Glacial Isostasy, Sea-Level and Mantle Rheology
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Instead of all areas around a load becoming depressed, the flexural rigidity of the lithosphere among other things causes some areas to actually be elevated during this same time. This is known as the peripheral bulge which forms a ring around the outside of the load Figure 2 ; Middleton and Wilcock, Effect and "Use" of Isostatic Rebound. Now that the "basics" of isostatic rebound have been established, the rest of this paper will focus on one effect of rebound and insight gained about the earth through the modeling of rebound. I will confine my view to the rebound processes associated with glacial isostasy, although other types of loads may have similar effects.
Sea level changes have left numerous signs to their occurrence such as abandoned shorelines, drowned rivers, marine extensions into continents, biological e. Plag, et. In fact, it was these signs of sea level change that led early geologists to think about isostatic rebound processes e. Jamieson, and Shaler, Today, the knowledge of eustatic sea level variations is very important in climatic studies.
However, since apparent sea level is affected by two different, but linked, processes, deciphering purely eustatic sea level can be complicated.
The processes affecting sea level are: 1 formation of continental ice and 2 glacial compression and rebound. One area where this complication is now obvious is the east coast of the United States. The North American continent was partially covered with the Laurentide ice sheet until about years ago Farrand, The east coast was the peripheral bulge of this ice sheet and the land there is now experiencing subsidence.
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This sinking makes it appear as though sea level were rising more quickly than it truly is. One solution to this problem of differentiating between eustatic sea level and apparent sea level rise caused by glacial rebound processes is to study an area that would not be affected by glaciation. A sea level curve from Barbados has been derived for this reason Peltier, However, there may be other problems associated with using this area e.
Another solution is one presented by Peltier In this study, Peltier compares the variations of sea level derived from tide gauge data short term with radiocarbon data long term which would record the variations of sea level over the past several millennia on the east coast of the United States Figure 3. Peltier felt that since the Laurentide ice sheet which had partially covered North America had disappeared by years ago, there should be no eustatic sea level change due to the melting of ice it should have already happened by this point since this time. The tide gauge data should reflect both apparent sea level change as well as any eustatic sea level change that we are presently experiencing.
Subtracting the long term, isostatically controlled rates of sea level change from the total change determined by the tide gauge data should result in the present eustatic sea level change. Using this method, Peltier obtained a eustatic sea level rise of about two millimeters per year. This is supported, generally, by other sources Plag, Much of the focus on isostatic rebound today is on the information that can be obtained about the earth's structure and processes through modeling.
Studies on the viscosity of the mantle abound, as well as studies on general mantle and lithospheric properties.
Most of the studies consist of mathematical and computer modeling; many assumptions and simplifications are necessary. Most models try to see the effect that certain conditions would have on rebound processes, and then check to see if the results are present geologically. Various properties of the mantle have been explored via modeling of the effects these properties have on postglacial rebound. These include stratification of the mantle both vertically and horizontally Fang and Hager, , mantle rheology Newtonian vs.
Power law behavior; Gasperini et al. Calculation : Mantle Viscosity Middleton and Wilcock, One way to decipher the viscosity of the mantle is to calculate how fast the surface of the earth rises after the load produced by an ice sheet is removed melts. Figure 4 shows a simple model of a two dimensional cross-section through the ice sheet the diamond shape and the asthenosphere neglecting lithospheric effects, apparently! Assuming that the ice sheet had reached isostatic equilibrium before melting, the pressure exerted by the ice sheet would be balanced by the buoyancy pressure exerted by the asthenosphere.
So if you take the partial derivatives of both sides of the equation with respect to both x and P, you obtain:.
Glacial Isostasy, Sea-Level and Mantle Rheology
Since u equals the average velocity between two parallel plates, equations can be given by the theory of Couette flow not given in this paper and one can get an equation for u based on these equations as well as the pressure gradient calculated in Part I. Now you can solve for viscosity m by filling in the "knowns" to the final v equation. L can be estimated from seismological data. Clark PU, Tarasov L. Closing the sea level budget at the Last Glacial Maximum.
Golledge NR, et al. Glaciology and geological signature of the Last Glacial Maximum Antarctic ice sheet. Ivins ER, et al. A deglacial model for Antarctica: geological constraints and glaciological modelling as a basis for a new model of Antarctic glacial isostatic adjustment. Bentley MJ, et al. Deglacial history of the West Antarctic Ice Sheet in the Weddell Sea embayment: constraints on past ice volume change.
Clark PU. Parrenin F, et al. A fully coupled 3-D ice-sheet-sea-level model: algorithm and applications. Model Dev. Modeling the evolution of Antarctic ice sheet over the last , years: Implications for altitude changes in the Vostok region. Geophys Res-Atmos. Huybrechts P.
Sea-level changes at the LGM from ice-dynamic reconstructions of the Greenland and Antarctic ice sheets during the glacial cycles. An assessment of forward and inverse GIA solutions for Antarctica.
Velicogna I, Wahr J. Measurements of time-variable gravity show mass loss in Antarctica.
Shepherd A, et al. A reconciled estimate of ice-sheet mass balance. Gunter BC, et al. Empirical estimation of present-day Antarctic glacial isostatic adjustment and ice mass change. Martin-Espanol A, et al.
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Spatial and temporal Antarctic Ice Sheet mass trends, glacio-isostatic adjustment, and surface processes from a joint inversion of satellite altimeter, gravity, and GPS data. Sasgen I, et al. Shepherd, A. Mass balance of the Antarctic Ice Sheet from to Nature , Groh A, et al. Barletta VR, et al. Observed rapid bedrock uplift in Amundsen Sea Embayment promotes ice-sheet stability. Bevis, M. Geodetic measurements of vertical crustal velocity in West Antarctica and the implications for ice mass balance. Thomas ID, et al. Widespread low rates of Antarctic glacial isostatic adjustment revealed by GPS observations.
Crustal and upper mantle structure beneath Antarctica and surrounding oceans. Morelli A, Danesi S. Seismological imaging of the Antarctic continental lithosphere: a review.
Glacial Isostasy, Sea-Level and Mantle Rheology - Semantic Scholar
Planet Change. Lloyd, A. Shen, W. The crust and upper mantle structure of central and West Antarctica from Bayesian inversion of Rayleigh wave and receiver functions. Schaeffer AJ, Lebedev S. Global shear speed structure of the upper mantle and transition zone. Computations of the viscoelastic response of a 3-D compressible Earth to surface loading: an application to Glacial Isostatic Adjustment in Antarctica and Canada. Lateral viscosity variations beneath Antarctica and their implications on regional rebound motions and seismotectonics.
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