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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 and 1000 quasi-Nye-modeled depth-age relationships for each of six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. (b) Distribution of relative differences between quasi-Nye-modeled and observed depth-age relationships (relative difference from core age). (c) Relationship between the relative age difference of the bounding reflections (age difference of bounding reflection pair divided by their mean age) and the relative difference from core age.
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fig06: (a) Observed and 1000 quasi-Nye-modeled depth-age relationships for each of six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. (b) Distribution of relative differences between quasi-Nye-modeled and observed depth-age relationships (relative difference from core age). (c) Relationship between the relative age difference of the bounding reflections (age difference of bounding reflection pair divided by their mean age) and the relative difference from core age.

Mentions: Figure 6a shows that, for the above scenario, quasi-Nye dating of these ice cores can deviate substantially from the reported depth-age scales. However, Figure 6b shows that these deviations are generally relatively small. Based on best fit Gaussian curves to the distributions of these deviations, the relative age uncertainty typically due to quasi-Nye dating is <3%. Quasi-Nye dating tends to underestimate the age of ice from the Last Glacial Period (LGP; 11.7–115 ka). For ice of this age, which is generally found deeper within the GrIS, flow may also result from simple shear, leading to decreasing vertical strain rates there and hence older ice relative to a Nye model [Dansgaard and Johnsen, 1969]. Conversely, where basal melting is known to occur (e.g., NorthGRIP), quasi-Nye dating overestimates ice age in a manner similar to that of a Nye model relative to a model that includes basal melting [Fahnestock et al., 2001b; Dahl-Jensen et al., 2003].


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 and 1000 quasi-Nye-modeled depth-age relationships for each of six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. (b) Distribution of relative differences between quasi-Nye-modeled and observed depth-age relationships (relative difference from core age). (c) Relationship between the relative age difference of the bounding reflections (age difference of bounding reflection pair divided by their mean age) and the relative difference from core age.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig06: (a) Observed and 1000 quasi-Nye-modeled depth-age relationships for each of six deep ice cores recovered from the GrIS. Depth is normalized by the local ice thickness. (b) Distribution of relative differences between quasi-Nye-modeled and observed depth-age relationships (relative difference from core age). (c) Relationship between the relative age difference of the bounding reflections (age difference of bounding reflection pair divided by their mean age) and the relative difference from core age.
Mentions: Figure 6a shows that, for the above scenario, quasi-Nye dating of these ice cores can deviate substantially from the reported depth-age scales. However, Figure 6b shows that these deviations are generally relatively small. Based on best fit Gaussian curves to the distributions of these deviations, the relative age uncertainty typically due to quasi-Nye dating is <3%. Quasi-Nye dating tends to underestimate the age of ice from the Last Glacial Period (LGP; 11.7–115 ka). For ice of this age, which is generally found deeper within the GrIS, flow may also result from simple shear, leading to decreasing vertical strain rates there and hence older ice relative to a Nye model [Dansgaard and Johnsen, 1969]. Conversely, where basal melting is known to occur (e.g., NorthGRIP), quasi-Nye dating overestimates ice age in a manner similar to that of a Nye model relative to a model that includes basal melting [Fahnestock et al., 2001b; Dahl-Jensen et al., 2003].

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