A review of the brittle ice zone in polar ice cores

Abstract

Maintaining ice-core quality through the brittle ice zone (BIZ) remains challenging for polar ice-core studies. At depth, increasing ice overburden pressurizes trapped air bubbles, causing fracture of cores upon exposure to atmospheric pressure. Fractured ice cores degrade analyses,
reducing resolution and causing contamination. BIZ encounters at 18 sites across the Greenland, West and East Antarctic ice sheets are documented. The BIZ begins at a mean depth of 545 ± 162 m (1 standard deviation), extending to depths where ductile clathrate ice is reached: an average
of 1132 ± 178 m depth. Ice ages in this zone vary with snow accumulation rate and ice thickness, beginning as young as 2 ka BP at Dye-3, Greenland, affecting ice >160 ka BP in age at Taylor Dome, Antarctica, and compromising up to 90% of retrieved samples at intermediate-depth sites.
Effects of pressure and temperature on the BIZ are explored using modeled firn-column overburden pressure and borehole temperatures, revealing complex associations between firn densification and BIZ depth, and qualitatively supporting expected thinning of the BIZ at low ice temperatures due
to shallower clathrate stability. Mitigating techniques for drilling, transport, sampling and analysis of brittle ice cores are also discussed.

Full text

A review of the brittle ice zone in polar ice cores
Peter D. NEFF
Antarctic Research Centre, Victoria University of Wellington, Wellington, New Zealand
E-mail: peter.neff@vuw.ac.nz
National Isotope Centre, GNS Science, Gracefield, New Zealand
ABSTRACT. Maintaining ice-core quality through the brittle ice zone (BIZ) remains challenging for
polar ice-core studies. At depth, increasing ice overburden pressurizes trapped air bubbles, causing
fracture of cores upon exposure to atmospheric pressure. Fractured ice cores degrade analyses,
reducing resolution and causing contamination. BIZ encounters at 18 sites across the Greenland, West
and East Antarctic ice sheets are documented. The BIZ begins at a mean depth of 545 162 m (1
standard deviation), extending to depths where ductile clathrate ice is reached: an average of
1132 178 m depth. Ice ages in this zone vary with snow accumulation rate and ice thickness,
beginning as young as 2 ka BP at Dye-3, Greenland, affecting ice >160 ka BP in age at Taylor Dome,
Antarctica, and compromising up to 90% of retrieved samples at intermediate-depth sites. Effects of
pressure and temperature on the BIZ are explored using modeled firn-column overburden pressure and
borehole temperatures, revealing complex associations between firn densification and BIZ depth, and
qualitatively supporting expected thinning of the BIZ at low ice temperatures due to shallower clathrate
stability. Mitigating techniques for drilling, transport, sampling and analysis of brittle ice cores are also
discussed.
KEYWORDS: clathrate hydrates, ice core, ice coring, paleoclimate, polar firn
INTRODUCTION
Deep ice drilling at both poles reveals valuable climate
records extending up to 800 ka into the past (EPICA
Community Members, 2004), in addition to unveiling much
about the physical structure of the Greenland, West and East
Antarctic ice sheets. In recent years, ice-core drilling,
transport, sampling and analytical procedures have con-
tinually improved, spurred by the goal of providing con-
tinuous, high-resolution records of atmospheric greenhouse
gases and climate and environmental proxies.
However, poor ice-core quality in the so-called brittle ice
zone (BIZ), where extensive fracturing of core samples is
caused by rapid relaxation (decompression) of the ice (e.g.
Gow, 1971), remains a technical challenge at all phases of
intermediate-depth and deep ice-core studies. At the drill
site, mechanical stresses from drilling and logging the core
can degrade initial core quality in this zone. Sampling at
freezer facilities often induces further fracturing as indi-
vidual brittle ice-core samples are cut with bandsaws.
Finally, brittle ice samples prove challenging for laboratory
procedures such as continuous-flow analysis (e.g. Osterberg
and others, 2006; Bigler and others, 2011), with fractures
allowing drilling fluid required for deep ice drilling to
penetrate into samples, contaminating major-ion chemistry,
trace chemistry and gas measurements.
While at relatively high-snow-accumulation deep drill
sites ice ages in the BIZ represent several thousand years of
the Holocene, high-snow-accumulation intermediate-depth
drill sites and low-snow-accumulation deep drill sites may
place many thousands of years of transitional and/or glacial
ice within this zone of compromised core quality. In the
case of drill sites with intermediate ice thickness, a large
fraction (90% or more) of the dated ice-core record will
likely be placed within the BIZ. Brittle ice-core quality,
through recovery, transport, sampling and analysis, is one of
the technical challenges identified by the International
Partnerships in Ice Core Sciences (IPICS), with implications
both for the ‘40k array’ initiative to gather a spatially
distributed, bipolar network of 40 ka ice-core records, and
any Antarctic ‘oldest ice’ site chosen to retrieve a continuous
1.5 Ma record (IPICS, 2005; Brook and others, 2006).
The term ‘brittle ice’ with respect to ice drilling originates
in the description by Gow and others (1968) of highly
fractured ice from depths between 400 and 900 m in the
Byrd Station ice core, the first drilled to bedrock in
Antarctica. Investigation of ice relaxation by Gow (1968,
1971), also in the Byrd Station ice core, indicates that the
linear increase in overburden pressure with depth accord-
ingly increases the in situ pressure within air bubbles in the
ice; once removed from this high-pressure environment,
bubble pressure exceeds the tensile strength of the confining
ice, causing cracking between bubbles and initiating wide-
spread fracturing in ice cores (Uchida and others, 1994;
IPICS, 2005). Immediately upon exposure at the surface,
fracturing begins explosively, propagating across- and along-
core on decimeter scales. Slower relaxation of ice cores over
periods of months to years after drilling (also explored by
Gow, 1971) may alleviate some or all remaining brittleness;
however, ice cores from Siple Dome, Antarctica, remain
brittle more than a decade after cores were retrieved
(personal communication from M. Twickler, 2013). Brittle
fracture in ice cores only fully diminishes at depths where air
bubbles are absorbed into the ice, transitioning to air-hydrate
crystals (clathrates). Here the ice is more ductile as
dissociation pressure and temperature is reached, incorpor-
ating gases into the crystal lattice (e.g. Miller, 1969; Shoji
and Langway, 1982; Pauer and others, 1995; Kuhs and
others, 2000; Lipenkov, 2000). This transition occurs gradu-
ally as bubble number-densities decrease and clathrates
become dominant (e.g. Uchida and others, 1994; Kipfstuhl
and others, 2001; Ueltzhöffer and others, 2010). It is
expected that clathrate formation, and thus onset of ductile
Annals of Glaciology 55(68) 2014 doi: 10.3189/2014AoG68A02372

ice and improved ice-core quality, occurs at shallower
depths as ice temperature decreases (Miller, 1969).
