Upper mantle viscosity structure and lithospheric thickness of Antarctica inferred from recent seismic models

2020 ◽  
Author(s):  
Douglas Wiens ◽  
Andrew Lloyd ◽  
Weisen Shen ◽  
Andrew Nyblade ◽  
Richard Aster ◽  
...  
Keyword(s):  
Antarctic Peninsula ◽  
Upper Mantle ◽  
Ice Sheet ◽  
West Antarctica ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Seismic Models ◽  
The Antarctic

<p>Upper mantle viscosity structure and lithospheric thickness control the solid Earth response to variations in ice sheet loading. These parameters vary significantly across Antarctica, leading to strong regional differences in the timescale of glacial isostatic adjustment (GIA), with important implications for ice sheet models.  We estimate upper mantle viscosity structure and lithospheric thickness using two new seismic models for Antarctica, which take advantage of temporary broadband seismic stations deployed across Antarctica over the past 18 years. Shen et al. [2018] use receiver functions and Rayleigh wave velocities from earthquakes and ambient noise to develop a higher resolution model for the upper 200 km beneath Central and West Antarctica, where most of the seismic stations have been deployed. Lloyd et al [2019] use full waveform adjoint tomography to invert three-component earthquake seismograms for a radially anisotropic model covering Antarctica and adjacent oceanic regions to 800 km depth. We estimate the mantle viscosity structure from seismic structure using laboratory-derived relationships between seismic velocity, temperature, and rheology. Choice of parameters for this mapping is guided in part by recent regional estimates of mantle viscosity from geodetic measurements. We also describe and compare several different methods of estimating lithospheric thickness from seismic constraints.</p><p>The mantle viscosity estimates indicate regional variations of several orders of magnitude, with extremely low viscosity (< 10<sup>19</sup> Pa s) beneath the Amundsen Sea Embayment (ASE) and the Antarctic Peninsula, consistent with estimates from GIA models constrained by GPS data.  Lithospheric thickness is also highly variable, ranging from around 60 km in parts of West Antarctica to greater than 200 km beneath central East Antarctica. In East Antarctica, several prominent regions such as Dronning Maude Land and the Lambert Graben show much thinner lithosphere, consistent with Phanerozoic tectonic activity and lithospheric disruption. Thin lithosphere and low viscosity between the ASE and the Antarctic Peninsula likely result from the thermal effects of the slab window as the Phoenix-Antarctic plate boundary migrated northward during the Cenozoic. Low viscosity regions beneath the ASE and Marie Byrd Land coast connect to an offshore anomaly at depths of ~ 250 km, suggesting larger-scale thermal and geodynamic processes that may be linked to the initial Cretaceous rifting of New Zealand and Antarctica. Low mantle viscosity results in a characteristic GIA time scale on the order of several hundred years, such that isostatic adjustment occurs on the same time scale as grounding line retreat.  Thus the associated rebound may lessen the effect of the marine ice sheet instability proposed for the ASE region. </p>

scholarly journals GRACE constraints on Earth rheology of the Barents Sea and Fennoscandia

2019 ◽  
Author(s):  
Marc Rovira-Navarro ◽  
Wouter van der Wal ◽  
Valentina R. Barletta ◽  
Bart C. Root ◽  
Louise Sandberg Sørensen
Keyword(s):  
Solid Earth ◽  
Upper Mantle ◽  
Barents Sea ◽  
Ice Sheet ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Mantle Viscosity ◽  
The Barents Sea ◽  
Grace Data ◽  
Geodynamic Models

Abstract. The Barents Sea is situated on a continental margin and was home to a large ice sheet at the Last Glacial Maximum. Studying the solid Earth response to the removal of this ice sheet (Glacial Isostatic Adjustment, GIA) can give insight in the sub-surface structure in this region. However, because the region is currently covered by ocean, uplift measurements from the center of the former ice sheet are not available, but GRACE data has been shown to be able to constrain GIA. Here we analyze GRACE data for the period 2003–2015 in the Barents Sea and use it to constrain a GIA models for the region. We study the effect of uncertainty in non-tidal ocean mass models that are used to correct GRACE data and find that it is not negligible and should be taken into account when studying solid Earth signals in oceanic areas from GRACE. We compare the obtained gravity rates with GIA model predictions for different ice deglaciation chronologies and infer a lower bound for the Earth's upper mantle viscosity of 2·1020 Pa·s. Following a similar procedure for Fennoscandia we find that the preferred upper mantle viscosity there is a factor 2 larger than in the Barents Sea for a range of lithospheric thickness values. This factor is shown to be consistent with the ratio of viscosities derived for both regions from global seismic models. The viscosity difference can serve as constraint for geodynamic models of the area.

