Foraminifera

Foraminifera are marine eukaryotic unicellular organisms that construct a shell or test. They use chemicals from their surroundings to construct calcareous or siliceous crystals, or particulate grains to form an agglutinated test. They are heterotrophic protoctists with granular reticulopods (pseudopodial networks) exhibiting two-way streaming. Foraminifers are Linnean classified by their chemistry, mineralogy, structure of the test walls, cytology, and DNA of protoplasm.

Foraminifers can be either benthic or planktonic.

Benthic foraminifers, in the shape of simple agglutinated tubes, lived in the Cambrian (500 million years ago). Planktonic foraminifers appear in the fossil record during the Jurassic (206 million years ago). Both variants provide excellent fossil records and are the most diverse group of shelled marine microorganisms on Earth. Most live in specific environments and cannot survive drastic environment changes. Most foraminifers evolve relatively quickly and only range in the geological record for a short time (approximately 105 years), making them useful for developing theories on evolution, origination, extinction, and biogeographic distribution of past and present environments projected into the future.

The ecological controls on foraminifers depend on if they are bathyal-abyssal open ocean or shallow-water foraminifers. Benthic foraminifers are affected by the input of organic matter and other food sources from the surface layer 328 ft. (100 m), amount of available oxygen, sediment influx, and seafloor currents, salinity, and temperature. Benthic foraminifers are used to evaluate surface productivity in the ocean. Controls on planktonic foraminifers include salinity, temperature, upwelling, and the productivity of the surface layer. Planktonic foraminifers are used to reconstruct ocean currents, circulation, paleotemperatures, and large-scale shifts in Earth’s surface thermal regime.

Stable oxygen and carbon isotopes, as well as trace elements taken up in tests of foraminifers, are used reconstruct gross past climate trends and temperature cycles. Foraminifers are sampled from marine sediment cores from around the globe. They are correlated with the magnetic stratigraphy and biostratigraphy of other microfossil groups among the Atlantic, Pacific, Indian, and Arctic oceans. Oxygen and carbon in biogenic carbonates are determined by the mass ratios of CO2. 16O and 18O for foraminifers are used for fractionation comparison. δ18O in seawater is linked to the hydrologic cycle (evaporation, atmospheric vapor transport, freshwater return to the oceans by precipitation, runoff or melting of icebergs, and long-term storage in aquifers and ice sheets).

Lighter isotopes evaporate first and precipitate last. Therefore, the farther the precipitation occurs from source waters, the more depleted in 18O the vapor becomes. Temperature and salinity can be deduced from 18O values. There are many factors that affect the fractionation of oxygen in foraminifer tests, including ontogenics, symbiotic photosynthesis, respiration, gametogenic calcite, and carbonate ion concentrations. All of these factors need to be taken into account when interpreting

δ18O in planktonic foraminifers and sometimes in benthic foraminifers.

Carbon isotopes are derived from organic matter and sediment carbonate reservoirs. Surface waters are enriched in 13C because of the fractionation that occurs in photosynthesis. In deep water, δ13C is controlled by the amount of organic decay, time of exposure at the sea floor, and rate of decay of organic matter. Change in depth of calcification of foraminifers is also recorded. 

Trace elements are also used to reconstruct past oceanic conditions, providing independent validation of other proxies such as stable isotope values and ratios. The calcite test of a foraminifer is composed of 99 percent CaCO3, with the remainder comprised of trace elements. The composition of a test reflects seawater composition and the biological and physical conditions of the environment. Paleotemperature proxies (Mg), seawater nutrients, carbon and carbonate levels (Cd, Ba), physical properties such as temperature and pressure (Mg, Sr, F, B), history of ocean chemistry (Li, U, V, Sr, Nd), and secondary post-depositional processes such as CaCO3 and SiO2 diagenesis (Mn) are combined for the development of paleoceanographic reconstructions.