This article was originally published in the Encyclopedia of Quaternary Science published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who you know, and providing a copy to your institutions administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institutions website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial Rohling E.J. (2013) Oxygen Isotope Composition of Seawater. In: Elias S.A. (ed.) The Encyclopedia of Quaternary Science, vol. 2, pp. 915-922. Amsterdam: Elsevier. © 2013 Elsevier Inc. All rights reserved. Author's personal copy Oxygen Isotope Composition of Seawater E J Rohling , The Australian National University, Canberra, ACT, Australia ã 2013 Elsevier B.V. All rights reserved. Introduction different isotopes impose subtle differences in their physico- chemical properties. The mass differences are particularly This article deals with processes that affect the ratio of the two important in light elements (low numbers in the Periodic most common stable isotopes of oxygen in seawater and ex- Table). Molecules vibrate with a fundamental frequency plores ways in which this has changed during the Quaternary. which depends on the mass of the isotopes from which they Both text and figures draw heavily on the water-based oxygen are composed. The resultant differences in dissociation energy isotope part of a previously published essay on stable oxygen of the light and heavy isotopes imply that bonds formed by and carbon isotopes in Foraminifera (Rohling and Cooke, light isotopes are weaker than those formed by heavy isotopes. 1999). In contrast to the current text, which offers only the Hence, as a rule of thumb, molecules comprised of the light fundamental key references, the essay of Rohling and Cooke isotopes react somewhat more easily than those comprised of (1999) contains a very extensive set of references, and would the heavy isotopes. therefore be a useful first step for further and more specialized The partitioning of isotopes between substances with dif- reading. ferent isotopic compositions is called ‘fractionation.’ The frac- tionation factor (a), which quantifies isotopic fractionation between two substances A and B, is defined as a RA/RB. ¼ Here, RA and RB are the heavy/light ratios between the abun- Natural Abundance, Measurement, and 18 16 dances of any two isotopes (e.g., O/ O) in the exchanging Physicochemical Behavior chemical compounds A and B, respectively. Fractionation mainly results from: (1) isotope exchange reactions and (2) Oxygen exists in nature in the form of three stable isotopes: kinetic effects. Isotope exchange reactions concern partitioning 16O, 17O, and 18O. Their relative natural abundances are 99.76, of isotopes between phases that are in equilibrium, and are 0.04, and 0.20%, respectively. Because of the higher abun- therefore also known as ‘equilibrium isotope fractionation’ dances and the greater mass difference between 16O and 18O, processes. Processes of equilibrium fractionation are essen- research on oxygen isotope ratios normally concerns 18O/16O tially temperature dependent. Kinetic effects cause deviations ratios. from the simple equilibrium processes due to different rates of The word isotope (Greek, meaning ‘equal places’) implies reaction for the various isotopic species (due directly to vibra- that the various isotopes occupy the same position in the tion differences, or indirectly through differences in bonding Periodic Table. The difference between the atomic masses of energies). Important kinetic effects are associated with the isotopic species for each element consists of a different diffusion. number of neutrons in the nucleus, but isotopes have the 16 same number of protons. The oxygen isotope 8O contains 8 protons and 8 neutrons, giving it an atomic mass of 16. The 18 isotope 8O contains 8 protons and 10 neutrons, giving it an Processes Controlling Oxygen Isotope Ratios in atomic mass of 18. Seawater While the absolute abundances of minor isotopes (such as 18 O) cannot be determined accurately, it is still possible to get The oxygen isotope ratio of seawater is intimately linked with quantitative results by comparing the result given for a known fractionation processes within the hydrological cycle. Schemat- external standard with that for the unknown sample. These ically, this cycle is comprised of evaporation, atmospheric differences in isotope ratios, known as d values, are defined as: vapor transport, precipitation, and subsequent return of fresh- water to the ocean (directly via