Ellen Thomas

Benthic foraminifera are an important component of the deep-sea biomass in the present oceans, adapted to its cold, dark, and extremely oligotrophic environments. Faunas are highly diverse, and many species have a cosmopolitan distribution. In addition to their interest as indicator species living in the largest habitat on earth, their tests have been used extensively in isotope and trace element analysis aimed at reconstruction of past environments. This section is designed to introduce the basics of what we benthic foraminiferal taxonomy, ecology and paleoecology and their use as a proxy for interpreting the state of past oceans and climates, specifically oceanic productivity and deep water oxygenation.

“The case of the three species of protozoan (I forget the names) which apparently select differently sized grains of sand, etc., is almost the most wonderful fact I ever heard of. One cannot believe that they have mental power enough to do so, and how any structure or kind of viscidity can lead to this result passes all understanding.”

Charles Darwin, letter to W.B. Carpenter, 1872

Why study deep-sea benthic foraminifera?

What is ‘deep-sea’? (van Morkhoven et al., 1985)

Note that ‘bathyal’ covers sites on continental margins as well as in open ocean (sea mounts)

What is a ‘foraminifer’?

Kingdom Granuloreticulosa, Phylum Foraminiferida

Genetic information indicates that unicellular eukaryotes, which were until recently classified into the Kingdom Protista (Other Kingdoms: Kingdom Prokaryotes, Kingdom Plantae, Kingom Animalia, Kingdom Fungi). Nowadays we recognize the Superkingdoms Archaea and Eubacteria (formerly Prokaryotes), and the Superkingdom Eukaryotes (Protists, mushrooms, plants, animals). The Superkingdom Eukarya is being reinterpreted, with many group formerly placed together in “protists’ now being elevated to the level of ‘Kingdom’. This means that we now think that, for instance, foraminifera are as different from other protists (e.g., dinoflagellates, radiolarians, diatoms) as cows are from oak trees. Genetic information also strongly indicates that Foraminifera together with test-less, freshwater forms form a monophyletic group, which split off from other life forms early on, maybe in the late Proterozoic.

Basic terminology for foram tests:

Chamber arrangements




Orders of Foraminifera (based on wall structure and chemistry; B. K. Sen Gupta, ed., 1999, Modern Foraminifera, Chapter 2 (p. 7-35), Kluwer Academic Publishers). (see also practical)

Presently most abundant groups in the deep sea are underlined

Genetic evidence suggests strongly that Allogromida (‘naked’) and Astrorhizida (agglutinated) are one order.

Phylogeny of deep-sea benthic foraminifera

What is the function of the test of benthic foramifera?

Granuloreticulate pseudopods, the main distinguishing character of foraminifera.

Pseudopodia: fundamental importance, mechanism through which forams interact with environment

What do forams eat? (almost everything)

Who eats foraminifera?

Reproduction: complex alternation of sexual-asexual generations

Note that the gametes may exist in free form for at least several days, and function as ‘propagules’, i.e., help in spreading benthic forms worldwide (hence many cosmopolitan taxa).


Test formation in forams:

(calcite, hyaline)

Foraminifera cover calcite wall of earlier chambers, and walls between chambers are (in bilamellar forms) existing of 4 layers (one from each adjoining chamber), so that one should be careful in using spot-analysis (laser-zapping) of foraminiferal subsequent chambers and septa in foraminifera with the aim of using these analysis for very high resolution records.

As an additional complexity: some (porcellaneous) foraminifera have been shown to take up a droplet of water within the cytoplasm, then use ions in the ‘internal pool’ to form thin craysallites within that droplet, thus causing chemical/isotopic heterogeneity. Other use ions from droplet, but appear to keep that droplet open to exchange with sea water.

Some species (e.g., hyaline Amphistegina) use ‘pooled ions’, others (e.g., porcellaneous Amphisorus) do not. We do not know whether these are typical for the larger groups or not; at least another few small hyaline species also used ‘pooled ions’.

What do we know about deep-sea forams?

Much early data on deep-sea benthic foraminifera (and on other deep-sea groups) were collected on the 1872-1876 Challenger Expedition (benthic foraminifera described by Brady, 1881, 1884). For updated taxonomy and re-publication of plates see Jones, 1994.

