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AbruptSLR

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Internal Layers of the Antarctic Ice Sheet
« on: August 01, 2014, 02:27:48 AM »
A-Team was kind enough to provide the following link to the British Antarctic Survey's repository of images of the internal layers of the Antarctic Ice Sheet:

http://www.antarctica.ac.uk/bas_research/data/access/res/data.php


The IceBridge program also provides survey data from Antarctica as well, some of which can be found at the following links:

http://nsidc.org/data/ilatm2
http://nsidc.org/data/icebridge/index.html

The internal structure of the WAIS will become increasingly important, particularly to the ASE marine glaciers, as the ice shelves are lost and the ice flow velocities increase this will accelerate the formation of crevasses in the ice streams (such as PIG and Thwaites) which possibly as early as 2050 may contribute to major calving events in the ASE marine glaciers forming a mélange in-front of the marine glaciers, very much as is currently occurring for Jakobshavn. 

A-Team, and others, are doing a fantastic job of documenting the kinematics of the ice movement (including flow, crevasses and calving) for Jakobshavn; all of which will provide valuable lessons for the ASE marine glaciers, particularly if surface meltwater starts to flow into the crevasses for Jakobshavn (this year or in the near future); which could occur in the ASE marine glaciers by mid-century.

Edit: For those who want a preview of what internal structure is for an ice sheet, I provide the attached image of striations from the British Antarctic Survey site.
« Last Edit: August 01, 2014, 03:47:42 PM by AbruptSLR »
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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #1 on: August 02, 2014, 08:05:01 PM »
Unfortunately, I do not have time, at the moment, to contribute discussion to this thread, on the internal ice structure of the AIS, according to the importance of this topic.  Nevertheless, I would like to follow-up on my first post here by briefly focusing on one of the pioneers in the field - Terence Hughes, and his concepts. 

I start this brief discussion by providing a link (and one extract), with a free access pdf, to/from Terence Hughes' 1981 reference entitled "The Weak Underbelly of the West Antarctic Ice-Sheet".  The extract (and the associate image from Hughes 1981) indicate that at that time the Thwaites Ice Tongue was 200-km long, which is an indication that " … the ice-stream velocity exceeds the iceberg calving rate, which is most likely towards the beginning of a surge." (see also the "Surge" thread in this Antarctic folder).  Since, 1981 the Thwaites Ice Tongue has completely collapsed (leaving only a pinned/grounded iceberg, disconnected from the Thwaites ice stream), in my opinion due to the advection of warm CDW into Pine Island Bay (rather than due to the surge of the Thwaites ice stream coming to an end). 

Hughes, Terence J., "The Weak Underbelly of the West Antarctic Ice-Sheet" (1981). Earth Science Faculty Scholarship. Paper 156. http://digitalcommons.library.umaine.edu/ers_facpub/156

http://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=1155&context=ers_facpub

Extract: "Thwaites Glacier terminates as a huge floating ice tongue 200 km long. Floating ice tongues form when the ice-stream velocity exceeds the iceberg calving rate, which is most likely toward the beginning of a surge."

With the previous linked reference (Hughes 1981) I have established that Thwaites Glacier is likely beginning a surge phase, and that buttressing from its ice tongue (and possibly buttressing from its ice shelf) has been lost, this implies that its ice flows may soon accelerate.  Now I would like to reference Reusch and Hughes (2003) with work on the Bryd Glacier, as I believe that the Byrd Glacier has many parallels to the Thwaites Glacier (except that Bryd's ice flow is constrained laterally, while in a few decades the ice flow from Thwaites could spread laterally), and thus lessons learned from Byrd can be applied to Thwaites. " Reusch and Hughes (2003) hypothesized that as Byrd Glacier transitions from sheet flow to stream flow, the ice surface undergoes changes in surface slope (‘surface waves’) that appear to be unrelated to bed topography. The implication is that these surface waves reflect variations in the coupling between ice and the bed and that they may move as individual ice columns and migrate through the glacier. According to this hypothesis, surface waves represent regions of high longitudinal tensile stresses on the ice surface and the dominant resistance for the flow of Byrd Glacier is due to these longitudinal stress gradients."  I believe this pattern of behavior can be seen to be developing in Thwaites now.

