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sidd

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Re: Antarctic Tectonics
« Reply #50 on: February 05, 2014, 05:45:54 AM »
i have issues with the Adhikari paper

1)i believe the rate of melt ice unloading will be order of magnitude faster than viscoelastic crust response

2)they ignore basal till erosion and transition to temperate beds "although quite rapid evacuation of soft sediments is now occurring at the bed of  Pine Island Glacier. "

3)treat basal hydrology changes amplifying sliding in optimistic manner, basal viscosity  and transition to temperate beds are underestimated, surfac melt is not the only factor amplifying sliding

sidd

AbruptSLR

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Re: Antarctic Tectonics
« Reply #51 on: February 05, 2014, 04:14:24 PM »
sidd,

I always appreciate your critiques of the various scientific references cited here.  I think that it is important for readers of this Antarctic folder realize that current models are both half full, and half empty, at the same time.  I believe that the Adhikari et al paper offers some new insight to me (the half full part) about the lower bound of future glacial isostatic rebound in the Pine Island Bay (at least 45 mm/year); however, in addition to the issues that you cite in your post I can add (the half empty part):

1) Their model does not fully account for the coming end of the negative phase of the PDO, nor do they consider the influence of the AMO on the ABSL, and thus any ocean coupling that they consider is too low.
2) The cracking/calving of the PIIS, the Thwaites Ice Tongue and the Thwaites Ice Shelf are all accelerating, thus the buttressing action in their model is too high.
3) Their model does not consider synergy between the PIG and the Thwaites Glacier, either through ocean advection, through the SW Tributary glacier, or through changes in the basin boundary.
4) Their model does not include ice mass loss through drainage of basal melt water.
5) Their modeled rate of ice mass loss does not include the instability calving effect illustrated by the Jakobshavn Effect, that has causes ice mass loss from Jakoshavn to increase several times in a few years; and when both the PIG and the Thwaites Glacier reach similar instability conditions, their rate of ice mass loss will likely accelerate several orders of magnitude faster that the GIA rebound, making any change in the negative bed slope irrelevant.
6) Their forcing functions assume old climate sensitivities that have recently been proven to be too low, raising the prospect of future surface melting, which has been shown mathematically to accelerate the Jakobshavn Effect type accelerated ice face calving.
7) I also believe that there is synergy between ocean water advection and basal water drainage into the ocean that their model does not consider.

But I have made all of these points before, and while I appreciate the insight from these new model results, I realize that it will be several decades before they make reasonable projections of future ice mass loss, and in the meantime we should all expect those projections to steadily increase, year after year.
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #52 on: March 02, 2014, 11:04:41 PM »
The linked research helps to provide a more complete picture of glacio-isostatic rebound in Antarctica:

White, D. A., and D. Fink (2014), Late Quaternary glacial history constrains glacio-isostatic rebound in Enderby Land, East Antarctica, J. Geophys. Res. Earth Surf., 119, doi:10.1002/2013JF002870.

http://onlinelibrary.wiley.com/doi/10.1002/2013JF002870/abstract

Abstract: "Measurements of the loss or gain of ice mass from large ice sheets are presently achieved through satellite-based techniques such as GRACE (Gravity Recovery and Climate Experiment). The accuracy of these satellite-based measurements to changes in modern ice sheet mass depends on our knowledge of present-day glacio-isostatic crustal uplift rates caused by past ice sheet changes. To improve models of glacio-isostatic rebound in East Antarctica, we investigated ice histories along Rayner Glacier, Enderby Land, and a little explored sector of the ice sheet where GRACE data had suggested significant mass gain during the last decade. Observations from a recent glacial geomorphic reconnaissance coupled with cosmogenic nuclide dating indicate that in the lower part of the Rayner Glacier, Enderby Land, ice heights lowered by at least 300 m and the calving margin retreated by at least 10 km in the early Holocene (~6 to 9 ka B.P.). The magnitude and timing of deglaciation are consistent with ice histories used to model the postglacial rebound corrections for present-day GRACE mass trends. These observations strengthen the body of evidence that suggests ice mass gain in Enderby Land is presently partly offsetting mass loss in other parts of Antarctica."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #53 on: April 24, 2014, 01:13:10 AM »
The following link leads to an interesting article about on-going research on Mount Erebus:

http://www.mnn.com/earth-matters/wilderness-resources/stories/antarcticas-sleeping-dragon-lava-lake-steams-amid-coldest
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Re: Antarctic Tectonics
« Reply #54 on: May 12, 2014, 04:23:22 PM »
While the following linked reference applies to the rapid bedrock uplift due to viscoelastic rebound associated with ice mass unloading in the Northern Antarctic Peninsula, the fact that this rapid uplift is primarily due to unexpected low upper-mantle viscosity, is potentially very bad news for the WAIS as it continues to lose ice mass.  Furthermore, it is possible that this low upper-mantle viscosity may be the reason that the GIA corrections of the GRACE date in the ASE glaciers has been difficult to reconcile with the rapid uplift in that area as well:

Grace A. Nield, Valentina R. Barletta, Andrea Bordoni, Matt A. King, Pippa L. Whitehouse, Peter J. Clarke, Eugene Domack, Ted A. Scambos, Etienne Berthier , (2014), “Rapid bedrock uplift in the Antarctic Peninsula explained by viscoelastic response to recent ice unloading”, Earth and Planetary Science Letters,  Vol 397, DOI: 10.1016/j.epsl.2014.04.0191 July, 2014. http://dx.doi.org/10.1016/j.epsl.2014.04.019 published online on 12th May 2014

http://www.sciencedirect.com/science/article/pii/S0012821X14002519

Abstract: "Since 1995 several ice shelves in the Northern Antarctic Peninsula have collapsed and triggered ice-mass unloading, invoking a solid Earth response that has been recorded at continuous GPS (cGPS) stations. A previous attempt to model the observation of rapid uplift following the 2002 breakup of Larsen B Ice Shelf was limited by incomplete knowledge of the pattern of ice unloading and possibly the assumption of an elastic-only mechanism. We make use of a new high resolution dataset of ice elevation change that captures ice-mass loss north of 66°S to first show that non-linear uplift of the Palmer cGPS station since 2002 cannot be explained by elastic deformation alone. We apply a viscoelastic model with linear Maxwell rheology to predict uplift since 1995 and test the fit to the Palmer cGPS time series, finding a well constrained upper mantle viscosity but less sensitivity to lithospheric thickness. We further constrain the best fitting Earth model by including six cGPS stations deployed after 2009 (the LARISSA network), with vertical velocities in the range 1.7 to 14.9 mm/yr. This results in a best fitting Earth model with lithospheric thickness of 100–140 km and upper mantle viscosity of 6 x 1017 – 2 x 1018 – much lower than previously suggested for this region. Combining the LARISSA time series with the Palmer cGPS time series offers a rare opportunity to study the time-evolution of the low-viscosity solid Earth response to a well-captured ice unloading event."
« Last Edit: May 12, 2014, 06:46:54 PM by AbruptSLR »
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Re: Antarctic Tectonics
« Reply #55 on: May 12, 2014, 07:51:47 PM »
The following quote from the linked article at the Reporting Climate Science website, indicates that the low viscosity of the upper mantle documented in the Northern Antarctic Peninsula (see my immediate prior post) is due to subtle changes in temperature, or chemical composition of the upper mantle.  This implies that older models for projecting Glacial Isostatic Adjustment, GIA, factors that do not include the influence of this subtle changes in temperature, or chemical composition, should not be relied upon for estimates of SLR based on satellite measurements (note that assuming greater rebound in the ASE (due to more magma flow) in interpreting GRACE measurements in the ASE would result in greater SLR projections):

http://www.reportingclimatescience.com/news-stories/article/antarctic-ice-loss-moves-the-earth-below.html

Quote: "And they have shown for the first time how the mantle below the Earth’s crust in the Antarctic Peninsula is flowing much faster than expected, probably due to subtle changes in temperature or chemical composition.  This means it can flow more easily and so responds much more quickly to the lightening load hundreds of miles above it, changing the shape of the land."

