Thomas S. James

Predictions of Antarctic crustal motions driven by present-day ice sheet evolution and by isostatic memory of the Last Glacial Maximum

Detectable crustal motion and secular rate of change of solid-surface gravity may be produced by the Earths response to present-day and past ice mass changes in Antarctica. Scenarios of present-day ice mass balance, previously utilized to explore the global geodetic signatures of the Antarctic ice sheet, produce elastic crustal responses that are typically bounded by uplift rates ≤5 mm/yr, horizontal motion ≤1 mm/yr, and solid-surface gravity change rates ≤1 μGal/yr. In a restricted locality, one scenario produces uplift rates slightly in excess of 10 mm/yr and correspondingly enhanced horizontal and gravity rates. In contrast, the viscoelastic response to ice mass changes occurring since Last Glacial Maximum (LGM) exceeds 5 mm/yr (uplift) over substantial portions of West Antarctica for a wide range of plausible choices of timing and magnitude of deglaciation and mantle viscosity. Similarly, viscoelastic gravity rate predictions exceed 1 μGal/yr (decrease) over large regions, confirming suggestions that a Global Positioning System (GPS) and absolute gravity-based program of crustal monitoring in Antarctica could potentially detect postglacial rebound. A published revision to the CLIMAP model of the Antarctic ice sheet at LGM, herein called the D91 model, features a substantially altered West Antarctic ice sheet reconstruction. This revision predicts a spatial pattern of present-day crustal motion and surface gravity change that diverges strikingly from CLIMAP-based models. Peak D91 crustal rates, assuming deglaciation begins at 12 kyr and ends at 5 kyr, are around 16 mm/yr (uplift), 2 mm/yr (horizontal), and −2.5 μGal/yr (gravity). Tabulated crustal response predictions for selected Antarctic bedrock sites indicate critical localities in the interior of West Antarctica where expected responses are large and D91 predictions differ from CLIMAP-based models by a factor of 2 or more. Observations of the postglacial rebound signal in Antarctica might help constrain Antarctic mass balance and contribution to sea level rise over the past 20,000 years.

Global geodetic signatures of the Antarctic Ice Sheet

Four scenarios of present day Antarctic ice sheet mass change are developed from comprehensive reviews of the available glaciological and oceanographic evidence. The gridded scenarios predict widely varying contributions to secular sea level change ranging from −1.1 to 0.45 mm/yr, and predict polar motion and time-varying low-degree gravitational coefficients that differ significantly from earlier estimates. A reasonably linear relationship between the rate of sea level change from Antarctica A and the predicted Antarctic is found for the four scenarios. This linearity permits a series of forward models to be constructed that incorporate the effects of ice mass changes in Antarctica, Greenland, and distributed smaller glaciers, as well as postglacial rebound (assuming the ICE-3G deglaciation history), with the goal of obtaining optimum reconciliation between observed constraints on and sea level rise . Numerous viable combinations of lower mantle viscosity and hydrologie sources are found that satisfy “observed” in the range of 1 to 2–2.5 mm/yr and observed for degrees 2, 3, and 4. In contrast, rates of global sea level rise above 2.5 mm/yr are inconsistent with available observations. The successful composite models feature a pair of lower mantle viscosity solutions arising from the sensitivity of to glacial rebound. The paired values are well separated at  mm/yr, but move closer together as is. increased, and, in fact, merge around =2-2.5 mm/yr, revealing an intimate relation between and preferred lower mantle viscosity. This general pattern is quite robust and persists for different solutions, for variations in source assumptions, and for different styles of lower mantle viscosity stratification. Tighter constraints for l > 2 may allow some viscosity stratification schemes and source assumptions to be excluded in the future. For a given total observed , the sea level rise from Antarctica A is tightly constrained and ranges from 0 to + 1 mm/yr (corresponding to an ablating ice sheet) as estimates of are raised from 1 to 2.5 mm/yr. However, when the degree 3 zonal harmonic constraint is removed, the solutions show little sensitivity to Antarctic mass balance, emphasizing the need for a well determined odd-degree secular zonal harmonic for determining polar ice mass balance.

Moments of inertia and rotational stability of Mars: Lithospheric support of subhydrostatic rotational flattening

A revised estimate of the spin axis precession rate of Mars has recently been obtained via analysis of range and range-rate data from the Viking and Pathfinder landers. When combined with existing estimates of the degree 2 spherical harmonic coefficients of the gravitational field, this yields a complete determination of the inertia tensor of Mars. Despite this progress, there are still numerous unresolved issues related to the internal structure and rotational dynamics of Mars. We compare results of two different approaches to this problem. In one approach, the observed gravitational field is conceptually partitioned into hydrostatic and nonhydrostatic contributions. In the other approach, the input to the system is partitioned into rotational and load components, and the internal structure (density and elastic rigidity) determines the response. We demonstrate that there is an important, and still unresolved trade-off between lithospheric thickness and the shape of the load component of the gravity field. As the lithospheric thickness is increased, the required load departs more from axial symmetry. The load corresponding to zero lithospheric thickness is nearly symmetric about an equatorial axis, but if the lithospheric thickness is closer to 100 km, the required load is a fully triaxial ellipsoid, with the intermediate moment of inertia halfway between the least and greatest moments. The symmetry of the load has considerable influence on the long-term rotational stability of Mars.

Temporal Geoid of a Rebounding Antarctica and Potential Measurement by the GRACE and GOCE Satellites

We model the present-day time-dependent gravity field of Antarctica driven by solid earth rebound response to deglaciation of a more extensive continental ice cover at Last Glacial Maximum (22–15 kyr BP). Among the various global components of late-Pleistocene and Holocene eustatic sea level rise, Antarctica’s contribution has been most contentious. Using new geological inferences provided by marine sediment cores and ice-volcanic deposits within the Antarctic continental interior, we compute predictive maps of geoid secular variation and uplift. Competing forward models produce quite distinguishable signatures in terms of the resolution and accuracy of the GRACE and GOCE satellite missions. Both the timing and amplitude of the ice sheet paleotopography and the structure of mantle viscosity are crucial inputs to the predicted temporal gravity field in Antarctica. In general, our new predictions indicate that the rebound component of Antarctica’s secularly varying geoid is at the 0.1 to 0.6 mm/yr level. These estimates may err on the conservative side if the ice sheet has been slowly deglaciating during the last two thousand years.

Polar motion of a viscoelastic Earth due to glacial cycle mass loading

The growth and decay of ice sheets can change the symmetry axis of the global mass distribution and thus excite motion of the rotation axis of the Earth. We develop a simple, normal mode expansion of the operator which converts surface load histories into polar motion histories, and use it to characterize the polar motion response to arbitrary surface loading excitation, in terms of gain and phase, for a wide range of Earth models and excitation time scales. Uncertainties in loading history presently limit the utility of constraints on mantle rheology which can be obtained from matching the present direction and rate of motion of the pole. Because of its magnitude and distance from the pole, the Laurentide ice sheet alone can induce motion of the rotation pole by over 100 km. This will influence the pattern of incident radiation on continents and oceans, and may play a significant role in limiting ice sheet growth. Differential motion of the mantle and core, on a glacial timescale, may also influence the geomagnetic field.

Climatic impact of glacial cycle polar motion: Coupled oscillations of ice sheet mass and rotation pole position

Precessional motion of Earths rotation axis relative to its orbit is a well-known source of long-period climatic variation. It is less well appreciated that growth and decay of polar ice sheets perturb the symmetry of the global mass distribution enough that the geographic location of the rotation axis will change by at least 15 km and possibly as much as 100 km during a single glacial cycle. This motion of the pole will change the seasonal and latitudinal pattern of temperatures. We present calculations, based on a diurnal average energy balance, which compare the summer and winter temperature anomalies due to a 1° decrease in obliquity with those due to a 1° motion of the rotation pole toward Hudson Bay. Both effects result in peak temperature perturbations of about 1° Celsius. The obliquity change primarily influences the amplitude of the seasonal cycle, while the polar motion primarily changes the annual mean temperatures. The polar motion induced temperature anomaly is such that it will act as a powerful negative feedback on ice sheet growth. We also explore the evolution of the coupled system composed of ice sheet mass and pole position. Oscillatory solutions result from the conflicting constraints of rotational and thermal stability. A positive mass anomaly on an otherwise featureless Earth is in rotational equilibrium only at the poles or the equator. The two polar equilibria are rotationally unstable, and the equatorial equilibrium, though rotationally stable, is thermally unstable. We find that with a plausible choice for the strength of coupling between the thermal and rotational systems, relatively modest external forcing can produce significant response at periods of 10 4 -10 6 years, but it strongly attenuates polar motion at longer periods. We suggest that these coupled oscillations may contribute to the observed dominance of 100 kyr glacial cycles since the mid-Pleistocene and will tend to stabilize geographic patterns that are suitable to glaciations.