Reusch, D. and Hughes, Terence J., "Surface "Waves" on Byrd Glacier, Antarctica" (2003). Earth Science Faculty Scholarship. Paper 94. http://digitalcommons.library.umaine.edu/ers_facpub/94  (citation: Reusch,. D. and Hughes, T.J., (2003), "Surface "Waves" on Byrd Glacier, Antarctica", Antarctic Science 15 (4): 547–555, DOI: 10.1017/S095410200301664)

http://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=1093&context=ers_facpub

Now I would like to move on to Hughes et al (2011) (see following link to a free access pdf and the abstract) that discusses the ice-bed coupling beneath and beyond ice streams using information related to a 2004 surge in the flow of the Byrd Glacier associated with the basal drainage of two large subglacial lakes.  I switch to this topic because Thwaites also has a well-developed subglacial basal water hydrological system and surges in the Thwaites ice stream flow (see the "Surge" thread) seem to also be related to episodic basal water discharges; and the Hughes et al 2011 reference shows how during such surges the ice flow accelerates above the bed as controlled by the floating friction (phi) that is influenced by the internal ice structure in the ice sheet (see the second attached image and associated caption)


Hughes, Terence J.; Sargent, Aitbala; and Fastook, James L., "Ice-Bed Coupling Beneath and Beyond Ice Streams: Byrd Glacier, Antarctica" (2011). Earth Science Faculty Scholarship. Paper 49. http://digitalcommons.library.umaine.edu/ers_facpub/49  (citation: Hughes, T., A. Sargent, and J. Fastook (2011), Ice‐bed coupling beneath and beyond ice streams: Byrd Glacier, Antarctica, J. Geophys. Res., 116, F03005, doi:10.1029/2010JF001896.)

http://digitalcommons.library.umaine.edu/cgi/viewcontent.cgi?article=1048&context=ers_facpub

Abstract: "Ice sheet thickness is determined mainly by the strength of ice‐bed coupling that controls holistic transitions from slow sheet flow to fast streamflow to buttressing shelf flow. Byrd Glacier has the largest ice drainage system in Antarctica and is the fastest ice stream entering Ross Ice Shelf. In 2004 two large subglacial lakes at the head of Byrd Glacier suddenly drained and increased the terminal ice velocity of Byrd Glacier from 820 m yr−1 to 900 m yr−1. This resulted in partial ice‐bed recoupling above the lakes and partial decoupling along Byrd Glacier. An attempt to quantify this behavior is made using flowband and flowline models in which the controlling variable for ice height above the bed is the floating fraction phi of ice along the flowband and flowline. Changes in phi before and after drainage are obtained from available data, but more reliable data in the map plane are required before Byrd Glacier can be modeled adequately. A holistic sliding velocity is derived that depends on phi, with contributions from ice shearing over coupled beds and ice stretching over uncoupled beds, as is done in state‐of‐the‐art sliding theories.

Caption for second attached image of the Byrd Glacier: "The geometrical force balance on an ice stream ending as a confined ice shelf. (top) Stresses that resist gravitational flow along x. The bed supports ice in the shaded area. Ice in the unshaded area is supported by “effective” basal water pressure. (middle) Gravitational forces at x represented as triangles and a rectangle are linked to specific resisting stresses. The area inside the thick border is linked to compressive stress sC. Heights hI, hW, and hF are measured from the bed for x > 0. (bottom) Resisting stresses and gravitational forces along Dx. Resisting and gravitational forces are balanced along x and Dx [see Hughes, 2009a]."


Unfortunately, I have run out of time, but I believe that this brief review of Terence Hughes' insights (from three of his papers focused on the Pine Island Bay marine glaciers and the Byrd Glacier) on the flow kinematics of ice sheets / ice streams (as related to internal ice structure, buttressing, bed conditions, subglacial basal water, etc.) are highly relevant to projecting ice mass loss from the AIS (and the ASE marine glaciers in-particular), over the coming decades and centuries.  Hopefully, in future posts I can discuss how internal ice structure can influence crevasse formation that can contribute to Jakobshavn type of calving behavior for the critical Thwaites Glacier in the coming decades.
« Last Edit: August 02, 2014, 11:06:28 PM by AbruptSLR »
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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #2 on: August 03, 2014, 12:03:43 AM »
I am putting this post in this thread (with the linked reference on facture-induced softening for large scale ice dynamics) because although it focuses on Antarctic ice shelves, it could well be applied to fracturing in grounded glaciers:

