Biological surface crusts

Figure 1-7. The Raman spectral stackplot shows the spectra obtained from a vertical transect through the surface crust, the biological hyaline layer, algal layer, depletion zone and basal rock, from the top. Endolithic colonisation of Beacon Sandstone, Mars Oasis, Antarctica, showing zonation. The lower interface of the algal zone dominated by cyanobacteria occurs 8 nun below the upper surface of the rock...

Figure 1-17. Sabkha surface crust with gypsum and halite crystals, Rhub-al-Khalil, Arabian desert cyanobacterial colonisation at interface with subsurface dolomitized calcite. The Raman spectrum shows the presence of scytonemin and carotene in the biological zone...

The peculiar properties of water are as important in biology as in chemistry life on this planet would be quite different were it not for these properties. For example, if ice were heavier than water, a large part of the earth would have quite a different climate. When water freezes an ice crust forms on its surface and to a large extent prevents further freezing, since the heat in the water is lost by conduction and radiation through the ice crust, a much slower process than if its surface were water. Again, the maximum in the density prevents the formation of ice. If water had a normal coefficient of expansion, cold water would sink and the warmer water rise to the surface. Actually, this process occurs only until a... 

Almost all the calcium carbonate that is formed both on the land surface and in the oceans today is produced by living organisms, and it is likely that most of the CaC03-reservoir of the crust is derived from biologically produced limestone (Monty in Westbroek, 1983) 80). Carbonate minerals are also by far the most widely utilized bioinorganic constituents. 

Soil is the product formed when the rocks of the earth s crust are exposed at the surface and are subjected to various physical, chemical, and, eventually, biological weathering processes. The minerals in these rocks are predominantly silicates, which dominate the characteristics of most soils. Table 1 shows those elements that are found in the crust above an average concentration of 1% and their corresponding soil content. The importance of aluminosilicates in soil is clear from the enrichment factors of approximately 1 for O, Si, and Al. Some loss occurs of K, Fe, Ca, Na, and Mg as a result of soil processes. But two elements, C and N, show considerable enrichment in soil because of the crucial role played by organic matter. 

Iron and manganese total 5% of the continental crust, with iron contributing 98% and manganese the remainder (Weaver and Tamey, 1984). They are subject to rapid changes in redox state mediated by both geochemical and biological processes. Iron atoms in near-surface Earth environments cycle between an oxidized or ferric state, Fe(III), and a reduced or ferrous state, Fe(II). Manganese exists in three redox states Mn(II), Mn(III), and Mn(IV). In this review, the abbreviation Mn(IV) is understood to represent Mn(IV) and Mn(III). 

The biological influence on sulfur in the lithosphere may be more fully appreciated by briefly reviewing geological evolution with reference to sulfide mineralisations as summarised by various authors (Watson, 1973 Lacy, 1975). The postulated early events in the sequence are accretion and separation into core, mantle and crust, accompanied by a decline in the thermal gradient with loss of radioactive heating sources. The early crust was very thin and mantle sulfide readily surfaced to be preserved as the volcanogenic... 

Diagenesis (sensu lato) all the chemical, mineralogical, physical and biological changes undergone by the sediment after its initial deposition and during and after its lithification, exclusive of surficial alteration (weathering) and metamorphism. These processes occur under conditions of pressure and temperature that are usual at the Earth s surfaces and in the outer part of the Earth s crust. 

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