Impact of glacial isostatic adjustment on the long-term stability of the Antarctic ice sheet

2021 ◽  
Author(s):  
Violaine Coulon ◽  
Kevin Bulthuis ◽  
Pippa Whitehouse ◽  
Sainan Sun ◽  
Frank Pattyn
Keyword(s):  
Sea Level ◽  
Viscoelastic Properties ◽  
Ice Sheet ◽  
West Antarctica ◽  
Antarctic Ice Sheet ◽  
Isostatic Adjustment ◽  
West Antarctic ◽  
Mantle Viscosity ◽  
The Antarctic

<p>Projections of the contribution of the Antarctic ice sheet to future sea-level rise remain highly uncertain, especially on long timescales. One of the reasons for this uncertainty lies in the uncertainty in the intensity of the feedbacks of glacial isostatic adjustment (GIA; i.e. the combination of bedrock adjustment and gravitationally-consistent sea-surface changes due to ice mass changes) on ice-sheet evolution. Indeed, the Antarctic ice sheet lies on a solid Earth that displays large spatial variations in rheological properties, with a thin lithosphere and low-viscosity upper mantle beneath West Antarctica and an opposing structure beneath East Antarctica (Morelli & Danesi, 2004; Lloyd et al., 2020). In addition to this West-East dichotomy, strong viscoelastic heterogeneities (sometimes by several orders of magnitude across relatively short spatial scales) exist within the East and West Antarctic regions (An et al., 2015). These lateral variations are known to have a significant impact on the ice-sheet grounding-line stability (Gomez et al., 2015; Konrad et al., 2015). However, large uncertainties remain in determining these viscoelastic properties with precision.</p><p>Here, we investigate the influence of GIA feedbacks on the uncertainty in assessing the long-term contribution of the Antarctic ice sheet to future sea-level rise (SLR). In this framework, we design an ensemble approach, taking advantage of the computational efficiency of the Elementary GIA model (Coulon et al., under review). The latter consists of a modified Elastic Lithosphere—Relaxing Asthenosphere model able to consider spatially-varying viscoelastic properties supplemented with an approximation of gravitationally-consistent geoid changes, allowing to approximate near-field relative sea-level changes. Using existing upper-mantle viscosity and lithosphere thickness maps, we produce a large range of plausible Antarctic viscoelastic properties by varying the level of lateral variability in the associated relaxation time and flexural rigidity. We thereby take into account (i) the important lateral variations in rheological properties observed beneath the Antarctic ice sheet as well as (ii) the strong uncertainty characterizing the estimation of Antarctic solid Earth properties. We investigate the potential stabilizing role of GIA effects as well as their influence on multi-centennial to multi-millenial SLR. In addition, we investigate whether GIA feedbacks are able to stabilize the Antarctic ice sheet on short or longer timescales for strong and intermediate mitigation climate scenarios. Preliminary results (Coulon et al., under review) show that the weak Earth structure observed beneath West Antarctica plays a significant role in promoting the stability of the West Antarctic ice sheet (WAIS). However, WAIS collapse cannot be prevented under high-emissions climate scenarios. The highest uncertainty arises from the East Antarctic ice sheet (EAIS) where ice retreat in the Aurora Basin is highly dependent on mantle viscosity.</p>

scholarly journals Mass balance of the Antarctic ice sheet 1992–2016: reconciling results from GRACE gravimetry with ICESat, ERS1/2 and Envisat altimetry

2021 ◽  
pp. 1-27
Author(s):  
H. Jay Zwally ◽  
John W. Robbins ◽  
Scott B. Luthcke ◽  
Bryant D. Loomis ◽  
Frédérique Rémy
Keyword(s):  
Mass Balance ◽  
Antarctic Peninsula ◽  
East Antarctica ◽  
Mantle Material ◽  
West Antarctica ◽  
Mantle Viscosity ◽  
Grounding Line ◽  
The Antarctic ◽  
Total Gain

Abstract GRACE and ICESat Antarctic mass-balance differences are resolved utilizing their dependencies on corrections for changes in mass and volume of the same underlying mantle material forced by ice-loading changes. Modeled gravimetry corrections are 5.22 times altimetry corrections over East Antarctica (EA) and 4.51 times over West Antarctica (WA), with inferred mantle densities 4.75 and 4.11 g cm−3. Derived sensitivities (Sg, Sa) to bedrock motion enable calculation of motion (δB0) needed to equalize GRACE and ICESat mass changes during 2003–08. For EA, δB0 is −2.2 mm a−1 subsidence with mass matching at 150 Gt a−1, inland WA is −3.5 mm a−1 at 66 Gt a−1, and coastal WA is only −0.35 mm a−1 at −95 Gt a−1. WA subsidence is attributed to low mantle viscosity with faster responses to post-LGM deglaciation and to ice growth during Holocene grounding-line readvance. EA subsidence is attributed to Holocene dynamic thickening. With Antarctic Peninsula loss of −26 Gt a−1, the Antarctic total gain is 95 ± 25 Gt a−1 during 2003–08, compared to 144 ± 61 Gt a−1 from ERS1/2 during 1992–2001. Beginning in 2009, large increases in coastal WA dynamic losses overcame long-term EA and inland WA gains bringing Antarctica close to balance at −12 ± 64 Gt a−1 by 2012–16.

scholarly journals Lithosphere and upper-mantle structure of the southern Baltic Sea estimated from modelling relative sea-level data with glacial isostatic adjustment

Solid Earth ◽  
2014 ◽  
Vol 5 (1) ◽  
pp. 447-459 ◽  
Author(s):  
H. Steffen ◽  
G. Kaufmann ◽  
R. Lampe
Keyword(s):  
Baltic Sea ◽  
Sea Level ◽  
Upper Mantle ◽  
Glacial Isostatic Adjustment ◽  
Relative Sea Level ◽  
Southern Baltic Sea ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Mantle Viscosity ◽  
Ice Model

Abstract. During the last glacial maximum, a large ice sheet covered Scandinavia, which depressed the earth's surface by several 100 m. In northern central Europe, mass redistribution in the upper mantle led to the development of a peripheral bulge. It has been subsiding since the begin of deglaciation due to the viscoelastic behaviour of the mantle. We analyse relative sea-level (RSL) data of southern Sweden, Denmark, Germany, Poland and Lithuania to determine the lithospheric thickness and radial mantle viscosity structure for distinct regional RSL subsets. We load a 1-D Maxwell-viscoelastic earth model with a global ice-load history model of the last glaciation. We test two commonly used ice histories, RSES from the Australian National University and ICE-5G from the University of Toronto. Our results indicate that the lithospheric thickness varies, depending on the ice model used, between 60 and 160 km. The lowest values are found in the Oslo Graben area and the western German Baltic Sea coast. In between, thickness increases by at least 30 km tracing the Ringkøbing-Fyn High. In Poland and Lithuania, lithospheric thickness reaches up to 160 km. However, the latter values are not well constrained as the confidence regions are large. Upper-mantle viscosity is found to bracket [2–7] × 1020 Pa s when using ICE-5G. Employing RSES much higher values of 2 × 1021 Pa s are obtained for the southern Baltic Sea. Further investigations should evaluate whether this ice-model version and/or the RSL data need revision. We confirm that the lower-mantle viscosity in Fennoscandia can only be poorly resolved. The lithospheric structure inferred from RSES partly supports structural features of regional and global lithosphere models based on thermal or seismological data. While there is agreement in eastern Europe and southwest Sweden, the structure in an area from south of Norway to northern Germany shows large discrepancies for two of the tested lithosphere models. The lithospheric thickness as determined with ICE-5G does not agree with the lithosphere models. Hence, more investigations have to be undertaken to sufficiently determine structures such as the Ringkøbing-Fyn High as seen with seismics with the help of glacial isostatic adjustment modelling.

scholarly journals Estimation of present-day glacial isostatic adjustment, ice mass change and elastic vertical crustal deformation over the Antarctic ice sheet

2017 ◽  
Vol 63 (240) ◽  
pp. 703-715 ◽  
Author(s):  
BAOJUN ZHANG ◽  
ZEMIN WANG ◽  
FEI LI ◽  
JIACHUN AN ◽  
YUANDE YANG ◽  
...  
Keyword(s):  
Antarctic Peninsula ◽  
Crustal Deformation ◽  
Ice Sheet ◽  
East Antarctica ◽  
Mass Change ◽  
Glacial Isostatic Adjustment ◽  
Antarctic Ice Sheet ◽  
Isostatic Adjustment ◽  
The Antarctic ◽  
Vertical Crustal Deformation

ABSTRACTThis study explores an iterative method for simultaneously estimating the present-day glacial isostatic adjustment (GIA), ice mass change and elastic vertical crustal deformation of the Antarctic ice sheet (AIS) for the period October 2003–October 2009. The estimations are derived by combining mass measurements of the GRACE mission and surface height observations of the ICESat mission under the constraint of GPS vertical crustal deformation rates in the spatial domain. The influence of active subglacial lakes on GIA estimates are mitigated for the first time through additional processing of ICESat data. The inferred GIA shows that the strongest uplift is found in the Amundsen Sea Embayment (ASE) sector and subsidence mostly occurs in Adelie Terre and the East Antarctica inland. The total GIA-related mass change estimates for the entire AIS, West Antarctica Ice Sheet (WAIS), East Antarctica Ice Sheet (EAIS), and Antarctic Peninsula Ice Sheet (APIS) are 43 ± 38, 53 ± 24, −23 ± 29 and 13 ± 6 Gt a−1, respectively. The overall ice mass change of the AIS is −46 ± 43 Gt a−1 (WAIS: −104 ± 25, EAIS: 77 ± 35, APIS: −20 ± 6). The most significant ice mass loss and most significant elastic vertical crustal deformations are concentrated in the ASE and northern Antarctic Peninsula.

scholarly journals GRACE constraints on Earth rheology of the Barents Sea and Fennoscandia

Solid Earth ◽  
2020 ◽  
Vol 11 (2) ◽  
pp. 379-395
Author(s):  
Marc Rovira-Navarro ◽  
Wouter van der Wal ◽  
Valentina R. Barletta ◽  
Bart C. Root ◽  
Louise Sandberg Sørensen
Keyword(s):  
Solid Earth ◽  
Upper Mantle ◽  
Barents Sea ◽  
Ice Sheet ◽  
Last Glacial ◽  
Lithospheric Thickness ◽  
Mantle Viscosity ◽  
The Barents Sea ◽  
Grace Data ◽  
The Last Glacial

Abstract. The Barents Sea is situated on a continental margin and was home to a large ice sheet at the Last Glacial Maximum. Studying the solid Earth response to the removal of this ice sheet (glacial isostatic adjustment; GIA) can give insight into the subsurface rheology of this region. However, because the region is currently covered by ocean, uplift measurements from the center of the former ice sheet are not available. The Gravity Recovery and Climate Experiment (GRACE) gravity data have been shown to be able to constrain GIA. Here we analyze GRACE data for the period 2003–2015 in the Barents Sea and use the data to constrain GIA models for the region. We study the effect of uncertainty in non-tidal ocean mass models that are used to correct GRACE data and find that it should be taken into account when studying solid Earth signals in oceanic areas from GRACE. We compare GRACE-derived gravity disturbance rates with GIA model predictions for different ice deglaciation chronologies of the last glacial cycle and find that best-fitting models have an upper mantle viscosity equal or higher than 3×1020 Pa s. Following a similar procedure for Fennoscandia we find that the preferred upper mantle viscosity there is a factor 2 larger than in the Barents Sea for a range of lithospheric thickness values. This factor is shown to be consistent with the ratio of viscosities derived for both regions from global seismic models. The viscosity difference can serve as constraint for geodynamic models of the area.

scholarly journals Reduced ice mass loss and three-dimensional viscoelastic deformation in northern Antarctic Peninsula inferred from GPS

2020 ◽  
Vol 222 (2) ◽  
pp. 1013-1022 ◽  
Author(s):  
Nahidul Hoque Samrat ◽  
Matt A King ◽  
Christopher Watson ◽  
Andrew Hooper ◽  
Xianyao Chen ◽  
...  
Keyword(s):  
Mass Loss ◽  
Antarctic Peninsula ◽  
Upper Mantle ◽  
Three Dimensional ◽  
Plate Motion ◽  
Viscoelastic Deformation ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Wide Range ◽  
Uplift Rates

SUMMARY We consider the viscoelastic rheology of the solid Earth under the Antarctic Peninsula due to ice mass loss that commenced after the breakup of the Larsen-B ice shelf. We extend the previous analysis of nearby continuous GPS time-series to include five additional years and the additional consideration of the horizontal components of deformation. They show strong uplift from ∼2002 to 2011 followed by reduced uplift rates to 2018. Modelling the GPS-derived uplift as a viscoelastic response to ongoing regional ice unloading from a new ice model confirms earlier estimates of low upper-mantle viscosities of ∼0.3–3 × 1018 Pa s in this region but allows a wide range of elastic lithosphere thickness. The observed and modelled north coordinate component shows little nonlinear variation due to the location of ice mass change to the east of the GPS sites. However, comparison of the observed and modelled east coordinate component constrains the upper-mantle viscosity to be less than ∼9 × 1018 Pa s, consistent with the viscosity range suggested by the uplift rates alone and providing important, largely independent, confirmation of that result. Our horizontal analysis showed only marginal sensitivity to modelled lithospheric thickness. The results for the horizontal components are sensitive to the adopted plate rotation model, with the estimate based on ITRF2014 suggesting that the sum of residual plate motion and pre-2002 glacial isostatic adjustment is likely less than ∼±0.5 mm yr−1 in the east component.

scholarly journals Lithosphere and upper-mantle structure of the southern Baltic Sea estimated from modelling relative sea-level data with glacial isostatic adjustment

2013 ◽  
Vol 5 (2) ◽  
pp. 2483-2507
Author(s):  
H. Steffen ◽  
G. Kaufmann ◽  
R. Lampe
Keyword(s):  
Baltic Sea ◽  
Sea Level ◽  
Upper Mantle ◽  
Glacial Isostatic Adjustment ◽  
Relative Sea Level ◽  
Southern Baltic Sea ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Mantle Viscosity ◽  
Ice Model

Abstract. During the last glacial maximum, a large ice sheet covered Scandinavia, and the Earth's surface was depressed by several 100 m. Beyond the limit of this Fennoscandian ice sheet, mass redistribution in the upper mantle led to the development of peripheral bulges around the glaciated region. These once uplifted areas subside since the begin of deglaciation due to the viscoelastic behavior of the mantle. Parts of this subsiding region are located in northern central Europe in the coastal parts of Denmark, Germany and Poland. We analyze relative sea-level (RSL) data of these regions to determine the lithospheric thickness and radial mantle viscosity structure for distinct regional RSL subsets. We load a one-dimensional Maxwell-viscoelastic earth model with a global ice-load history model of the last glaciation. We test two commonly used ice histories, RSES from the Australian National University and Ice-5G from the University of Toronto. Our results indicate that the lithospheric thickness varies, depending on the ice model used, between 60 and 160 km. The lowest values are found in the Oslo Graben area and the western German Baltic Sea coast. In between, thickness increases by at least 30 km tracing the Fyn High. In Poland, lithospheric thickness values up to 160 km are reached. However, the latter values are not well constrained due to a low number of RSL data from the Polish area. Upper-mantle viscosity is found to bracket [2–7] × 1020 Pa s when using Ice-5G. Employing RSES much higher values of 2 × 1021 Pa s yield for the southern Baltic Sea, which suggests a revision of this ice-model version. We confirm that the lower-mantle viscosity in Fennoscandia can only be poorly resolved. The lithospheric structure inferred partly supports structural features of regional and global lithosphere models based on thermal or seismological data. While there is agreement in eastern Europe and southwest Sweden, the structure in an area from south of Norway to northern Germany shows large discrepancies for two of the tested models. It thus remains challenging to sufficiently determine the Fyn High as seen with seismics with the help of glacial isostatic adjustment modelling.

scholarly journals Brief Communication: Upper air relaxation in RACMO2 significantly improves modelled interannual SMB variability in Antarctica

2015 ◽  
Vol 9 (5) ◽  
pp. 4981-4995
Author(s):  
W. J. van de Berg ◽  
B. Medley
Keyword(s):  
Regional Climate Model ◽  
Interannual Variability ◽  
Antarctic Peninsula ◽  
Climate Model ◽  
Regional Climate ◽  
Ice Sheet ◽  
West Antarctica ◽  
Steep Topography ◽  
The Antarctic

Abstract. The regional climate model RACMO2 has been a powerful tool for improving SMB estimates from GCMs or reanalyses. However, new yearly SMB observations for West Antarctica show that the modelled interannual variability in SMB is poorly simulated by RACMO2, in contrast to ERA-Interim, which resolves this variability well. In an attempt to remedy RACMO2 performance, we included additional upper air relaxation (UAR) in RACMO2. With UAR, the correlation to observations is similar for RACMO2 and ERA-Interim. The spatial SMB patterns and ice sheet integrated SMB modelled using UAR remain very similar to the estimates of RACMO2 without UAR. We only observe an upstream smoothing of precipitation in regions with very steep topography like the Antarctic Peninsula. We conclude that UAR is a useful improvement for RCM simulations, although results in regions with steep topography should be treated with care.

Antarctic glacial isostatic adjustment: a new assessment

2005 ◽  
Vol 17 (4) ◽  
pp. 541-553 ◽  
Author(s):  
ERIK R. IVINS ◽  
THOMAS S. JAMES
Keyword(s):  
Mass Balance ◽  
Antarctic Peninsula ◽  
Ross Sea ◽  
Ice Sheet ◽  
Gravity Change ◽  
Glacial Isostatic Adjustment ◽  
Crustal Motion ◽  
Isostatic Adjustment ◽  
Time Space ◽  
The Antarctic

The prediction of crustal motions and gravity change driven by glacial isostatic adjustment (GIA) in Antarctica is critically dependent on the reconstruction of the configuration and thickness of the ice sheet during the Late Pleistocene and Holocene. The collection and analysis of field data to improve the reconstruction has occurred at an accelerated pace during the past decade. At the same time, space-based imaging and altimetry, combined with on-ice velocity measurements using Global Positioning System (GPS) geodesy, has provided better assessments of the present-day mass balance of the Antarctic ice sheet. Present-day mass change appears to be dominated by deglaciation that is, in large part, a continuation of late-Holocene evolution. Here a new ice load model is constructed, based on a synthesis of the current constraints on past ice history and present-day mass balance. The load is used to predict GIA crustal motion and geoid change. Compared to existing glacioisostatic models, the new ice history model is significantly improved in four aspects: (i) the timing of volume losses in the region ranging from the Ross Sea sector to the Antarctic Peninsula, (ii) the maximum ice heights in parts of the Ellsworth and Transantarctic Mountains, (iii) maximum grounding line position in Pine Island Bay, the Antarctic Peninsula, and in the Ross Sea, (iv) incorporation of present-day net mass balance estimates. The predicted present-day GIA uplift rates peak at 14–18 mm yr−1 and geoid rates peak at 4–5 mm yr−1 for two contrasting viscosity models. If the asthenosphere underlying West Antarctica has a low viscosity then the predictions could change substantially due to the extreme sensitivity to recent (past two millennia) ice mass variability. Future observations of crustal motion and gravity change will substantially improve the understanding of sub-Antarctic lithospheric and mantle rheology.

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