Interpretation of detailed density profiles of alpine ice
cores as past environment proxy signal

KANAMORI Syosaku
Ph. D. Dissertation
Graduate School of Environmental Science
Hokkaido University
March 2008

Contents
Contents ............................................................................................................................ 1
1.

2.

Introduction .............................................................................................................. 3
1.1.

Temporal resolution in ice core studies .............................................................. 3

1.2.

Density profiles of ice cores ............................................................................... 4

1.3.

Objectives of this study ...................................................................................... 6

Ice coring, field measurements and ice core measurements..................................... 8
2.1.

Ice coring ............................................................................................................ 8

2.2.

Spatial distribution of annual accumulation rate at the Summit Caldera, Mt.

Wrangell ..................................................................................................................... 14

3.

4.

5.

2.3.

Accumulation measurements for individual storms on Mt. Wrangell.............. 14

2.4.

Density measurements...................................................................................... 20

Interpretation of detailed density profiles at Mt. Wrangell ............................. 29
3.1.

Relationship between surface condition and density profile ............................ 29

3.2.

Effect of densification on detailed density profiles .......................................... 34

Detailed dating of Mt. Wrangell cores.................................................................... 37
4.1.

Estimation of surface condition at Mt. Wrangell from weather station data.... 37

4.2.

Detailed dating of the Mt. Wrangell ice cores .................................................. 43

4.3.

Monthly variation of parameters in ice core data ............................................. 50

Effect of snow drift at alpine ice core sites ............................................................ 56
5.1.

Comparison of annual accumulation rates ....................................................... 56

5.2.

Effect of erosions to ice core parameters.......................................................... 60

5.3.

Bending in depth-density profile ...................................................................... 62

5.4.

Redistribution of accumulated snow at the Summit Caldera of Mt. Wrangell. 63
16.

Conclusions ............................................................................................................ 66
6.1.

Interpretation of detailed density profiles as past surface condition proxy data
66

6.2.

Detailed dating using density profiles .............................................................. 66

6.3.

Accumulation environment of alpine ice core sites.......................................... 67

6.4.

Meaning of this study ....................................................................................... 67

Reference ........................................................................................................................ 69
Appendix A Detection of snow burial dates at the snow accumulation measurement ... 73
June 2005.................................................................................................................... 73
July 2005 .................................................................................................................... 74
August 2005................................................................................................................ 76
September 2005 .......................................................................................................... 80
October 2005 .............................................................................................................. 81
November 2005 .......................................................................................................... 83
December 2005........................................................................................................... 84
January to May 2006 .................................................................................................. 86
Appendix B..................................................................................................................... 88
Acknowledgements ........................................................................................................ 91

2

1. Introduction
1.1. Temporal resolution in ice core studies
Ice core studies unveil past climate history with various timescales. Ice coresfrom the Greenland ice sheet or Antarctic ice sheet date back to past hundreds thousands
years covering glacial and interglacial cycle [e.g. Dansgaard et al., 1993; GreenlandIce-core Project Members, 1993; Petit et al., 1999]. Annual layer thicknesses are thin insuch polar ice cores. For example, annual layer thicknesses in Dome Fuji ice core,
which is one of the oldest ice cores was less than 40 mm in ice thickness (Watanabe etal., 1999). Alpine ice cores from high accumulation sites cover ice for hundreds years,
which enable to reconstruct El Niño-Southern Oscillation Events (ENSO) or Pacific
Decadal Oscillations (PDO) [e.g. Thompson, 1984; Kang, 2005]. Annual layer thickness
are thick in such mountain ice cores. For example, annual layer thicknesses in Mt.
Wrangell ice core where is located in south central Alaska were a few meters in ice
equivalent. Such ice cores have higher potential for analysis with high temporal
resolution.
The highest time resolution in dating ice core was annual to seasonal ones.
Schwikowski et al. [1999], for example, dated an ice core from Fiescherhorn glacier,Swiss Alps using annual layer counting of seasonally changing parameters such astritium or stable isotope ratio. Nakazawa et al. [2005] determined four seasonal layers
using various species of pollen grains at Belukha glacier in Russia's Altai Mountains. In
such alpine sites, surface melting events, episodic precipitation, wind erosion and snow
drift make dating of ice core difficult and high temporal resolution is scarcely detained..
Analysis of ice core with higher temporal resolution as precise as monthly
3scale enlarges the potential of ice core study; one of the advantages of ice cores as past
climate proxy is that ice core contains past transported materials in atmosphere such as
Kosa dust (e.g. Goto-Azuma et al., 2003) which comes by episodic events. The datingwith monthly time scale makes it possible to examine such events with a help of
atmospheric observations and/or model results and helps understanding of interactions
to climate- or eco-systems. The combined analysis based on monthly scale dating of anice core also helps understanding of other ice core parameters such as stable isotope
ratio.
The key factors which limit temporal resolution of ice core dating arevariation of accumulation rate caused by episodic precipitation events, wind erosion andsnow drift. Although effect of melting is also the problem for ice core dating, this can be
avoided by site selection in the case of alpine glaciers at high latitude, which is thetarget of this thesis. The author focused detailed density profile to examine amount ofaccumulation in each event which is affected by episodic precipitation event, winderosion and snow drift. At a snow surface, textures and density depends on conditions
during and after the accumulation (Akitaya and Yamada, 1991). In detail, shapes ofprecipitated snow crystals and weather conditions changes snow densities in theaccumulation event. Temperature and overburden pressure changes snow densities after
the accumulation event. Hence, detailed density profile of ice core reflects history of
surface condition directly.

