Shallow-Water Carbonates

Shallow-water carbonates are formed either by sediments made up of a variety of carbonate par- 

These platforms are the second important tropical to subtropical environment where high amounts of carbonate are produced and accumulated at water depths shallower than 50 m. The areal extension is about 0.8-10 km In contrast to reefs, on carbonate platforms production is mainly carried out by benthic red/green algae, mollusks and benthic foraminifera. Estimates of biotic and, to a much lesser extent, abiotic carbonate production on platforms range between 300-500 g CaCO m yr , which amounts to 4-10 mol yr on a global scale. Accumulation of platform carbonate is difficult to assess because a lot of it is dissolved or can be found as exported material in several 10 to 100 m 

Only very little knowledge exists concerning the quantitative carbonate production, export and its accumulation on continental shelves. Two shelf types, carbonate-rich and carbonate-poor shelves, are distinguished by Milliman and Droxler (1996). 

Their areas amount to 15 and 10-10 km, respectively (Hay and Southam 1977). However for the two shelf types well-constrained estimates of how much carbonate is produced are missing. In the context of shelves it may be important to separate two other specific bioherms which could have a great potential in shallow-water carbonate production. These are sedimentaiy carbonates exclusively built up by the calcareous green algae Halimeda in tropical latitudes (e.g. Roberts and Macintyre 1988) and extensive biotic cold-water carbonate reefs or banks as described above for mid to high latitudes. For Halimeda bioherms total carbonate production and accumulation is about 1.510 mol yr (Table 9.1) while the estimates for open shelves given by Wollast (1994) and Milliman and Droxler (1996) do not differentiate a 

Habitat Area Cd CO3 Production Accumulation Dissolution/Erosion 

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Shallow water carbonate (reefs carbonate muds) Reservoir quality governed by diagenetic processes and structural history (fracturing). Prolific production from karstified carbonates. High and early water production possible. Dual porosity systems in fractured carbonates. Dolomites may produce H S. 

About 25% of the carbonates deposited in shallow water are eventually eroded and carried downslope by bottom and turbidity currents to become part of the shelf and pelagic sediments. Shallow-water carbonates are also notable for their mineral composition. In addition to calcite and aragonite, some shallow-water calcifiers deposit hard parts containing high percentages of magnesium. These are referred to as magnesium-rich calcites. 

Only a few evaporites have been found that are more than 800 miUion years old, indicating that most of the salt formed prior to this period has been recycled via uplift and weathering. No evaporites of Archean age have as yet been discovered. The oldest known chemical sediments were deposited 3.45 bybp in what is now western Australia. They appear to have precipitated as shallow-water carbonates. This suggests that sulfate concentrations during the Archean were much lower than present day, probably because of limited oxygenation of the atmosphere and ocean. 

Shallow water, carbonate-rich sediments are largely confined today to the subtropic and tropic climatic zones, but are found even at high latitudes. Their... 

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. 

Bernstein L.D. and Morse J.W. (1985) The steady-state calcium carbonate ion activity product of recent shallow water carbonate sediments in seawater. Mar. Chem. 15, 311-326. 

Walter L.M. (1983) The dissolution kinetics of shallow water carbonate grain types Effects of mineralogy, microstructure and dolution chemistry. Ph.D. dissertation, Univ. Miami, Miami, FL. 

Kempe, S. (1990) Alkalinity the fink between anaerobic basins and shallow water carbonates Naturwiaaenschaffer 7, 426-427. 

The Upper Chaudian deposits are the most developed different facies of them are spread over almost the entire Black Sea coast. In the reference section on Cape Chauda they are represented by shallow-water carbonate sediments with abundant fauna consisting of Pliocene relics (Tschaudia tschaudae), Black Sea endemic species (Didacna pseudocrassa, D. olla, and others), and fresh-water elements (Dreissena polymorpha). In other sections of the... 

