Dissolution Of Deepsea Carbonates

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Throughout the deep ocean basins of the world, a major factor affecting preservation of carbonate tests is the rate of dissolution at depth. The oceans are predominantly undersaturated with respect to calcium carbonate at all depths below the upper mixed layer (the zone above the thermocline; Olausson, 1965, 1967). After death of the organisms, deposition of the test on the ocean floor leads to dissolution in the undersaturated water (Adelsack and Berger, 1977). Pteropod tests (composed of calcium carbonate in the form of aragonite) are most susceptible to solution and are the first to disappear; pteropods are thus only found in relatively shallow waters where undersaturation is not pronounced (Berger, 1977). At greater depth, dissolution of tests made of calcite (e.g., foraminifera and coccoliths) becomes apparent. The level at which calcite dissolution is at a maximum is the lyso-cline (Berger, 1970, 1975) generally encountered at 2500-4000 m depth in the

Below Lysocline

FIGURE 6.32 Schematic diagram illustrating selective dissolution of planktonic foraminiferal species at depth, due to undersaturation of the water with respect to calcium carbonate. Dark circles represent the resistant species Globoratalia tumida. Open circles represent Globigerinoides ruber, that is dissolved relatively easily. Qobigerina bulloides (open circle with a line) is intermediate in resistance. Dissolution alters the species composition of the sediment so it may not be representative of species in the overlying water column. At depths below the compensation depth only the occasional Globoratalia tumida may survive. Changes in the depth of the lysodine and compensation depth through time may offset the sediment species composition, due to differential dissolution (Be, 1977).

FIGURE 6.32 Schematic diagram illustrating selective dissolution of planktonic foraminiferal species at depth, due to undersaturation of the water with respect to calcium carbonate. Dark circles represent the resistant species Globoratalia tumida. Open circles represent Globigerinoides ruber, that is dissolved relatively easily. Qobigerina bulloides (open circle with a line) is intermediate in resistance. Dissolution alters the species composition of the sediment so it may not be representative of species in the overlying water column. At depths below the compensation depth only the occasional Globoratalia tumida may survive. Changes in the depth of the lysodine and compensation depth through time may offset the sediment species composition, due to differential dissolution (Be, 1977).

oceans (Fig. 6.32). In the Atlantic, there is evidence that this corresponds to the boundary between North Atlantic Deep Water and the deeper Antarctic Bottom Water (Berger, 1968). Below this level, calcite dissolution rates increase markedly, until at extreme depths the water is so corrosive to calcite that virtually no tests survive to be deposited. The depths at which the dissolution rate equals the rate of supply of carbonate tests from the overlying water column is the calcite compensation depth (CCD) (Berger, 1970). This can be envisioned as analogous to a snowline on land; deep ocean basins below the compensation depth are devoid of carbonate sediments, and higher levels are increasingly blanketed by microfossil tests (Berger, 1971). Because the calcite compensation depth is a function of both the rate of supply of carbonate tests and the dissolution rate, its actual depth varies from one area to another (Fig. 6.33) though generally it is <4000 m (Berger and Winterer, 1974). As vast areas of the ocean floor are below 4000 m, particularly in

Aragonite Compensation Depth

the Pacific Basin, this phenomenon greatly restricts the area in which foraminiferal studies can be usefully carried out (see Fig. 6.2). Even in less deep areas of the ocean, sediments accumulating below the lysocline are subject to significant dissolution. Most importantly, dissolution does not affect all species uniformly; selective removal of the more fragile, thin-walled species may significantly alter the original assemblages (biocoenoses), leaving behind thanatocoenoses which are unrepresentative of productivity in the overlying water column. Assemblages may be enriched with resistant species, which tend to be deep-dwelling, secreting their relatively thick tests in water that is significantly cooler than that near the ocean surface (Ruddiman and Heezen, 1967; Berger, 1968). Similarly, in populations of a particular species, the thicker-walled, more robust individuals, which are preferentially preserved, tend to build their shells in deeper, colder water and are therefore isotopically heavier than their more fragile counterparts (Hecht and Savin, 1970, 1972; Berger, 1971).

Studies of the relative abundances of different foraminiferal species, in cores from various depths, have demonstrated these effects well (Fig. 6.34) and enabled species to be ranked according to their relative susceptibility to solution. Similar

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