Plankton calcareous

Biogenic Ma.teria.ls, Deep ocean calcareous or siUceous oo2es are sediments containing >30% of biogenic material. Foraminifera, the skeletal remains of calcareous plankton, are found extensively in deep equatorial waters above the calcium carbonate compensation depth of 4000 to 5000 m. 

In the preceding sections, we have discussed the marine processes that control calcium carbonate s formation, dissolution, and delivery to the seafloor. Their combined effects determine the geographic distribution of calcium carbonate in marine sediments seen in Figure 15.5. As noted earlier, the global distribution of calcareous sediments does not seem to follow that of plankton production. This points to the overriding importance of the processes that control the dissolution and sedimentation of calcium carbonate. 

The model provided in Figure 20.1 is for an ocean basin whose abyssal plains all lie below the CCD. This most closely resembles the conditions in the North Pacific, whereas the rest of the ocean basins have a significant portion of their abyssal plains lying above the CCD, and, hence, contain some calcareous oozes. From a global perspective, calcareous oozes are more abundant than siliceous oozes. This is caused by two phenomena (1) all seawater is undersaturated with respect to opal, whereas all surface waters and 20% of the deep waters are saturated with respect to calcite, and (2) siliceous plankton are dominant only in upwelling areas. 

What has happened to the bicarbonate and calcium delivered to the ocean by river runoff As described later, these two ions are removed from seawater by calcareous plankton because a significant fraction of their hard parts are buried in the sediment. In contrast, the only sedimentary way out of the ocean for chloride is as burial in pore waters or precipitation of evaporites. The story with sodium is more complicated— removal also occurs via hydrothermal uptake and cation exchange. Because the major ions are removed from seawater by different pathways, they experience different degrees of retention in seawater and uptake into the sediments. Another level of fractionation occurs when the oceanic crust and its overlying sediments move through the rock cycle as some of the subducted material is remelted in the mantle and some is uplifted onto the continents. 

The process of removal of calcium by marine organisms in the water column is well known. Production of calcium carbonate by water column biological processes may be estimated from primary productivity and from the mean chemical composition of plankton. After death of the organisms and removal of the organic protective layer, the skeletons may undergo dissolution if they encounter water undersaturated with respect to their mineral composition. Active dissolution of calcium carbonate occurs mainly near the sediment-water interface in deep waters that are undersaturated with respect to both calcite and aragonite (see Chapter 4). Thus, calcium is regenerated from calcareous skeletons and, finally, only a small fraction of the initial production of these materials accumulates in sediments. An... 

The organic biomarker proxy for past sea surface temperatures ( U37 ) came to paleoceano-graphy from an unexpected direction. Nearly aU paleoceanographic tools rely on some aspect of the fossilized hard parts of marine organisms. Thus, assemblages of calcareous microplankton such as foraminifera and coccoliths, or of siliceous plankton such as radiolaria and diatoms, provided the basis for the CLIMAP reconstruction of the Ice Age ocean (CLIMAP, 1976, 1981). Additionally, a host of chemical methods relies on the same hard parts to furnish isotopic and trace element... 

Carbonate particles embedded in the faecal pellets of planktonic crustaceans are protected from dissolution by an organic capsule. This mechanism is lEirgely responsible for the presence of remains of calcareous nannoplemk-ton in deep ocean sediments below the compensation level for carbonate (Honjo, 1976). Similarly, carbonate particles suspended in sea water are apparently coated by a layer of organic matter. After this layer is experimentally removed by a strong oxidant (e.g. hypochlorite or H2O2), the rate of carbonate dissolution increases significantly (Chave and Suess, 1967,... 

The actual calcium concentration in the marine waters is about 30 times higher than in the riverine waters. This is owed to the limited solubility of calcium carbonate and, especially, related to the extensive calcium uptake by planktonic organisms followed by calcium deposition as pellets. These processes have facilitated the vast accumulation of calcium as a component of massive layers of limestone, dolomite, marl, calcareous clay, and other Ca-containing rocks (see Box 8). 

Perhaps the best known of the mass extinctions is that associated with the demise of the dinosaurs at the Cretaceous—Tertiary boundary, which has been attributed to a meteorite impact (Alvarez et al. 1980 Kruge et al. 1994 Skelton et al. 2003), although natural climate change may also have been a factor. Planktonic organisms (particularly foraminiferans, calcareous phytoplankton and radiolarians) were also severely affected. On land, trees seem to have been devastated, with the recovery period marked by the proliferation of ferns. The pollen record is dominated by fern spores, and hence known as the fern spike, which is observed... 

Certain ocean precipitates incorporate the Mg2+ ion in their makeup. In calcite (CaC03), it has been shown that the magnesium content enhances calcite solubility, which, in turn, slows crystal growth. This raises concerns about increasing the magnesium content of seawater (as from oil production), which can have adverse implications for calcareous marine organisms such as plankton. 

Hay, W. W. 2004. Carbonate fluxes and calcareous nanno-plankton. In Thierstein, H. R. Young, J. R. (eds) Coccolithophores - From Molecular Processes to Global Impacts. Springer, Berlin, 509-528. 

Although calcareous plankton presently dominate marine carbonate precipitation,... 

Fig. 1.6 SEM photographs of the important sediment-forming planktonic organisms in various stages of decay. Above. Siliceous centered diatoms. Middle. Siliceous radiolarian. Below. Calcareous coccolithophorid of single placoliths = coccoliths and lutitic abrasion of coccoliths. Identical scale of 5 pm.

Table 1.5). The distribution of these sediments in the three great oceans shows a considerable degree of variation (Fig. 1.14). The distribution patterns strongly depend on the water depth, i.e. the position of the CCD. Calcareous ooze, primarily consisting of foraminiferal oozes and nanno-plankton oozes, covers vast stretches of the sea-... 

This section primarily focuses on the description of the deposition and accumulation of carbonates in shallow waters and in the deep ocean. The main depocenters for calcium carbonates are the continental shelf areas, as well as island arcs or atolls, which are the typical shallow water environments for massive carbonate formation, and the pelagic deep-sea sediments above the calcite compensation depth catching the rain of small calcareous tests formed by marine plankton in the surface waters. 

---