Note on estimates of the glacial-isostatic decay spectrum for Fennoscandia

For more than 30 years, Sauramo's (1958) shoreline diagram of the Fennoscandian uplift has been used in geophysical studies for estimates of the glacial-isostatic decay spectrum in order to infer from it the viscosity stratification in the Earth's mantle below Fennoscandia. The intent of the present note is to point out that more recent geological studies suggest that Sauramo's shoreline diagram is an incorrect representation of the Fennoscandian uplift. Geophysical interpretations based on the diagram may therefore require revision.     

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.     

Material versus isobaric internal boundaries in the Earth and their influence on postglacial rebound

Most previous earth models used to calculate viscoelastic relaxation after the removal of the Late Pleistocene ice loads implicitly assume that there is no exchange of mass across the mantle density discontinuities on periods of tens of thousands of years (the material boundary formulation). In the present study, simple incompressible models are used to determine the Earth's behaviour in the case where the density discontinuity remains at a constant pressure rather than deforming with the material (the isobaric boundary formulation). The calculation of the movement of the boundary is more rigorous than in earlier studies and uses the local incremental pressure calculated at the depth of the boundary and allows for the vertical deformation caused by the change in volume as material changes phase. It is shown that the buoyancy modes associated with the density discontinuities decrease in strength and increase in relaxation time analogous to what results when the density contrast is reduced. Also, two viscoelastic modes arise from an isobaric boundary, which is also predicted when there is a contrast in rigidity or viscosity across a material boundary. The difference in predicted radial deformation between the isobaric boundary model and the material boundary model is largest for long-wavelength loads for which the material incremental pressure at depth is largest. If the isobaric boundary model is appropriate for the treatment of the mineral phase changes in the mantle on glacial rebound timescales, then previous inferences of the deep-mantle to shallow-mantle viscosity ratio based on large-scale deformation (spherical harmonic degree < 10) of the Earth and including data from the early part of the glacio-isostatic uplift are too small.     

Effect of viscosity structure on fault potential and stress orientations in eastern Canada

Previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern Canada have focused on the mode of failure and the observed timing of the pulse of earthquake/faulting activity following deglaciation. In this study, the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times. It is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 10²¹ Pa s mantle.
The effects of viscosity structure on the above predictions are also examined. It is shown that, since most of the above observations are found within the ice margin, they are not very sensitive to lithospheric thickness. Also, the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust. High viscosity (10²² Pa s) in the lower mantle is rejected by the stress orientation and rotation observations. A low-viscosity (6 times 10²⁰Pa s) upper mantle with 1.6 times 10²¹ Pa s in the upper part of the lower mantle and 3 times 10²¹ Pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations.     

On the choice of timescale in glacial rebound modelling: mantle viscosity estimates and the radiocarbon timescale

Most glacial rebound studies have been carried out with respect to the radiocarbon timescale, whose departures from the calendar timescale are becoming increasingly well established. In consequence, it has sometimes been argued that the choice of the radiocarbon timescale may invalidate some of the conclusions drawn from rebound and sea-level analyses. The purpose of this note is to compare rebound model results based on both timescales, using the British Isles data for the test. The results indicate that the choice of timescale is unimportant provided that the time dimension of viscosity is appropriately defined. The results confirm that the radiocarbon viscosities are about 15 per cent less than the corresponding calendar-time viscosities. Also, provided that consistency of timescales is maintained in the analysis, and that the time-accuracy estimates of the radiocarbon data reflect the departures from a linear timescale, the use of the radiocarbon timescale does not impinge on inferences drawn about the timing of melting of ice sheets or eustatic sea-level change.     

1980～2007年喜马拉雅东段洛扎地区冰川变化遥感监测

利用1980年地形图和多年遥感资料,采用目视解译方法手工提取了喜马拉雅东段洛扎地区4个时期的冰川信息,对冰川时空分布特征和变化与不确定性进行了分析,并结合近28年(1980～2007年)的气温、降水量资料对研究区的冰川变化原因进行了研究.结果表明:(1)1980～2007年,洛扎地区冰川面积从491.64 km<'2>...     

The onset of Pleistocene glaciation in the Barents Sea: implications for glacial isostatic adjustment

In this paper the effect of a delayed onset of glaciation in the Barents Sea on glacial isostatic adjustment is investigated. The model calculations solve the sea-level equation governing the total mass redistributions associated with the last glaciation cycle on a spherically symmetric, linear, Maxwell viscoelastic earth for two different scenarios for the growth phase of the Barents Sea ice sheet. In the first ice model a linear growing history is used for the Barents Sea ice sheet, which closely relates its development to the build-up of other major Late Pleistocene ice sheets. In the second ice model the accumulation of the Barents Sea ice sheet is restricted to the last 6 ka prior to the last glacial maximum.
The calculations predict relative sea levels, present-day radial velocities, and gravity anomalies for the area formerly covered by the Weichselian ice sheet. The results show that observed relative sea levels in the Barents Sea are appropriate for distinguishing between the different glaciation histories. In particular, present-day observables such as the free-air gravity anomaly over the Barents Sea, and the present-day radial velocities are sensitive to changes in the glaciation history on this scale.
A palaeobathymetry derived from relative sea-level predictions before the last glacial maximum based on the second ice model essentially agrees with a palaeobathymetry derived by Lambeck (1995). The additional emerged areas provide centres for the build-up of an ice sheet and thus support the theory of Hald, Danielsen & Lorentzen (1990) and Mangerud et al. (1992) that the Barents Sea was an essentially marine environment shortly before the last glacial maximum.