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The Holocene portion of the Siple Dome (Antarctica) ice core was dated by interpreting the electrical, visual and chemical properties of the core. The data were interpreted manually and with a computer algorithm. The algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the GISP2 (Greenland Ice Sheet Project 2) ice core, 10Be stratigraphic ties to the dendrochronology 14 C record and the dated volcanic stratigraphy. The algorithm interpretation is more consistent and better quantified than the tedious and subjective manual interpretation.

A new ice core from Siple Dome, Antarctica suggests the surface temperature increased by ∼6°C in just several decades at approximately 22 ka BP. This abrupt change did not occur 500 km away in the Byrd ice core, or in climate proxy records in the Siple Dome core indicative of the mid-latitude Pacific. This demonstrates there was significant spatial heterogeneity in the response of the Antarctic climate during the last deglaciation and draws attention to unexplained mechanisms of abrupt climate change in Antarctica.

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

Old and older, cold and colder Greenland surface air temperatures changed dramatically during the last deglaciation. The exact amount is unknown, which makes it difficult to understand what caused those changes. Buizert et al. report temperature reconstructions for the period from 19,000 to 10,000 years before the present from three different locations in Greenland and interpret them with a climate model (see the Perspective by Sime). They provide the broad geographic pattern of temperature variability and infer the mechanisms of the changes and their seasonality, which differ in important ways from the traditional view. Science, this issue p. 1177 ; see also p. 1116

Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.

The abrupt warming that initiated the Bølling-Allerød interstadial was the penultimate warming in a series of climate variations known as Dansgaard-Oeschger events. Despite the clear expression of this transition in numerous palaeoclimate records, the relative timing of climate shifts in different regions of the world and their causes are subject to debate. Here we explore the phasing of global climate change at the onset of the Bølling-Allerød using air preserved in bubbles in the North Greenland Eemian ice core. Specifically, we measured methane concentrations, which act as a proxy for low-latitude climate, and the 15N/ 14N ratio of N2, which reflects Greenland surface temperature, over the same interval of time. We use an atmospheric box model and a firn air model to account for potential uncertainties in the data, and find that changes in Greenland temperature and atmospheric methane emissions at the Bølling onset occurred essentially synchronously, with temperature leading by 4.5+21-24 years. We cannot exclude the possibility that tropical climate could lag changing methane concentrations by up to several decades, if the initial methane rise came from boreal sources alone. However, because even boreal methane-producing regions lie far from Greenland, we conclude that the mechanism that drove abrupt change at this time must be capable of rapidly transmitting climate changes across the globe. © 2014 Macmillan Publishers Limited.

Reconstruction of atmospheric CO2 during times of past abrupt climate change may help us better understand climate-carbon cycle feedbacks. Previous ice core studies reveal simultaneous increases in atmospheric CO2 and Antarctic temperature during times when Greenland and the northern hemisphere experienced very long, cold stadial conditions during the last ice age. Whether this relationship extends to all of the numerous stadial events in the Greenland ice core record has not been clear. Here we present a high-resolution record of atmospheric CO2 from the Siple Dome ice core, Antarctica for part of the last ice age. We find that CO2 does not significantly change during the short Greenlandic stadial events, implying that the climate system perturbation that produced the short stadials was not strong enough to substantially alter the carbon cycle., Whether all rapid climate events during the last ice age impacted the global carbon cycle is not clearly understood. Ahn and Brook present a high-resolution record of atmospheric CO2 from Antarctica and suggest that only Greenland stadials associated with massive iceberg discharge influenced atmospheric CO2.

Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources. The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial-interglacial and millennial timescales. It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations. Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming.

We present successful (81)Kr-Kr radiometric dating of ancient polar ice. Krypton was extracted from the air bubbles in four ∼350-kg polar ice samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA). The (81)Kr radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 ± 2.5 ka. Our experimental methods and sampling strategy are validated by (i) (85)Kr and (39)Ar analyses that show the samples to be free of modern air contamination and (ii) air content measurements that show the ice did not experience gas loss. We estimate the error in the (81)Kr ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (Marine Isotope Stage 5e, 130-115 ka before present) can be found in abundance near the surface of Taylor Glacier. Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA (81)Kr analysis requires a 40-80-kg ice sample; as sample requirements continue to decrease, (81)Kr dating of ice cores is a future possibility.

