Daniela Jansen1, I. Weikusat1,2, T. Kleiner1, F. Wilhelms1,3 , D. Dahl-Jensen4, A. Frenzel1, Nicolas Stoll1, T. Binder1, J. Eichler1, S. H. Faria5,6, S. Sheldon4, C. Panton4, S. Kipfstuhl1, and H. Miller1
Borehole shape
In situ-measurement of ice deforma1on from repeated borehole logging of the EPICA Dronning Maud Land (EDML) ice core, East Antarc1ca.
Introduc1on
The EPICA Dronning Maud Land ice core was drilled between 2001 and 2006 at the Kohnen Sta1on, Antarc1ca. During the drilling process the borehole was logged repeatedly, measuring temperature and pressure as well as azimuth and inclina1on (Wilhelms et al., 2015) Repeated logging of the borehole shape delivers an es1ma1on of the in-situ deforma1on within the ice sheet, in par1cular shear strain rates for the lower part, which are essen1al for the effec1veness of ice transport from the inner con1nent towards the ocean.
References:
Gundestrup et al. 1994. The UCPH borehole logger. Mem. Natl Inst ,Polar Res. ,Spec. Issue ,49, 224-233.
Weikusat et al., 2017. Physical analysis of an Antarc1c ice core—towards an integra1on of micro- and macrodynamics of polar ice. Phil. Trans. R. Soc. A 375:
20150347. h[p://dx.doi.org/10.1098/rsta.2015.0347
Wilhelms et al., 2015. The EPICA Dronning Maude Land deep drilling opera1on. Annals of Glaciology 55(68) 2014 doi: 10.3189/2014AoG68A189
BREMERHAVEN Am Handelshafen 12 27570 Bremerhaven Telefon 0471 4831-0 www.awi.de Affilia1ons:
(1) Alfred-Wegener-Ins1tut Helmholtz-Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany (daniela.jansen@awi.de), (2) Department of Geosciences, Eberhard Karls University, Tübingen, Germany, (4) CIC, Niels Bohr Ins1tute, University of Copenhagen, Copenhagen, Denmark, (3) Georg-August-Universität Gögngen, Gögngen, Germany, (5) Basque Centre for Climate Change (BC3), University of the Basque Country, Leioa, Spain, (6) IKERBASQUE, Basque Founda1on for Science, Bilbao, Spain
Instrumenta1on
The logging system developed by the University of Copenhagen (Gundestrup et al., 1994) recorded the 1lt of the borehole with respect to the ver1cal (inclina1on) as well as the heading of the borehole with respect to magne1c north (azimuth) by means of two fluxgate magnetometers.
Conclusions
• The girdle fabric indicates a layer of ice hardly affected by simple shear, which is sandwiched between a layer with strong shear below, and with moderate shear above.
• The larger grains are most likely due to less deforma1on, as the isotope curve show impuri1es content should be similar to finer grained layers above and directly below.
• Thus, it appears this layer might be a shadow zone due to the localized shear above and below.
Acknowledgements:
D.J. was funded by the HGF junior research group “The effect of deforma1on mechanism for ice sheet dynamics” (VHNG 802). This work is a contribu1on to the European Project for Ice Coring in Antarc1ca (EPICA), a joint European Science Founda1on/European Commission scien1fic programme, funded by the EU (EPICA-MIS) and by na1onal contribu1ons from Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Sweden, Switzerland and the UK. The main logis1c support was provided by IPEV and PNRA (at Dome C) and AWI (at Dronning Maud Land).
X 5.48
CL1.1 /AS 4.18 CR 2.8 EGU2017-16368
Contact: daniela.jansen@awi.de
Ice microstructure
Deforma1on regimes
From the two borehole shape records we could derive a ver1cal profile of the velocity gradient. This can be used to es1mate the local ver1cal shear strain rate. It appears that there is a transi1on from rela1vely high shear strain rate at the base, which reaches a minimum at 2372 m depth, where the ver1cal velocity gradient is as low as zero, indica1ng a pure shear environment.
As the ice flow velocity at the posi1on of the EDML core is rela1vely slow (about 0.75 m a-1), the changes of borehole shape between the logs during the drilling period were very small and thus difficult to interpret. Thus, the site has been revisited in the Antarc1c summer season 2016/2017 and logged again using the same measurement system. Here we present the change of the borehole shape between November 2005 and December 2016, and correlate our findings with observed ice microstructure
-3000 50 -50 40 -2500 -2000
-40 30
-1500
Depth (m)
North (m) -30
-1000
20
East (m) -500
-20 0
-10 10
0 0
10
-1900
-2000
-2100
4 2 -2200
Depth (m)
0 East (m) -2300
-2 -2400
-4 -6 -2500
No 0
Magne1c north
East Azimuth
Inclina1on
Depth increment
Evalua1on of ice thin sec1on in the depth region around the shis in the velocity gradient showed, that the c-axes are distributed in a girdle, a fabric typical for the upper part of ice divides, and an indicator for pure shear. Weikusat et al.
show that at EDML the girdle fabric is changing into a single maximum fabric at approx. 2000 m depth.
Around a depth of 2370 m the girdle reappears for a narrow depth interval of about ten meters.
At the same 1me the grain size is increasing significantly. Very large grains with an area of about 1000 mm2 are found in all samples measured from this depth region.
The grains are too large to es1mate a mean grain size, as many of the large grains are cut at the sample edges. Above and below this area grain size is much smaller.
As an indicator for the impurity content the δ18O ra1o is plo[ed.
From this and also from line scan images (not shown) we assume that the impurity content within the small grained samples shown here is not very different from the girdle region. The decrease of δ18O
below indicates the end of MIS 5.5. 1 cm
1 cm
1 cm 2360 m
2374 m
2380 m
This dataset provides the basis for a 3-D reconstruc1on of the borehole shape by integra1ng over all depth increments.
The data were processed with Matlab, with a script developed by C. Panton.
Addi1onal to the 3D borehole shape the ver1cal velocity gradient du/dz was calculated. In the top part of the borehole there are some undula1ons, but in the lower part where simple shear is the domina1ng deforma1on the shape appears to be more even.
The shape of the borehole describes a gentle curve, first poin1ng west north west and then at a depth of 1500 m turning towards south. The overall horizontal distance of 42 m from the top to the base of the borehole is small in comparison to the depth scale.
0 2 4 6
du/dz [s-1] ×10-11
-54 -52 -50 -48 -46 -44 -42-2438 -2413 -2388 -2363 -2338 -2313 -2288 -2268
Depth [m]
-2 0 2 4 6
du/dz [s-1] ×10-11
-54-52-50-48-46-44-42-2450 -2425 -2400 -2375 -2350 -2325 -2300 -2280
Depth [m]
δ 18O [‰]"
Deforma1on regimes Ver1cal velocity gradient Isotope ra1o
Dominated by simple shear
Dominated by simple shear
Dominated by
pure shear 2374 m
2360 m
2380 m
2360 m
1 cm 2374 m
2380 m Weikusat et al., 2017
1 cm 1 cm
Color code for c-axis orienta1on