T. Albrecht and A. Levermann, 2014, "Fracture-induced softening for large-scale ice dynamics", The Cryosphere, 8, 587–605, www.the-cryosphere.net/8/587/2014/, doi:10.5194/tc-8-587-2014

http://www.the-cryosphere.net/8/587/2014/tc-8-587-2014.pdf

Abstract. "Floating ice shelves can exert a retentive and hence stabilizing force onto the inland ice sheet of Antarctica. However, this effect has been observed to diminish by the dynamic effects of fracture processes within the protective ice shelves, leading to accelerated ice flow and hence to a sea level contribution. In order to account for the macroscopic effect of fracture processes on large-scale viscous ice dynamics (i.e., ice-shelf scale) we apply a continuum representation of fractures and related fracture growth into the prognostic Parallel Ice Sheet Model (PISM) and compare the results to observations. To this end we introduce a higher order accuracy advection scheme for the transport of the two-dimensional fracture density across the regular computational grid. Dynamic coupling of fractures and ice flow is attained by a reduction of effective ice viscosity proportional to the inferred fracture density. This formulation implies the possibility of non-linear threshold behavior due to self-amplified fracturing in shear regions triggered by small variations in the fracture-initiation threshold. As a result of prognostic flow simulations, sharp across-flow velocity gradients appear in fracture-weakened regions. These modeled gradients compare well in magnitude and location with those in observed flow patterns. This model framework is in principle expandable to grounded ice streams and provides simple means of investigating climate-induced effects on fracturing (e.g., hydro fracturing) and hence on the ice flow. It further constitutes a physically sound basis for an enhanced fracture-based calving parameterization."

See also related commentary at:
http://www.the-cryosphere-discuss.net/7/C3138/2014/tcd-7-C3138-2014.pdf

« Last Edit: August 03, 2014, 07:59:04 PM by AbruptSLR »
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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #3 on: August 03, 2014, 04:54:42 AM »
The first attached figure is from Bassis & Jacobs (2013) for an idealize glacier and shows the relationship of ice thickness, water depth and the water pressure induced propagation of fractures

J. N. Bassis & S. Jacobs, (2013), "Diverse calving patterns linked to glacier geometry"; Nature Geoscience; doi:10.1038/ngeo1887

While the Bassis & Jacobs (2013) results are for an idealized glacier they are directly applicable to fractures in the Thwaites Glacier.

Further in this regards, the September 2012 surge of the Thwaites ice stream feeding the former Thwaites ice tongue so thinned the ice thickness that extensive fracturing occurred in the ice surface as shown in the January 2013 Landsat-7 photo (from Logan et al 2013) of this area showing the fracturing (surface crevasses) landward of the grounding line, leading to the calving of block-shaped icebergs from the former ice tongue as the ice flowed into the ocean.

Liz LOGAN, Ginny CATANIA, Luc LAVIER, Eunseo CHOI, (2013), "A novel method for predicting fracture in floating ice"; Journal of Glaciology, Vol. 59, No. 216, 2013 doi:10.3189/2013JoG12J210

Lastly, I would like to note that while surface crevasses are currently weakening the resistance to flow of the ice in the Thwaites Gateway, the third and fourth attached images show the shear strain within the internal structure of the Thwaites Glacier according to MacGregor et al (2013).  Particularly, note the location of the eastern shear margin of the Thwaites Glacier, which will grow as the Thwaites ice flow velocity accelerates and this shear margin will likely intersect with the shear margin of the Southwest (SW) Tributary glacier that currently feeds into the PIG.  When these two shear margins intersect (particularly see the fourth image), it is likely that the Thwaites Glacier will cross a tipping point of instability leading to accelerated collapse behavior.

