Grain‐scale processes, folding, and stratigraphic disturbance in the GISP2 ice core

Alley, Richard B.; Gow, Anthony J.; Meese, D. A.; Fitzpatrick, Joan J.; Waddington, Edwin D.; Bolzan, John F.

doi:10.1029/96jc03836

Cited by 117 publications

(78 citation statements)

References 38 publications

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“…Their stratigraphic record is considered reliable back to about 110,000 yr B.P. The ice below that level seems to be affected by glacier tectonics (Alley et al, 1997) and surface melting. It may also have been deposited at substantially lower altitude than today, in a different atmospheric circulation regime (Cuffey and Marshall, 2000).…”

Section: Polar Icementioning

confidence: 99%

Last Interglacial Climates

Kukla¹,

et al. 2002

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The last interglacial, commonly understood as an interval with climate as warm or warmer than today, is represented by marine isotope stage (MIS) 5e, which is a proxy record of low global ice volume and high sea level. It is arbitrarily dated to begin at approximately 130,000 yr B.P. and end at 116,000 yr B.P. with the onset of the early glacial unit MIS 5d. The age of the stage is determined by correlation to uranium-thorium dates of raised coral reefs. The most detailed proxy record of interglacial climate is found in the Vostok ice core where the temperature reached current levels 132,000 yr ago and continued rising for another two millennia. Approximately 127,000 yr ago the Eemian mixed forests were established in Europe. They developed through a characteristic succession of tree species, probably surviving well into the early glacial stage in southern parts of Europe.

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Section: Polar Icementioning

confidence: 99%

Last Interglacial Climates

Kukla¹,

et al. 2002

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“…In order to provide folding between layers, a shear strain should be superimposed on the axial strain, and some irregularities in the viscosity between layers should exist (Gow and Williamson, 1976;Alley et al, 1997). Such irregularities, from microstructure and fabric observations, appear here at a small scale, and could be inherited from initial variability (conjugate effect of dust content, initial fabric and strain history).…”

Section: Comparison With the Grip And Ngrip Ice Coresmentioning

confidence: 99%

Fabric along the NEEM ice core, Greenland, and its comparison with GRIP and NGRIP ice cores

et al. 2014

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Abstract. Fabric (distribution of crystallographic orientations) along the full NEEM ice core, Greenland was measured in the field by an automatic ice texture analyzer every 10 m, from 33 m down to 2461 m depth. The fabric evolves from a slightly anisotropic fabric at the top, toward a strong single maximum at about 2300 m, which is typical of a deformation pattern mostly driven by uniaxial compression and simple shearing. A sharp increase in the fabric strengthening rate is observed at the Holocene to Wisconsin (HW) climatic transition. From a simple model we estimate that this depth is located at a transition from a state dominated by vertical compression to a state dominated by vertical shear. Comparisons are made to two others ice cores drilled along the same ridge; the GRIP ice core, drilled at the summit of the ice sheet, and the NGRIP ice core, drilled 325 km to the NNW of the summit along the ridge, and 365 km upstream from NEEM. This comparison tends to demonstrate that the ice viscosity change with the HW climatic transition must be associated with the shear-dominated state to induce the abrupt fabric strengthening observed at NEEM. This comparison therefore reflects the increasing role of shear deformation on the coring site when moving NW along the ridge from GRIP to NGRIP and NEEM. The difference in fabric profiles between NEEM and NGRIP also evidences a stronger lateral extension associated with a sharper ridge at NGRIP. Further along the core, centimeter scale abrupt texture (fabric and microstructure) variations are observed in the bottom part of the core. Their positions are in good agreement with the observed folding layers in Dahl-Jensen et al. (2013).

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“…The crystal orientation fabrics (COFs) in ice sheets are classified based on the c axis projection distribution on Schmidt diagrams. Vertical single-maximum, elongated single-maximum, and vertical girdle are three typical COF types usually found in ice cores from ice sheets, mainly drilled at divides and domes (Alley et al, 1997;Matsuoka et al, 2003;Fujita et al, 2006;Eisen et al, 2007;Montagnat et al, 2012;Weikusat et al, 2017;see Faria et al, 2014a for a review). For single-maximum COF, all c axes align up in a cone centered along or close to the z axis, and in the classical glaciological projection into the horizontal the points in Schmidt diagrams (stereographic projections) concentrate close to the center of the circle as this center represents the vertical direction (z axis).…”

Section: Circular Data Analysismentioning

confidence: 99%

Multi-channel and multi-polarization radar measurements around the NEEM site

González

Leuschen

et al. 2018

The Cryosphere

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Abstract. Ice properties inferred from multi-polarization measurements, such as birefringence and crystal orientation fabric (COF), can provide insight into ice strain, viscosity, and ice flow. In 2008, the Center for Remote Sensing of Ice Sheets (CReSIS) used a ground-based VHF (very high frequency) radar to take multi-channel and multi-polarization measurements around the NEEM (North Greenland Eemian Ice Drilling) site. The system operated with 30 MHz bandwidth at a center frequency of 150 MHz. This paper describes the radar system, antenna configurations, data collection, and processing and analysis of this data set. Within the framework derived from uniaxial ice crystal model, we found that ice birefringence dominates the power variation patterns of co-polarization and cross-polarization measurements in the area of 100 km 2 around the ice core site. The phase shift between ordinary and extraordinary waves increases nonlinearly with depth. The ice optic axis lies in planes that are close to the vertical plane and perpendicular or parallel to the ice divide depending on depth. The ice optic axis has an average tilt angle of about 11.6 • vertically, and its plane may rotate either clockwise or counterclockwise by about 10 • across the 100 km 2 area, and at a specific location the plane may rotate slightly counterclockwise as depth increases. Comparisons between the radar observations, simulations, and ice core fabric data are in very good agreement. We calculated the effective colatitude at different depths by using azimuth and colatitude measurements of the c axis of ice crystals. We obtained an average effective c axis tilt angle of 9.6 • from the vertical axis, very comparable to the average optic axis tilt angle estimated from radar polarization measurements. The comparisons give us confidence in applying this polarimetric radio echo sounding technique to infer profiles of ice fabric in locations where there are no ice core measurements.

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Grain‐scale processes, folding, and stratigraphic disturbance in the GISP2 ice core

Cited by 117 publications

References 38 publications

Last Interglacial Climates

Last Interglacial Climates

Fabric along the NEEM ice core, Greenland, and its comparison with GRIP and NGRIP ice cores

Multi-channel and multi-polarization radar measurements around the NEEM site

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