Grain-scale processes, folding, and stratigraphic disturbance in the GISP2 ice core
Alley, R.B., A.J. Gow, D.A. Meese, J.J. Fitzpatrick, E.D. Waddington, and J.F. Bolzan
Flow disturbances have been shown to alter stratigraphic order in the lower part of the ice sheet in central Greenland. Vertical thin sections of the GISP2 ice core show that in the lower 30%, the expected c-axis- vertical fabric is interrupted by planes of grains ("stripes") with c-axis oriented approximately in the dip directions of the planes. Stripe-parallel shear produces small-scale folds. The stripes can be explained qualitatively by a simple nucleation-and-growth model based on the strong anisotropy of ice-crystal deformation. Nucleation probably is sensitive to spatially variable rates of polygonization or grain growth, producing spatially variable stripe densities. Stripes are modeled to affect the ice viscosity, so variations in stripe density may contribute to viscosity contrasts that might produce larger deformational features and loss of stratigraphic order.
Visual stratigraphy of the GISP2 ice core: Basis, reproducibility, and persistence
Alley, R.B., C.A. Shuman, D.A. Meese, A.J. Gow, K.M. Cuffey, K.C. Taylor, J.J. Fitzpatrick and B. Elder.
The dating of the GISP2 ice core for the most recent 50,000 years is based on multi-parameter layer counting (Meese et al, 1994), including heavy reliance on visible annual layering. Visible stratigraphy is a long-established, well-known technique, but the accuracy of the visible annual signal in the GISP2 ice core and its persistence through 80% of the ice-sheet thickness have surprised some workers. In the Holocene ice, the technique is based on the recognition of summertime hoar sequences formed very near the surface as a direct result of the effects of surface heating. Prior to the Holocene, visible recognition of “cloudy bands” —possibly dust layers — is important. Both hoar layers and cloudy bands are visible in ice from the early Holocene, allowing calibration of one visual signal to the other. Intercomparisons of observations from different workers, and of observations by one worker taken at different times and under different conditions, show 1% reproducibility in the Holocene over century-length intervals. This is similar to the absolute accuracy of cumulative ages from visible stratigraphy, based on comparison to independent dates of certain events. In the late glacial, reproducibility is not as good and decreases with increasing age and depth; however, the apparent accuracy remains as good as that of independent measures to which we can compare. The youngest 50,000 years of the working GISP2 depth-age scale (Meese et al, 1994) are based on all available data for given intervals of the core. Not all annual-layer indicators are available at all depths, owing to disturbances in the brittle ice, diffusion of stable isotopes, etc. Careful inspection of the combined data set allows one to distinguish where indicators were added or subtracted, probably owing to changes in ability to recognize “extreme” years. This variability in estimated accumulation rates might complicate time-series analyses of this long data set. To avoid such complications, we have prepared an accumulation-rate time series based solely on visible layering. Comparison to the multi-parameter record shows no significant differences over century-length intervals in the Holocene.
Aspects of climate variability in the north Atlantic sector: discussion and relation to the GISP2 high
resolution deuterium signal.
Barlow, L.K., Barry, R.G., Rogers, J.C., Serreze, M.C., and White, J.W.C.
This paper presents a review of aspects of climate dynamics and intra-annual to decadal climate variability in the North Atlantic Sector which possibly influence the seasonal scale to decadal scale deuterium isotopic signal in the GISP2 ice core. Interpretation of the isotopic signal and its spatial applicability at the seasonal level, and over decadal time periods, requires investigation into synoptic scale climatology. We use a combination of meteorological data and literature review to discuss possible influences on the isotopic signal of seasonal and longer interval variability in: 1) the amount and likely source regions of precipitation reaching the GISP2 site; 2) the characteristics of cyclonic activity over the Greenland region; 3) changes in major features such as the mean sea level Icelandic Low and Azores High pressure systems, the North Atlantic Oscillation, and the Baffin trough. We use the strength of the correlation with coastal temperature time series to demonstrate the influence of synoptic scale climatology on the GISP2 isotopic record for the years 1858 to 1985, therefore providing guidelines for interpretation of the high resolution isotopic record further back in time.
Asian provenance of last-glacial maximum dust in the GISP2 ice core, Summit, Greenland.
Biscaye, P.E., F.E. Grousset, M. Revel, S. Van der Gaast, G. A. Zielinski, A. Vaars, and G. Kukla
Samples of dust from the GISP2 ice core, Summit, Greenland, dated within marine isotope Stage 2 (between 23,340 and 26,180 calendar years BP), around the time of the coldest local last-glacial temperatures, have been analyzed to determine their provenance. To accomplish this, we have compared them with approximately coeval aeolian sediments (mostly loesses) sampled in possible source areas (PSAs) from around the Northern Hemisphere. The <5 mm grain-size fraction of those samples was analyzed on the rationale that it corresponds to the atmospheric dust component of that time and locale that was sufficiently fine grained to be transported over long distances. Based on comparison of the clay mineralogy and Sr-, Nd-, and Pb- isotope composition between ice dust and PSAs, and assuming that we have sampled the most important PSAs, we have determined that the probable source area of these GISP2 dusts was in eastern Asia. The dust was not derived from either the mid-continental USA or the Sahara, two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling. Except for a brief period during an interstadial when dust transport was exceptionally low (for glacial times) and had a mineralogical composition indicative of a slightly more southern provenance, the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size, or lower fluxes of dust with smaller mean grain size. This includes the high-dust period that correlates with the Heinrich-2 of major iceberg discharges in to the North Atlantic. Variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size.
Reconstruction of past accumulation rates from the GISP2 ice-core data.
Bolzan, J.F., N.A. Cutler, E.D. Waddington, C.F. Raymond, R.B. Alley, and D.A. Meese
The high-resolution stratigraphic record in the GISP2 ice core has enabled an accurate depth-age relation to be established based on the identification of annual layers and the measurement of the stable oxygen isotope ratio of the entrapped air. The latter data also imply the variation in annual layer thickness with depth below the zone where annual horizons can be identified stratigraphically. Here we describe two methods for reconstructing the variation of the accumulation rate with time from the annual layer thickness data. One approach computes the total layer thinning along reconstructed particle trajectories ending at various depths in the core. This generates the dilation factor as a function of depth, which is the ratio of surface layer thickness to the thickness measured in the core. The other method exploits the relation between accumulation rate and ice thickness implied by Glen’s flow law and mass continuity. An assumed initial accumulation rate history is used to calculate the resulting thickness history. This thickness history is then used to unstrain the measured layers and produce a new accumulation history; the process being repeated until satisfactory convergence is reached. We infer the likely accumulation record from a comparison of the two results, and estimate the uncertainty from sensitivity studies. Finally the implications for changes in the ice sheet configuration in the region are discussed.
Temperature, Accumulation, and Ice Sheet Elevation in central Greenland Through the Last Deglaciation
Cuffey, K. M., G.D. Clow
We present a combined heat- and ice-flow model, constrained by measurements of temperature in the GISP2 borehole and by the GISP2 depth-age scale, which determines a history of temperature, accumulation rate, and ice sheet elevation for the past 50,000 years in central Greenland, Important results are: that the temperature increase from average glacial to Holocene conditions was large, approximately 15°C, with a 20°C warming from late glacial to Holocene; that the average accumulation rate during the last glacial maximum (between 15 kabp and 30 kabp) was 5.5 to 7 cm/ a; that long-term (500-1000a) averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the Holocene; and that the Greenland ice sheet probably thickened during the deglacial transition. The inverse correlation of accumulation rate and temperature in the mid and late Holocene suggests that the Greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought, and demonstrates that accumulation rate is not a reliable proxy for temperature. The elevation history of the ice sheet is poorly constrained by the model, and independent evidence is needed. We also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins; this was approximately 1900 years for central Greenland during deglaciation.
Air-snow exchange investigations at Summit, Greenland: An overview
Dibb, J. E. and J.-L. Jaffrezo
The GISP2 and GRIP deep drilling programs at Summit, Greenland included support (both logistical and scientific) of extensive investigation of atmospheric transport and air-snow exchange processes of gases and particles relevant to the interpretation of the ice core records. Much of the sampling for the air-snow exchange investigations was conducted at a unique solar-powered camp 30 km southwest of the GISP2 drill camp (even further from the GRIP camp), and was characterized by a high degree of international collaboration and co-operation. The wide range of expertise and analytical capabilities of the 20 plus investigators participating in these studies has provided important insight into the meteorological, physical and chemical processes which interact to determine the composition of snow and firn at Summit. Evolving understanding of this system will allow improved reconstruction of the composition of the atmosphere over Greenland in the past from the detailed Summit ice core records. This paper provides an overview of air-snow exchange investigations at Summit, including their development through the course of the drilling programs (1989-1993), significant findings related to both air-snow exchange issues and the present state of the Arctic free troposphere, as well as the major outstanding questions which are being addressed in ongoing experiments at Summit.
Physical and structural properties of the GISP2 ice core
Gow, A.J., D.A. Meese, R.B. Alley, J.J. Fitzpatrick, S. Anandakrishnan, G.A. Woods and B.C. Elder
Substantial data sets have been collected on the relaxation characteristics, density, grain size, c-axis fabrics and ultrasonic velocities of the GISP2 core to its contact with bedrock at 3053.5 m. Changes in all these properties paralleled closely those found in cores from Byrd Station, Antarctica and Dye-3, Greenland. Density increased progressively with depth to a maximum of 0.921 Mg/m3 at about 1400 m, at which depth the ice became bubble free. Below about 2000 m in situ densities began to decrease in response to increasing ice sheet temperatures. Densities remeasured at intervals since drilling revealed significant volume expansion (relaxation) due to microcracking and the exsolving of enclathratized gases, especially in the brittle ice zone from between 600 and 1400 m. Grain size increased linearly to about 1000 m, thereafter remaining fairly constant until the Younger Dryas event at 1678 m where a 2 to 3 fold decrease in grain size occurred. These grain size changes were accompanied by a progressive clustering of crystal c-axes towards the vertical including a small but significant increase in c-axis concentration across the Younger Dryas/Holocene boundary. Increased dust levels in the Wisconsin ice have contributed to the maintenance of a fine-grained texture which, with its strong vertical c-axis fabric, persisted to nearly 3000 m. However, beginning at about 2800 m layers of coarse-grained ice are observed. Below 3000 m the ice became very coarse-grained. This change, attributed to annealing recrystallization at elevated temperatures in the ice sheet, was accompanied by a ring-like redistribution of the c-axes about the vertical. Ultrasonic measurements of vertical and horizontal p-wave velocities made at 10 m intervals along the entire length of the GISP2 core fully confirmed the results of the crystallo-optical observations. A return to fine-grained ice coincided with the first appearance of brown, silty ice 13 m above bedrock.
18O/16O Variability in Greenland Snow and Ice with 10-3 to 105 Yr Time
Grootes, P.M., M. Stuiver, E. Steig, K. Cuffey, J. W. C. White, L. Barlow
The distribution of the main isotopic forms of water, H216O, HD16O, H218O, in ice cores has been used extensively as a proxy-indicator of temperature in the past. With a growing data base of ever more detailed and precise isotope measurements has come the realization that even in the relatively simple, strongly temperature-controlled polar environment local temperature is not the only factor influencing the isotopic abundance ratios. However, in a compendium paper Jouzel et al. show, via modeling of the isotopes in the water cycle, that local temperature is still a dominant factor, normally accounting for some 70% of the variance, and clarify the role of changes in the water vapor source regions. Alley et al. confirm modeling predictions by showing good agreement between long-term isotope fluctuations and temperature fluctuations deduced from bore-hole temperatures and from the occurrence of melt layers at GISP2, and between isotopes in accumulation and satellite/AWS data. We report measurements of isotopes in water vapor, precipitation, firn, and ice over the GISP2 field seasons, and discuss the significance of some of the results for reconstructing climate changes in the past from ice cores.
Seasonal variability of present-day atmospheric transport to Summit, Greenland.
Kahl, J.D.W., D.A. Martinez, H. Kuhns, C. Davidson, J.L. Jaffrezo, J.M. Harris
The seasonal variation in atmospheric transport patterns to Summit, Greenland is examined using a 44- year record of daily, 10-day, isobaric back trajectories at the 500 hPa level. Over 24,000 modeled trajectories are aggregated into distinct patterns using cluster analysis. Ten-day trajectories reaching Summit are longest during winter, with 67% extending upwind (westward) as far back as Asia or Europe. Trajectories are shortest during summer, with 46% having 10-day origins over North America. During all seasons a small percentage (3-7%) of trajectories originate in West Asia/Europe and follow a meridional path over the Arctic Ocean before approaching Summit from the northwest. Trajectories at the 700 hPa level tend to be shorter than at 500 hPa, with many of the 700 hPa trajectories from North America tracking over the North Atlantic and approaching Summit from the south. The long-range transport climatology for Summit is similar to a year-round climatology prepared for Dye 3, located 900 km to the south. An analysis of several aerosol species measured at Summit during summer, 1994 reveals examples of the usefulness and also the limitations of using long-range air trajectories to interpret chemical data.
Measurements of in-situ 14C concentrations in GISP2, <17ky BP: implications to ice flow dynamics.
Lal, D. and A.J.T Jull
The recent finding that nuclear interactions of cosmic rays with oxygen in ice directly contribute to 14C concentration in polar ice, has led to two principal realizations: (i) the absolute amount of in situ 14C generally exceeds that incorporated in ice along with trapped air, and (ii) that the in situ 14C can be used as a tracer for determining ice accumulation/ablation rates as well as secular changes in cosmic ray flux. In view of (i), the conventional 14C age estimated without considering contributions due to in situ 14C production would underestimate the true age. The concentration of in situ 14C in ice depends on several factors: (i) altitude and latitude of the site, (ii)ice accumulation/ablation rate, and (iii) degrees of retention of in situ produced 14C in the ice sample. We have a fairly accurate knowledge of the in situ 14C production rates for the steady state galactic cosmic ray flux, and can also make fair estimates for the temporal changes in flux with solar activity Consequently, if we can understand well the characteristics of trapping of in situ 14C in firn and in ice, the studies of 14C in polar ice can be used to determine past accumulation/ablation rates and changes in cosmic ray flux. Several studies of polar ice samples from Greenland and Antarctica have provided a reasonable baseline for understanding the characteristics of the in situ 14C. We review here the studies to date of a few GISP2 and several Antarctic ice samples.
Major features and forcing of high latitude northern hemisphere atmospheric circulation over the last
Mayewski, P.A., J.E. Dibb, W.B. Lyons, L.D. Meeker, M.C. Morrison, S. O`Brien, M. Prentice, J. Putscher, J. Thomas, M.S. Twickler, C.P. Wake, S. Whitlow, Q. Yang , G. Zielinski
Our glaciochemical investigations have contributed to the overall GISP2 effort by providing information valuable to: site selection; general understanding of the chemistry of Greenland snow (e.g., level, input timing) and relation to snow chemistry in other regions; dating of the GISP2 record; calibration of other ice core measurements; statistical and mathematical analysis of ice core and environmental time series; the impact of anthropogenic activity on the chemistry of the atmosphere; volcanic event and biomass burning records; characterization of climate change during the Little Ice Age, Younger Dryas, Dansgaard/Oeschger events and other abrupt changes in atmospheric circulation; documentation n of sea ice distribution and other environmental boundary conditions; and estimates of the relative influence of a variety of climate forcing factors (eg., insolation changes due to Earth orbit cycles, solar variability, volcanic events, aerosol loading, anthropogenic activity) over the last glacial/interglacial cycle.
A Lumped Parameter Model for the Atmosphere-to-Snow Transfer Function for Hydrogen Peroxide
McConnell, J.R., R.C. Bales, J.R. Winterle, H. Kuhns, C.R. Stearns
Of the main atmospheric oxidants, only hydrogen peroxide (H2O2) is preserved in polar ice cores. To make use of the peroxide record, however, requires a quantitative understanding of the "transfer function" or relation between atmospheric concentrations of H2O2 and those preserved in the ice core. Snow-pit H2O2 profiles adjacent to three automatic snow depth gages from Summit, Greenland were used to estimate parameters and evaluate the performance of a lumped parameter model to relate concentrations in the atmosphere with those in surface snow and shallow firn. Three of the model parameters define an equilibrium partitioning coefficient between snow and atmosphere as a nonlinear function of depositional temperature. Model parameters yielded a function that closely matched previous laboratory estimates (Conklin et al., 1993). A fourth parameter reflects the disequilibrium that may be preserved during periods of rapid accumulation. The final model parameter describes the exchange of H2O2 between near- surface snow and the atmosphere, allowing already buried snow to either take up or release H2O2 as conditions in and above the snowpack change. We simulated snow pit profiles by combining this transfer function model with a finite-difference model of gas-phase diffusion in the snowpack. Two application for this transfer function are (1) to estimate the local seasonal or annual atmospheric H2O2 concentration in the past from snow-pit and ice-core records and (2) to invert snow-pit and ice core H2O2 profiles to obtain estimates of the seasonal or annual accumulation time series. In the first case, an independent estimate of snow accumulation in needed, and in the second application, an independent estimate of the annual H2O2 atmospheric cycle is needed.
A 110ka history of change in continental biogenic source strength and related atmospheric circulation
Meeker, L.D., P.A. Mayewski, M.S. Twickler and S.I. Whitlow
A history of continental biogenic emissions is revealed from a 110,000 year long record of ammonium concentration recovered from a central Greenland ice core. Periodic components observed in the ammonium series are compared to previously derived records of Earth’s orbital cycles, sub-harmonics or combination tones of these cycles, and solar variability. Continental biogenic source emissions are closely linked to increases in both solar insolation and solar output. However, during the cold portions of rapid climate change events, when continental biogenic source strength is low, small increases in ammonium are still evident in Greenland as a consequence of expansion into these source regions by polar air masses.
The GISP2 Depth-Age Scale: Methods and Results
Meese, D.A., A.J. Gow, R.B. Alley, P. M. Grootes, M. Ram, K.C. Taylor, G.A. Zielinski, J.F. Bolzan, P.A. Mayewski, E.D. Waddington
The GISP2 depth-age scale is presented on a new multiparameter continuous count method. This is the first ice core or geological material to be continuously dated, with minor breaks beyond 110,000 yBP. Parameters used to date the core include visual stratigraphy, oxygen isotopes, electrical conductivity measurements, laser light scattering from dust, volcanic signals and major ion chemistry. Comparisons with deep sea cores, tree rings and varies using radiochemical methods calibrated to corals show agreement within the expected error for each method. Additional comparisons are also made to the GRIP ice core and a model based on the 18O of O2 combined with SPECMAP.
10Be Concentrations in the GISP2 Ice Core From 3-40 kabp.
Nishiizumi, K. and R. Finkel
Cosmogenic radionuclide concentrations in ice cores can be used to study the history of the galactic cosmic ray flux, of solar activity, of the geomagnetic field and of climate. With these objectives in mind, we have measured a continuous profile of 10Be and 36Cl in the 3.05 km-long GISP2 ice core, from the ice cap summit in central Greenland. The time resolution of our sampling varies from annual in the upper part of the core to centennial in the lower part. The annual samples show variations correlated with the 11-year Schwabe solar activity cycle. The 10Be and 36Cl concentrations also show significant climate- related variations. These variations are correlated with the climate swings (Oldest Dryas - BŅlling-AllerŅd and Younger Dryas - Preboreal) which occurred at the end of the last glacial period. In the deeper part of the core we observed an increase in 10Be concentration which is well correlated in time with the 10Be peaks observed in the Vostok, Antarctica core. Using the high resolution depth-age relation determined by annual layer counting on the GISP2 core , we have calculated radionuclide fluxes on a sample by sample basis in order to examine the role of changing annual layer thickness in determining the changes in radionuclide concentration. We have also revisited the variability in the 10Be/36Cl ratio in ice cores. We find that the ratios observed in these samples are significantly less variable than has been observed in cores from other sites in Greenland.
Continuous dust concentration profile of pre-holocene ice from the GISP2 ice core: dust stadial,
interstadials, and the Eemian.
Ram, M. and G. Koenig
In this paper, we discuss and interpret the Laser-Light Scattering (LLS) measurements we performed on the GISP2 ice core which allowed us to obtain a unique record of dust concentration along the core from surface down to bedrock. Measurements on Holocene ice were carried by scattering laser-light off core meltwater. For the glacial, bubble-free ice, we used a new technique involving scattering directly off the ice. LLS measurements were calibrated by comparison with Elzone‘ measurements done on select sections of ice core. The record exhibits sharp peaks along the entire length of the core which we interpret as spring-summer dust maxima. These seasonal peaks were very helpful in dating the core. The maximum resolution of our measurements was 3 mm for scattering off meltwater and 0.5 mm for scattering off ice. A smoothed, five year running average of our results was plotted and showed that the average dust concentration varied very little, for the most part, during the Holocene. A very different picture emerges for glacial ice, however. Large changes in dust concentration that correlate extremely well with corresponding changes in ECM and oxygen isotope measurements are observed to have occurred during glacial times. The dust record clearly shows the Younger Dryas transition, the BŅlling- AllerŅd and all the Dansgaard-Oeschger events that are seen in other records with periods of high dust concentration going hand in hand with cold events. The dust measurements also shed some important light on the relationship between the ECM signal and dust concentration. Breaking the continuity of our record are small sections where ice was lost during core retrieval and the large segment of brittle ice which was not measurable by laser-light scattering. Elzone™ measurements of dust concentration are presented for select sections of brittle ice.
Methanesulfonate in GISP2 ice core
Saltzman, E.S., Pai-Yei Whung, and P.A. Mayewski
A detailed record of methanesulfonate has been obtained from the GISP2 ice core. Methanesulfonate (MSA) is an oxidation product of atmospheric dimethylsulfide (DMS), which is the predominant biogenic sulfur gas emitted from the surface oceans. The MSA record in the GISP2 core therefore contains information about the emissions, transport, and atmospheric transformations of DMS, and about the relationship between the biogenic sulfur cycle and climate change. The most striking aspect of the GISP2 MSA record is that there is a slight decrease in MSA concentrations in glacial ice compared to Holocene ice. This trend is very different from that observed in the the East Antarctic Vostock ice core, where glacial conditions were characterized by elevated MSA concentrations.
The glacial/interglacial trend in MSA in the GISP2 core is markedly different from that of other ionic species (Mayewski et al., this volume), including sodium (Na+), sulfate (SO42-), chloride (Cl -), and calcium (Ca2+), which generally exhibit large increases during glacial times. The dramatic decrease in MSA/non-seasalt sulfate ratios in glacial ice supports the idea that the large SO42- concentrations in Greenland glacial ice are derived from terrestrial emissions, presumably in the form of continental dust, rather than marine gas emissions. MSA/Na ratios in the glacial ice of the GISP2 core are also significantly lower then Holocene values. Since both MSA and Na+ in Greenland ice are ultimately derived from air/sea exchange processes at the sea surface, this evidence suggests that there must have been a significant decrease in DMS concentrations in surface waters of the glacial North Atlantic Ocean.
Detection and monitoring of annual indicators and temperature trends at GISP2 using passive microwave
remote sensing data.
Shuman, C.A., R.B. Alley, M.A. Fahnestock, P.J. Fawcett, R.A. Blindschadler, J.W.C. White, P.M. Grootes, S. Anandakrishnan, and C. R. Stearns
Satellite, passive microwave sensors provide a sensitive means of studying ice-sheet surface processes that assists ice-core interpretation and can extend local observations across regional scales. Analysis of specie sensor microwave/imager (SSM/I) brightness temperature (TB) data supports ice-core research in two specific ways. Fist, the summer hoar complex layers used to date the Holocene portion of the Greenland Ice Sheet Project II (GISP2) ice core can be defined temporally and spatially by SSM/I 37 GHz V TB data can be correlated with proxy temperature trends from stable-isotope- ratio (18O and D) profiles from snow pits which allows the assignment of dates to specific snow depths.
The CO2 concentration of air trapped in GISP2 ice formed during periods of rapid climate change
Smith, H.J., M. Wahlen, D. Mastoianni, D. Taylor, P.A. Mayewski.
The CO2 content of air occluded in GISP2, Greenland, ice formed across two separate stadials, centered at approximately 46 kyBP and 69 kyBP, is as much as 90 ppm more during warm intervals than during cold. These CO2 variations are superimposed on changes in annual layer thickness and 18O of the ice and do not show the 200-700 year offsets which would be expected for concurrent variations in the atmosphere and the ice. The CO2 concentration during the stadials are similar to the atmospheric values recorded by Antarctic ice of the same age, so processes occurring in the ice after bubble enclosure must be enriching the air trapped in GISP2 ice formed during the interstadials. This conclusion is supported by ca content and ECM data for the ice, which show that adequate carbonate is present to produce these enrichments and that CO2 content is high only when the electrical conductivity (a proxy for H+ concentration) is high. High resolution mapping of one 4 cm section of ice reveals a 200 ppm increase in the CO2 content of the trapped air, from approximately 275 ppm to 475 ppm. Analyses of the TIC of ice from both the LGM and Holocene show that the 13C approaches that of soil and marine carbonates and imply that most of the Ca in the ice is from CaCO3. These results suggest hat the CO2 record preserved in ice can be altered by in- situ decarbonation reactions and that only ice containing either abundant or essentially no carbonate contains a reliable record of paleoatmospheric CO2.
An inter-laboratory comparison of techniques for extracting and analyzing
trapped gases in ice cores
Sowers, T., E. Brook, J. Chappellaz, J. M. Barnola, T. Blunier, A. Fuchs, M. Whalen and D. Etheridge
We are assessing the various analytical techniques and reference standards used to extract and analyze trapped gases in ice cores. The impetus for the study arose from a discussion amongst the GISP II and GRIP principle investigators at the Annecy meeting (3/93) concerning the comparison of the paleo- atmospheric records generated by various laboratories. Such an effort was deemed necessary to determine how the data sets compare to one another. It was agreed that each laboratory would analyze as many constituents as possible. The intercalibration effort has been divided into two parts:
1) Samples of compressed air were sent to all laboratories for analysis. Each laboratory was tasked to analyze their flask for as many different properties as possible. The results of this phase of the intercomparison will demonstrate how well the various working standards compare to one another. Preliminary results from the Grenoble laboratory indicate that their working CH4 standard has a nominal concentration which is 2.1% higher than the NOAA scale. Results from the other laboratories are forthcoming.
2) The next phase of the intercalibration involves the analyses of trapped air in ice. Ice from the GISP II F core was utilized for this purpose. Six meters of ice between 116 and 140 mbs were cut and transported to various laboratories for analysis. Two separate core configurations were used. In one configuration, 10-33cm pieces of core were split in half. Each of the two pieces from a 33cm interval was transported to a different laboratory. In this way, every core interval was analyzed by at least two laboratories. Enough ice was distributed so multiple measurements could be made on each 33 cm piece of ice. The second configuration involved cutting neighboring sections of ice and sending full core sections (~20cm) to each lab. This configuration was essential for those labs who crush ice using the “cheese grater” crusher. Again, only limited data are available at time of writing so no conclusions can be drawn as to the compatibility of the results.
In this manuscript we will provide a detailed discussion of the various extraction techniques. Blank problems, system configurations, and general extraction techniques will be covered in detail. The results of the intercalibration will provide the information necessary to accurately compare gas records generated by different laboratories and therefore improve understanding of atmospheric history derived from ice cores.
Electrical measurements on the GISP2 core
Taylor, K., R.B. Alley, G.W. Lamorey, P.A. Mayewski
The ability of the GISP2 core to conduct a DC electrical current was continuously measured along the length of the core with a resolution of 3mm. This measurement is related to the concentration of the H+ ion in the ice. A seasonal signal is observed in the Holocene which is related to seasonal changes in nitric acid. A seasonal signal is also observed in the late Wisconsin and is related to seasonal changes in the concentration of alkaline dust. These seasonal signals were used to assist in the identification of annual layers. Biomass burning in distant regions can produce ammonium which neutralizes the H+ ion in the ice. The ammonium causes a characteristic short duration decrease in the electrical conductivity. These characteristics can be used to develop a record of biomass burning. Volcanic activity can result in increased levels of HCL and H2SO4 in the ice which increases the electrical conductivity. This has been used to identify evidence of volcanic fallout in the core and for the development of stratigraphic ties between cores. The windy time periods associated with cold conditions deposited greater amounts of alkaline dust on the ice sheet which results in a 40 fold decrease in conductivity. Major climate changes occurred in less than a decade.
The climate signal in the stable isotopes of Summit, Greenland snow: Results of comparisons with modern
White, J.W.C., L.K. Barlow and D. Gorodetzky
Measurements of deuterium excess made on the GISP2 ice core have been used infer conditions of evaporation over the North Atlantic for the past 1000 years. Environmental factors influencing deuterium excess include the sea surface temperature (SST), the humidity over the ocean and the wind speeds over the ocean. Data for the last 100 years reveal a strong inverse correlation with Jakobshaven air temperatures. This is interpreted as reflecting an inverse relationship between SSTs in the North Atlantic and Jakobshaven temperatures, which is part of the East-West temperature “seesaw” between Oslo and Jakobshaven. Assuming deuterium excess can be used to reconstruct SSTs, isotopes in the GISP2 core can be used to infer a North-South isotope temperature seesaw in the North Atlantic region. The inferred record is presented for the last 1000 years. With the same assumption, deuterium excess values can also be used to correct D and 18O values for moisture source temperature effects. This correction is derived and applied to isotopic values from recent ice, the glacial interglacial transition, and the Dansgaard-Oescher Events.
Wilson, A, and Long
A review of techniques for recovering samples of paleoatmospheres from polar ice cores.
No Abstract on File
Volcanic aerosol records and tephrochronology of the Summit, Greenland, ice cores
Zielinski, G.A., P.A. Mayewski, L.D. Meeker, K. Grönvold, M.S. Germani, S. Whitlow, M.S. Twickler, and K. Taylor
The recently collected GISP2 and GRIP ice cores from summit, Greenland, provide lengthy and highly
resolved records of the deposition of both the aerosol (H2SO4) and silicate (tephra) components of past
volcanism. Both types of data are very beneficial in developing the entire volcanism-climate system. The
continuous time series of volcanic SO42- for the last 110,000 years show a strong relationship between
periods of increased volcanism and periods of climatic change. The greatest number of volcanic SO42-
signals, many of very high magnitude, occur during and after the final stages of deglaciation (6000-
17,000 years ago) possibly reflecting the increased crustal stresses that occur with changing volumes of
continental ice sheets and with the subsequent changes in the volume of water in ocean basins (sea level
change). The increase in the number of volcanic SO42- signals at 27,000-36,000 and 79,000-85,000
years ago may be related to initial ice sheet growth prior to the glacial maximum and prior to the
beginning of the last period of glaciation, respectively. A comparison of the electrical conductivity (ECM)
of the GISP2 core with that of the volcanic SO42- record for the Holocene indicates that only about half of
the larger volcanic signals are coincident in the two records. Other volcanic acids besides H2SO4 and
other SO42-sources can complicate the comparison although the threshold level picked to make such
comparisons is especially critical. Tephra as been found in both cores with a composition similar to that
originating from the Vatnaöldur eruption that produced the Settlement Layer in Iceland (mid-A.D. 870s),
from the Icelandic eruption that produced the Saksunarvatn ash (~10,300 years ago) and from the
Icelandic eruption(s) that produced the Z2 ash zone in North Atlantic marine cores (~52,7000 years ago).
Presence of these layers provide absolute time lines for correlation between the two cores and for
correlation with proxy records from marine sediment cores and terrestrial deposits containing these same
tephras. The presence of both rhyolitic and basaltic shards in the Z2 ash in the GISP2 core and the
composition of the basaltic grains lends support to multiple Icelandic sources (Torfajökull area and Katla)
for the Z2 layer. Deposition of the Z2 layer occurs at the beginning of a stadial event further reflecting the
possibility of a volcanic triggering by the effects of changing climatic conditions.