Joseph A. MacGREGOR, Ginny A. CATANIA, Howard CONWAY, Dustin M. SCHROEDER, Ian JOUGHIN, Duncan A. YOUNG, Scott D. KEMPF, & Donald D. BLANKENSHIP, (2013), "Weak bed control of the eastern shear margin of Thwaites Glacier, West Antarctica"; Journal of Glaciology, Vol. 59, No. 217, doi: 10.3189/2013JoG13J050
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #4 on: August 03, 2014, 04:31:43 PM »
Returning to Terence Hughes insights about AIS kinematics and internal structure, in the following linked reference (with a free access pdf), he expands on his earlier theory of thermal convection in the AIS, citing new evidence and proposed new tests to verify his insight.  The new evidence again focuses on the Byrd Glacier (see the first image and associated caption) about converging and dispersing tributary flows which changes ice flow speeds, boundary conditions and thermal energy that influences convection rolls shown in the second attached image (see also the associated caption).  Obviously, ice is not a liquid therefore its thermal convection is heavily influence by creep as indicated by the third and fourth attached images (and associated captions).  If this theory is verified by field measurements it could have a profound influence on understanding ice sheet flow and how heat (say from friction, geothermal, surface meltwater penetrating surface crevasses, or other sources) feedback mechanisms could accelerate local ice streams (such as Thwaites and other ASE marine glaciers) leading to possible ice sheet (say WAIS) destruction:

Terence J. Hughes, (2012), " Thermal convection in ice sheets: New data, new tests", Natural Science, Vol.4 No.7, Article ID:20743,10 pages DOI:10.4236/ns.2012.47056

http://file.scirp.org/Html/1-8301678_20743.htm

Abstract: "Thermal convection in the Antarctic Ice Sheet was proposed in 1970. Demonstrating its existence proved to be elusive. In 2009, tributaries to ice streams were postulated as the surface expression of underlying thermal convection rolls aligned in directions of advective ice flow. Two definitive tests of this hypothesis are now possible, using highly accurate ice elevations and velocities provided by the European, Japanese, and Canadian Space Agencies that allow ice-stream tributaries and their velocities to be mapped. These tests are 1) measuring lowering of tributary surfaces to see if lowering is due only to advective ice thinning, or also requires lowering en masse in the broad descending part of convective flow, and 2) measuring transverse surface ice velocities to see if ice entering tributaries from the sides increases while crossing lateral shear zones, as would be required if this flow is augmented by convective flow ascending in the narrow side shear zones and diverted into tributaries by advective ice flow. If 1) and 2) are applied to tributaries converging on Byrd Glacier, the same measurements can be conducted when tributaries pack together to become “flow stripes” down Byrd Glacier and onto the Ross Ice Shelf to see if 2) is reduced when lateral advection stops. This could determine if thermal convection remains active or shuts down as ice thins. Thermal convection in the Antarctic Ice Sheet would raise three questions. Can it cause the ice sheet to self-destruct as convective flow turns on and off? Does it render invalid climate records extracted at depth from ice cores? Can the ice sheet be studied as a miniature mantle analogous in some respects to Earth’s mantle?"

Selected extracts: "Recrystallization begins at a creep strain of about 10 percent for a given applied stress. The strain rate, initially infinite, gives an infinite Rayleigh number. Both decrease over time and the Rayleigh number can fall below its critical value before slow steady state creep is established. Then convective flow stops. If the Rayleigh number remains above its critical value, recrystallization can take place and allow fast steady state creep to produce a higher Rayleigh number that allows faster convective flow. This component of ice-sheet flow can therefore turn on and off, and thereby regulate the 90 percent of ice discharged by ice streams in the Antarctic Ice Sheet. In the extreme, the faster discharge may allow the ice sheet to self-destruct, if more ice leaves than can be replaced by precipitation over the surface.

As in rifted crustal ridges, ice pulls away from ice divides, but ice is too slow and soft to produce rifts. Ice converges on ice streams that supply ice shelves, which calve instead of collide like crustal plates. This understanding of ice-sheet dynamics would draw wide interest, and be a great boon to glaciology. The glaciology of ice sheets would undergo a Scientific Revolution comparable to plate tectonics if convection rolls were shown to underlie ice-stream tributaries."

Caption for the first image of the Byrd Glacier Flow Pattern: "A Radarsat image of Byrd Glacier, including converging tributaries at its head and lateral rifts where it enters Ross Ice Shelf. The trace of tributaries continues on Byrd Glacier and onto the Ross Ice Shelf as flow stripes."

Caption of the second image of theoretical convective ice stream flow pattern: "A cartoon showing thermal convection rolls in transverse cross-section beneath ice-stream tributaries. Letters T, C, and S show respective regions of tensile, compressive, and shear flow caused by convection. Top: For an isolated tributary, lateral advective flow moves down the side slopes into the tributary and augments convective flow into the tributary. Similar flow of subglacial water may produce a lake under the tributary. Bottom: Lateral advective flow stops when tributaries get packed together as they enter an ice stream."

