GRIP Ice Core Research Articles

Ice-fabrics study in the upper 1500 m of the Dome C (East Antarctica) deep ice core

AbstractA study of c-axis orientations in the upper 1500m of the Dome C (East Antarctica) deep ice core has been carried out using an automatic ice-fabric analyzer (AIFA). Twenty-nine vertical and a few horizontal thin sections from different depths in the core have been analyzed. Several statistical parameters describing fabric strength and fabric shapes have been calculated from the c-axis orientation data. The fabric diagrams display a near-random c-axis orientation distribution in the uppermost parts of the ice sheet. A tendency of c-axis rotation towards a broad single-maximum fabric is observed in the lowest part of the studied interval. The fabric development at Dome C thus appears typical for ice-sheet summit and dome sites. The fabric development at Dome C is compared with the fabric evolution in the Dome F and GRIP ice cores, and data on crystal size obtained with image-analysis techniques are presented. Studies of misorientation angles between the c axes of neighbouring crystals reveal little evidence for polygonization, but microscopic observations show that sub-grain boundaries are present in half of the grains at any depth.

Evaluating controls on ice dynamics in the north-east Atlantic using an event stratigraphy approach

An event stratigraphy approach is used to examine controls on late Devensian-age glacial events in Ireland. The event stratigraphy, based on onshore geological evidence, is constructed from relative-age relationships of cross-cutting bedforms found behind ice margins, and radiometric dates from ice-marginal landforms. The event stratigraphy can be compared with the Greenland GRIP ice core δ 18O proxy climate record and high-resolution marine records in the north-east Atlantic. These records reflect different aspects of the coupled ice–ocean–atmosphere system on different scales. Prior to Heinrich event 1 at ∼14.5 14C kyr BP (cal. ∼17 kyr BP) the glacial event stratigraphy in Ireland more closely matches events recorded in North Atlantic marine cores. After Heinrich event 1 the Ireland glacial event stratigraphy better fits with the Greenland δ 18O record. This shift in phasing relations in the north-east Atlantic following Heinrich event 1 reflects a range of variables including onshore ice retreat, changes in sea-ice cover and strengthened thermohaline circulation. The GRIP event stratigraphy may therefore not always be the most appropriate ‘standard’ against which to compare all glacial/climate records in the north-east Atlantic region. Late Devensian glacial events in Ireland match better to shifts observed in the marine record, suggesting an ocean driver of glacial events in north-west Europe.

Abrupt millennial climatic changes from Nussloch (Germany) Upper Weichselian eolian records during the Last Glaciation

Increasing attention has been focussed on the massive iceberg discharges in the North Atlantic, Heinrich events, during the Last Glaciation, and their worldwide terrestrial counterparts. However, these events are particular episodes among more regular cyclic events named Dansgaard–Oeschger events in ice cores or Bond cycles in marine cores. Here we examine grain size, δ 13C, and magnetic susceptibility data from Nussloch, one of the most complete west European eolian sequence examined so far. These indices indicate that, during the 31–19 kyr interval (using chronology provided by radiocarbon, OSL and TL dates), millennial scale variations occurred affecting both precipitation and vegetation. Furthermore, these are associated with oscillations in the wind strength matching the dust deposition rate in Greenland, as recorded in the GRIP ice core. Our study shows that the abrupt climatic changes, the Dansgaard–Oeschger events, are also recorded in the west European loess sequences. This implies an atmospheric mechanism linking dust deposition over Greenland and Europe, which is probably connected to global climatic events during the Last Glaciation.

Radiolarian-based paleotemperatures during the last 160 kyr at ODP Site 1089 (Southern Ocean, Atlantic Sector)

Two cores, Site 1089 (ODP Leg 177) and PS2821-1, recovered from the same location (40°56′S; 9°54′E) at the Subtropical Front (STF) in the Atlantic Sector of the Southern Ocean, provide a high-resolution climatic record, with an average temporal resolution of less than 600 yr. A multi-proxy approach was used to produce an age model for Core PS2821-1, and to correlate the two cores. Both cores document the last climatic cycle, from Marine Isotopic Stage 6 (MIS 6, ca. 160 kyr BP, ka) to present. Summer sea-surface temperatures (SSSTs) have been estimated, with a standard error of ca. ±1.16°C, for the down core record by using Q-mode factor analysis (Imbrie and Kipp method). The paleotemperatures show a 7°C warming at Termination II (last interglacial, transition from MIS 6 to MIS 5). This transition from glacial to interglacial paleotemperatures (with maximum temperatures ca. 3°C warmer than present at the core location) occurs earlier than the corresponding shift in δ 18O values for benthic foraminifera from the same core; this suggests a lead of Southern Ocean paleotemperature changes compared to the global ice-volume changes, as indicated by the benthic isotopic record. The climatic evolution of the record continues with a progressive temperature deterioration towards MIS 2. High-frequency, millennial-scale climatic instability has been documented for MIS 3 and part of MIS 4, with sudden temperature variations of almost the same magnitude as those observed at the transitions between glacial and interglacial times. These changes occur during the same time interval as the Dansgaard–Oeschger cycles recognized in the δ 18O ice record of the GRIP and GISP ice cores from Greenland, and seem to be connected to rapid changes in the STF position in relation to the core location. Sudden cooling episodes (‘Younger Dryas (YD)-type’ and ‘Antarctic Cold Reversal (ACR)-type’ of events) have been recognized for both Termination I (ACR-I and YD-I events) and II (ACR-II and YD-II events), and imply that our core is located in an optimal position in order to record events triggered by phenomena occurring in both hemispheres. Spectral analysis of our SSST record displays strong analogies, particularly for high, sub-orbital frequencies, to equivalent records from Vostok (Antarctica) and from the Subtropical North Atlantic ocean. This implies that the climatic variability of widely separated areas (the Antarctic continent, the Subtropical North Atlantic, and the Subantarctic South Atlantic) can be strongly coupled and co-varying at millennial time scales (a few to 10-ka periods), and eventually induced by the same triggering mechanisms. Climatic variability has also been documented for supposedly warm and stable interglacial intervals (MIS 1 and 5), with several cold events which can be correlated to other Southern Ocean and North Atlantic sediment records.

Northern Hemisphere glaciation and its sensitivity to basal melt water

A finite element model for ice sheets has been modified to include an accounting of basal melt water. The calculated amount of basal water is used to find the velocity of ice sliding over the bed. The interplay of the basal water and sliding velocity is investigated by a series of modeling experiments. Simulations of the interception, growth, and de-glaciation of the Northern Hemisphere are considered. The simulations are driven with a climate controlled by the GRIP ice core data. The experiments contrast a number of different mechanisms for relating the amount of basal water and the sliding velocity. The mechanisms range from no sliding whatsoever, to a scheme in which the sliding velocity is proportional to the amount of basal water. The modeled results demonstrate that choice of sliding mechanism has a dramatic effect on the glaciation cycle. Furthermore, modeling results indicate that the sliding velocity must be related to the basal water depth to correctly simulate a complete glacial cycle. The relation is important to inception and all other phases of a glacial cycle. The volume of the ice at the last glacial maximum can be compared to the estimated sea level rise to further constrain the new modeling approaches.

Geologically and geomorphologically constrained numerical model of Laurentide Ice Sheet inception and build-up

The locations, shapes and masses of evolving ice sheets during interglacial to glacial transitions remain elusive, but need to be clarified for identification of the climatic feedbacks that amplified low-amplitude insolation changes to become the major global climatic shift of the last glacial maximum. The best way to explore probable spatial patterns of ice sheet inception and build-up is through ice sheet modelling, and validating model output against geomorphological and geological data. We have modelled the 120–55 kyr BP evolution of the Laurentide Ice Sheet, focusing on the locations, outline, flow patterns, and basal thermal regimes of its precursor ice centres. For this, we employed a time-dependent mass-balance-driven finite element model, forced with GRIP ice core oxygen isotope values calibrated to temperature. The model achieves a good fit to relict ice flow systems in Keewatin, the Interior Plains, and the Hudson Bay lowland. Thus, our model indicates that the first coherent ice centre in North America was situated over Baffin Island and the Melville and Boothia Peninsulas, and expanded onto mainland Canada at 90–75 kyr BP. A previously unknown 200-km-long moraine zone near the Thlewiaza River, Keewatin, may demarcate the marginal position of this early Central Arctic Ice Sheet. Cordilleran and Keewatin ice merged, and the Hudson Bay lowland was inundated from the east (from Quebec), by 65 kyr BP.

A First Step to Reconciling the GRIP and GISP2 Ice-Core Chronologies, 0–14,500 yr B.P.

δ18O records from the GISP2 and GRIP ice cores are widely used as benchmark paleoclimate chronologies, but there are significant differences between the two time scales. The present study shows that offsets between the two chronologies over the past 14,500 yr do not accumulate gradually, but appear over two short intervals of 100–200 yr. An initial offset of 80 yr occurs close to 3300–3400 yr B.P., and another 100 yr of slip appears at the start of the Younger Dryas. Since these discrepancies are localized, resolving them may be far easier than if large sections of both cores required reexamination.

Observations of polar ice from the Holocene and the glacial period using the scanning electron microscope

AbstractSamples taken from the Dome C ice core, Antarctica, and the GRIP ice core, Greenland, are examined using the scanning electron microscope to determine their microstructure. In both cores, samples are taken from two differing climatic periods: the Holocene and the last glacial period. Many of the usual features observed in similar samples under the light microscope are observed, including: bubbles, grain boundaries and clathrate hydrates. Features not resolvable using the light microscope are also found. Dust particles are found in situ. Eighty-five per cent of those observed contained silicon, which was generally associated with aluminium and magnesium. An estimation is made of the relative proportions of dust particles located at grain boundaries and in the bulk of the ice grain. At Dome C a higher proportion than expected from a random distribution of particles was found located at grain boundaries, although in Greenland this was not found to be the case for most samples. Direct evidence is also presented indicating the role of dust particles and microscopical inclusions in impeding or ``pinning’’ grain-boundary migration. Soluble impurities are also detected at some triple junctions and grain boundaries.

Internal layer tracing and age‐depth‐accumulation relationships for the northern Greenland ice sheet

Clues to previous ice sheet structure and long‐term glaciological processes are preserved in the internal layering configuration of the Greenland ice sheet. Information about these internal layers has been retrieved over many parts of the ice sheet with the University of Kansas ice‐penetrating radar. We report on the coherence of these layers over very large distances, describe a method of tracing these layers along thousands of kilometers of flight line, and do so for one flight during the 1999 Program for Arctic Regional Climate Assessment (PARCA) aircraft campaign. We determine the ages of these layers, based on information at the GRIP ice core site, and extend these ages along the flight line to Camp Century, where they are compared to modeled‐derived age estimates. These ages agree with each other to between 2 and 15%, differences that can be substantially reduced with minor changes to the model parameters (accumulation rate and shear layer depth). Finally, we are able to derive estimates of accumulation rates along the flight line by fitting the age‐depth data from layer tracing to a Dansgaard‐Johnsen model with a minimization technique, providing estimates that match recent accumulation patterns within a few centimeters per year.

Geomagnetic intensity and inclination variations at Hawaii for the past 98 kyr from core SOH-4 (Big Island): a new study and a comparison with existing contemporary data

Reliable Thellier–Thellier paleointensity determinations were obtained from 321 (out of 766 examined) samples from the upper 200 units (675 m) of core SOH-4 drilled in the Kilauea volcano. The main trend of the SOH-4 record is a marked decrease from values higher than present in the last 4 kyr to values about 40% smaller from about 20 kyr to the bottom of the sampled interval. Three main lows, coinciding with shallow or negative inclinations, observed in the record were identified with an excursion previously documented in Hawaii, and the Mono Lake and Laschamps events. For the last 15 kyr, the SOH-4 intensities are largely in agreement with the worldwide distribution of volcanic data. In the 15–75 kyr interval, the general lack of volcanic data makes comparison with the SOH-4 data difficult. The SOH-4 record is similar to a composite record of paleointensity largely based on a recent North Atlantic sedimentary paleointensity stack (NAPIS-75). The SOH-4 record is also generally consistent with a record of geomagnetic field intensity obtained from a study of 36 Cl in the GRIP ice core, calculated with the assumption that the production rate of 36 Cl is controlled only by the geomagnetic field. One pronounced discrepancy between the different records is observed around 50 kyr BP, when the sedimentary and cosmogenic based records document high intensities which are not observed in SOH-4. These high intensities are also absent in a recent record based on the analyses of sea-floor magnetic anomalies. The histogram of VADMs obtained from SOH-4 is different from that obtained from the sedimentary or 36 Cl based records and is more skewed than the distribution given by Kono and Tanaka for the last 2.5 million years.

Rapid climate change during the last glacial–interglacial transition; implications for stratigraphic subdivision, correlation and dating

The frequent and often abrupt climatic changes that occurred during the transition from the last cold stage to the present interglacial (the ‘Late-glacial period’) pose particular problems for stratigraphy and correlation, while difficulties are also encountered in developing a coherent independent dating framework for this important time interval. It has been recommended that the Greenland GRIP ice core should constitute a stratotype for the late Pleistocene, and that stratigraphic sequences should be correlated with the GRIP oxygen isotope signal on the basis of climatically defined ‘events’. Three approaches to the development of quantified climate records (using pollen, coleopteran and chironomid data) are examined and links to the GRIP ice core discussed. Despite its limitations, radiocarbon remains the most universal method for establishing a timescale for the Late-glacial, and the paper concludes with a discussion of an experimental approach designed to establish the reliability and comparability of Late-glacial radiocarbon dates. Evaluatory exercises of this nature are now required if radiocarbon dating is to provide the necessary high-resolution chronology for time-stratigraphic correlation between terrestrial sequences, marine records and the GRIP stratotype.

Response of the pelagic environment to palaeoclimatic changes in the central Mediterranean Sea during the Late Quaternary

Three central Mediterranean deep-sea cores have been studied to reconstruct the palaeoclimatic history of the basin over the time interval 34–0 kyr bp. The intensity and duration of the climatic events that occurred in the Mediterranean Sea during the last glacial-postglacial transition were estimated by observing compositional changes in the planktonic foraminifera and calcareous nannoplankton (coccolithophores) assemblages, together with a reconstruction of sea surface temperatures (using the Modern Analogue Technique and U 37 k′ index), the δ 18O signal and pteropod fluxes. The application of two independent and well established techniques for the determination of the past SST led to a number of considerations about the accuracy and efficacy of the use of such methodologies in the Mediterranean Sea, notoriously dominated by local factors and characterised by a number of independent environments. The recognition of millennial to centennial climatic instabilities, in both the SST and microfossil records, was possible because of the high resolution of the study. A succession of nine main biozones and six sub-zones, based on the major changes in the planktonic foraminifera records, has been recognised and compared with GRIP and GISP2 ice cores δ 18O values and with records from four other studies from the central Mediterranean Sea. During the early phase of the Holocene, a period characterised by relatively higher temperatures and lighter δ 18O values has been recognised as being coeval with the ‘Climatic Optimum’ (between 10.5 and 6.1 kyr bp, calendar age). This interval was characterised by an abrupt drop in the pteropods relative abundances and fluxes. The pteropodal fossil assemblage may have been affected by a possible event of selective dissolution of the aragonite driven by the shoaling of the ACD. A coeval change in the position of the pycnocline may have been responsible for a strong relative increase in abundance of Neogloboquadrina pachyderma r.c., and a decrease in the abundance of Globorotalia inflata in the western sector of the Mediterranean Sea.

Characterization of luminescence properties of insoluble mineral grains extracted from the Greenland Summit GRIP ice core, and their potential for luminescence dating

We describe several optically stimulated luminescence (OSL) and thermoluminescence (TL) properties of insoluble mineral grains extracted from the Greenland summit GRIP ice core in order to examine the potential of these techniques for age determination of these materials. Since the dust grains examined in this work were exposed to natural and artificial light at many stages prior to luminescence analysis, the purpose of the experiments reported herein was not to make age determinations, but rather to evaluate the general suitability of dust grains extracted from ice cores for luminescence dating. The measurements indicate that the fine-grained dust has a wide dynamic dose response range, with limited saturation effects being seen until absorbed doses of ∼1000 Gy. Furthermore, the sample's TL and OSL signals were stable over the time intervals evaluated in this study and readily susceptible to bleaching by exposure to sunlight. These three properties form a stable base for the development of luminescence dating procedures for insoluble mineral grains recovered from ice cores. The experiments demonstrate that such procedures can be carried out on sample masses of less than 1mg.

Presence of the Solar de Vries Cycle (∼205 years) during the Last Ice Age

Certain characteristic periodicities in the Δ14C record from tree rings, such as the well‐known 11‐yr Schwabe cycle, are known to be of solar origin. The origin of longer‐period cycles, such as the 205‐yr de Vries cycle, in the Δ14C record was less certain, and it was possible to attribute it either to solar or climatic variability. Here, we demonstrate that the de Vries cycle is present in 10Be data from the GRIP ice core during the last ice age (25 to 50 kyr BP). Analysis of the amplitude of variation of this cycle shows it to be modulated by the geomagnetic field, indicating that the de Vries cycle is indeed of solar, rather than climatic, origin.

Correlation between high-resolution climate records from a Nanjing stalagmite and GRIP ice core during the last glaciation

A 400-mm-long stalagmite from Tangshan Cave, Nanjing has been analyzed by a high-precision TIMS-U series dating method and also determined for oxygen and carbon stable isotopic compositions. The results provided a high-resolution paleoclimate record for eastern China during a time interval (from 54 000 to 19 000 aBP) of the last glaciation. The continuous record of oxygen-18 variations in the stalagmite, indicating a precipitation history of the East Asian monsoon, shows not only signals of the Heinrich events, but also the Dansgaard-Oeschger cycles which are first found in the last glacial climate record of the East Asian monsoon area. Although the stalagmite-based climatic signals match well with the GRIP ice core record, some differences between the two records can be recognized: (1) The last glacial climate changes in eastern China exhibited a long-term remarkably cooling trend, superimposed on which were four successive Bond’s cycles illustrated by the δ18O curve. This strong cooling tendency may be an effect of the strong summer monsoon event during the MIS 3 over the Tibetan Plateau. (2) There exist some phase differences of 1000–2000 years between the cooling events in the stalagmite-based climate signal and the GRIP ice core record. Such differences should be further verified by calibrations of multiple dating methods

Rate controlling processes in the creep of polar ice, influence of grain boundary migration associated with recrystallization

Information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from Greenland and Antarctica. It is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration (gbm). A deformation model based on an equilibrium between work-hardening and recovery processes has been developed. The decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization. The value of the dislocation density along the Byrd and GRIP ice cores obtained thanks to the model is about 1×10 11 m −2. This value is in agreement with data from synchrotron X-ray diffraction on samples taken along the core. This deformation model can account for the deformation of polar ice at low stresses. It is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process.

Reconstruction of the geomagnetic field between 20 and 60 kyr BP from cosmogenic radionuclides in the GRIP ice core

A pure physical model for the simulation of cosmic ray particle interactions with the Earth’s atmosphere was used to investigate the effects of a changing geomagnetic field on the production rates of cosmogenic nuclides. Analytical dependencies of the production rates of 3H, 7Be, 10Be, 14C and 36Cl on geomagnetic field intensity were developed. Applying those relations to the 10Be and 36Cl fluxes measured in the GRIP ice core, the geomagnetic field intensity for the period between 20 and 60 kyr BP was reconstructed. Comparison with remnant magnetism records from marine sediment cores shows excellent agreement. This validates the use of cosmogenic nuclides in ice cores to reconstruct geomagnetic field variations.

Chlorine-36 evidence for the Mono Lake event in the Summit GRIP ice core

A distinct peak has been discovered in the 36Cl data from the GRIP ice core between the Dansgaard Oeschger (D–O) events 6 and 7 at approximately 32 kyr BP. This peak can be attributed to a minimum of the geomagnetic dipole field associated with the Mono Lake event. Since the 36Cl peak reflects a higher production rate of all cosmogenic radionuclides, it has an impact on the 14C dating of the last ice age. Furthermore, it provides an additional time marker similar to a peak found earlier corresponding to the Laschamp event at approximately 39 kyr BP.

Warm Period Growth of Travertine during the Last Interglaciation in Southern Germany

Late-Quaternary travertine at two sites near Stuttgart formed entirely during interglacial periods. The travertine contains structures from growth induced by bacteria, and such structures have been dated by 230Th/U mass spectrometry. The resulting ages from both sites imply growth episodes of short duration, with growth rates up to 5 mm yr−1, at 99,800 ± 1300 yr B.P. (2ς; n = 8) and 105,900 ± 1300 yr B.P. (2ς; n = 7). These episodes were likely part of marine isotope stage (MIS) 5.3. Deposition of silt interrupted travertine growth at one of the sites ∼105,000 yr B.P. Likely correlatives of this silt are the St. Germain I-B stade recorded in the Grand Pile peat bog and a cold episode ∼1000 yr long recorded by δ18O values in the GRIP ice core. Travertine also formed during stage 5.5 (∼115,000 yr) and during the early Holocene. We found no evidence for travertine accumulation in stages 2, 3, 4, and 5.1. At both sites, the Sr/U ratio and the initial 234U/238U activity ratio resemble those of modern spring water. However, the sites differ in the chemical composition of spring water and in stratigraphic sequence of travertine accumulation.

Oxygen isotopes of lake marl at Gerzensee and Leysin (Switzerland), covering the Younger Dryas and two minor oscillations, and their correlation to the GRIP ice core

The ratio of oxygen isotopes is a temperature proxy both in precipitation and in the calcite of lacustrine sediments. The very similar oxygen-isotope records from Greenland ice cores and European lake sediments during the Last Glacial Termination suggest that the drastic climatic changes occurred quasi-simultaneously on an extra-regional, probably hemispheric scale. In order to study temporal relations of the different parameters recorded in lake sediments, for example biotic response times to rapid climatic changes, a precise chronology is required. In unlaminated lake sediments there is not yet available a method to provide a high-resolution chronology, especially for periods with radiocarbon plateaux. Alternatively, an indirect time scale can be constructed by linking the lake stratigraphy with other well-dated climate records. New oxygen-isotope records from Gerzensee and Leysin, with an estimated sampling resolution of between 15 and 40 years, match the Greenlandic isotope record in many details. Under the assumption that the main variations in temperature and thus in oxygen isotopes occurred about simultaneously in Greenland and Switzerland, we have assigned a time scale to the lake sediments of Gerzensee and Leysin by wiggle-matching their stable-isotope records with those of Greenland ice cores, which are among the best dated climatic archives. We estimate a precision of 20 to 100 years during the Last Glacial Termination.