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Formation of the Ice Core Isotopic Composition Title Formation of the Ice Core Isotopic Composition Author(s) Ekaykin, Alexey A.; Lipenkov, Vladimir Ya. 低温科学, 68(Supplement), 299-314 Citation Physics of Ice Core Records II : Papers collected after the 2nd International Workshop on Physics of Ice Core Records, held in Sapporo, Japan, 2-6 February 2007. Edited by Takeo Hondoh Issue Date 2009-12 Doc URL http://hdl.handle.net/2115/45456 Type bulletin (article) Note IV. Chemical properties and isotopes File Information LTS68suppl_022.pdf Instructions for use Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP Formation of the Ice Core Isotopic Composition Alcxcy A. Ekaykin·· •• , Vladimir Va. Lipcnkov" '" Hokkaido Ulliversily, Sapporo, Japan, ekaykill@aari,lIl1'.ru •• Arctic and All/arctic Research Instilule, SI. Petersburg. Russia. [email protected] Abstract: Main processes of the ice core isotopic In this work we overview the processes leading to the composition formation are overviewed. Theory of formation of the vertical profile of isotopic composition isotope-temperature relationship is discussed and of an ice core. using the extensive data set obtained at confirmed by a number of experimental data. The the Russian Vostok Station, central Antarctica. At first , factors related to wind-driven spatial snow we present the theoretical background of the redistribution and post-depositional isotopic changes relationship between the stable water isotope content in that may aller or weaken this relationship, are also precipitation and air temperature (Section 2). In Section considered. For high-resolution isotopic time-series 3 we consider the limitations of isotopic method due to obtatned at sites wilh low accumulation of snow, Ihe the fonnation of non-climatic noise as a result of signal-to-noise ratio is shown to be as low as 0.25, depositional and post-depositional processes in the which means that noise accounts for about 80 % of the upper snow thickness. 1n order to estimate the signal-to­ total variance. [t is demonstrated that "classical isotopic noise ratio in vertical isotopic profiles, the data on local method" (based on the present-day geographical spatial isotopic distribution in the vicinity of Vostok isotope-temperature slope) underestimates the amplitude Station are used. Temporal variability of isotopic of past temperature changes in Antarctica. The most content of the Vostok precipitation and snow thickness likely reason for the discrepancy is the change in the is then compared with the instrumental record of air moisture source conditions. After correction for the temperature (Section 4) to derive the local isotope­ latter, the paleo-temperature reconstructions produced temperature calibration coefficient needed for the deep by the isotopic method become consistent with those ice core-based paleo-reconstructions. We also use the obtained from borehole temperature measurements. We high-resolution isotopic profi les from deep snow pits to show that in the case of the Vostok ice core, both reconstruct Vostok temperature history over the past approaches lead to the same temperature shift of 1000C 200 years thus demonstrating the validity of the method between LGM and the present time. The isotopic within a shorter time scale. Finally, the isotopic record composition of the basal part of the Vostok ice core, from the deep Vostok borehole is considered in Section comprising frozen subg[ac ial Lake Vostok water, is also 5. The paleotemperature reconstruction obtained with discussed. isotope approach is compared with that inferred from borehole temperature measurements. The most reliable Key words: Antarctica. Vostok station, ice core, estimate of the amplitude of the past temperature isotopic composition, paleo-climate. Lake Vostok changes at Vostok, consistent with results of both methods, is presented. Section 6 of the paper deals with formation of the isotopic composition of the basal I. lntroduction and outline section of the Vostok ice core comprising frozen water of subglacial Lake Vostok. Stable water isotopic composition (Of) and 8 80) of ice cores is commonly known as one of the most 2. Theory of isotope-temperature valuable paleo-climate proxies for temperature relationship. Modeling isotopic content of conditions of the precipitation fonnation in the past [15]. precipitation The observed relationship between air temperature and isotopic content of the precipitation (both geographical In this section we discuss the theoretical background and temporal) has been robustly con finned of the relationship between the stable water isotope experimentally and explained theoretically (see review content of precipitation and air temperature. The main in [20], for example). Isotopic data extracted from deep principles of modeling the isotopic composition of ice cores has allowed reconstructing paleo-temperature liquid, mixed and solid precipitation are also given. over about 120 ka in Greenland [60] and 800 ka in The relationship between isotopic composition of Antarctica [40]. precipitation and tempemture of its fo nnation is based On the other hand, the limitations of the isotopic on the "isotopic depletion" of moisture in the paleo-thermometer method are also known [2, 33 , 39, precipitating air mass due to isotopic fractionation 46. 