Caption of the third image of creep curves: "Creep curves for simple shear in polycrystalline ice at −3˚C for applied shear stresses of 117 kPa (curve A) and 55 kPa (curve B). Transient creep dominates for the first 50 to 100 hours of strain, depending on the applied stress. Recrystallization begins at about 20 percent strain. Shear displacements dz were measured on planes normal to x, so ezx = 1/2 (¶dz/¶x + ¶dx/¶z) gives strain ¶dz/¶x = 2ezx. At 55 kPa, recrystallization would begin when 2ezx = 2000 hours, about 17 days."

Caption of the fourth image of creep spectrum: "The viscoplastic creep spectrum for both slow and fast steady-state creep respectively before and after recrystallization. Viscoplastic viscosity hV is the tangent-slope to curves at applied stress s. At plastic yield stress sO the strain rate is change in epsilono for all values of n. Recrystallization produces an easyglide fabric for which sO is reduced. Two viscoplastic yield criteria are shown for ice at viscoplastic yield stress sV at n = 3. The maximum stress-curvature criterion gives sV = 0.386sO. The stress-intercept tangent line at change in epsilono  gives sV = 0.667sO."
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AbruptSLR

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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #5 on: August 04, 2014, 04:29:09 PM »
In Reply # 414 of the Jakobshavn thread in the Greenland folder, A-team presents a detailed list of paleo-events that would leave signatures in the layers of the GIS, together with the depths that this signature events can be identified in the various GIS ice cores indicated on his list.  This list of events is also relevant to the AIS and prior comparisons have been made between the signatures in ice cores from both the GIS and the AIS, see the first attached image.  However, the Antarctic ice core record goes back further in time than the Greenland ice core record, as best exemplified by the WAIS Divide Ice Core program, and the second attached images shows the location of the WAIS Divide ice core (together with the Byrd, and Siple Dome, ice cores).  The WAIS Divide location was selected due to good snowfall, a relatively flat bedmap, minimal surface melting, and as horizontal ice flow has made minimal disruption to the local internal ice structure (see my last post about T. Hughes insights about ice flow from divides to sheet flow to stream flow).  Furthermore, the WAIS Divide location is relatively close to the critical ASE marine glaciers.  The third attached image shows an October 12 2012 IceBridge radar image (the fourth attached image shows the flight path) from the Thwaites Gateway (where the ice is too thin to support the Thermal Convection postulated by T. Hughes, and thus the bottom bed topology [indicated by the red line] dominates [note the purple line shows the firn line, and intermediate striations may be dust, or ash]).  While interpolating between boreholes and radar image of the ice structure is beyond my skill level, nevertheless the WAIS Divide ice core provides other valuable insights. However, due to the value of the WAIS Divide Ice Core, that analysis of the core is going slowly, but the following link leads to recent associated publications (including the two listed below) that provide insight about other signature events including worldwide volcanic eruptions and comparisons with other ice core findings:

http://www.waisdivide.unh.edu/Publications/index.shtml

Sigl, M., McConnell, J.R., Toohey, M., Curran, M., Das, S.B., Edwards, R., Isaksson, E., Kawamura, K., Kipfstuhl, S., Kruger, K., Layman, L., Maselli, O., Motizuki, Y., Motoyama, H., Pasteris, D.R. and Severi, M. (2014), "Insights from Antarctica on volcanic forcing during the Common Era" Nature Climate Change, p. 1 – 5, doi: 10.1038/nclimate2293

Fudge, T.J., Waddington, E.D., Conway, H., Lundin, J.M.D. and Taylor, K. (2014), "Interpolation methods for Antarctic ice-core timescales: application to Byrd, Siple Dome and Law Dome ice cores", Climate of the Past, 10, p. 1195 – 1209, doi: 10.5194/cp-10-1195-2014

