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Author Topic: Subglacial Lake and Meltwater Drainage Systems  (Read 56550 times)

Lennart van der Linde

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Re: Subglacial Lake and Meltwater Drainage Systems
« Reply #50 on: July 10, 2015, 11:13:35 PM »
New paper by Fisher et al 2015 on geothermal heat flux under WAIS:
http://advances.sciencemag.org/content/advances/1/6/e1500093.full.pdf



AbruptSLR

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Re: Subglacial Lake and Meltwater Drainage Systems
« Reply #52 on: July 11, 2015, 01:46:47 AM »
Lennart,

Thank you for the excellent references.  For those who are interested, I provide the following extracts:

http://www.scientificamerican.com/article/high-heat-measured-under-antarctica-could-support-substantial-life/
Extract: "In 2014 scientists reported that one major West Antarctic ice stream, called Thwaites Glacier, sits atop several local hotspots (inferred using ice-penetrating radar and computer modeling). These could melt water and lubricate the glacier, says Duncan Young, a glaciologist with the University of Texas at Austin who was part of that study. The hotspots sit beneath several critical spots in the glacier’s inland tributaries, potentially increasing the supply of ice that is poured into the main trunk of the glacier—and eventually, the ocean, where it contributes to sea level rise. Thwaites Glacier is of particular interest because it is already accelerating and thinning in response to rising temperatures.
Much remains to be learned about the vast landscape hidden beneath the West Antarctic Ice Sheet but one possibility is becoming increasingly clear. Aerial surveys using ice-penetrating radar show numerous isolated high spots in the subglacial topography. These often correspond with strong magnetic anomalies—a marker of iron-rich lava rocks. “There have been at least three subglacial volcanoes identified under the ice sheet now,” Young says—“and we have suspicions of a bunch more”—perhaps hundreds. Dozens of these suspected volcanoes possess unusually squat profiles, suggesting that they actually erupted and grew while buried under the crushing weight of the ice sheet. At least one subglacial volcano is thought to be active right now—a submerged peak named Mount Casertz.
The upper surface of the ice sheet dips 50 meters as it flows over the buried crest of this volcano. Maintaining that low spot year after year is no small thing, because the crushing mass of the surrounding ice sheet should ooze inward and fill even a shallow depression. Calculations suggest that Mount Casertz exudes 700 million watts of geothermal heat—roughly equal to the energy produced by a medium-size nuclear power plant. It maintains the topographic depression above it by melting 70 million tons of water off the bottom of the ice sheet each year.
It’s entirely possible that Casertz or another of these hidden volcanoes could erupt in the future. No one believes that even a catastrophic eruption would rip apart the ice sheet—it’s simply too massive. But the meltwater that it produces could still cause a large glacier like Thwaites to speed up in a way that’s never been seen before."
“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: Subglacial Lake and Meltwater Drainage Systems
« Reply #53 on: July 29, 2015, 01:26:54 AM »
The linked reference discusses efforts to improve modelling of subglacial lakes, and as Antarctic subglacial lakes are subject to rapid drainage events, improved models could have predictive value.

Gudlaugsson, E., Humbert, A., Kleiner, T., Kohler, J., and Andreassen, K.: The influence of a model subglacial lake on ice dynamics and internal layering, The Cryosphere Discuss., 9, 3859-3886, doi:10.5194/tcd-9-3859-2015, 2015.

http://www.the-cryosphere-discuss.net/9/3859/2015/tcd-9-3859-2015.html

Abstract: "As ice flows over a subglacial lake, the drop in bed resistance leads to an increase in ice velocities and a subsequent draw-down of isochrones and cold ice from the surface. The ice surface flattens as it adjusts to the lack of resisting forces at the base. The rapid transition in velocity induces changes in temperature and ice viscosity, releasing deformation energy which raises the temperature locally. Recent studies of Antarctic subglacial lakes indicate that many lakes experience very fast and possibly episodic drainage, during which the lake size is rapidly reduced as water flows out. A question is what effect this would have on internal layers within the ice, and whether such past events could be inferred from isochrone structures downstream.

Here, we study the effect of a subglacial lake on the dynamics of a model ice stream as well as the influence that such short timescale drainage would have on the internal layers of the ice. To this end, we use a Full–Stokes, polythermal ice flow model. An enthalpy gradient method is used to account for the evolution of temperature and water content within the ice.

We find that the rapid transition between slow-moving ice outside the lake, and full sliding over the lake, releases large amounts of deformational energy, which has the potential to form a temperate layer at depth in the transition zone. In addition, we provide an explanation for a characteristic surface feature, commonly seen at the edges of subglacial lakes, a hummocky surface depression in the transition zone between little to full sliding. We also conclude that rapid changes in lake geometry or basal friction create a travelling wave at depth within the isochrone structure that transfers downstream with the advection of ice, thus indicating the possibility of detecting past events with ice penetrating radar."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

A-Team

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Re: Subglacial Lake and Meltwater Drainage Systems
« Reply #54 on: July 29, 2015, 05:55:57 AM »
It's an interesting enough idea and indeed ice sheets carry very long histories in their isochronal depositional layer lines. I am not familiar enough with Antarctic lakes to know if any are strategically positioned, ie where this history would be important in predicting 'trending' issues of our times.

Glancing at the full text graphics, it didn't seem like there were any available ice penetrating radar tracks over any of the lakes down there (it's a big place and radar coverage is not nearly as dense as Greenland).
 
If someone is up to submitting a comment pdf, it would not take ten minutes to chase down Cresis or other radar over what few lakes are available for Greenland, either holding the authors' feet to the fire for a missed prediction or applauding for a validatable one (here they should add you as a co-author for doing their homework).

On the technical side, for a really old lake, how much detail could the isochrons be expected to retain for how long? For what is a localized effect, I would expect some sort of relaxation scale in which the unaffected enveloping layers dampen out the effect over some distance/time, similar to what an errant region on a drum membrane experiences.

AbruptSLR

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Re: Subglacial Lake and Meltwater Drainage Systems
« Reply #55 on: December 03, 2015, 09:57:49 PM »
The linked (open access) reference discusses the subglacial hydrology of the Ross Ice Streams:

S. Goeller, V. Helm, M. Thoma, and K. Grosfeld (2015), "Subglacial hydrology indicates a major shift in dynamics of the West Antarctic Ross Ice Streams within the next two centuries", The Cryosphere Discuss., 9, 3995–4018, doi:10.5194/tcd-9-3995-2015

http://www.the-cryosphere-discuss.net/9/3995/2015/tcd-9-3995-2015.pdf

Abstract: "The mass export of the West Antarctic Ice Sheet (WAIS) is dominated by fast flowing ice streams. Understanding their dynamics is a key to estimate the future integrity of the WAIS and its contributions to global sea level rise. This study focuses on the Ross Ice Streams (RIS) at the Siple Coast. In this sector, observations reveal a high variability of ice stream pathways and velocities which is assumed to be driven by subglacial hydrology. We compute subglacial water pathways for the present-day ice sheet and verify this assumption by finding high correlations between areas of enhanced basal water flow and the locations of the RIS. Moreover, we reveal that the ice flow velocities of the individual ice streams are correlated with the sizes of the water catchment areas draining underneath. The future development of the subglacial hydraulic environment is estimated by applying ice surface elevation change rates observed by ICESat and CryoSat-2 to the present-day ice sheet geometry and thus assessing prognostic basal pressure conditions. Our simulations consistently indicate that a major hydraulic tributary of the Kamb and Whillans Ice Stream (KIS and WIS) will be redirected underneath the Bindschadler Ice Stream (BIS) within the next two centuries. The water catchment area feeding underneath the BIS is estimated to grow by about 50% while the lower part of the stagnated KIS becomes increasingly separated from its upper hydraulic tributaries. We conclude, that this might be a continuation of the subglacial hydraulic processes which caused the past stagnation of the KIS. The simulated hydraulic rerouting is also capable to explain the observed deceleration of the WIS and indicates a possible future acceleration of the BIS accompanied by an increased ice drainage of the corresponding ice sheet interior."

“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: Subglacial Lake and Meltwater Drainage Systems
« Reply #56 on: December 06, 2015, 02:38:22 PM »
The linked reference focuses on the findings of a synthetic setup designed to represent a simplified Recovery Ice Stream and catchment with one overdeepening; however, the findings are relevant to a good number of Antarctic subglacial lake filling and drainages systems including those for the Byrd Glacier and for the Thwaites Glacier (particularly in the Ice Tongue area), and describes the mechanism for cyclic filling & emptying of such subglacial Antarctic lake systems:


Dow, C. F., Werder, M. A., Nowicki, S., and Walker, R. T.: Modeling Antarctic subglacial lake filling and drainage cycles, The Cryosphere Discuss., 9, 6545-6579, doi:10.5194/tcd-9-6545-2015, 2015.

http://www.the-cryosphere-discuss.net/9/6545/2015/tcd-9-6545-2015.html

Abstract. The growth and drainage of active subglacial lakes in Antarctica has previously been inferred from analysis of ice surface altimetry data. We use a subglacial hydrology model applied to a synthetic Antarctic ice stream to determine internal controls on the filling and drainage of subglacial lakes and their impact on ice stream dynamics. Our model outputs suggest that the highly constricted subglacial environment of the ice stream, combined with relatively high rates of water flow funneled from large catchments, can combine to create a system exhibiting slow-moving pressure waves. Over a period of years, the accumulation of water in the ice stream onset region results in a buildup of pressure creating temporary channels, which then evacuate the excess water. This increased flux of water through the ice stream drives lake growth. As the water body builds up, it too steepens the hydraulic gradient and allows greater flux out of the overdeepened lake basin. Eventually this flux is large enough to create channels that cause the lake to drain. Due to the presence of the channels, the drainage of the lake causes high water pressures around 50 km downstream of the lake rather than immediately in the vicinity of the overdeepening. Following lake drainage, channels again shut down. Lake drainage depends on the internal hydrological development in the wider system and therefore does not directly correspond to a particular water volume or depth. This creates a highly temporally and spatially variable system, which is of interest for assessing the importance of subglacial lakes in ice stream hydrology and dynamics.
“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: Subglacial Lake and Meltwater Drainage Systems
« Reply #57 on: February 29, 2016, 06:53:42 PM »
The linked reference describes how height-change satellite-altimeter changes can be used to identify episodic subglacial meltwater discharge with sub-annual changes in ice velocities:

Matthew R. Siegfried, Helen A. Fricker, Sasha P. Carter & Slawek Tulaczyk (27 February 2016), "Episodic ice velocity fluctuations triggered by a subglacial flood in West Antarctica", Geophysical Research Letters, DOI: 10.1002/2016GL067758

http://onlinelibrary.wiley.com/doi/10.1002/2016GL067758/abstract

Abstract: "Height-change anomalies in satellite-altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes on timescales of months to years. These signals have been mapped throughout Antarctica, but only broad connections have been made between active lakes and ice dynamics. We present the first high-frequency observations of ice velocity evolution due to cascading subglacial lake drainage events using five years (2010–2015) of Global Positioning System data on Whillans and Mercer ice streams, West Antarctica. We observed three episodic ice velocity changes over two years, where flow speed increased by up to 4%. We also observed an eleven-month disruption of the tidally-modulated stick-slip cycle that dominates regional ice motion. Our observations reveal that basal conditions of an Antarctic ice stream can rapidly evolve and drive a dynamic ice response on sub-annual timescales, which can bias observations used to infer long-term ice-sheet changes."
“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: Subglacial Lake and Meltwater Drainage Systems
« Reply #58 on: April 28, 2016, 04:49:38 PM »
The linked (open access) reference discusses how some Antarctic subglacial lakes drain through sediment-floor canals:

Carter, S. P., Fricker, H. A., and Siegfried, M. R.: Antarctic subglacial lakes drain through sediment-floored canals: Theory and model testing on real and idealized domains, The Cryosphere Discuss., doi:10.5194/tc-2016-74, in review, 2016.

http://www.the-cryosphere-discuss.net/tc-2016-74/

Abstract. Over the past decade, satellite observations of ice surface height have revealed that active subglacial lake systems are widespread under the Antarctic ice sheet, including the ice streams. For some of these systems, additional observations of ice stream motion have shown that lake activity can affect ice-stream dynamics. Despite all this new information, we still have insufficient understanding of the lake-drainage process to incorporate it into ice sheet models. Process models for drainage of ice-dammed lakes based on conventional "R-channels" incised into the base of the ice through melting are unable to reproduce the timing and magnitude of drainage from Antarctic subglacial lakes estimated from satellite altimetry given the low hydraulic gradients along which such lakes drain. We have developed an alternative process model, in which channels are mechanically eroded into the underlying deformable subglacial sediment. When applied to the known active lakes of the Whillans/Mercer ice stream system, the model successfully reproduced both the inferred magnitudes and recurrence intervals of lake volume changes, derived from Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data for the period 2003–2009. Water pressures in our model changed as the flood evolved: during drainage, water pressures initially increased as water flowed out of the lake primarily via a distributed system, then decreased as the channelized system grew, establishing a pressure gradient that drew water away from the distributed system. This evolution of the drainage system can result in the observed internal variability of ice flow over time. If we are correct that active subglacial lakes drain through canals in the sediment, this mechanism also implies that active lakes are typically located in regions underlain by thick subglacial sediment, which may explain why they are not readily observed using radio-echo sounding techniques.
“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: Subglacial Lake and Meltwater Drainage Systems
« Reply #59 on: November 05, 2016, 05:54:50 PM »
The linked reference confirms that subglacial drainage of basal meltwater can induce rapid ice flow in West Antarctica:

Cooper W. Elsworth & Jenny Suckale (31 October 2016), "Subglacial drainage may induce rapid ice flow rearrangement in West Antarctica", Geophysical Research Letters, DOI: 10.1002/2016GL070430

http://onlinelibrary.wiley.com/doi/10.1002/2016GL070430/abstract

Abstract: "Ice streams are corridors of rapid ice flow draining the ice sheets. They can exhibit astonishing spatial variability on annual to centennial time scales. We propose that changes in the subglacial drainage of meltwater could induce these sudden rearrangements of ice streams. We develop a two-dimensional, thermo-mechanical model representing an ice stream cross-section and couple it to a plastically deforming bed with spatially variable meltwater influx. We find that where ice flows over deformable sediments and lacks significant topographic control, the efficiency of subglacial water drainage exerts direct control on the velocity, location and width of ice streams. This implies that meltwater percolation at the meter scale could have a significant effect on the short-term variability in ice loss from a continental-scale ice sheet. We verify our model against previous analytical results and validate it against surface observations from the Siple Coast of West Antarctica."
“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: Subglacial Lake and Meltwater Drainage Systems
« Reply #60 on: November 05, 2016, 06:05:18 PM »
The linked reference introduce sub-glacial hydrology mechanisms into the PISM ice model which they calibrate to Heinrich Events and then apply the calibrated model to ice in the Siple Coast region of West Antarctica:

Feldmann, J. and Levermann, A.: From Heinrich Events to cyclic ice streaming: the grow-and-surge instability in the Parallel Ice Sheet Model, The Cryosphere Discuss., doi:10.5194/tc-2016-235, in review, 2016.

http://www.the-cryosphere-discuss.net/tc-2016-235/


Abstract. Here we report on a cyclic, physical ice-discharge instability in the Parallel Ice Sheet Model, simulating the flow of a three-dimensional, inherently buttressed ice-sheet-shelf system which periodically surges on a millennial timescale. The thermo-mechanically coupled model on 1 km horizontal resolution includes an enthalpy-based formulation of the thermodynamics, a non-linear stress-balanced based sliding law and a very simple sub-glacial hydrology. The simulated unforced surging is characterized by rapid ice streaming through a bed trough, resulting in abrupt discharge of ice across the grounding line which is eventually calved into the ocean. We identify and visualize the central feedbacks that dominate the sub-sequent phases of ice build-up, surge and stabilization which emerge from the interaction between ice dynamics, thermodynamics and the sub-glacial till layer. A reduction in the surface mass balance or basal roughness yields a damping of the feedback loop which suggests that thinner ice sheets may be less susceptible to surging. The presented mechanisms underlying our simulations of self-maintained, periodic ice growth and destabilization may play a role in large-scale ice-sheet surging, such as the surging of the Laurentide Ice Sheet, which is associated with Heinrich Events, and ice-stream shut-down and reactivation, such as observed in the Siple Coast region of West Antarctica.

Also, in the Science folder & the "Sea Level Rise: New Iceberg Theory" thread, sidd posted the following related Reply #2:

http://forum.arctic-sea-ice.net/index.php/topic,455.0.html


"Nice model from Feldmann and Levermann giving ice surges with a realistic (PISM) ice model. Open access

doi:10.5194/tc-2016-235

"During the surge phase mainly the process of hydraulic runaway (positive feedback between basal melt water production and flow acceleration; Fowler and Johnson, 1995) is in effect. It is complemented by creep instability (positive feedback between strain heating and ice deformation; Clarke et al., 1977), which additionally promotes rapid ice streaming [Ref. 20] (Figs. 4 and 5). The modeled cyclic alternation of ice streaming and stagnation provides a simple example of ice-stream shutdown and re-activation, a phenomenon which is characteristic for the dynamics of some of the Siple Coast outlets in West Antarctica.

The period duration of a full surge cycle in our model of about 1.8 kyr is very close to results from other recent studies (Bougamont et al., 2011; Robel et al., 2016) which is surprising considering the differences in degree of physical approximations, [Ref 25] parameterizations, and setup complexity between the three studies"."
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson

Adam Ash

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Re: Subglacial Lake and Meltwater Drainage Systems
« Reply #61 on: December 13, 2016, 12:21:05 PM »
This stunning Antarctic lake is buried in ice. And that could be bad news
www.smh.com.au/environment/this-stunning-antarctic-lake-is-buried-in-ice-and-that-could-be-bad-news-20161212-gt9oco.html

Another mechanism for deep penetration of heat into ice shelves.  Sigh.