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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

Depth-age relationships for the six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. Solid lines are the ice core-derived depth-age relationships (Table 2) and surrounding fill is the assigned age uncertainty. Circles are reflection depths/ages that were dated at NorthGRIP only. Triangles are non-core-intersecting reflections whose ages were estimated using regions where they overlapped with NorthGRIP-dated reflections. (a) Non-core-intersecting reflections (triangles) dated using 1-D (vertical) linear interpolation and extrapolation of depth-age relationships. (b) Non-core-intersecting reflections dated using quasi-Nye dating. Note that when dating the entire radiostratigraphy, all the ice core depth-age relationships are used.
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fig07: Depth-age relationships for the six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. Solid lines are the ice core-derived depth-age relationships (Table 2) and surrounding fill is the assigned age uncertainty. Circles are reflection depths/ages that were dated at NorthGRIP only. Triangles are non-core-intersecting reflections whose ages were estimated using regions where they overlapped with NorthGRIP-dated reflections. (a) Non-core-intersecting reflections (triangles) dated using 1-D (vertical) linear interpolation and extrapolation of depth-age relationships. (b) Non-core-intersecting reflections dated using quasi-Nye dating. Note that when dating the entire radiostratigraphy, all the ice core depth-age relationships are used.

Mentions: The suitability of dating part of this radiostratigraphy using the quasi-Nye method is evaluated using two approaches. First, we compare quasi-Nye-inferred reflection ages to those inferred by 1-D (vertical) linear interpolation and extrapolation of depth-age relationships (Figure 7). In this evaluation, we use only the depth-age relationship from NorthGRIP to infer the depth-age relationship at the other ice core sites. Linear reflection dating tends to overestimate the age of reflections as compared to the logarithmic depth-age relationship predicted by equation (17) and is generally inadequate, a result similar to that of Lhomme et al. [2005] and Fudge et al. [2014]. Reflection ages calculated using quasi-Nye dating tend to better match ice core depth-age relationships and are more self-consistent, particularly at shallower depths and especially at Camp Century. This behavior is consistent with expectations. Vertical strain rates are generally predicted to be uniform in the upper portion of the ice sheet and are more likely to decrease closer to the bed, e.g., within the basal shear layer [Dansgaard and Johnsen, 1969]. Quasi-Nye dating tends to perform better in the upper portion of the ice sheet and worse closer to the bed, because it may assume a uniform vertical strain rate over a larger depth than is physically reasonable (e.g., Figure 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)

Depth-age relationships for the six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. Solid lines are the ice core-derived depth-age relationships (Table 2) and surrounding fill is the assigned age uncertainty. Circles are reflection depths/ages that were dated at NorthGRIP only. Triangles are non-core-intersecting reflections whose ages were estimated using regions where they overlapped with NorthGRIP-dated reflections. (a) Non-core-intersecting reflections (triangles) dated using 1-D (vertical) linear interpolation and extrapolation of depth-age relationships. (b) Non-core-intersecting reflections dated using quasi-Nye dating. Note that when dating the entire radiostratigraphy, all the ice core depth-age relationships are used.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig07: Depth-age relationships for the six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. Solid lines are the ice core-derived depth-age relationships (Table 2) and surrounding fill is the assigned age uncertainty. Circles are reflection depths/ages that were dated at NorthGRIP only. Triangles are non-core-intersecting reflections whose ages were estimated using regions where they overlapped with NorthGRIP-dated reflections. (a) Non-core-intersecting reflections (triangles) dated using 1-D (vertical) linear interpolation and extrapolation of depth-age relationships. (b) Non-core-intersecting reflections dated using quasi-Nye dating. Note that when dating the entire radiostratigraphy, all the ice core depth-age relationships are used.
Mentions: The suitability of dating part of this radiostratigraphy using the quasi-Nye method is evaluated using two approaches. First, we compare quasi-Nye-inferred reflection ages to those inferred by 1-D (vertical) linear interpolation and extrapolation of depth-age relationships (Figure 7). In this evaluation, we use only the depth-age relationship from NorthGRIP to infer the depth-age relationship at the other ice core sites. Linear reflection dating tends to overestimate the age of reflections as compared to the logarithmic depth-age relationship predicted by equation (17) and is generally inadequate, a result similar to that of Lhomme et al. [2005] and Fudge et al. [2014]. Reflection ages calculated using quasi-Nye dating tend to better match ice core depth-age relationships and are more self-consistent, particularly at shallower depths and especially at Camp Century. This behavior is consistent with expectations. Vertical strain rates are generally predicted to be uniform in the upper portion of the ice sheet and are more likely to decrease closer to the bed, e.g., within the basal shear layer [Dansgaard and Johnsen, 1969]. Quasi-Nye dating tends to perform better in the upper portion of the ice sheet and worse closer to the bed, because it may assume a uniform vertical strain rate over a larger depth than is physically reasonable (e.g., Figure 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