Presentation on theme: "Lecture 6 Factors controlling the distribution of foraminifera."— Presentation transcript:
Lecture 6 Factors controlling the distribution of foraminifera
Ecology Numerous foraminifera inhabit the benthic environment. Some move freely over the sea-bed or in the first few millimeters of sediment. Others use their pseudopodia or calcareous secretions to attach themselves to supports such as rocks, shells and seaweed. Most are marine and stenohaline (they can tolerate only very small variations in the salinity of the water). Certain groups having a porcelaneous test (e.g. the miliolines Spirolina, Peneroplis and Aveolinella) can live equally well in hyperhaline environments lagoons with a salinity › 35 parts per mille (‰). Certain types such as the agglutinates (e.g. Eggerella) and hyalines (e.g. Nonion) prefer water with a low salinity e.g. brackish lagoons and estuaries. Still others (e.g. Trochammina and Elphidium) can adjust to considerable vaiations in salinity and may be found in all environments with exception of lakes where foraminifera never live.
Food Foraminifera play an important role in marine ecosystems as micro-omnivores, i.e. they feed on small bacteria, algae, protests and invertebrates (Lipps & Valentine,1970). Some are scavengers, feeding on dead organic particles. Certain foraminifera from reef and carbonate shoal environments appear to benefit from endosymbiotic algae in much the same way as do the corals, e.g. Archaias and Elphidium. It is possible that the fossil "larger foraminifera" achieved their great size in this way, the algae providing nutrients fom photosynthesis and favoring maximum CaCO 3 precipitation by the uptake of CO 2. High diversity foraminiferid assemblages strongly suggest a wide range of available food resources.
Predation Benthic foraminifera stand a very high chance of being ingested by creatures such as worms, crustaceans, gastropods, echinoderms and fish that browse on the sediments and organisms upon the sea floor. As yet, the effects of such predation on living foraminiferid populations is little known and may serve either to raise or lower diversity.
Substrate Silty and muddy substrates are often rich in organic debris and the small pore spaces contain bacterial blooms. Such substrates are therefore attractive to foraminifera and support large populations. Many of these species are thin-shelled, delicate and elongate forms. The larger pore spaces of sands and gravels contain fewer nutrients and therefore support sparser populations. Foraminifera from these coarser substrates may be thicker- shelled, heavily ornamented and of biconvex or fusiform shape. Although foraminifera have been found living up to 200 mm below the sediment surface, the majority are feeding within the top 10 mm or so, the depth of burial varying between species.
Light The zone of light penetration in the oceans ( the photic zone) is affected by water clarity and the incident angle of the Sun's rays. Hence the photic zone is deeper in tropical waters (<200 m) and decreases in depth towards the poles where it also varies marked seasonality. Primary production of nutrients by planktonic and benthic algae render this zone attractive to foraminifera, especially the porcelaneous Miliolina and the larger forms.
Temperature Each species is adapted to a certain range of temperature conditions. Stratification of the oceans results in the lower layers of water being cooler, as for example in tropical waters where the surface may average 28ºC but the bottom waters of the abyssal plains may average less than 4ºC. These cooler, deeper waters may be characterized by cool-water benthic assemblages that otherwise are found at shallower depths nearer the Poles. Planktonic foraminifera are also adopted to different oceanic layers of particular temperatures and densities. In several planktonic species (e.g. Globigerina pachyderma) warm and cool populations can be distinguished by a predominance of right-hand (dextral) or left-hand (sinstral) coiling. The sequence of Pleistocene temperature fluctuations has been determined from studies of these and similar foraminifera obtained in deep-sea cores.
Benthic and planktonic foraminiferid assemblages changes with depth and latitude, especially in relation to temperature.
Oxygen Oxygen concentrations do not vary greatly in present seas and oceans, with a few exceptions such as the Black Sea. Anaerobic assemblages are typified by small, thin-shelled unornamented. Calcareous or agglutinated assemblages. Although low O 2 decreases the ability to secrete CaCO 3 it can increase its subsequent chances of preservation, unless conditions are also acidic.
Salinity The majority of foraminifera are adopted to normal marine salinities (about 35 ‰) and it is in such conditions that the highest diversity assemblages are found. The low salinity of brackish lagoons and marshes favors low diversity assemblages of agglutinated foraminifera (mostly with non-labyrinthic walls and organic, siliceous or ferruginous cements, e.g. Reophax) and certain Rotaliacea (e.g. Ammonia). The soft, tectinous Allogrominia are found in fresh and brackish waters, but they are rarely encountered as fossils. The high CaCO 3 concentrations of hypersaline waters favor the porcelaneous Miliolina (especially the Nubecularidae and Miliolidae, e.g. Triloculina) but deter most other groups. Triangular plots of the relative proportions of Textulariina, Miliolina and Rotaliina have proved useful as indices for paleosalinities.
Benthic and planktonic foraminiferid abundance and general composition change with depth and salinity.
CaCO 3 The solubility of CaCO 3 is less in warm than in cool waters. CaCO 3 solubility also increases with pressure (i.e. depth). The ratio of CO 2 to O 2 increases with depth because algae cannot photosynthesis below the photic zone, although animals continue to respire. This leads to a decrease in pH with depth, from about 8.2 to as low as 7.0. The level at which CaCO 3 solution equals CaCO 3 supply is called the calcium carbonate compensation depth (or CCCD). The net result is a drop in the number of calcareous organisms with depth, there being few below 3000 m. For this reason, the agglutinated foraminifera dominate populations from abyssal depths.
Foraminifera and sedimentology Planktonic foraminifera are important contributors to deep sea sedimentation and, with coccoliths, account for more than 80% of modern carbonate deposition in seas and oceans. At present the foraminifera contribute more than the cocoolithophores, although this was not the case with earlier chalks and oozes. Three factors are important in controlling the deposition of Globigerina ooze (i.e. ooze in which over 305 of sediment is globigerinacean): climate, depth of the lysocline and terrigenous sediment supply. Globigerina oozes cannot accumulate where there is an influx of terrigenous clastics, hence they are rarely found on continental shelves. At present such ooze are accumulating mainly between 50° N and 50° S at depths between about 200 and 5000 m, especially along the mid-oceanic ridges.
Foraminifer nannofossil ooze Deep sea red clay Clayey calcareous ooze Calcareous ooze