GISP2 Cores Research Articles

Ice ages: Evidence for past melting at the base of the GISP2 ice core from uranium-thorium disequilibria dating

In order to further develop 238U-234U-230Th disequilibria techniques for dating polar ice, we measured these nuclides by mass spectrometric methods for silty ice samples from the base of the GISP2 central Greenland ice core. We separated the samples by filtration and analyzed the <50 nm (truly dissolved + particulate) and >200 nm (particulate) filtered fractions. 230Th/234U activity ratios are quite low in the <50 nm fraction (0.18-0.24), and since natural liquid water is characterized by 230Th/234U activity ratios ≪1, these results suggest that recent melting/freezing event(s) have occurred at the base of the GISP2 core. The particulate fraction is characterized by 230Th/234U activity ratios of 1.12-1.23 which are also consistent with recent (<380 ka) fractionation of thorium from uranium in these ice samples.We have modeled these results using a two component mass balance calculation, with dissolved and particulate pools for each radionuclide. Although the model results vary slightly depending on assumptions, corrected dates for when the samples were last frozen range from 3500-8300 years ago, with typical 2σ uncertainties of 600-2300 years. These results are generally consistent with other geochemical and chronological data for Greenland ice cores and the notion of ice melting at the base of large continental ice sheets, even in areas that are currently frozen. Based on our U-series results and gas data in the literature, prior melting of the GISP2 ice core has evidently occurred for most of the 13 m thick silty basal layer but is most pronounced for the deepest 2 m. We also suggest that this new 230Th ingrowth method for dating previously melted ice may be more generally applicable to samples with a range of particulate matter concentrations.

Using Holo-Hilbert spectral analysis to quantify the modulation of Dansgaard-Oeschger events by obliquity

Astronomical forcing (obliquity and precession) has been thought to modulate Dansgaard-Oeschger (DO) events, yet the detailed quantification of such modulations has not been examined. In this study, we apply the novel Holo-Hilbert Spectral Analysis (HHSA) to five polar ice core records, quantifying astronomical forcing's time-varying amplitude modulation of DO events and identifying the preferred obliquity phases for large amplitude modulations. The unique advantages of HHSA over the widely used windowed Fourier spectral analysis for quantifying astronomical forcing's nonlinear modulations of DO events is first demonstrated with a synthetic data that closely resembles DO events recorded in Greenland ice cores (NGRIP, GRIP, and GISP2 cores on GICC05 modelext timescale). The analysis of paleoclimatic proxies show that statistically significantly more frequent DO events, with larger amplitude modulation in the Greenland region, tend to occur in the decreasing phase of obliquity, especially from its mean value to its minimum value. In the eastern Antarctic, although statistically significantly more DO events tend to occur in the decreasing obliquity phase in general, the preferred phase of obliquity for large amplitude modulation on DO events is a segment of the increasing phase near the maximum obliquity, implying that the physical mechanisms of DO events may be different for the two polar regions. Additionally, by using cross-spectrum and magnitude-squared analyses, Greenland DO mode at a timescale of about 1400 years leads the Antarctic DO mode at the same timescale by about 1000 years.

Ice-sheet bed 3-D tomography

AbstractInformation on bed topography and basal conditions is essential to developing the next-generation ice-sheet models needed to generate a more accurate estimate of ice-sheet contribution to sea-level rise. Synthetic aperture radar (SAR) images of the ice–bed can be analyzed to obtain information on bed topography and basal conditions. We developed a wideband SAR, which was used during July 2005 to perform measurements over a series of tracks between the GISP2 and GRIP cores near Summit Camp, Greenland. The wideband SAR included an eight-element receive-antenna array with multiple-phase centers. We applied the MUltiple SIgnal Classification (MUSIC) algorithm, which estimates direction of arrival signals, to single-pass multichannel data collected as part of this experiment to obtain fine-resolution bed topography. This information is used for producing fine-resolution estimates of bed topography over a large swath of 1600 m, with a 25 m posting and a relative accuracy of approximately 10 m. The algorithm-derived estimate of ice thickness is within 10 m of the GRIP ice-core length. Data collected on two parallel tracks separated by 500 m and a perpendicular track are compared and found to have difference standard deviations of 9.1 and 10.3 m for the parallel and perpendicular tracks, respectively.

