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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) Apparent thickness of Eemian ice (≥115 ka) within the GrIS. Gray dots represent all mapped and dated internal reflections inferred to be 115 ka (the end of the Eemian period) or older. (b) Uncertainty in thickness of Eemian ice, expressed as the combination of the uncertainty in the depth of the 115 ka isochrone (Figure 9d) and the uncertainty in the gridded ice thickness [Bamber et al., 2013].
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fig10: (a) Apparent thickness of Eemian ice (≥115 ka) within the GrIS. Gray dots represent all mapped and dated internal reflections inferred to be 115 ka (the end of the Eemian period) or older. (b) Uncertainty in thickness of Eemian ice, expressed as the combination of the uncertainty in the depth of the 115 ka isochrone (Figure 9d) and the uncertainty in the gridded ice thickness [Bamber et al., 2013].

Mentions: The spatial extent of Eemian-aged reflections and the apparent thickness of Eemian ice are shown in Figure 10. Similarly, Animation S2 suggests the presence of Eemian ice mostly along or near the central ice divide. Close to the bed, where fewer reflections were traced, our estimate of Eemian ice thickness depends partly on the quality of quasi-Nye dating there. The vertical strain rate inferred between reflection pairs shallower in the ice column will tend to overestimate the vertical strain rate closer to the bed [Dansgaard and Johnsen, 1969], suggesting that our approach will tend to overestimate the thickness of Eemian or older ice. Furthermore, in the process of normalizing the age structure, we cannot easily account for occasional overturning, as observed at NEEM [NEEM community members, 2013]. Hence, we qualify as “apparent” the thickness of Eemian ice reported here and reemphasize both the inherent challenge of dating deep ice and the specific challenge of locating Eemian ice with the GrIS. Based on the success of the flattening of most of the reflections (Animation S1), particularly of the higher-quality data from later P3 campaigns, there are several portions of the GrIS interior with relatively simple depth-age relationships. Near the bed, some of this ice is probably Eemian aged, and such locations should be evaluated further as potential sites for future deep ice cores.


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) Apparent thickness of Eemian ice (≥115 ka) within the GrIS. Gray dots represent all mapped and dated internal reflections inferred to be 115 ka (the end of the Eemian period) or older. (b) Uncertainty in thickness of Eemian ice, expressed as the combination of the uncertainty in the depth of the 115 ka isochrone (Figure 9d) and the uncertainty in the gridded ice thickness [Bamber et al., 2013].
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig10: (a) Apparent thickness of Eemian ice (≥115 ka) within the GrIS. Gray dots represent all mapped and dated internal reflections inferred to be 115 ka (the end of the Eemian period) or older. (b) Uncertainty in thickness of Eemian ice, expressed as the combination of the uncertainty in the depth of the 115 ka isochrone (Figure 9d) and the uncertainty in the gridded ice thickness [Bamber et al., 2013].
Mentions: The spatial extent of Eemian-aged reflections and the apparent thickness of Eemian ice are shown in Figure 10. Similarly, Animation S2 suggests the presence of Eemian ice mostly along or near the central ice divide. Close to the bed, where fewer reflections were traced, our estimate of Eemian ice thickness depends partly on the quality of quasi-Nye dating there. The vertical strain rate inferred between reflection pairs shallower in the ice column will tend to overestimate the vertical strain rate closer to the bed [Dansgaard and Johnsen, 1969], suggesting that our approach will tend to overestimate the thickness of Eemian or older ice. Furthermore, in the process of normalizing the age structure, we cannot easily account for occasional overturning, as observed at NEEM [NEEM community members, 2013]. Hence, we qualify as “apparent” the thickness of Eemian ice reported here and reemphasize both the inherent challenge of dating deep ice and the specific challenge of locating Eemian ice with the GrIS. Based on the success of the flattening of most of the reflections (Animation S1), particularly of the higher-quality data from later P3 campaigns, there are several portions of the GrIS interior with relatively simple depth-age relationships. Near the bed, some of this ice is probably Eemian aged, and such locations should be evaluated further as potential sites for future deep ice cores.

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