Sea-Level Change (1990) / Chapter Skim
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4 Glacial Isostatic Adjustment and Relative Sea-Level Change
4 Glacial Isostatic Adjustment and Relative Sea-Level Change
Pages 73-87
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From page 73... ...
RICHARD PELTIER University of Toronto ABSTRACT Secular trends of relative sea level revealed on tide-gauge records have recently been interpreted as requiring some eustatic increase of sea level to have occulted during the past century. Although allowance in interpreting these data is usually made for a contribution due to thermal expansion of ocean volume, it is not generally recognized that the continuing influence of glacial isostatic disequilibrium also contributes significantly to the observed secular trends.
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From page 74... ...
Individual relict beaches in the sequence may be dated using the radiocarbon method and their heights above present-day sea level plotted as a function of their age to obtain a local history of relative sea level such as that shown in Figure 4.1b for the Richmond Gulf sequence. Such data make up the primary geophysical database for the inference of mantle viscosity, a physical parameter that is fundamental to the understanding of continental
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From page 75... ...
An excellent example of a drowned coast produced by glacial peripheral bulge collapse is provided by the east coast of the continental United States, all of which is peripheral to the huge Laurentide ice mass that covered Canada 18 ka. An illustrative set of '4C-controlled RSL data from sites along this coast is shown in Figure 4.2.
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From page 76... ...
These coastlines are however located, respectively, on the forebulges of the ancient Laurentian and Fennoscandian ice sheets so that, as discussed above, sea level in these regions will appear to be rising as a consequence of the sinking of the land with respect to the geoid that accompanies the isostatic readjustment of the surface following deglaciation. If it were possible to correct these passive margin data for the influence of glacial isostatic disequilibrium, the residual trends so obtained might be particularly useful as constraints on the current rate of eustatic water rise (fall)
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From page 77... ...
east coast caused by glacial isostatic disequilibrium may be as high as 2 mm/yr. The field theory is then employed to filter this effect from the secular trends observed on all tide gauges located along both the east and west coasts of the continental United States.
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From page 78... ...
Inspection of these comparisons shows that the RSL data at sites inside the ice margin are reasonably well fit by the theoretical model and that the RSL variations at these sites are rather insensitive to changes of lithospheric thickness. This is entirely expected as the spatial scale of the Laurentian ice sheet (Figure 4.3)
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From page 79... ...
Figure 4.6 shows a comparison of observed and predicted present-day peak free-air anomalies for Laurentia and Fennoscandia for a number of Earth models having fixed lithospheric thickness of 120.7 km, and an upper mantle viscosity of 102~ Pa s as required by the sea-level data discussed previously. The Earth models employed differ from one another only in terms of their elastic structures, and lower mantle viscosity is varied through the same sequence of values for all models.
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From page 80... ...
and predicted peak free-air gravity anomalies for both Laurentia and Fennoscandia. Predictions of the peak free-air anomaly are shown as a function of lower mantle viscosity with the upper mantle value held fixed at 102' Pa s and the lithospheric thickness fixed at 120 km.
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From page 81... ...
, this holds true for homogeneous Earth models, since in this limit the isostatic adjustment and rotational contributions to the rotational forcing exactly annihilate one another. For radially stratified models, however, the dynamical symmetry that underlies this cancellation is broken and polar wander can occur even at a time when the surface load is steady.
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From page 82... ...
Model L1F includes, in addition, the influence of the density jump at 670 km depth in the Earth based on the assumption that this discontinuity is capable of inducing a buoyant restoring force when it is displaced from equilibrium by the applied surface loads. The effect of this internal buoyancy in the mantle is to further increase the speed prediction in the model with 102i Pa s uniform mantle viscosity.
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From page 83... ...
The manner in which the interpreta tions of both of these rotational data are influenced by small ice sheets and glaciers has been discussed in detail by Pettier (19881. SECULAR VARIATIONS OF RELATIVE SEA LEVEL WITH GLACIAL ISOSTASY REMOVED Given that the global model of glacial isostasy, described above, is able to explain much of the observed variability in the record of RSL change over the past 104 yr, it is natural to employ it to filter from the recent historical record of tide-gauge observations of secular sealevel change that component which is due to this cause.
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From page 84... ...
Figure 4.10 shows the present-day rate of sea-level rise and fall predicted for North America and northwestern Europe using an Earth model with 1066B elastic structure. The viscous component of themodel is one that has a lithospheric thickness of 200 km, an upper-mantle viscosity of 102i Pa s, and a lower-mantle viscosity of 2 x 102' Pa s.
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From page 85... ...
The tide-gauge observations employed to construct this figure consist of the secular trends extracted from the individual U S EAST COAST INFER ~ (a 4 _ 85 time series of observations at each gauge over the time interval 1940 to 1980 as published in the recent catalogue of the National Ocean Service (19831. Figure 4.11(a)
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From page 86... ...
In regions that were once ice covered, RSL is currently falling at a rate near 1 cm/yr due to this cause. In the immediately peripheral region of the collapsing forebulge, sea level would appear to be rising at rates in excess of 1 mm/yr due to the same process of glacial isostatic adjustment, if this were the only effect operative.
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From page 87... ...
Glacial isostatic adjustment and the free-air gravity anomaly as a constraint on deep mantle viscosity, Geophys.
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