1.2. Density profiles of ice cores
At glaciers with dry snow facies, density monotonically increases with depth
by densification processes in broad view. At the first stage of densification, where4density is less than 550 kgm-3, mechanical packing of ice grains dominates thedensification process. The second stage, where density is 550-730 kgm-3, densification
process occurs by the mechanical packing and plastic deformation of ice grains. At thethird stage, where density is 730-820 or 840 kgm-3 which close off air bubbles, plastic
deformation dominates the densification process. At the fourth stage, where density ishigher than the close off density, shrinkage of the air bubble dominates the densification
process [Maeno and Kuroda, 1986] .
In conventional volumetric density measurement for the ice core, resolution
was 0.5-1.0 m along core length because it depends on length of each core pieces. This
is too coarse to detect density variations caused by intra-annual accumulation events.
Density measurement methods using gamma ray or X-ray dramatically improved theresolution of measurement. Gerland et al. [1999] non-destructively measured density
profile of 181m-long ice core from Berkner Island, Antarctica using gamma ray. The
resolution was 3mm along core length and the absolute accuracy was 2%. Hori et al.
[1999] measured density profile of Dome Fuji ice core using X-ray. The resolution was
1mm along core length and the absolute accuracy was 1%. Both of the density profiles
show that density fluctuations in millimeter scale were maintained through densification
process. Although variations of density disappear at deeper part in the fourth stage ofdensification processes, the record remains at least in the firn part. The age of firn partdiffers from site to site by local condition; it is almost determined by accumulation rate
and temperature empirically [Herron and Langway, 1980].
In pit studies at glacier surface, seasonal variation of density has been
examined. In Greenland, typical summer layer is coarse grained and low-density layers
and typical winter layer is relatively fine grained, homogeneous and high-density.
5Summer layers has more variability than winter layer in stratigraphy and density[Benson, 1962]. On the other hand, Koerner [1971] reported summer layer was denser
at the Plateau station in Antarctica. Density profiles reflect local accumulation condition
and the characteristics of the density profiles at the surface vary from place to place.
Zwally and Li [2002] explained seasonality in density profile and seasonalityof surface elevation at the summit of Greenland by seasonality of densification rate.
Gerland et al. [1999] found that denser tendency in winter layer at shallower part
inversed at deeper part in the density profile of Barkner Island core, Antarctica. Hence,
density profile of ice core can reflect not only surface conditions but also densification
process. Both effect should be considered in interpretations of density profiles.

1.3. Objectives of this study
The main purpose of this study is to read climatic and/or environmental
histories with the highest temporal resolution from ice cores to connect ice core data andobserved data and/or model data in timing. An alpine glacier filling the Summit Calderaof Mt. Wrangell, Alaska (62°N, 144°W; 4100 m a. s. l.) was selected as the ice coringsite because high accumulation rate of 2.5 m w. e. a-1 allows analysis with high temporal
resolution.
Three objectives were pursued in the thesis. The first objective is to interpretdetailed density profiles of ice cores as a past accumulation event proxy data. In situ
accumulation measurement at the Summit Caldera of Mt. Wrangell and detailed density
profiles of ice cores with millimeter scale at the corresponding period were compared toestablish the relationship between the surface conditions and characteristics of thedensity profile at the surface layers. Then density variations through densification6

process were examined by comparison with density profiles before and after thedensification.
The second objective is to date the Mt. Wrangell ice core with monthly timescale. The result of in situ accumulation measurement at the Summit Caldera of Mt.
Wrangell was correlated to precipitation and temperature data at regional weather
stations which can date back to early 20th century. Then surface events at the Summit
Caldera of Mt. Wrangell which formed characteristic features in the density profiles atdeeper part were connected with correlated events in regional weather data and thedensity features were dated by the timing of the correlated events in the regional
weather data.
The third objective is to evaluate accumulation environment of alpine ice coredrilling sites to understand condition of the sites for the analysis with high time
resolution. Windy weather is expected at high mountain sites. For example, weather
meter at 5770 m a. s. l. near the Summit of Mt. McKinley was often destroyed after
winter season because of windy storms [Okura, personal commun.]. Areas with bared
rocks in glaciers which are often seen at mountain sites suggest precipitated snow isn’t
caught there. Saddle locations or ice caps are often selected as drilling sites in mountainsites because they tend to have relatively thick ice and gentle surface which imply older
record with less effect of glacier flow. But such locations would have windy weather
and would be affected by wind erosions or snow drift. The King Col, Mt. Logan is oneof typical alpine ice core sites located at a saddle in high elevation. Detailed density
profile and accumulation time series at the King Col is examined in this context toevaluate condition of a typical alpine ice core site.

