Measured velocities Joughin et al
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(2) IPCC 5 (2013). Measured velocities Joughin et al. (2017). Modelled velocities Aschwanden et al. (2016). •. Models are still not able to predict solid ice discharge and ice sheet contribution well enough. •. Significant uncertainties remain regarding the magnitude and rate of ice stream contribution towards sea-level rise ! ice streams.
(3) Motivation. NASA (2017), edited.
(4) Motivation Melting at top and base. • •. Negative mass balance contributes to sea level rise Ice streams account for 50% of mass loss in Greenland ! need for better understanding of rheology of ice. •. Analysis of microstructures and physical properties of fast flowing ice at Northeast Greenland Ice Stream (NEGIS). Ice surface elevation. Ice surface velocities ! !-!. Discharge of solid ice. Bedrock elevation. < 10m/a (“sheetish”) > 50m/a (“streamish”). Bamber et al. (2013,) Joughin et al. (2016), Illustration: D. Jansen (AWI).
(5) EastGRIP First physically-motivated ice core " new insights into: " Deformation mechanisms " Inclusions/ impurities " Large scale structures " Size and shape of grains and folding " Subgrain structures " C-axis orientation. logistically motivated. paleo-climate motivated. Vallelonga et al. (2014), edited. NASA (2016).
(6) Motivation • Different planes in crystal ! easiest deformation along basal plane (perpendicular to c-axis) • Schmidt diagrams project c-axes as pole figures, core axis is represented through the centre of the circle • Eigenvalues portray c-axis distribution as the three principal axes of an ellipsoid. Eichler (2013). Moldflow Insight (2017). Faria et al. (2013).
(7) Motivation • Deformation of crystals ! ice sheets flow and deform ! flow behaviour depends on crystal preferred orientation (CPO), mode and direction of. basal slip. random. deformation. • Evolution of fabric depends mainly on dominant cluster/ solid-cone. strain conditions ! c-axis distribution rotates towards compression axes. „[...] a depth-varying fabric implies corresponding depth-varying rheological behavior. Determining this depth-varying rheological structure is critical for modeling flow near ice divides and interpreting ice core records (Gusmeroli et al., 2012).. SingleMaximum. Gusmeroli et al. (2012), edited.
(8) Work in the “lab” • International project in NE-Greenland, aiming to retrieve an ice core from NEGIS • Worldwide cooperation in the field and during the following analyses, managed by Centre for Ice and Climate (Denmark) • Major partners: Germany, Japan, Norway, US, France. Greenland.net (30.11.2017).
(9) Work in the “lab” • Camp consists of airstrip, science trenches, accomodations and workshops ! logistical hub for other projects • Ice thickness of about 2550m and rather undisturbed layers • Surface velocity of ~65 m/yr ! EastGRIP camp moves 1 diameter/day.
(10) Work in the “lab” Physical properties (PP) work station. thin section (300 µm). Sledge microtome. Large area scanning macroscope (LASM) Photos by J. Kerch and D. Jansen (AWI). Fabric Analyser (FA).
(11) Work in the “lab” • Combined analysis of crystal fabric and microstructure maps by making and examining thin sections (~every 10-30m of depth) • Large area scanning macroscope (LASM): specialized scanner for ice core research ! air inclusions, texture and deformation-related features • Fabric Analsyer (FA): Automated polarized-light microscope ! textural parameters (i.e. fabric) and microstructure. EastGRIP steering committee (2016).
(12) Work in the “lab” •. Measurement time (science trench, -15°C): " LASM: cutting and sample preparation (15 min) + sublimation (>4 h) + scanning (5 min) " Fabric Analyser: sample preparation (20 min) + sublimation (>3) + analysis of fabric (50 min) ! approx. 8-9 samples/day. Photos by N. Stoll and J. Kerch (AWI). 212m. 2506_6 1378m.
(13) Fabric overview • 2017: 275 samples measured ≈ resolution of one full measurement every 10m ! representative for upper ~250 m + some areas with higher resolution • 2018: 522 samples measured ! measurements every 10-15m, including lower brittle zone and nine volume cuts • Total: 744 vertical samples 53 horizontal samples.
(14) Fabric overview • major findings: 1) rapid evolution of c-axes anisotropy compared to lower dynamic sites 2) partly novel characteristics in crystal prefered orientation (CPO) patterns Equal Area. Kamb Contours. Lower Hemisphere. C.I. = 2.0 Sigma. 234m cross-girdle. Equal Area Lower Hemisphere. N = 740 Kamb Contours C.I. = 2.0 Sigma. 1680m horizontal maximum/ hourglass. N = 3397. Eigenvalues from Orientation tensor (2nd order).
(15) Fabric overview • major findings: 1) rapid evolution of c-axes anisotropy compared to lower dynamic sites Orientation tensor (2nd order) Eigenvalue definition.
(16) Fabric overview Weikusat et al. (2017) EDMLevolution 1) Rapid of c-axes anisotropy NEEM Eichler (2013), Montagnat et al. (2014) compared todata lower dynamic sites EGRIP (g-w) new. GRIP. Thorsteinsson et al. (1997). 0.8. eigenvalue. 0.6. 0.4. 0.2. 500. 1000. 1500 depth. 2000.
(17) Fabric overview New characteristic: Two larger orientation tensor´s eigenvalues fluctuate in 50-150m intervals # „wavy“ form.
