Radial Resolution in the Inference of Mantle Viscosity from Observations of Glacial Isostatic Adjustment

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.

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Parameters controlling mid-Holocene highstand in Glacial Isostatic Adjustment modelling

10.5194/egusphere-egu21-4224 ◽

2021 ◽

Author(s):

Tanghua Li ◽

Stephen Chua ◽

Nicole Khan ◽

Patrick Wu ◽

Benjamin Horton

Keyword(s):

Southeast Asia ◽

Sea Level ◽

Late Holocene ◽

Ice Sheets ◽

Thickness Variation ◽

Glacial Isostatic Adjustment ◽

Lithospheric Thickness ◽

Isostatic Adjustment ◽

Mantle Viscosity ◽

Open Questions

Holocene relative sea-level (RSL) records from regions distal from ice sheets (far-field) are commonly characterized by a mid-Holocene highstand, when RSL reached higher than present levels. The magnitude and timing of the mid-Holocene highstand varies spatially due to hydro-isostatic processes including ocean syphoning and continental levering. While there are open questions regarding the timing, magnitude and source of ice-equivalent sea level in the middle to late Holocene.Here, we compare Glacial Isostatic Adjustment (GIA) model predictions to a standardized database of sea-level index points (SLIPs) from Southeast Asia where we have near-complete Holocene records. The database has more than 130 SLIPs that span the time period from ~9.5 ka BP to present. We investigate the sensitivity of mid-Holocene RSL predictions to GIA parameters, including the lateral lithospheric thickness variation, mantle viscosity (both 1D and 3D), and deglaciation history from different ice sheets (e.g., Laurentide, Fennoscandia, Antarctica).We compute gravitationally self-consistent RSL histories for the GIA model with time dependent coastlines and rotational feedback using the Coupled Laplace-Finite Element Method. The preliminary results show that the timing of the highstand is mainly controlled by the deglaciation history (ice-equivalent sea level), while the magnitude is dominated by Earth parameters (e.g., lithospheric thickness, mantle viscosity). We further investigate whether there is meltwater input during middle to late Holocene and whether the RSL records from Southeast Asia can reveal the meltwater source, like Antarctica.

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Glacial isostatic adjustment and mantle viscosity

Journal of Geophysical Research Atmospheres ◽

10.1029/jb093ib06p06397 ◽

1988 ◽

Vol 93 (B6) ◽

pp. 6397 ◽

Cited By ~ 16

Author(s):

Charles B. Officer ◽

Walter S. Newman ◽

John M. Sullivan ◽

Daniel R. Lynch

Keyword(s):

Glacial Isostatic Adjustment ◽

Isostatic Adjustment ◽

Mantle Viscosity

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Glacial isostatic adjustment, relative sea level history and mantle viscosity: reconciling relative sea level model predictions for the U.S. East coast with geological constraints

Geophysical Journal International ◽

10.1093/gji/ggv066 ◽

2015 ◽

Vol 201 (2) ◽

pp. 1156-1181 ◽

Cited By ~ 40

Author(s):

Keven Roy ◽

W.R. Peltier

Keyword(s):

Sea Level ◽

East Coast ◽

Glacial Isostatic Adjustment ◽

Relative Sea Level ◽

Isostatic Adjustment ◽

Mantle Viscosity ◽

Model Predictions ◽

Level Model ◽

Geological Constraints ◽

The U.S

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3D glacial-isostatic adjustment models using geodynamically constrained Earth structures

10.5194/egusphere-egu21-13479 ◽

2021 ◽

Author(s):

Meike Bagge ◽

Volker Klemann ◽

Bernhard Steinberger ◽

Milena Latinović ◽

Maik Thomas

Keyword(s):

