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= Foraminifera as a Climate Proxy = Foraminifera (informally referred to as "forams") are useful climate proxies due to their distinct speciation and broad temporal and spatial distribution. During their lifetime, foraminifera intake seawater and precipitate calcareous shells. Upon death, they sink to the seafloor and preserve the seawater isotopic composition. Foraminiferal isotopic measurements were first utilized as a climate proxy by Cesare Emiliani in 1955.

Sample Collection Methods
There are three main methods for collecting foraminifera samples for research.

Global expeditions are taken to various oceanic sites to collect ancient samples from deep sea sediments. Oceanic sediments retrieved on these expeditions in drill cores are sieved to separate sediment grains into size categories ranging from 150μm to 400μm. Foraminifera are identified, removed from the sediment, and organized by species. Different species of foraminifera exhibit distinct isotopic signatures depending on the depth at which they lived in the water column.

For modern samples, researchers can culture foraminifera in seawater tanks while controlling temperature and pH. Foraminifera can also be harvested from the ocean directly.

Modern samples are often compared to ancient samples as modern seawater conditions can be measured while ancient seawater conditions must be inferred.

Once collected, foraminifera are cleaned and crushed in bulk into a fine powder and their isotopic compositions are measured through mass spectrometry.

Mg/Ca and δ18O Thermometry
Mg/Ca ratios can be used to infer seawater temperature as the incorporation of Mg in shells is temperature dependent. Combining this with different species of foraminifera allows one to reconstruct seawater temperature throughout an entire water column and its evolution through time. This can be used to infer the precipitation regime, ocean mixing, the magnitude and frequency of glacier and interglacial events, and the timing of the opening and closing of seaways. An important caveat is that other factors affect the incorporation of Mg/Ca beyond temperature. The uptake of δ18O is also affected by temperature. Mg/Ca and δ18O can be measured in the same phase and sample to compare observed trends.

Boron
Studies to further characterize the ocean and carbonate system have made use of boron isotopes from foraminifera, which incorporate boron into their tests (shells). Globigerinoides ruber and Globigerinoides sacculifer are two species which are abundant in the water column and are widely used in boron isotope analysis.

The resonance time of boron is in the order of millions of years and is beneficial when looking at broad time periods over various glacial cycles. Both δ11Β and B/Ca ratios for useful for climate science research.

Boron isotope (δ11Β) composition in benthic and planktonic foraminifera has been established as a proxy for seawater pH and is used to reconstruct atmospheric CO2. This is applicable to areas where seawater CO2 composition is similar to its corresponding atmospheric composition. The pH of seawater is responsible for variance in isotope composition from borate species and studies have observed a direct relationship between a decrease in δ11Β with increasing depth.

Planktonic foraminifera have been used for borate/bicarbonate values [B(OH)4-/HCO3-] and benthic foraminifera for deep water [CO32-] reconstructions. Boron/bicarbonate ratios, converted from B/Ca ratios, are indicators in change of alkalinity, dissolved inorganic carbon and pH.

Factors Which Affect Isotopic Signatures
The use of foraminifera in paleoclimate reconstructions is dependent on the assumption that isotopic composition of foraminifera shells reflects the isotopic composition of seawater in which the calcareous shell was formed. However, in order to accurately interpret stable isotope data obtained from foraminifera in deep-sea sediments, certain environmental and biological factors must be considered.

Post depositional alteration of sediments in which foraminifera are preserved through processes such as bioturbation can have a significant impact on how stable isotopic signatures are interpreted. Bioturbation of oceanic sediments by mobile benthic organisms can result in mixing of fossil assemblages of foraminifera. This places a limit on the temporal resolution of foraminiferal stable isotope data and can obscure the distinction between measurements indicating gradual changes in paleonvioronmental conditions and measurements of rapid changes which have been “smoothed out” by sediment mixing. Diagenetic effects such as selective dissolution of foraminifera tests can also influence measured stable isotope values.

Biological factors such as species specific preferences for depth and seasonal habitats can also impact isotopic signatures. Benthic foraminifera may display an oxygen isotope signature depleted in δ18O as a result of lowered pH in water present in the pore space of sediment where the organisms lived. Interpreting isotopic data can be complicated further as a result of biological processes which cause foraminifera to calcify out of isotopic thermodynamic equilibrium with the surrounding seawater. For example, oxygen isotope composition may vary as a function of shell size in species of foraminerifera harboring photosynthetic algal symbionts. Increased photosynthetic activity consumes carbon dioxide resulting in a greater calcification rate and a depletion in δ18O. These biological processes which can override the environmental conditions influencing isotopic signatures preserved in biominerals are known as vital effects. Understanding the source of vital effects in biominerals and how to account for them in measurements is essential to making accurate interpretations of stable isotope data for the purpose of paleoclimate reconstruction.

Carbonate Clumped Isotopes
Foraminifera are a common sample source for clumped isotope thermometry. Carbonate clumped isotope thermometry is a useful proxy because it is based on thermodynamic equilibrium rather than biogeochemical proxies which, as discussed above, may be subject to biological processes and the influence of fluid composition. Clumped isotopes are formed when heavy isotopes of carbon and oxygen clump together to form bonds within the carbonate mineral lattice. The formation of bonds between these heavy isotopes is temperature dependent, and therefore measurements of clumped isotopes can be used as a paleothermometer. Foraminifera clumped isotope measurements are used in conjunction with δ18O measurements to provide information about temperature as well as the oxygen isotopic composition of the water in which the test formed. Large abundances of foraminifera sample material are required for analysis to obtain precise results. To be used for clumped isotope analysis, sample foraminifera must be cleaned to prevent contamination and crushed into a fine powder.