Chronology reconstruction for the disturbed bottom section of the GISP2 and the GRIP ice cores: Implications for Termination II in Greenland

We have reconstructed chronology for the disturbed bottom parts of the GRIP and GISP2 ice cores using the combined paleoatmospheric records of CH4 concentration and δ18Oatm in the trapped gases. Our reconstructed ages for basal ice samples are based on comparison of published measurements of CH4 and δ18Oatm from the disturbed section of the GRIP and GISP2 cores with the same properties in the Vostok ice core. NGRIP δ18Oice values are also used to constrain the chronology during the end of marine isotope stage 5e. For each sample, we assign an age that represents the unique or most probable time of gas trapping, given its gas composition. Of 157 samples with CH4 and δ18Oatm data, 10 give unique ages. Twenty‐five newly measured values of the triple isotope composition of O2 from the disturbed section of the GISP2 core add a third time‐dependent gas property that agrees with our reconstruction. Our reconstruction supports earlier conclusions of Landais et al. (2003) that the disturbed section primarily includes ice from the last interglacial (MIS 5e) and the penultimate glacial period (MIS 6). The oldest ice in the basal layer of GISP2 and GRIP has an age ≥237 ka. The climate history we derive suggests that the last interglacial at Summit, Greenland, around 127 ka was slightly warmer than the current interglacial period. Reduction of various ion concentrations in ice and thickening of the ice sheet during Termination II was similar to that in Termination I.

Orbital tuning chronology for the Vostok climate record supported by trapped gas composition

We present data on the O 2/N 2 ratios of trapped gas samples for the entire length of the Vostok climate record. As in other cores, O 2/N 2 ratios in these samples are less than the atmospheric ratio, by a small and variable amount, because O 2 is selectively excluded during the gas trapping process, and because O 2 is also preferentially lost in poorly preserved core samples. Samples younger than 150 ka have large and variable O 2 depletions. Samples older than 200 ka have O 2/N 2 ratios that replicate well and vary smoothly with depth. We plot O 2/N 2 ratios of well replicated samples older than 160 ka, using a chronology derived by matching the δ 18O of paleoatmospheric O 2 (δ 18O atm) to northern hemisphere June insolation. On this timescale, O 2/N 2 varies coherently with local (78°S) summertime insolation. Based on time series analysis of the O 2/N 2 record and the dynamics of snow metamorphism at the surface, we conclude that summertime insolation influences physical properties of ice grains that control the degree of O 2 exclusion during bubble closeoff. O 2/N 2 in Vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core. We show that the δ 18O atm chronology, supported by the coincidence of O 2/N 2 ratios with insolation, is also compatible with recent radiometric dating of corals from high sea stands. We further successfully test the δ 18O atm tuning chronology by showing that it predicts a chronology for the GISP2 core which is essentially indistinguishable from the standard GISP2 chronology and, therefore, in excellent agreement with the radiometric chronology of Hulu Cave, China. An accurate chronology for the Vostok ice core is now in place.

The Two‐Mile Time Machine: Ice Cores, Abrupt Climate Change and Our Future

Enhanced greenhouse warming attributable to our profligate consumption of fossil fuels has established itself as perhaps the most contentious environmental issue currently being debated in the international political arena. To a large extent, the debate over whether societies should undertake actions to mitigate potentially adverse effects of climate change focuses on uncertainties in climate predictions and societal impacts. In an effort to reduce some of the uncertainty the climate community has expended considerable efforts to obtain detailed records of past climates, recognizing that the past may hold the key to our future.The Two‐Mile Time Machine by Richard Alley tells the captivating story of how one such record reconstructed from the GISP2 core, drilled through the more than 3‐km thick Greenland Ice Sheet, has revealed startling clues about past climates and shattering many of our ideas on how climate operates.

Interannual variability of elevation on the Greenland ice sheet: effects of firn densification, and establishment of a multi-century benchmark

AbstractIn order to interpret measurements of ice-sheet surface elevation changes in terms of climatic or dynamic trends, it is necessary to establish the range of stochastic variability of elevation changes resulting from interannual fluctuations of accumulation rate and firn density. The analyses presented here are intended to facilitate such interpretations by defining benchmarks that characterize elevation-change variability in central Greenland, in the current climate and over the past millennium. We use a time- dependent firn-densification model coupled to an ice- and heat-flow model, forced by annual accumulation rate and temperature reconstructions from the Greenland Ice Sheet Project II (GISP2) ice core, to examine the elevation changes resulting from this climatic forcing. From these results, effective firn densities are calculated. These are factors that convert water-equivalent accumulation-rate variability to surface elevation variability. A current-climate benchmark is defined by applying this conversion to Van der Veen and Bolzan’s water-equivalent statistics, and to a 50 year accumulation variability estimate from the GISP2 core. Elevation-change statistics are compiled for the past millennium to define longer-term benchmarks, which show that multi-century variability has been substantially larger than current variability estimated by Van der Veen and Bolzan. It is estimated here that the standard deviation of net elevation change over 5 and 10 year intervals has been 0.27 and 0.38 m, respectively. An approximate method for applying these quantitative results to other dry-snow sites in Greenland is suggested.

