GRIP Ice Core Research Articles

Solar forced Dansgaard/Oeschger events?

It has been suggested that the strong climatic changes during the last ice age, the so‐called Dansgaard/Oeschger (D/O) events, could have been prompted by solar activity changes. This hypothesis is based on the apparent cyclic occurrence of the D/O events and the solar influence on climate during the Holocene on similar time scales. We test this hypothesis by comparing the 10Be and δ18O records from the GRIP ice core. A superimposed epoch analysis allows us to reduce the noise in the data and to extract estimates of solar activity changes in connection to the D/O events. This comparison does not provide convincing evidence for a persistent solar influence on these strong climatic oscillations during the last ice age.

What is what in the ice and the ocean?

Recently released data from the North Greenland Ice core Project (NGRIP) document several rapid, abrupt climate changes affecting the Northern Hemisphere in the last 110,000 years. In particular, the new core shows high-resolution succession of expressed warm and cold episodes, which occurred during substages of Marine Isotope Stage MIS 5. Some of these variations were reported earlier from the GISP2 and GRIP ice cores. In the NGRIP core, following the INTIMATE group recommendations, the oscillations were given labels, which are in part the same as in the isotope system of deep-sea sediments, although to some extent they are not coeval. Here we recommend honoring the originally published marine designations wherever possible, but distinguishing them by a prefix referring to their recognition in the ice.

A new Greenland ice core chronology for the last glacial termination

We present a new common stratigraphic timescale for the North Greenland Ice Core Project (NGRIP) and GRIP ice cores. The timescale covers the period 7.9–14.8 kyr before present and includes the Bølling, Allerød, Younger Dryas, and early Holocene periods. We use a combination of new and previously published data, the most prominent being new high‐resolution Continuous Flow Analysis (CFA) impurity records from the NGRIP ice core. Several investigators have identified and counted annual layers using a multiparameter approach, and the maximum counting error is estimated to be up to 2% in the Holocene part and about 3% for the older parts. These counting error estimates reflect the number of annual layers that were hard to interpret, but not a possible bias in the set of rules used for annual layer identification. As the GRIP and NGRIP ice cores are not optimal for annual layer counting in the middle and late Holocene, the timescale is tied to a prominent volcanic event inside the 8.2 kyr cold event, recently dated in the DYE‐3 ice core to 8236 years before A. D. 2000 (b2k) with a maximum counting error of 47 years. The new timescale dates the Younger Dryas‐Preboreal transition to 11,703 b2k, which is 100–150 years older than according to the present GRIP and NGRIP timescales. The age of the transition matches the GISP2 timescale within a few years, but viewed over the entire 7.9–14.8 kyr section, there are significant differences between the new timescale and the GISP2 timescale. The transition from the glacial into the Bølling interstadial is dated to 14,692 b2k. The presented timescale is a part of a new Greenland ice core chronology common to the DYE‐3, GRIP, and NGRIP ice cores, named the Greenland Ice Core Chronology 2005 (GICC05). The annual layer thicknesses are observed to be log‐normally distributed with good approximation, and compared to the early Holocene, the mean accumulation rates in the Younger Dryas and Bølling periods are found to be 47 ± 2% and 88 ± 2%, 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.

Developing a bubble number-density paleoclimatic indicator for glacier ice

AbstractPast accumulation rate can be estimated from the measured number-density of bubbles in an ice core and the reconstructed paleotemperature, using a new technique. Density increase and grain growth in polar firn are both controlled by temperature and accumulation rate, and the integrated effects are recorded in the number-density of bubbles as the firn changes to ice. An empirical model of these processes, optimized to fit published data on recently formed bubbles, reconstructs accumulation rates using recent temperatures with an uncertainty of 41% (P < 0.05). For modern sites considered here, no statistically significant trend exists between mean annual temperature and the ratio of bubble number-density to grain number-density at the time of pore close-off; optimum modeled accumulation-rate estimates require an eventual ~2.02 ± 0.08 (P < 0.05) bubbles per close-off grain. Bubble number-density in the GRIP (Greenland) ice core is qualitatively consistent with independent estimates for a combined temperature decrease and accumulation-rate increase there during the last 5 kyr.

Geomagnetic field intensity during the last 60,000 years based on 10Be and 36Cl from the Summit ice cores and 14C

Based on 10Be, 36Cl and 14C records we reconstruct past changes in the geomagnetic field intensity under the assumption that radionuclide records are mainly influenced by changes in their production rates. Subsequent comparison of these records with independent geomagnetic field reconstructions allows us to assess the reliability of our approach and to identify periods where our method or the geomagnetic field reconstructions might be problematic. The geomagnetic field derived from the 10Be-flux to Summit in Central Greenland agrees well with independent geomagnetic field intensity reconstructions. However, compared to independent palaeomagnetic data the radionuclide record suggests increased geomagnetic field intensities especially for the period from 20,000 to 30,000 years BP. 36Cl from the GRIP ice core agrees well with the 10Be record for the last ice age and hence also the reconstructed geomagnetic field. Based on the tree-ring radiocarbon record it is possible to reconstruct geomagnetic field intensity changes during the Holocene. A comparison of radiocarbon-based and independent geomagnetic field reconstructions indicates that changes in the carbon cycle significantly influenced the atmospheric radiocarbon concentration before 10,000 years BP.

The duration of the Bølling-Allerød period (Greenland Interstadial 1) in the GRIP ice core

AbstractA new dating of the Bølling-Allerød period (Greenland Interstadial event 1) in the GRIP ice core is presented. Newly measured profiles of δD and δ18O, as well as existing profiles of Ca2+, NH4+, dust and NO3–, have been used for the dating. As seasonal variations can be observed in all six components, it was possible to simultaneously count annual layers in the profiles in order to obtain a multi-parameter dating. The new data presented in this study include a total of 36.85 m of stable-isotope profiles of 1 cm resolution from five sections of the Bølling-Allerød period in the GRIP ice core. The annual-layer counting suggests a duration of the complete Bølling-Allerød period, as revealed in the GRIP ice core, of 1627 ± 52 years. This estimate contrasts with an earlier finding from the same GRIP ice core, where the Bølling-Allerød was found to span 2168 years (Hammer, in press). This estimate was based on layer counting, using dust concentrations only.

Ice fabric evolution process understood from anisotropic distribution of a-axis orientation on the GRIP (Greenland) ice core

AbstractTo better understand how ice fabric evolves in polar ice sheets, we use X-ray diffraction to measure ice crystal orientations. X-ray measurement equipment which can measure the orientation of the c axis and a axis of each crystal in a thin section with high measurement accuracy was developed. In this study, we present a-axes orientation distribution of the deep part of the GRIP (Greenland summit) ice core. At some depths, we find an anisotropic distribution of a-axes orientation. Long-term uniaxial compression tests are also carried out on the GRIP ice core to investigate the ice fabric evolution process. The c-axis orientation distribution develops into a stronger single maximum as the strain increases up to about 20% strain. We find that the a axes of each grain also tend to cluster close to nearly a mutual direction. We discuss the development process of ice fabrics, taking into consideration the distribution of the a-axis orientations. It is suggested that these fabrics may be attributed to a local simple shear deformation in the deep part of an ice sheet.

Inter-profile correlation of the Chinese loess/paleosol sequences during Marine Oxygen Isotope Stage 5 and indications of pedogenesis

Low-field magnetic susceptibility has been widely used to determine the pedostratigraphy of the Chinese loess/paleosol sequences. However, uncertainties remain in correlating between the loess magnetic susceptibility and the marine oxygen isotope records because susceptibility variations are affected by both global and local paleoclimatic changes. To provide a more sound paleoclimatic interpretation of magnetic susceptibility variations, age models across Marine Oxygen Isotope Stage (MIS) 5 for the Jiuzhoutai (JZT) and Yuanbao (YB) sections, western Chinese Loess Plateau, were constructed through an integrated approach by linking the major pedostratigraphic boundaries of the loess profiles to the SPECMAP oxygen isotope curve, and by correlating relative magnetic paleointensity records with both the SINT800 global paleointensity stack from marine sediments and 36Cl records from the GRIP ice core. Results indicate good correlation of SIRM 60 mT (a residual remanence of saturation isothermal remanent magnetization after a 60 mT alternating field demagnetization) variations between these two sites, which agree well with fluctuations in subtropical Atlantic sea surface temperatures. All cooling events recorded by ice-core and Atlantic marine sediments within MIS5 have counterparts in SIRM 60 mT . SIRM 60 mT is partially controlled by the degree of low-temperature oxidation, which is strongly temperature dependent. However, strong pedogenesis can decrease SIRM 60 mT due to further oxidation of partially oxidized magnetites above some critical points. Therefore, we propose that SIRM 60 mT is best suited to record paleotemperature changes in loess profiles from the western Chinese Loess Plateau, where pedogenesis is the weakest. Furthermore, by inter-profile correlation between the YB and JZT sections, we note that the seemingly uniform sub-paleosol unit with a broad susceptibility peak (previously assigned to MIS5c) between ∼34.4 and ∼37.4 m in the YB profile actually consists of two independent units (lower part of S1L1/MIS5b and S1S2/MIS5c). This indicates that susceptibility values can be strongly affected by local factors (e.g., mainly precipitation). Therefore, beside the simplistic traditional paleoclimatic interpretation of variations in loess susceptibility involving only cold/dry and warm/humid scenarios, cold/humid and warm/dry scenarios should also be considered.

Marine reservoir age variability and water mass distribution in the Iceland Sea

Lateglacial and Holocene tephra markers from Icelandic source volcanoes have been identified in five sediment cores from the North Icelandic shelf and correlated with tephra layers in reference soil sections in North Iceland and the GRIP ice core. Land-sea correlation of tephra markers, that have been radiocarbon dated with terrestrial material or dated by documentary evidence, provides a tool for monitoring reservoir age variability in the region. Age models developed for the shelf sediments north of Iceland, based on offshore tephrochronology on one hand and on calibrated AMS 14C datings of marine molluscs on the other, display major deviations during the last 4500 years. The inferred temporal variability in the reservoir age of the regional water masses exceeds by far the variability expected from the marine model calculations. The observed reservoir ages are generally considerably higher, by up to 450 years, than the standard model ocean. It is postulated that the intervals with increased and variable marine reservoir age reflect incursions of Arctic water masses derived from the East Greenland Current to the Iceland Sea and the North Icelandic shelf.

Causes and timing of the 8200 yr BP event inferred from the comparison of the GRIP 10Be and the tree ring Δ 14C record

We analyse the 8200 yr BP cold event by comparing the high-resolution 10Be record from the GRIP ice core from Central Greenland with the well-known tree ring Δ 14 C record. By transferring the absolute dated tree ring chronology to the ice core time scale, we show that the coldest phase in the GRIP record occurred around 8150 yr BP. Furthermore, this method allows us to disentangle production and climate effects on 10Be and 14C with important implications for the reconstruction of past solar activity, and changes in the carbon cycle and 10Be transport. We show that, in principle, it is possible to infer changes in ocean circulation by comparing 10Be and 14C records. However, the duration of the 8200 yr BP event is too short to assign unambiguously a significant change in atmospheric 14C concentration to changes in the global ocean circulation. Based on the comparison of 10Be with climate records, one could argue that the 8200 yr BP cold event is triggered by a change towards lower solar activity. However, this link is questioned by the fact that around this period there are other similar and even stronger changes in solar activity that have no apparent connection to climate changes.

The Preboreal climate reversal and a subsequent solar‐forced climate shift

AbstractAccurate chronologies are essential for linking palaeoclimate archives. Carbon‐14 wiggle‐match dating was used to produce an accurate chronology for part of an early Holocene peat sequence from the Borchert (The Netherlands). Following the Younger Dryas–Preboreal transition, two climatic shifts could be inferred. Around 11 400 cal. yr BP the expansion of birch (Betula) forest was interrupted by a dry continental phase with dominantly open grassland vegetation, coeval with the PBO (Preboreal Oscillation), as observed in the GRIP ice core. At 11 250 cal. yr BP a sudden shift to a humid climate occurred. This second change appears to be contemporaneous with: (i) a sharp increase of atmospheric 14C; (ii) a temporary decline of atmospheric CO2; and (iii) an increase in the GRIP 10Be flux. The close correspondence with excursions of cosmogenic nuclides points to a decline in solar activity, which may have forced the changes in climate and vegetation at around 11 250 cal. yr BP. Copyright © 2004 John Wiley & Sons, Ltd.

Identification of Aniakchak (Alaska) tephra in Greenland ice core challenges the 1645 BC date for Minoan eruption of Santorini

Minute shards of volcanic glass recovered from the 1645 ± 4 BC layer in the Greenland GRIP ice core have recently been claimed to originate from the Minoan eruption of Santorini [Hammer et al., 2003]. This is a significant claim because a precise age for the Minoan eruption provides an important time constraint on the evolution of civilizations in the Eastern Mediterranean. There are however significant differences between the concentrations of SiO2, TiO2, MgO, Ba, Sr, Nb and LREE between the ice core glass and the Minoan eruption, such that they cannot be correlatives. New chemical analyses of tephra from the Late Holocene eruption of the Aniakchak Volcano in Alaska, however, show a remarkable similarity to the ice core glass for all elements, and this eruption is proposed as the most likely source of the glass in the GRIP ice core. This provides a precise date of 1645 BC for the eruption of Aniakchak and is the first firm identification of Alaskan tephra in the Greenland ice cores. The age of the Minoan eruption of Santorini, however, remains unresolved.

A Radiocarbon Perspective on Greenland Ice-Core Chronologies: Can we Use Ice Cores for 14C Calibration?

Some of the most valuable paleoclimate archives yet recovered are the multi-proxy records from the Greenland GISP2 and GRIP ice cores. The crucial importance of these data arises in part from the strong correlations that exist between the Greenland δ18O records and isotopic or other proxies in numerous other Northern Hemisphere paleoclimate sequences. These correlations could, in principle, allow layer-counted ice-core chronologies to be transferred to radiocarbon-dated paleoclimate archives, thus providing a 14C calibration for the Last Glacial Maximum and Isotope Stage 3, back to the instrumental limits of the 14C technique. However, this possibility is confounded by the existence of numerous different chronologies, as opposed to a single (or even a “best”) ice-core time scale. This paper reviews how the various chronologies were developed, summarizes the differences between them, and examines ways in which further research may allow a 14C calibration to be established.

Volcanic ash retrieved from the GRIP ice core is not from Thera

Tephra found in an ice core from Greenland (GRIP) has been claimed to be from the Minoan eruption of Thera (Santorini), Greece. If true, this would date the eruption, thereby resolving a decades‐long debate in chronology. Herein, it is shown that the methods used to match the Greenlandic tephra with Thera are flawed and that the geochemical data imply the tephra is not from Thera.

Air-hydrate crystal growth in polar ice

Based on the theory of precipitation from supersaturated solutions proposed by Lifshitz and Slyozov (J. Phys. Chem. Solids 19 (1/2) (1961) 35), we develop a mathematical description of post-formation growth (ripening) of mixed air clathrate-hydrate crystalline inclusions in polar ice sheets. The growth is controlled by oxygen and nitrogen diffusion through the ice matrix. Hydrate populations in general go through three sequential stages: (1) a short transient characterized by the rapid composition relaxation and dissolution of the smallest hydrates, (2) a slow transformation of the resulting size distributions towards a steady-state pattern that is an attribute of (3) the asymptotic stage of ripening. A regularization procedure is used to numerically solve the initial value problem. Computer simulations of the hydrate size distributions are compared to the data from a 3300-m ice core from Vostok Station, East Antarctica. The asymptotic stage is likely unattainable in natural conditions. Data from the GRIP ice core (central Greenland) suggest that the activation energy of hydrate growth increases at the elevated temperature near the ice-sheet bottom. The theory predicts extinction of the climatically induced fluctuations in the hydrate number-concentration and mean-radius profiles in ice sheets with depth.

Climate variations since the last interglacial recorded in Czech loess

Magnetic susceptibility, organic matter content, and grain size were measured on a loess/palaeosol sequence deposited since the last interglacial at the Dolni Vestonice section in the Czech Republic. High values in magnetic susceptibility and organic matter content in palaeosols PK0, PK2, and PK3 indicate that they were strongly influenced by long‐term pedogenesis and high vegetation density linked to interglacials or interstadials. Grain‐size variations record a series of cold wind‐strengthened events during the last glacial, which we interpret as a record of the distinctive ‘Younger Dryas’ episode and millennial‐scale climate oscillations that correlate with the GRIP ice core δ18O record and North Atlantic Heinrich events. These climate events, as indicated by grain‐size peaks, combined with previous climate records at French Massif Central and Lago Grande di Monticchio in Italy, suggest that the climate over these areas might have undergone synchronous oscillations with the shifts in sea‐surface temperatures of the North Atlantic and air temperature changes above Greenland.

Strontium isotope measurements in Greenland ice from the last glacial maximum to the early Holocene

Strontium isotopic composition and concentration have been measured by thermal ionisation mass spectrometry on a suite of samples from the GRIP ice core drilled at Summit, Greenland. The sample ages range from ∼24 to ∼7.3 ky BP extending from the last glacial maximum into the early Holocene. Less than 10 g of sample was used for each analysis. No attempt was made to separate soluble/insoluble species in the samples. Sr concentrations are between ∼950 and ∼1,550 pg.g -1 over the period ∼24 ky to -14 ky BP but fall dramatically to generally less than 150 pg.g -1 between -14 to ∼7.3 ky BP. The 87 Sr/ 86 Sr ratio shows a general rising trend from ∼0.712 to ∼0.715 over the entire period however there are a number of significant deviations from this trend which are most likely due to changing source regions for aerosol dust input to Greenland. Sr isotopic composition shows a strong correlation with δ 18 O suggesting that climate plays a strong role in determining regions for dust release.

Paleomagnetic correlations between scandinavian ice-sheet fluctuations and greenland dansgaard–oeschger events, 45,000–25,000 yr B.P.

Two paleomagnetic excursions, the Skjong correlated with the Laschamp (about 41,000 GISP2 yr B.P.) and the Valderhaug correlated with the Mono Lake (about 34,000 GISP2 yr B.P.), have been identified in stratigraphic superposition in laminated clay deposited in ice-dammed lakes in three large caves in western Norway. During both periods the margin of the Scandinavian Ice Sheet advanced and reached the continental shelf beyond the outermost coastline. The mild, 4000-yr-long Ålesund interstade, when the coast and probably much of the hinterland were ice-free, separated the two glacial advances. The two paleomagnetic excursions have also been indirectly identified as increased fluxes of 36Cl and 10Be in the GRIP ice core, Greenland. This article presents a correlation between ice-margin fluctuations of the Scandinavian Ice Sheet and the stratigraphy of GRIP/GISP cores, using the paleomagnetic excursions and the 36Cl and 10Be peaks and thus circumventing the application of different dates or time scales. Some of the fluctuations of the Scandinavian Ice Sheet were of the “Allerød/Younger Dryas type” in the sense that its margin retreated during mild interstades on Greenland and readvanced during cold stades. However, some fluctuations were apparently not in phase with the Greenland climate.

Devensian Lateglacial environmental changes in Britain: a multi-proxy environmental record from Llanilid, South Wales, UK

A multi-proxy environmental record for the Devensian (Weichselian) Lateglacial and early Holocene periods, which includes pollen, plant macrofossil, coleopteran, geochemical and stable-isotope data, is described from a site at Llanilid, South Wales, UK. The geochronology of the sequence is derived from two age–depth models which show a broad measure of agreement over much of the profile. This data set provides the basis for a detailed reconstruction of changing environmental conditions in western Britain during the transition from the Last Cold Stage to the present (Holocene) interglacial (ca 15,000–10,000 cal yr BP). Two marked cooling episodes are recorded during the course of the Lateglacial Interstadial (Greenland Interstadial 1), with mean July temperatures falling by more than 5°C from a maximum of around 20°C during the early Interstadial, and by a further 4–5°C around 13,100 cal yr BP. The initial drop in temperature led to a reduction in Juniperus scrub, while the second, and more abrupt temperature decline resulted in a significant contraction in areas of Betula woodland. A relatively slight and short-lived warming during the later Interstadial enabled tree birch to expand once again. Mean July temperatures of 10–11°C characterised the Loch Lomond/Younger Dryas Stadial (Greenland Stadial 1) between ca 12,600 and 11,400 cal yr BP, during which time a scrub tundra with Betula, Salix and a range of open-habitat taxa became established locally. The onset of the Holocene Interglacial at ca 11,400 cal yr BP is marked by an abrupt temperature rise of the order of 9°C, and by the rapid expansion of Betula woodland. The Llanilid palaeoclimate record is similar to that from the Gransmoor site in northeast England, and also to the climatic sequence inferred from the GRIP ice core, particularly during the later part of the Lateglacial Interstadial (ca 14,000–12,600 cal yr BP), during the Loch Lomond/Younger Dryas Stadial and in the early Holocene. The Llanilid chronology is less secure during the early part of the Lateglacial Interstadial (pre-14,000 cal yr BP), but it is possible that warming may have begun earlier in this part of western Britain than in Greenland.