The BIZ, though observed at all deep drilling sites, has
seldom been specifically examined and is only approxi-
mately defined as a zone beginning several hundred meters
below the ice surface and extending to depths between
1000 and 1500 m. This is in part due to the inherently
qualitative definition of brittle ice, wide-ranging metrics for
ice-core quality, rounding of reported depths, and the
entangled effects of ice physical properties and stresses
(mechanical and thermal) induced during drilling, on-site
handling and transport. BIZ depths collated here are likely
accurate to 50 m. The first sustained decline in ice-core
quality should be reported, and is considered here, as the
top depth of the BIZ, with the bottom depth marked by a
return to consistently excellent ice-core quality (e.g. fig. 10
of Souney and others, 2014). This paper compiles reported
information regarding the BIZ for 18 deep (>1500 m) and
intermediate-depth (500–1500 m) polar ice-drilling projects
(Fig. 1a and b). Two effects are explored with possible
controls on brittle ice onset and relief. First, the effect of
varying firn-column thickness on overburden pressure at
depth is considered using modeled firn-column densities at
all sites. Second, reported brittle ice depths are examined
with respect to clathrate stability using in situ ice tempera-
ture from borehole measurements and modeled overburden
pressures. Techniques are detailed that have been or could
be employed to reduce brittle fracture in ice cores during
drilling, transport and sampling, and challenges pertaining
to analysis of brittle ice samples are also discussed.
BRITTLE ICE ZONE DEPTH
Reported BIZ depths and drill site conditions – snow accu-
mulation rate, mean annual surface air temperature and firn/
ice transition (FIT) depth – are summarized in Tables 1, 2 and
3 for sites across the Greenland ice sheet (GIS), West
Antarctic ice sheet (WAIS) and East Antarctic ice sheet (EAIS),
respectively. BIZ top and bottom depths, as well as FIT depth
(assuming a density of 830 kg m
–3
), are displayed for all sites
in Figure 2. While intermediate-depth drilling projects at
Siple Dome, Berkner Island, Roosevelt Island, Law Dome
and Taylor Dome experienced brittle ice conditions, the ice/
bed interface was encountered before reaching pressures
and temperatures suitable for clathrate formation and the
transition to ductile ice. Some ice-core quality improvement
was anecdotally observed in the final several meters of the
Siple Dome, Berkner Island and Roosevelt Island ice cores,
possibly due to higher temperatures near the bed (Gow and
Meese, 2007; Mulvaney and others, 2007; personal com-
munication from N. Bertler, 2013).
Pressure and temperature effects: brittle zone onset
Snow accumulation rate and surface air temperature are
primary controls on the densification of polar firn (e.g.
Herron and Langway, 1980; Ligtenberg and others, 2011),
although the firnification process is not fully understood
(Hörhold and others, 2011). Firn densification is relevant
here for its role in isolating air bubbles from porous firn and
Fig. 1. Locations of polar drill sites in (a) Greenland and
(b) Antarctica discussed in text. Map data from Timmermann and
others (2010).
Table 1. Site information, brittle ice zone (BIZ) depths (bold), mean annual snow accumulation, mean surface air temperature, firn/ice
transition depth (density 830 kg m
–3
reached) and BIZ ice ages for Greenland ice sheet ice-drilling sites. GRIP: Greenland Ice Core Project;
GISP2: Greenland Ice Sheet Project 2; NGRIP: North Greenland Ice Core Project; NEEM: North Greenland Eemian Ice Drilling
Camp Century Dye-3 GRIP GISP2 NGRIP NEEM
Coordinates 77.17° N,
61.13° W
65.18° N,
43.82° W
72.58° N,
37.63° W
72.60° N,
38.50° W
75.10° N,
42.32° W
77.45° N
51.60° W
Years drilled 1963–66
1
1979–81
7
1989–92
9
1989–93
14
1996–2004
9
2008–12
19
Surface elevation (m) 1887
2
2490
7
3238
11
3200
14
2917
9
2450
19
Drill depth (m) 1387
1
2037
7
3029
11
3053
14
3090
9
2540
19
Ice thickness (m) 1387
1
2037
7
3029
11
3053
14
3090
9
2540
19
BIZ top (m) 600
3
800
8,9
800
11
650
15,16
790
9
609
20
BIZ bottom (m) 1150
3
1200
8,9
1300
11,12
1400
15,16
1200
9
1281
20
BIZ thickness (m) 550 400 500 750 410 671
Snow accumulation (m ice a
–1
) 0.33
2
0.56
9
0.23
9
0.22
9
0.19
9
0.22
19
Mean air temperature (°C) –24
4
–20
9
–32
9
–32
4
–32
9
–29
19
Firn/ice transition (m) 72
5
67.5
4
78
13
77
4
78
18
78.8
21
BIZ top age (ka BP) 2.3
6
2
10
4
9
3
15,17
4.7
9
3
22
BIZ bottom age (ka BP) 10
6
3.8
10
7.1
9
8.3
15,17
8
9
9
22
1
Ueda and Garfield (1968);
2
Drinkwater and others (2001);
3
Shoji and Langway (1987);
4
Cuffey and Paterson (2010);
5
Kovacs and others (1969);
6
Dansgaard
and others (1969);
7
Gundestrup and Hansen (1984);
8
Shoji and Langway (1982);
9
Vinther and others (2006);
10
Langway and others (1985);
11
Dansgaard and
others (1993);
12
Pauer and others (1995);
13
Schwander and others (1993);
14
Alley and others (1993);
15
Gow and others (1997);
16
M. Twickler (personal
communication, 2013);
17
The Greenland Summit Ice Cores CD-ROM (1997);
18
Martinerie and others (2009);
19
NEEM Community Members (2013);
20
Warming and others (2013);
21
Buizert and others (2012);
22
Rasmussen and others (2013).
