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Radiostratigraphy and age structure of the Greenland Ice Sheet.

MacGregor JA, Fahnestock MA, Catania GA, Paden JD, Prasad Gogineni S, Young SK, Rybarski SC, Mabrey AN, Wagman BM, Morlighem M - J Geophys Res Earth Surf (2015)

Bottom Line: The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet.Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography.Phase information predicts reflection slope and simplifies reflection tracingReflections can be dated away from ice cores using a simple ice flow modelRadiostratigraphy is often disrupted near the onset of fast ice flow.

View Article: PubMed Central - PubMed

Affiliation: Institute for Geophysics, The University of Texas at Austin Austin, Texas, USA.

ABSTRACT

: Several decades of ice-penetrating radar surveys of the Greenland and Antarctic ice sheets have observed numerous widespread internal reflections. Analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections. Here we present a comprehensive deep radiostratigraphy of the Greenland Ice Sheet from airborne deep ice-penetrating radar data collected over Greenland by The University of Kansas between 1993 and 2013. To map this radiostratigraphy efficiently, we developed new techniques for predicting reflection slope from the phase recorded by coherent radars. When integrated along track, these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing. Core-intersecting reflections were dated using synchronized depth-age relationships for six deep ice cores. Additional reflections were dated by matching reflections between transects and by extending reflection-inferred depth-age relationships using the local effective vertical strain rate. The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet. Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography. Disrupted radiostratigraphy is also observed in a region north of the Northeast Greenland Ice Stream that is not presently flowing rapidly. Dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly. This radiostratigraphy provides a new constraint on the dynamics and history of the Greenland Ice Sheet.

Key points: Phase information predicts reflection slope and simplifies reflection tracingReflections can be dated away from ice cores using a simple ice flow modelRadiostratigraphy is often disrupted near the onset of fast ice flow.

No MeSH data available.


Related in: MedlinePlus

Example radargrams showing patterns qualitatively similar to sheath folds in rock from transects collected on (a) 29 March 2011 (76.4°N, 50.1°W) and (b, c) 29 April 2011 (79.5°N, 51.6°W; and 79.6°N, 50.2°W, respectively). Both transects were collected during the 2011 P3 campaign (Table 1). Magenta arrows point to “eye”-like features. The color of the traced reflections has no physical meaning.
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fig13: Example radargrams showing patterns qualitatively similar to sheath folds in rock from transects collected on (a) 29 March 2011 (76.4°N, 50.1°W) and (b, c) 29 April 2011 (79.5°N, 51.6°W; and 79.6°N, 50.2°W, respectively). Both transects were collected during the 2011 P3 campaign (Table 1). Magenta arrows point to “eye”-like features. The color of the traced reflections has no physical meaning.

Mentions: For several radargrams, the structure of the basal layer qualitatively resembles that of sheath folds in rock, which can form due to flow in the vicinity of rheologically weak layers or slip surfaces [e.g., Reber et al., 2012]. These folds are three-dimensional structures whose vertical cross section (as they would be observed by ice-penetrating radar) varies depending on the cross section's orientation with respect to the main axis of the fold. Sheath folds can generate characteristic “eye” structures, which are observed occasionally near the base of the GrIS (Figure 13). If these features are indeed sheath folds, then these observations appear to support the hypothesis presented by NEEM community members [2013] explaining the formation of disrupted basal ice. They hypothesized that the interface between rheologically weaker ice from the LGP and stiffer ice from the Eemian period leads to the development of a slip surface there. This slip surface is equivalent to a false bed, which has also been identified in the West Antarctic Ice Sheet at Siple Dome [Pettit, 2003]. Reflections within and bounding these structures are often indistinct and difficult to trace except by purely manual methods, so we did not map them exhaustively. The apparent slopes of these reflections sometimes exceed that which can be imaged by airborne radar sounding [Holschuh et al., 2014], so their resemblance to sheath folds may be partly due to aliasing of backscatter from these complex features.


Radiostratigraphy and age structure of the Greenland Ice Sheet.

