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2,000 4,000 6,000 8,000 10,000 12,000 Calendar Years B.P.

FIGURE 5.1 5 Accumulation at Summit, Greenland (l00-yr running mean). Following an increase in accumulation from the LGM to the early Holocene, conditions became relatively stable, averaging 0.24 m ±5% on this timescale for the last ~9000 yr (see Fig. 5.33) (Meese et al., 1994).

2,000 4,000 6,000 8,000 10,000 12,000 Calendar Years B.P.

FIGURE 5.1 5 Accumulation at Summit, Greenland (l00-yr running mean). Following an increase in accumulation from the LGM to the early Holocene, conditions became relatively stable, averaging 0.24 m ±5% on this timescale for the last ~9000 yr (see Fig. 5.33) (Meese et al., 1994).

A more detailed examination of the last 200 yr reveals considerable variability, but no overall trend in accumulation. One problem with accumulation is that it is far less spatially coherent than temperature, making it difficult to correlate with other records. Hence it is perhaps not surprising that the GISP2 record shows few similarities with earlier studies of accumulation changes over the last 800-1500 yr at Dye-3, Milcent, or even Crête (Reeh et al., 1978). Nevertheless, all studies seem to support the conclusion that there has not been any significant long-term change in accumulation over much of the Greenland Ice Sheet over (at least) the last 1400 yr. This is similar to the conclusion reached by Koerner (1977) for the Devon Island Ice Cap.

5.3.4 Theoretical Models

Dating ice at great depth poses severe problems which cannot be easily resolved by the methods previously described. At present, the method most widely used to date pre-Holocene ice is to calculate ice age at depth by means of a theoretical ice-flow model (Dansgaard and Johnsen, 1969; Reeh, 1989; Johnsen and Dansgaard, 1992). Such models describe mathematically the processes by which ice migrates through an ice sheet. Snow accumulating on an ice sheet is slowly transformed into ice during densification of the firn. As more snow accumulates the ice is subjected to vertical compressive strain in which each layer is forced to thin, and is advected laterally towards the margins of the ice sheet (Fig. 5.16). Hence, a core from any site, apart from the ice divide, will contain ice deposited up-slope, with the oldest and deepest ice originating at the summit. Because summit temperatures are cooler, if the core is not recovered from the highest part of the ice sheet, 8lsO values at depth must be corrected for this altitude effect, which will be present regardless of whether any long-term climatic fluctuation has occurred. Because of the nature of ice flow and

Distance (km)

FIGURE 5.16 Schematic cross section to show flow in an ice sheet. Snow deposited on the surface is transformed to ice and follows the flowlines indicated. Ice thins by plastic deformation under compression by the overlying ice. Hence an ice core from site X will contain a record of ice originating upstream, requiring adjustment for the colder conditions in that region. An ice core from the summit will have fewer problems, providing the ice divide has not varied over time. Samples recovered at the surface in ice-sheet margins may represent the same paleo-environmental record as that from an ice core through the ice sheet (modified, Reeh, 1991).

Distance (km)

FIGURE 5.16 Schematic cross section to show flow in an ice sheet. Snow deposited on the surface is transformed to ice and follows the flowlines indicated. Ice thins by plastic deformation under compression by the overlying ice. Hence an ice core from site X will contain a record of ice originating upstream, requiring adjustment for the colder conditions in that region. An ice core from the summit will have fewer problems, providing the ice divide has not varied over time. Samples recovered at the surface in ice-sheet margins may represent the same paleo-environmental record as that from an ice core through the ice sheet (modified, Reeh, 1991).

deformation, most of the time period recorded in an ice core is found in the lowest 5-10% of the record. This means that even small differences in an age-depth model can result in large discrepancies in age estimates for the lowest part of deep ice cores (see e.g., revisions made in the Dye-3 chronology by Dansgaard et al., 1982 and the discussion of age uncertainties in Reeh, 1991).

Simple models can provide a rough estimate of ice age at depth, but for more accurate age estimates, some knowledge of past changes in ice thickness and temperature, accumulation rates, flow patterns, and ice rheology (which changes with dust content) is required (Paterson, 1994). Many of these problems are minimized in the case of ice cores from ice divides (e.g., the GRIP core at Summit, Greenland) or in cores that penetrate very thick ice sheets to depths well above the bed (e.g., Vostok, Antarctica). Nevertheless, even in these cases, uncertainties related to past ice sheet dimensions and the stability of ice divides, changes in ice sheet thickness and especially changes in accumulation rate, can change age-depth relationships very significantly in the deepest sections of a core. On the other hand, if ice can be dated independently by some other means (see Sections 5.4.4 and 5.4.5), flow models can then be constrained and used to estimate changes in those parameters, such as accumulation rate, which would otherwise be problematical (Dahl-Jensen et al., 1993). In this way, iterative changes in models, using best estimates of various parameters and how they might realistically have varied in the past, together with the ages of certain fixed points (such as tephras of known age, or the ~35,000 yr B.P. 10Be anomaly) can be used to refine and improve an ice core chronology.

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