Marine environments, carbonate deposition

Most of the above diagenetic processes can occur in any of three environments. In the marine environment, the deposit is in contact with sea-water, which contains dissolved magnesium and may be either un-saturated, or super-saturated with respect to calcium carbonate species. In the ground-water environment, it is in contact with water, which is low in dissolved magnesium and is generally unsaturated with calcium carbonate. In the burial environment it is subject to high pressures and possibly elevated temperatures (or hydrothermal conditions) and is in contact with water, which may have a widely varying composition. 

Carbonate sediments deposited in shallow marine environments are often exposed to the influence of meteoric waters during their diagenetic history. Meteoric diagenesis lowers 8 0- and 8 C-values, because meteoric waters have lower 8 0-values than sea water. For example. Hays and Grossman (1991) demonstrated that oxygen isotope compositions of carbonate cements depend on the magnitude of depletion of respective meteoric waters. 5 C-values are lowered because soil bicarbonate is C-depleted relative to ocean water bicarbonate. 

In this chapter, we introduced the reader to some basic principles of solution chemistry with emphasis on the C02-carbonate acid system. An array of equations necessary for making calculations in this system was developed, which emphasized the relationships between concentrations and activity and the bridging concept of activity coefficients. Because most carbonate sediments and rocks are initially deposited in the marine environment and are bathed by seawater or modified seawater solutions for some or much of their history, the carbonic acid system in seawater was discussed in more detail. An example calculation for seawater saturation state was provided to illustrate how such calculations are made, and to prepare the reader, in particular, for material in Chapter 4. We now investigate the relationships between solutions and sedimentary carbonate minerals in Chapters 2 and 3. 

In this chapter and the following, shoal-water ("shallow-water") carbonate sediments, and the seawaters they form from, are examined. Emphasis is on the biogeochemical processes affecting carbonate materials in this global marine environment, not on the general sedimentology of shoal-water carbonate deposits. The latter subject is discussed in innumerable publications such as "Carbonate Depositional Environments" (Scholle et al., 1983), to which the reader is referred. 

Continental shelves and slopes comprise approximately 10 percent of the Earth s surface, and contain over half the sediments in the ocean (Heezen and Tharp, 1965 Gregor, 1985). Recent estimates of marine carbonate burial rates (e.g., Hay and Southam, 1977 Sundquist, 1985) indicate that between about 35 to 70 percent of Holocene carbonate deposition has taken place on continental shelves. In spite of their importance for carbonate accumulation and the global CO2 cycle, relatively few studies have been made on the chemical controls of calcium carbonate accumulation in these sediments, and most of these studies have been confined to near-shore environments. 

The chemistry of the carbonic acid system in seawater has been one of the more intensely studied areas of carbonate geochemistry. This is because a very precise and detailed knowledge of this system is necessary to understand carbon dioxide cycling and the deposition of carbonate sediments in the marine environment. A major concept applicable to problems dealing with the behavior of carbonic acid and carbonate minerals in seawater is the idea of a constant ionic medium. This concept is based on the observation that the salt in seawater has almost constant composition, i.e., the ratios of the major ions are the same from place to place in the ocean (Marcet s principle). Possible exceptions can include seawater in evaporative lagoons, pores of marine sediments, and near river mouths. Consequently, the major ion composition of seawater can generally be determined from its salinity. It has been possible, therefore, to develop equations in which the influence of seawater composition on carbonate equilibria is described simply in terms of salinity. 

This type is usually related to marine organic matter deposited in a reducing environment with medium to high sulfrir content. The hydrogen to carbon ratio and the oil and gas potential are lower than observed for type I kerogen but still very important. 

Shallow marine environments include coral and algal reefs as well as other bioherms and many favour calcification by benthic fauna. Stromatolites and stromatolitic environments are also typical shallow marine formations. The shallow marine carbonate environment may be subdivided into more or less agitated waters with dominantly benthic fauna, calm shallow areas with carbonate muds (e.g. Bahama Banks) with ooids as typical forms of deposits and reef areas with their complicated patterns of calcification and deposition (Bathurst, 1975 Kinsey and Davies, Chapter 2.5). 

Most weathering processes, to be sure, take place in an environment where aerobic bacteria are active, unless there are considerable quantities of nitrogenous organic matter present. Anaerobic conditions, which usually also imply pH values close to (or less than) 7, are encountered in certain marine muds, guano deposits, and elsewhere. The anaerobe, Clostridium acidiurici (Liebert), which is known to occur in soils, produces ammonia, carbon dioxide and acetic acid from uric acid, guanine and xanthine (Baker and Beck, 1942), while Streptococcus allantoicus forms — in addition to ammonia and carbon dioxide — urea, oxamic acid, etc. Oxalic acid is a common product in the decomposition of guano. 

Occurrences of elemental sulfur in peat, coal, and petroleum are described in Chapter 6.4. The role of sulfate reducers in these environments is suggested by the fact that fossil fuels formed in marine environments, where sulfate is in abundant supply, have significantly more sulfide and native sulfur than those formed under freshwater conditions. In fact, a general geological feature of native sedimentary sulfur deposits is their location in sulfate-carbonate rocks and proximity to oil-gas-bearing strata and hydrologic zones where sulfate waters mix with chloride brines (Ivanov, 1964). 

The soluble form of calcium can be precipitated in the marine environment to form rock by some physical conditions such as warming of the water (carbon dioxide is less soluble in warm water than in cold water and thus calcium carbonate is precipitated), by the use of carbon dioxide by marine plants, or by alterations in the pH of water by ammonia-producing bacteria which also lowers the solubility of calcium carbonate. However, the majority of calcium carbonate deposits are formed from skeletal fragments of organisms living in the marine environment. Some of these organisms inhabit reefs but the majority float free in water. Figure 2.13 shows various shapes of shells formed by Coccolithophorides which can be spherical coccospheres some, such as dicoaster, are star shaped. 

Van Kranendonk, M.J., Webb, G.E., and Kamber, B.S., 2003. Geological and trace element evidence for a marine sedimentary environment of deposition and biogenicity of 3.45 Ga stromatolitic carbonates in the Pilbara Craton and support for a reducing Archaean ocean. Geobiology, 1, 91-108. 

The structural molecules of the skeletons and shells of invertebrates, which function as physical defenses against predation, are important in marine environments, where they produce carbonate and silicate rocks. Deposition in anaerobic environments has also been the basis for the formation of the extensive deposits of gas and oil that now fuel modern industrial societies. Removal of carbon from the biosphere by organisms to produce carbonate rocks, coal, oil, and hydrocarbon gases has been responsible for the presence of oxygen in the atmosphere of the Earth. Reversal of this process by human consumption of fossil fuels has already produced a detectable increase in atmospheric carbon dioxide. 

A nalytical determinations of the quantities and types of pollutant hydro-carbons entering the marine environment are essential for an understanding of the fate and effects of these compounds in marine systems. Since substantial amounts of these hydrocarbon compounds are deposited in marine sediments, research studies have examined the content of polycyclic aromatic hydrocarbons (PAHs) and other petroleum hydrocarbons in marine sediment samples. 

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