimated that the oceans cover about 70% of the earth s area and contain about 80% of the water on or in the earth. Seawater concentration varies from 25,000 mg/E of total dissolved solids up to over 50,000 mg/E in the Arabian Gulf. The average composition of seawater is about 35,000 mg/E with the major ions being shown in Table 4.9. 

Figure 7. (a) Idealized oxygen isotope profile of altered, sediment-covered oceanic crust based on Gregory and Taylor s (1981) study of the Ibra section of the Oman ophiolite and data for marine sediments taken from data sources cited in the text. The vertical black bar marks the range in 5 0 t ical of mantle peridotites the vertical white bar marks the range in 5 0 typical of fresh oceanic basalts, (b) VMues of typical of various major rock types in the Earth s cmst. Data sources are listed in the text. These ranges emphasize typical, representative values and purposefully exclude extreme examples of many rock types. The isotopic composition of most mantle peridotites (Fig. 8) and all NMORBs (Fig. 9) spans a range equal to the thickness of the vertical black hne. 

The sediments in the deep sea consist of only few basic types which in their manifold combinations are suited for the description of a varied facial pattern (Table 1.4). The characteristic pelagic deep-sea sediment far from coastal areas is deep-sea red clay, an extremely fine-grained (median < 1 pm) red-brown clay sediment which covers the oceanic deep-sea basins below the Calcite Compensation Depth (CCD). More than 90 % is composed of clay minerals, other hydrogenous minerals, like zeolite, iron-manganese precipitates and volcanic debris. Snch sediment composition demonstrates an anthigenic origin. The... 

Deep-sea sediments cover more than 50% of the earth s surface and consist of carbonates, red clay and siliceous ooze (cf. Chap. 1). On average, red clay covers about 31% of the world s ocean basins but its abundance is much higher in the Pacific (49%) than in the Atlantic (26%) and Indian (25%) Oceans (Glasby 1991). Carbonates act as a diluent for the transition elements in deep-sea sediments because of the low contents of these elements in them and the composition of deep-sea sediments is therefore often presented on a carbonate-free basis. 

As apparent from the properties listed in Table 1, ocean chemists have a variety of tracers at hand, covering a range of residence times and chemical behaviors. Those tracers varying due to radioactive decay have distinct isotopic compositions in their various source materials (Table 2). This makes them particularly useful both as water mass tracers, and to reconstruct the flux from these various sources into the oceans. It may be surprising to And the cosmogenic nuclide °Be in this list of otherwise radiogenic tracers. The reason is that Be behaves very similarly to the other tracers in that the ratio °Be/ Be is distinct in different water masses. Given that °Be is the only tracer of which the flux into the oceans is known, t can be calculated precisely from its water column concentration. Further, the continent-derived isotope Be is the only tracer of which the flux into the oceans can be calculated from the °Be/ Be ratio. 

Today, the more external part of the crust or lithosphere constitutes the superficial covering of the earth. Two kinds of crust are easily distinguished by composition, thickness and consistency continental crust and oceanic crust. Continental crust has a thickness that, in mountain chains, may reach 40 kilometers. It is composed mainly of metamorphic rock and igneous blocks enriched with potassium, uranium, thorium and silicon. This forms the diffuse granitic bedrock of 45 % of the land surface of the earth. The oceanic crust has a more modest thickness, in the order of 5-6 kilometers, and is made up of basaltic blocks composed of silicates enriched with aluminium, iron and manganese. It is continuously renewed along mid-ocean ridges (cf Table 2.2). 

The vast layer of salty water that covers so much of the planet is in actuality one large connected body and is generally constant in composition. For this reason, oceanographers speak of a world ocean rather than of the separate oceans we learn about in geography books. 

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