133 research outputs found
True polar wander driven by late-stage volcanism and the distribution of paleopolar deposits on Mars
The areal centroids of the youngest polar deposits on Mars are offset from
those of adjacent paleopolar deposits by 5-10 degrees. We test the hypothesis
that the offset is the result of true polar wander (TPW), the motion of the
solid surface with respect to the spin axis, caused by a mass redistribution
within or on the surface of Mars. In particular, we consider TPW driven by
late-stage volcanism during the late Hesperian to Amazonian. There is
observational and qualitative support for this hypothesis: in both North and
South, observed offsets lie close to a great circle 90 degrees from Tharsis, as
expected for polar wander after Tharsis formed. We calculate the magnitude and
direction of TPW produced by mapped late-stage lavas for a range of
lithospheric thicknesses, lava thicknesses, eruption histories, and prior polar
wander events. If Tharsis formed close to the equator, the stabilizing effect
of a fossil rotational bulge located close to the equator leads to predicted
TPW of <2 degrees, too small to account for observed offsets. If, however,
Tharsis formed far from the equator, late-stage TPW driven by low-latitude,
late-stage volcanism would be 6-33 degrees, similar to that inferred from the
location of paleopolar deposits. 4.4+/-1.3x10^19 kg of young erupted lava can
account for the offset of the Dorsa Argentea Formation from the present-day
south rotation pole. This mass is consistent with prior mapping-based estimates
and would imply a mass release of CO2 by volcanic degassing similar to that in
the atmosphere at the present time. The South Polar Layered Deposits are offset
from the spin axis in the opposite sense to the other paleopolar deposits. This
can be explained by an additional contribution from a plume beneath Elysium. We
conclude with a list of observational tests of the TPW hypothesis.Comment: Accepted by Earth and Planetary Science Letters. 3 tables, 8 figure
A reappraisal of postglacial decay times from Richmond Gulf and James Bay, Canada
Decay times inferred from relative sea-level (RSL) histories of previously glaciated regions provide a potentially important constraint on mantle rheology. We present a new compilation of RSL data from Richmond Gulf and James Bay, Canada. This recompilation reveals errors in previous compilations that led to inaccurate estimates for the Richmond Gulf decay time in a series of recently published articles. We derive updated estimates for the decay time at Richmond Gulf and James Bay using a methodology that incorporates errors in both the age and the height of the sea-level markers. This exercise is guided by a series of synthetic RSL calculations that show that decay time estimates in the region can be significantly biased if the RSL time-series are not corrected for global eustatic sea-level trends, or if the estimates are based on composite RSL histories derived by combining data from both the Richmond Gulf and the James Bay regions. Our decay time analysis for Richmond Gulf applies the pioneering approach of Walcott (1980) to a large database and we derive a value of 4.0-6.6 kyr, where the range is defined by a misfit tolerance 10 per cent higher than the minimum. Our analysis for James Bay is based on the uplift curve derived by Hardy (1976), and we estimate a decay time of about 2.0-2.8 kyr. The difference between our estimates for Richmond Gulf and James Bay may be due to errors in the observational record from these regions, but could also be influenced by lateral variations in lithospheric structure associated with the assembly of Laurentia
Constraining proposed combinations of ice history and Earth rheology using VLBI determined baseline length rates in North America
We predict the present-day rates of change of the lengths of 19 North American baselines due to the glacial isostatic adjustment process. Contrary to previously published research, we find that the three dimensional motion of each of the sites defining a baseline, rather than only the radial motions of these sites, needs to be considered to obtain an accurate estimate of the rate of change of the baseline length. Predictions are generated using a suite of Earth models and late Pleistocene ice histories, these include specific combinations of the two which have been proposed in the literature as satisfying a variety of rebound related geophysical observations from the North American region. A number of these published models are shown to predict rates which differ significantly from the VLBI observations
Studies of regional and global tectonics and the rotation of the earth using very-long-baseline interferometry
Work is continuing on the study of atmospheric gradients. We include a preprint entitled 'The effect of turbulence on atmospheric gradient parameters determined from ground-based radiometric and space geodetic measurements'. Work has begun on a study of solid Earth tidal deformations using the VLBI data set. We have examined deformations at the semi-diurnal tidal period using the IRIS data set
Laurentide-Cordilleran Ice Sheet saddle collapse as a contribution to meltwater pulse 1A
The source or sources of meltwater pulse 1A (MWP-1A) at ~14.5 ka, recorded at widely distributed sites as a sea-level rise of ~10-20 m in less than 500 years, is uncertain. A recent ice modeling study of North America and Greenland (Gregoire et al., 2012) has suggested that the collapse of an ice saddle between the Laurentide and Cordilleran Ice Sheets, with a eustatic sea-level equivalent (ESLE) of ~10 m, may have been the dominant contributor to MWP-1A. To test this suggestion, we predict gravitationally self consistent sea-level changes from the Last Glacial Maximum to the present-day associated with the Gregoire et al. (2012) ice model. We find that a combination of the saddle collapse scenario and melting outside North America and Greenland with an ESLE of ~3 m yields sea-level changes across MWP-1A that are consistent with far-field sea-level records at Barbados, Tahiti and Sunda Shelf
Crustal loading near Great Salt Lake, Utah
Two sites of the BARGEN GPS network are located ∼30 km south of Great Salt Lake (GSL). Lake-level records since mid-1996 indicate seasonal water elevation variations of ∼0.3 m amplitude superimposed on a roughly “decadal” feature of amplitude ∼0.6 m. Using an elastic Green's function and a simplified load geometry for GSL, we calculate that these variations translate into radial crustal loading signals of ±0.5 mm (seasonal) and ±1 mm (decadal). The horizontal loading signals are a factor of ∼2 smaller. Despite the small size of the expected loading signals, we conclude that we can observe them using GPS time series for the coordinates of these two sites. The observed amplitudes of the variations agree with the predicted decadal variations to <0.5 mm. The observed annual variations, however, disagree; this difference may be caused by some combination of local precipitation-induced site motion, unmodeled loading from other nearby sources, errors in the GSL model, and atmospheric errors
Time-dependent rotational stability of dynamic planets with elastic lithospheres
True polar wander (TPW), a reorientation of the rotation axis relative to the solid body, is driven by mass redistribution on the surface or within the planet and is stabilized by two aspects of the planet's viscoelastic response: the delayed viscous readjustment of the rotational bulge and the elastic stresses in the lithosphere. The latter, following Willemann (1984), is known as remnant bulge stabilization. In the absence of a remnant bulge, the rotation of a terrestrial planet is said to be inherently unstable. Theoretical treatments have been developed to treat the final (equilibrium) state in this case and the time-dependent TPW toward this state, including nonlinear approaches that assume slow changes in the inertia tensor. Moreover, remnant bulge stabilization has been incorporated into both equilibrium and linearized, time-dependent treatments of rotational stability. We extend the work of Ricard et al. (1993) to derive a nonlinear, time-dependent theory of TPW that incorporates stabilization by both the remnant bulge and viscous readjustment of the rotational bulge. We illustrate the theory using idealized surface loading scenarios applied to models of both Earth and Mars. We demonstrate that the inclusion of remnant bulge stabilization reduces both the amplitude and timescale of TPW relative to calculations in which this stabilization is omitted. Furthermore, given current estimates of mantle viscosity for both planets, our calculations indicate that departures from the equilibrium orientation of the rotation axis in response to forcings with timescale of 1 Myr or greater are significant for Earth but negligible for Mars
Reciprocity and sensitivity kernels for sea level fingerprints
Reciprocity theorems are established for the elastic sea level fingerprint
problem including rotational feedbacks. In their simplest form, these results
show that the sea level change at a location x due to melting a unit point mass
of ice at x' is equal to the sea level change at x' due to melting a unit point
mass of ice at x. This identity holds irrespective of the shoreline geometry or
of lateral variations in elastic Earth structure. Using the reciprocity
theorems, sensitivity kernels for sea level and related observables with
respect to the ice load can be readily derived. It is notable that calculation
of the sensitivity kernels is possible using standard fingerprint codes, though
for some types of observable a slight generalisation to the fingerprint problem
must be considered. These results are of use within coastal hazard assessment
and have a range of potential applications within studies of modern-day sea
level change.Comment: Paper submitted to Geophysical Journal Internationa
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The Mid-Pliocene sea-level conundrum: Glacial isostasy, eustasy and dynamic topography
Determining eustatic sea level during the Mid-Pliocene warm period (~ 3.3 to 2.9 Ma) has been a central but elusive goal in the study of past warm climates. Estimates of eustatic sea level based on geologic data span a broad range; variation that we now recognize is due in part to geographically varying post-depositional displacement caused by glacial isostatic adjustment and dynamic topography. In this study, we combine field observations and glacial isostatic adjustment modeling to estimate the dynamic topography signal in three areas that are important to paleo-sea level studies of the Mid-Pliocene warm period (South Africa, West Australia and southeastern United States). We show that dynamic topography played a significant role in the post-depositional displacement of Pliocene, and even younger Pleistocene, shorelines. In this regard, we provide a robust paleo-sea level elevation data set, corrected for glacial isostatic adjustment, that can be used to evaluate predictions from mantle flow models of dynamic topography
On postglacial sea level—III. Incorporating sediment redistribution
We derive a generalized theory for gravitationally self-consistent, static sea level variations on earth models of arbitrary complexity that takes into account the redistribution of sediments. The theory is an extension of previous work that incorporated, into the governing equations, shoreline migration due to local sea level variations and changes in the geometry of grounded, marine-based ice. In addition, we use viscoelastic Love number theory to present a version of the new theory valid for spherically symmetric earth models. The Love number theory accounts for the gravitational, deformational and rotational effects of the sediment redistribution. As a first, illustrative application of the new theory, we compute the perturbation in sea level driven by an idealized pulse of sediment transport into the Gulf of Mexico. We demonstrate that incorporating a gravitationally self-consistent water load in this case significantly improves the accuracy of sea level predictions relative to previous simplified treatments of the sediment redistribution
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