MacGregor JA, Fahnestock MA, Catania GA, Paden JD, Prasad Gogineni S, Young SK, Rybarski SC, Mabrey AN, Wagman BM, Morlighem M - J Geophys Res Earth Surf (2015)

Example radargrams showing patterns qualitatively similar to sheath folds in rock from transects collected on (a) 29 March 2011 (76.4°N, 50.1°W) and (b, c) 29 April 2011 (79.5°N, 51.6°W; and 79.6°N, 50.2°W, respectively). Both transects were collected during the 2011 P3 campaign (Table 1). Magenta arrows point to “eye”-like features. The color of the traced reflections has no physical meaning.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4508962&req=5

fig13: Example radargrams showing patterns qualitatively similar to sheath folds in rock from transects collected on (a) 29 March 2011 (76.4°N, 50.1°W) and (b, c) 29 April 2011 (79.5°N, 51.6°W; and 79.6°N, 50.2°W, respectively). Both transects were collected during the 2011 P3 campaign (Table 1). Magenta arrows point to “eye”-like features. The color of the traced reflections has no physical meaning.
Mentions: For several radargrams, the structure of the basal layer qualitatively resembles that of sheath folds in rock, which can form due to flow in the vicinity of rheologically weak layers or slip surfaces [e.g., Reber et al., 2012]. These folds are three-dimensional structures whose vertical cross section (as they would be observed by ice-penetrating radar) varies depending on the cross section's orientation with respect to the main axis of the fold. Sheath folds can generate characteristic “eye” structures, which are observed occasionally near the base of the GrIS (Figure 13). If these features are indeed sheath folds, then these observations appear to support the hypothesis presented by NEEM community members [2013] explaining the formation of disrupted basal ice. They hypothesized that the interface between rheologically weaker ice from the LGP and stiffer ice from the Eemian period leads to the development of a slip surface there. This slip surface is equivalent to a false bed, which has also been identified in the West Antarctic Ice Sheet at Siple Dome [Pettit, 2003]. Reflections within and bounding these structures are often indistinct and difficult to trace except by purely manual methods, so we did not map them exhaustively. The apparent slopes of these reflections sometimes exceed that which can be imaged by airborne radar sounding [Holschuh et al., 2014], so their resemblance to sheath folds may be partly due to aliasing of backscatter from these complex features.

Bottom Line: The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet.Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography.Phase information predicts reflection slope and simplifies reflection tracingReflections can be dated away from ice cores using a simple ice flow modelRadiostratigraphy is often disrupted near the onset of fast ice flow.

View Article: PubMed Central - PubMed

Affiliation: Institute for Geophysics, The University of Texas at Austin Austin, Texas, USA.

ABSTRACT

: Several decades of ice-penetrating radar surveys of the Greenland and Antarctic ice sheets have observed numerous widespread internal reflections. Analysis of this radiostratigraphy has produced valuable insights into ice sheet dynamics and motivates additional mapping of these reflections. Here we present a comprehensive deep radiostratigraphy of the Greenland Ice Sheet from airborne deep ice-penetrating radar data collected over Greenland by The University of Kansas between 1993 and 2013. To map this radiostratigraphy efficiently, we developed new techniques for predicting reflection slope from the phase recorded by coherent radars. When integrated along track, these slope fields predict the radiostratigraphy and simplify semiautomatic reflection tracing. Core-intersecting reflections were dated using synchronized depth-age relationships for six deep ice cores. Additional reflections were dated by matching reflections between transects and by extending reflection-inferred depth-age relationships using the local effective vertical strain rate. The oldest reflections, dating to the Eemian period, are found mostly in the northern part of the ice sheet. Within the onset regions of several fast-flowing outlet glaciers and ice streams, reflections typically do not conform to the bed topography. Disrupted radiostratigraphy is also observed in a region north of the Northeast Greenland Ice Stream that is not presently flowing rapidly. Dated reflections are used to generate a gridded age volume for most of the ice sheet and also to determine the depths of key climate transitions that were not observed directly. This radiostratigraphy provides a new constraint on the dynamics and history of the Greenland Ice Sheet.

Key points: Phase information predicts reflection slope and simplifies reflection tracingReflections can be dated away from ice cores using a simple ice flow modelRadiostratigraphy is often disrupted near the onset of fast ice flow.

No MeSH data available.


Related in: MedlinePlus