Satellite Imagery Reveals the Road Map for “Horizontal Ice Coring”

Published 27-08-2020

NASA and The Geological Survey of Denmark and Greenland (GEUS) have used high-resolution satellite imagery to push glaciologist Niels Reeh’s theory of horizontal ice coring into reality over a large area of the ice sheet.

Niels Reeh collecting ice surface samples at the Paakitsoq profile in 1994. Photo: Henrik Højmark Thomsen.

For ten years, between 1985 and 1998, Niels Reeh, a glaciologist at GEUS, travelled all around the margin of the Greenland Ice Sheet. He was trying to understand how patterns in the color of the ice visible at the ice-sheet surface were related to the age of the ice itself.

When snow falls in the center of the ice sheet, it becomes buried and turns into ice, as it descends vertically down into the ice sheet. Over time, horizontal flow brings this ice from the ice-sheet center towards its margin. Eventually, this ice will be exposed again at the ice-sheet surface in the ablation area, which is snow-free by the end of every summer.

During his decade of Greenland trips, Reeh explored the idea of a “horizontal ice core”. He found that the patterns of ice bands exposed in the snow-free ablation area could allow dating individual ice bands. Unfortunately, at the time it was difficult to map individual ice bands over large areas with oblique aerial photos from helicopter windows.

New study pushes the theory toward reality

In a new open-access study in the Journal of Glaciology, however, glaciologist Joseph MacGregor at NASA Goddard Space Flight Center, in collaboration with GEUS researchers William Colgan and Kristian Kjeldsen, have used high-resolution satellite imagery to push Reeh’s ideas on horizontal ice coring into reality over a large area of the ice sheet.

The new study uses ESA Sentinel-2 and DigitalGlobe WorldView imagery to map coherent ice bands across North Greenland, represented approximately 20% of the total ice-sheet margin. This satellite imagery is readily capable of distinguishing subtle color and brightness variations between ice bands. These subtle variations, ranging from dark blue to light brown, are mostly related to atmospheric dust concentration when the ice was deposited as snowfall. Alternating light and dark bands track alternating warm and cold periods, respectively.

Satellite images — The horizontal sequences in ice band colors revealed by high-resolution satellite imagery align with the dust concentration record measured in the North GRIP deep ice core. The satellite images depict the snow-free ice-sheet ablation area in the vicinity of Warming Land, North Greenland, which contains the oldest surface-exposed ice. Inset shows the location of the North GRIP ice core and the ice flow path towards Warming Land. Illustration: Joseph MacGregor, NASA Goddard Space Flight Center

Subtle strata

The horizontal ice bands identified in the satellite imagery can produce good, qualitative matches to patterns in dust concentration previously measured in the NorthGRIP deep ice core. The horizontal color patterns of the ice bands are also correlated with the oxygen isotope concentrations laboriously sampled by Reeh and his colleagues along horizontal transects at dozens of sites around the ice-sheet margin nearly two decades ago.

“The ever-increasing availability of high-resolution satellite imagery has revolutionized our ability to discern and map subtle features in the ice sheet's margin stratigraphy, features you could only see with the naked eye from low-flying aircraft,” explains Joseph MacGregor from NASA Goddard Space Flight Center.

The ice bands mapped by the project provide a literal road map for horizontal ice coring in North Greenland. Most notably, this road map highlights that the oldest surface-exposed ice in North Greenland – may to be up to 55,000 years old – is likely located within two kilometer-long portions of the margin adjacent to Warming Land at 81ºN. This new information drastically improves the opportunity for bulk recovery of paleoclimatically valuable ice from Greenland.

The legacy of Niels Reeh

“This work really builds on the theoretical foundation for horizontal ice coring that was developed by Niels Reeh. Resolving the age of ice parcels in such exquisite detail will transform our work in the North Greenland ablation area. Visually, it is probably even more beautiful than Reeh would have expected,” says GEUS Senior Researcher William Colgan.

Niels Reeh worked at the Geological Survey of Greenland (GGU) and GEUS in various positions over several decades. He passed away in May 2009. His idea for horizontal ice coring is one of many of his hypotheses that continue to be explored by newly developed instruments and methods. The current study built on the foundation of extensive in situ sampling undertaken by Reeh two decades ago, with the assistance of many GEUS colleagues at the time.