pelagic carbonates

pelagic carbonates Pelagic carbonates are composed of coccolithophores(plant), foraminifera (zooplankton) and, less commonly, pteropods (marine molluscs). All these organism secrete calcareous tests (skeletons), although pteropod skeletons are composed of a high-magnesium calcite (aragonite). Accumulation of pelagic carbonates occurs throughout the ocean basins above the carbonate compensation depth, or CCD, but predominates within the Atlantic Ocean.

Coccolithophores, commonly called mannofossils, are spherical, photosynthetic plants between 10 and 50 μm (micrometres) in diameter. Coccolithophores secrete minute plates of calcite (coccoliths) between about 2 and 10 μm across, and form coccolith oozes. Intact coccolithophores or coccospheres are occasionally preserved within sediments. Coccolithophores inhabit the euphotic zone (the upper 100 m of the ocean), and commonly exhibit vertical species stratification. Globally their diversity increases from a minimum in subpolar waters to a maximum in tropical and equatorial waters. Like most phytoplankton, their species distribution is closely linked to water masses. Fossil assemblages can thus help to identify the distribution of past water masses. The shells of colder-water species are generally more robust than their warmer-water counterparts. This often leads to bias in preserved floras, especially at water depths at or near the CCD, for these sediments are primarily affected by thermohaline circulation changes during glacial–interglacial transitions.

Foraminifera (size range from 30 μm to 1 mm) are a diverse group of marine organisms composed of both planktonic and benthonic species. Planktonic species can be subdivided into spinose (Globigerinidae) and nonspinose (Globorotaliidae) forms. The shape of the test (shell) takes a variety of forms as adaptations to maintain buoyancy, and the distribution of species is generally determined by the availability of food and light intensity. Most planktonic forms inhabit the euphotic zone. Thus, areas of increased productivity, typically below latitudes of 60°, are characterized by high numbers of foraminifera. The diversity of planktonic foraminifera decreases with increasing latitude, and the occurrence of specific species is sometimes used to infer the presence or absence of particular water masses.

Benthonic foraminifera are either mobile or sessile, infaunal or epifaunal, and their shape and wall texture differentiate them from planktonic species. Their habitats include both shelf seas and the abyssal plain, and although they occur at all latitudes, their diversity is greatest in the tropics. Individual species show a strong correlation with depth. However, the variety of depth-dependent environmental factors (e.g. light availability, nutrient concentrations, temperature, salinity, oxygen and carbon dioxide contents), often makes it difficult to determine which is the limiting factor for any particular species. Some abyssal plain species are believed to be associated with particular bottom water masses and can be used to chart their palaeoceanographic courses.

Oxygen isotope analysis of planktonic foraminifera tests gives estimates for past sea-surface temperatures and salinities, making it possible to reconstruct palaeo-surface circulation patterns. Isotope data from benthonic foraminifera tests similarly make it possible to reconstruct of bottom water mass history, and therefore the thermohaline structure of the oceans.

Pteropods (planktonic gastropods) form only a minor component of biogenic carbonate sediments. Their thin aragonitic shells come in a variety of shapes, with lengths from 0.3 to 10 mm. Most pteropod species typically inhabit the upper 500 m of tropical and subtropical water masses. In sea water, aragonite is less stable than calcite, and the aragonite compensation depth (ACD) is shallower than the CCD, typically less than about 2–3 km. Pteropods are useful as biostratigraphical markers and water mass indicators, especially in semi-enclosed and marginal seas, such as the Mediterranean Sea and the Red Sea.

R. B. Pearce

Bibliography

Open University (1991). Ocean chemistry and deep-sea sediments (2nd edn). Pergamon Press, Oxford.