Foraminifera • Foraminifers are found in all marine environments. Some are adapted to brackish water. There are none in fresh water. Two life modes are recognized: • Benthic . These are forms that live on the sea floor, either on the surface of the sediment (epifauna) or buried in it (infauna). • Planktonic . These are forms which float passively, moved only by currents but capable of vertica migration. Some planktonic foraminifers (their largest dimension on the order of a half-millimeter). Small benthic foraminifers (their largest dimension on the order of a half-centimeter). Some large benthic foraminifers (their largest dimension on the order of a centimeter). Planktonic Foraminifera Globigerina bulloides Globorotalia menardii d'Orbigny (Parker, Jones and Brady) Pliocene-Recent Pliocene-Recent Globigerina bulloides d'Orbigny Hantkenina alabamensis Cushman, 1927 Pliocene-Recent Eocene Globigerinoides ruber Pseudohastigerina micra d'Orbigny (Cole, 1927) Miocene-Recent Eocene-Oligocene Globorotalia inflata Globorotalia cerro- d'Orbigny azulensis Cole, 1928 Pliocene-Recent Eocene Globorotalia menardii Parasubbotina (Parker, Jones and pseudobulloides Brady) (Plummer, 1926) Pliocene-Recent Lower-Middle Palaeocene Planktonic Foraminifera Racemiguembelina Subbotina triloculinoides fructicosa (Egger) (Plummer, 1926) Middle-Uppper lower-upper Palaeocene Maastrichtian (Upper Cretaceous) Pseudotextularia elegans (Rzehak) Abathomphalus mayaroensis (Bolli) Campanian- Maastrichtian (Upper Upper Maastrichtian Cretaceous (Upper Cretaceous) Hedbergella delrioensis (Carsey) Coniacian-Santonian Globotruncana linneiana (Upper Cretaceous) (d'Orbigny) Santonian-Maastrichtian Heterohelix globulosa (Upper Cretaceous) (Ehrenberg) Coniacian-Maastrichtian (Upper Cretaceous) Benthonic Foraminifera (calcareous wall) Spiroloculina ornata Nodosaria sp. (d'Orbigny) Jurassic -Recent Elphidium macellum Vaginulina bernardi (Fichtel and Moll) Paalzow -Recent Oxfordian (Jurassic) Brizalina alata (Seguenza) Quinqueloculina impressa Reuss -Recent ?-Eocene Lagena striata (d'Orbigny) Nonionella opima ??-Recent (Cushman) Cyclamina acutidorsata ??-Recent (von Hantken) Oligocene-Miocene Benthonic Foraminifera (agglutinated wall) Larger Benthic Discocyclina (Discocyclina) marginata (Cushman) ?Eocene Discocyclina sp. Gumbel Nummulites sp. Lamarck Mid Paleocene-Upper Paleocene-Holocene Eocene Alveolina sp. d'Orbigny Upper Paleocene-Upper Eocene • Classification Foraminifera are classified primarily according to the composition and morphology of the test. Three basic wall compositions are recognised, organic (protinaceous mucopolysaccharide i.e. the allogromina), agglutinated and secreted calcium carbonate (or more rarely silica): • Agglutinated forms, i.e the Textulariina, may be composed of randomly accumulated grains or grains selected based on specific gravity, shape or size; some forms arrange particular grains in specific parts of the test. • Secreted-test foraminifera are again subdivided into three major groups, microgranular (i.e. Fusulinina), porcelaneous (i.e. Miliolina) and hyaline (i.e. Globigerinina). Microgranular wall forms (commonly found in the late Paleozoic) are composed of equidimensional subspherical grains of crystalline calcite. Porcelaneous forms have a wall composed • of thin inner and outer veneers enclosing a thick middle layer of crystal laths, they are imperforate and made from high magnesium calcite. The hyaline foraminifera add a new lamella to the entire test each time a new chamber is formed; various types of lamellar wall structure have been recognized, the wall is penetrated by fine pores and hence termed perforate. A few "oddities" are also worth mentioning, the Suborder Spirillinina has a test constructed of an optically single crystal of calcite, the Suborder Silicoloculinina as the name suggests has a test composed of silica. Another group (the Suborder Involutina) have a two chambered test composed of aragonite. The Robertinina also have a test composed of aragonite and the Suborder Carterina is believed to secrete spicules of calcite which are then weakly cemented together to form the test. Foraminiferal suborders and their envisaged phylogeny. Redrawn from Tappan and Loeblich (1988). Among the suborders shown only the Fusulinina are extinct. Applications • As previously mentioned, foraminifera have been utilized for biostratigraphy for many years, and they have also been valuable in paleoenvironmental reconstructions, most recently for paleoceanographic and paleoclimatologic purposes. For example palaeobathymetry, where assemblage composition is used and paleotemperature where isotope analysis of foraminiferal tests is a standard procedure. In terms of biostratigraphy, foraminifera have become extremely useful, different