Modern Foraminifera

Large-scale biogeographic distributions of benthic Foraminifera of the world’s shelves exhibit a pattern that indicates a major controlling role for temperature at this scale. Cold-, temperate-, and warm-water groupings of provinces have been recognized around the world, with a greater number of provinces delineated in warmer regions. Modern provincial patterns, however, have also been influenced by differences in regional geological histories.

The continental margins of North and Central America have been subject to the most intensive biogeographic investigations, and eleven benthic foraminiferal provinces have been recognized in the neritic zone. Their distribution and boundaries can be closely related to the distribution and boundaries of major water masses. Thus, the provincial scheme for modern benthic Foraminifera is similar to that for modern shallow marine macroorganisms. The differences suggest that benthic Foraminifera may be somewhat less sensitive to the changes in the marine climate than other benthic organisms of the neritic zone.

No center or centers of origin can be identified for the modern benthic Foraminifera of the North American Atlantic continental margin. First stratigraphic occurrences are both temporally and geographically widespread, and suggest rapid dispersal.

There are several important unresolved issues in the area of modern planktonic foraminiferal biogeography. The large-scale latitudinally symmetrical faunal provinces do not appear to show a consistent relationship to comparably-scaled hydrographic features. The origins of these provinces are therefore likely to be understood by reference to other causal factors. The first of these is the degree to which smaller-scale hydrographic features such as current, gyre, and frontal systems combine to play a role in determining larger-scale latitudinal distribution patterns. The second is the role historical processes — particularly, latitude-dependent rates of speciation and extinction, and the development of tectonic barriers — have played in establishing faunal provinces and the global diversity gradient. A number of additional factors, including depth habitats, bipolarity, pore size, coiling direction, and iterative evolution, are indirectly related to physicochemical characteristics of the foraminiferal habitat.

As our understanding of the basic governing principles of modern foraminiferal biogeography develops, it becomes easier to interpret the fossil record in the light of those principles. By the same token, modern distribution patterns, since they are partly the result of historical processes, cannot be fully explained without reference to their past development. We are so accustomed to hearing that the present is the key to the past that we sometimes forget that the past can also be the key to understanding the present. It follows that biogeographic studies of modern and fossil Foraminifera must necessarily proceed hand-in-hand.

Hundreds of known benthic foraminifer species live in coastal marine environments. Most of them are rare. The dominant species are widely distributed, many across major biogeographic barriers. The transoceanic distribution of some abundant marsh and estuarine species is hard to explain, except by accidental transport and high tolerance to environmental variables. The transition from a brackish to a normal-marine nearshore fauna is generally marked by increases in species diversity and the proportion of calcareous species in the community. A few calcareous genera are represented by the same or sibling species on the soft, clastic substrates of many inner continental shelves, spanning large latitudinal and longitudinal ranges. Hard substrates and marine vegetation in the tropics support a large variety of taxa, including nearly all living species of larger Foraminifera. With a few exceptions, the biogeographic imprint on nearshore, open-marine faunas is best seen in the composition of the entire assemblage, rather than in the presence or absence of a few dominant species.

In summary, certain benthic Foraminifera from various water depths inhabit oxygen-poor and even anoxic environments. It is established that at least some foraminiferal species survive anoxia and even sulfidic conditions for periods up to a few weeks, but the tolerance of most species to oxygen depletion is unknown. Furthermore, the physiological mechanisms enabling foraminiferal species to survive exposure to anoxia and/or sulfidic conditions are not yet identified. The available data suggest, however, that all Foraminifera are aerobic for at least part of their life, and, in all likelihood, some species are facultative anaerobes. Obligate anaerobes have not been identified among foraminiferal species. The information necessary to understand the diverse aspects of foraminiferal adaptation to oxygen-depleted environments must come from experimental studies. Only with such biological information, it will be possible to construct more accurate databases for use in other disciplines such as paleoecology and paleoceanography.

Trace elements in foraminifer shells are controlled by seawater composition and the physical and biological conditions during calcification. Because it is possible to calibrate shell composition against the controlling factors, foraminiferal trace elements provide researchers with a toolbox of powerful proxies to investigate the chemical, physical, and biological evolution of the oceans. Some of the most important challenges faced by trace element researchers are to (1) establish precise relationships among shell chemistry, seawater composition, and physical and biological factors; (2) to apply elemental tracers to the past and take into account the range of potential complications that affect proxy interpretation; and (3) to establish which secondary factors such as diagenesis may affect post-depositional shell chemistry. With a precise knowledge of the factors that control shell composition, and in conjunction with stable isotopes, foraminiferal trace elements should provide a very powerful tool to study ocean and climate evolution.