7

2. Ice coring, field measurements and ice core measurements
2.1. Ice coring
2.1.1. Ice core sites
Ice cores were drilled at two sites, which have different characteristics in
topography in the Wrangell-St. Elias Mountains. One is the Summit Caldera, Mt.
Wrangell (62°N, 144°W; 4100 m a. s. l.) [Shiraiwa et al., 2004] and the other is King
Col, Mt. Logan (61°N, 141°W; 4135 m a. s. l.) [Goto-Azuma et al., 2003; Shiraiwa etal., 2003] (Fig. 2.1). These areas are extend into the dry snow facies [Benson, 1968;
Partington, 1998] and there is no significant melting higher than 4000 m approximately.
Several ice cores have been obtained from this region to study past climate [Benson,
1984; Fisher et al., 2004; Goto-Azuma et al., 2003; Holdsworth et al., 1992; Shiraiwa etal., 2004; Yalcin et al., 2006; Zagorodnov et al., 2005; Fig. 2.1]. The Summit Caldera of
Mt. Wrangell is unique in topography. The ice filled caldera at the summit is like a bowl
gathering snow. Such caldera filled by aged ice is rare and number of the reports of ice
coring in a caldera is limited [e.g. Shiraiwa et al., 2001]. On the other hand, saddle
location of King Col, Mt. Logan is one of typical sites in alpine ice coring. Ice core sites
shown in Figure 2.1 are all located at saddle except Mt. Wrangell and Eclipse Ice Field.
Flat surfaces and relatively thick ice, which are important prerequisites for ice coring
because effect of glacier flow is small and older ice can be extracted, are found in
saddle locations often. Detailed density profiles were examined at Mt. Wrangell
carefully then compared with the King Col data.

8140ºW

144ºW

62ºN

4000
3000
2000
1000
0
[m a.s.l.]

Mt. Wrangell

Gulkana

Bona-Churchill
EclipseIce Field

ValdezMt. Logan;

60ºN{ King Col

NW Col, PR Col,

Cordova

Gulf of Alaska0

Map location

100

200[km]Ice core sites
Weather stations

60ºN
180ºW

Yakutat

150ºW

Figure 2.1: Map of the Wrangell-St.Elias Mountains showing the location of ice
coring site at Mt. Wrangell (4100 m), King Col (4135 m) , other ice core sites
and weather stations. The ice core sites in the map include Northwest Col (5340m) and Prospector-Russell Col (5340 m) in Mt. Logan, Eclipse Ice Field (3017m)
and Bona-Churchill (4400m).

2.1.2. The Summit Caldera, Mt. Wrangell
Mt. Wrangell has a broad summit area of more than 35 km2 above 4000 m
altitude and is dominated by a large 4 x 6 km ice-filled Summit Caldera with
post-caldera craters along its rim (Fig. 2.2). Most of ice in the Summit Caldera is
drained through Long glacier at the south side. Various glaciological and volcanologicalstudies have been conducted by Carl Benson and his colleagues from 1960s on Mt.
Wrangell [e.g. Benson and Mothyka, 1978; Benson et al., 2007]. The 10 m-deep
temperatures at the Summit Caldera were -20±0.5ºC in the 1960s and in 1976 [Benson
and Motyka, 1978] and –18.9ºC in a recent study [Shiraiwa et al., 2004]. Snow cover at
the Summit Caldera is slightly below the dry snow line [Benson, 1968] and a few ice
9lenses which have several millimeter thick were found in most of annual layers. Benson
obtained 44m-deep ice core at north rim of the Summit Caldera in 1982 [Benson, 1984].
This is the longest and the best dated ice core among Benson’s works. New seven ice
cores were obtained at the Summit Caldera from 2003 to 2006 as the author describe
later. The ice thickness is thought to exceed 900 m, but a definite bottom has not yet
been detected [Clarke et al., 1989; Benson et al., 2007]. The deepest ice core, which was
216 m-deep and drilled in 2004 didn’t reach the bedrock.

Mt. Wrangell

Summit Caldera

Figure 2.2: Aerial photograph of Mt. Wrangell (4317m) from the south, with Mt.
Sanford (4950m) taken by the US Air Force for the USGS in 1948. Original
photograph was presented by Benson et al. [2007].

10

Detailed density measurements were carried out for five new ice cores. The
drilled depth, year, and location of each core is shown in Table 2.1 and Figure 2.3. The
ice cores deeper than 50 m were drilled by electro-mechanical drills. The 2003 core wad
drilled by the Dokodemo-drill, a portable ice core drilling system developed by ILTS,
Hokkaido University [Kohshima et al., 2002; Shiraiwa et al., 2004]. The 2004 core wasdrilled using the titanium electro-mechanical drill developed by GeoTech. Co., Ltd,
Japan. Other shallower cores were drilled by hand augers manufactured by ILTS.

Table 2.1: Ice cores drilled at the Summit Caldera of Mt Wrangell and the King Col,
Mt. Logan
Year

Depth
[m]

X-ray density
sample depth[m]Location(WGS84)

Other measured
properties

Reference

The Summit Caldera of Mt. Wrangell
1982

44

2003

50

2004

216

2005
(2005a)2005(2005b)
2006(2006-1)
2006(2006-2)
2006(2006-82)

9
17
8
23
14

62°00’13”N,
144°01’54”W
61°59’54”N,
144°02’30”W
61°59’29”N,
144°02’15”W
61°59’29”N,
144°02’15”W
61°59’29”N,
144°02’15”W
61°59’29”N,
144°02’15”W
61°59’29”N,
144°02’15”W
62°00’13”N,
144°01’54”W

Benson [1984;
personal commun.]
Shiraiwa et al. [2004],
Yasunari et al. [2007]-

β-activity, δ18O

0.79-50.31

δD, Tritium

1.54-40.37

-

-

-

δD

Matoba [personalcommun.]0.71-16.80--

0.80-8.33

δD-

3.54-22.92

-

-

-

δD

-

δ18O, MSA,
Tritium

Goto-Azuma[personal commun.]

The King Col, Mt. Logan
2002

220

60°35’20”N,
144°36’15”W

1.30-70.39

11

1982 and 2006 core [2006-82 core] site
Acc. rate: 0.85 m w.e. a-1 (1951-1982)
0.8-1.0 m w.e. a-1 (2005-2006)
Summit (4317 m)
North Crater

2003 core site
Acc. rate: 2.49 m w.e. a -1 (1992-2003)

West Crater

Tower site (4100 m)
2004, 2005 and 2006 [2006-1,2] core site
Acc. rate: 2.66 m w.e. a-1 (1992-2004)

1 km

North

MT. WRANGEL L
Date of photogrammetry: 1975/8/27
Conter interval: 10 m

Figure 2.3: Topographic map of the Summit Caldera, Mt. Wrangell. Recent Ice coresites where ice cores were drilled in 2003-2006 and the 1982 core site by Benson[1984] are shown. Tower for the accumulation measurement was built at the same
location with 2004-2006 core site. Averaged accumulation rate at each core site
are shown with their locations. The original map was presented by Benson andFollett [1986].

2.1.3. King Col, Mt. Logan
King Col is snow-clad saddle between Mt. Logan (5959m) and King Peak
(4137m). It is 0.5 x 0.5 km flat snow field and open for east-west direction [Fig. 2.4aand 2.4b]. The ice thickness is estimated to be 250 m by radio echo sounding [Kanamori,unpublished data]. An ice core was drilled to 220 m-deep using an electro-mechanicaldrill system developed by Takahashi [1996] in 2002 [Goto-Azuma et. al, 2003; Shiraiwa
et al., 2003].

12

6718500

Northing UTM 7VD-04 [m]6718000

6717500

6717000

6716500

6716000

6715500
520500521000521500522000522500523000523500Easting UTM 7VD-04 [m]Figure 2.4: (a)Aerial photograph of the King Col (4135m) with the King Peak
(5173m) from northeast side taken by Takayuki Shiraiwa in 2002. Schoening Ridge
continue up to the King Peak from east side. (b)Topographic map of the King Col.
“x” indicates the drilling site. Contour interval is 50m. The original map waspresented by Shiraiwa [2003].

13

2.2. Spatial distribution of annual accumulation rate at the Summit Caldera, Mt.
Wrangell
Spatial distribution of accumulation rate is important to examine spatial
representation of the ice core information. Averaged annual accumulation rates for morethan ten years are clearly known at three points in the Summit Caldera, Mt. Wrangell.
The locations and the accumulation rates are shown in Fig. 2.3. The dating of a tephra
layer from the 1992 Mt. Spurr eruption allowed a determination of averaged annual
accumulation of 2.49 m w. e. (1992-2003) at the 2003 core site [Yasunari et al., 2007].
The same tephra layer was found at 52.6 m-depth in the 2004 core, which results in an
averaged accumulation rate of 2.66 m w. e. (1992-2004) there. The 1982 core obtainedby Benson was dated using the β-activity peak from nuclear tests in 1963. In addition,
stable isotope profile shows annual periodicity and annual layer counting of stable
isotope ratio gave dates before 1963 [Benson, unpublished data]; the averaged
accumulation rate was 0.85 m w. e. (1951-1982). Another shallow surface core wasobtained at proximity to 1982 core site in 2006. Hydrogen isotope ratio was analyzed
using a mass spectrometer (JASCO International Co. Ltd., Isoprime PyrOH) every 0.05
m along core length for the ice core. The accumulation rate was then found to be 0.8-1.0
m w. e. (2005-2006) according to annual layer counting of the stable isotope ratio
profile.

2.3. Accumulation measurements for individual storms on Mt. Wrangell
2.3.1. Method
To date individual accumulation events and examine characteristic features indensity profile, which corresponds to snow surface condition, in situ accumulation14

measurement was carried out at the Summit Caldera, Mt. Wrangell through a year. The
location of measurements was set at the 2004 core site as shown in Figure 2.3. In June
2005 a metal tower was constructed and a total of 32 temperature sensors was installedon it so that snow accumulation could be measured by noting the time when snow burial
suppressed the diurnal temperature cycle. The conventional method of using an acoustic
sounder to measure the distance between the snow surface and the sensor above thesurface was unsuitable at Mt. Wrangell because rime ice could disturb the sensor. A
further problem is that rime ice and stormy weather at the summit might destroy thetower. Actually, heavy rime ice caused serious problem for meteorological instruments
and snow survey poles in 1961 [Benson, 1968]. To reduce the risk of collapse, only a
top five-meters of the tower was exposed above the snow surface in June 2005 (Fig.
2.5a). Although this was less than the expected annual accumulation of 6 to 7 m snow, it
still covered most of the annual accumulation. An avalanche beacon (Ortovox, F-1
focus) was mounted at the middle of the tower with additional batteries to help us findthe tower in the following year, in case of complete burial (Fig. 2.5a).