precipitation and via runoff or Rsam Rstd dsam À 1000 iceberg melting). Long-term storage of freshwater in aquifers ¼ Rstd  and especially ice sheets is also important for seawater isotope Here, sam is the sample value and std is the standard or ratios (Figure 1). Formation and melting of seasonal sea ice reference value. These variations in composition are given in imposes strong local variability. Finally, the spatial distribution delta (d) notation and are reported in parts per thousand (per of oxygen isotopes in the world’s oceans depends on processes mille, %). R stands for the heavy/light ratio between the abun- of advection and mixing of water masses from different source 18 16 dances of any two isotopes (e.g., O/ O). A positive d value regions with different isotopic signatures. The various influ- indicates enrichment in the heavy isotope, relative to the stan- ences are discussed below, but it should be noted that this dard, and conversely, depletion is shown by a negative d value. forms only a basic introduction, and specialist texts and re- The standard used today in the analysis of d18O in water is the views should be consulted before advanced applications. Key Vienna Standard Mean Ocean Water (VSMOW). texts and overviews can be found in Craig and Gordon (1965), All isotopes of a given element contain the same number Dansgaard (1964), Garlick (1974), Gonfiantini (1986), Hoefs and arrangement of electrons, and so broadly display similarity (1997), Hoffmann and Heimann (1997), Joussaume and in their chemical behavior. However, the mass differences of Jouzel (1993), Jouzel et al. (1975), Majoube (1971), Merlivat 915 Encyclopedia of Quaternary Science, (2013), vol. 2, pp. 915-922 Author's personal copy 916 PALEOCEANOGRAPHY, PHYSICAL AND CHEMICAL PROXIES | Oxygen Isotope Composition of Seawater Rayleigh distillation Precipitation Predominantly equilibrium Precipitation effects Equilibrium and kinetic (snow formation) effects Evaporation Equilibrium and kinetic effects Ice storage Iceberg calving Runoff (104–105 years) Enrichment Depletion Depletion from melting Groundwater storage 4 (10 years) Storage effects ‘fix’ 16O: cause 18O enrichment throughout oceans Figure 1 Schematic presentation of the hydrological cycle influences on oxygen isotope ratios. Effects on seawater are described in italics (reproduced from Rohling EJ and Cooke S (1999) Stable oxygen and carbon isotope ratios in foraminiferal carbonate. In: Sen Gupta BK (ed.) Modern Foraminifera, pp. 239–258. Dordrecht, The Netherlands: Kluwer Academic). The ‘fix’ comment refers to the storage of preferentially 16O-enriched precipitation in ice sheets and groundwater, which constitutes a preferential removal of 16O from the oceans and thus a relative 18O-enrichment in the oceans. and Jouzel (1979), Rohling and Cooke (1999), Rozanski et al. Since diffusion rates depend on property gradients, the final (1982, 1993), and Stewart (1975). isotopic composition of newly evaporated water also depends 18 on the isotopic composition (d Oatm) of vapor already pre- sent in the turbulent region of the atmosphere, and on d18Oof Evaporation the surface water (Gonfiantini, 1986): The isotopic exchange at the sea–air interface is given by: 18 18 1 d OSea surface e 18 d OE À hd Oatm De 16 18 18 16 ¼ 1 h De a À À H2 Oliquid H2 Ovapor H2 Oliquid H2 Ovapor À þ þ , þ Here, the 18O, , and values, normally reported in , are to d De e % Molecules composed of lighter isotopes have higher vapor 3 be used in true form, that is, value 10À . It will be obvious pressures and the lighter molecular species are therefore pref-  that the preferential uptake of the lighter isotope during evap- erentially enriched in the vapor phase. The fractionation factor 18 18 16 18 16 oration causes a shift to heavier d O values in the remaining for the equilibrium exchange is al v [ O/ O]l/[ O/ O]v. À ¼ surface waters. The most commonly used relationship between al v and tem- À perature during evaporation is that given by Majoube (1971): 2 3 1 3 al v exp 1:137 TÀ 10 0:4156 TÀ 2:0667 10À Precipitation and Atmospheric Vapor Transport À ¼ fð Þ Àð ÞÀ  g where T is in Kelvin. This relationship illustrates a decrease Fractionation processes during the formation of droplets are in fractionation with increasing temperature. The fractionation basically the same as during evaporation, but work in the causes a difference between d18O of seawater and d18O of vapor opposite direction. Droplets are normally near equilibrium 18 18 3 evaporated from that seawater equal