Furthermore, the following linked reference provides new findings from the WAIS Divide and the EDML ice cores, Antarctica, and indicates that the atmospheric methane record in Antarctica is much different than that for Greenland, indicating a local source of methane (such as marine methane hydrates around the Southern Ocean basin):

http://waisdivide.unh.edu/Publications/DisplayArticle.shtml?REF_ID=1365


Winstrup, M., Vinther, B.M., Sigl, M., McConnell, J., Svensson, A.M. and Wegner, A. (2014)
Development and comparison of layer-counted chronologies from the WAIS Divide and EDML ice cores, Antarctica, over the last glacial transition (10-15 ka BP) EGU General Assembly 2014, held 27 April - 02 May 2014 in Vienna, Austria, id. EGU2014-12193-1

Also, the following partial summary about ice drilling for the WAIS Divide program (from the following website) discusses how the GHG record from the ice core indicates the risk of abrupt local climate change:
http://waisdivide.unh.edu/

"Innovations in Ice Drilling Enable Abrupt Climate Change Discoveries
A revolutionary drilling system leads to the retrieval of additional ice for evidence of abrupt climate change from the Antarctic Ice Sheet.
Deep within ice sheets in the polar regions is an archive of evidence about the climate of the past. Ice cores drilled in the past have yielded amazing scientific discoveries, for example that climate can change abruptly in less than ten years, and that the CO2 in the atmosphere now is higher than evidenced from the last 800,000 years. At the WAIS Divide site, a cold area of the West Antarctic Ice Sheet where the abundant snowfall rarely melts, the ice contains many tens of thousands of years of annual information about past climate. At specific depths in the ice sheet, including from times of abrupt climate change in the past, scientists are investigating past greenhouse gas records and other evidence from the ice that will help to understand why and how abrupt changes occur. …"

Lastly, I would like to note that the borehole information from Lake El'gygytgyn in Russia has comparable timescales as that from the WAIS Divide ice core program, and as indicated in the following linked article, the researchers have compared findings from the two different hemispheres and they have found indications that paleo - Polar Amplification has been greater than previously recognized raising the likelihood that Earth System Sensitivity is likely greater than previously recognized:

Brigham-Grette, Julie; Melles, Martin; Minyuk, Pavel, et al., (2013) "Millennial scale change from Lake El’gygytgyn, NE Russia: Did we step or leap out of the Warm Pliocene into the Pleistocene?"
http://instaar.colorado.edu/meetings/AW2013/abstract_details.php?abstract_id=78

The Lake El’gygytgyn region of Russia seems to have been considerably warmer during MIS 11c [the Holsteinian peak] than it was during MIS 5e [the Eemian peak]. This is despite the fact that summer solar radiation was less intense (though the season was longer) and greenhouse gas concentrations were similar. The researchers of sediment in the lake write, “Consequently, the distinctly higher observed [temperature and precipitation] at MIS 11c cannot readily be explained by the local summer orbital forcing or GHG concentrations alone, and suggest that other processes and feedbacks contributed to the extraordinary warmth at this interglacial, and the relatively muted response to the strongest forcing at MIS 5e.”  The Arctic is especially sensitive to climate changes (through the loss of reflective snow and ice, for example), and what happens there affects the rest of the planet as well. Figuring out which feedbacks could account for the warm temperatures during MIS 11c could be useful.  Seeing how climate responds to many different situations helps researchers obtain a deeper understanding of the climate system. And therein lies the value in climate records from disparate regions. As the researchers put it, “The observed response of the region’s climate and terrestrial ecosystems to a range of interglacial forcing provides a challenge for modeling and important constraints on climate sensitivity and polar amplification.”
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AbruptSLR

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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #6 on: August 05, 2014, 04:15:14 AM »
Obviously, the internal structure of the AIS is dependent on its history; however, I am having a hard time getting a clear answer as to how old the bottom ice is for the WAIS Divide ice core.  I believe that when the project was initially announced they had a goal of retrieving a core with bottom ice with about a 100 ka age.  However, the following abstract from a Nineteenth Annual WAIS Workshop, 2012, paper indicates that the ice at the bottom of the recovered core was about 62 kya younger than expected.  It this is report is correct, then either the bottom of the ice sheet in this area is melting much faster than expected (which may well be the case as high basal melt rates have been documented in this area), or that during the Eemian much more of the ice in this area melted and was slow to comeback:

"High Basal Melt at the WAIS--‐Divide ice--‐core site T.J. Fudge, Gary Clow, Howard Conway, Kurt Cuffey, Michelle Koutnik, Tom Neumann, Kendrick Taylor, and Ed Waddington

We use the depth-age relationship and borehole temperature profile from the WAIS-Divide ice core site to determine the basal melt rate and corresponding geothermal flux. The drilling of the WAIS-Divide ice core has been completed to 3400 m depth, about 60 m above the bed. The age of the deepest ice is 62 ka, younger than anticipated, with relatively thick annual layers of ~1 cm.  The borehole temperature profile shows a large temperature gradient in the deep ice. We infer a basal melt rate of 1.5 (±0.5) cm yr-1 using a 1-D ice flow model constrained by these data sets. The melt rate implies a geothermal flux of ~230 mW m-2, three times the measured value of 70 mW m-2 at Siple Dome.