On the characteristics of cosmogenic in situ 14C in some GISP2 Holocene and late glacial ice samples

In previous studies, we have shown that information on ice accumulation processes is recorded by the in situ production of 14C, from nuclear interactions of cosmic rays with oxygen in ice (Lal et al., 1997). In this report, we discuss a study of a number of Holocene and several glacial ice samples from the GISP2 core whose accumulation rates are known and which should be little affected by uncertainties in ice flow models (Cuffey et al., 1997), to determine trapping efficiencies of in situ 14C. We present new results along with discussion of the earlier results of Lal et al. (1997), which include results on partitioning of in situ 14C among the CO and CO 2 phases through time in ice. The usefulness of the in situ 14C data to measurements of ice accumulation rates and ages are discussed.

GISP2 Oxygen Isotope Ratios

The GISP2 oxygen isotope record, with its high-resolution detail, yields crucial information on past climate change. The glacial δ18O oscillations of the GISP2 core, with their very fast onsets, are templates of a prototype oscillation of variable duration with an amplitude of 3.9‰. The halfway mark of the cold–warm transition is reached in 2 years; the top is reached in 50 years. The δ18O–time gradient of the leading front is about 7.8‰ per 100 yr. After reaching the top, δ18O slowly declines by −0.14‰ per 100 yr. The duration of δ18O decline varies from a couple of centuries for fast oscillations to about 4000 yr for slower ones. The subsequent δ18O downturn during the warm–cold transition has a δ18O–time gradient of −3.2‰ per 100 yr and lasts about 80 yr.

10Be and36Cl concentrations in the GISP2 and Siple Dome ice cores

10Be and36Cl concentrations in the GISP2 and Siple Dome ice cores

Anisotropic behavior of GRIP ices and flow in Central Greenland

Mechanical tests have been performed on strongly textured ice samples coming from a wide range of depths (from 1328 down to 2868 m) of the GReenland Ice core Project (GRIP) for different sample orientations with respect to the prescribed stress. In this way, two directional viscosities, corresponding to the “easy glide” and to the “hard glide” orientations, were determined along the core. The viscoplastic anisotropy gradually increases down to a depth of ∼2600 m and slightly decreases below, revealing a clear correlation between rheology and texture. The experimental mechanical response compares well with that predicted by the ViscoPlastic Self-Consistent (VPSC) model. The VPSC model is also applied to ice samples that exhibit an axisymmetric texture to show in more detail the sensitivity of the rheology to specific texture parameters. This leads to a number of recommendations for future mechanical tests on anisotropic samples. A large-scale ice flow model is finally used to estimate the influence of ice anisotropy on the flow along the GRIP–GISP2 flow line. The particular mechanical behavior of deep GRIP ices in the stress regime corresponding to an ice divide leads to deformation rates that are highly sensitive to bedrock topography and texture pattern. This feature is likely to favour the formation of stratigraphic disturbances in deep ice layers, as observed in the last 300 m of both GRIP and GISP2 cores.

Temperature history and accumulation timing for the snowpack at GISP2, central Greenland

AbstractPrevious research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated thatδD andδ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of theδDrecord at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2data. The slope of the multi-yearTvsδcorrelation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.

Temperature history and accumulation timing for the snowpack at GISP2, central Greenland

AbstractPrevious research has documented a close association between high-resolution snow-pit profiles of hydrogen and oxygen stable-isotope ratios and multi-year Special Sensor Microwave/lmager (SSM/I) 37 GHz brightness temperature data in central Greenland. Comparison of the SSM/I data to profiles obtained during the 1989-91 field seasons indicated that δD and δ18O data from the near-surface snow at the Greenland summit are a reliable, high-resolution temperature proxy. To test this new technique further, additional stable-isotope data were obtained from a 2 m snow pit constructed during late-June 1995 near the GISP2 site.This new profile, supported by pit stratigraphy and chemistry data, confirms the utility of comparing stable-isotope records with SSM/I brightness temperatures. The sub-annual variation of the δD record at the GISP2 site was determined using 15 match points, from approximately December 1991 through June 1995 and was guided in part by time-constrained hoar layers. The close association of these temperature proxies supports the assertion that snow accumulation occurs frequently through the year and that the isotope record initially contains temperature information from many times of the year. This is also independently confirmed by analysis of H2O2 data. The slope of the multi-year T vs δ correlation was evaluated along with the sub-annual variation in the amount, rate and timing of accumulation. These new results are consistent with those from the previous study and they also demonstrate that the snow in this area initially contains temperature and chemical records with sub-annual resolution. This encourages confident interpretation of the paleoclimatic signal variations in the GISP2 and GRIP deep cores.

The Holocene-Younger Dryas Transition Recorded at Summit, Greenland

Analysis of ice from Dye-3, Greenland, has demonstrated that the transition between the Younger Dryas and Holocene climate periods occurred over a 40-year period. A near annually resolved, multiparameter record of the transition recorded in the GISP2 core from Summit, Greenland, shows that most of the transition occurred in a series of steps with durations of about 5 years. Some climate proxies associated with mid-latitude sources appear to have changed about 15 years before climate proxies associated with more northern regions. Changes in atmospheric water vapor are likely to have played a large role in the climate transition.

Sulphur emissions to the stratosphere from explosive volcanic eruptions

Two methods were used to quantify the flux of volcanic sulphur (as the equivalent mass of SO2) to the stratosphere over different timescales during the Holocene. A combination of satellite-based measurements of sulphur yields from recent explosive volcanic eruptions with an appropriate rate of explosive volcanism for the past 200 years constrains the medium-term (∼102 years) flux of volcanic sulphur to the stratosphere to be ∼1 Mt a–1, with lower and upper bounds of 0.3 and 3 Mt a–1. The short-term (∼10- to 20-year) flux due to small magnitude (1010–1012 kg) eruptions is of the order of 0.4 Mt a–1. At any time the instantaneous levels of sulphur in the stratosphere are dominated by the most recent (0–3 years) volcanic events. The flux calculations do not attempt to address this very short timescale variability. Although there are significant errors associated with the raw sulphur emission data on which this analysis is based, the approach presented is general and may be readily modified as the quantity and quality of the data improve. Data from a Greenland ice core support these conclusions. Integration of the sulphate signals from presumed volcanic sources recorded in the GISP2 core provides a minimum estimate of the 103–year volcanic SO2 flux to the stratosphere of 0.5–1 Mt a–1 over the past 9000 years. The short-term flux calculations do not account for the impact of rare, large events. The ice-core record does not fully account for the contribution from small, frequent events.

Greenland ice core records and rapid climate change

Long ice cores from Greenland yield records of annually resolved climate change for the past ten to twenty thousand years, and decadal resolution for one hundred thousand years or more. These cores are ideally suited to determine the rapidity with which major climate changes occur. The termination of the Younger Dryas, which marks the end of the last glacial period, appears to have occurred in less than a human lifetime in terms of oxygen isotopic evidence (a proxy for temperature), in less than a generation (20 years) for dust content and deuterium excess (proxies for winds and sea-surface conditions), and in only a few years for the accumulation rate of snow. Similarly rapid changes have been observed for stadial-interstadial climate shifts (Dansgaard-Oeschger cycles) which punctuate the climate of the last glacial period. These changes appear to be too rapid to be attributed to external orbital forcings, and may result from internal instabilities in the Earth’s atmosphere-ocean system or periodic massive iceberg discharges associated with ice sheet instability (Heinrich events). In contrast, the Holocene climate of the Arctic appears to have been relatively stable. However, the potential for unstable interglacials, with very rapid, shortlived climatic deteriorations, has been raised by results from the lower part of the GRIP ice core. These results have not been confirmed by other ice cores, notably the nearby GISP2 core. Evidence from other records of climate during the Eemian interglacial have yielded mixed results, and the potential for rapid climate change during interglacial periods remains one of the most intriguing gaps in our understanding of the nature of major Quaternary climate change.

Climate correlations between Greenland and Antarctica during the past 100,000 years

THE ice cores recovered from central Greenland by the GRIP1,2 and GISP23 projects record 22 interstadial (warm) events during the part of the last glaciation spanning 20–105 kyr before present. The ice core from Vostok, east Antarctica, records nine interstadials during this period4,5. Here we explore links between Greenland and Antarctic climate during the last glaciation using a high-resolution chronology derived by correlating oxygen isotope data for trapped O2 in the GISP2 and Vostok cores. We find that interstadials occurred in east Antarctica whenever those in Greenland lasted longer than 2,000 years. Our results suggest that partial deglaciation and changes in ocean circulation are partly responsible for the climate teleconnection between Greenland and Antarctica. Ice older than 115 kyr in the GISP2 core shows rapid variations in the δ18O of O2 that have no counterpart in the Vostok record. The age–depth relationship, and thus the climate record, in this part of the GISP2, core appears to be significantly disturbed.