(18) Fabric overview 2) partly novel characteristics in CPO patterns. Equal Area Lower Hemisphere. Kamb Contours C.I. = 2.0 Sigma. Common CPO patterns Broad single maximum 118m. N = 1468. Broad girdle Equal Area Lower Hemisphere. Kamb Contours C.I. = 2.0 Sigma. Fully developed girdle ≈ 1400m at ice divides N = 894. 701m. Equal Area. Kamb Contours. Lower Hemisphere. C.I. = 2.0 Sigma. Strong girdle. 1367m. N = 1545.
(19) CPO in detail 2) partly novel characteristics in CPO patterns. Compression Equal Area Lower Hemisphere. Thorsteinsson (1996), modified. Kamb Contours C.I. = 2.0 Sigma. Common CPO patterns Broad single maximum # vertical compression from overlying layers or temperature-gradient snow metamorphosis. Fully developed girdle # extension along flow and ice deforms rather than being translated by rigid block movement. 118m. N = 1468. Equal Area Lower Hemisphere. Kamb Contours C.I. = 2.0 Sigma. N = 894. 701m. Extension Equal Area. Kamb Contours. Lower Hemisphere. C.I. = 2.0 Sigma. Strong girdle. 1367m. N = 1545. Paterson (1994), modified.
(20) CPO in detail 2) partly novel characteristics in CPO patterns Novel CPO patterns from EGRIP • •. “butterfly shaped” cross girdle broad “hourglass shaped” girdle Equal Area Lower Hemisphere. N. Equal Area Lower Hemisphere. Kamb Contours C.I. = 2.0 Sigma. W. Kamb Contours C.I. = 2.0 Sigma. E. 1664m. 234m S. N = 698. N = 2973.
(21) Hypotheses Law et al. (2014). Join me at the following poster session (OMp29) for more information. ISSM : Rückkamp & Humbert, Illustration: Jansen. Kerch (EGU 2018). Alley (1992).
(22) Small-scale changes Top. 9.6cm. 1360m. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. 1366m. N = 1193. N = 1220. N = 1129. N = 1124. N = 1268. N = 957. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Equal Area. Kamb Contours. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. Lower Hemisphere. C.I. = 2.0 Sigma. N = 1388. N = 1890. N = 1702. N = 1545. N = 1647. N = 1818.
(23) Small-scale changes Eigenvectors. Maximum Eigenvector V1 Intermediate Eigenvector V2 Minimum Eigenvector V3.
(24) Evolution of grain properties Grain size Evolution of a grain-size dependent anisotropy in the first 350m of ice core. •. Bulk anisotropy caused by deformation and early recrystallisation (?) Deformation by small grains in shallow part. Deformation by large grains in deeper part. 0.50 0.45. c−axis eigenvalue. 0.55. 0.60. •. 0.40. large grains small grains 150. e3 = measure for bulk anisotropy strength. 200. 250 bag top depth [m]. 300. 350.
(25) Evolution of grain properties Grain size. •. 200 300 400. Constant GS between 1400-1714m GS variability extreme between 550–900m GS variability smaller in glacial and rather constant between 1400-1714m. Grain-size increases. •. 100. 500 600 700 800 900 1000 1100. Grain-size decreases. •. Mean grain-size (GS) increases until 550m, decreases afterwards. Depth in m. •. 1200 1300 1400 1500 1600 1700. 10. 15. Mean grain diameter in mm. 20. 25.
(26) Evolution of grain properties Grain clustering • Small grains with similar orientations seem to cluster around large grains with different orientation # „core and mantle structure“. Passchier & Trouw (2005). Medium GS. Small GS. Medium GS 277m. 1648m.
(27) Evolution of grain properties Grain shape • Characteristic are also amoeboid grain shapes and sutured grain boundaries, typical features of grain boundary migration. Passchier & Trouw (2005). 338m. 828m.
(28) Evolution of grain properties Grain irregularity PR decreases. • Larger perimeter ratios (PR) than in EDML core • PR decreases until 722m, then increases linearly until final depth • Large, but constant PR variability. Grain size. EDML. PR increases. PR = measure for grain irregularity. Weikusat et. al (2009).
(29) Take home message Vertical eigenvector V1 and broad single maximum CPO Equal Area Lower Hemisphere. Tipping of eigenvectors, “crossing” of eigenvalues and butterfly cross-girdle. 100 Roman Warm. Kamb Contours C.I. = 2.0 Sigma. Period. 200 300 400. Holocene Climate Optimum. N = 1468. 500. “Wavy” eigenvalue evolution. 600. 5.9k event. Horizontal girdle CPO and flipping eigenvector. Depth in m. 700 800. Holocene Climate Optimum. 900. 8.2k event Brittle Zone. 1000 1100 1200 1300. Strong decrease of grain-size. 1400 1500. Last Glacial 1600 1700 0.0. Small perimeter ratio. 0.2. 0.4. 0.6. Eigenvalue. 0.8. Age model: Sune Rasmussen, Interpolated from 900m on.
(30) Take home message.
(31) Take home message Vertical eigenvector V1 and broad single maximum CPO Equal Area Lower Hemisphere. Tipping of eigenvectors, “crossing” of eigenvalues and butterfly cross-girdle. 100 Roman Warm. Kamb Contours C.I. = 2.0 Sigma. Period. 200 300 400. Holocene Climate Optimum. N = 1468. 500. “Wavy” eigenvalue evolution. 600. 5.9k event. Horizontal girdle CPO and flipping eigenvector. Depth in m. 700 800. Holocene Climate Optimum. 900. 8.2k event Brittle Zone. 1000 1100 1200 1300. Strong decrease of grain-size. 1400 1500. Last Glacial 1600 1700 0.0. Small perimeter ratio. 0.2. 0.4. 0.6. Eigenvalue. 0.8. Age model: Sune Rasmussen, Interpolated from 900m on.
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