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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

(a) Observed along-track bed slope (α(H)) within 5 km intervals across the GrIS. (b) Along-track bed slope predicted by the depth profile of the slope of internal reflections (α′(H)) within the same 5 km intervals. (c) Absolute value of the residual between the observed and predicted bed slopes (/α(H) − α′(H)/). (d–f) /α(H) − α′(H)/ within selected regions outlined by magenta dashed lines in Figure 11c. Background shading is the satellite-measured surface speed [Joughin et al., 2010]. For all panels, bed slopes and differences are plotted in ascending order to better resolve larger values.
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fig11: (a) Observed along-track bed slope (α(H)) within 5 km intervals across the GrIS. (b) Along-track bed slope predicted by the depth profile of the slope of internal reflections (α′(H)) within the same 5 km intervals. (c) Absolute value of the residual between the observed and predicted bed slopes (/α(H) − α′(H)/). (d–f) /α(H) − α′(H)/ within selected regions outlined by magenta dashed lines in Figure 11c. Background shading is the satellite-measured surface speed [Joughin et al., 2010]. For all panels, bed slopes and differences are plotted in ascending order to better resolve larger values.

Mentions: We find several regions with large differences between the observed and predicted bed slopes (equation (22)). These regions are typically located at the onset of fast ice flow, such as within and upstream of Petermann Glacier and Jakobshavn Isbræ (Figure 11). Low bed slopes in these regions are qualitatively consistent with observations of lower bed roughness there, particularly for Petermann Glacier [Rippin, 2013]. The lack of bed-conforming radiostratigraphy in these regions suggests strong and long-term organization of ice flow that is consistent with pattern of surface velocity [Joughin et al., 2010] and may be further modulated by spatiotemporal variation in basal friction [e.g., Hindmarsh et al., 2006; Christianson et al., 2013, 2014]. This pattern is distinct from observations of some Antarctic glaciers that contain disrupted radiostratigraphy, which has been interpreted as evidence of fast ice flow in the past [e.g., Catania et al., 2005; Rippin et al., 2006].


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)

(a) Observed along-track bed slope (α(H)) within 5 km intervals across the GrIS. (b) Along-track bed slope predicted by the depth profile of the slope of internal reflections (α′(H)) within the same 5 km intervals. (c) Absolute value of the residual between the observed and predicted bed slopes (/α(H) − α′(H)/). (d–f) /α(H) − α′(H)/ within selected regions outlined by magenta dashed lines in Figure 11c. Background shading is the satellite-measured surface speed [Joughin et al., 2010]. For all panels, bed slopes and differences are plotted in ascending order to better resolve larger values.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig11: (a) Observed along-track bed slope (α(H)) within 5 km intervals across the GrIS. (b) Along-track bed slope predicted by the depth profile of the slope of internal reflections (α′(H)) within the same 5 km intervals. (c) Absolute value of the residual between the observed and predicted bed slopes (/α(H) − α′(H)/). (d–f) /α(H) − α′(H)/ within selected regions outlined by magenta dashed lines in Figure 11c. Background shading is the satellite-measured surface speed [Joughin et al., 2010]. For all panels, bed slopes and differences are plotted in ascending order to better resolve larger values.
Mentions: We find several regions with large differences between the observed and predicted bed slopes (equation (22)). These regions are typically located at the onset of fast ice flow, such as within and upstream of Petermann Glacier and Jakobshavn Isbræ (Figure 11). Low bed slopes in these regions are qualitatively consistent with observations of lower bed roughness there, particularly for Petermann Glacier [Rippin, 2013]. The lack of bed-conforming radiostratigraphy in these regions suggests strong and long-term organization of ice flow that is consistent with pattern of surface velocity [Joughin et al., 2010] and may be further modulated by spatiotemporal variation in basal friction [e.g., Hindmarsh et al., 2006; Christianson et al., 2013, 2014]. This pattern is distinct from observations of some Antarctic glaciers that contain disrupted radiostratigraphy, which has been interpreted as evidence of fast ice flow in the past [e.g., Catania et al., 2005; Rippin et al., 2006].

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