Bipolar Signature Atmospheric methane has increased approximately 2.5-fold since the start of the industrial revolution, a consequence of human activity. However, a smaller and more gradual rise began around 6000 years ago, near the time when human agriculture began to develop and expand. Mitchell et al. (p. 964 ) present two, high-resolution ice core methane records of the past 2500 years, one from each pole. Methane emissions were primarily from the tropics, with secondary contributions from the higher latitudes where most humans lived. Thus, both natural and human sources are needed to explain the late-Holocene atmospheric methane record.

Measurements of helium isotopes in particles separated from polar ice demonstrate that extraterrestrial ³He dominates the ³He flux at the GISP2 (Greenland) and Vostok (Antarctica) ice core sites. Replicate measurements of late Holocene ice samples yield ³He fluxes of 0.62±0.27×10−12 cm³ STP cm−2 ka−1 (GISP2) and 0.77 ± 0.25 × 10−12 cm³ STP cm−2 ka−1 (Vostok), similar to results from marine sediments. These are the first detailed measurements of ³He in particles from ice core samples, and they demonstrate the utility of the ice core record for evaluating the temporal history of the extraterrestrial dust flux. Results from Vostok samples from 1096–1403 m depth (75–97 ka B.P.) are similar to the late Holocene data, with the exception of two highly anomalous results from 1307 m. The latter probably indicate the presence of rare, large or gas-rich extraterrestrial particles.

According to one controversial idea, increases in atmospheric greenhouse-gas concentrations due to human activities can be detected as early as several thousand years ago. Eight years after the publication of this hypothesis, controversy continues.

The last glacial period was terminated by an abrupt warming event in the North Atlantic approximately 15,000 years before the present, and warming events of similar age have been reported from low latitudes. Understanding the mechanism of this termination requires that the precise relative timing of abrupt climate warming in the tropics versus the North Atlantic be known. Nitrogen and argon isotopes in trapped air in Greenland ice show that the Greenland Summit warmed 9 +/- 3 degrees C over a period of several decades, beginning 14,672 years ago. Atmospheric methane concentrations rose abruptly over a approximately 50-year period and began their increase 20 to 30 years after the onset of the abrupt Greenland warming. These data suggest that tropical climate became warmer or wetter (or both) approximately 20 to 80 years after the onset of Greenland warming, supporting a North Atlantic rather than a tropical trigger for the climate event.

Central Greenland ice cores provide evidence of abrupt changes in climate over the past 100,000 years. Many of these changes have also been identified in sedimentary and geochemical signatures in deep-sea sediment cores from the North Atlantic, confirming the link between millennial-scale climate variability and ocean thermohaline circulation. It is shown here that two of the most prominent North Atlantic events—the rapid warming that marks the end of the last glacial period and the Bølling/Allerød–Younger Dryas oscillation—are also recorded in an ice core from Taylor Dome, in the western Ross Sea sector of Antarctica. This result contrasts with evidence from ice cores in other regions of Antarctica, which show an asynchronous response between the Northern and Southern Hemispheres.

Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 ± 3 °C colder than today.

We undertook an interlaboratory comparison of techniques used to extract and analyze trapped gases in ice cores. The intercomparison included analyses of standard reference gases and samples of ice from the Greenland Ice Sheet Project 2 (GISP2) site. Concentrations of CO2, CH4, the δ;18O of O2, the δ15N of N2, and the O2/N2, and Ar/N2 ratios were measured in air standards and ice core samples. The standard reference scales for CO2 and CH4 were consistent at the ±2% level. The δO2/N2 and δ18O of O2 measurements showed substantial deviations between the two laboratories able to measure these ratios. The deviations are probably related to errors associated with calibration of the working standards. The δArN2 and δ15N of N2 measurements were consistent. Five laboratories analyzed the CH4 concentration in a 4.2-m section of the GISP2 ice core. The average of 20 discrete CH4 measurements was 748±10 parts per billion by volume (ppbv). The standard deviation of these measurements was close to the total analytical uncertainty associated with the measurements. In all cases, those laboratories employing a dry extraction technique determined higher CH4 values than laboratories using a wet extraction technique. The origin of this difference is unclear but may involve uncertainties associated with blank corrections. Analyses of the CO2 concentration of trapped gases showed extreme variations which cannot be explained by analytical uncertainties alone. Three laboratories measured the [CO2] on 21 discrete depths yielding an average value of 283±13 parts per million by volume (ppmv). In this case, the standard deviation was roughly a factor of 2 greater than the analytical uncertainties. We believe the variability in the measured [CO2] results from impurities in the ice which may have compromised the [CO2] of trapped gases in Greenland ice. © Copyright 1997 by the American Geophysical Union.