Importance of deep-sea benthic foraminifera

Fundamental niche: species could theoretically exist under these condition

Realized niche: species really exists under these conditions (smaller space than fundamental niche)

Interpretation of deep-sea benthic foraminiferal assemblages:

In the 1970s, Lohmann first recognized that the water masses in the North Atlantic (e.g., AntArctic Bottom Water, AABW, and North Atlantic Deep Water, NADW) were characterized by typical foraminiferal assemblages recognized in multivariate analysis (Lohmann, 1978). It turned out, however, that it was not possible to typify global water masses by faunal assemblages consistently, leading to disappointment in the 1980s. In the 1990s, however, many new studies of recent faunas were directly or indirectly linked to the JGOFS (Joint Global Ocean Flux Studies), and led to recognition of the importance of food in the life of foraminifera: they depend upon food delivered from primary productivity in the surface waters, 1000s of meters away.

Delivery of food to ocean floor:

Marine snow: particles mm-cm sized, consisting of dead and dying phytoplankton, zooplankton exoskeletons, fecal matter). These fall at a speed of 102-103 m/day; a single unicellular alga would probably not even sink to the sea floor, being re-suspended many times.

Seasonality of productivity at pelagic mid latitudes: pulse of phytodetritus, followed by rapid growth-reproduction of some benthic foraminifera

Relatively high, continuous supply along continental margins; there freshly produced organic matter is augmented with more refractory organic material derived from lateral transport

Food from surface to bottom:

Very little (~1% or less) primary produced material reaches sea floor; follows seasonal productivity (‘fresh phytodetritus)

Ballasted by silica (diatoms), carbonate (foraminifera), dust; in fecal pellets; in glutinous material (diatoms, cyanobacteria); in ‘giant balls of mucus’, larvacean (tunicate) houses; carrion falls (‘dead whales’); lateral transport (refractory organic matter)

Discrepancy between food requirements of faunas and supply in sediment traps: faunas need more than what is delivered

In the present world we thus see bentho-pelagic coupling, in which the benthic faunas reflect what happens at the ocean surface where their food is produced.

Jorissen et al., 1995: TROX model (TR - trophic, food; OX - oxygen). Food is main limiting/determining factor in low food regions, where all organic matter is used up at the sediment/water interface. In such regions there is no food for infaunal ) within sediment) species. In very high food regions, the foraminifera and other organism do not eat everything raining down, and the sediment pore waters become anoxic (oxidation of some organic matter); here also foraminifera can not live within the sediment. In mesotrophic regions foraminifera may live down until 10-15 cm, with epifaunal, shallow infaunal, middle infaunal and deep infaunal taxa.



Benthic Foraminifera in the deep oceans: is the present the key to the past?

Can we use present benthic foraminifera as source for proxy information on such environmental parameters as temperature, salinity, depth, primary productivity, oxygenation? If  we can, is that information valid for reconstruction of past environments?

Benthic Foraminifera in the DEEP oceans:

Benthic forams: what is proxy for what?

  1. Planktic/Benthic ratio: paleodepth, dissolution, surface productivity
  2. Benthic Foraminiferal Accumulation Rate: surface productivity
  3. Species % abundance, Species Diversity: paleodepth, oxygenation of bottom waters, productivity, seasonality of productivity, labile/refractory organic matter, water masses, current activity, CaCO3 corrosivity
  4. Morphotypes’: microhabitats (infaunal/epifaunal) oxygenation, productivity

1. Planktic/Benthic

How to distinguish planktic and benthic foraminifera:

2. Benthic Foraminiferal Accumulation Rate (BFAR): how does ‘food reaching bottom’ relate to ‘primary productivity’?

3.a Species % abundance

3b. Species Richness, Diversity

4. ‘Morphotypes’: Infaunal/Epifaunal

How to define infaunal and epifaunal:

Average Living Depth (ALD10), Jorissen et al., 1995:

Example of difficulties: e.g., what is environmental significance of faunas dominated by small, thin-walled specimens?

Just on faunas, not possible to decide which is most important factor in specific case

Cenozoic benthic foraminiferal events:

Note: diversity high globally in greenhouse world, drops, and diversity gradient may have been established at formation on Antarctic ice sheets (Thomas and Gooday, 1996; Thomas et al., 2000).  Various groups of common deep-sea benthic foraminifera (Epistominella exigua, indicator of fresh phytodetritus deposition; Nuttallides umbonifera, indicator of AABW) only became common at Eocene-Oligocene transition (establishment Antarctic ice cap.

What is typical in these non-analog ‘Greenhouse’ faunas?    

Greenhouse Faunas Contradiction”:

Higher primary productivity on sea floor

K/T boundary: no benthic foram extinction (Culver, 2003)

WHY no serious consequences of collapse productivity’ on food-starved deep-sea biota, in presence of bentho-pelagic coupling?

Paleocene/Eocene Benthic Foraminiferal Extinction Event:

What caused the global benthic foraminiferal extinction?

Cenozoic benthic foraminiferal faunas:

My personal opinion on some points: