Pacific Ocean sediment calcitic

Figure 17. Log of the dissolution rate vs. total carbonate ion concentration for synthetic aragonite, pteropods, calcitic Pacific Ocean sediment, and foraminifera in the 125-500 iim size fraction. (A) indicates ihe aragonite equilibrium total carbonate ion concentration at 25°C, 1 atm (26). (C) indicates the calcite equilibrium total carbonate ion concentration at 25°C, 1 atm (25).

Figure 18. Ratio of measured rates of dissolution per gram to the initial dissolution for pteropods and calcite Pacific Ocean sediment (57)...

Figure 2 Rate of carbonate dissolution from deep-sea sediment versus (1 — O). SoUd Une from Hales and Emerson (1996), dotted line from Keir (1980), dashed line from Atlantic Ocean, and dotted and dashed line from Pacific Ocean sediment results of Morse (1978). Note that Hales and Emerson (1996) used a different calcite solubility product.

A third example is the increasing solubility of calcite with depth in the ocean. Surface seawater is generally supersaturated with respect to calcite, whereas at depths below about 500 m the oceans become undersaturated with calcite (cf. Drever 1988). For this reason there is a tendency for carbonate materials to dissolve and disappear with increasing depth as they settle into ocean bottom sediments. Reasons for the shift to undersaturation with increasing ocean depth include declining temperatures, increased pressure (see Chap. 1, Section 1.6.2), and increasing CO2 concentrations. The depth at which a pronounced reduction in the carbonate content of ocean sediments is observed is termed the calcite compensation depth (CCD). Because of the slow rate at which the calcite dissolves, the CCD is found well below the approximately 500 m depth at which calcite becomes undersaturated. The CCD is thus at about 3.5 km depth in the Pacific Ocean and 5 km depth in the Atlantic Ocean (Drever 1988). 

They suggested that most of the carbonate dissolution in the deep ocean (Fig. 9.5) occurs within the sediments (85 %). The extension of their results from Pacific and Indian Ocean to the Atlantic Ocean leading to 120 10 molyr of global dissolved carbon fluxes from sediments may, however, be critical because of the completely different deep-water conditions in the Indo-Pacific and the Atlantic. Deep ocean waters in the Indian and Pacific Oceans are known to be much older and depleted in CO implying that a much higher proportion of calcite dissolution contributes to the total alkalinity input there. However, despite this problem of different bottom-... 

Based on thermodynamic considerations, sediments that lie at depths below the saturation horizon should have 0% CaCOj. This then explains why calcareous oozes are restricted to sediments lying on top of the mid-ocean ridges and rises and why the sediments of the North Pacific are nearly devoid of calcite and aragonite. (The low %CaCOj in the sediments of the continental margin is a result of dilution by terrestrial clay minerals.)... 

Observations from studies of surface sediments have allowed definition of regionally varying levels in the ocean at which pronounced changes in the presence or preservation of calcium carbonate result from the depth-dependent increase of dissolution on the seafloor. The first such level to be identified was simply the depth boundary in the ocean separating carbonate-rich sediments above from carbonate-free sediments below. This level is termed the calcite (or carbonate) compensation depth (CCD) and represents the depth at which the rate of carbonate dissolution on the seafloor exactly balances the rate of carbonate supply from the overlying surface waters. Because the supply and dissolution rates of carbonate differ from place to place in the ocean, the depth of the CCD is variable. In the Pacific, the CCD is typically found at depths between about 3500 and 4500 m. In the North Atlantic and parts of the South Atlantic, it is found... 

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