54], which makes the absolute values of isotope­ during phase transfonnation of water. Since saturation temperature reconstructions somewhat uncertain. water vapor pressure is less for heavy water molecules 16 18 16 (HD 0 and H2 0) than for light molecules (H2 0), the -299- concentration of heavy isotopes in a liquid phase is mean value of a from the beginning of the process to the higher than in a vapor phase equilibmted with this liquid. current moment. So, the isotopic content of water vapor of an air mass The equilibrium fmctionation coefficient a (aD for fomled over the ocean is reduced compared to that of deuterium and a l8 for oxygen IS) is by definition equal the ocean water. As the cooling of the air mass proceeds, to ratio of heavy isotope concentmtion in liquid to its the water vapor condenses and new portions of concentmtion in water vapor being in equilibrium with precipitation are enriched in heavy isotopes with respect the liquid: to the vapor remaining in the air mass thus making the R lil(!lid vapor more and more isotopically depleted (Fig. I). a = --- (always > I), (3) Obviously, in the course of further cooling both vapor R,·up-o, and condensate become isotopically lighter due to the and Can also be determined as mtio of saturation washing out of heavy water molecules during pressure of light molecules to that of heavy ones. In turn, precipitation formation. the fractionation coefficients are tempemture-dependent. The temperature functions of fractionation coefficients are discussed in (70]. condensation 1--<'I '\., Unlike the condensation in the atmosphere, the evaporation of water from the ocean takes place under non-equilibrium conditions characterized by under­ '"Of'O transport qoY,ransport G saturated water vapor above the sea surface. As a -to ,," .u- I ~ consequence, a "kinetic isotopic effect" that occurs due to the slower diffusion of heavy molecules leads to evapou"tion precipitation §if ?:~~~~...-----1 larger effective fractionation coefficients as compared _ -20"" ice sheet with those at equilibrium conditions. The first ::::: theoretical model satisfactorily describing the kinetic 0 .. effect was developed by Merlivat and Jouzel (1979) [57]. According to the authors, the isotopic composition ocean continent ) of water vapor forming over the sea surface, li,{), equals E UATOR POLE to: I I-k 8,., = (I + 8~ ) --- - I , (4) Figure I, Nalwal water cycle and isotope a I - kh Fac/iolla/ioll of oxygen 18 (Fom [20]) where ooe is the isotopic composition of the ocean water, Cl is the equilibrium fractionation coefficient, II is Thus, the isotopic composition of precipitation is the relative humidity of the air over the water surface proportional to the mtio F of current amount of water in and k is the coefficient that accounts for the molecular the air mass to its initial amount. In turn, F depends on and turbulent diffusivity of water vapor. The latter the difference of condensation temperature at the coefficient differs by 12 % for D and 180, and is current time and at the beginning of the distillation dependent on wind regime, though in some cases it can process. Isotopic composition of liquid precipitation (op) be took constant and equal to 0.006 (for oxygen IS) can be sufficiently well predicted by a Rayleigh [36]. distillation approach which is based on the assumption The equation (4) is based on the assumption that the that condensation takes place in the dynamic and air mass is fomled in a closed system with no vapor isotopic equilibrium and that new portions of being in the air before evaporation begins, which may condensate are removed immediately from the air mass be far from reality [II]. It was shown however that [15J' using this assumption for estimating the initial vapor 8 =.!!..... Fa.. -'_ I (I) isotopic composition leads to systematic differences , a, ' compared to simulations conducted with three dimensional isotopic-atmospheric general circulation where li (liD or liMO) is isotope composition models (IGeM) [54]. The drawback was corrected in expressed as ratio of heavy isotope concentmtion (mole fmction) in the sample (SA) to its concentration in [70] by replacing k by k· = k+ A(I-k), where A accounts standard water (ST), in per mil: for the involvement of outer water
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