forms have shown evolutionary bursts at different periods and generally if one form is not available to be utilized for biostratigraphy another is. For example preservation of calcareous walled foraminifera is dependent on the depth of the water column and Carbonate Compensation Depth (the depth below which dissolution of calcium carbonate exceeds the rate of its deposition), if calcareous walled foraminifera are therefore not preserved agglutinated forms may be. • The oldest rocks for which foraminifera have been biostratigraphically useful are Upper Carboniferous to Permian strata, which have been zoned using the larger benthic fusulinids. Planktic foraminifera have become increasingly important biostratigraphic tools, especially as petroleum exploration has extended to offshore environments of increasing depths. The first and last occurrences of distinctive "marker species" from the Cretaceous to Recent (particularly during the Upper Cretaceous) has allowed the development of a well-established fine scale biozonation. • Benthic foraminifera have been used for paleobathymetry since the 1930's and modern studies utilize a variety of techniques to reconstruct paleodepths. For studies of relatively recent deposits simple comparison to the known depth distribution of modern extant species is used. For older material changes in species diversity, planktic to benthic ratios, shell-type ratios and test morphology have all been utilized. Variations in the water temperature inferred from • oxygen isotopes from the test calcite can be used to reconstruct paleoceanographic conditions by careful comparison of changes in oxygen isotope levels as seen in benthic forms (for bottom waters) and planktic forms (for mid to upper waters). This type of study has allowed the reconstruction of oceanic conditions during the Eocene- Oligocene, the Miocene and the Quaternary. • Benthic foraminifera have been divided into morphogroups based on the test shape and these groups used to infer paleo-habitats and substrates; infaunal species tending to be elongate and streamlined in order to burrow into the substrate and epifaunal species tending to be more globular with one relatively flatter side in order to facilitate movement on top of the substrate. It should be remembered, however, that a large variety of morphologies and possible habitats have been recognized making such generalizations of only limited use. Studies of modern foraminifera have recognized correlations between test wall type (for instance porcelaneous, hyaline, agglutinated), paleodepths and salinity by plotting them onto triangular diagrams. Biostratigraphic interest of foraminifera • The chronostratigraphic range of the foraminifers extends from the Early Cambrian to the current epoch (Fig. 5). The first forms to appear had organic tests or were simple agglutinated tubes. Until Devonian times species are rare. Microgranular calcitic tests are at a maximum in the Carboniferous and Permian with the development of the Order Fusulinida that disappears at the end of the Paleozoic. In Carboniferous strata porcelaneous tests appear with the Cornuspirids. During Mesozoic times they are followed by many-chambered agglutinated tests (Lituolids in the Jurassic and Orbitolinids in the Cretaceous) and by the radiation of hyaline tests (among which are the Nodosariids in the Jurassic). The forms first to appear are all benthic. Planktonic forms do not exist before the Jurassic. The high sea levels and warm climates of the Cretaceous favored diversification of plankton but many of the numerous forms were decimated by the major extinctions at the end of the Mesozoic. An evolutive explosion occurred at the base of the Cenozoic with the appearance of morphotypes similar to those of existing plankton. The larger benthic foraminifers also diversified in the Paleogene with the Alveolinids, the Nummulites and the "orbitoidiforms". Fig. 5. Stratigraphic ranges of some families of foraminifers (after Bignot, 2001, modified). • The "orbitoidiforms" have disappeared at the beginning of the Neogene. The diversity of plankton that diminished at the end of the Cretaceous period was reversed by brief recrudescence in the warm periods of the Paleogene and Neogene. The distribution of test types and families during time cannot be used to construct a detailed stratigraphic scale. Precision can be attained only by working at the species level and also requires on a very detailed knowledge of the phyletic relationships of the species (microevolution) in order to calibrate as exactly as possible the ranges of the several taxa. • Biostratigraphy is the study of sedimentary strata based on their fossil content, rather than on lithology or other geologic parameters. Micropaleontologists divide sedimentary