15

(a) June 2005(b) June 2006

Surface level

5m

1.5 m

Avalanche
beaconDataloggerTop of the tower
Figure 2.5: Pictures of the accumulation measurement tower at the Summit Calderaof Mt. Wrangell. (a)After installing the tower in June 2005. (b)After one year
measurement in June 2006.

Thirty two sets of copper-constantan thermocouples were mounted on thetower at interval of 0.15 m, starting at 0.24 m above the June 3 snow surface. Hourly
temperatures were recorded with a CR-10X (Campbell Scientific, Inc.) data logger anda multiplexer (AM16/32, Campbell Scientific, Inc.). An internal sensor of the datalogger measured the reference temperature for the thermocouples. The data logger andmultiplexer were kept in an insulated box. The accuracy of each temperature
measurement is within ±1.5ºC. The temperature time series showed large variations indaily temperatures until they were buried by snow accumulation, which allowed thedates of burial to be determined. The detectable snow-height change by this method is16

0.15 m and its time-resolution is one day, because the suppression of the daily
temperature cycle can only be detected within about one day. The data was recovered inJune 2006. At the time, the top of the tower was buried by 1.5 m of snow (Fig. 2.5b). To
convert from snow depth to water-equivalent depth, a density profile was measured
using a snow pit and ice cores to a depth of 8.3 m close to the tower’s location in June2006.

2.3.2. Results
Temperature data was obtained from 3 June 2005 to 22 June 2006. The
thermocouples at 0.54, 2.79, 3.84 and 4.59 m above the initial snow surface had stopped
working before their snow burials, but all others collected enough long records todetermine the snow burial date. This temperature profiles were converted to snowheight time series (Fig. 2.6; See Appendix A for temperature time series and theconversion). Total snow accumulation in the period was 6.48 m and the top of tower
was buried by snow on 8 December 2005. Using the measured density profile, 6.48 m insnow was found to corresponded to 2.66 m w. e. The exact days of snow burial were
determined for 70% of the total accumulation in water equivalent. In mid-August 2005,
the snow height temporarily decreased. This was due to snow compaction caused bysurface melting. At that time, thermocouples that had been buried recorded an increaseof snow temperature to the melting point. In addition, ice lenses were found at thecorresponding depth when the tower was dug out for data recovery, and in the ice core,
there were a 10 mm thick granular snow layer and two 2 mm thick ice lenses. This was
the only event in which any surface-lowering that exceeded the 0.15 m detection
threshold was recorded.
17

Figure 2.6: Snow height from 3 June 2005 to 22 June 2006 at the summit of Mt.
Wrangell. The top of the tower became buried on 8 Dec 2005. The gray area showsrange of estimated value between burial of the tower to the snow surface of 22
June 2006, which was 6.48 m above the snow surface of 3 June 2005.

Snow height time series after complete snow burial of the tower was estimatedusing the thermal diffusion equation
∂T
∂ 2T
=k 2
∂t
∂z

(2.1),

where T is temperature, t is time, k is thermal diffusivity and z is depth. The thermal
diffusivity k=K/ρc, where K is thermal conductivity, ρ is density and c is specific heat
capacity. Thermal conductivity of snow was given by Devaux [1933], which gives
empirical value for density range of 100-600 kgm-3. Vertical temperature profiles with
0.15 m interval were calculated by the thermal diffusion equation for the period after the
tower burial. The initial temperature was given by measured value just before the snow
burial. The time step was one hour. Surface temperature was given by estimated snow
surface temperature calculated by a regression curve from Gulkana temperature data
shown in Figure 4.1 (See chapter 4 for the temperature estimate). The 10m-deep18

temperature was given -18.9 °C, which is measured value in 2003 [Shiraiwa et al.,2004] .
The calculated value by the model at each depth and measured value at the topof tower was compared (Fig. 2.7). The top of tower was estimated to be located at thedepth range between two similar wave forms. Possible range of snow height was shownin Figure 2.6. There was a surface lowering event on mid March which appeared as
fluctuation in relative short time of measured temperature profile.

Figure 2.7: Comparison of measured temperature at the top of accumulation
measurement tower after snow burial on December 2005 and calculated temperatures
which expects snow temperature at certain depths. The top of tower is expected
to be located at the depth temperature of that shows most similar fluctuations.
The scale of calculated temperatures are same with the scale of measured
temperature but vertically lagged for comparison in the figure. The arrow
indicates timing of erosion event which is assumed from sharp fluctuation in themeasured temperature profile.

192.4. Density measurements
2.4.1. Method
Setting of the measurement system
Detailed density profiles were measured by the X-ray transmission method[Hori et al., 1999]. In this method, intensity of transmitted X-ray through an ice coresample is converted to the density value of the ice core sample using a calibration curve
which describe transmitted X-ray intensities as a function of thickness of pure bulk ice.
Figure 2.8 shows a schematic picture of the measurement system which is operated at-20 °C. A thick ice core sample on the sample stage moves through the X-ray beam
perpendicularly with constant velocity and the transmitted X-ray is measured by ascintillation counter continuously. The value was averaged every 1mm along core length.
The X-ray generator was operated at 30 kV. The width of optical slit was limited to 0.05
mm for a divergence slit, 0.2 mm for a scattering slit and 0.2 mm for a receiving slit.
The height of the scattering slit was 20 mm.

20Figure 2.8: A schematic picture of the X-ray transmission measurement system(Hori et al., 1999;Partly modified).

Ice core samples
Detailed density was measured for the Mt. Wrangell ice cores drilled in 2003,
2004, 2005, 2006 and the King Col ice core drilled in 2002. The measured ice cores andtheir lengths are shown in Table 2.1. Figure 2.9 shows cross sections of each ice core.
The thick sections of ice core samples were cut by bandsawing then planed by
microtome. The samples were typically about 500 mm long and 35 mm thick. Thicknessof each sample was measured by a vernier caliper with an accuracy of 0.1 mm fordensity calculations.

21

12mm

(a)

35mm

94 mm
32mm

(b)

(c)

47mm

50mm
35mm 36mm

35mm 47mm

89mm

94mm

(d)

45mm
35mm 45mm

89mm

Figure 2.9: Cross sections of the ice core samples. Lines in each circle
corresponding to cutting line by a band saw. Gray areas were used detailed
density measurement. Scale is 1:1 for Fig. 2.9a and 1:2 for other three pictures.
(a)King Col core. (b)Mt. Wrangell 2003 core. (c)Mt. Wrangell 2004 core. (d)Mt.
Wrangell 2005, 2006-1 and 2006-2 core.

22

Data conversion and calibration
The density ρ is calculated asρ = ρ ice

t eff

(2.2)

ts

where ρice is density of pure bulk ice, 918.3 at -20°C and ts is thickness of the coresample. The value of teff is assumed from transmitted X-ray intensities using acalibration curve.
Because continuous X-ray is used for the measurement, wavelength
dependence of X-ray absorption have to be taken into account in the calibration. The
measured fraction transmitted, f is described as;∞∫λ I (λ ) exp(− μ (λ )t )dλ
0

f =

eff

min

∞

(2.3).

∫ I (λ )dλ0

λminHere, I0(λ) is intensity of incident X-ray, which is function of wavelength λ. The
minimum λ in the spectrum is λmin. μ(λ) is the linear absorption coefficient of pure ice,
which is dependent of λ. teff is effective thickness of the sample. Because f cannot becalculated theoretically, the relation between teff and f is assumed using a calibration
curve,

f = A1 exp(− B1t eff ) + A2 exp(− B2 t eff ) + y 0(2.3).

Here, A1, A2, B1, B2 and y0 are fitting parameters. Although f should be expressed byinfinite number of the exponential terms theoretically, only two exponential terms were
calculated to avoid over fitting for limited number of data samples.
Figure 2.10 shows the calibration curve. Samples for the calibration wereobtained from clear, no visible air bubble part of ice thickness of which was considered23

as teff. The fitting parameters are also shown in Fig. 2.10. The R2 value of the calibration
was 0.99924 and χ/df was 0.00004, hence f was almost described as induced variable ofteff and error in the fitting is relatively small than other errors described in the next

X-ray fraction transmitted; fsubsection.

0.6
0.5
0.4
0.3
0.2
10

15
20
25
30
Effective thickness [mm] ; teff

Figure 2.10: Calibration curve for the density calculation. The equation andfitting parameters are;A 1: 0.4813
A 2 : 0.4813
teff
teff
B
:
17.18895
B+ A2 exp
f = A1 exp+ y0
1
2 : 17.18895
B1
B2
y 0 : 0.07694

χ2/DoF = 0.00004 and R2 = 0.99924.

ErrorsPrincipally, the following two errors are considered;
1. errors associated with the sample thickness measurement by a vernier caliper,
2. statistical variations of X-ray intensity.
Error 1 is within 1% of the thickness. Thickness of the sample was about 35 mm and theaccuracy of the measurement was within 0.1 mm. Error 2 is within 0.5% of X-ray

24

intensity. Statistical variations of X-ray are known to be 1/N0.5 (N: intensities of X-ray)empirically, and N at the measurement was more than 6×104 counts. Therefore the total
error is considered to be within 1%.

2.4.2. Results and discussion

Figure 2.11 shows X-ray density profiles and conventional volumetric density
profiles at the Summit Caldera of Mt. Wrangell and King Col. The X-ray density
profiles overlaps volumetric density profiles well. Although density seems
monotonically increased with depth in the volumetric density profiles, it is fluctuated in
the high resolution profiles by the X-ray transmission method. Most of positive spikes
in the profiles correspond to melt features which appeared as ice lenses in visualstratigraphy. The higher densities in the King Col profile, in contrast to those of Mt.
Wrangell’s at same depths seems to be caused by higher initial density and lower
accumulation rate, which keeps time passed denser firn in shallower part at King Col.
.

