Lithosphere oceanic

The outer shell of the earth, consisting of the upper mantle and the crust (Figure I4. lO), is formed of a number of rigid plates. These plates are 20 in number and are shown in Figure 14.1 I. Of these, six or seven are major plates, as can be seen in the map. The edges of these plates define their boundaries and the arrows indicate the direction of their movement. These plates contain the continents, oceans and mountains. They almost float on the partially molten rock and metal of the mantle. The outer shell, known as the lithosphere, is about 70 to 1,50 km thick. It has already moved great distances below the etirth s surface, ever since the earth was formed and is believed to be in slow and continuous motion all the time. The plates slide on the molten mantle and move about lO to 100 mm a year in the direction shown by the arrows. The movement of plates is believed to be the cause of continental drifts, the formation of ocean basins and mountains and also the consequent earthquakes and volcanic eruptions. 

Under low-dose conditions, forest ecosystems act as sinks for atmospheric pollutants and in some instances as sources. As indicated in Chapter 7, the atmosphere, lithosphere, and oceans are involved in cycling carbon, nitrogen, oxygen, sulfur, and other elements through each subsystem with different time scales. Under low-dose conditions, forest and other biomass systems have been utilizing chemical compounds present in the atmosphere and releasing others to the atmosphere for thousands of years. Industrialization has increased the concentrations of NO2, SO2, and CO2 in the "clean background" atmosphere, and certain types of interactions with forest systems can be defined. 

Oxygen is the most abundant element on the earth s surface it occurs both as the free element and combined in innumerable compounds, and comprises 23% of the atmosphere by weight, 46% of the lithosphere and more than 85% of the hydrosphere ( 85.8% of the oceans and 88.81% of pure water). It is also, perhaps paradoxically, by far the most abundant element on the surface of the moon where, on average, 3 out of every 5 atoms are oxygen (44.6% by weight). 

As a specific example, consider oceanic sulfate as the reservoir. Its main source is river runoff (pre-industrial value 100 Tg S/yr) and the sink is probably incorporation into the lithosphere by hydrogeothermal circulation in mid-ocean ridges (100 Tg S/yr, McDuff and Morel, 1980). This is discussed more fully in Chapter 13. The content of sulfate in the oceans is about 1.3 X lO TgS. If we make the (im-realistic) assumption that the present runoff, which due to man-made activities has increased to 200 Tg S/yr, would continue indefinitely, how fast would the sulfate concentration in the ocean adjust to a new equilibrium value The time scale characterizing the adjustment would be To 1.3 X 10 Tg/(10 Tg/yr) 10 years and the new equilibrium concentration eventually approached would be twice the original value. A more detailed treatment of a similar problem can be found in Southam and Hay (1976). 

Carbon is released from the lithosphere by erosion and resides in the oceans ca. 10 years before being deposited again in some form of oceanic sediment. It remains in the lithosphere on the average 10 years before again being released by erosion (Broecker, 1973). The amount of carbon in the ocean-atmosphere-biosphere system is maintained in a steady state by geologic processes the role of biological processes is, however, of profound importance... 

Comparison of Figs 13-6a and 13-6b clearly demonstrates the degree to which human activity has modified the cycle of sulfur, largely via an atmospheric pathway. The influence of this perturbation can be inferred, and in some cases measured, in reservoirs that are very distant from industrial activity. Ivanov (1983) estimates that the flux of sulfur down the Earth s rivers to the ocean has roughly doubled due to human activity. Included in Table 13-2 and Fig. 13-6 are fluxes to the hydrosphere and lithosphere, which leads us to these other important parts of the sulfur cycle. 

Almost all the Earth s carbon is found in the lithosphere as carbonate sediments that have precipitated from the oceans. Shells of aquatic animals also contribute CaC03 to the lithosphere. Carbon returns to the hydrosphere as carbonate minerals dissolve in water percolating through the Earth s crust. This process is limited by the solubility products for carbonate salts, so lithospheric carbonates represent a relatively inaccessible storehouse of carbon. 

Lassiter JC, Hauri EH (1998) Osnhum-isotope variahons in Hawaiian lavas evidence for recycled oceanic lithosphere in the Hawaiian plume. Earth Planet Sci Lett 164 483-496 LaTourrehe TZ, Kennedy AK, Wasserbuig GJ (1993) Thorium-uraiuum frachonation by garnet evidence for a deep source and rapid rise of oceanic basalts. Science 261 739-742 Liu M, Chase CG (1991) Evoluhon of Hawaiian basalts a hotspot melhng model. Earth Planet Sci Leh 104 151-165... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Figure 5.5 Least-square mixing hyperbola for the isotopic data on Heard Island of Barling and Goldstein (1989). Data from Table 5.10. The 87Sr/86Sr value of the MORB source ( 0.7025) lies below the horizontal asymptote. Asthenosphere and oceanic lithosphere are unlikely source components of Heard Island basalts.

Figure 15. Diagram showing the major components of the global calcium cycle with b Ca values (denoted as 5). The modem residence time of Ca in the oceans is about 1 million years (Holland 1978 1984). Abbreviations used are SW = seawater, Sed = sedimentation, clastic = clastic sediments, carb = marine carbonate sediments, hydrol = mid-ocean ridge hydrothermal systems, lith = continental lithosphere.

Changes in ocean-floor depth, sediment thickness, and hydrothermai circuiation with increasing age of the lithosphere, (a) Heat loss is by advection, (b) heat loss is by advection and conduction, and (c) heat loss is by conduction alone. Source From Sclater, J. G. (2003). Nature 421, 590-591. 

Hydrosphere The water porUon of the earth, as distinguished from the solid (lithosphere) and gaseous (atmosphere) parts. This includes water in lakes, ponds, streams, rivers, glaciers, icebergs, the ocean, pore waters, and that which is trapped in crustal rocks. 

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