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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 sequence for prediction of radiostratigraphy using the Doppler centroid wavenumber (kD). (a) Amplitude of the along-track Fourier transform of a short segment (<250 m) of the same transection as shown in Figure 3. Vertical red dashed lines represent the edges of the search region for a kD maxima and the vertical white dashed line is zero wavenumber. Blue dots in the inset panel represent distinct kD maxima. (b) The kD-predicted radiostratigraphy overlain on the SAR-focused returned amplitude. The format follows Figure 3d.
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fig04: Example sequence for prediction of radiostratigraphy using the Doppler centroid wavenumber (kD). (a) Amplitude of the along-track Fourier transform of a short segment (<250 m) of the same transection as shown in Figure 3. Vertical red dashed lines represent the edges of the search region for a kD maxima and the vertical white dashed line is zero wavenumber. Blue dots in the inset panel represent distinct kD maxima. (b) The kD-predicted radiostratigraphy overlain on the SAR-focused returned amplitude. The format follows Figure 3d.

Mentions: We apply a Fourier transform in the along-track direction to short overlapping segments (<250 m) of the complex radar data (Figure 4). These segments must be sufficiently short that reflection slopes within them can be assumed to be uniform; otherwise, signal aliasing may occur [Holschuh et al., 2014]. The amplitude of englacial reflections in this two-dimensional traveltime-wavenumber space, i.e., their Doppler spectrum, is often narrowly peaked. This observation further indicates that such reflections are highly specular. We take the Doppler centroid wavenumber kD to be the wavenumber of this peak, which can be converted directly to Δϕ/Δx and again related to Δt/Δx following equation (5).


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 sequence for prediction of radiostratigraphy using the Doppler centroid wavenumber (kD). (a) Amplitude of the along-track Fourier transform of a short segment (<250 m) of the same transection as shown in Figure 3. Vertical red dashed lines represent the edges of the search region for a kD maxima and the vertical white dashed line is zero wavenumber. Blue dots in the inset panel represent distinct kD maxima. (b) The kD-predicted radiostratigraphy overlain on the SAR-focused returned amplitude. The format follows Figure 3d.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig04: Example sequence for prediction of radiostratigraphy using the Doppler centroid wavenumber (kD). (a) Amplitude of the along-track Fourier transform of a short segment (<250 m) of the same transection as shown in Figure 3. Vertical red dashed lines represent the edges of the search region for a kD maxima and the vertical white dashed line is zero wavenumber. Blue dots in the inset panel represent distinct kD maxima. (b) The kD-predicted radiostratigraphy overlain on the SAR-focused returned amplitude. The format follows Figure 3d.
Mentions: We apply a Fourier transform in the along-track direction to short overlapping segments (<250 m) of the complex radar data (Figure 4). These segments must be sufficiently short that reflection slopes within them can be assumed to be uniform; otherwise, signal aliasing may occur [Holschuh et al., 2014]. The amplitude of englacial reflections in this two-dimensional traveltime-wavenumber space, i.e., their Doppler spectrum, is often narrowly peaked. This observation further indicates that such reflections are highly specular. We take the Doppler centroid wavenumber kD to be the wavenumber of this peak, which can be converted directly to Δϕ/Δx and again related to Δt/Δx following equation (5).

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