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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.
Living foraminiferid (Spirillina vivipara)
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 CaCO3
precipitation by the uptake of CO2.
• 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 thickershelled, 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 O2 decreases the ability to
secrete CaCO3 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 CaCO3 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.
CaCO3
• The solubility of CaCO3 is less in warm than in cool
waters. CaCO3 solubility also increases with pressure
(i.e. depth). The ratio of CO2 to O2 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 CaCO3 solution
equals CaCO3 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
Foraminiferal ooze with pteropods