Our knowledge of long-term human effects on greenhouse gas levels in the atmosphere comes from air trapped in ice cores taken from polar ice sheets. These ice core samples allow researchers to place modern changes in the context of natural variations over hundreds of thousands of years. In his Perspective, [Brook][1] discusses results reported in the same issue by [ Siegenthaler et al .][2] and by [ Spahni et al .][3] based on new samples obtained by the European Project for Ice Coring in Antarctica (EPICA). The new long records of carbon dioxide, methane, and nitrous oxide from EPICA extend the window on greenhouse gas levels to 650,000 years. The results confirm that the modern atmosphere is highly anomalous and reinforce the view that greenhouse gases and climate are intimately related. [1]: http://www.sciencemag.org/cgi/content/full/310/5752/1285 [2]: http://www.sciencemag.org/cgi/content/full/310/5752/1313 [3]: http://www.sciencemag.org/cgi/content/full/310/5752/1317

International audience; Ancient air trapped inside bubbles in ice cores can now be analysed for methane concentration utilising a laser spectrometer coupled to a continuous melter system. We present a new ultra-high resolution record of atmospheric methane variability over the last 1800 yr obtained from continuous analysis of a shallow ice core from the North Greenland Eemian project (NEEM-2011-S1) during a 4-week laboratory-based measurement campaign. Our record faithfully replicates the form and amplitudes of multi-decadal oscillations previously observed in other ice cores and demonstrates the detailed depth resolution (5.3 cm), rapid acquisition time (30 m day-1) and good long-term reproducibility (2.6%, 2σ) of the continuous measurement technique. In addition, we report the detection of high frequency ice core methane signals of non-atmospheric origin. Firstly, measurements of air from the firn-ice transition region and an interval of ice core dating from 1546-1560 AD (gas age) resolve apparently quasi-annual scale methane oscillations. Traditional gas chromatography measurements on discrete ice samples confirm these signals and indicate peak-to-peak amplitudes of ca. 22 parts per billion (ppb). We hypothesise that these oscillations result from staggered bubble close-off between seasonal layers of contrasting density during time periods of sustained multi-year atmospheric methane change. Secondly, we report the detection of abrupt (20-100 cm depth interval), high amplitude (35-80 ppb excess) methane spikes in the NEEM ice that are reproduced by discrete measurements. We show for the first time that methane spikes present in thin and infrequent layers in polar, glacial ice are accompanied by elevated concentrations of carbon- and nitrogen-based chemical impurities, and suggest that biological in-situ production may be responsible.

3He,10Be and26Al surface exposure ages were determined for rock samples from late Quaternary drift sheets deposited on the coast of McMurdo Sound, Antarctica, by the Ross Sea Ice Sheet. The youngest drift, previously believed to be Late Wisconsin in age, has exposure ages of 8–106 kyr (19 samples). These results suggest that the apparent ‘Late Wisconsin’ ice margin may represent a number of ice sheet advances during the last glacial period.3He exposure ages from analogous, stratigraphically older drift range from 104 to 567 kyr (seven samples), and clearly distinguish the youngest drift from older deposits. Comparison of10Be and3He concentrations in Ross Sea Drift quartz samples indicates significant3He loss in only one sample, suggesting that cosmogenic3He is generally retained in young Antarctic quartz samples (exposure age < 100 kyr) of appropriate grain size. In two of the five quartz samples analyzed, significant excesses of26Al relative to10Be preclude meaningful26Al exposure age calculations. These excesses may be due to nucleogenic26Al produced as a byproduct of uranium and thorium decay.

[1] During the last glacial period atmospheric carbon dioxide and temperature in Antarctica varied in a similar fashion on millennial time scales, but previous work indicates that these changes were gradual. In a detailed analysis of one event we now find that approximately half of the CO2increase that occurred during the 1500-year cold period between Dansgaard-Oeschger (DO) events 8 and 9 happened rapidly, over less than two centuries. This rise in CO2 was synchronous with, or slightly later than, a rapid increase of Antarctic temperature inferred from stable isotopes.

[1] We report a decadally resolved record of atmospheric CO2 concentration for the last 1000 years, obtained from the West Antarctic Ice Sheet (WAIS) Divide shallow ice core. The most prominent feature of the pre-industrial period is a rapid ∼7 ppm decrease of CO2 in a span of ∼20–50 years at ∼1600 A.D. This observation confirms the timing of an abrupt atmospheric CO2 decrease of ∼10 ppm observed for that time period in the Law Dome ice core CO2 records, but the true magnitude of the decrease remains unclear. Atmospheric CO2 variations over the time period 1000–1800 A.D. are statistically correlated with northern hemispheric climate and tropical Indo-Pacific sea surface temperature. However, the exact relationship between CO2 and climate remains elusive due to regional climate variations and/or uneven geographical data density of paleoclimate records. We observe small differences of 0 ∼ 2% (0 ∼ 6 ppm) among the high-precision CO2 records from the Law Dome, EPICA Dronning Maud Land and WAIS Divide Antarctic ice cores. However, those records share common trends of CO2 change on centennial to multicentennial time scales, and clearly show that atmospheric CO2 has been increasing above preindustrial levels since ∼1850 A.D.

surface 14 C activity that differs by up to 25% from the activity calculated using an ablation-only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model twodimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future 14 C and ice flow studies at ice margin

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO 2 and CH 4 to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.

[1] We present a new high-precision, high-resolution record of atmospheric methane from the West Antarctic Ice Sheet (WAIS) Divide ice core covering 1000–1800 C.E., a time period known as the late preindustrial Holocene (LPIH). The results are consistent with previous measurements from the Law Dome ice core, the only other high-resolution record of methane for this time period, and confirm most of the observed variability. Multidecadal variability in methane concentrations throughout the LPIH is weakly correlated or uncorrelated with reconstructions of temperature and precipitation from a variety of geographic regions. Correlations with temperature are dominated by changes in Northern Hemisphere high latitude temperatures between 1400 and 1600 C.E. during the onset of the Little Ice Age. Times of war and plague when large population losses could have reduced anthropogenic emissions are coincident with short periods of decreasing global methane concentrations.

In situ-produced cosmogenic helium (3Hec) provides a new tool for constraining histories of Quaternary geomorphic surfaces. Before general application of the technique, however, the systematics and production rates of 3Hec must be well understood. In a companion study, 3He and 10Be data from sandstone and granite boulders in the Dry Valleys region of Antarctica have been used to constrain the ages of an important moraine sequence formed by the Taylor Glacier. Data from these deposits also provide information about the systematics of 3He in quartz that has important implications for geochronology based on 3Hec. In contrast to previous results from olivine and clinopyroxene, crushing quartz in vacuo releases helium with high 3He/4He ratios (up to 148 × Ra, where Ra is the atmospheric 3He/4He ratio), indicating that crushing cannot be used to determine the isotopic composition of trapped (i.e., noncosmogenic) helium in quartz. Analysis of 3He in different size fractions of the same samples indicates significant 3 He loss not predicted by existing 3He diffusion data for quartz. The origin of the discrepancy is not clear, but loss from these samples is not as significant as suggested by the limited data of previous studies.

In situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits. In situ3He and 10Be in quartz sandstone boulders from Arena Valley, southern Victoria Land, Antarctica, provide chronological constraints for a sequence of moraines ("Taylor II-IVb" moraines) related to expansions of Taylor Glacier and the East Antarctic Ice Sheet. Mean 3He ages are 113,000 ± 45,000 yr, 208,000 ± 67,000 yr, 335,000 ± 187,000 yr, and 1.2 ± 0.2 myr, for Taylor II,III,IVa, and IVb moraines, respectively (mean ± 1σ). Corresponding mean 10Be ages for Taylor II and IVb moraines are 117,000 ± 51,000 yr and 2.1 ± 0.1 myr. For the older deposits the 3He ages are probably lower limits due to diffusive loss. Although the exposure ages appear consistent with the few previous age estimates, particularly with an isotope stage 5 age for Taylor II, each moraine exhibits a broad age distribution. The distribution probably results from a variety of factors, which may include prior exposure to cosmic rays, 3He loss, erosion, postdepositional boulder movement, and radiogenic production of 3He. Nonetheless, the exposure ages provide direct chronological constraints for the moraine sequence, and suggest a maximum thickening of Taylor Glacier relative to the present ice surface of ∼500 m since the late Pliocene-early Pleistocene.

We have measured cosmic ray produced 10Be (t½= 1.5 My) and 26Al (t½ = 0.72 My) as functions of depth in a core of quartz sandstone bedrock collected in South Victoria Land, Antarctica. These data were used to place limits on the exposure age and erosion rate of the exposed surface, and to calculate the effective attenuation lengths of cosmic rays producing these nuclides. These latter results indicate that the production attenuation length for 26Al is comparable to that for 10Be . This suggests that the production ratio 26Al:10Be does not vary substantially with depth in a rock, and that its value at the surface of exposed rock is not strongly affected by erosional losses, allowing wider use of the ratio in determining exposure ages.

[1] The global biogeochemical cycle of methane has received wide attention because of methane's role as a greenhouse gas. Measurements of methane in air trapped in Greenland ice cores provide a high-resolution record of methane levels in the atmosphere over the past ∼100 ka, providing clues about what controls the methane cycle on geologic timescales. Remarkable similarity between local temperature recorded in Greenland ice cores and changes in global methane concentrations has been noted in previous studies, with the inference that the local temperature variations have global significance, but the resolution of sampling and measurement precision limited fine-scale comparison of these variables. In this work a higher-precision (∼2 ppb) methane data set was obtained from the Greenland Ice Sheet Project 2 (GISP2) ice core for the time interval between 86 and 68 ka, encompassing three large abrupt warming events early in the last glacial period: Dansgaard-Oeschger (D-O) events 19, 20, and 21. The new data set consists of duplicate measurements at 158 depths, with average time resolution of 120 years. Such detailed measurements over D-O 21, the longest in Greenland records, have not yet been reported for other ice cores. The new data set documents short-term variability (∼20 ppb typical amplitude), which is remarkably persistent, and in many cases similar features are observed in the most detailed published δ18Oice record. High-precision GISP2 δ15N data show that changes in Greenland temperature are synchronous with the methane variations at the onset of D-O events 19, 20, and 21, supporting previous results from the Greenland Ice Core Project ice core for D-O 19 and 20. Cross-spectral analysis quantifies the extremely close similarity between the new methane record and the δ18Oice record. Because methane sources are widely distributed over the globe, this work further validates δ18Oice at Greenland summit as a geographically broad climate indicator on millennial to multicentennial timescales.

The cause of a large increase of atmospheric methane concentration during the Younger Dryas–Preboreal abrupt climatic transition (~11,600 years ago) has been the subject of much debate. The carbon-14 ( 14 C) content of methane ( 14 CH 4 ) should distinguish between wetland and clathrate contributions to this increase. We present measurements of 14 CH 4 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured 14 CH 4 values were higher than predicted under any scenario. Sample 14 CH 4 appears to be elevated by direct cosmogenic 14 C production in ice. 14 C of CO was measured to better understand this process and correct the sample 14 CH 4 . Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas–Preboreal CH 4 rise.

Concentrations of 10Be (t12 = 1.5 × 106 y) and 26A1 (t12 = 0.72 × 106 y) have been determined by accelerator mass spectrometry (AMS) in a suite of quartz samples taken from sandstone boulders in several moraines in Arena Valley, a dry valley adjacent to the Taylor Glacier in the Quatermain Mountains, Southern Victoria Land, East Antarctica. These isotopes are produced in surficial quartz by cosmic ray spallation of O and Si. The concentrations in these samples ranged from 6.1 × 105 to 3.0 × 107 at g−1 for 10Be and from 9.4 × 106 to 1.2 × 108 at g−1 for 26A1, depending upon the extent of exposure at the surface. Analyses of 10Be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons; the value of 142+96−41 g cm−2 is consistent with other published values. Production rates of 17+6−4 at g−1 y−1 for 10Be and 113+54−16 at g−1 y−1 for 26A1 at 1300 m and 87°S and a 26Al: 10Be production ratio of 6.5+1.3−1.3 were calculated from the data. These values correspond to sea-level production rates at high geomagnetic latitude of 6.4 at g−1 y−1 and 41.7 at g−1 y−1 for 10Be and 26A1, respectively, consistent with determinations based on ~ 11 Ky glacially polished surfaces in the Sierra Nevada in California. These production rates imply exposure ages for the various moraines ranging from 50 Ky to 2.5 My, in accordance with other geological evidence. The 10Be and 26Al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3He. Examination of the 3He concentrations in conjunction with the present data set yields an estimate of 3He production of 230+8540 at g−1 y−1, corresponding to 85+31−15 at g−1 y−1 at sea level. This is lower than previously reported values of 240 ± 60 at g−1 y−1 and ~190 at g−1 y−1 (scaled to sea-level and high geomagnetic latitude) based, respectively, on volcanic flows less than 2000 years old at low latitude, and samples collected from 14,400-year-old volcanic flows. This discrepancy is likely due to diffusive losses which may become significant (up to ~50%) for surface exposure ages on the order of 2.0 My. Nevertheless, the general correlations among the three isotopes suggest that 3He does not suffer gross losses through diffusion.

Boulders from the Gruda Moraine, which is associated with the maximum extent of the Scandinavian Ice Sheet (SIS) during the last glaciation, and the Baltija (also referred to as the South Lithuanian), the Middle and North Lithuanian moraines, which are associated with recessional stages of the SIS, were sampled for surface exposure dating using (10)Be. By combining these data with existing radiocarbon ages, we developed a chronology for the retreat of the SIS margin in Lithuania. Our new (10)Be ages suggest that the SIS margin began to retreat from its maximum extent at 18.3 +/- 0.8 (10)Be kyr. Based on a probable correlation of the Baltija Moraine with the Pomeranian Moraine in Poland, we infer that the Baltija Moraine was formed following a re-advance of the SIS margin. The ice margin retreated from the Baltija position at 14.0 +/- 0.4 (10)Be kyr. The SIS-margin retreat paused at least two more times to form the Middle Lithuanian Moraine at 13.5 +/- 0.6 (10)Be kyr and the North Lithuanian Moraine (tentatively correlated to the Pajuris Moraine) at 13.3 +/- 0.7 (10)Be kyr. Subsequent ice-margin retreat from the North Lithuanian Moraine represented the final deglaciation of Lithuania. Direct dating of these moraines better constrains the relation of ice-margin positions in Lithuania to those in adjacent countries as well as the SIS response to climate change.

[1] New ice core analyses show that the prominent rise in atmospheric methane concentration at Dansgaard-Oeschger event 21 was interrupted by a century-long 20% decline, which was previously unrecognized. The reversal was found in a new ∼100-year resolution study of methane in the GISP2 ice core, encompassing the beginning of D-O event 21, which also corresponds to the transition from MIS 5b to 5a. Although a corresponding reversal (within age uncertainty) is observed in climate proxies measured in GISP2 ice, including δ18Oice, electrical conductivity, light scattering, and several ions, this feature has not been discussed previously. Abrupt changes in methane are paralleled by changes in δ15N of trapped air, a quantity that reflects local temperature change at Greenland summit. The reversal described here supports the hypothesis that climate can be unstable during major transitions, as was previously described for the last deglaciation.

[1] Using new and existing ice core CO2 data from 65 ∼ 30 ka a new chronology for CO2 is established and synchronized with Greenland ice core records to study how high latitude climate change and the carbon cycle were linked during the last glacial period. Atmospheric CO2 rose several thousand years before abrupt warming in Greenland associated with Dansgaard-Oeschger events, 8, 12, 14, 17, four large warm events that follow Heinrich events. The CO2 rise terminated at the onset of Greenland warming for each of these events. Atmospheric CO2 is strongly correlated with the Antarctic isotopic temperature proxy with an average time lag of 720 ± 370 yr (mean ± 1σ) during the time interval studied. The new data and chronology should provide a better target for models attempting to explain CO2 variability and abrupt climate change.

[1] Ice core records show atmospheric methane mixing ratio and interpolar gradient varying with climate. Changes in wetland sources have been implicated as the basis for this observed variation in the record, but more recently, modeling studies suggest that changes in the CH4 sink resulting from changes in sea surface temperature (SST) and emissions of other volatile organic carbon (VOC) compounds by vegetation must also be considered. We use the Goddard Institute for Space Studies (GISS) General Circulation Model (GCM) with the GISS Tropospheric Chemistry Model to study the response of the methane mixing ratio to source changes during the Holocene and also to a changing chemical sink during glacial-interglacial transitions. We combine model results with ice core data to demonstrate a method that provides constraints on changes in northern and tropical methane sources. Results show that within the Holocene, changes in the atmospheric methane mixing ratio and latitudinal gradient are not linear with respect to changing methane emissions. Tropical and northern emissions varied from preindustrial levels by as much as 38% and 15%, respectively, within the Holocene. At glacial-interglacial transitions the methane mixing ratio is sensitive to changes in both VOC and tropical methane emissions and the sensitivity also depends strongly on the assumed SST shift. Our findings suggest that changes in the ice core methane record are likely the result of changes in both the source and the sink. Changes in the sink become especially important when changes in methane mixing ratio and/or climate are large.

How fast and how much climate can change has significant implications for concerns about future climate changes and their potential impacts on society. An abrupt climate change 8200 years ago (8.2 ka event) provides a test case to understand possible future climatic variability. Here, methane concentration (taken as an indicator for terrestrial hydrology) and nitrogen isotopes (Greenland temperature) in trapped air in a Greenland ice core (GISP2) are employed to scrutinize the evolution of the 8.2 ka event. The synchronous change in methane and nitrogen implies that the 8.2 ka event was a synchronous event (within 74 years) at a hemispheric scale, as indicated by recent climate model results [Legrande, A. N., Schmidt, G. A., Shindell, D. T., Field, C. V., Miller, R. L., Koch, D. M., Faluvegi, G., Hoffmann, G., 2006. Consistent simulations of multiple proxy responses to an abrupt climate change event. Proceedings of the National Academy of Sciences 103, 837–842]. The event began with a large-scale general cooling and drying around � 8175730 years BP (Before Present, where Present is 1950 AD). Greenland temperature cooled by 3.371.11C (decadal average) in less than � 20 years, and atmospheric methane concentration decreased by � 80725 ppb over � 40 years, corresponding to a 1575% emission reduction. Hemispheric scale cooling and drying, inferred from many paleoclimate proxies, likely contributed to this emission reduction. In central Greenland, the coldest period lasted for � 60 years, interrupted by a milder interval of a few decades, and temperature subsequently warmed in several steps over � 70 years. The total duration of the 8.2 ka event was roughly 150 years. r 2007 Elsevier Ltd. All rights reserved.

International audience; We have reconstructed chronology for the disturbed bottom parts of the GRIP and GISP2 ice cores using the combined paleoatmospheric records of CH 4 concentration and d 18 O atm in the trapped gases. Our reconstructed ages for basal ice samples are based on comparison of published measurements of CH 4 and d 18 O atm from the disturbed section of the GRIP and GISP2 cores with the same properties in the Vostok ice core. NGRIP d 18 O ice 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 CH 4 and d 18 O atm data, 10 give unique ages. Twenty-five newly measured values of the triple isotope composition of O 2 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.

As an initial attempt to constrain the geometry and history of the southern part of the Scandinavian ice sheet, in situ 10 Be concentrations in bedrock samples have been measured from four sites in southeastern Norway. Among our results are a number of pre-Last Glacial Maximum (LGM) exposure ages. Bedrock outcrops in high elevation blockfields typically give surface exposure ages older than 20 10 Be ka, whereas glacially moulded bedrock surfaces at lower elevations yield surface exposure ages younger than 15 10 Be ka. These results clearly indicate the preservation of surfaces and landforms predating the LGM. Most ice-sheet models predict a thick ice mass in southeastern Norway at the LGM. Pre-LGM 10 Be ages either contradict the models, supporting geomorphological reconstructions of a thin LGM ice sheet, or indicate preservation of previously exposed surfaces beneath cold-based ice. In the latter case, models of 10 Be production incorporating regional glacial chronology and periodic burial suggest that the surfaces with the highest 10 Be concentrations may have existed for more than one million years.

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.

Over the course of Earth history it is generally believed that atmospheric carbon dioxide (CO2) and climate are closely coupled ( 1 ). The most direct evidence comes from polar ice cores. Snow falling in Antarctica and Greenland gradually compacts to form solid ice and trap air. Polar ice also records past temperatures in the ratio of heavy to light isotopes in the water molecule. Ice core analyses have shown that Antarctic temperature and atmospheric CO2 concentrations are highly correlated over the large-scale climate cycles of the past 800,000 years ( 2 ). But which came first? Does CO2 drive climate cycles or is it a feedback in the system that contributes to warming? On page 1060 of this issue, Parrenin et al. ( 3 ) address this question in a study of CO2 concentrations and Antarctic temperatures during the last deglaciation.

International audience

[1] We have measured the CO2 concentration of air occluded during the last 40,000 years in the deep Siple Dome A (hereafter Siple Dome) ice core, Antarctica. The general trend of CO2 concentration from Siple Dome ice follows the temperature inferred from the isotopic composition of the ice and is mostly in agreement with other Antarctic ice core CO2 records. CO2 rose initially at ∼17.5 kyr B.P. (thousand years before 1950), decreased slowly during the Antarctic Cold Reversal, rose during the Younger Dryas, fell to a local minimum at around 8 kyr B.P., and rose continuously since then. The CO2 concentration never reached steady state during the Holocene, as also found in the Taylor Dome and EPICA Dome C (hereafter Dome C) records. During the last glacial termination, a lag of CO2 versus Siple Dome isotopic temperature is probable. The Siple Dome CO2 concentrations during the last glacial termination and in the Holocene are at certain times greater than in other Antarctic ice cores by up to 20 ppm (μmol CO2/mol air). While in situ production of CO2 is one possible cause of the sporadic elevated levels, the mechanism leading to the enrichment is not yet clear.

Atmospheric methane (CH4) recorded in Antarctic ice cores represents the closest ice proxy available for Greenland temperature changes beyond times when Greenland climate records are available. The record over four climatic cycles from the Vostok ice core offers the opportunity to study the phase relationship between Greenland and Antarctic climate changes through detailed CH4 profiles. Combining American and French analytical efforts, we have improved the time resolution of the existing CH4 record from Petit et al [1999]. Spectral analyses reveal high- and low-frequency variability (including a strong precessional component). The phase relationship between CH4 and the Antarctic temperature proxy (δD) shows a systematic lag of CH4 versus temperature by 1100 ± 200 years, on long timescales (50-400 kyr) and a more complex behavior over shorter timescales (i.e., ≤25 kyr), suggesting that Dansgaard/Oeschger-type of climatic variability and associated interhemispheric linkage are robust features of late Quaternary climate. Copyright 2004 by the American Geophysical Union.

Between about 24,000 years ago and today, the large ice sheets covering most of Canada and parts of Europe and Asia melted away, sea level rose by 120 m, Earth warmed by about 5°C, and rainfall and vegetation patterns shifted, sometimes abruptly. This dramatic natural climate experiment was set in motion by cyclic variations in the geometry of Earth's orbit, but a complex system of feedbacks governed the transition from a glacial to an interglacial state. One of the most important of these feedbacks was a well-documented change in the atmospheric greenhouse gas carbon dioxide (CO2) ( 1 ). On page 711 of this issue, Schmitt et al. ( 2 ) provide important new carbon isotopic data that help to explain these changes.

Corrigendum: An ice core record of near-synchronous global climate changes at the Bolling transition