Many studies of the impact of chemical diagenesis on the carbonate chemistry of anoxic sediments have focused primarily on the fact that sulfate reduction results in the production of alkalinity, which can cause precipitation of carbonate minerals (see previous discussion). However, during the early stages of sulfate reduction (—2-35%), this reaction may not cause precipitation, but dissolution of carbonate minerals, because the impact of a lower pH is greater than that of increased alkalinity (Figure 4). Carbonate ion activity decreases rapidly as it is titrated by CO2 from organic matter decomposition leading to a decrease in pore-water saturation state. This process is evident in data for the Fe-poor, shallow-water carbonate sediments of Morse et al. (1985) from the Bahamas and has been confirmed in studies by Walter and Burton (1990), Walter et al. (1993), and Ku et al. (1999) for Florida Bay, Tribble (1990) in Checker Reef, Oahu, and Wollast and Mackenzie (unpublished data) for Bermuda sediments. 

Walter and co-workers (Walter and Burton, 1990 Walter et al., 1993 Ku et al., 1999) have made extensive efforts to demonstrate the importance of dissolution of calcium carbonate in shallow-water carbonate sediments. Up to — 50% carbonate dissolution can be driven by the sulfate reduction-sulfide oxidation process. In calcium carbonate-rich sediments there is often a lack of reactive iron to produce iron sulfide minerals. The sulfide that is produced by sulfate reduction can only be buried in dissolved form in pore waters, oxidized, or can diffuse out of the sediments. In most carbonate-rich sediments the oxidative process strongly dominates the fate of sulfide. Figure 6 (Walter et al., 1993) shows the strong relationship that generally occurs in the carbonate muds of Florida Bay between total carbon dioxide, excess dissolved calcium (calcium at a concentration above that predicted from salinity), and the amount of sulfate that has been reduced. It is noteworthy that the burrowed banks show much more extensive increase in calcium than the other mud banks. This is in good agreement with the observations of Aller and Rude (1988) that in Long Island Sound siliciclas-tic sediments an increased bioturbation leads to increased sulfide oxidation and carbonate dissolution. 

Kendall, C.G.St.C. and Skipworth, P.A.D E., 1969. Geomorphology of a recent shallow water carbonate province Khor A1 Bazam, Trucial Coast, South West Persian Gulf. Bull. Geol. Soc. Am., 80 865—891. 

Sedimentary calcium carbonates are formed as the shells of marine plants and animals. Biologically produced CaCOs consists primarily of two minerals aragonite and calcite. Shallow-water carbonates, primarily corals and shells of benthic algae (e.g. Halimeda) are heterogeneous in their mineralogy and chemical composition but are composed mainly of aragonite and magnesium-rich calcite (see Morse and Mackenzie (1990) for a discussion). Carbonate tests of microscopic plants and animals, most of which hve in the surface ocean (there are also benthic animals that produce carbonate shells), are primarily made of the mineral calcite, which composes the bulk of the CaCOs... 

Formation, which characterizes a period of relative tectonic quiescence. The passive marine margin sequence (Albian to Recent) is characterized by the development of a shallow-water carbonate platform (Macae Formation), which prograded into deepwater siliciclastics (Campos Formation). 

The rate of total neritic carbonate production in the modern ocean is roughly 25-10 mol yr from which 60 % (15 10 molyr ) accumulate as shallow-water carbonates. The difference, 10-10 molyr, is the contribution of the neritic environment to the pelagic environment, either in the form of flux of total dissolved inorganic carbon or particulate accumulation on continental slopes and in the deep sea. 

This chapter summarizes the most recent compilations of carbonate reservoir size in the ocean and sediments, as well as the particulate and dissolved fluxes (Fig. 9.5) provided by the above mentioned authors. Coral reefs are probably the best documented shallow-water carbonate environment. Carbonate production on reef flats range as high as 10.000 g CaCOj m yr , with a global mean of about 1800 g CaCOj myr . Totally this amounts to 24.5 10 molyr (Table 9.1) from which 14.5-10 mol yr accumulate and 10-10 mol yr are transported to the deep-sea either by particulate or dissolved export. One of the most uncertain numbers in all these budget calculations are the estimates of the global carbonate production in... 

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