25

Figure 2.11: Density profiles by X-ray transmission method and volumetric method
at the Summit Caldera of Mt. Wrangell and the King Col, Mt. Logan.

Density range of 820-840 kgm-3, which corresponds to close-off depth,
appeared at about 90m-deep at Mt. Wrangell profiles. Such a deep close-off depth had
appeared only in inland of ice sheets [Paterson, 1994], where 10m depth temperatures
are less than -50°C. Hence, Mt. Wrangell is a unique case. Probably, this is because ofhigh accumulation rate which is about 2.5 m w. e. a-1 at the Summit Caldera of Mt.
Wrangell. The value is about 103 times higher than inland of ice sheets. Close-off depth
is determined by combination of initial density, accumulation rate and densification rate.
At Mt. Wrangell, effect of accumulation rate seems to be large; i.e. a surface layer goesdeep by highly accumulated snow in short period. The ice age at the close-off depth isestimated to be 30 years. This is much younger than the value of the inland of ice sheets
which is more than 1k years old.
26

The King Col profile is also unique because of its bending at the profile. The
slope of depth-density curve increased at depth of about 40 m. This is unusual and neverappears under steady state condition. Another point to be mentioned in the King Col
profile is fluctuations appeared in the density range over 900 kgm-3. The fluctuation ofdensity is still detectable in the density range. Error range in the relative density value is
within 0.5% because it depends on error in statistical variations of X-ray intensity.
Hence, some of the fluctuations in the density range over 900 kgm-3 are significant.
Figure 2.12 shows density profiles at the Summit Caldera of Mt. Wrangell for
four years. All ice cores were drilled at almost same place except the 2003 core which is
700 m NNW from the other sites. The four profiles are overlapped. It means that the
profiles agree with Sorge’s law [Bader, 1954] which states depth-density profile isinvariant with time under conditions of constant accumulation rate and time. However,fluctuations in the density profiles doesn’t overlap. This means detailed density profiles
reflects variations of accumulation rate and temperatures in short periods such asmonthly to daily time scale. Fluctuations with millimeter scale wavelength seems tocorrespond to daily scale changes in surface conditions.

27

(a) 2003 vs. 2004(b) 2003 vs. 2005(c) 2003 vs. 2006

Figure 2.12: Detailed density profiles at the Summit Caldera of Mt. Wrangell. (a)density profiles of the 2003 core and the 2004 core. (b)density profiles of the2003 core and the 2005(2005b) core. (c)density profiles of the 2003 core and the2006(2006-1,2) core.

28

3. Interpretation of detailed density profiles at Mt. Wrangell
3.1. Relationship between surface condition and density profile

In order to connect characteristics in density profile and surface conditions,detailed density profile and visual stratigraphy of the Mt. Wrangell 2006 core werecompared with the measured accumulation time series and temperature profiles.
Temperature profiles of snow cover and accumulation time series estimated by thetemperature profile were recorded from 3 June 2005 to 22 June 2006. Daily
accumulation data were extracted until 7 December 2005 when top of the measurement
tower was varied by snow. Exact date of each depth is not directly recorded in the core
because accumulation rate at each event in water equivalent is unknown. Therefore thedate is assumed from the comparison. In the ice core, dates of two depths was identified
clearly. One is 3 June 2005 which the measurement tower was built on and the other is
10 August 2005 when the melt event occurred on. The surface level on 3 June is
obviously determined by the tower basement. Melt features in an ice core is formed
after some percolation [Benson, 1962]. In the case of the 2006 core, granular snow layer
appeared just above the clear ice lenses in visual stratigraphy. Therefore the granularsnow layer was considered as the surface level of the melt event. In addition, surface
level of the tower burial day was estimated from water equivalent depth. According toSorge’s law, depth-density curve is invariant with time. At the time of tower burial,snow height from the bottom of tower height was 4.89 m and it correspond to 1.92 m w.
e. at the site. Therefore 1.92 m w. e. above the tower basement depth in the ice core wasestimated to be surface level at the tower burial. These three points were controls on thedating of the 2006 core.
Figure 3.1 shows integrated value of the measured daily accumulation and the29ice core data. Boundaries of measured accumulation events in the integrated
accumulation data indicate position of approximate corresponding boundaries in the icecore depth. If density of newly accumulated snow were constant, this approximation
would become accurate. The corresponding depths to the accumulation event
boundaries were estimated from the approximate position by the measured
accumulation data and visual stratigraphy of the ice core (Fig. 3.1; See Appendix B for
detail of the visual stratigraphy). The stratigraphic features that were estimated to becorrespond to accumulation event boundaries were shown in table 3.1. It seems that
estimated boundaries of accumulation events by individual storms appears as stepwisechanges in detailed density profiles. The author assume that boundaries of accumulation
events appear as stepwise change in density profile because such density differences is
the most possibly formed at storm event boundaries as following conditions. Just before
snow accumulation event, snow surface tend to be denser because of sintering, windpacking or a surface crust layer. Density of new snow is relatively low; therefore such
structure with stepwise change of density can be formed. Another possible cause of
stepwise change in a density profile is wind erosion. When wind erosion occurs, softer
surface snow is most likely eroded and denser layer can remain at the surface. In
addition, thick denser layer can be formed by wind compaction. Actually, rectangularshape of density profile (Fig. 3.1) is assumed to be formed by erosion event, which
occurred in mid March (Fig. 2.6). This pattern is also included in boundaries ofaccumulation events.

