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Author Topic: Under the Ice: A closer look at recent Antarctica and Greenland Ice Melt  (Read 13517 times)

prokaryotes

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I've compiled an excerpt from a recent research letter about ice melt in Antarctica nd Greenland.


Summary

Antarctica

  • Satellite radar altimetry since 2002 shows accelerated thinning (Amundsen Sea, Pine Island and Thwaites glacial ice streams)
    Laser altimetry shows thinning on 20 of 54 Antarctic ice shelves
    Ice shelves buttress their tributary glaciers, melt-induced thinning of the ice shelves drives a corresponding thinning and acceleration of the upstream glaciers
    Heat for basal melting occurs from wind-forced incursions of deeper and warmer water and from local surface waters warmed by summer sun
    Extensive melt-induced subglacial channels under Pine Island Glacier
    Bottom melt influences the structural integrity of the entire glacier
    Inland course and extent of, for example, troughs under Pine Island Glacier, follow tectonic rifts
    The rift systems, some of them sloping inward (landward), represent preferred routes for warm water penetration
    Basal melting has eroded and expanded a cavity under the Pine Island Ice Shelf, allowing more warm seawater (as warm as 4C) to access the underside
    Meltwater input to the surrounding ocean appears to have increased by 50% over a decade
    A newly discovered large subglacial basin deep in the interior of the Weddell Sea, under the present day Filchner Ice Shelf and its tributary glaciers
    Plausible redirection of warm coastal ocean currents into the Filchner trough beneath the Filcher-Ronneshelves As a consequence, basal melting increases by a factor of 20
    In general, a consistent picture emerges around Antarctica of ice and ice shelves responding rapidly via the ocean to changes in Southern Hemisphere wind pattern
    Patterns that themselves vary on timescales of years to decades in concert with global features such as El Niño–Southern Oscillation (ENSO)

Greenland

  • Marine-terminating glaciers drain nearly 90% of the Greenland ice mass
    Under-ice motions (basal sliding) play a very large role in dynamics of ice sheet’s
    Vertical uplift, in excess of post glacial rebound, due to rapid crustal response to recent ice mass losses
    Uplift ‘pulses’ correlated with short-lived events such as seasonal surface melt anomalies
    Greenland Ice Sheet interacting extensively and rapidly with surrounding ocean (see Fig) and overlying atmosphere

http://climatestate.com/magazine/2013/07/under-the-ice-a-closer-look-at-recent-antarctica-and-greenland-ice-melt/
« Last Edit: July 27, 2013, 01:37:16 AM by prokaryotes »

AbruptSLR

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Hi Prokaryotes,

The title of this article confuses me, because no where in the article do I see any mention that any "Coastal Antarctic Permafrost" is melting at all, let alone faster than expected.  Additionally, the I find the editorial style confusing.  Nevertheless, I find the references to the article helpful, so thanks for the post.

Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

prokaryotes

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The title of this article confuses me, [..] I find the editorial style confusing.
Fixed the headline. What in particular is confusing to you?  Thanks for your input.

AbruptSLR

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For one example, the article states:

"Basal melting has eroded and expanded a cavity under the Pine Island Ice Shelf, allowing more warm seawater (as warm as 4C) to access the underside."

I believe that this temperature must be potential temperature (or the difference between the temperature of the water and the melting temperature of the ice); because I do not believe that the CDW is 4C.

For another example, I am concerned that some readers might think that the following quote implies that the basal melt rate beneath the FRIS has already increased by a factor of 20 (as this has not happened yet but is projected to occur within several decades):

"Plausible redirection of warm coastal ocean currents into the Filchner trough beneath the Filcher-Ronneshelves As a consequence, basal melting increases by a factor of 20."
 
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

prokaryotes

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Notice that all the data there is referenced (see bottom article linkage)

"We conclude that the basal melting has exceeded the increase in ice inflow, leading to the formation and enlargement of an inner cavity under the ice shelf within which sea water nearly 4 °C above freezing can now more readily access the grounding zone." http://www.nature.com/ngeo/journal/v4/n8/full/ngeo1188.html

"Here we show that a redirection of the coastal current into the Filchner Trough and underneath the Filchner–Ronne Ice Shelf during the second half of the twenty-first century would lead to increased movement of warm waters into the deep southern ice-shelf cavity. Water temperatures in the cavity would increase by more than 2 degrees Celsius and boost average basal melting from 0.2 metres, or 82 billion tonnes, per year to almost 4 metres, or 1,600 billion tonnes, per year. Our results, which are based on the output of a coupled ice–ocean model forced by a range of atmospheric outputs from the HadCM35 climate model, suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern Weddell Sea due to thinning of the formerly consolidated sea-ice cover. The projected ice loss at the base of the Filchner–Ronne Ice Shelf represents 80 per cent of the present Antarctic surface mass balance6. Thus, the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of Antarctic ice streams and ice shelves, and global sea level rise." http://www.nature.com/nature/journal/v485/n7397/full/nature11064.html

AbruptSLR

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Prokaryotes,

I just hope that most readers realize that the freezing point of water under pressure is suppressed (see the attached phase diagram for water) so that when the article says "... nearly 4oC above freezing ..." it does not mean 4oC.

Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

prokaryotes

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Ok, what do you suggest i should add as a notice then? I think adding that graphic would be a bit to much?
 
The entire content is an excerpt from the linked research letter.

And are YOU sure you understand this correctly, is the water "really" under pressure?

"enlargement of an inner cavity under the ice shelf" - why should be there a special pressure? Isn't this how isostatic rebound functions? Cavities make room for the rebound, because there is a lack of pressure there.
« Last Edit: July 27, 2013, 02:41:12 AM by prokaryotes »

sidd

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Cavities would make room for rebound if they were empty. Unfortunately they are filled with sea water which is heavier than ice ...


prokaryotes

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Cavities would make room for rebound if they were empty. Unfortunately they are filled with sea water which is heavier than ice ...

Unfortunately i need some more evidence, since the data in question is published on Nature. To me it is not clear that these cavities are entirely filled with sea water.

Please provide some background information - a study if possible, thanks!

AbruptSLR

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Prokaryotes,

First, regarding the temperature of the warm CDW accelerating the melting of the subglacial cavities, according to Wikipedia (see link below) its temperature typically ranges between 1oC and 2oC; and many measurements in the trough outside of PIG have recorded temperatures of 1.6oC (see my other posts on this matter).  The difference between 1.6oC temperature and 4oC temperature difference that you cited is due to the suppression of the freezing point of water (which changes with depth).  The important implication of this is that, in the near future, as the grounding line, GL, of PIG and Thwaites Glacier, TG, retreat down into their respective basins the water depth will become greater so the temperature difference between the CDW and the melting point of the ice will become slightly larger resulting in more melting.  While of even greater importance, is that as the TG GL retreats down into the Byrd Subglacial Basin, BSB, the width and height of the contact area between the CDW and the ice near the GL will increase markedly, resulting much larger future volumes of ice mass loss.

http://en.wikipedia.org/wiki/Circumpolar_deep_water

Second, regarding your discussions with Sidd about bottom pressure relief contributing to Post-Glacial Rebound (or GIA): (a) It goes without saying that the subglacial cavities are filled with water, because it is the warmth of the water in the cavities that is forming the cavities in the first place (and there is no way that air could get down to those depths); (b) the reason for the rebound is the thinning of the ice that is, or recently was, resting on the ground; but what counts is the volume (or weight) of ice above floatation, VAF, as this is the only weight that is removed from the earth's crust as the ice thins; because once the ice floats, the weight of the seawater on the crust is more or less constant no matter how much further the ice shelf/tongue thins.  For example, before the subglacial cavity was formed the ice used to be resting on the ground and now it is not so there is rebound, but this rebound is relative limited as the ice just upstream of the GL was so thin that it was almost ready to float; but when you look at the GIA corrections that I posted in the "Tectonic" thread you will see that in the BSB the corrections are made thousands of kilometers upstream from the GL because the ice is thinning there also, just like a mound of honey thins everywhere as it spreads out.  When the WAIS collapses, the rebound will be much greater at the deepest parts of the BSB than along the current coastal areas, because the VAF near the middle of the BSB is very large.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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prokaryotes

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and there is no way that air could get down to those depths

There are tiny air bubbles which gets released during the melting. Further does the warm water penetration depending on local topography creates tunnels with flawing ice discharge and a mix of warmer ocean current and melt water.

Update
With tunnels i meant a combination of rifts.

Here we report the discovery of a subglacial basin under Ferrigno Ice Stream up to 1.5 kilometres deep that connects the ice-sheet interior to the Bellingshausen Sea margin, and whose existence profoundly affects ice loss. We use a suite of ice-penetrating radar, magnetic and gravity measurements to propose a rift origin for the basin in association with the wider development of the West Antarctic rift system. The Ferrigno rift, overdeepened by glacial erosion, is a conduit which fed a major palaeo-ice stream on the adjacent continental shelf during glacial maxima6. The palaeo-ice stream, in turn, eroded the ‘Belgica’ trough, which today routes warm open-ocean water back to the ice front7 to reinforce dynamic thinning. We show that dynamic thinning from both the Bellingshausen and Amundsen Sea region is being steered back to the ice-sheet interior along rift basins. We conclude that rift basins that cut across the WAIS margin can rapidly transmit coastally perturbed change inland, thereby promoting ice-sheet instability.
http://www.nature.com/nature/journal/v487/n7408/full/nature11292.html

the reason for the rebound is the thinning of the ice that is, or recently was, resting on the ground; but what counts is the volume (or weight) of ice above floatation, VAF, as this is the only weight that is removed from the earth's crust as the ice thins; because once the ice floats, the weight of the seawater on the crust is more or less constant no matter how much further the ice shelf/tongue thins.

But the ice does not float (unless a threshold of fundamental structural integrity creates motion), there are tunnels down there and local topographical features, the water is just creating a way through the lowest elevation.

.. when you look at the GIA corrections that I posted in the "Tectonic" thread
Already on my reading list.

I appreciate your input AbruptSLR, however as i pointed out above i rather stick to the published paper, until you or someone else can present equal validated data.
 
« Last Edit: July 27, 2013, 06:36:04 PM by prokaryotes »

AbruptSLR

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Prokaryotes,

You are free to post any information that you want on your own website, and furthermore it is not possible for me to cite any reference that would state that the subglacial cavities do not have air inside of them, because no scientist would even think it necessary to discuss thus a low probability possibility.  You need to realize that the subglacial cavities are formed because of a buoyancy driven effect of light fresh melt water floating upwards (and then out of the cavity); which then draws warm CDW along the bottom of the trough to the grounding line, where the heat of the CDW melts more ice causing more fresh water that results in a sustaining pump action that advects the warm CDW into (and then out of) the subglacial cavities.  Any air coming out of the ice would further float out faster than the fresh water thus accelerating the pumping action (not forming any meaningful air pockets).

With regard to your statements about VAF: (a) the glacial ice is currently flowing due to gravity, which causes it to thin (just like honey thins as it flows due to gravity); (b) when this ice thins sufficiently (due to gravitational thinning, or basal ice melting) it floats downstream of the grounding line (which is why the grounding line is found at the upstream tip of the subglacial cavity); lastly (c) the downstream ends of marine terminating glaciers and all of marine ice sheets are held down (grounded) on the seafloor by the weight of the VAF, and while the grounded ice can calve at the face of the marine glacier and fall directly into the ocean (and float away as in the case of the Jakobshaven Effect, or my postulated Thwaites Effect (see the "PIG/Thwaites 2102 to 2040-2060" thread), it is currently more common (see the Ross Ice Shelf, the Filchner Ronne Ice Shelf, the Thwaites Ice Tongue, the Pine Island Ice Shelf, etc) for the glacial ice to thin sufficiently for the ice downstream of the grounding line, GL, to float (as an ice shelf or an ice tongue) while still being physically connected to the grounded glacial ice upstream of the GL.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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prokaryotes

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You are free to post any information that you want on your own website
Yes, and i choose to publish the peer reviewed research rather than a forum post, which is not backed up by hard evidence or any kind of source to help evaluate.

Feel free to add your commentary under said article, you are welcome.
« Last Edit: July 27, 2013, 07:26:15 PM by prokaryotes »

AbruptSLR

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In an attempt to reduce confusion: (a) I concur with your peer reviewed paper that some rebound is occurring in the seafloor of the subglacial cavities, because the GL used to be downstream of the present location of the cavity; which implies that some weight has been removed from the seafloor so some rebound must be occuring; nevertheless, (b)  I agree with Sidd's point that this rebound is limited because the weight of the water in the cavities is comparable to the weight of the ice immediately upstream of the GL.  As more and more ice is melted, and the GL retreats more and more, there will be more and more rebound.

All of this discussion is directed towards your quote below:

"And are YOU sure you understand this correctly, is the water "really" under pressure?

"enlargement of an inner cavity under the ice shelf" - why should be there a special pressure? Isn't this how isostatic rebound functions? Cavities make room for the rebound, because there is a lack of pressure there."

In direct response to your question: Yes, I am very certain that the water in the subglacial cavities are under hydrostatic pressure (which increases with depth); but no I do not think that there is any "special pressure" within the water with the cavities, as the freezing point of sea water is suppressed everywhere in ocean according to the hydrostatic pressure of the water at depth according to the phase diagram that I provided.  The earth's crust does not care whether the weight on it is water, ice, or some other solid weight; so if you change the weight the crust with either move up or down depending on the change in weight, and the historical changes in weight, and the local 3D mantle mechanics.

Best,
ASLR
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

prokaryotes

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Re speculation from above: subglacial cavities are always filled with water


Source http://homepages.ulb.ac.be/~desamyn/Tsanfleuron%20Glacier%2006.html

prokaryotes

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Depositional model in subglacial cavities, Killiney Bay, Ireland. Interactions between sedimentation, deformation and glacial dynamics

Abstract
Subglacial meltwater drainage and sedimentary processes play a major role in ice-sheet dynamic but there is a lack of study of subglacial environment because modern ice-sheet beds remain inaccessible. Previous authors already intended to provide diagnostic criterion and recent investigations suggest that fluid pressure variations are a key factor in subglacial environment. This paper investigated the late Devensian sedimentary record in order to describe subglacial sedimentological facies associations and deformation features related to fluid overpressures. We used an integrated approach, based on stratigraphy, sedimentology and deformations styles to demonstrate a subglacial depositional model. The studied interval is composed of stratified gravel and sand interbedded with diamicton and boulder pavement, deposited in depressions formed by irregularity of the upper surface of diamicton. Deformation structures include convolutes, dykes and normal micro-faulting. Dykes show different dip directions from vertical, oblique to subhorizontal from which both directions of shortening and extension can be determined. Vertical dykes are formed under pure shear strain related to ice weight only. Oblique dykes imply both ice-bed coupling and simple shear strain between ice and substrate induced by flowing ice related to progressively increasing meltwater drainage intensity. Horizontal dykes are formed when minimum strain is vertical and therefore the overpressure exceeds the weight of overburden. They are associated with high meltwater drainage intensity and ice-bed uncoupling and refer to hydrofracturing. Overall, depositional and deformation sequence also illustrates the increasing intensity of meltwater drainage in small cavity as high energy channelised deposits, and in large cavities where subaqueous fan are deposited under hydraulic jump conditions. Moreover, large cavities represent lee-side cavities formed by fast-flowing ice over an obstacle. Hydrofracturing is likely to occur when a dense fluid, potentially associated with catastrophic drainage of an upstream cavity enters the low-pressure confined environment of a downstream cavity and is injected under pressure in the soft substrate. The studied glacial sequence represents a regional pattern probably related to an allocyclic control on sedimentation linked to long-term glacial dynamics rather than local depositional conditions. Based on these results, we provided a synthetic model linking depositional and deformation processes during ice-sheet growth and decay, but also valid at different timescales from annual to seasonal scale.
http://www.sciencedirect.com/science/article/pii/S0277379111003866

prokaryotes

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Here is a bit more on subglacial cavieties and water pressure...

Drainage through subglacial water sheets
Timothy T. Creyts
Department of Earth and Planetary Science, University of California, Berkeley, California, USA
Christian G. Schoof
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada
Abstract.
Subglacial drainage plays an important role in controlling the coupling be-
tween glacial ice and the underlying bed. Here, we study the ∞ow of water in thin, macro-
porous sheets between ice and bed. Previous work shows that small perturbations in wa-
ter depth for a nearly parallel-sided water Ølm between ice and bed grow unstably be-
cause areas with greater water depth have enhanced viscous dissipation that leads to en-
hanced melting of the ice roof. We argue that in the presence of bed protrusions bridg-
ing a water sheet, the sheet can be stabilized by downward motion of the ice roof around
protrusions. This should be the case provided the rate of roof closure increases faster
with water depth than the rate of viscous dissipation within the sheet. We therefore mod-
ify existing theory to include protrusions that partially support the mass of the over-
lying glacier. DiÆerences in the pressure driving on these protrusions relative to the wa-
ter sheet drive roof closure. The roof closure rate includes both the eÆects of regelation
and creep normal to the bed as mechanisms by which the overlying ice can move down-
ward, closing the ice{bed gap occupied by the water. The roof closure rate includes the
mechanisms of both regelation and creep normal to the bed allowing ice to move down-
ward, closing the ice{bed gap occupied by the water. In order to account for multiple
protrusion sizes at the bed (for instance, resulting from an assortment of various-sized
sediment grains), we incorporate a method for partitioning overburden pressure among
the diÆerent protrusion size classes and the water sheet. This method allows prediction
of the rates of downward motion for the ice roof. Rates are dependent on the degree of
ice-protrusion contact and therefore on water depth. We then investigate the possibil-
ity of stable, steady sheet conØgurations for reasonable parameter choices, and Ønd that
these steady stats can occur for modest sheet thicknesses at very low eÆective pressures,
as is appropriate for instance of ice streams
. Moreover, we Ønd that multiple steady sheet
thicknesses exist allowing for the possibility of hydraulic switches between low and high
conductivity regimes for the subglacial hydrological system.
http://www.eos.ubc.ca/~cschoof/sheetflow-preprint.pdf

Further


Friction at the glacier bed is deter- mined in large part by eÆective pressure, usually deØned as the diÆerence between ice overburden and subglacial water pressure. This is the case for both deformable and rigid glacier beds [ e.g., Paterson , 1994, Chaps. 7,8]. For glaciers and ice sheets with water at the bed, any predictive theory of ice dynamics requires a component that describes evolu- tion of eÆective pressure, that is, a theory for drainage at the ice{bed interface. To determine the distribution of eÆective pressure at the glacier bed requires an understanding of the morphology of the subglacial drainage system and of the relationship between water discharge, eÆective pressure, and hydraulic gradient in individual drainage elements. A drainage sys- tem can consist of diÆerent types of individual elements: for instance, channels, linked cavities, canals, englacial or groundwater ∞ow, or a combination of any of these [ e.g., Fountain and Walder , 1998; Hubbard and Nienow , 1997]. While theories exist for the behavior of individual drainage elements, interactions between any of these elements are not to grow. Consequently, linked cavity systems tend to form a distributed drainage network.


Once a drainage system is established subglacially, its re- sponse to water input is determined by the relationship be- tween ∞ux on one hand and eÆective pressure and hydraulic gradient on the other. Usually, systems such as R-channels that contain more water at high rather than low eÆective pressure will also transmit higher ∞uxes at high rather than low eÆective pressure. Conversely, distributed drainage sys- tems such as linked cavities, in which water storage is facili- tated by low eÆective pressures, need not have such a simple, monotonic relationship between eÆective pressure and ∞ux. For instance, the linked cavity theory of Fowler [1987] pre- dicts increasing ∞ux with increasing eÆective pressure (as R- channel theory does) while the canal theory of Walder and Fowler [1994] predicts the opposite. To model a drainage system thus requires the physics that determines water ∞ux at a given eÆective pressure to be understood. In short, a theory for subglacial drainage must incorpo- rate two fundamental pieces: a functional relationship be- tween water storage and eÆective pressure, and a means of determining water ∞ux in terms of eÆective pressure and hydraulic gradient,


Further i assume that AbruptSLR is following Weertman and Walder, rather than the newer approach by Timothy T. Creyts & Christian G. Schoof
Sheet-like drainage elements have been considered previ- ously, for instance by Weertman [1972] and Walder [1982]. The main diÆerence between their notion of a water Ølm and our notion of a water sheet is that we consider an ice roof that is partially supported by contact with the bed | as is also the case in a linked cavity system | while in Weertman's and Walder's cases, ice and bed are everywhere separated by water, so the ice is eÆectively a∞oat on a thin water Ølm. In general, we expect complete ∞otation of the ice on a thin water Ølm not to occur, ut unevenness in the bed to lead to partial contact.

As we shall outline next, this is a crucial diÆerence which allows our water sheet to remain stable while Walder's Ølm conØguration necessarily leads to channelization. From this point forward, when discussing subglacial drainage, we make the distinction that a water sheet has partial contact between the ice roof and sediment ∞oor. On the contrary, a water Ølm everywhere supports the overlying ice as described by Walder [1982] and Weertman [1972].


If the ice is at the pressure melting point throughout, then a pressure diÆerence ¢ æ in the ice near the contact with the bed protrusion will cause a tem- perature gradient that leads to a melt/freeze pattern that allows the ice to move downward [ Paterson , 1994, Chap. 7]. This temperature gradient will be of magnitude Ø ¢ æ=r e , where r e is a radius of ice{protrusion contact area and Ø is the rate of change of ice melting temperature with pres- sure from the Clausius-Clapeyron relationship [ e.g., Wagner et al. , 1994]. If K denotes thermal conductivity, then the associated heat ∞ux is KØ ¢ æ=r e , leading to a regelation velocity v r = KØ ¢ æ Ω i Lr e


The regelation process described here is driven by the pressure diÆerence ¢ æ around the contact area between ice and the bed protrusion. This is related to eÆective stress, and we expect ¢ æ to increase with æ e . More accurately, we can estimate ¢ æ as follows. Consider an arbitrary area S i of the lower boundary of the ice that will be partially in contact with the bed and partially supported by the water sheet.
Let overburden stress æ i act normal to this area. De- note by S s the part of this area that is in contact with the bed protrusions, and let æ s be normal stress at these ice{bed contacts. Then S w = S i ° S s is the part of the ice roof in contact with water, and we denote water pressure by p w . Force balance requires S i æ i = S w p w + S s æ s ; (11) so that ¢ æ = æ s ° p w = S i S s ( æ i ° p w ) = S i S s æ e : (12) In other words, the driving pressure diÆerence ¢ æ in the regelation process is eÆective pressure divided by the frac- tion of the ice roof occupied by ice{bed contacts.

Equation (10) therefore becomes v r = ØK Ω i Lr e S i S s æ e : (13) The total downward motion v of the ice is then simply given by the superposition of viscous creep and regelation, v = v c + v r :


Conclusions

Here, we have extended previous work [ e.g., Walder , 1982; Weertman , 1972] to show that distributed water sheets can be stable to much greater depth than previously quan- tiØed. The presence of protrusions that bridge the ice{bed gap can stabilize distributed sheets. Stabilization occurs because areas of greater water depth (and therefore those areas that are actively increasing water depth due to ice melt from enhanced viscous dissipation) can be oÆset by en- hanced downward closure of an ice roof. This mechanism relies on a Ønite diÆerence between overburden and water pressure ( i.e., a Ønite eÆective pressure) driving downward closure. This feature stands in contrast to water Ølms with- out bed protrusions Walder [ e.g., 1982] where only water pressure balances ice overburden.

In constructing our theory, we have developed a recursive formulation for computing the partition of stresses between diÆerent protrusion sizes that exist at the bed and related these stresses to the downward motion of the ice through both viscous creep and regelation mechanisms. As a result, we are able to relate the closure velocity of the ice roof above the water sheet to eÆective pressure and sheet thickness. A steady state water sheet can then be formed if the melt rate of the ice roof due to viscous dissipation in the sheet bal- ances the closure velocity. Steady state sheets of this form can, however, only persist if they are also stable , that is, if a small departure from steady state thickness leads to a negative feedback that returns thickness to its steady state value. This requires that a small thickening of the sheet from steady state should lead to a larger increase in down- ward ice velocity than the corresponding increase in melt rate. In turn, this is the case if a thickening of the sheet leads to a signiØcant loss of contact between ice and bed protrusions.

Our theory predicts that such stable steady states do ex- ist, and in fact, for beds with multiple protrusion sizes, mul- tiple stable steady states can exist. Switches between these steady states can then lead to abrupt switches in water dis- charge in the drainage system. Future work will extend our theory to take account of spatial variations in eÆective pres- sure and hydraulic gradient, and to understand the eÆects of potential hydraulic switches



Example of "hydrostatic water pressure exceeded the ice pressure at the bottom of the cavity"

Detection of a subglacial lake in Glacier de Teˆte Rousse (Mont Blanc area) http://lgge.obs.ujf-grenoble.fr/IMG/pdf/j11J179.pdf




Re 4C temp - so still a question to what degree near 4C temps can be considered "slightly above the freezing temp"

In warm-based glaciers, also called temperate glaciers - with the bottom near or slightly above the freezing temperature, water flows in or on the bottom of the glacier forming subglacial streams in channels, that finally join at the snout, forming a glacier outlet.

There are three types of subglacial channels, depending on such factors like glacier movement, bedrock topography and lithology:

N-channel or Nye-channel: incised in the underlying bedrock.

R-channel or Röthlisberger-channel: incised in the ice (and so not found in "fossil" form).

C-channel or Clarke-channel: partly incised in the bedrock and ice, a combination of the formerly mentioned types.
http://rockglacier.blogspot.de/2009/09/geologists-who-say-nye.html
« Last Edit: July 27, 2013, 10:47:00 PM by prokaryotes »

prokaryotes

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Flotation and free surface ow in a model for subglacial drainage. Part I: Distributed drainage

(Received 30 January 2012)
We present a continuum model for melt water drainage through a spatially distributed
system of connected subglacial cavities, and consider in this context the complications in-
troduced when eff ective pressure or water pressure drops to zero
. Instead of unphysically
allowing water pressure to become negative, we model the formation of a partially vapour-
or air- lled space between ice and bed. Likewise, instead of allowing sustained negative
e ective pressures, we allow ice to separate from the bed at zero e ective pressure. The
resulting model is a free boundary problem in which an elliptic obstacle problem deter-
mines hydraulic potential, and therefore also determines regions of zero e ective pressure
and zero water pressure. This is coupled with a transport problem for stored water, and
the coupled system bears some similarities with Hele-Shaw and squeeze lm models.
We present a numerical method for computing time-dependent solutions, and nd close
agreement with semi-analytical travelling wave and steady state solutions. As may be
expected, we nd that ice-bed separation is favoured by high uxes and low ice surface
slopes and low bed slopes, while partially lled cavities are favoured by low uxes and
high slopes. At the boundaries of regions with zero water or e ective pressure, disconti-
nuities in water level are frequently present, either in the form of propagating shocks or
as stationary hydraulic jumps accompanied by discontinuities in potential gradient.
http://www.sfu.ca/~mawerder/docs/Schoof_etal_2012.pdf

LOL

prokaryotes

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I assembled everything i collected so far about this topic, in a blog post...


A closer look at subglacial glacier cavities and water pressure http://climatestate.com/magazine/2013/07/a-closer-look-at-subglacial-glacier-cavities-and-water-pressure/

AbruptSLR

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Prokayotes,

Typepad just ate my more thoughtful and detailed response (which is too bad because I am not going to re-create a post of that length); but I will say that I am mostly thinking of cases that apply to the WAIS because I am mostly concerned about the risk of abrupt SLR; and essentially all glaciers in West Antarctica drain into the ocean, so that air would not enter them, such as the mountain glaciers that you refer to in your posts.  Also, I have the bad habit of referring to "subglacial cavities" as only being those formed by advective melting processes associate with warm ocean water (as in the case of PIG/PIIS); where the pressure in the cavity matches that of the adjoining ocean (where the warm water comes from), and in these cases rebound occurs.  In most WAIS subglacial hydrological systems (subglacial lakes/swamps/streams) the water pressure is close to the weight of the ice above it, so in cases like Thwaites Glacier where this ice is thinning both from the gravity flow and from basal ice melting there is rebound; but where that ice is not thinning there is no new rebound (unless it is recovering from paleo-ice mass loss).

Most of your posts seem more relent to land based glaciers, rather than to the WAIS which is a marine ice sheet.
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dorlomin

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Interesting links there prokaryotes, lots of reading. Many thanks for your time.
Take it for granted you are wrong.
Just try to work out what about and why.

AbruptSLR

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As there are essentially no subglacial hydrological systems in the Antarctic that are free draining to the atmosphere, as discussed by Prokayotes; I believe that his interesting links would be better posted in either the Mountain Glaciers folder, or the Greenland folder, and not in the Antarctic folder.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

sidd

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My post was specifically about the cavity under PIG shelf. I am quite sure thats fulla water, I think they tried sending
a sub in

prokaryotes

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As there are essentially no subglacial hydrological systems in the Antarctic that are free draining to the atmosphere

Citation needed.

Beneath the Antarctic Ice Sheet, these subglacial drainage channels are connected to numerous subglacial lakes
http://climatestate.com/magazine/2013/07/glacier-hydrology-an-introduction/#SECTION_1

Several hundred (379) subglacial lakes have now been mapped [..]These lakes, including Lake Vostok, appear to drain and fill slowly.[..]
NASA’s repeat-track Ice, Cloud and Land Elevation Satellite (ICEsat) uses laser altimeter data to measure height changes at a high resolution. ICEsat satellite observations of subglacial lakes has shown that the ice surface above subglacial lakes is constantly changing, suggesting that water is flowing between lakes[5,8]. These lakes, identified in regions of ice-stream onset zones, and characterised by changes in elevation, are considered active, whilst the lakes identified by RES under thicker ice under Dome C, for example, are considered inactive[5]. Smith et al. (2009) recently identified that, out of the 280 lakes mapped, there were 124 active lakes under the Antarctic Ice Sheet. These lakes are rapidly changing in elevation, filling and draining on timescales of months to years.

The drainage of these lakes has been linked to accelerated ice flow rates, for example, in the Byrd Glacier system (10% faster for more than one year)[5, 11].  Lakes may exchange water with other lakes, or with the surrounding glacier environment, via englacial or subglacial conduits.
http://www.antarcticglaciers.org/modern-glaciers/subglacial-lakes/

If there is drainage, especially of subglacial lakes near coastel areas, there ought to be open cavities, since the water is following the lowest topography.
« Last Edit: July 28, 2013, 11:58:16 AM by prokaryotes »

prokaryotes

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My post was specifically about the cavity under PIG shelf. I am quite sure thats fulla water, I think they tried sending a sub in


What Lies Beneath: NASA Antarctic Sub Goes Subglacial http://climatestate.com/2013/07/28/what-lies-beneath-nasa-antarctic-sub-goes-subglacial/

prokaryotes

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Yes, I am very certain that the water in the subglacial cavities are under hydrostatic pressure (which increases with depth); but no I do not think that there is any "special pressure" within the water with the cavities, as the freezing point of sea water is suppressed everywhere in ocean according to the hydrostatic pressure of the water at depth according to the phase diagram that I provided.  The earth's crust does not care whether the weight on it is water, ice, or some other solid weight; so if you change the weight the crust with either move up or down depending on the change in weight, and the historical changes in weight, and the local 3D mantle mechanics.


In addition to the possibility of diff erent styles of drainage, models also have to contend with channels and cavities that may only be partially filled with water, or even with no water at all (see e.g. Fowler 1987, for a discussion). This is most likely when melt water input is low or under thin ice, and is commonly observed near glacier termini. Most current models do not describe partially filled channels and cavities, and instead unphysically predict negative water pressures, a notable exception being the partially filled channel model of Schuler & Fischer (2009). At the opposite extreme, existing models often predict large negative eff ective pressures in response to increases in water input, corresponding to water pressure signi ficantly exceeding overburden (e.g. Pimentel & Flowers 2010; Schoof 2010). The physics built into these models is however not intended to capture the rapid opening of an ice-water gap that should ensue, which has been modelled as an elastic hydrofracture problem in Tsai & Rice (2010).
http://www.sfu.ca/~mawerder/docs/Schoof_etal_2012.pdf

AbruptSLR

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Prokaryotes,

Like Sidd, in my quote that you cite in the immediately preceeding post, I thought that we were talking about the "subglacial cavity" beneath the Pine Island Ice Shelf", and/or comparable cases that are directly connected to the ocean; with water pressures in equilibrium with those in the connected ocean.  In all of my preceeding posts in the Antarctic folder, I have used the term "subglacial cavity" rather sloppily to refer only to cavities directly connected to the ocean and created by the advective process driven by the buoyancy of melt water melted by warm CDW; and all the cases that you are referring to I have generally called "subglacial hydrological systems"; which in the Antarctic generally have hydraulic heads greater than in the Southern Ocean.

Further it appears to me that you are now mixing references about mountain glaciers with references about Antarctic Ice Sheets; therefore, I must conclude that either you are very confused, or that you are a Troll.  So either do better homework, or I will stop wasting my time responding.
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prokaryotes

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I thought that we were talking about the "subglacial cavity" beneath the Pine Island Ice Shelf
No, this entire discussion is not especially about the Pine Island Ice Shelf.

Further it appears to me that you are now mixing references about mountain glaciers with references about Antarctic Ice Sheets
Again citation needed.


I must conclude that either you are very confused, or that you are a Troll.
It is you who comes here and question the sourced peer reviewed science and it is you who post content without citation and now you begin to resort to ad hominem. Reported.

Neven

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Gentlemen, don't let the heat get to you. There's no need to lose patience over this.  :)
Il faut cultiver notre jardin

sidd

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"If there is drainage, especially of subglacial lakes near coastel areas, there ought to be open cavities, since the water is following the lowest topography."

1)once a cavern has drained and no more water is coming in or being generated, it will close thru ice creep and macroscopic motion in roughly
t= (linear dimension of lake)/ (ice flow velocity)

this is seen in greenland for example, the conduits draining a melt lake usually do not persist into the next year, in some cases close faster, melt lakes appear at new locations, which sculpt new conduits und so weiter.

2) As i remarked in another thread, subglacial water can flow uphill. Off the top of my head, the ice-water interface slope is about 10 times the ice-air interface slope, and in the opposite direction. So if you have a horizontal bed with a little hole in it fulla water, and the ice air surface above the hole slopes down in the downstream (ice flow) direction,  the surface of the water in the hole slopes up 10 times as much in the downstream direction. So the downstream side of the hole has to higher than the upstream side to confine the water. One can work out that in the case of retrograde bed (slopes downward upstream) that the slope has to be 10 times the ice air slope in downstream direction to confine water beneath.

Please do work the pressure/angle relations out yourself, I am quoting from memory, as always, I could be wrong.

AbruptSLR

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On the hope that the following information is constructive:

(1) In the first attached image of the Bedmap2 Antarctic topology, all the areas shaded in blue have bed elevations below sea level.  Thus it is reasonable to believe that any subglacial hydrological system in any of these blue areas would drain directly into the ocean, thus it is not reasonable to believe that there would be any basal cavities filled with air to allow rebound in any of these blue areas.  From this image it is clear that the majority of areas in both East and West Antarctica are below sea level.
(2) The second attached image shows a map of most documented subglacial hydrological systems; while the third attached image shows a computer projection of where one would expect to find subglacial hydrological systems based on bed warmth.  Again this indicates that the great majority of subglacial hydrological systems are below sea level.
(3) The average austral summer temperature in the Antarctic (besides portions of the Antarctic Peninsula) is below the freezing point of water at STP; therefore any water trying to drain from a subglacial hydrological systems located in any of the non-blue area of the figure, would freeze, forming a plug that would stop, or limit the assumed drainage.
(4) If for some reason a subglacial hydrological system were to drain so fast that the leaking water did not freeze, then most likely the cavity would collapse due to the weight of the glacial ice above the cavity.
(5) If none of the cases listed above are relevant in a given Antarctic situation, then it is physically possible for melt water to drain from a non-collapsing subglacial hydrological cavity and for air to remain in such a cavity for some period of time.
« Last Edit: July 29, 2013, 06:08:16 PM by AbruptSLR »
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AbruptSLR

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I believe that in the larger picture, it is of limited importance whether a subglacial hydrological system drains directly into the ocean, or whether it first drains onto land and then runs-off into the ocean (which may be more relevant in the Antarctic when its average austral summer temperature increases to that currently found in the summer in Greenland); as in either case the water mass change in the conduit is relatively small, while the mass change from the upstream melt source is potentially significant (with regard to measurable GIA and SLR contributions).  While it is not likely that surface ice melt will be a significant source of ice mass loss from the Antarctic anytime before 2050 - 2070; nevertheless, I believe that the GRACE mass loss measurements indicate that basal ice melt from large portions of the WAIS (including the BSB) may be currently significant, and will likely become more significant in the future.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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prokaryotes

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Identification and control of subglacial water networks under Dome A, Antarctica

Subglacial water in continental Antarctica forms by melting of basal ice due to geothermal or frictional heating. Subglacial networks transport the water from melting areas and can facilitate sliding by the ice sheet over its bed. Subglacial water flow is driven mainly by gradients in overburden pressure and bed elevation. We identify small (median 850 m) water bodies within the Gamburtsev Subglacial Mountains in East Antarctica organized into long (20-103 km) coherent drainage networks using a dense (5 km) grid of airborne radar data. The individual water bodies are smaller on average than the water bodies contained in existing inventories of Antarctic subglacial water and most are smaller than the mean ice thickness of 2.5 km, reflecting a focusing of basal water by rugged topography. The water system in the Gamburtsev Subglacial Mountains reoccupies a system of alpine overdeepenings created by valley glaciers in the early growth phase of the East Antarctic Ice Sheet. The networks follow valley floors either uphill or downhill depending on the gradient of the ice sheet surface. In cases where the networks follow valley floors uphill they terminate in or near plumes of freeze-on ice, indicating source to sink transport within the basal hydrologic system. Because the ice surface determines drainage direction within the bed-constrained network, the system is bed-routed but surface-directed. Along-flow variability in the structure of the freeze-on plumes suggests variability in the networks on long (10s of ka) timescales, possibly indicating changes in the basal thermal state.
http://adsabs.harvard.edu/abs/2013JGRF..118..140W

AbruptSLR

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While the following reference, abstract and attached image are generic in nature; the boundary conditions (which are comparable to that for the Ferrigno Glacier) assumed in this work indicate that "subglacial cavities" (or "ice cavities" as noted on the figure) are likely to grow in the Bellingshausen coastal sector, in a comparable fashion to the advective driven "subglacial cavity" that is extending beneath PIG from the PIIS:

On the Role of Coastal Troughs in the Circulation of Warm Circumpolar Deep Water on Antarctic Shelves

by: Pierre St-Laurent, John M. Klinck and Michael S. Dinniman; 2012, JPO.

Abstract:
"Oceanic exchanges across the continental shelves of Antarctica play an important role in biological systems and the mass balance of ice sheets. The focus of this study is on the mechanisms responsible for the circulation of warm Circumpolar Deep Water (CDW) within troughs running perpendicular to the continental shelf. This is examined using process oriented numerical experiments with an eddy-resolving (1 km) 3–D ocean model that includes a static and thermodynamically active ice shelf. Three mechanisms that create a significant onshore flow within the trough are identified: (1) a deep onshore flow driven by the melt of the ice shelf, (2) interaction between the longshore mean flow and the trough, and (3) interaction between a Rossby wave along the shelf break and the trough. In each case the onshore flow is sufficient to maintain the warm temperatures underneath the ice shelf and basal melt rates of O(1myr−1). The third mechanism in particular reproduces several features revealed by moorings from Marguerite Trough (Bellingshausen Sea): the temperature maximum at mid-depth, a stronger intrusion on the downstream edge of the trough, and the appearance of warm anticyclonic anomalies every week. Sensitivity experiments highlight the need to properly resolve the small baroclinic radii of these regions (5 km on the shelf): simulations at 3 km resolution cannot reproduce mechanism 3 and the associated heat transport."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Lennart van der Linde

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I'm not sure where to post this, but have a look at this paper (abstract) by Bassis & Jacobs just out:
http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1887.html

It is mentioned by Joe Romm here:
http://thinkprogress.org/climate/2013/07/30/2341511/greenland-antarctica-rapid-ice-loss/

Bassis & Jacobs conclude:
"[R]apid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration."

AbruptSLR

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For those interested, I have posted about the excellent Bassie & Jacobs 2013 paper in reply 46 in the PIG/Thwaites 2012 to 2040-2060 thread here:

http://forum.arctic-sea-ice.net/index.php/topic,72.0.html#lastPost
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Lennart van der Linde

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Ah, of course this had been posted already! Sorry for not checking more carefully :)

prokaryotes

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In regards to melting and temperature, Rignot 2017 has some explanation in regards to the freezing point https://youtu.be/AAPPq43iRLs?t=15m04s

TerryM

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I'm afraid I have not followed this thread.


Was the issue of freezing point at pressure finally resolved?


ASLR's excellent chart (reply #5), should provide all the needed information.


The freezing point of water remains virtually static from 10mbar to ~500bar (right scale). Since 500 bar =~ 5,000 meters of sea water depth, I think it's unlikely that water pressure is having much of a measurable effect on the freezing point.


Terry
If this has been resolved, or I've misunderstood the back and forth, please ignore this post.