ISSN:0148-0227    

DOI:10.1029/97JC02835

年代:1997

数据来源: WILEY

摘要:

A study of the isotopic and gas composition of the basal silty ice recovered by the Greenland Ice Core Project (GRIP) core indicates that local ice formed in the absence of the Greenland Ice Sheet is still preserved at Summit. Such ice developed most probably within a peat deposit in a permafrost environment. This local ice was subsequently intimately mixed with glacier ice from an advancing ice sheet progressing on the site. This is in agreement with the “highland origin and windward growth” hypothesis for ice sheet development, not for an in situ or regional growth from snowbanks. The basal ice from the GRIP core possibly dates back to the original buildup of the Greenland Ice Sheet 2.4 million years

ISSN:0148-0227    

DOI:10.1029/96JC01558

年代:1997

数据来源: WILEY

摘要:

High‐resolution continuous profiles were obtained on the Greenland Ice Core Project (GRIP) ice core using two different electrical methods. After correction for temperature and density, the electrical conductivity method (ECM) technique responds only to acidity, while dielectric profiling (DEP) responds to acid, ammonium, and chloride. Detailed chemistry on a section of glacial‐age ice allows us to confirm the calibration factor for chloride in DEP. Acidity dominates the DEP variability in the Holocene, Allerod/Bolling, and larger interstadials; ammonium dominates in the Younger Dry as, while chloride is the major contributor in cold periods including smaller interstadials. From the electrical signals plotted on a linear timescale we can deduce the background (nonvolcanic) acidity of the ice, varying from always acidic in the Holocene to always alkaline in the cold periods. In the interstadials, the ice is close to neutral, with most of it acidic in larger interstadials, most of it alkaline in smaller ones, and rapid alternations within interstadials. It is not clear whether neutralization of individual acidic particles occurred in the atmosphere or whether acid and alkaline particles coexisted until deposition in the snowpack. The changes in acidity observed at GRIP apply at least to all of Greenland and probably to much of North America. There would have been ecological effects and important changes in the uptake of some chemicals onto ice. If acidic sulfate particles were neutralized and removed from the atmosphere, which remains uncertain, then there are atmospheric chemistry and radiative effects that require further investigat

ISSN:0148-0227    

DOI:10.1029/96JC02223

年代:1997

数据来源: WILEY

摘要:

This paper reviews aspects of climate variability in the North Atlantic sector that may influence the seasonal to decadal scale isotopic signal in the Greenland Ice Sheet Project 2 (GISP2) ice core. Interpretation of the isotopic signal and its spatial applicability at the seasonal level requires investigation into synoptic scale climatology. We discuss possible climatic influences of (1) likely source regions of precipitation reaching the GISP2 site, (2) the characteristics of cyclone activity over the North Atlantic sector, and (3) changes in major atmospheric features such as the mean sea level Icelandic Low and Azores High pressure systems, the North Atlantic Oscillation, and the Baffin trough. Next, we evaluate correlations between the GISP2 deuterium isotopic signal and coastal temperatures with atmospheric pressure patterns, thereby deriving climatic interpretations of the high‐resolution isotopic record. The GISP2 site is influenced by both the Icelandic Low to the southeast and Davis Strait/Baffin Bay storms to the southwest and west. The North Atlantic Oscillation influences the GISP2 isotopic signal through the seesaw in winter temperatures between west Greenland and northern Europe. Agreement in excursion directions of GISP2 isotopes and east Greenland and Iceland temperature records is associated with different positions of the Baffin trough in winter and also with anomalous 500‐mbar net geostrophic flow. Linkages between isotopic excursion direction and atmospheric variability need to be explored further by comparison with additional seasonal data sets from other Greenland ice co

ISSN:0148-0227    

DOI:10.1029/96JC02401

年代:1997

数据来源: WILEY

摘要:

The Greenland Ice Sheet Project 2 glaciochemical series (sodium, potassium, ammonium, calcium, magnesium, sulfate, nitrate, and chloride) provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid‐low latitudes of the northern hemisphere. Interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants, volcanic and biomass burning events, storminess over marine surfaces, continental aridity and biogenic source strength plus information related to the controls on both high‐ and low‐frequency climate events of the last 110,000 years. Climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long‐term behavior of atmospheric circulation patterns through changes in ice volume (sea level), events such as the Heinrich events (massive discharges of icebergs first identified in the marine record) that are found to operate on a 6100‐year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics, and rapid climate change events (massive reorganizations of atmospheric circulation) that are demonstrated to operate on 1450‐year cycles. Changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events. Explanation for the exact timing and global synchroneity of these events is, however, more complicated. Preliminary evidence points to possible solar variability‐climate associations for these events and perhaps others that are embedded in our ice‐core‐derived atmospheric ci

ISSN:0148-0227    

DOI:10.1029/96JC03365

年代:1997

数据来源: WILEY

摘要:

Annual layers are visible in the Greenland Ice Sheet Project 2 ice core from central Greenland, allowing rapid dating of the core. Changes in bubble and grain structure caused by near‐surface, primarily summertime formation of hoar complexes provide the main visible annual marker in the Holocene, and changes in “cloudiness” of the ice correlated with dustiness mark Wisconsinan annual cycles; both markers are evident and have been intercalibrated in early Holocene ice. Layer counts are reproducible between different workers and for one worker at different times, with 1% error over century‐length times in the Holocene. Reproducibility is typically 5% in Wisconsinan ice‐age ice and decreases with increasing age and depth. Cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50,000 years. Visible observations are not greatly degraded by “brittle ice” or many other core‐quality problems, allowing construction of long, consistently sampled time series. High accuracy requires careful study of the core by dedi

ISSN:0148-0227    

DOI:10.1029/96JC03837

年代:1997

数据来源: WILEY

摘要:

We present a combined heat‐ and ice‐flow model, constrained by measurements of temperature in the Greenland Ice Sheet Project 2 (GISP2) borehole and by the GISP2 δ18O record and depth‐age scale, which determines a history of temperature, accumulation rate, and ice sheet elevation for the past 50,000 years in central Greenland. Important results are: that the temperature increase from average glacial to Holocene conditions was large, approximately 15°C, with a 20°C warming from late glacial to Holocene; that the average accumulation rate during the last glacial maximum (between 15 and 30 kyr B. P.) was 5.5 to 7 cm yr−1, approximately 25% of the modern accumulation rate; that long‐term (500–1000 years) averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the Holocene; and that the Greenland Ice Sheet probably thickened during the deglacial transition. The inverse correlation of accumulation rate and temperature in the mid and late Holocene suggests that the Greenland Ice Sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature. The elevation history of the ice sheet is poorly constrained by the model, and independent evidence is needed. We also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins. This was approximately 1900 years for central Greenland durin

ISSN:0148-0227    

DOI:10.1029/96JC03981

年代:1997

数据来源: WILEY

摘要:

Over 70,000 samples from the 3029‐m‐long Greenland Ice Core Project (GRIP) ice core drilled on the top of the Greenland Ice Sheet (Summit) have been analyzed for δ8O. A highly detailed and continuous δ8O profile has thus been obtained and is discussed in terms of past temperatures in Greenland. We also discuss a three‐core stacked annual δ8O profile for the past 917 years. The short‐term (<50 years) variability of the annual δ8O signal is found to be 1‰ in the Holocene, and estimates for the coldest parts of the last glacial are 3‰ or higher. These data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the Eemian. Spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the δ8O signal, which for the Holocene ice is found to be considerably stronger than expected. We suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix. Deconvolution techniques were employed for restoring with great confidence the highly attenuated annual δ8O signal in the Holocene. We confirm earlier findings of dramatic temperature changes in Greenland during the last glacial cycle. Abrupt and strong climatic shifts are also found within the Eem/Sangamon Interglaciation, which is normally recorded as a period of warm and stable climate in lower latitudes. The stratigraphic continuity of the Eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not

ISSN:0148-0227    

DOI:10.1029/97JC00167

年代:1997

数据来源: WILEY

摘要:

The Greenland Ice Sheet Project 2 (GISP2) depth‐age scale is presented based on a multiparameter continuous count approach, to a depth of 2800 m, using a systematic combination of parameters that have never been used to this extent before. The ice at 2800 m is dated at 110,000 years B.P. with an estimated error ranging from 1 to 10% in the top 2500 m of the core and averaging 20% between 2500 and 2800 m. Parameters used to date the core include visual stratigraphy, oxygen isotopic ratios of the ice, electrical conductivity measurements, laser‐light scattering from dust, volcanic signals, and major ion chemistry. GISP2 ages for major climatic events agree with independent ages based on varve chronologies, calibrated radiocarbon dates, and other techniques within the combined uncertainties. Good agreement also is obtained with Greenland Ice Core Project ice core dates and with the SPECMAP marine timescale after correlation through the δ18O of O2. Although the core is deformed below 2800 m and the continuity of the record is unclear, we attempted to date this section of the core on the basis of the laser‐light scattering of dust in t

ISSN:0148-0227    

DOI:10.1029/97JC00269

年代:1997

数据来源: WILEY

摘要:

Recent efforts to link the isotopic composition of snow in Greenland with meteorological and climatic parameters have indicated that relatively local information such as observed annual temperatures from coastal Greenland sites, as well as more synoptic scale features such as the North Atlantic Oscillation (NAO) and the temperature seesaw between Jakobshaven, Greenland, and Oslo, Norway, are significantly correlated with δ18O and δD values from the past few hundred years measured in ice cores. In this study we review those efforts and then use a new record of isotope values from the Greenland Ice Sheet Project 2 and Greenland Ice Core Project sites at Summit, Greenland, to compare with meteorological and climatic parameters. This new record consists of six individual annually resolved isotopic records which have been average to produce a Summit stacked isotope record. The stacked record is significantly correlated with local Greenland temperatures over the past century (r=0.471), as well as a number of other records including temperatures and pressures from specific locations as well as temperature and pressure patterns such as the temperature seesaw and the North Atlantic Oscillation. A multiple linear regression of the stacked isotope record with a number of meteorological and climatic parameters in the North Atlantic region reveals that five variables contribute significantly to the variance in the isotope record: winter NAO, solar irradiance (as recorded by sunspot numbers), average Greenland coastal temperature, sea surface temperature in the moisture source region for Summit (30°–20°N), and the annual temperature seesaw between Jakobshaven and Oslo. Combined, these variables yield a correlation coefficient ofr=0.71, explaining half of the variance in the stacked isotope r

ISSN:0148-0227    

DOI:10.1029/97JC00162

年代:1997

数据来源: WILEY