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Stable Isotope Records from Greenland Deep Ice Cores: The Climate Signal and the Role of Diffusion

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Abstract

Glaciers and ice caps exert a strong influence on Earth’s environment and climate. They can grow and decay, change the sea level, control the albedo and shape the mountains and continents on which they reside. During the last glacial period the expanding glaciers surged frequently and brought an enormous number of icebergs into the North Atlantic which caused a whole suite of violent climatic oscillations (Heinrich events, Dansgaard-Oeschger cycles) (Bond et al., 1992; Broecker et al., 1985; Johnsen et al., 1972).

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... Fig. 1a shows the 10 Be concentration from 7800 to 8500 yr BP from the GRIP ice core (Muscheler et al., 2004). The data are shown on the ss08c time scale which is based on layer counting and which is the best estimate of the GRIP time scale for the Holocene period (Johnsen et al., 1999). For the last ice age it is indispensable to take the variable accumulation rate into account to estimate past variations in 10 Be production (Alley et al., 1995;Finkel and Nishiizumi, 1997;Yiou et al., 1997;Wagner et al., 2001b). ...
... This comparison indicates that the GRIP time scale agrees with the tree ring chronology around 7900 yr BP. For the 8200 yr BP event it suggests that the GRIP ss08c time scale has to be shifted by approximately 20-30 years towards younger ages which is within the errors of this ice core time scale (Johnsen et al., 1999). The differences between modelled and measured D 14 C as, for example, around 8350 yr BP complicate the matching when it comes to small D 14 C wiggles. ...
Article
We analyse the 8200yr BP cold event by comparing the high-resolution 10Be record from the GRIP ice core from Central Greenland with the well-known tree ring Δ14C record. By transferring the absolute dated tree ring chronology to the ice core time scale, we show that the coldest phase in the GRIP record occurred around 8150yr BP. Furthermore, this method allows us to disentangle production and climate effects on 10Be and 14C with important implications for the reconstruction of past solar activity, and changes in the carbon cycle and 10Be transport. We show that, in principle, it is possible to infer changes in ocean circulation by comparing 10Be and 14C records. However, the duration of the 8200yr BP event is too short to assign unambiguously a significant change in atmospheric 14C concentration to changes in the global ocean circulation. Based on the comparison of 10Be with climate records, one could argue that the 8200yr BP cold event is triggered by a change towards lower solar activity. However, this link is questioned by the fact that around this period there are other similar and even stronger changes in solar activity that have no apparent connection to climate changes.
... The best linear fits to group 1 and group 3 are displayed. [Johnsen et al., 1999], this d 18 O increasing trend translates into a warming trend to first order of ∼3.0 ± 1.2°C over the past 40 years as comparison no significant trend is seen in the mean annual coastal temperatures from Ilulissat over the last 40 years. However, the Ilulissat temperatures have over the last 10 years increased with ∼2.2 ± 1.6°C. ...
... For the NEEM07S3 record the numbers are +3.3°C and +1.3°C, respectively. This calculated warming of the site region follows the previous reported warming (section 3.1) using the empirical relationship between mean annual isotope value and temperature reported by [Johnsen et al., 1999] (∼3.0 ± 1.2°C for the last ∼40 years). ...
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Samples of precipitation and atmospheric water vapor were collected together with shallow firn/ice cores as part of the new deep drilling project in northwest Greenland: the NEEM project. These samples were analyzed for their isotope composition to understand the processes affecting the climatic signal archived in the water stable isotope records from the NEEM deep ice core. The dominant moisture source for the snow deposited at the NEEM-site may be originating as far south as 35°N from the western part of the Atlantic Ocean. The surface atmospheric water vapor appears in isotopic equilibrium with the snow surface indicating a large water exchange between the atmosphere and snowpack. The interannual variability of NEEM shallow firn/ice cores stable isotope data covering the last ∼40 years shows an unexpectedly weak NAO signal. Regional to global atmospheric models simulate a dominant summer precipitation in the NEEM area, suggesting that the intermittency of modern winter precipitation is responsible for the lack of a strong NAO imprint. The interannual variability of NEEM isotope data however shows a strong correlation with interannual variations of Baffin Bay sea ice cover, a relationship consistent with air mass trajectories. NEEM deep ice core isotopic records may therefore provide detailed information on past Baffin Bay sea ice extent. NEEM stable water isotope content increasing trend points to a local warming trend of ∼3.0°C over the last 40 years.
... [11] Firn diffusion in the top $55 meters of the ice cores tends to smooth out d 18 O seasonal variations [Johnsen et al., 1999]. Hence back-diffused d 18 O data is preferable in comparison with raw data when examining seasonal signals in the d 18 O records. ...
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The winter delta18O signal is extracted from 7 Greenland ice cores covering the past ~700 years. To filter out noise and local variations in the 7 isotope records a principal component analysis is carried out on the ice core data. A comparison between the time series of the first principal component (PC1) with 67 years of winter (December to March) temperature measurements from 3 southern Greenland synoptic stations shows highly significant correlations. Southern Greenland winter temperatures are known to be greatly influenced by the North Atlantic Oscillation (NAO). A good proxy for southern Greenland temperatures is therefore expected to reveal at least parts of the NAO signal. It is shown that the PC1 time series indeed is significantly correlated to the NAO during the winter months.
... [5] In order to derive annual accumulation rates from the observed annual layer thicknesses, the data had to be corrected for densification and thinning of the ice layers due to ice flow. This was done by using a flow model Johnsen et al., 1999] also accounting for firnification at the top of the ice. In this way we obtained cross-dated chronological time series of annual accumulation rates over the latest two millennia, with relative dating errors being at most a few years. ...
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In the accumulation zone of the Greenland ice sheet the annual accumulation rate may be identified through identification of the annual cycle in the isotopic climate signal and other seasonally varying parameters. On an annual basis the accumulation rate in different Greenland ice cores is highly variable, and the degree of correlation between the accumulation series from different ice cores is low. When using multiyear averages of the different accumulation records the correlation however increases significantly. A statistical model has been developed estimating the common climate signal in the different records through optimization of the signal to noise variance ratio. A common Greenland accumulation record with five years resolution for the past 1800 years has been extracted. The record establishes a climatic record which points to a possible cause for the disappearance of the Norse settlements in Greenland.
... Precipitation on present-day polar ice caps in the Barents Sea area is associated with the passage of cyclones, and rapid changes in 18 O have been observed in winter when the cold arctic air is broken by a cyclone bringing moisture with a high 18 O from mid-latitudes (Simões, 1990). Although the amplitude of the 18 O signal decreases with depth in ice due to diffusion effects (Johnsen et al., 1999), some of the 18 O difference observed in the Cape Shpindler massive ice might be an inherited seasonal signal. Second, silty basal ice from the GRIP ice core on Greenland is significantly isotopically heavier (7–12%) than the above-lying glacier ice. ...
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A massive ground ice body at Cape Shpindler on Yugorski Peninsula, southern Kara Sea coast, Russia, was studied with regard to large-scale internal structures, its stratigraphic context and contacts to surrounding sediments, in order to highlight its origin. The massive ground ice contains deformation structures and deformed sediment rafts that show a consistent direction of deforming force. It is bounded upwards with a sharp and unconformable thaw contact to overlying till. The stratigraphical and structural evidence suggests that the massive ground ice body is relict glacier ice. Examination of data from a separate study on ice crystallography and isotopic composition of the massive ice body does not contradict this conclusion. The isotope composition and profiles conform with what can be expected for deformed basal ice. The chronology for the Shpindler Cape sequence implies that the glacier ice might be older than 250 ka years. Consequently, permafrost has preserved the relict glacier ice for the duration of at least two interglacials (Eemian and Holocene), as well as several Saalian and Weichselian interstadials, illustrating the preservation potential of the permafrost. Copyright © 2003 John Wiley & Sons, Ltd.
... We compared the raw data of our two longest lake records (cores NAUJG1.1 and SFL4.1) with the Summit Greenland Ice Core Project (GRIP) and Greenland Ice Sheet Project 2 (GISP2) δ 18 O ice-core records for the Holocene (Dansgaard et al., 1993;Grootes et al., 1993;Meese et al., 1994;Stuiver et al., 1995;Johnsen et al., 1998) (Fig. 4A). Values of δ 18 O in ice are believed to reflect surface-air temperature at the precipitation site , and stacked annual Summit δ 18 O records correlate well with West Greenland coastal temperatures, demonstrating its long-distance significance for the past century . ...
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Article
The available methods for dating of ice cores are based on radioactive decay, ice-flow calculations, or stratigraphic observations. The two former categories are broadly outlined, and special emphasis is given to stratigraphic methods. Reference horizons are established back to A.D. 1783, in the form of elevated electrical conductivities due to fallout of soluble volcanic debris. Seasonal variations in the concentrations of insoluble microparticles and/or stable isotopes are measured over the entire 400 m lengths of three ice cores, recovered by Greenland Ice Sheet Program (GISP). The resulting absolute time scales are probably accurate within a few years per thousand. Techniques are outlined for re-establishing the approximate, original shape of heavy-isotope profiles that have been more or less smoothed by diffusion in firn and ice. Annual-layer thickness measurements on 24 increments down to 1130 m depth in the Camp Century ice core determine a flow pattern, consistent with that suggested by Dansgaard and Johnsen (1969), and a Camp Century time scale with an estimated uncertainty better than 3% back to 10000 years B.P.
Article
We calibrate the δ 18O paleothermometer for central Greenland using borehole temperatures, a thermal model forced by a measured δ 18O record and a formal inverse technique. The calibration is determined mostly by temperature fluctuations of the last several centuries, including the Little Ice Age. Results are generally insensitive to model variables, including initial condition, basal boundary condition, parameterization of snow thermal properties, ice thickness and likely errors in temperature and isotope measurements. Results of this borehole calibration also seem to be in agreement with modern spatial gradients of δ 18O and temperature. We suggest that calibrations of isotopic paleothermometers using borehole temperatures are a useful paleoclimate tool, because they are independent of spatial gradients and include the effects of prehistoric temperatures over ice sheets.
Article
Modelling the temperature profile along the GRIP deep bore at the summit of the Greenland ice sheet leads to conversion factors that allow interpretation of the dated stable isotope profile as a climatic temperature record spanning the last 113,000 years. When corrected for surface elevation changes, the late glacial to Boreal temperature shift appears to have been 22 °C in central Greenland. The warming at the end of the last glaciation probably began earlier in Greenland, than in Antarctica. DOI: 10.1034/j.1600-0889.47.issue5.9.x
Chapter
Nitrate is present in polar ice mainly in form of HNO3, which is a dominant component of the acidity of the ice (in Greenland this is only valid for Holocene ice, because the ice age ice is alkaline).
Chapter
The bedrock topography of a 1400 square kilometer area around the Dye 3 radar station on the Greenland Ice Sheet is obtained from radio-echo soundings. The subgîaciaî landscape is very mountainous with large changes in bedrock relief. The compiled map of the bedrock shows that the deep ice core from Dye 3 is on the eastern slope of a valley that trends northwest-southeast. In addition there are large undulations upstream of the drill site. Measurements were made by both an airborne 10 kw sounder, and a 1 kw lightweight surface sounder. Corrections for navigational errors were made to the airborne measurements when data from both were combined to produce topographical maps. The two-way dielectric absorption in the ice is calculated using a measured temperature profile in the borehole. It indicates the reflection coefficient at the ice-bedrock interface to be very low. A list of satellite-obtained locations of map control points is included.
Chapter
Different components and properties of the ice deposits on the earth (glaciers, ice sheets) store information on the climate and environment. We can mainly distinguish between three storage types: a) properties and isotopie composition of the ice itself, b) solid (and liquid) trace substances in the ice, and c) gaseous and volatile components. This article reviews some aspects of the transfer of gaseous components from the atmosphere to the ice.
Article
ECM profiles over the upper 297 m of the EUROCORE and GRIP ice cores, drilled at the Summit location in Central Greenland are presented. Special emphasis is given to the calibration of the ECM in terms of acidity and the chemical composition of the acids in the ice. The records reveal several acid signals caused by past volcanic eruptions over the period 740–1989 AD. The use of the ECM profiles as an indicator of volcanic acid deposition is discussed and the Summit record is compared to the record from Crête, 150 km south of the Summit region. Finally the ECM profile is discussed as an indicator of past upper atmospheric volcanic H2SO4 load which is believed to be a key parameter for modelling the impact of past volcanism on past climate.
Chapter
Although there are needs for a better understanding of the atmosphere-snow and ice connection and for improving deep ice dating polar ice cores are providing multiple proxy records of climate and related parameters. They show evidences of anthropogenic impact on aerosol concentrations in Greenland snow (i.e. SO4 and NO3) and, on a global scale, on greenhouse gases such as CH4 and CO2. Over the last climatic cycle (i.e. ~ 150 ky) they indicate temperature changes up to about 10°C with periodicities characteristic of earth orbital parameters. Very cold climatic stages are associated with lower snow accumulation and high concentrations of marine and continental aerosols, likely reflecting strengthened sources and atmospheric transport. Greenland ice has recorded rapid changes of climate during the last ice age and deglaciation. Antarctic cores have shown a close association between CO2 and climate throughout the last climatic cycle. New results on methane and sulfates further support the picture of large chemical atmospheric modifications, some of them involving the biosphere, which have likely contributed in glacial interglacial climatic changes.
Article
Fini and ice cores from seven Greenland ice sheet locations were analysed for dust concentration by means of the light scattering method and the Coulter counter technique. Comparison with Lamb's dust veil indices suggests that the deposition of dust in South Greenland is essentially of nonvolcanic origin, at least since AD 1780. Etudes de la teneur en poussière de carottes de glace du Groenland Résumé. On a mesuré la teneur en poussière de carottes de névé et de glace prélevées en sept stations du Groenland par diffusion de la lumière et à l'aide d'un compteur de Coulter. La comparaison avec les indices de voile de Lamb suggère que le dépôt de poussière au Groenland du sud est essentiellement d'origine non volcanique, au moins depuis 1780.
Article
If it were possible to properly extract seasonal information from ice-core isotopic records, paleoclimate researchers could retrieve a wealth of new information concerning the nature of climate changes and the meaning of trends observed in ice-core proxy records. It is widely recognized, however, that the diffusional smoothing of the seasonal record makes a “proper extraction" very difficult. In this paper, we examine the extent to which seasonal information (specifically the amplitude and shape of the seasonal cycle) is irrecoverably destroyed by diffusion in the firn. First, we show that isotopic diffusion firn is reasonably well understood. We do this by showing that a slightly modified version of the Whillans and Grootes (1985) theory makes a tenable a priori prediction of the decay of seasonal isotopic amplitudes with depth at the GISP2 site, though a small adjustment to one parameter significantly improves the prediction. Further, we calculate the amplitude decay at various other ice-core sites and show that these predictions compare favorably with published data from South Pole and locations in southern and central Greenland and the Antarctic Peninsula. We then present numerical experiments wherein synthetic ice-core records are created, diffused, sampled, reconstituted and compared to the original. These show that, alter diffusive mixing in the entire fini column, seasonal amplitudes can be reconstructed to within about 20% error in central Greenland but that all information about sub-annual signals is permanently lost there. Furthermore, most of the error in the amplitude reconstructions is due to the unknowable variations in the sub-annual signal. Finally, we explore how these results can be applied to other locations and suggest that Dye 3 has a high potential for meaningful seasonal reconstructions, while Siple Dome has no potential at all. An optimal ice-core site for seasonal reconstructions has a high accumulation rate and a low temperature.
Article
S(lsO)-cycles with wavelengths shorter than 20 cm of ice are obliterated during firnification. This corresponds to a diffusion length of 7-8 cm of ice, independent of accumulation rate and temperature, according to a model presented. Another combined ice flow-diffusion model describes the observed smoothing of S( 180)-gradients at great depths. Based on digital deconvolution techniques, a method is described that allows re-establishing apparently lost S(180)-cycles. This may considerably extend the range of ice core dating by annual S(lsO)-cycles.
Article
Because of snow drifting, two time series of any variable derived from two adjacent ice cores will differ considerably. The size and statistical nature of this noise element is discussed for two kinds of measured substance. A theory is developed and compared to data from Greenland and Canadian Arctic ice cores. In case 1, the measured substance can diffuse and the seasonal cycle degrade with time and depth, e.g. δ( ¹⁸ O). In case 2, the measured substance cannot diffuse, e.g. microparticles. The case 2 time series contain drift noise proportional to that in the accumulation series. For accumulation series, the spectral power is concentrated at the high frequencies, i.e. is “blue”. Such noise can be easily reduced by taking relatively short time averages. The noise in the case 1 time series, however, starts out “blue” but quickly diffuses to have a “red” character with significant power at longer wavelengths, and many decades of such series must be averaged to reduce the noise level. Because the seasonal amplitude of any given variable is an important input to the drift noise and because the seasonal amplitudes of some variable types are latitude-dependent, some sites have inherently less drift noise than others.
Article
Detailed studies of the last 20 km of the flow-line leading to the core hole at Dye 3 Greenland, provide a description of ice flow over and around basal hills. The surface pattern is very simple. Velocity vectors are nearly parallel to one another and the largest variations in velocity are speed changes along the direction of flow. The surface elevation is stepped and the speed is faster than average where the surface slope is steepest. These positions correspond to basal highs, and the surface velocity increases as expected, based on the decrease in ice thickness, which indicates that most of the ice thickness must vary in velocity as does surface ice. Further support for this comes from the form of an internal radio-reflecting layer, which, in general, has the same shape as the bed but with much reduced relief. The damping of the relief is the same both along and across the flowline, suggesting that lateral velocity fluctuations are not important and that flow around and between obstacles is not well developed at the surface or at depth. At two sites, however, the internal layer does not match the bed and at one of these there must be important third-dimensional flow at depth.
Article
A new deep ice-core drilling site has been identified in north Greenland at 75.12°N, 42.30°W, 316 km north-northwest (NNW) of the GRIP drill site on the summit of the ice sheet. The ice thickness here is 3085 m; the surface elevation is 2919 m. The North GRIP (NGRIP) site is identified so that ice of Eemian age (115-130 ka BP, calendar years before present) is located as far above bedrock as possible and so the thickness of the Eemian layer is as great as possible. An ice-flow model, similar to the one used to date the GRIP ice core, is used to simulate the flow along the NNW-trending ice ridge. Surface and bedrock elevations, surface accumulation-rate distribution and radio-echo sounding along the ridge have been used as model input. The surface accumulation rate drops from 0.23 m ice equivalent year-1 at GRIP to 0.19 m ice equivalent year-1 50 km from GRIP. Over the following 300 km the accumulation is relatively constant, before it starts decreasing again further north. Ice thicknesses up to 3250 m bring the temperature of the basal ice up to the pressure-melting point 100-250 km from GRIP. The NGRIP site is located 316 km from GRIP in a region where the bedrock is smooth and the accumulation rate is 0.19 m ice equivalent year-1. The modeled basal ice here has always been a few degrees below the pressure-melting point. Internal radio-echo sounding horizons can be traced between the GRIP and NGRIP sites, allowing us to date the ice down to 2300 m depth (52 ka BP). An ice-flow model predicts that the Eemian-age ice will be located in the depth range 2710-2800 m, which is 285 m above the bedrock. This is 120 m further above the bedrock, and the thickness of the Eemian layer of ice is 20 m thicker, than at the GRIP ice-core site.
Article
We calibrate the δ18O paleothermometer for central Greenland using borehole temperatures, a thermal model forced by a measured δ18O record and a formal inverse technique. The calibration is determined mostly by temperature fluctuations of the last several centuries, including the Little Ice Age. Results are generally insensitive to model variables, including initial condition, basal boundary condition, parameterization of snow thermal properties, ice thickness and likely errors in temperature and isotope measurements. Results of this borehole calibration also seem to be in agreement with modern spatial gradients of δ18O and temperature. -from Authors
Article
A conceptually simple method for power estimation in maximum entropy spectral analysis, based on evaluation of complex residues of the spectral density estimator, is suggested. Numerical integration of the peaks of the power density function is thus avoided. The agreement in simple cases with conventional estimates is demonstrated, and the explicit performance is analyzed in detail in a series of examples. The close connection between the residue power estimate and the estimate proposed recently by Pisarenko is pointed out. The method is particularly suitable for spectral decomposition of low noise time series with several harmonic components, because it allows a direct listing of frequency and power estimates, provides an indication of the purity of the obtained harmonic components and enhances the resolution of the maximum entropy spectral density estimator.
Article
The air trapped in the bubbles of natural ice is not the same age as the surrounding ice. This is due to the fact that the air is enclosed in isolated bubbles only at the depth of the firn-ice transition. Within the overlying porous firn layer the air is able to mix and to exchange to a certain degree with the atmosphere. The age difference between ice and air is given by the age of the ice at pore close-off, less the mixing delay. Also, there is an age distribution due to diffusive smoothing and due to the gradual enclosure of the air at the firn-ice transition. Knowledge of this age relation is necessary for the interpretation of climatic parameters measured on ice cores. This work concentrates on the effect of diffusive mixing. We report on measurements of the diffusivity of CO2 and O2 (in N2) in firn samples from Siple Station, Antarctica. It is shown that the dominant mixing process is molecular diffusion. The diffusion coefficient depends approximately linearly on the porosity. A one-dimensional diffusion model has been used to calculate the air mixing in firn at Siple Station (Antarctica), at the South Pole, and at Station Crete (Greenland). An exchange time of between 10 and 50 years is obtained.
Article
Over 70,000 samples from the 3029-m-long Greenland Ice Core Project (GRIP) ice core drilled on the top of the Greenland Ice Sheet (Summit) have been analyzed for delta18O. A highly detailed and continuous delta18O profile has thus been obtained and is discussed in terms of past temperatures in Greenland. We also discuss a three-core stacked annual delta18O profile for the past 917 years. The short-term (
Article
Selected segments from the Eemian period of the Greenland Ice Core Project (GRIP) core from Summit, Greenland, have been analyzed by ion chromatography and Coulter Multisizer. The results will be discussed with special emphasis on the sudden cooling events, event 1 and event 2, in the record as observed in theSO profile (GRIP members, 1993; Dansgaard et al., 1993). Whether the sudden cooling events of the Eemian represent a real climatic signal or they are a consequence of disturbed ice core stratigraphy is still a matter of debate (Chappellaz et al., this issue). However, several features of the chemical profiles across these cooling events are difficult to explain if they were a result of disturbed stratigraphy. We will focus our discussion on the unique profiles of nitrate, methane sulfonic acid (MSA), and ammonium across event 1 and argue that even if we allow for movement of ions by diffusion or displacement of ions by physical or chemical interactions, events 1 and 2 are not likely to consist of ice from other climatic periods which have been inserted in the Eemian strata by folding processes. Furthermore, our records do not show any layers in the vicinity of the Eemian strata which can be a possible source of the ice in event 1. We believe that the events represent a climate signal, and we will discuss other possible explanations of the profiles. Our conclusion that the cold events in the Eemian represent a real climatic signal runs counter to other evidence from gas measurements. However, whatever their cause, the unusual chemical signals require further discussion and explanation.
Article
Ice cores contain information on climatic variations and their causes. Recent results obtained on the new deep ice core drilled in 1981 at Dye 3, South Greenland, in the frame of the US-Danish-Swiss Greenland Ice Sheet Program are: Comparison of the δ 180 variations in the Greenland ice cores with those in European lake carbonate exhibits strong similarities and provides time marks (13,000, 11,000, 10,000 B.P.) for the Late-Glacial section of the ice cores; CO2 concentration measurements in the occluded air indicate low (180–200 ppm) CO2 concentrations 30,000 to 15,000 B.P. and an increase to ca. 300 ppm around 13,000 B.P.,. The CO2 increase might reflect a change in the ocean circulation at the end of the last glaciation and could have contributed to the establishment of the Holocene environmental conditions; 10Be concentration measurements on samples covering the last 50,000 yaers show a correlation with δ 180, low δ 180 values corresponding to high 10Be concentrations (atoms per g of ice). Probably this mainly reflects changes in the rate of precipitation in the northern hemisphere. Based on the ice core information climatic events during the Glacial-Postglacial transition are discussed.
Article
Molecular mixing in cold snow and firn is ordinarily controlled by vapor diffusion. Diffusion through or along the ice matrix is slow, whereas diffusion from the air to within separate grains is comparatively rapid. Thus mixing occurs in the vapor phase, and exchange with the local ice involves all the water molecules. A quantification of these concepts is applied to the diffusion of oxygen-isotopic depth profiles in southern Greenland and at Dome C, Antarctica. It successfully describes the smoothing of measured profiles. The smoothing rate is strongly dependent on temperature and density of the firn.
Article
A comprehensive study of textures and fabrics has been carried out on the Greenland Ice Core Project (GRIP) ice core. Crystal sizes and c axis orientations have been measured on thin sections with conventional techniques, yielding new information on the growth, rotation and recrystallization of ice crystals in the Greenland Ice Sheet. Normal grain growth is found to persist to a depth of 700 m in the core, where the onset of polygonization due to increasing strain prevents a further increase in grain size in the Holocene ice. Smaller crystals are observed in the Wisconsin ice, larger crystals are found in the Eemian ice, and the crystal size is found to vary with climatic parameters in these periods. This dependence, which probably results from variable impurity content in the ice, persists to a depth of 2930 m. Coarse-grained ice, probably resulting from rapid growth of crystals at comparatively high temperatures, is found in the lowest 100 m of the core. The data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet. A significant strengthening is not observed at the Holocene-Wisconsin transition. The fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m. The evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear. Evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth. Stretched fabrics in coarse-grained ice in the lowest 100 m could be due to tensional stresses; this ice is unlikely to be undergoing any significant horizontal deformation at the present time.
Article
Shallow temperature profiles (50-100 m) from polar ice caps contain information about recent climatic changes. Correlation with 8(I80)-records and temperature records is possible by using a method described in this paper. The method helps to understand the climatic information of S( 180)-records, and allows the climatic regime of ice cap stations to be established. Profils des teneurs en isotopes stables en comparaison de profils des températures dans le névé et d'enregistrements historiques des températures Résumé. Les profils de températures jusqu'à 50 ou 100 m de profondeur obtenus dans les calottes polaires renferment des informations sur les changements climatiques récents. Une corrélation entre les enregistrements des teneurs en isotopes (SlsO) et des températures est possible en utilisant la méthode décrite dans ce travail. Cette méthode aide à la compréhension des informations climatiques fournies par les profils isotopiques et permet d'établir le régime climatique des stations sur la calotte glaciaire. S(180)-profiles along firn and ice cores from the Greenland ice sheet contain long-term climatic information, which has been shown to correlate with variations known from other parts of the North Atlantic region. The purpose of this work is to show that the S(180)-profiles also reveal to some extent local short-term climatic changes. This is demonstrated by comparison with measured temperature profiles along the holes left after drilling the cores. Temperature profiles from suitable sites on stable, high polar ice sheets are essentially determined by the past temperature history of the ice sheet. The actual shape of temperature profiles depends, furthermore, on how the thermal properties of firn change with depth. Long-term climatic variations are reflected in the temperature profile at great depths, whereas recent short-term climatic variations determine the upper part of the temperature profiles considered here. The climatic information contained in a measured temperature profile is derived by comparison with profiles calculated by assuming a temperature history for the surface and adjusting the assumption to the best obtainable fit. Below 10 m depth, the vertical temperature gradients are often small and to a certain degree influenced by the annual temperature cycle, at least down to 20 m depth, which may be corrected for. The calculated temperature profiles are obtained by numerical integration of the differential equation for heat conduction in moving firn and ice, which also takes into account how the thermal parameters of firn and ice are dependent on temperature and density : — = — ill ^llYf - 1^ _ dT IdK
Article
An empirical model of firn densification from the surface to the zone of pore close-off has been constructed. Fundamental rate equations have been derived for the first two stages of densification. In the first stage, for densities less than 0.55 Mg m ⁻³ , the densification rate is proportional to the mean annual accumulation times the term ( ρi − ρ ), where ρ is the density of the snow and ρi is the density of pure ice. The densification rate in the second stage, where 0.55 Mg m ⁻³ < ρ < 0.8 Mg m ⁻³ , is proportional to the square root of the accumulation rate and to ( ρ i− ρ ). Depth–density and depth–age calculations from this model are compared with observation. Model accumulation rates are within about 20% of values obtained by other techniques. It is suggested that depth intervals of constant density in some Antarctic cores may represent a synchronous event in the 1880 ’s when ten times the normal accumulation fell within a year or two.
Article
Acidity profiles along well dated Greenland ice cores reveal large volcanic eruptions in the Northern Hemisphere during the past 10,000 yr. Comparison with a temperature index shows that clustered eruptions have a considerable cooling effect on climate, which further complicates climatic predictions.
Article
The available methods for dating of ice cores are based on radioactive decay, ice-flow calculations, or stratigraphic observations. The two former categories are broadly outlined, and special emphasis is given to stratigraphic methods. Reference horizons are established back to A.D. 1783, in the form of elevated electrical conductivities due to fallout of soluble volcanic debris. Seasonal variations in the concentrations of insoluble microparticles and/or stable isotopes are measured over the entire 400 m lengths of three ice cores, recovered by Greenland Ice Sheet Program (GISP). The resulting absolute time scales are probably accurate within a few years per thousand. Techniques are outlined for re-establishing the approximate, original shape of heavy-isotope profiles that have been more or less smoothed by diffusion in firn and ice. Annual-layer thickness measurements on 24 increments down to 1130 m depth in the Camp Century ice core determine a flow pattern, consistent with that suggested by Dansgaard and Johnsen (1969), and a Camp Century time scale with an estimated uncertainty better than 3% back to 10000 years B.P.
Article
The chemical composition of soluble impurities along the Dome C ice core covering approximately the last 30 000 years is reported and interpreted in terms of atmospheric contributions. Terrestrial and sea-salt inputs are known to have been much higher during the Last Glacial Maximum (LGM) than during the Holocene period. For this reason, the gas-derived compounds (mainly H 2 SO 4 and HNO 3 ) which dominate the chemistry of present-day snow are minor components in LGM snow. The exact calculation of each of the various contributions has been made possible by the determination of all major ions (H ⁺ , Na ⁺ , K ⁺ , NH 4⁺ , Mg ²⁺ , Ca ²⁺ , NO 3⁻ , SO 4²⁻ and Cl−) in the samples. Three additional deep ice cores from other Antarctic areas have also been analyzed, but in a less comprehensive manner than the Dome C core. The differences observed at the four study sites increase the general understanding of the past atmospheric chemistry of the Southern Hemisphere.
Article
The suggestion of climatic instability during the last interglacial period (Eem), based on the bottom 10% of the Greenland Ice core Project (GRIP) isotopic profile, has been questioned because the bottom record from the neighboring Greenland Ice Sheet Project 2 (GISP2) core (28 km away) is strikingly different over the same interval and because records of the delta18O of atmospheric O2 from both cores showed unexpected rapid fluctuations. Here we present detailed methane records from the Vostok (Antarctica), GRIP, and GISP2 cores over the relevant intervals. The GRIP and GISP2 data show rapid and large changes in methane concentration, which are correlative with variations of the delta18O of the ice, while the Vostok record shows no such variations. This discrepancy reinforces the suggestion that the bottom sections of the Greenland records are disturbed. By combining the methane data with measurements of delta18O of O2 in the same samples, we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples. Our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation.
Article
The temperature distribution through the Greenland ice sheet at the Dye 3 borehole is a record of the past climatic changes in the Arctic. The numerical model of the temperature distribution now presented reproduces the observed temperature distribution within 0.03 K, and shows that the basal ice is still cooled 5 K by the cold ice-age climate. The results suggest a mean ice-age temperature of −32 ± 2°C, which is 12 K colder than the present temperature, and a precipitation rate 50 ± 25% of the present rate. Calculations of a more detailed temperature history through the present inter-glacial period reveal evidence of the AD 1920–50 maximum, the little ice age, and the Atlantic period.
Article
The Camp Century, Greenland, deep ice core reveals seasonal variations in the isotopic composition of the ice back to 8,300 years BP. This is not the case for the Byrd Station, Antarctica, deep ice core. Both cores show long-term perturbations in isotopic composition reflecting climatic changes from before the beginning of the last glaciation. But the complexity of the glaciological regime at Byrd Station precludes a rational choice of a time scale. Pole-to-pole correlations of the palaeoclimatic data therefore become speculative except for the more pronounced features and general trends.