Solid Earth ◽

Sea Level ◽

Ice Sheet ◽

Sea Level Change ◽

Glacial Isostatic Adjustment ◽

Level Change ◽

Isostatic Adjustment ◽

Mantle Viscosity ◽

The Earth ◽

Earth Structures

The interaction between ice sheets and the solid Earth plays an important role for ice-sheet stability and sea-level change and hence for global climate models. Glacial-isostatic adjustment (GIA) models enable simulation of the solid Earth response due to variations in ice-sheet and ocean loading and prediction of the relative sea-level change. Because the viscoelastic response of the solid Earth depends on both ice-sheet distribution and the Earth&#8217;s rheology, independent constraints for the Earth structure in GIA models are beneficial. Seismic tomography models facilitate insights into the Earth&#8217;s interior, revealing lateral variability of the mantle viscosity that allows studying its relevance in GIA modeling. Especially, in regions of low mantle viscosity, the predicted surface deformations generated with such 3D GIA models differ considerably from those generated by traditional GIA models with radially symmetric structures. But also, the conversion from seismic velocity variations to viscosity is affected by a set of uncertainties. Here, we apply geodynamically constrained 3D Earth structures. We analyze the impact of conversion parameters (reduction factor in Arrhenius law and radial viscosity profile) on relative sea-level predictions. Furthermore, we focus on exemplary low-viscosity regions like the Cascadian subduction zone and southern Patagonia, which coincide with significant ice-mass changes.

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Using magnetotelluric and seismic geophysical observations to infer viscosity for Glacial Isostatic Adjustment calculations

10.5194/egusphere-egu2020-9826 ◽

2020 ◽

Author(s):

Florence Ramirez ◽

Kate Selway ◽

Clinton Conrad

Keyword(s):

Grain Size ◽

Strain Rate ◽

Water Content ◽

Seismic Attenuation ◽

Conversion Process ◽

Glacial Isostatic Adjustment ◽

Partial Melt ◽

Isostatic Adjustment ◽

Mantle Viscosity ◽

The Impact

A physical property that is important for understanding the geodynamics of Earth&#8217;s lithosphere and asthenosphere is the effective viscosity &#951;eff (the ratio of stress and strain rate). This is particularly important for accurate Glacial Isostatic Adjustment (GIA) calculations, which are increasingly crucial for estimating ice loss and sea level rise from the Greenland and Antarctic ice sheets. Mantle viscosity cannot be measured directly, but can be inferred from strain rate, for example as observed by ground uplift following deglaciation or a seismic event. Empirically, mantle strain rate is mainly controlled by stress, temperature, grain size, and composition (water content and partial melt). The influence of these controlling parameters can be inferred from geophysical observations such as seismic and magnetotelluric (MT) measurements, which are useful for imaging the subsurface of the Earth but do not directly constrain viscosity. These observations can be used to improve constraints on viscosity using a three-step conversion process: (1) constrain temperature from MT, seismic, and other data; (2) constrain compositional structure from MT and seismic data (water content of nominally anhydrous minerals from MT, partial melt content from MT and seismics); and finally, (3) convert the calculated thermal and compositional structures into a constrained viscosity structure. In each conversion process, we can assess and quantify the involved uncertainties. Furthermore, we determine the dominant deformation regime in order to accurately interpret the sensitivity of viscosity to its controlling parameters. For instance, water content strongly affects viscosity for the dislocation-accommodated grain-boundary sliding (dis-GBS) and dislocation creep regimes, while diffusion creep and dis-GBS are highly sensitive to grain size. Stress and grain size are important parameters for determining where these critical transitions may occur. Although neither MT nor seismic velocity observations place strong constraints on grain size, information about seismic attenuation or tectonic history can potentially provide information about grain&#8211;size. Overall, we find that seismic and MT observations together can significantly improve estimates of mantle viscosity, and in particular can place useful constraints on the amplitude of regional variations in mantle viscosity. Such constraints will be particularly useful for studies to estimate the impact of such variations on GIA processes.

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

Solid Earth Discussions ◽

10.5194/sed-5-2483-2013 ◽

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.

