Factors controlling

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Lecture 6
Factors controlling the distribution of
foraminifera
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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.

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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.

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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.

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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.

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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 (lt200 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.

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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.

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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.

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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.

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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.

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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.

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