On the fringe of Antarctica lies the deepest canyon found on any continent in the world. The canyon bottoms out at 3.5 kilometers (11,500 feet) below sea level. But you can’t see the bottom because the canyon is completely filled by ice from the Denman Glacier. Located in East Antarctica, the Denman Glacier measures 12 miles across, and its glacial ice flows the length of the canyon from the continent and into the ocean. Thus, Denman Canyon forms a dangerous conduit from the ocean into the heart of Antarctica.
The glaciers and ice shelves of West Antarctica garner significant attention. Studies like the recent one investigating the huge Thwaites Glacier have raised awareness of instabilities in the West Antarctic ice sheets. Some types of instabilities affect the way these vast ice deposits respond to climate change and global warming. These instabilities ultimately affect when will the ice melts and how fast the melt
West Antarctica is important in understanding future sea-level rise. But East Antarctica ice reserves are nine times greater than those of West Antarctica. Therefore, glaciers like Denman pose the most significant threat to sea-level rise in terms of their meltwater contribution.
Retrograde bed slope
The geometry of Denman Canyon results in the base of the glacier sloping downward away from the ocean. Therefore, the glacier’s base becomes deeper as the glacier moves further inland. This configuration is called a retrograde bed slope.
Another important component of glaciers like Denman is the grounding line. This line refers to the location where the base of a glacier ceases to float on ocean water and the glacial ice contacts bedrock.
Antarctic glaciers and ice sheets are currently melting from both above and below. Warm summers melt the ice on top, and then warm ocean waters melt the ice below. So, under attack from both directions, the ice retreats towards the continent.
When the grounding line retreats inland on a retrograde slope glacier, the thickness of the “floating ice” increases. Glacial mechanics dictate that the increased thickness then causes an increase in the rate of seaward ice movement from the glacier. Thus feeding more ice into the ocean, ice that eventually becomes more meltwater.
Why does the technical stuff matter?
The grounding line for the Denman Glacier has retreated inland about five kilometers (three miles) in the past 20 years. This retreat means that ocean waters warm enough to melt ice advanced five kilometers inland. The inland retreat of the grounding line also increases the thickness of “floating ice” and then the seaward flow of glacial ice increases.
Improvements in our understanding of glacial dynamics change our understanding of how Antarctica responds to climate change. Unfortunately, this newfound knowledge indicates that melting rates for Antarctic ice are higher than initially anticipated. Since Antarctica contains the largest volume of ice on the planet, its melt rate affects predictions of sea-level rise.
The base of the Denman Canyon is far below sea level. So, as the grounding line retreats, seawater can penetrate far inland into the heart of the continent. This unseen source of basal heat directly results in more ice melt beneath the glacier. But it also results in the acceleration of ice flow to the ocean. So, seawater penetrating far beneath the base of Antarctica’s ice does not bode well for current predictions of sea-level rise. Those original predictions appear on the low side.
The collapse of Antarctic ice shelves (Source: ArcheanWeb) – https://archeanweb.com/2020/02/14/collapse-of-antarctic-ice-shelves/ Also:
Scientists just discovered a massive new vulnerability in the Antarctic ice sheet (By Chris Mooney; Washington Post) – https://www.washingtonpost.com/climate-environment/2020/03/23/denman-glacier-climate-change/ Also:
The stability of grounding lines on retrograde slopes (By: G. H. Gudmundsson, J. Krug, G. Durand, L. Favier, and O. Gagliardini; The Cryosphere) – https://www.the-cryosphere.net/6/1497/2012/tc-6-1497-2012.pdf Also:
Feature Image: Shape of bedrock under Denman Glacier (Source NASA) – https://commons.wikimedia.org/wiki/File:Shape_of_bedrock_under_Denman_Glacier.png – This file is in the public domain in the United States because it was solely created by NASA. NASA copyright policy states that “NASA material is not protected by copyright unless noted”.