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Inferring mantle viscosity through data assimilation of relative sea level observations in a glacial isostatic adjustment model

10.5194/egusphere-egu21-1261 ◽

2021 ◽

Author(s):

Reyko Schachtschneider ◽

Jan Saynisch-Wagner ◽

Volker Klemann ◽

Meike Bagge ◽

Maik Thomas

Keyword(s):

Data Assimilation ◽

Sea Level ◽

Glacial Isostatic Adjustment ◽

Layer Model ◽

Relative Sea Level ◽

Isostatic Adjustment ◽

Mantle Viscosity ◽

Level Data ◽

The Time Domain ◽

Spectral Finite Element

We suggest to apply data assimilation in glacial isostatic adjustment (GIA) to constrain the mantle viscosity structure based on sea level observations. We apply the Parallel Data Assimilation Framework (PDAF) to assimilate sea level data into the time-domain spectral-finite element code VILMA in order to obtain better estimates of the mantle viscosity structure. In a first step, we reduce to a spherically symmetric earth structure and prescribe the glaciation history. A particle filter is used to propagate an ensemble of models in time. At epochs when observations are available, each particle's performance is estimated and the particles are resampled based on their performance to form a new ensemble that better resembles the true viscosity distribution.Using this algorithm, we show the ability to recover mantle viscosities from a set of synthetic relative sea level observations. Those synthetic observations are obtained from a reference run with a given viscosity structure that defines the target viscosity values in our experiments. The viscosity estimation is applied to a three-layer model with an elastic lithosphere and two mantle layers, and to a multi-layer model with a smoother viscosity profile. We use various subsets of realistic observation locations (e.g. only observations from Fennoscandia) and show that it is possible to obtain the target viscosity values in those cases. We also vary the time from which observations are available to evolve the test cases towards a realistic scenario for the availability of relative sea level observations. The most relevant cases start at 26.5ka BP and at 10ka BP as they mark the beginning of the maximum glaciation and the end of deglaciation with a larger amount of observations following, respectively, and end at present day.

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Optimal locations of sea-level indicators in glacial isostatic adjustment investigations

Solid Earth ◽

10.5194/se-5-511-2014 ◽

2014 ◽

Vol 5 (1) ◽

pp. 511-521 ◽

Cited By ~ 7

Author(s):

H. Steffen ◽

P. Wu ◽

H. Wang

Keyword(s):

Sea Level ◽

Glacial Isostatic Adjustment ◽

Load History ◽

Lithospheric Thickness ◽

Ice Load ◽

Isostatic Adjustment ◽

Mantle Viscosity ◽

Almost Everywhere ◽

Time Period ◽

Lateral Heterogeneities

Abstract. Fréchet (sensitivity) kernels are an important tool in glacial isostatic adjustment (GIA) investigations to understand lithospheric thickness, mantle viscosity and ice-load model variations. These parameters influence the interpretation of geologic, geophysical and geodetic data, which contribute to our understanding of global change. We discuss global sensitivities of relative sea-level (RSL) data of the last 18 000 years. This also includes indicative RSL-like data (e.g., lake levels) on the continents far off the coasts. We present detailed sensitivity maps for four parameters important in GIA investigations (ice-load history, lithospheric thickness, background viscosity, lateral viscosity variations) for up to nine dedicated times. Assuming an accuracy of 2 m of RSL data of all ages (based on analysis of currently available data), we highlight areas around the world where, if the environmental conditions allowed its deposition and survival until today, RSL data of at least this accuracy may help to quantify the GIA modeling parameters above. The sensitivity to ice-load history variations is the dominating pattern covering almost the whole world before about 13 ka (calendar years before 1950). The other three parameters show distinct patterns, but are almost everywhere overlapped by the ice-load history pattern. The more recent the data are, the smaller the area of possible RSL locations that could provide enough information to a parameter. Such an area is mainly limited to the area of former glaciation, but we also note that when the accuracy of RSL data can be improved, e.g., from 2 m to 1 m, these areas become larger, allowing better inference of background viscosity and lateral heterogeneity. Although the patterns depend on the chosen models and error limit, our results are indicative enough to outline areas where one should look for helpful RSL data of a certain time period. Our results also indicate that as long as the ice-load history is not sufficiently known, the inference of lateral heterogeneities in mantle viscosity or lithospheric thickness will be interfered by the uncertainty of the ice model.

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