We compile radio-echo sounding data sets to assess the spatial extent of high melt. Deep internal layers are the most useful for inferring spatial patterns of basal melt. Unfortunately, the IceBridge WAIS-core flight and two site-selection surveys did not image consistent reflectors deeper than Old Faithful (2420 m and 17.8 ka). A ground-based survey by CReSIS (Laird et al., 2010) was able to image consistent layers as deep as 3000 m, but the survey is not oriented along the ice-flow direction making interpretation more difficult. There is no obvious draw down of deep internal layers that would indicate an area of localized melt. While this suggests a uniform melt rate within the survey, it might also indicate that other factors (e.g. accumulation gradients, rough bed topography) obscure the influence of basal melt on the internal layer depths."
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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #7 on: August 05, 2014, 04:29:31 PM »
Seeing as I implied in my last post that the age of the bottom ice at the WAIS Divide ice core might by as young as 100,000 - 62,000 = 38,000 years old, and that this might be due to a combination of high local basal melt rates and/or a slow recovery from a collapse of the ice sheet in the Byrd Subglacial Basin, BSM, during the Eemian; it seems like I should provide the most recent (and most accurate) estimate of the basal melt rates in the BSB as indicated in the attached image from Schroeder et al 2014 (see reference below).

Schroeder, D.M., Blankenship, D.D., Young, D.A. and Quartini, E., (2014), "Evidence for elevated and spatially variable geothermal flux beneath the West Antarctic Ice Sheet", PNAS, doi: 10.1073/pnas.1405184111

As there is no surface depression near the WAIS Divide ice core site, it is reasonable to conclude that any basal melting effect is influencing the entire basin, and that as the ice flow velocity in the BSB accelerates it will result in more internal ice friction, which in-turn will resulting in more internal ice melting, which in-turn will contribute to a more developed subglacial hydrological system, which in-turn will accelerate the ice flows even more, which in-turn will lead to more internal ice fracturing and crevasse formation; which (I estimate) will in-turn lead to Jakobshavn type calving (see the attached image of Jakobshavn's Southern Calving front on August 3 2014, posted by A-Team, showing extensive fracturing and crevasse formation) in the ASE marine glaciers circa 2050.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #8 on: August 05, 2014, 06:53:19 PM »
While the subject in this thread is internal ice layering (and internal ice structure), nevertheless in my past two posts I have tied the internal ice structure with increasing ice flow velocity to increasing friction and internal ice melting and to the subglacial hydrological system.  Therefore, the accompanying figures are from Livingstone et al (2013), and they show the extensive subglacial hydrological systems in Antarctica (with increasing warming [due to: surface, ocean, basal, basal friction, albedo, surface melting] these systems should become more extensive and important in the future):

S. J. Livingstone, C. D. Clark, and J. Woodward, (2013), "Predicting subglacial lakes and meltwater drainage pathways beneath the Antarctic and Greenland ice sheets", The Cryosphere Discuss., 7, 1177–1213, www.the-cryosphere-discuss.net/7/1177/2013/, doi:10.5194/tcd-7-1177-2013

The caption for the first image is: "In (B), the blue colour illustrates regions below the pressure melting point. This is used as a simple mask to remove all subglacial lakes that fall within the cold-bedded zones. Note, the subglacial drainage network is still treated as though the bed was wholly warm based."

The second image is a close-up of an area taken from the first image, but focused on the WAIS area, and which shows subglacial hydrological systems in the ASE drainage basins.
With regard to the last figure of the ASE drainage basins I would like to emphasize how close they are to each other, and as ice mass lost from one drainage base it will influence and accelerate the ice mass lost in the adjoining basins due to lost lateral support and increased lateral shear in the ice on the drainage boundaries.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

AbruptSLR

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Re: Internal Layers of the Antarctic Ice Sheet
« Reply #9 on: August 07, 2014, 08:09:07 PM »
The first attached image from the following link indicates how steep the slope of the surface elevation is for the Thwaites Glacier, which provides plenty of driving force to deform the internal ice structure and to potentially accelerate the ice flow:

http://bprc.osu.edu/rsl/IST/documents/Cryosat_orbit_comp.pdf

The second attached image from the following linked reference compares the surface and bed elevations of various different WAIS ice streams (including Thwaites):

Neil Ross, Robert G. Bingham, Hugh F. J. Corr, Fausto Ferraccioli, Tom A. Jordan, Anne Le Brocq, David M. Rippin, Duncan Young, Donald D. Blankenship & Martin J. Siegert, (2012), "Steep reverse bed slope at the grounding line of the Weddell Sea sector in West Antarctica", Nature Geoscience, 5, 393–396, doi:10.1038/ngeo1468

http://www.nature.com/ngeo/journal/v5/n6/full/ngeo1468.html

Abstract: "The bed of the West Antarctic Ice Sheet is, in places, more than 1.5 km below sea level. It has been suggested that a positive ice-loss feedback may occur when an ice sheet’s grounding line retreats across a deepening bed. Applied to the West Antarctic Ice Sheet, this process could potentially raise global sea level by more than 3 m. Hitherto, attention has focussed on changes at the Siple Coast and Amundsen Sea embayment sectors of West Antarctica. Here, we present radio-echo sounding information from the ice sheet’s third sector, the Weddell Sea embayment, that reveals a large subglacial basin immediately upstream of the grounding line. The reverse bed slope is steep, with about 400 m of decline over 40 km. The basin floor is smooth and flat, with little small-scale topography that would delay retreat, indicating that it has been covered with marine sediment and was previously deglaciated. Upstream of the basin, well-defined glacially carved fjords with bars at their mouths testify to the position of a former ice margin about 200 km inland from the present margin. Evidence so far suggests that the Weddell Sea sector of the West Antarctic Ice Sheet has been stable, but in the light of our data we propose that the region could be near a physical threshold of substantial change."

The next linked reference (with a free access pdf), provides numerous images and data on the bed of Thwaites for modeling purposes; however, I note that Rignot et al (2014) demonstrated that there are no pinning point for the Thwaites bed to stop the Thwaites ice flow from accelerating:

John A. GOFF, Evelyn M. POWELL, Duncan A. YOUNG, Donald D. BLANKENSHIP, (2014), "Instruments and Methods Conditional simulation of Thwaites Glacier (Antarctica) bed topography for flow models: incorporating inhomogeneous statistics and channelized morphology", Journal of Glaciology, Vol. 60, No. 222, doi: 10.3189/2014JoG13J200


http://www.igsoc.org/journal/60/222/j13J200.pdf


ABSTRACT: "Thwaites Glacier, Antarctica, is experiencing rapid change and its mass could, if disgorged into the ocean, lead to _1m of global sea-level rise. Efforts to model flow for Thwaites Glacier are strongly dependent on an accurate model of bed topography. Airborne radar data collected in 2004/05 provide 35,000 line km of bed topography measurements sampled every 20m along track. At _15 km track spacing, this extensive dataset nevertheless misses considerable important detail, particularly: (1) resolution of mesoscale channelized morphology that can guide glacier flow; and (2) resolution of small-scale roughness between the track lines that is critical for determining topographic resistance to flow. Both issues are addressed using a conditional simulation that merges a stochastic realization (an unconditional simulation) with a deterministic surface. A conditional simulation is a non-unique interpolation that reproduces observed statistical behavior without affecting data values. Channels are resolved in the deterministic surface using an interpolation algorithm designed for sinuous channels.  Small-scale roughness is resolved using a statistical analysis that accounts for heterogeneity, including an abrupt transition between ‘lowland’ and ‘highland’ morphology. Multiple realizations of the unconditional simulation can be generated to sample the probability space and allow error characterization in flow modeling."

Edit: For convenience, I thought that I should provide the last two attached images from the Goff et al 2014 reference illustrating the internal ice structure and the bed elevations, respectively, for Thwaites.
« Last Edit: August 07, 2014, 08:16:43 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