In 11111 i u 11 I ill nu

Figure 3.7 Multiparameter sequence between 270 and 275 m in the GISP2 ice core. Twenty strati-graphic layers and ¿>180 peaks were counted in conjunction with 22 electrical conductivity method (ECM) and 23 laser-light scattering from dust (LLS) peaks. Annual layer markers are shown as vertical white lines, corresponding to the spring/summer inputs for each parameter. (Modified from Meese et al., 1997)

Figure 3.7 Multiparameter sequence between 270 and 275 m in the GISP2 ice core. Twenty strati-graphic layers and ¿>180 peaks were counted in conjunction with 22 electrical conductivity method (ECM) and 23 laser-light scattering from dust (LLS) peaks. Annual layer markers are shown as vertical white lines, corresponding to the spring/summer inputs for each parameter. (Modified from Meese et al., 1997)

oxygen ratios allowed correlation between the two ice-core records and with other ice core and marine deep-sea records. The central Greenland ice cores have clearly demonstrated the occurrence of large, rapid, regional to global-scale climate oscillations during most of the last 110,000 years, on a scale not recorded in modern times (Figs 3.8 and 3.9).

Most of the glacial-interglacial differences normally occur over decades, while some indicators of atmospheric circulation change in only 1-3 years. These millennial-scale events over Greenland were quite large, with temperature fluctuations of up to 20°C, doubling of the snow accumulation, significant changes in wind-blown dust and sea-salt loading, and approximately 100 ppbv variations in methane (CH4) concentration. In addition, the analyses of the recent central Greenland ice cores have given information about the origin of the ice sheet and its basal conditions, reconstruction of atmospheric circulation patterns and their temporal variations from chemical indicators and dust sources, and the anthropogenic influence on the chemical composition of the atmosphere. In addition, the cores have given data on glacier physics and flow modelling, solar influences on climate, and former size and atmospheric response of volcanic eruptions.

A study of the isotope and gas composition of the basal silty ice in the GRIP ice core indicates that local ice formed in the absence of the ice sheet is still preserved (Souchez, 1997). The ice probably formed in a peat deposit under permafrost conditions. This local ice was subsequently mixed with ice from an advancing ice sheet, according to the 'highland origin and windward growth' hypothesis for development of ice sheets (see Fig. 1.1).

In the GRIP core, continuous profiles of electrical conductivity were obtained (Fig. 3.10) (Wolff et al., 1997). After having been corrected for temperature and density, the electrical conductivity reflects acidity variations, while g 60 -

Figure 3.8 Calculated temperature change at the GRIP and GISP2 sites over the last 110,000 years. The GRIP data are from Johnsen et al (1995), while the GISP2 data are from Cuffey et al (1995). (Adapted from Jouzel et al, 1997)

Figure 3.8 Calculated temperature change at the GRIP and GISP2 sites over the last 110,000 years. The GRIP data are from Johnsen et al (1995), while the GISP2 data are from Cuffey et al (1995). (Adapted from Jouzel et al, 1997)

dielectric profiling yields acid, ammonium and chloride. Acidity dominates the variations in dielectric profiling during the Holocene, Allerod/Boiling, and larger interstadials. Ammonium dominates during the Younger Dryas, whereas chloride contributes most in cold periods and minor interstadials. The ice varies from acidic during the Holocene to alkaline in the cold periods. During the interstadials, however, the ice is close to neutral.

Electrical conductivity measurements (ECM) in the GISP2 ice core reflect the +H concentration in the core (Taylor et al, 1997). Seasonal variations in the nitrate concentration were used together with annual layer counting to date the core, and to correlate the GRIP and

Cal. bp

Figure 3.9 Holocene temperature variations recorded in the GISP2 ice core. (From Cuffey et al, 1992,1994)

Cal. bp

Figure 3.9 Holocene temperature variations recorded in the GISP2 ice core. (From Cuffey et al, 1992,1994)

Figure 3.10 The electrical records and the oxygen isotope record for the last 100,000 years in the GRIP core. From the top: dielectrical profiling (DEP) signal; electrical conductivity measurements (ECM) on a linear and on a logarithmic scale; and oxygen isotope signal. Climatic periods marked are the Holocene (Hoi), Younger Dryas (YD), Allerad/Boiling (A/B), interstadial (IS) and cold periods between IS 18 and 19 (cold 18/19). (Adapted from Wolff et al, 1997)

Figure 3.10 The electrical records and the oxygen isotope record for the last 100,000 years in the GRIP core. From the top: dielectrical profiling (DEP) signal; electrical conductivity measurements (ECM) on a linear and on a logarithmic scale; and oxygen isotope signal. Climatic periods marked are the Holocene (Hoi), Younger Dryas (YD), Allerad/Boiling (A/B), interstadial (IS) and cold periods between IS 18 and 19 (cold 18/19). (Adapted from Wolff et al, 1997)

0 20,000 40,000 60,000 80,000 100,000

Years bp

Figure 3.11 The <5lsO series and major ion series in the GISP2 ice core covering the last 110,000 years. (Adapted from Mayewski et al, 1997)

0 20,000 40,000 60,000 80,000 100,000

Years bp

Figure 3.11 The <5lsO series and major ion series in the GISP2 ice core covering the last 110,000 years. (Adapted from Mayewski et al, 1997)

Thousands of years bp

Figure 3.12 Temperature history from the GISP2 ice core according to calibrated isotope values and corrected for elevation changes. The data are smoothed with a 250-year triangular filter visualizing the effect of different elevation corrections. (Modified from Cuffey and Clow, 1997)

Thousands of years bp

Figure 3.12 Temperature history from the GISP2 ice core according to calibrated isotope values and corrected for elevation changes. The data are smoothed with a 250-year triangular filter visualizing the effect of different elevation corrections. (Modified from Cuffey and Clow, 1997)

GISP2 ice cores. Volcanic eruptions and biomass burning events are also recorded by ECM.

Chemical analyses of sodium, potassium, ammonium, calcium, magnesium, sulphate, nitrate and chloride in the GISP2 core (Fig. 3.11) give a record of the atmospheric chemical composition and the history of atmospheric circulation in the mid-high latitudes of the northern hemisphere (Mayewski et al., 1997). The record documents anthropogenic pollution, volcanic events, biomass burning, stormi-ness over the oceans, continental aridity, and information related to the forcing of both high- and low-frequency climate events during the last 110,000 years (orbital cycles, Heinrich events and insolation variations).

Cuffey and Clow (1997) presented a model based on temperature, 61 O variations, and a depth-age scale in the GISP2 ice core to obtain records of temperature, rate of accumulation, and the elevation of the ice sheet over the past 50,000 years (Fig. 3.12). Their model indicates that the temperature increased about 15°C from mean glacial to Holocene conditions. In addition, the average accumulation rate during the last glacial maximum

(15,000-30,000 yr bp) was around 25 per cent of the present accumulation rate, and long-term averaged accumulation rate and temperature correlate inversely over the last 7000 years. Interestingly, the Greenland ice sheet may have thickened during the glacial-deglacial transition, although the elevation history of the ice sheet is poorly constrained by the model. Studies of air content in the GRIP ice core (Raynaud et al., 1997) seem to confirm a thickening of central Greenland during the Weichselian/Holocene transition. Figure 3.13 shows Holocene accumulation changes in the GISP2 ice core (Alley et al, 1993). The frequency of melt layers in the GISP2 ice core decreased significantly from a maximum 7500-7000 yr bp (Fig. 3.14). Less frequent melt features after 7000 yr bp probably reflects a general summer temperature cooling (Alley and Anandakrishnan, 1995).

Dahl-Jensen et al. (1998) presented a 50,000-yr temperature history at GRIP and a 7000-yr history at Dye 3, using measured temperature profiles through the boreholes. The last glacial maximum, the Holocene climatic optimum, the Medieval period, the Little Ice Age, and a

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Years BP

Figure 3.13 Holocene accumulation changes in the GISP2 ice core. (Alley et al., 1993)

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 12000

Years BP

Figure 3.13 Holocene accumulation changes in the GISP2 ice core. (Alley et al., 1993)

warm period around ad 1930 are recorded in the GRIP reconstruction, with amplitudes of -23, +2.5, +1, -1, and +0.5 Kelvin, respectively. The temperature in Dye 3 is similar to the GRIP history, but the amplitude is 1.5 times larger, suggesting greater climatic variability at that site.

The GRIP and GISP2 central Greenland ice cores provide evidence of abrupt climate changes during the last 100,000 years. Several of these variations have also been identified in deep-sea sediments from the North Atlantic. Steig et al. (1998) demonstrated that two of the most significant North Atlantic events (the

Age (years before ad 1950)

Figure 3.14 Melt layers according to age in the GISP2 ice core. The curve shows the 100-yr running mean of melt frequency (number of melt features per 100 years). (Modified from Alley and Ananda-krishnan, 1995)

Age (years before ad 1950)

Figure 3.14 Melt layers according to age in the GISP2 ice core. The curve shows the 100-yr running mean of melt frequency (number of melt features per 100 years). (Modified from Alley and Ananda-krishnan, 1995)

Thousand years before present

Figure 3.15 The GISP2 record of ammonium concentrations over the last 110,000 years. (Modified from Meeker et al, 1997)

Thousand years before present

Figure 3.15 The GISP2 record of ammonium concentrations over the last 110,000 years. (Modified from Meeker et al, 1997)

rapid warming marking the end of the last glacial period, and the B0lling/Aller0d-Younger Dryas oscillation) are also recorded in an ice core from Taylor Dome, located in the western Ross Sea sector of Antarctica. The results from Taylor Dome contrast with data presented from ice cores in other regions of Antarctica, indicating asynchronous response between the northern and southern hemispheres.

A 110,000-year record of ammonium concentration in the GISP2 ice core (Fig. 3.15) has been used to infer terrestrial biological production and the pattern of atmospheric transport of ammonium (Meeker et al., 1997). During warm periods, ammonium transport to Greenland was similar to the present. During extremely cold conditions, low ammonium levels in the ice core are the result of southerly excursions of the zonal polar circulation.

The completion of the ice drilling at the Vostok station in east Antarctica (78°S, 106°E, elevation 3488 m, mean annual temperature -55°C) in January 1998 (length of ice core 3623 m) has allowed the extension of the ice record of atmospheric composition and climate to the past four glacial-interglacial cycles (e.g. Petit et al., 1999). The records from the Vostok ice core (Fig. 3.16) indicate that climate, within certain bounds, has almost always been in a state of change for the last 420,000

years. Features of the last glacial-interglacial cycle are also seen in previous cycles. During each of the last four glacial terminations, properties change in the following sequence: the temperature and atmospheric concentrations of C02 and CH4 rise steadily, whereas the dust input decreases. During the last half of the temperature rise, CH4 increases rapidly. The results from the Vostok ice core suggest climate forcing from orbital parameters followed by greenhouse gases and an albedo effect. The temperatures in Antarctica were higher during interglacials 5.5 and 9.3 (see Fig. 3.16) than during the Holocene or interglacial 7.5.

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