Neff: The brittle ice zone in polar ice cores 73

because the thickness and density of the firn column controls
the exact overburden pressure imposed on ice and air
bubbles at depth. Overburden pressure may affect where
brittle fracture of ice cores commences, as thin firn columns
at warmer, higher-snow-accumulation sites should reach full
ice density (920 kg m
–3
) more rapidly and thus exert more
overburden pressure on ice and air bubbles at depth, and
vice versa. Additionally, overburden pressure and in situ ice
temperature control clathrate stability and are expected to
affect the depth of the transition out of the bubbly BIZ and
into the bubble-free, ductile clathrate-ice zone below, with
clathrates stabilizing at lower (higher) pressures when
temperatures are low (high) (e.g. Miller, 1969).
To quantify the influence of firn column thickness on
overburden pressure at depth, while also converting BIZ
depths to equivalent overburden pressures, firn-column
density is modeled using the University of Washington Firn
Model Intercomparison Experiment online Herron and
Langway (1980) model (FirnMICE, http://firny.ess.washing-
ton.edu/communityfirnmodel/). A surface snow density of
390 kg m
–3
is assumed for all sites, and mean annual surface
air temperature and snow accumulation rate (ice equivalent)
are input into the model to construct 1 m resolution depth–
density profiles from the surface to 300 m for each site
(Fig. 3a). Constant ice density of 920 kg m
–3
is assumed
below 300 m, although for bubbly ice this may be an
overestimate of several kg m
–3
. Snow density measurements
from Taylor Dome, Berkner Island and WAIS Divide agree
well with model data generated for these sites (density
measurements provided with the FirnMICE online model, not
shown). Overburden pressure at a given depth is calculated
as the sum of overlying snow and ice with density prescribed
by the FirnMICE model output. One cubic meter of ice
(density 920 kg m
–3
) exerts 9.02 10
–3
MPa overburden
pressure (Fig. 3b). Modeled overburden pressure at 300 m,
the depth at which the thickest modeled firn column (Dome
Fuji) reaches ice density of 920 kg m
–3
, is presented in
Figure 3c for all sites, plotted against modeled and observed
FIT depths (830 kg m
–3
density). The mean misfit between
observed and modeled FIT depth is –4%, with minimum
misfits of +1% and –2% (EPICA Dome C and WAIS Divide,
respectively) and a maximum misfit of –25% (Siple Dome).
Table 2. Site information, BIZ depths (bold), mean annual snow accumulation, mean surface air temperature, FIT depth (density 830 kgm
–3
reached) and BIZ ice ages for WAIS ice-drilling sites
Byrd Station Siple Dome Berkner Island WAIS Divide Roosevelt Island
Coordinates 80.02° S, 119.52° W 81.65° S, 148.81° W 79.55° S, 45.68° W 79.47° S, 112.09° W 79.36° S, 161.71° W
Years drilled 1966–68
1
1997–99
5
2003–05
7
2006–11
10
2011–12
Surface elevation (m) 1530
1
620
5
890
7
1766
10
550
Drill depth (m) 2164
1
1004
5
948
7
3405
10
764
Ice thickness (m) 2164
1
1004
5
948
7
3455
10
764
BIZ top (m) 400
1,2
400
5
450
7
650
11
475
BIZ bottom (m) 900
1,2
1000
5
948
7
1300
11
764
BIZ thickness (m) 500 600 498 650 289
Snow accumulation (m ice a
–1
) 0.14
3
0.11
3
0.13
7
0.22
10
0.23
13
Mean air temperature (°C) –28
3
–25
3
–26.5
7
–30
10
–23
13
Firn/ice transition (m) 64
3
57.5
3
57
8
76.5
12
52
13
BIZ top age (ka BP) 3.5
4
5
6
4.5
9
2.7
10
4
13
BIZ bottom age (ka BP) 9.5
4
57
6
95
9
6
10
>40
13
1
Gow (1968);
2
Gow (1971);
3
Cuffey and Paterson (2010);
4
Blunier and Brook (2001);
5
Gow and Meese (2007);
6
Brook and others (2005);
7
Mulvaney and
others (2007);
8
Gerland and others (1999);
9
R. Mulvaney (personal communication (2014);
10
WAIS Divide Project Members (2013);
11
Souney and others
(2014);
12
Kreutz and others (2011);
13
N. Bertler (unpublished information).
Fig. 2. Firn/ice transition (FIT; depth where density 830 kg m
–3
reached; circles), BIZ top depths (inverted triangles) and BIZ bottom depths
(triangles) at ice-drilling sites, ordered by increasing FIT depth. Open triangles denote BIZ bottom depths which represent the ice/bed
interface, rather than transition to the bubble-free, ductile ice zone. RICE: Roosevelt Island Climate Evolution; WSD: WAIS Divide; GISP2:
Greenland Ice Sheet Project 2; GRIP: Greenland Ice Core Project; NGRIP: North Greenland Ice Core Project; NEEM: North Greenland
Eemian Ice Drilling; EDML: EPICA Dronning Maud Land; EDC: EPICA Dome C.
Neff: The brittle ice zone in polar ice cores74

Across the 18 drill sites, 300 m overburden pressure
varies from 2.36 MPa at Dome Fuji (modeled FIT depth
114 m, +12% misfit) to 2.56 MPa at Siple Dome (modeled
FIT depth 46 m, –25% misfit; Fig. 3b and c). Poor FIT depth
reproduction for these end-member sites (Fig. 3c) suggests
an overestimate of the possible range of overburden
pressures; more conservative is 2.4 MPa at EPICA Dome C
(modeled FIT depth 101 m, +1% misfit) to 2.55 MPa at
Berkner Island (modeled FIT depth 52 m, –10% misfit). This
overburden pressure fluctuation at depth, a maximum
difference of 0.15–0.2 MPa between the thickest firn
column (slower firnification, lower overburden pressure at
depth) and the thinnest (faster firnification, higher over-
burden pressure at depth), is equivalent to 16.5–22.2 m of
ice overburden. If the firn column is ignored and full ice
density is assumed from the surface, overburden pressure at
depth (2.7 MPa at 300 m) is overestimated by 0.14–
0.34 MPa or 15.5–37.7 m of ice (dashed line, Fig. 3a and
b). This results in an underestimation of the depth for
theoretical clathrate stability. The BIZ does generally occur
at shallower depths where the firn/ice transition is shallower
(e.g. Fig. 2) and exerts greater overburden pressure at depth.
This is most clearly observed at GIS and WAIS drill sites,
although with low significance, as is expected due to low
precision of reported BIZ depths and complex temperature
and accumulation effects on firnification and clathrate
formation processes. Reported BIZ top depths begin
10 m deeper per 1 m thickening of the FIT at the 11 GIS
and WAIS drill sites (regression of GIS and WAIS FIT depths
and BIZ top depths gives R
2
= 0.54, not shown). As
overburden pressure alone cannot explain such a deepen-
ing, this suggests that unidentified firnification processes
likely exert significant control on BIZ onset depth. Propor-
tional deepening of BIZ onset is not observed at all EAIS drill
sites, especially not those on the EAIS plateau, perhaps
because of increasing tensile strength of ice at smaller grain
sizes and lower temperatures (e.g. Butkovich, 1954;
Petrovich, 2003), as well as other unknown effects associ-
ated with extreme low-temperature and low-accumulation
firn densification, grain growth and air-bubble formation.
Pressure and temperature effects: brittle zone relief
In situ ice temperature determines the depth (pressure) of
clathrate stability and thus should affect the depth of the
transition from bubbly, brittle ice to bubble-free, ductile ice.
This is supported by observations of shallower appearance of
clathrates and disappearance of air bubbles in ice cores at
low-temperature sites (e.g. observations from Dye-3 and
GRIP versus Vostok and Dome Fuji; Ikeda-Fukazawa and
others, 2001). While temperature effects controlling BIZ
onset are less clear, it is expected that the BIZ will be relieved
at shallower depths (lower pressures) where ice temperatures
are lower. Using overburden pressure as calculated above,
and borehole temperature data from selected drill sites
(Table 4), the BIZ can be evaluated with respect to
temperature and pressure and thus compared more accur-
ately with theoretical clathrate stability (Fig. 4). BIZ onset in
many cases begins at pressures (depths) where clathrates
should already begin to stabilize, but brittle fracture is not
relieved until ductile conditions are reached as bubble
number densities become sufficiently low and clathrates
dominate, strengthening ice cores. While it is tempting to
suggest that BIZ bottom pressure indeed decreases with
lower ice temperatures, this is not a statistically significant
feature including all reported BIZ data (excluding intermedi-
ate-depth sites where clathrates do not stabilize before the
ice/bed interface is reached). Certainly the BIZ in the
Greenland summit ice cores and at WAIS Divide (1200–
1400 m BIZ bottom depths; 10.5–12.5 MPa) extends sev-
eral hundred meters deeper than that of Dome Fuji, Vostok,
EPICA Dronning Maud Land and Talos Dome (840–1050 m
BIZ bottom depths, 7.2–9.2 MPa), but the EPICA Dome C ice
Table 3. Site information, BIZ depths (bold), mean annual snow accumulation, mean surface air temperature, FIT depth (density 830 kgm
–3
reached) and BIZ ice ages for EAIS ice-drilling sites
Law Dome Taylor Dome Vostok station EPICA Dome C Dome Fuji EPICA Dronning
Maud Land
Talos Dome
Coordinates 66.77° S,
112.80° E
77.70° S,
159.07° E
78.47° S,
106.87° E
75.10° S,
123.35° E
77.32° S,
39.70° E
75° S, 0° E 72.78° S,
159.07° E
Years drilled 1991–93
1
1993–94
4
1990–98,
2005–12
7
1999–2004
12
1993–96,
15
2003–07
16
2000–06
20
2005–07
25
Surface elevation (m) 1370
1
2375
5
3488
8
3233
13
3810
15
2892
21
2318
26
Drill depth (m) 1200
1
554
4
3769
9
3260
12
3035
16
2774
20
1620
26
Ice thickness (m) 1220
1
554
4
3769
9
3275
12
3035
16
2774
20
1795
26
BIZ top (m) 552
2
335
4
250
10
600
14
500
17
500
22
667
27,28
BIZ bottom (m) 1200
2
554
4
900
10
1200
14
840
17
1050
22
1002
27,28
BIZ thickness (m) 648 291 650 600 340 550 334
Snow accumulation (m ice a
–1
) 0.7
1
0.06
5
0.022
11
0.036
11
0.03
15
0.064
21
0.08
26
Mean air temperature (°C) –22
1
–43
5
–55
8
–54
11
–58
15
–44
21
–41
26
Firn/ice transition (m) 66
3
72
4
95
11
100
11
100
18
83
23
66
27
BIZ top age (ka BP) 0.9
1
10
6
10.5
8
29
13
19.3
19
7.7
24
11
26
BIZ bottom age (ka BP) 20
1
>160
6
61.5
8
80.5
13
46
19
24.3
24
30
26
1
Morgan and others (1997);
2
Morgan and others (1994);
3
Etheridge and others (1996);
4
Fitzpatrick (1994);
5
Morse and others (1999);
6
Steig and others (2000);
7
Vasiliev and others (2011);
8
Petit and others (1999);
9
Jouzel (2013);
10
Uchida and others (1994);
11
Cuffey and Paterson (2010);
12
Parrenin and others (2007);
13
EPICA Community Members (2004);
14
Parrenin and others (2012);
15
Watanabe and others (1999);
16
Motoyama (2007);
17
Fujii and others (2002);
18
Hondoh
and others (1999);
19
Kawamura and others (2007);
20
Severi and others (2007);
21
Ueltzhöffer and others (2010);
22
F. Wilhelms (personal communication,
2014);
23
Oerter and others (2004);
24
EPICA Community Members (2010);
25
TALDICE (Talos Dome Site Information; http://www.taldice.org/project/site/index.
php);
26
Stenni and others (2011);
27
TALDICE, 2006/07 field season (http://www.taldice.org/site/0607/index.php);
28
Schilt and others (2010);
29
Frezzotti and
others (2004).
Neff: The brittle ice zone in polar ice cores 75

core remained highly fractured to 1200 m depth (10.5 MPa).
Additionally, the BIZ at Camp Century, Dye-3 and Byrd
Station transitions to ductile ice at shallow depths, with
pressures only 1–2 MPa greater than that required for initial
clathrate stability, while most sites transition out of the BIZ at
pressures 3–5 MPa in excess of requirements for theoretical
onset of clathrate stability (Fig. 4). This raises the interesting
prospect that ice-flow advection of cold ice towards the
surface at flank sites could encourage shallower formation of
clathrates (e.g. Camp Century, Byrd Station; Shoji and
Langway, 1987). However, without disentangling the effects
Fig. 3. Results of FirnMICE firn column density modeling (Herron
and Langway (1980) model) and overburden pressure calculations.
(a) Density–depth profiles generated for the 18 drill-site tempera-
ture and snow accumulation regimes, shaded according to mod-
eled FIT depth (light grey: model FIT <70 m; grey: 70m < model FIT
< 80 m; black: model FIT >80 m); bold dashed line indicates ice
density (920 kg m
–3
). (b) Ice overburden pressure versus depth in
the firn column (0–300 m) for the 18 drill sites (light grey: model FIT
<70 m; grey: 70 m < model FIT < 80 m; black: model FIT >80 m);
bold dashed line indicates overburden pressure assuming constant
ice density from the surface. (c) 300 m depth overburden pressure
versus FIT depth from modeled (open circles) and measured (filled
circles) firn-column density data.
Fig. 4. BIZ top (inverted triangles) and bottom pressures (triangles)
plotted at respective in situ ice temperature from borehole
temperature measurements (Table 4). Theoretical clathrate stability
curves are plotted for N
2
(solid), O
2
(dotted) and air (dashed)
hydrates (Miller, 1969; Kuhs and others, 2000). The stability curve
of Kuhs and others (2000) is indicated with ‘x’. Open triangles
denote BIZ bottom pressures at the ice/bed interface, thus not the
full transition from the bubbly, brittle-ice zone to the bubble-free,
ductile ice zone below. WSD: WAIS Divide; EDC: EPICA Dome C.
Table 4. In situ ice temperature (borehole temperature; °C) at BIZ
top and bottom depths (see Tables 1–3) for selected sites.
Temperatures are 0.5°C, estimated from published graphical data
where original datasets could not be accessed. Temperature data
for Berkner Island are modeled
Drill site Temperature Source
BIZ
top depth
BIZ
bottom depth
Camp Century –23 –17 Shoji and Langway (1987)
Dye-3 –20.7 –20.6 Shoji and Langway (1987)
GRIP –31.4 –32.2 The Greenland Summit Ice
Cores CD-ROM (1997)
GISP2 –31 –32 The Greenland Summit Ice
Cores CD-ROM (1997)
NorthGRIP –32 –33 Dahl-Jensen and others
(2003)
Byrd Station –28.4 –28.7 Ueda and Garfield (1969),
T.J. Fudge (personal
communication, 2014)
Siple Dome –19.1 –2.75 G. Clow (personal
communication (2014)
Berkner Island –21 –10 Mulvaney and others
(2007)
WAIS Divide –29.8 –30.2 Cuffey and Clow (2014)
Law Dome –22 –7 Van Ommen and others
(1999)
Taylor Dome –33 –26.1 G. Clow (personal
communication, 2014)
Vostok station –55.5 –50.1 Salamatin and others
(1994)
EPICA Dome C –50 –42 Pol and others (2010)
Dome Fuji –52 –49 Ikeda-Fukuzawa and
others (2001)
Neff: The brittle ice zone in polar ice cores76

of imprecise BIZ records, drill performance and ice-core
handling/transport, it is difficult to use reported brittle ice
depths to further evaluate this correlation between clathrate
stability and the bottom of the BIZ.
To improve understanding of the mechanisms involved in
BIZ onset and relief, it may prove useful to investigate ice
physical properties including bubble number density,
bubble size, micro-bubbles and ice fabric (e.g. grain size,
crystal anisotropy). Take the anomalously shallow BIZ at
Vostok, for example: Uchida and others (1994) observed a
reduction of core quality caused by fractures as shallow as
100 m (likely linked to thermal drilling at Vostok), becoming
heavily fractured from 250 to 750 m with progressive
improvement to 900 m where core quality again became
excellent. This shallowest occurrence of the BIZ may be
related to a rapid increase in bubble number density (bubbles
cm
–3
) observed in the Vostok ice core beginning at 300 m
depth (increasing from 400 to 800 bubbles cm
–3
; Uchida
and others, 1994; Ueltzhöffer and others, 2010). Such an
increase in bubble number density may effectively reduce
bubble pressures required for fracture propagation by
narrowing interstitial ice between bubble cavities. Bubble
number densities reported in the BIZ at Byrd Station and
WAIS Divide were relatively more constant at 200 and
450 bubbles cm
–3
, respectively (Gow, 1971; Fitzpatrick
and others, in press). Micro-bubbles formed through sublim-
ation–condensation processes may also play a role at Vostok
(e.g. Lipenkov, 2000), and are observed in the EPICA Dome
C and Dronning Maud Land ice cores (Ueltzhöffer and
others, 2010). The second shallowest occurrence of the BIZ,
335 m at Taylor Dome, may be anomalously shallow due to
high strain rates at this site; elongated bubbles were observed
from 360 to 390 m (Fitzpatrick, 1994).
BRITTLE ICE ZONE AGE
Figure 5 displays observed BIZ top and bottom ages from
published age scales developed for the 18 drill sites
discussed above (detailed ages in Tables 1–3). At deep ice-
core sites where snow accumulation rates are relatively high
(e.g. GIS and inland WAIS sites), the age of ice within the BIZ
is relatively young, dating from as few as 2 ka BP (Dye-3;
Langway and others, 1985) to 9.5 ka BP (Byrd Station; Blunier
and Brook, 2001). These ages represent 10% or less of the
age of dated ice-core records developed from these sites.
While the Holocene marks the beginning of relatively stable
global climate, with the exception of the 8.2 ka BP event (e.g.
Alley and others, 1997), continued emphasis on under-
standing natural climate variability precludes classifying ice
of this age as scientifically less interesting than older
transitional and glacial ice (e.g. Mayewski and others,
2004; Marcott and others, 2013; Steig and others, 2013).
A wide range of ice ages is found within the BIZ at deep
ice-core sites with low snow accumulation rates (e.g. EAIS
plateau sites: Taylor Dome, Vostok, Dome C, Dronning
Maud Land, Dome Fuji, Talos Dome), and intermediate-
depth ice-core sites with higher snow accumulation rates
(e.g. coastal Antarctic sites: Siple Dome, Berkner Island,
Roosevelt Island, Law Dome). In intermediate-depth ice-
core records (all at intermediate ice-thickness coastal
Antarctic locations), ice at the top of the BIZ dates to
between approximately 4 ka BP at Roosevelt Island (personal
communication from N. Bertler, 2013) and 9 ka BP at Law
Dome (Morgan and others, 1997). Ice at these coastal
Antarctic sites remains brittle to the bed, dating to a
minimum of 40 ka BP (Roosevelt Island; personal commu-
nication from N. Bertler, 2013) and in all cases placing at
least 90% of the dated ice-core record within the BIZ. Brittle
ice at EPICA Dome C dates from 29 to 80.5 ka BP (EPICA
Community Members, 2004). While not a large fraction of
the entire ice-core record (6% of the 800 ka record), this
section of the Dome C ice core is highly detailed when
compared to deeper ice where ice-flow thinning impairs
resolution at this extremely low-accumulation site. From the
surface to 2000 m depth, 55 cm long sections of the Dome
C ice core exhibit temporal resolution of 50–100 years or
less, while below 2000 m similar sections contain 200–1000
years or more (Pol and others, 2010). Taylor Dome, a very
shallow site, became brittle from a depth of 335 m, dated to
10 ka BP, remaining brittle to the bed and spanning ice ages
in excess of 160 ka BP (Steig and others, 2000). Ice within the
BIZ at these coastal Antarctic and EAIS plateau sites spans
ages of fundamental interest to paleoclimate research,
relevant to investigations into regional timing of the onset
of deglaciation during the late-Pleistocene and rapid climate
anomalies occurring during this transition (e.g. Younger
Dryas, Antarctic Cold Reversal; Alley and others, 1993;
WAIS Divide Project Members, 2013).
MITIGATING BRITTLE ICE IMPACTS
Drilling techniques
Currently, drilling through the BIZ produces ice-core
sections with several to many breaks, fractures and hairline
cracks, commonly affecting more than half of recovered ice
in the middle of this zone. Drilling fluid required at these
depths pervades all cracks and fractures, contaminating
many chemical analyses. At WAIS Divide, where BIZ core
quality was the best of any recent United States-led drilling
project, 1 m long core sections from 900 to 1200 m were on
Fig. 5. BIZ top (inverted triangles) and bottom (triangles) age (ka BP) at drill sites, ordered by region and date of drilling. Open triangles
denote BIZ bottom ages which represent the maximum dated age (ice/bed interface) at these sites. WSD: WAIS Divide; RICE: Roosevelt
Island Climate Evolution; EDC: EPICA Dome C; EDML: EPICA Dronning Maud Land.
Neff: The brittle ice zone in polar ice cores 77

average between ‘good’ (containing zero to three breaks or
50% unfractured) and ‘fair’ quality (containing >10 cm of
core length without fractures; Souney and others, 2014).
As the only step performed at in situ pressures within the
ice sheet (minimum of 2.0 MPa, maximum 12.4 MPa in the
BIZ), with the added benefit of damping effects associated
with a liquid-filled borehole, the drilling process provides
important opportunities to reduce the major component of
brittle fracture in ice cores by performing mechanically
severe steps that might damage cores if performed at surface
pressures (700 hPa, 0.07 MPa). For example, drilling at
WAIS Divide employed a unique strategy specifically for
brittle ice, performing three core breaks per 2.5 m drill run
before returning the drill sonde to the surface (detailed in
Souney and others, 2014). This technique of performing
several core breaks per run at in situ pressure results in ice-
core sections that are ready to ship, without subjecting
brittle ice cores at surface pressure to the vibration and high
stresses of making circular-saw section cuts. A similar
downhole technical innovation suggested by Ueda (2002)
is the development of a drill sonde that captures and seals
cores in a vessel at in situ pressures, potentially allowing for
a more gradual transition to atmospheric pressure than the
usual minutes-to-hours-long transition as the drill sonde is
brought to the surface. Slowing the pressure and tempera-
ture transition by hoisting brittle ice-drill runs slowly to the
surface, reducing drill penetration rate and considering drill
fluid pressure balance have also been proposed (see the
discussion by J. Schwander and others in IPICS, 2005).
Maintaining temperatures within drilling and on-site
storage structures similar to those at depth in the borehole
is a commonly practised technique in ice drilling (e.g.
Souney and others, 2014). At Roosevelt Island, an actively
cooled storage cave kept ice at –23°C despite surface
temperatures reaching as much as –5°C (personal commu-
nication from D. Mandeno, 2014). Temperature gradients
may reach several tens of degrees between in situ ice
temperature and surface drill structure temperatures, which
will cause differential heating of ice cores brought to
the surface, inducing temperature and stress gradients as
the ice-core surface warms (and expands) more quickly than
the interior. However, it is difficult to overstate the primary
impact of the pressure gradient between the BIZ and the
surface, which, as a ratio, is a minimum of 30 : 1 at the
shallowest observed BIZ onset (Vostok: 2.0 MPa at 250 m,
0.07 MPa surface air pressure) and grows to more than a
100 : 1 ratio at most ice-core drill sites (reaching a maximum
BIZ bottom-to-surface-pressure ratio of 177 : 1 at the bottom
of the Greenland Ice Sheet Project 2 (GISP2) BIZ: 1400 m
depth, 12.4 MPa).
Transport techniques
Reduction of mechanical shock during core transport from
the drill site to laboratories has been achieved primarily by
sheathing brittle ice cores in tight-fitting nylon netting
immediately upon removal from the drill, and delaying
shipment of brittle ice for as long as logistically possible to
allow for maximum relaxation. Use of netting was innovated
at Berkner Island (Mulvaney and others, 2007), and the
same was used successfully at WAIS Divide (Souney and
others, 2014) and Roosevelt Island (personal communica-
tion from N. Bertler, 2013). While itself not preventing
initial fracture of brittle ice cores, nylon netting holds
badly fragmented sections in place, preventing any loss of
stratigraphic order, and protects cores from further damage
due to vibration during shipment.
Relaxation of ice cores after drilling is examined
thoroughly by Gow (1971), measuring significant post-
drilling volume expansion (density reduction) at all depths
in the Byrd Station ice core. Cores from the BIZ around
800 m depth exhibit greatest expansion: 0.2% (0.002 kg
m
–3
density decrease) after 8 months, 0.4% (0.004 kg m
–3
decrease) after 16 months, and up to 0.6% (0.006 kg m
–3
decrease) after 27 months (see fig. 2 of Gow, 1971). Cores
from other depths expanded by an average of 0.2%
(0.002 kg m
–3
decrease) after 27 months. Nearly identical
relaxation characteristics were observed in the GISP2 ice
core (Gow and others, 1997). Ice cores retrieved using a hot-
water drill at Siple Dome relaxed very quickly, likely due to
the thermal drilling technique, but cores were also stored at
relatively high surface temperatures after drilling (Engelhardt
and others, 2000; Gow and Meese, 2007). Ice cores
retrieved by PICO (Polar Ice Coring Office) mechanical
drilling at Siple Dome showed very little relaxation, and
remain brittle to date (Gow and Meese, 2007; personal
communication from M. Twickler, 2013).
Much of this relaxation in brittle ice is attributed to slow
dilation of highly pressurized bubbles abundant in this zone.
This lends support to the practice of overwintering brittle
ice, most recently performed at WAIS Divide (Souney and
others, 2014). At Roosevelt Island, 2 m long drill runs were
sheathed in netting and stored in a refrigerated snow cave
for up to 14 days before making 1 m section cuts for
shipment. Improvements were noted in bandsaw cuts after
even this short period of relaxation, although some
propensity for fracturing remained (personal communication
from N. Bertler, 2013). It is important to note that
overwintering or long-term storage of brittle ice cores delays
sampling and analysis of this section of ice. While this delay
is less significant on the time frame of a multi-year deep
drilling project, for smaller endeavors, especially at inter-
mediate-depth coastal sites where a significant portion of
the dated ice-core record lies within the BIZ, delaying
shipment and/or sampling of ice cores is more challenging.
Sampling techniques
Ice-core sampling is commonly performed at –20 to –25°C,
making preliminary longitudinal cuts using a horizontal
bandsaw and subsequent sampling with common vertical
bandsaws. While a simple tool, the bandsaw applies
consistent force at the cutting teeth, especially if tracking
of the saw along-core is automated to steadily move the saw
blade through the ice. After sufficient relaxation – allowing
for slow dilation of air bubbles long after initial violent
cracking and fracturing observed immediately after drilling –
brittle ice may feed through a bandsaw with little additional
fracturing. Brittle ice below depths of 475 m from Roosevelt
Island proved prohibitively brittle after 6 months stored at
–30°C, with several instances of near-catastrophic fracturing
during horizontal cutting due to vibration from the saw
blade. At this depth, a conventional vertical bandsaw used
to make 0.035 m 0.035 m 1.0 m rods of ice also began
to add many fractures to previously flawless ice-core
samples, which had immediately prior been cut into
0.1 m 0.035 m 1.0 m slabs without damage. Core
sampling was halted at 500 m and the remaining ice stored
at –18°C for an additional year, after which cutting
proceeded without significant challenges.
Neff: The brittle ice zone in polar ice cores78

Other cutting instruments may prove more conducive to
processing brittle ice-core samples. Tison (1994) describes
the use of a diamond-wire saw for preparing thin sections of
debris-rich or brittle ice, an option attractive for its reduced
vibration levels. However, with slow cutting rates and the
high cost of diamond wires, this option may require
significant development before being applicable to high-
volume ice-core sample processing.
Analytical techniques
Many analyses performed in polar ice-core studies depend
on relatively unbroken samples to prevent contaminants
from altering original paleoclimate or paleo-environmental
signals. Measurements of atmospheric gases preserved in
bubbles ideally require avoidance of section cuts and other
breaks present in ice-core samples, in order to exclude
modern atmospheric gases from analysis. Major-ion and
trace chemistry, analyzed in longitudinal samples from the
center of ice cores in order to avoid modern contaminants
imparted during drilling, shipment and sampling, requires
removal of outer sample surfaces and cleaning of any
exposed surfaces including section cuts and fractures.
Drilling fluid pervades all fractures in ice-core samples –
especially fluids with low volatility, such as Estisol-240
(Dow Haltermann, Germany) coconut oil extract used at
NEEM (North Greenland Eemian Ice Drilling) and Roosevelt
Island – rendering chemical analyses extremely difficult,
especially in ice from the BIZ.
Continuous-flow analysis (CFA) systems gravity-feed
longitudinal ice samples through a sectioned heating plate,
pumping meltwater directly into online instruments and/or
fraction collectors to create discrete sub-samples (e.g.
Osterberg and others, 2006; Bigler and others, 2011).
When analyzing highly fractured samples in CFA systems,
contamination affects not only fractured core sections, but
also subsequent ice as relatively high-concentration con-
taminants wash out of sample lines and instruments.
Additionally, vertical guide systems for gravity-feeding
samples struggle to accommodate ice samples containing
fractures, especially high-angle longitudinal fractures,
which commonly wedge against plastic guides, disrupting
sample flow and accurate depth logging.
Fractured sample sections are commonly removed
entirely from CFA campaigns, such that CFA for major-
ion chemistry of brittle ice from the WAIS Divide ice core
only processed 62% of ice from the depths of 577 m to
1300 m (personal communication from D. Ferris, 2014). An
optical drill fluid detection system identified 27 instances
of drill fluid contamination in CFA tubing while analyzing
175 m of brittle ice from the NEEM ice core (Warming
and others, 2013). Using this detection system, particular
negative impacts were noted for dust, conductivity,
ammonium, hydrogen peroxide and sulfate datasets in
brittle ice from the NEEM ice core. While technically
challenging, feasible solutions have been developed for
analyzing brittle ice-core samples with little sample loss,
such as the high-resolution CFA water stable-isotope
analysis of 13 mm 13 mm 1.0 m rods of ice from the
WAIS Divide ice core (B. Vaughn and others, unpublished
information). This approach used tightly fitting square
acrylic tubes to protect the fragile ice rods during shipment,
and also support them vertically in a sample rack during
analysis – while light vibration successfully prevented
wedging against the acrylic tubing during melting, even
in highly fractured brittle ice (personal communication
from B. Vaughn, 2014).
CONCLUSION
At 18 intermediate-depth and deep polar ice-core drilling
sites across the Greenland ice sheet and West and East
Antarctic ice sheets, the BIZ of poor-quality, highly fractured
ice cores extends from a mean top depth of 545 162 m
to a mean bottom depth of 1132 178 m (excluding
intermediate-depth sites where the ice/bed interface is
reached before the transition to ductile ice).
Firn-column thickness, controlled primarily by site
temperature and snow accumulation rate, determines the
precise overburden pressure at depth, quantified here to
demonstrate that thicker (thinner) firn columns apply less
(more) pressure at depth. Both reported BIZ top and bottom
depths at GIS and WAIS sites are in fact deeper where FIT
depth is similarly deep, as could be expected due to
fluctuating overburden pressure; however, the deepening of
the BIZ is greater than can be explained from overburden
pressure alone. Additionally, the absence of this relationship
at extremely cold, dry EAIS plateau sites suggests that other
factors associated with firn densification and grain growth,
affecting eventual clathrate formation and stability, are likely
involved. While it is expected, due to pressure and
temperature controls on clathrate stability, that the BIZ
should transition to ductile, bubble-free ice at lower
pressures (shallower depths) when ice temperatures are
lower, this is not a quantitative feature of reported BIZ
bottom depths including all deep drill sites. Although
shallower stability of clathrates and shallower disappear-
ance of air bubbles is observed in ice cores from colder sites,
BIZ bottom depths do not clearly behave similarly. This is
likely due to imprecision in reported BIZ depths, as well as
the convolution of ice-core fracture caused purely by
physical properties during relaxation and fracture due to
additional stresses induced during retrieval (e.g. drill
performance, handling techniques).
Consideration of this 531 138 m thick zone (mean BIZ
top minus bottom depth excluding intermediate-depth sites),
where pressurized air bubbles and ice relaxation upon
retrieval of cores cause extensive and sometimes explosive
fracturing, is pertinent to all projects developing records
from ice cores recovered at depths greater than 400 m (i.e.
mean BIZ top depth, 545 m, less one standard deviation,
162 m, gives 383 m). Relatively high snow accumulation
rates and thick ice ensure that ice from the BIZ at inland GIS
and WAIS sites is restricted to Holocene ages, and spans
<10% of the completed ice-core records. EAIS plateau and
coastal Antarctic drill sites, however, place a considerably
larger amount of dated ice-core records in the BIZ. Low
snow accumulation rates and large ice thicknesses at EAIS
plateau sites place 20 000–50 000 years of glacial ice from
these ice-core records in the BIZ, affecting the resolution of
these valuable records which are already challenged by low
snow accumulation rates. Conversely, high snow accumu-
lation rates and limited ice thickness at coastal Antarctic
drill sites place the majority of recovered ice and dated ice-
core records within the BIZ, at least 90% of the records at all
coastal Antarctic drill sites.
Innovative drilling and transport strategies have had
recent successes in minimizing fracturing in ice cores from
the BIZ, including the WAIS Divide brittle-ice drilling
Neff: The brittle ice zone in polar ice cores 79

technique, netting core after removing from the drill, and
relaxing ice prior to shipment and/or sampling. However,
comparatively little has been done to develop better
sampling methods and techniques for analyzing highly
fractured ice-core samples. Consideration of mitigating
strategies at any stage can have beneficial impacts on
downstream work phases, most importantly potentially
improving final scientific results. Targeting of ice-core sites
is increasingly focused by refined scientific questions,
specific research interests and desire to infill areas of sparse
geographical coverage. Advancing understanding of the
physical mechanisms controlling the BIZ has the potential to
significantly improve the continuous recovery and develop-
ment of ice-core paleoclimate and environmental records,
as this zone affects samples of ages spanning periods of
fundamental scientific interest at many potential drill sites.
ACKNOWLEDGEMENTS
The author thanks R. Dadic and N. Bertler for thoughtful
discussion of the manuscript. Thorough critique, discussion
and encouragement from two anonymous reviewers signifi-
cantly improved on the original text. The online University
of Washington FirnMICE model (http://firny.ess.washington.
edu/communityfirnmodel/) was used to model firn density
and estimate overburden pressures. The author has been
funded by the New Zealand Ministry of Business,
Innovation, and Employment Grants (RDF-VUW-1103,
CO5X0202), Victoria University and GNS Science. This
work is a contribution to the Roosevelt Island Climate
Evolution (RICE) Programme, funded by national contribu-
tions from New Zealand, Australia, Denmark, Germany,
Italy, the People’s Republic of China, Sweden, United
Kingdom and the United States of America. The main
logistic support for RICE was provided by Antarctica New
Zealand (K049) and the US Antarctic Program (I-209M).
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