The pdf of the PPT at the following link provides an idea of the prior state of the art on this topic in 2013, which indicates that it may not be reasonable to rely upon the projections of the old models for rebound:

ftp://sidads.colorado.edu/pub/projects/waisworkshop/2013/presentations/session3/Wiens.pdf
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sidd

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Re: Antarctic Tectonics
« Reply #56 on: May 13, 2014, 12:27:40 AM »
It is not clear to me how the SLR results will be affected by this new result on a less vicsous mantle in the North Antarctic Peninsula. I prefer to await the recalculation.

sidd

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Re: Antarctic Tectonics
« Reply #57 on: May 13, 2014, 12:45:57 AM »
sidd,

I agree that it is a good idea to await the recalculation of the current SLR projections.

Nevertheless, my point is that existing SLR projection based on GRACE measurement of mass loss in the ASE area are likely too low because some of the estimated ice mass loss is being replaced by mass from less viscous upper mantle material squeezing back under the glaciers from beneath the adjoining seafloor.  Once the correct amount of rebound is determined (there is currently a GPS program measuring the rebound), it is possible (probable in my opinion) that we will find that the ice mass loss from the ASE is accelerating faster that we previously thought (based on GRACE measurements).

Best,
ASLR
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wili

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Re: Antarctic Tectonics
« Reply #58 on: May 13, 2014, 01:04:41 AM »
As water leaves the polar ice sheets, doesn't much of it move toward the equator. Could that added weight kind of squeeze the whole earth, possibly making it more likely that we will uplift and eruptions near the poles?

Just a wild idea.
« Last Edit: May 13, 2014, 01:43:51 AM by wili »
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Re: Antarctic Tectonics
« Reply #59 on: May 13, 2014, 02:06:50 AM »
wili,

Certainly the redistribution of was mass associated with RSLR will (progressively) be sufficient to accelerate seismic activity in many parts of the world, including near the AIS and the GrIS.  It is already sufficient to change the axis of rotation of the earth.

Best,
ASLR
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sidd

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Re: Antarctic Tectonics
« Reply #60 on: May 13, 2014, 05:56:15 AM »
Very briefly: GRACE data

1)is reported as (global) spectral spherical harmonic coefficients, we play with these to adjust for local crustal rebound

2)another approach is to use known masscons (including ice mass loss) and move them around to reproduce GRACE derived gravitational changes, This gets tricky very quickly (stiff DEs) but can be pirouetted around using variational technique with appropriate cost function

both of these approaches will give different SLR fingerprint, a la Mitrovica et seq.

so when you put in a less viscous mantle in north antarctic peninsula, this will cause different response   in global spherical harmonic fits or  masscon change. Is not clear to me that a local change in mantle viscocity in north antarctic peninsula will result in larger estimate of ice mass waste over all of antarctica, for all i know it might give change estimate of subterranean aquifer withdrawal in Ogallala or north india ...

I will await the peer reviwed literature.

sidd

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Re: Antarctic Tectonics
« Reply #61 on: May 13, 2014, 04:47:15 PM »
sidd,

I concur that the topic of the influence of ice mass loss on tectonic response is a complex, and rather confusing, topic; and in that sense it is a very good idea to wait for peer reviewed research to clarify these consequences.

And to address one possible area of confusion, I am not implying that I believe that a reduction in upper mantle viscosity associated with ice mass loss in the Northern Antarctic Peninsula is going to have a major impact of the rest of the WAIS, let alone on the rest of the world (it will have some but not major).  However, I do mean to note that if the viscosity of the upper mantle in the Northern Antarctic Peninsula can be reduced due to changes in temperature and chemistry associated with ice mass loss about the upper mantle; then such a similar process may be beginning in the ASE, and that I believe that researchers should examine this possibility in their future peer reviewed publications.

Best,
ASLR
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #62 on: May 14, 2014, 12:54:18 AM »
The following linked research is about the upper mantle beneath Antarctica; however, it is preliminary in nature and focuses on East Antarctic; so we will need to wait until these researchers publish findings on the West Antarctic upper mantle temperature model:

Ward Stolk, Mikhail Kaban, Wouter van der Wal, and Doug Wiens, (2014), "An improved temperature model of the Antarctic uppermost mantle for the benefit of GIA modelling", Geophysical Research Abstracts, Vol. 16, EGU2014-12787-1, EGU General Assembly 2014


http://meetingorganizer.copernicus.org/EGU2014/EGU2014-12787-1.pdf


Abstract: "Mass changes in Antarctica’s ice cap influence the underlying lithosphere and upper mantle. The dynamics of the solid earth are in turn coupled back to the surface and ice dynamics. Furthermore, mass changes due to lithosphere and uppermost mantle dynamics pollute measurements of ice mass change in Antarctica. Thus an improved understanding of temperature, composition and rheology of the Antarctic lithosphere is required, not only to improve geodynamic modelling of the Antarctic continent (e.g. glacial isostatic adjustment (GIA) modelling), but also to improve climate monitoring and research.

Recent field studies in Antarctica have generated much new data. These data, especially an improved assessment of crustal thickness and seismic tomography of the upper mantle, now allow for the construction of an improved regional temperature model of the Antarctic uppermost mantle. Even a small improvement in the temperature models for the uppermost mantle could have a significant effect on GIA modelling in Antarctica. 

Our regional temperature model is based on a joint analysis of a high resolution seismic tomography model (Heeszel et al., forthcoming) and a recent global gravity model (Foerste et al., 2011). The model will be further constrained by additional local data where available. Based on an initial general mantle composition, the temperature and density in the uppermost mantle is modelled, elaborating on the the methodology of Goes et al. (2000) and Cammarano et al. (2003). The gravity signal of the constructed model is obtained using forward gravity modelling. This signal is compared with the observed gravity signal and differences form the basis for the compositional model in the next iteration. The first preliminary results of this study, presented here, will focus on the cratonic areas in East-Antarctica, for which modelling converges after a few iterations."

Cammarano, F. and Goes, S. and Vacher, P. and Giardini, D. (2003) Inferring upper-mantle temperatures from seismic velocities, Physics of the Earth and Planetary Interiors, 138, 197-222

Foerste et al. (2011) EIGEN-6 - A new combined global gravity field model including GOCE data from the collaboration of GFZ-Potsdam and GRGS-Toulouse. In: Geophysical Research Abstracts, volume 13, EGU2011-3242-2.

Goes, S. and Govers, R. and Vacher, P. (2000) Shallow mantle temperatures under Europe from P and S wave tomography, Journal of Gephysical Research, 105, B5, 11,153-11,169.
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #63 on: May 14, 2014, 01:20:14 AM »
The following linked reference, attached image and following quote, indicate the relatively unique nature of the upper mantle beneath the Antarctic.  Also one might want to review Reply #9 to see how the impact of magma flow in the uppermost mantle can influence GRACE satellite estimates of ice mass loss from the ASE:


http://www.luomus.fi/en/largest-lava-eruptions-earth


Figure Caption: " Schematic cross-section of the Karoo continental flood basalt province c. 180 million years ago. 1) Mantle melts extensively and the 2) melts intrude the lithosphere (=crust + brittle upper mantle), where they form large magma chambers and mix with it. 3) The contaminated melts proceed upwards and 4) erupt from shield volcanoes or fissures. 5) Some rare melts do not assimilate lithosphere and preserve the original mantle-derived geochemical signature. Image: Luomus / Jussi Heinonen"

Quote: "Our latest findings indicate that the enormous melt generation was caused by at least two processes: 1) Gondwana supercontinent functioned like a "lid on a cooking pot" and prevented the cooling of the sublithospheric mantle. High amount of accumulated heat caused more efficient melting of the mantle (Heinonen et al., 2010). 2) Some portions of the sublithospheric mantle were relatively Fe-rich and melted more efficiently than ambient mantle materials. Such portions were formed by mixing with ancient parts of oceanic crust that sank in to the mantle at subduction zones (Heinonen et al., 2013, 2014)."
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Re: Antarctic Tectonics
« Reply #64 on: May 20, 2014, 05:38:32 PM »
The linked article (copied below), adds new information to the risk that Antarctic (and elsewhere) ice mass loss could trigger new earthquakes and volcanic activity:

http://www.delhidailynews.com/news/Heightened-Antarctic-ice-loss-may-prompt-volcanic-activity--Study-1400594247/


Extract: "There is added reason to be worried about the melting Antarctic ice sheets as climate change is bringing about the deformation in the Earth's crust which poses threat of volcanic activity.  This, in turn would lead to a rise in the global sea-level, study suggests.

The researchers from the Newcastle University, UK with the help of the Global Positioning System (GPS) stations analysed the effect of the breakdown of the massive Larsen B ice sheet in the year 2002.
It also led them to understand how the Earth's mantle responded to the relatively unexpected loss of billions of tonnes of ice as glaciers accelerated.

Professor King said in a statement, "It's like the earth in 2002 was prodded by a stick, a very big stick, and we've been able to watch how it responded. We see the earth as being tremendously dynamic and always changing, responding to the forces."

He further commented, "It's one of the big unknowns: If something starts to happen with one of those volcanoes, our estimates of what sea levels might be like in the future may have a significant revision. Fire and ice generally don't go well together".
Such dynamism involves rocks that are hundreds of kilometers below the surface moving swiftly and could pose implications for volcanoes in the area.

In the words of Professor King, "It's a big 'if' - but if a volcano erupted from underneath the ice sheet, it would dramatically accelerate the ice melt and the flows into the oceans."

The latest Intergovernmental Panel on Climate Change report in the year 2013 projected that the global sea levels could rise between about 0.5 and 1 meter by the year 2100, depending on high rates of greenhouse gas emissions.

With a rapid breakdown of the Antarctic ice sheets, the western region of the continent, could witness much higher sea-level rises.

The new research, published in the Earth and Planetary Science Letters this month, may also impact regions with somewhat similar geology, such as Alaska.

Professor Matt King said in a statement, "The Alaskan glaciers are melting and the upper mantle is slightly runnier as well."

He further said that an earthquake of a greater intensity is expected in the region with the tectonic plates coming into contact."

See also:
http://www.smh.com.au/environment/fire-and-ice-melting-antarctic-poses-risk-of-volcanic-activity-study-shows-20140520-zrj06.html
« Last Edit: May 20, 2014, 09:08:53 PM by AbruptSLR »
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #65 on: May 29, 2014, 10:50:27 PM »
The following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm

It is particularly interesting that Wilson et al 2014 indicate that the magma beneath Marie Byrd Land has very low viscosity:

70A1149
The POLENET-ANET integrated GPS and seismology approach to understanding glacial isostatic adjustment and ice mass change in Antarctica

Terry WILSON, Michael BEVIS, Stephanie KONFAL, Richard ASTER, Julien CHAPUT, David HEESZEL, Douglas WIENS, Sridhar ANANDAKRISHNAN, Ian DALZIEL, Audrey HUERTA, Eric KENDRICK
Corresponding author: Terry Wilson
Corresponding author e-mail: wilson.43@osu.edu

Abstract: "The POLENET-ANET project is simultaneously resolving crustal motions, measured by GPS, and Earth structure and rheological properties, mapped by seismology. Measured vertical and horizontal crustal motion patterns are not explained by extant glacial isostatic adjustment (GIA) models. These models have ice histories dominated by ice loss following the Last Glacial Maximum (LGM) and rely on 1-D Earth models, with rheological properties varying only radially. Seismological results from POLENET-ANET are revealing significant complexity in lateral variation in Earth properties. For example, crustal thickness variations occur not only across the East-West Antarctic boundary, but also between crustal blocks within West Antarctica. Modeling of mantle viscosity based on shear wave velocities shows a sharp lateral gradient from high to low viscosity in the Ross Embayment, a much more gradual gradient in the Weddell Embayment, and very low viscosities below Marie Byrd Land and the Amundsen Sea Embayment (ASE). Remarkable vertical and horizontal bedrock crustal motion velocity magnitudes, directions and patterns correlate spatially, in many aspects, with Earth property variations mapped by seismology. Within the ASE, extremely high upward velocities are flanked by subsiding regions – neither predicted by GIA models. Given the thin crust and low mantle viscosity, it is likely that this is not an LGM signal, which would have already relaxed, and uplift due to the elastic response to modern ice mass change clearly is important. As in other regions where rapid GIA-induced uplift has been measured, the crustal velocities in the Amundsen Embayment may also record a viscoelastic response to ice loss on decadal–centennial timescales. Along the East-West Antarctic boundary in the Ross Embayment, GIA-induced horizontal crustal motions are toward rather than away from the principal ice load center, correlating spatially with the strong lateral gradient in mantle viscosity. In the Weddell Embayment region, where crustal thickness is intermediate between East and West Antarctica and mantle viscosity values are moderate, crustal motions show the best match with predictions of GIA models. It is clear that lateral variations in Earth properties fundamentally control the isostatic response to ice mass changes in Antarctica. Ongoing integrated seismic-GPS studies are critical to developing the next generation of GIA models."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #66 on: May 30, 2014, 12:37:33 AM »
The following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm

The Chunchum et al 2014 reference indicates the dramatic importance of refining the GIA model to use in different regions of Antarctic in order to determine correct ice mass balance measurements using GRACE observations.

70A0928
Antarctic ice sheet mass balance measured by GRACE gravity satellite and the uncertainties
Chunchun GAO, Yang LU, Chuandong ZHU
Corresponding author: Yang Lu
Corresponding author e-mail: luyang@whigg.ac.cn

Abstract: "The Gravity Recovery and Climate Experiment (GRACE) mission opened a new era in gravimetry for estimating the mass balance of the Antarctic ice sheet since 2002. Using Release 5.0 (RL05) GRACE monthly gravity fields for January 2003 through April 2013 from CSR (118 total), temporal and spatial variation of Antarctic ice sheet mass is recovered in two ways: the optimizing averaging kernel method and the two-step filter method. The results reveal that the mass of the ice sheet has decreased significantly for the past 10 years, the changes of –131±55, –97±48 and –43±35 Gt a–1 for three GIA models (GW13, IJ05, W12a), with an acceleration of –12±8 Gt a–2, and most of this mass loss came from the southeast Pacific sector of West Antarctica and the Antarctic Peninsula. In addition, we analyze the uncertainties in GRACE estimates of ice-sheet mass balance with emphasis, indicating that the largest sources of error in Antarctic ice-sheet mass balance are GIA correction. Comparison of the results from the two different methods shows that when the same time span and a consistent set of corrections are used, different GRACE post-processing methods produce consistent ice mass-balance estimates."
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Re: Antarctic Tectonics
« Reply #67 on: June 09, 2014, 05:45:50 PM »
The following linked reference cites evidence of low upper mantle velocities inland of the Amundsen Sea.  Such low-velocity zones indicate the presence of a significant degree of partial melting, and thus to potential for rapid rebound when ice mass is lost from the Byrd Subglacial Basin:

Natalie J. Accardo, Douglas A. Wiens, Stephen Hernandez, Richard C. Aster, Andrew Nyblade, Audrey Huerta, Sridhar Anandakrishnan, Terry Wilson, David S. Heeszel and Ian W. D. Dalziel, (2014), "Upper mantle seismic anisotropy beneath the West Antarctic Rift System and surrounding region from shear wave splitting analysis", Geophys. J. Int. (2014) doi: 10.1093/gji/ggu117

http://gji.oxfordjournals.org/content/early/2014/05/21/gji.ggu117.abstract

Abstract: "We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37 broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked to determine the best fitting splitting parameters for each station. Overall, fast anisotropic directions are oriented at large angles to the direction of Antarctic absolute plate motion in both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent reflects multiple mantle regimes. We suggest that the observed anisotropy along the central Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of the largest zones of extended continental crust on Earth, results from asthenospheric mantle strain associated with the final pulse of western WARS extension in the late Miocene. Strong and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred extension direction, a result unlike reports from the East African rift system and rifts within the Basin and Range, which show much greater variation. We contend that ductile shearing rather than magmatic intrusion may have been the controlling mechanism for accumulation and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but shows a consistent fast direction, possibly representative of anisotropy that has been ‘frozen-in’ to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea appear to be radial to the dome and may indicate radial horizontal mantle flow associated with an MBL plume head and low upper mantle velocities in this region, or alternatively to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern end of the WARS."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #68 on: June 09, 2014, 05:48:19 PM »
The linked reference cites both extensive rifting in the ASE, a possible branch of the West Antarctic Rift System in the Amundsen Sea Embayment:

Thomas Kalberg and Karsten Gohl, (2014), "The crustal structure and tectonic development of the continental margin of the Amundsen Sea Embayment, West Antarctica: implications from geophysical data", Geophys. J. Int., doi: 10.1093/gji/ggu118

http://gji.oxfordjournals.org/content/early/2014/05/20/gji.ggu118.abstract

Abstract: "The Amundsen Sea Embayment of West Antarctica represents a key component in the tectonic history of Antarctic–New Zealand continental breakup. The region played a major role in the plate-kinematic development of the southern Pacific from the inferred collision of the Hikurangi Plateau with the Gondwana subduction margin at approximately 110–100 Ma to the evolution of the West Antarctic Rift System. However, little is known about the crustal architecture and the tectonic processes creating the embayment. During two ‘RV Polarstern’ expeditions in 2006 and 2010 a large geophysical data set was collected consisting of seismic-refraction and reflection data, ship-borne gravity and helicopter-borne magnetic measurements. Two P-wave velocity–depth models based on forward traveltime modelling of nine ocean bottom hydrophone recordings provide an insight into the lithospheric structure beneath the Amundsen Sea Embayment. Seismic-reflection data image the sedimentary architecture and the top-of-basement. The seismic data provide constraints for 2-D gravity modelling, which supports and complements P-wave modelling. Our final model shows 10–14-km-thick stretched continental crust at the continental rise that thickens to as much as 28 km beneath the inner shelf. The homogenous crustal architecture of the continental rise, including horst and graben structures are interpreted as indicating that wide-mode rifting affected the entire region. We observe a high-velocity layer of variable thickness beneath the margin and related it, contrary to other ‘normal volcanic type margins’, to a proposed magma flow along the base of the crust from beneath eastern Marie Byrd Land—West Antarctica to the Marie Byrd Seamount province. Furthermore, we discuss the possibility of upper mantle serpentinization by seawater penetration at the Marie Byrd Seamount province. Hints of seaward-dipping reflectors indicate some degree of volcanism in the area after break-up. A set of gravity anomaly data indicate several phases of fully developed and failed rift systems, including a possible branch of the West Antarctic Rift System in the Amundsen Sea Embayment."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #69 on: August 11, 2014, 08:41:13 PM »
The linked reference (and associated attached image) provides the first evident that remote earthquakes (i.e. Chile) can trigger icequakes in Anatarctica:


Zhigang Peng, Jacob I. Walter, Richard C. Aster, Andrew Nyblade, Douglas A. Wiens & Sridhar Anandakrishnan, (2014), "Antarctic icequakes triggered by the 2010 Maule earthquake in Chile", Nature Geoscience, doi:10.1038/ngeo2212


http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2212.html


Abstract: "Seismic waves from distant, large earthquakes can almost instantaneously trigger shallow micro-earthquakes and deep tectonic tremor as they pass through Earth’s crust. Such remotely triggered seismic activity mostly occurs in tectonically active regions. Triggered seismicity is generally considered to reflect shear failure on critically stressed fault planes and is thought to be driven by dynamic stress perturbations from both Love and Rayleigh types of surface seismic wave. Here we analyse seismic data from Antarctica in the six hours leading up to and following the 2010 Mw 8.8 Maule earthquake in Chile. We identify many high-frequency seismic signals during the passage of the Rayleigh waves generated by the Maule earthquake, and interpret them as small icequakes triggered by the Rayleigh waves. The source locations of these triggered icequakes are difficult to determine owing to sparse seismic network coverage, but the triggered events generate surface waves, so are probably formed by near-surface sources. Our observations are consistent with tensile fracturing of near-surface ice or other brittle fracture events caused by changes in volumetric strain as the high-amplitude Rayleigh waves passed through. We conclude that cryospheric systems can be sensitive to large distant earthquakes."

Also see:

http://www.cbsnews.com/news/distant-earthquake-triggers-icequake-in-antarctica/
« Last Edit: August 11, 2014, 08:52:11 PM by AbruptSLR »
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Re: Antarctic Tectonics
« Reply #70 on: January 30, 2015, 05:32:14 PM »
The linked article discusses how as glaciers melt the reduction in pressure on the mantle can cause magma to form that contributes to an increase in volcanic activity.  As we are seeing direct evidence today of the increase in volcanic activity in Iceland due to the associated glacial ice melting (see quote below), you can image what will happen in to volcanic activity in Western Antarctica when (not if) the WAIS starts to collapse:

http://time.com/3687893/volcanoes-climate-change/

Quote: “As the glaciers melt, the pressure on the underlying rocks decreases,” Compton said in an e-mail to TIME. “Rocks at very high temperatures may stay in their solid phase if the pressure is high enough. As you reduce the pressure, you effectively lower the melting temperature.” The result is a softer, more molten subsurface, which increases the amount of eruptive material lying around and makes it easier for more deeply buried magma chambers to escape their confinement and blow the whole mess through the surface.
“High heat content at lower pressure creates an environment prone to melting these rising mantle rocks, which provides magma to the volcanic systems,”
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solartim27

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Re: Antarctic Tectonics
« Reply #71 on: February 10, 2015, 10:36:40 PM »
Just nice pictures, no new information
http://earthobservatory.nasa.gov/IOTD/view.php?id=85238
« Last Edit: February 11, 2015, 01:39:36 AM by solartim27 »
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Re: Antarctic Tectonics
« Reply #72 on: February 11, 2015, 12:27:50 AM »
Just nice pictures, no new information

I note that not only is Mount Sidley the highest volcano in Antarctica, it is also "...  the youngest volcano in the Executive Committee Range to rise above the ice sheet. Below the ice sheet, however, seismologists have detected new volcanic activity 30 miles from Sidley, according to a 2013 news report ."

See attached map and photo of the Executive Committee Range
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LRC1962

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Re: Antarctic Tectonics
« Reply #73 on: February 11, 2015, 03:03:32 PM »
With the new data of how fast the East antarctic is sliding into the west, --about half an inch--per year, would not that result in the next while moe volcanic activity when you add it to the rebound of lose of ice?
This makes for a very complex situation where lose of ice causes tectonic activity to move horizontally and vertically at the same time and not because one motion is caused by the other.
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #74 on: December 22, 2015, 11:55:11 PM »
The cited reference discusses evidence for relatively high heat anomalies beneath the Bentley Subglacial Trench and the Marie Byrd Land Dome:

A seismic transect across West Antarctica: Evidence for mantle thermal anomalies beneath the Bentley Subglacial Trench and the Marie Byrd Land Dome by Andrew J. Lloyd, Douglas A. Wiens, Andrew A. Nyblade, Sridhar Anandakrishnan, Richard C. Aster, Audrey D. Huerta, Terry J. Wilson, Ian W.D. Dalziel, Patrick J. Shore and Dapeng Zhao published in Geophysical Research Letters Solid Earth DOI: 10.1002/2015JB012455


Abstract: " West Antarctica consists of several tectonically diverse terranes, including the West Antarctic Rift System, a topographic low region of extended continental crust. In contrast, the adjacent Marie Byrd Land and Ellsworth-Whitmore mountains crustal blocks are on average over 1 km higher, with the former dominated by polygenetic shield and stratovolcanoes protruding through the West Antarctic ice sheet and the latter having a Precambrian basement. The upper mantle structure of these regions is important for inferring the geologic history and tectonic processes, as well as the influence of the solid earth on ice sheet dynamics. Yet this structure is poorly constrained due to a lack of seismological data. As part of the Polar Earth Observing Network, 13 temporary broadband seismic stations were deployed from January 2010 to January 2012 that extended from the Whitmore Mountains, across the West Antarctic Rift System, and into Marie Byrd Land with a mean station spacing of ~90 km. Relative P and S wave travel time residuals were obtained from these stations as well as five other nearby stations by cross correlation. The relative residuals, corrected for both ice and crustal structure using previously published receiver function models of crustal velocity, were inverted to image the relative P and S wave velocity structure of the West Antarctic upper mantle. Some of the fastest relative P and S wave velocities are observed beneath the Ellsworth-Whitmore mountains crustal block and extend to the southern flank of the Bentley Subglacial Trench. However, the velocities in this region are not fast enough to be compatible with a Precambrian lithospheric root, suggesting some combination of thermal, chemical, and structural modification of the lithosphere. The West Antarctic Rift System consists largely of relative fast uppermost mantle seismic velocities consistent with Late Cretaceous/early Cenozoic extension that at present likely has negligible rift related heat flow. In contrast, the Bentley Subglacial Trench, a narrow deep basin within the West Antarctic Rift System, has relative P and S wave velocities in the uppermost mantle that are ~1% and ~2% slower, respectively, and suggest a thermal anomaly of ~75 K. Models for the thermal evolution of a rift basin suggest that such a thermal anomaly is consistent with Neogene extension within the Bentley Subglacial Trench and may, at least in part, account for elevated heat flow reported at the nearby West Antarctic Ice Sheet Divide Ice Core and at Subglacial Lake Whillans. The slowest relative P and S wave velocity anomaly is observed extending to at least 200 km depth beneath the Executive Committee Range in Marie Byrd Land, which is consistent with warm possibly plume-related, upper mantle. The imaged low-velocity anomaly and inferred thermal perturbation (~150 K) are sufficient to support isostatically the anomalous long-wavelength topography of Marie Byrd Land, relative to the adjacent West Antarctic Rift System."

See also:

http://www.reportingclimatescience.com/news-stories/article/more-evidence-of-volcanic-heating-under-antarctic-ice-sheet.html

Extract: "The most interesting finding, Lloyd said, is the discovery of a hot zone beneath the Bentley Subglacial Trench."

Edit: Caption for attached image showing location of the Bentley Subglacial Trench in the Byrd Subglacial Basin: "The topography of West Antarctica below the ice sheet as viewed from above, looking toward the Antarctic Peninsula. Much of West Antarctica is a basin that lies below sea level (blue), although it is currently filled with ice, not water. West Antarctica was stretched and thinned as it moved away from East Antarctica, forming one of the world’s largest continental rift systems."
« Last Edit: December 23, 2015, 04:05:16 PM by AbruptSLR »
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #75 on: March 23, 2016, 03:29:45 PM »
The linked reference offers a new tool (gravity) to enhance the geologic investigation of Antarctica.

M. Scheinert, F. Ferraccioli, J. Schwabe, R. Bell, M. Studinger, D. Damaske, W. Jokat, N. Aleshkova, T. Jordan, G. Leitchenkov, D. D. Blankenship, T. M. Damiani, D. Young, J. R. Cochran, T. D. Richter. New Antarctic Gravity Anomaly Grid for Enhanced Geodetic and Geophysical Studies in Antarctica. Geophysical Research Letters, 2016; DOI: 10.1002/2015GL067439

http://onlinelibrary.wiley.com/doi/10.1002/2015GL067439/abstract;jsessionid=8EBF7D992980C94EA9ECA21E8208A025.f01t04

Abstract: "Gravity surveying is challenging in Antarctica because of its hostile environment and inaccessibility. Nevertheless, many ground-based, airborne, and shipborne gravity campaigns have been completed by the geophysical and geodetic communities since the 1980s. We present the first modern Antarctic-wide gravity data compilation derived from 13 million data points covering an area of 10 million km2, which corresponds to 73% coverage of the continent. The remove-compute-restore technique was applied for gridding, which facilitated leveling of the different gravity data sets with respect to an Earth gravity model derived from satellite data alone. The resulting free-air and Bouguer gravity anomaly grids of 10 km resolution are publicly available. These grids will enable new high-resolution combined Earth gravity models to be derived and represent a major step forward toward solving the geodetic polar data gap problem. They provide a new tool to investigate continental-scale lithospheric structure and geological evolution of Antarctica."
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plinius

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Re: Antarctic Tectonics
« Reply #76 on: March 23, 2016, 08:09:10 PM »
While I am very fond of that - is there a good quantification of reactions to geologically recent changes in ice coverage? I mean the lithosphere reacts quite slowly to the ice mass changes which I think leaves an imprint in gravity measurements.

AbruptSLR

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Re: Antarctic Tectonics
« Reply #77 on: March 23, 2016, 09:05:47 PM »
While I am very fond of that - is there a good quantification of reactions to geologically recent changes in ice coverage? I mean the lithosphere reacts quite slowly to the ice mass changes which I think leaves an imprint in gravity measurements.


If you go to the following websites, you will see that this is a practicable and useful new database:

http://research-in-germany.org/en/research-landscape/news/2016/01/2016-01-22-new-gravity-dataset-will-help-unveil-the-antarctic-continent.html

&

https://doi.org/10.1594/PANGAEA.848168

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AbruptSLR

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Re: Antarctic Tectonics
« Reply #78 on: December 12, 2016, 01:32:23 AM »
The linked reference evaluates the implications of more accurately considering a 3-D viscoelastic Earth models as opposed to the less accurate assumption of elastic response on the sea-level fingerprint implications of an abrupt collapse of the WAIS.  Their findings conclude that "… when viscous effects are included, the peak sea-level fall predicted in the vicinity of WAIS during a melt event will increase by ~25% and ~50%, relative to the elastic case, for events of duration 25 years and 100 years, respectively."  This is important w.r.t. global sea level rise as the further the local sea-level drops around West Antarctica, the higher sea level will raise at distance away from West Antarctica.

Carling C. Hay, Harriet C. P. Lau, Natalya Gomez, Jacqueline Austermann, Evelyn Powell, Jerry X. Mitrovica, Konstantin Latychev, and Douglas A. Wiens (2016), "Sea-level fingerprints in a region of complex Earth structure: The case of WAIS", Journal of Climate, DOI: http://dx.doi.org/10.1175/JCLI-D-16-0388.1


http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0388.1


Abstract: "Sea-level fingerprints associated with rapid melting of the West Antarctic Ice Sheet (WAIS) have generally been computed under the assumption of a purely elastic response of the solid Earth. We investigate the impact of viscous effects on these fingerprints by computing gravitationally self-consistent sea-level changes that adopt a 3-D viscoelastic Earth model in the Antarctic region consistent with available geological and geophysical constraints. In West Antarctica, the model is characterized by a thin (~65 km) elastic lithosphere and sub-lithospheric viscosities that span three orders of magnitude, reaching values as low as ~4 × 1018 Pa s beneath WAIS. Our calculations indicate that sea-level predictions in the near field of WAIS will depart significantly from elastic fingerprints in as little as a few decades. For example, when viscous effects are included, the peak sea-level fall predicted in the vicinity of WAIS during a melt event will increase by ~25% and ~50%, relative to the elastic case, for events of duration 25 years and 100 years, respectively. Our results have implications for studies of sea-level change due to both ongoing mass loss from WAIS over the next century and future, large scale collapse of WAIS on century-to-millennial time scales."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #79 on: May 21, 2017, 01:56:37 AM »
The linked reference examines the abrupt collapse of the Last Weichselian Icelandic ice sheet to better understand the risk for future collapses of existing marine glaciers / ice sheets.  They find that the geothermal conditions beneath such ice sheets can control the rate of ice mass loss particularly during phases of rapid retreat (see the post just before this one: #78):

Henry Patton, Alun Hubbard, Tom Bradwell, Anders Schomacker. The configuration, sensitivity and rapid retreat of the Late Weichselian Icelandic ice sheet. Earth-Science Reviews, 2017; 166: 223 DOI: 10.1016/j.earscirev.2017.02.001

http://www.sciencedirect.com/science/article/pii/S001282521630246X

Abstract: “The fragmentary glacial-geological record across the Icelandic continental shelf has hampered reconstruction of the volume, extent and chronology of the Late Weichselian ice sheet particularly in key offshore zones. Marine geophysical data collected over the last two decades reveal that the ice sheet likely attained a continental shelf-break position in all sectors during the Last Glacial Maximum, though its precise timing and configuration remains largely unknown. Within this context, we review the available empirical evidence and use a well-constrained three-dimensional thermomechanical model to investigate the drivers of an extensive Late Weichselian Icelandic ice-sheet, its sensitivity to environmental forcing, and phases of deglaciation. Our reconstruction attains the continental shelf break across all sectors with a total ice volume of 5.96 × 105 km3 with high precipitation rates being critical to forcing extensive ice sheet flow offshore. Due to its location astride an active mantle plume, a relatively fast and dynamic ice sheet with a low aspect ratio is maintained. Our results reveal that once initial ice-sheet retreat was triggered through climate warming at 21.8 ka BP, marine deglaciation was rapid and accomplished in all sectors within c. 5 ka at a mean rate of 71 Gt of mass loss per year. This rate of ice wastage is comparable to contemporary rates observed for the West Antarctic ice sheet. The ice sheet subsequently stabilised on shallow pinning points across the near shelf for two millennia, but abrupt atmospheric warming during the Bølling Interstadial forced a second, dramatic collapse of the ice sheet onshore with a net wastage of 221 Gt a−1 over 750 years, analogous to contemporary Greenland rates of mass loss. Geothermal conditions impart a significant control on the ice sheet's transient response, particularly during phases of rapid retreat. Insights from this study suggests that large sectors of contemporary ice sheets overlying geothermally active regions, such as Siple Coast, Antarctica, and NE Greenland, have the potential to experience rapid phases of mass loss and deglaciation once initial retreat is initiated.”

See also:
 https://www.sciencedaily.com/releases/2017/04/170424093950.htm


Extract: “"We found that, at certain times, the Icelandic ice sheet retreated at an exceptionally fast rate -- more than double the present-day rate of ice loss from the much larger West Antarctic ice sheet -- causing global sea level to rise significantly."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #80 on: August 13, 2017, 07:34:48 PM »
The linked study finds that the West Antarctic Rift System is the largest volcanic region on Earth, and that some of these volcanic may very well become more active should the WAIS collapse:

Title: "Scientists find what they think is largest volcanic region on Earth hidden in Antarctica after student's idea"

http://www.independent.co.uk/environment/antarctica-west-antarctic-rift-system-volcanoes-university-of-edinburgh-max-van-wyk-de-vries-a7891206.html

Extract: "A remote survey discovered 91 volcanoes ranging in height from 100m to 3,850m in a massive region known as the West Antarctic Rift System.

Geologists and ice experts say the range has similarities to east Africa's volcanic ridge, currently acknowledged to be the densest concentration of volcanoes in the world.

Researchers from the University of Edinburgh remotely surveyed the underside of the ice sheet for hidden peaks of basalt rock, like those of other volcanoes in the region whose tips push above the ice.

Previous studies have suggested that volcanic activity may have occurred in the region during warmer periods and could increase if Antarctica's ice thins in a warming climate.

Dr Robert Bingham, of the University of Edinburgh's School of GeoSciences, said: "It is fascinating to uncover an extensive range of volcanoes in this relatively unexplored continent.

"Better understanding of volcanic activity could shed light on their impact on Antarctica's ice in the past, present and future, and on other rift systems around the world.""
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gerontocrat

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Re: Antarctic Tectonics
« Reply #81 on: August 16, 2017, 02:36:44 PM »
I thought to have a look at where these volcanoes are. I have to say I did not have a clue about how much of Antarctica is below sea level. Wikipedia Maps is super.
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RoxTheGeologist

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Re: Antarctic Tectonics
« Reply #82 on: August 16, 2017, 04:26:03 PM »

Note that for ever km of ice removed you get 1/2.8 km of elevation (roughly, depending on density of the rock and depth of isostatic compensation, maybe flexural considerations). I'm not sure if anyone has produced a map where the ice thickness has been converted into base elevation of the continent.

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Re: Antarctic Tectonics
« Reply #83 on: August 16, 2017, 04:37:03 PM »
Just to emphasize that people have been aware of the activity of the West Antarctic Rift System, WARS, for some time now, I provide the following repost below citing 2011 research:

"The following from the Eighteenth Annual WAIS Workshop (2011), indicates how volcanically active the WAIS is now (Behrendt recommends that its potential contribution not be ignored when assessing the risks of coming SLR), and has been in the recent past:

"Geophysical evidence of Ice-Magma interactions beneath the West Antarctic Ice Sheet in the West Antarctic Rift System
John C. Behrendt
INSTAAR University of Colorado, Boulder
And USGS, Denver
Radar Ice Sounding and Aeromagnetic Surveys reported over the West Antarctic Ice Sheet WAIS have been interpreted as evidence of subglacial eruptions. Several active volcanoes have shown evidence of eruption through the WAIS and several other active volcanoes are present beneath the WAIS (e.g. Corr et al., 2009; Blankenship et al., 1993) reported from radar ice sounding and aeromagnetic data (Behrendt et al., 1995; 2004). Aeromagnetic profiles (>10,000 km) acquired in the early 1960s over the West Antarctic Ice Sheet (WAIS) combined with coincident aeromagnetic and radar ice sounding in 1978-79 indicated numerous high-amplitude, shallow-source, magnetic anomalies over a very extensive area of the volcanically active West Antarctic rift system interpreted as caused by subglacial volcanic rocks. These early aerogeophysical surveys defined this area as >500,000 km2. Five-kilometer spaced coincident aeromagnetic and radar ice sounding surveys since 1990 provide three dimensional characterization of the magnetic field and bed topography beneath the ice sheet. These 5-50-km width, semicircular magnetic anomalies range from 100->1000 nT as observed ~1 km over the 2-3 km thick ice have been interpreted as evidence of subglacial eruptions. Behrendt et al, (2005, 2008) interpreted these anomalies as indicating >1000 "volcanic centers." requiring high remanent normal (and at least 10% reversed) magnetizations in the present field direction. These data have shown that >80% of the anomaly sources at the bed of the WAIS, have been modified by the moving ice into which they were injected, requiring a younger age than the WAIS (about 25 Ma). Although exposed volcanoes surrounding the WAIS extend in age to ~34 m.y., Mt Erebus, (<1 Ma) Mt. Melbourne, (<0.26 Ma), and Mt. Takahae (<0.1 Ma) are examples of exposed active volcanoes in the WAIS area. However, the great volume of volcanic centers is buried beneath the WAIS. If only a very small percentage of these >1000 volcanic, magnetic-anomaly sources are active today, or in the recent past, in the drainage area of the WAIS, subglacial volcanism may still have a significant effect on the dynamics of the WAIS. Interpreted active subglacial volcanism is revealed by aerogeophysical data reported by Blankenship et al., (1993, Mt. Casertz), and Corr and Vaughan, (2008, near Hudson Mts.), who raised the question of possible volcanic effects on the regime of the WAIS. Wingham et al. (2009) reported an average rate of volume loss from 2.6 to 10.1 km3 /yr from 1995 to 2006 for the Pine Island Glacier in the vicinity of the active subglacial volcano near the Hudson Mts. Probably wet-based areas of the WAIS would be the most likely to be impacted. Here I discuss these geophysical data over the WAIS, and conclude that even if there is a very low probability, future effects on the stability of the WAIS and associated sea-level rise should not be ignored, as changes observed in the past 20 years resulting from global warming, could be accelerated by subglacial volcanism."

The attached figures are also from a Behrendt 2011 pptx."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #84 on: August 16, 2017, 04:46:14 PM »
I'm not sure if anyone has produced a map where the ice thickness has been converted into base elevation of the continent.

As a public service, I re-post the attached image showing what Antarctica might look like if all the ice were removed and all the isostatic rebound had already occurred.  When compared to the map in Reply #81, clearly, this rebound could contribute meaningfully to sea level rise in the future.
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RoxTheGeologist

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Re: Antarctic Tectonics
« Reply #85 on: August 16, 2017, 07:56:42 PM »

Thank you AbruptSLR!

Here is an interesting paper describing how the Ice Sheets, isostatic rebound, and SL all interact:

Current state and future perspectives on coupled ice-sheet – sea-level modelling

http://www.sciencedirect.com/science/article/pii/S0277379116306023

Abstract
The interaction between ice-sheet growth and retreat and sea-level change has been an established field of research for many years. However, recent advances in numerical modelling have shed new light on the precise interaction of marine ice sheets with the change in near-field sea level, and the related stability of the grounding line position. Studies using fully coupled ice-sheet – sea-level models have shown that accounting for gravitationally self-consistent sea-level change will act to slow down the retreat and advance of marine ice-sheet grounding lines. Moreover, by simultaneously solving the ’sea-level equation’ and modelling ice-sheet flow, coupled models provide a global field of relative sea-level change that is consistent with dynamic changes in ice-sheet extent. In this paper we present an overview of recent advances, possible caveats, methodologies and challenges involved in coupled ice-sheet – sea-level modelling. We conclude by presenting a first-order comparison between a suite of relative sea-level data and output from a coupled ice-sheet – sea-level model.

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Re: Antarctic Tectonics
« Reply #86 on: August 17, 2017, 01:17:32 PM »
...clearly, this rebound could contribute meaningfully to sea level rise in the future.

Is not the rebound a see-saw effect, wherein a bit of the Earth's crust rises as another part sinks?

Does not the rebound require the rebounding slab to have some latent buoyancy due to loading of the crust somewhere not too far away?  When the load on the depressed slab is reduced, then must not there be a corresponding settlement of the nearby loading mass, which would simply neutralise to some degree the impact of the rebound on sea levels. 

The effect of the rebound is also constrained by the shapes of the rising and falling crustal slabs, and their position relative to the ocean surface.  In an extreme case consider a very narrow skyscraper as the load - mostly above the existing ocean level, and a broad 'table' as the currently-depressed crust slab below sea level.  As the load comes off the slab it rises displacing water as it rises from the ocean floor approaching the water's surface. The falling load tower has minimal positive displacement as it moves down, so the net change is an increase in the displaced volume, and thus some sea level rise.  But as soon as any element of the rising slab breaches the sea surface, then no matter how much higher it goes, that column of the slab does not contribute any more to displacement.  So the slab (aka Antarctica) can rise to a considerable height without displacing any more ocean beyond that displaced at the stage where it pierces the sea surface.   

There are some funny games to be played in how this all works, but it certainly does not appear obvious that isostatic rebound in a given location will automatically lead to displacement of sea water and hence to SLR.

Which reminds me of a recent paper (mentioned somewhere in this forum) which indicated that when Greenland is completing its rebound, then Iceland will have sunk to several hundred metres below sea level as its mass provides the downward moment which is used to shove Greenland up as the ice cap is removed.  I don't know if that paper calculated the planet-wide net impact of that rise+fall event.

Not as simple as it sounds, but of course I guess nature will contrive to ensure that all feedbacks are net 'bad' for us just to keep us in our intellectual place!

gerontocrat

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Re: Antarctic Tectonics
« Reply #87 on: August 17, 2017, 01:55:11 PM »

Is not the rebound a see-saw effect, wherein a bit of the Earth's crust rises as another part sinks?


The Article I read about this used an analogy. Imagine a big lump of dough on your table top. Plonk a heavy weight in the middle. The middle sinks and the periphery outside the weight rises. Remove the weight - the centre rises and the periphery sinks - slowly.

But I am a "wish I had been" geologist
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RoxTheGeologist

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Re: Antarctic Tectonics
« Reply #88 on: August 17, 2017, 04:27:45 PM »
And there is also the effect of gravitational pull of the ice sheet. We tend to assume that gravitational forces only work straight up and down giving 'airy' isostacy, but the airy formula is only the first term in a series that describes the gravitational effects of mass at a distance above that mass (Parker, R.L 1972  http://topex.ucsd.edu/geodynamics/parker.pdf)

Everest, during the survey of India, showed that the Himalaya were causing distortion of maps because of their mass. The plumb lines being using for their zenith telescopes were not 'true'. The surface of the geoid (an equi-potential surface, that the plumb line hangs perpendicular to) wasn't parallel to the surface of the earth. This is true even though, as a whole, the Himalaya are in isostatic equilibrium.

Ice sheets 'pull' the ocean towards them, so when ice sheets melt there are lots of things going on, all based around the redistribution of mass of the water, Isostatic rebound, flexural compensation of mass, local gravitational changes, and, of course, addition of water to the ocean.

AbruptSLR

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Re: Antarctic Tectonics
« Reply #89 on: August 17, 2017, 04:34:05 PM »

Is not the rebound a see-saw effect, wherein a bit of the Earth's crust rises as another part sinks?


The Article I read about this used an analogy. Imagine a big lump of dough on your table top. Plonk a heavy weight in the middle. The middle sinks and the periphery outside the weight rises. Remove the weight - the centre rises and the periphery sinks - slowly.

But I am a "wish I had been" geologist

The situation for the marine glacial portions of Antarctica is different than that for most isostatic rebound cases around the world.  If one thinks of the Laurentide Ice Sheet removal from North America, then as the middle of the continent rebounds, the coastal perimeter sinks, thus increasing the holding capacity of the ocean basins (at the current sea level thus causing sea level to drop slightly).  However, for the great majority of the marine glacial portions of Antarctica the 'seafloor' would move upwards, while the dry land portions of the continent would reduce its elevation but not go underwater.  Thus isostatic rebound due to the loss of the Antarctic marine glaciers (effectively the only portions of the AIS that is at risk of being lost in the coming decades/centuries) would decrease the water holding capacity of the ocean basins (at current sea level, thus causing future sea level to increase).
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #90 on: August 17, 2017, 06:45:16 PM »
The following re-posted information indicates the tectonic behavior in the WAIS is more complex as the mantle below the Byrd Subglacial Basin (BSB) and Marie Byrd Land has magma with very low viscosity, as discussed in the two re-posted Replies from the Antarctic Tectonic thread (also the rate of ice mass loss from the BSB is exceptionally high).

The following abstract comes from the International Glacial Society Proceeding 65 at the following link:

http://www.igsoc.org/symposia/2014/chamonix/proceedings/procsfiles/procabstracts_65.htm

It is particularly interesting that Wilson et al 2014 indicate that the magma beneath Marie Byrd Land has very low viscosity:

70A1149
The POLENET-ANET integrated GPS and seismology approach to understanding glacial isostatic adjustment and ice mass change in Antarctica

Terry WILSON, Michael BEVIS, Stephanie KONFAL, Richard ASTER, Julien CHAPUT, David HEESZEL, Douglas WIENS, Sridhar ANANDAKRISHNAN, Ian DALZIEL, Audrey HUERTA, Eric KENDRICK
Corresponding author: Terry Wilson
Corresponding author e-mail: wilson.43@osu.edu

Abstract: "The POLENET-ANET project is simultaneously resolving crustal motions, measured by GPS, and Earth structure and rheological properties, mapped by seismology. Measured vertical and horizontal crustal motion patterns are not explained by extant glacial isostatic adjustment (GIA) models. These models have ice histories dominated by ice loss following the Last Glacial Maximum (LGM) and rely on 1-D Earth models, with rheological properties varying only radially. Seismological results from POLENET-ANET are revealing significant complexity in lateral variation in Earth properties. For example, crustal thickness variations occur not only across the East-West Antarctic boundary, but also between crustal blocks within West Antarctica. Modeling of mantle viscosity based on shear wave velocities shows a sharp lateral gradient from high to low viscosity in the Ross Embayment, a much more gradual gradient in the Weddell Embayment, and very low viscosities below Marie Byrd Land and the Amundsen Sea Embayment (ASE). Remarkable vertical and horizontal bedrock crustal motion velocity magnitudes, directions and patterns correlate spatially, in many aspects, with Earth property variations mapped by seismology. Within the ASE, extremely high upward velocities are flanked by subsiding regions – neither predicted by GIA models. Given the thin crust and low mantle viscosity, it is likely that this is not an LGM signal, which would have already relaxed, and uplift due to the elastic response to modern ice mass change clearly is important. As in other regions where rapid GIA-induced uplift has been measured, the crustal velocities in the Amundsen Embayment may also record a viscoelastic response to ice loss on decadal–centennial timescales. Along the East-West Antarctic boundary in the Ross Embayment, GIA-induced horizontal crustal motions are toward rather than away from the principal ice load center, correlating spatially with the strong lateral gradient in mantle viscosity. In the Weddell Embayment region, where crustal thickness is intermediate between East and West Antarctica and mantle viscosity values are moderate, crustal motions show the best match with predictions of GIA models. It is clear that lateral variations in Earth properties fundamentally control the isostatic response to ice mass changes in Antarctica. Ongoing integrated seismic-GPS studies are critical to developing the next generation of GIA models."

Also see:
The following linked reference cites evidence of low upper mantle velocities inland of the Amundsen Sea.  Such low-velocity zones indicate the presence of a significant degree of partial melting, and thus to potential for rapid rebound when ice mass is lost from the Byrd Subglacial Basin:

Natalie J. Accardo, Douglas A. Wiens, Stephen Hernandez, Richard C. Aster, Andrew Nyblade, Audrey Huerta, Sridhar Anandakrishnan, Terry Wilson, David S. Heeszel and Ian W. D. Dalziel, (2014), "Upper mantle seismic anisotropy beneath the West Antarctic Rift System and surrounding region from shear wave splitting analysis", Geophys. J. Int. (2014) doi: 10.1093/gji/ggu117

http://gji.oxfordjournals.org/content/early/2014/05/21/gji.ggu117.abstract

Abstract: "We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37 broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked to determine the best fitting splitting parameters for each station. Overall, fast anisotropic directions are oriented at large angles to the direction of Antarctic absolute plate motion in both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent reflects multiple mantle regimes. We suggest that the observed anisotropy along the central Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of the largest zones of extended continental crust on Earth, results from asthenospheric mantle strain associated with the final pulse of western WARS extension in the late Miocene. Strong and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred extension direction, a result unlike reports from the East African rift system and rifts within the Basin and Range, which show much greater variation. We contend that ductile shearing rather than magmatic intrusion may have been the controlling mechanism for accumulation and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but shows a consistent fast direction, possibly representative of anisotropy that has been ‘frozen-in’ to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea appear to be radial to the dome and may indicate radial horizontal mantle flow associated with an MBL plume head and low upper mantle velocities in this region, or alternatively to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern end of the WARS."
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #91 on: September 03, 2017, 07:05:26 PM »
As a follow-on to Replies #80 thru #90: after reading about the linked research that indicates that CO₂ from subsea volcanism (as Greenland separated from Europe) that drove the PETM; I cannot help but to wonder how much CO₂ would be emitted from subsea volcanism in West Antarctica if the WAIS were to largely collapse this century:

Title: "Study says ancient extreme global warming event was caused by C02 buildup from massive eruptions"

https://summitcountyvoice.com/2017/09/03/can-volcanoes-overheat-the-earth/

Extract: "The role of volcanoes on Earth’s climate is complex. It’s well known that aerosol particles from eruptions like Mt. St. Helens can cool temperatures by blocking a small part of the sun’s energy, but a new study suggests that an extreme global warming event about 56 million years ago was caused by massive volcanic C02 emissions.

The research, led by the University of Southampton found evidence that atmospheric C02 doubled within a relatively short period of time, on the geological scale, resulting in a global temperature spike that drove mass extinctions. The eruptions happened during formation of the Atlantic Ocean, according to the study published in the journal Nature.

The PETM  coincided with the formation  large stretches of ocean floor coated in lava, resulting from of a series of huge eruptions. These occurred as Greenland first started separating from north-western Europe, thereby creating the North Atlantic Ocean, the vestiges of which are still continuing in miniature in Iceland today. What has been missing is evidence linking these huge volcanic outpourings to the carbon release and warming that marks the PETM.

Professor Andy Ridgwell from University of California, Riverside, said, “Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, as these provide an indication of its source. When we force a numerical global climate model to take into account both sets of changes, the results point to the large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM.”

The team found that the PETM was associated with a total input of more than 10,000 petagrams of carbon from a predominantly volcanic source. This is a vast amount of carbon – some 30 times larger than all the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves.

In their computer model simulations, it resulted in the concentration of atmospheric CO2 increasing from 800 parts per million to above 2000 ppm. The Earth’s mantle contains more than enough carbon to explain this dramatic rise and it would have been released as magma, pouring from volcanic rifts at the Earth’s surface.

“How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change,” said Professor Gavin Foster, from the University of Southampton.

“For instance, we found that Earth’s warming at the PETM was about what we would expect given the CO2 emitted and what we know about the sensitivity of the climate system based on Intergovernmental Panel on Climate Change (IPCC) reports. However, compared with today’s human-made carbon emissions, the rate of carbon addition during the PETM was much slower, by about a factor of 20.”
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Antarctic Tectonics
« Reply #92 on: September 05, 2017, 12:30:26 AM »
The linked article is entitled: "Massive Antarctic volcanic eruptions linked to abrupt Southern hemisphere climate changes", and indicates that abrupt climate change ~17.7 kya was associated with a series of halogen rich eruptions from Mt Takahe in the BSB.  Who knows what lies in mankind's future should a collapse of the WAIS this century should trigger similar volcanic eruptions in West Antarctica:

https://phys.org/news/2017-09-massive-antarctic-volcanic-eruptions-linked.html

Extract: ""Detailed chemical measurements in Antarctic ice cores show that massive, halogen-rich eruptions from the West Antarctic Mt. Takahe volcano coincided exactly with the onset of the most rapid, widespread climate change in the Southern Hemisphere during the end of the last ice age and the start of increasing global greenhouse gas concentrations," according to McConnell, who leads DRI's ultra-trace chemical ice core analytical laboratory.

Climate changes that began ~17,700 years ago included a sudden poleward shift in westerly winds encircling Antarctica with corresponding changes in sea ice extent, ocean circulation, and ventilation of the deep ocean. Evidence of these changes is found in many parts of the Southern Hemisphere and in different paleoclimate archives, but what prompted these changes has remained largely unexplained.

"We know that rapid climate change at this time was primed by changes in solar insolation and the Northern Hemisphere ice sheets," explained McConnell. "Glacial and interglacial cycles are driven by the sun and Earth orbital parameters that impact solar insolation (intensity of the sun's rays) as well as by changes in the continental ice sheets and greenhouse gas concentrations."
"We postulate that these halogen-rich eruptions created a stratospheric ozone hole over Antarctica that, analogous to the modern ozone hole, led to large-scale changes in atmospheric circulation and hydroclimate throughout the Southern Hemisphere," he added. "Although the climate system already was primed for the switch, we argue that these changes initiated the shift from a largely glacial to a largely interglacial climate state. The probability that this was just a coincidence is negligible."

See also the subject reference:

Joseph R. McConnell el al., "Synchronous volcanic eruptions and abrupt climate change ∼17.7 ka plausibly linked by stratospheric ozone depletion," PNAS (2017). www.pnas.org/cgi/doi/10.1073/pnas.1705595114


See also: "Antarctica: What Would Happen if All the Volcanoes Buried Beneath the Ice Erupted?"

http://www.newsweek.com/antarctica-subglacial-volcanoes-eruption-risk-sea-ice-659537
« Last Edit: September 05, 2017, 06:21:04 PM by AbruptSLR »
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson