Greenland subglacial topography

[A-Team]

Below continues the overall assembly from the Petermann Grid down to NEEM, an additional 187 km to the south (first two images tile). While upheavals continue to occur, they no longer have the extended axes of features closer to the glacier. The lack of a regular grid also diminishes the value of these flight paths.

However there are some rather unusual configurations and some true giants that call for explanation. The lower images assemble these flight segments near original resolution. The first upheaval is 1278 m high (4193') by 14.1 km wide. The second has radar reflectors seemingly outlining a large 'floating chunk.

[A-Team]

The images below show the 15 north-south transects of the Petermann Grid. These are along-flow so complement the across-flow images above. The 2010 portion, flown in a DC8 instead of a P3 Orion, has inherently poor contrast that cannot be fully remediated. Most of the images tile 2-3 consecutive flight segments; some are not fully straight.

The 3 most western transects become increasingly perfect layer cake -- that continues throughout the Humboldt Glacier area and also on the northeast side (not shown). Thus the upheavals in this region are restricted to the more actively flowing regions of Petermann. Three of them are spectacular.

The data set at original resolution consists of 15 image files, each a 1600 x 527 pixel grayscale or 12.6 million pixels, not a large file but too large to display properly here. It is stored off-site at http://tinyurl.com/lhqol27. I am experimenting with a few concepts below and will be modifying this post until a halfway satisfactory blog display is reached. The animation, which precedes from west to east across the north-souuth grid lines, will take a click to get going.

The Petermann jelly roll (second image, 20110429_02_009) runs for over 48 km and reaches a peak height of ~900 m and north of it a major upheaval of a different character (20110429_02_00) adds more complexity. It seems downright silly to model this glacier using only surface elevation and bedrock topography. Note the latter has reverse-slope throughout the Grid -- the ice must go uphill to reach Nares Strait.

The third image, the next flight line west (W19 N12 in the coordinates of post #77) shows what the inside of this sheath fold looks like. The vertical scale is exaggerated in all this imagery to keep a practical width but still...

The last image assembles the separate north-south flight lines as an overlay on the Petermann Grid (the glacier is to the left). This, when rotated 90º clockwise, meshes with the west-east flight line assembly above. Together these cross slices provide some idea of the anatomy of upheavals in this region.

[sidd]

here is a Morlinghem bed contour quikpic of how Jacobshawn, Petermann connect to the inland depression. Green is 0, blues are deeper (200 m contours)

[A-Team]

Greenland being sideways. My interpretation: the tremendous weight of the ice along the center ridge has squished the underlying asthenosphere off to the side according to isostasy, creating the large central depression of the bedrock contrasted with coastal uplift.

Jakobshavn, being farther south and warmer, has been moving faster and been more active in excavating bedrock than the northern glaciers and has markedly encroached on the central depression, as well as trenching out to the continental shelf during peak glacier times. Jakobshavn is where it is because of a rare natural break in the coastal mountain range.

Prior to glaciation, the land drained to the north, exiting at Peterman. Jakobshavn has captured Petermann's watershed, at least in central Greenland but this is moot because there is no water in the Grand Canyon nor ice flowing along it. In lower Jakobshavn, the ice flow is still down one of the old channels (ie controlled by basal topography).

In upper Jakobshavn the situation is more of a hybrid: some generic ice dome flow but still persistence of channeling up to the confluence and perhaps some distance beyond. Radar transects of Jakobshavn never show shear zones at the walls in the manner of NEGIS which has not however excavated a deep channel along its length.

[Espen]

Jakobshavn is where it is because of a rare natural break in the coastal mountain range.

The same sentence can be used for Zachariae?

[A-Team]

The same sentence can be used for Zachariae?

It is certainly a break with respect to the formidable mountain range that extends along east coast of Greenland. However the ice does seem to struggle finding a low resistance exit: some of the basin discharges to a glacier to the south, Niag involves zigzags around barriers, and Zach an unfavorable sharp right turn.

If this ice stream is truly attributable to an excursion in the geothermal gradient coincidentally near the summit headwall, then its location needs to be considered relative to the overall geometry of the basin. That is, if the heat were instead uniformly applied, how would the ice flow be different? I would say a lot slower but probably the same exit points.

The same question comes up with paleo accumulation history which could change the position of the summit ridge as well as east side accumulation rates, which right now are low. It is the depth and slope that drive the flow. Field work last summer at mid-NEGIS and analysis of radar stratigraphy do not support major changes in these over the last 125 kyr.+

[Shared Humanity]

Geoff, good questions. The subglacial lakes are near-surface, so not much off-topic for the subglacial forum (though we have a full plate already with bedrock and radar isochrons, ie deep englacial) but the discussion of them so far has been mostly over at 'What's New in Greenland'. If someone would like to pursue the two dozen or so melt papers of 2014-25, probably best to put it all in a dedicated forum.

I would suggest Time is on the right side of the issue but is somewhat ahead of developments. There is definitely a concern that too-rapid climate warming will bring too much meltwater too soon -- and with it latent heat, greater susceptibility of warmer ice to stress, and bedrock lubrication -- into the glacier faster than it can be safely dissipated.

Wouldn't the fact that the subglacial lakes are near surface suggest that the melt water encounters a difficult to permeate barrier as it reaches the harder ice features deep within the ice sheet? Could it be the latent heat from the melt water can more easily carve out channels through the softer ice but as the melt water penetrates deeply into the ice sheet, both the loss of heat to the surrounding ice and increasingly dense, hard ice causes the melt water to pool and collect into these lakes? What would that suggest about the future behavior of the ice sheet as a whole?