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Oxygen Isotopes in Foraminifera: Overview and Historical Review

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Oxygen Isotopes in Foraminifera: Overview and Historical Review OXYGEN ISOTOPES IN FORAMINIFERA: OVERVIEW AND HISTORICAL REVIEW PAUL N. PEARSON School of Earth and Ocean Sciences, Main Building, Cardiff University, Park Place, Cardiff, CF10 3AT, United Kingdom [email protected] ABSTRACT.—Foraminiferal tests are a common component of many marine sediments. The oxygen iso- tope ratio (δ18O) of test calcite is frequently used to reconstruct aspects of their life environment. The δ18O depends mainly on the isotope ratio of the water it is precipitated from, the temperature of calcifica- tion, and, to a lesser extent, the carbonate ion concentration. Foraminifera and other organisms can poten- tially preserve their original isotope ratio for many millions of years, although diagenetic processes can alter the ratios. Work on oxygen isotope ratios of foraminifera was instrumental in the discovery of the orbital theory of the ice ages and continues to be widely used in the study of rapid climate change. Com- pilations of deep sea benthic foraminifer oxygen isotopes have revealed the long history of global climate change over the past 100 million years. Planktonic foraminifer oxygen isotopes are used to investigate the history of past sea surface temperatures, revealing the extent of past 'greenhouse' warming and global sea surface temperatures. INTRODUCTION Note: A number of textbooks provide succinct accounts of oxygen isotope systematics and THE MEASUREMENT of oxygen isotope ratios measurement; this section is based mainly on of biogenic calcite is one of the longest- Faure and Mensing (2005), Allègre (2008), and established and most widely used of all paleocli- Hoefs (2009). mate proxies. It principally provides information Oxygen has three stable isotopes, 16O, 17O, on the temperature or oxygen isotope ratio of and 18O, which occur on Earth in the approximate seawater at the time of calcification if the other proportions 99.757%, 0.038%, and 0.205%, re- parameter is known or assumed. The signal can, spectively (Rosman and Taylor, 1998; other in principle, survive for hundreds of millions of sources give slightly different figures). These dif- years in fossils. Although many types of organ- ferent abundances reflect the fact that the three isms produce calcite skeletons, foraminifera have isotopes are produced by different synthetic path- been employed particularly widely because of ways in stars. The proportions vary somewhat in their abundance and diversity in marine sediment, natural Earth materials because each substance especially deep-sea oozes where many of the has its own prior history of fractionation (proc- longest and most continuous paleoclimate records esses that sorted or partitioned the isotopes) and are found. Here, the development of the proxy in mixing (processes that combined or assimilated both benthic and planktonic foraminifera is re- the isotopes). Fractionations occur in two main viewed in two parts. Part 1 is an overview of the ways, isotope-exchange reactions and kinetic ef- principles of the technique and its early develop- fects. Because molecules with a heavy isotope ment, together with some of its complications and have slightly greater covalent bond strengths and limitations. Part 2 outlines some of the major ap- lower vibrational frequencies than their lighter plications in paleoclimate studies from the 1970s counterparts, they are slightly less reactive. They to the present. are also slower to diffuse along concentration gradients and across membranes. Examples of PART 1: PRINCIPLES AND HISTORY fractionation processes that affect oxygen isotopes in water are evaporation, in which the light iso- Oxygen isotopes tope 16O is slightly preferred, and condensation, In Reconstructing Earth’s Deep-Time Climate—The State of the Art in 2012, Paleontological Society Short Course, November 3, 2012. The Paleontological Society Papers, Volume 18, Linda C. Ivany and Brian T. Huber (eds.), pp. 1–38. Copyright © 2012 The Paleontological Society. THE PALEONTOLOGICAL SOCIETY PAPERS, VOL. 18 where the heavy isotope 18O is slightly preferred. torical reason for the adoption of this is described The physical mixing of two water masses with below). The original standards were simply different isotopic ratios (e.g. when a river flows SMOW and PDB, but because original supplies of into the sea) will produce water with an interme- both standards ran out and some inter-laboratory diate ratio. differences emerged in defining what the precise Because most natural fractionation and mix- values were relative to other available reference ing processes are strictly mass dependent, the iso- materials, a worldwide convention was deter- topes 17O and 18O fractionate and combine rela- mined (at a meeting in Vienna) (Gonfiantini, tive to 16O proportionally according to their re- 1984; see Coplen, 1994, for further discussion). spective masses (18O fractionating twice as much Deviations from VSMOW tend to be used in stud- as 17O), so for most applications, there is little to ies of the hydrological cycle and also for high- be gained by measuring both isotopes. The normal temperature processes, such as in metamorphic procedure is to measure the ratio of 18O to 16O in a rocks, and studies of phosphates and silica. Devia- sample. By convention, the isotope ratio, R, is tions from VPDB tend to be used in studies of defined as the abundance of the heavier isotope low-temperature carbonates, including foramini- over the abundance of the lighter isotope. For the fera. Note that carbon isotope ratios in carbonates, global average proportions given above, this is 13C/12C, are also quoted relative to the same expressed as follows: VPBD standard (as δ13C ‰). 18O 0.205 The oxygen isotope ratios of carbonate sam- R = = = 0.002055 ples are usually measured using a gas source mass 16O 99.757 spectrometer. The sample is reacted with 100% phosphoric acid, producing CO2 gas that is ion- Because natural fractionations are, in practice, ized in a vacuum chamber by electron bombard- quite small, and 18O is always very much rarer ment. The mass spectrometer accelerates the ions than 16O, the isotopic ratios of natural materials under high voltage, and a magnetic field splits are generally quite close to this average value. them into streams of different isotope ratio that Small differences between small numbers are un- generate electrical currents in the detectors. The wieldy, so (as is the convention for other stable ratio of these currents is proportional to the iso- isotope systems) oxygen isotope ratios are gener- tope ratio of the sample. By alternately switching ally quoted as deviations (delta values) from the between a standard of known isotope ratio (gener- 18 oxygen isotope ratio (δ O) of a standard sub- ally a CO2 reference gas supplied by the National stance in parts per thousand ('per mil', sometimes Institute of Standards and Technology) and an ‘per mille’, signified by the symbol, ‘‰’). unknown sample, the isotope ratio of the un- known can be calculated. In this way, the follow- ing masses are usually measured: 12C16O16O Rsample - Rstandard (mass 44), 13C16O16O (mass 45), and 12C18O16O δ18O = x 1000 (mass 46) (higher masses are created from other Rstandard combinations of 13C and 18O, but these are rare and not routinely measured). From these ratios, the δ18O and δ13C of the sample are calculated There are also good, practical reasons for this simultaneously and quoted relative to VPDB. In convention because isotope ratios of sample mate- reality, some complications have to be taken into rials are almost always measured relative to a account, notably subtraction of the contribution of laboratory standard rather than as an absolute ra- molecules containing 17O to the above masses, tio, which is much more difficult to determine and the effect of isotopic fractionation associated accurately. Note that if a sample has a positive with the phosphoric acid reaction. δ18O, it is said to be enriched in the heavy isotope relative to the standard and, if negative, it is said to be depleted. Foraminiferal calcite It so happens that two different standard val- Foraminifera are single-celled eukaryotic or- ues are in widespread use from which these delta- ganisms belonging to the Phylum Granuloreticu- values are quoted: Vienna Standard Mean Ocean losa, which comprises amoeboid organisms char- Water (VSMOW) and Vienna Pee Dee Belemnite acterized by pseudopodia that have a granular (VPDB, which is a carbonate standard; the his- texture to the flowing cytoplasm (Lee et al., 2 PEARSON: OXYGEN ISOTOPES IN FORAMINIFERA FIGURE 1.—Foram art: calcite tests of selected benthic (left) and planktonic (right) foraminifera. These are from exceptionally well-preserved Paleogene sediments of Tanzania (33–45 Ma). Scale is approximate; diameters are from about 0.20–0.75 mm. Images: P. N. Pearson and I. K. McMillan. Note the literature is split between those who use the adjective 'planktonic' versus 'planktic' and between those who use 'benthonic' versus 'benthic'. It so happens that planktonic and benthic are clearly in the ascendancy as of 2012, by roughly 10:1 and 30:1 respectively, as shown by a word search on abstracts. It has been argued that planktic is the correct Greek form of the adjective (Rodhe, 1974; Emiliani, 1991) but it has been pointed out that planktonic, like electronic, is perfectly good English, however ugly it may be in Greek (Hutchinson, 1974). The majority usage is followed here. 2000). Most foraminifera are marine, and many cessive chambers added episodically throughout secrete a test (or shell) made of calcium carbonate life, starting with a first-formed chamber (prolo- (CaCO3; generally low-Mg calcite, but high-Mg culus). The chambers all have small openings or calcite in porcelaneous species and aragonite in foramina (singular: foramen) that provide internal some groups). Some foraminifera live among the connections between them and allow cytoplasm ocean plankton distributed in the upper part of the flow inside and outside the test (hence 'foramini- water column; others are benthic, living directly fera', which means 'bearers of foramina').
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