Biochemical Sedimentary Rocks

Biochemical sedimentary rocks consist of sediments formed from the remains or secretions of organisms. They include fossiliferous limestone, coquina (limestone composed of shells and coarse shell fragments), chalk (porous, fine-textured variety of limestone composed of calcareous shells), lignite (brown coal), and bituminous (soft) coal. 

Bituminous—Still more energy is packed into bituminous coal, sometimes referred to as scft coal. In the United States, it is found primarily east of the Mississippi River in midwestern states such as Ohio and Illinois and in the Appalachian mountain range from Kentucky to Pennsylvania. 

Anthracite— Anthracite is the hardest coal and gives off a great amount of heat when it burns. Unfortunately, in the United States, as elsewhere in the world, there is little anthracite coal to be mined. The U.S. reserves of anthracite are located primarily in Pennsylvania. 

A basin is a large area with thick sedimentary rocks. It is where most natural gas is found. 

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Syn-sedimentary chemical deposits form by chemical and biochemical precipitation of valuable metal components carried in solution, concomitant with the formation of the enclosing sedimentary rock. The manner of such deposition depends on the concentration of the metal in the solvent, the solubility of the precipitating product, the solution chemistry, and the deposition environment. Iron, manganese, phosphorus, lead, zinc, sulfur and uranium are some of the elements that have formed economically valuable deposits by chemical precipitation during sedimentation. 

A clear avenue of future research is to explore the S-Fe redox couple in biologic systems. Bacterial sulfate reduction and DIR may be spatially decoupled, dependent upon the distribution of poorly crystalline ferric hydroxides and sulfate (e.g., Canfield et al. 1993 Thamdrup and Canfield 1996), or may be closely associated in low-suUate environments. Production of FIjS from bacterial sulfate reduction may quickly react with Fefll) to form iron sulfides (e.g., Sorensen and Jeorgensen 1987 Thamdrup et al. 1994). In addition to these reactions, Fe(III) reduchon may be coupled to oxidation of reduced S (e.g., Thamdrup and Canfield 1996), where the net result is that S and Fe may be cycled extensively before they find themselves in the inventory of sedimentary rocks (e.g., Canfield et al. 1993). Investigation of both S and Fe isotope fractionations produced during biochemical cycling of these elements will be an important future avenue of research that will bear on our understanding of the isotopic variations of these elements in both modem and ancient environments. 

G.L. Grandjean, E. (eds.) Mdssbauer spectroscopy applied to inorganic chemistry. Plenum Publ. Corp., 3 417-444 Webb, J. Macey, D.J. Mann, S. (1989) Biomineralization of iron in molluscan teeth. In Mann, S. Webb, J. Williams, R.J.P. (eds.) Biomineralization Chemical and biochemical perspectives. VCH Weinheim, 345-387 Webster, J.G. Swedlund, P.J. Webster, K.S. (1998) Trace metal adsorption onto an acid mine drainage iron(lll) oxy hydroxy sulfate. Environ. Sci.Techn. 32 1361-1368 Wedepohl, K.H. (1969) Composition and abundance of common igneous rocks. In Wedepohl, K.H. (ed.) Handbook of geochemistry. Springer, Berlin, 1 227-249 Wedepohl, K.H. (1969a) Composition and abundance of common sedimentary rocks. 

Because sedimentary carbonates represent primarily chemical and biochemical precipitates from seawater, and because they make up 20% of the common sedimentary rock record, these rock types have been particularly good sources of chemical and mineralogical data for interpretation of the secular and cyclic evolution of the Earth s surface environment. This carbonate rock record as a function of geological age is now explored as are age trends in other rock types and sediment properties. With this information as background material, we can then discuss what these relationships tell us about the history of carbonates and the exogenic system throughout geologic dme. 

As has already been mentioned, oxidation of methane led to the formation of free carbon which apparently was involved in biochemical cycles. Therefore the oldest sedimentary rocks could have contained both juvenile carbon, produced in chemical oxidation of methane, and biogenic carbon, formed in the decomposition of organisms. 

Sedimentary rocks are classified depending on how they were formed. They are divided into four groups (a) clastic sedimentary rocks , (b) biochemical (or biogenic) sedimentary rocks, (c) chemical sedimentary rocks and (d) other sedimentary rocks formed by impact, volcanism and other processes. 

Biochemical or biogenic sedimentary rocks are formed from calcareous skeletons of organisms. The main constituent mineral is calcite (CaCO ). Limestone is the main rock type in this category. 

The chemical components of calcium carbonate — dissolved calcium ions and carbon dioxide — are widely distributed. Calcium is the fifth most common element in the earth s crust (after oxygen, silicon, aluminium and iron). It was extracted from early igneous rocks by the combined effects of erosion by the weather and corrosion by acidic gases (oxides of sulfur, oxides of nitrogen and carbon dioxide dissolved in rain water). Carbon dioxide makes up about 0.03 % by volume of the earth s atmosphere and is dissolved in both fresh and sea water. Combination of dissolved calcium ions and carbon dioxide resulted in the sedimentary deposition of calcium carbonate, which was subsequently converted into limestone rock. Early limestones (Precambrian — Table 2.1) are believed to have been deposited as precipitates of CaCOa, and/or as a result of the biochemical activity of very simple organisms, such as bacteria. 

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