30

(No data)

Accumulation
Date

Dec 7

A

Accumulation
Dec 5
Date

Accumulation
Date

Nov 15

Aug 22B

Oct 6

Sep 23-24

Sep 12

Sep 7

Sep 4

Aug 19Aug 17 Aug 4-5

Jul 31 - Aug 1

Jul 26

Jul 17C

Dec 5

Aug 24

Jun 19D

Figure 3.1: Comparison of integrated value of measured snow accumulation and ice
core data. Upper chart: Integrated value of snow accumulation from 3 June to 8
December in 2005 (blocks with date) and no accumulation period between
accumulation events (vertical bars). Lower chart: Detailed density p rofile(black solid line) and layer boundaries in visual stratigraphy (vertical b reaklines) of the Mt. Wrangell 2006-1 core. A: Characteristic density profile whichis assumed to be correspond to an erosion event in mid March. B: Estimatedsurface depth at the tower burial on December 8 2005. C: Depth of surface melt
event on August 10-13. D: Depth of snow surface on June 3 2005. Scale of theupper chart and lower chart are fitted at the point of June 3 and December 8.
Corresponding points in both charts are indicated by arrows.

31

32

3.36

3.12

2.63

2.27

2.14

1.57

0.00

2005/9/24-10/6

2005/10/6-11/15

2005/11/15-12/5

2005/12/5-7

2005/12/7

2006/3/12-14

2006/6/22

4.95

2005/8/22-24

3.73

5.36

2005/8/19-22

2005/9/12-23

5.54

2005/8/17-19

4.10

5.72

2005/8/5-17

2005/9/7-12

5.86

2005/8/1-4

4.31

5.94

2005/7/26-31

2005/9/4-7

6.18

2005/7/17-7/26

4.54

6.26

2005/6/19-7/17

2005/8/24-9/4

6.48

[m]

2005/6/3

Date

0.00

0.52

0.74

0.79

0.94

1.14

1.25

1.41

1.57

1.66

1.76

1.95

2.14

2.23

2.30

2.36

2.40

2.51

2.55

2.66

[m w.e.]DepthControl

Erosion event

Estimate

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

Melt feature

Storm event boundary

Storm event boundary

Storm event boundary

Storm event boundary

ControlType

Melt layer by solar radiation or high temperature;
Exposed to the surface for a while
Wind packed layer; exposed to the surface for a while
Exposed to the surface for a while
Surface layer at melt events
Boundary of individual storms

Dense layer including large grain; may be melt feature
Dense layer with fine grains; texture is different fromlayer above
Texture is coarser at the layer above; stepwise change indensity
Granular snow just above a clear ice lensBoundary of coarse grain layers (above) and a fine grain
layer (below)
Top side of various texture layers Bottom side of a thicklayer with small density variation
Top side of a thick layer with small density variation;
Bottom side of various texture layers

Boundary of individual storms
Boundary of individual storms

Wind packed layer; exposed to the surface under windy
condition for a while
Wind packed layer; exposed to the surface under windy
condition for a while
Boundary of individual storms
Boundary of individual storms
Erosion event which was assumed from temperature profile-

Stepwise change in the density profile
Stepwise change in the density profile; Bottom ofvarious texture layers
Stepwise change in the density profile; layer boundary invisual stratigraphy
Dense layer with fine grains; Top of layers with finegrainsClear stepwise change in the density profile; top of a
thick layer with fine grains
Stepwise change in the density profile
Higher density structure at the expected depths range ofthe event boundary
Estimated depth of the surface at the tower burial
Rectangle shape of the density profile; fine grains
Surface level at the ice coring

Boundary of individual storms

Boundary of individual storms

Thin coarse grain layer; Top of various texture layers

Boundary of individual storms

Boundary of individual storms

Wind packed layer; exposed to the surface for a while

Dense layer with fine grains; texture is different fromlayer above

Supposed condition
Surface at the field work in 2005

Descriptions
Depth of the tower baseTable 3.1: Dated depths on the Mt. Wrangell 2006-1 core

Another kind of density peaks which is broader positive peak consists ofseveral small peaks appeared in the detailed density profile. One can see such featuresin accumulation event on August 19, August 24, September 12 and November 15 (Fig.
3.1). These were nonuniform snow layers in visual stratigraphy associated with larger
density variations. The small density peaks correspond to snow layers which consists ofround large grains (diameter: 1-2 mm) and smaller particles (See appendix B for detail).
These series of small peaks were supposed to be formed in an individual storm event
according to the result of dating. Figure 3.2 shows temperature profiles at theaccumulation measurement tower in the storm events in August and September 2006.
Some high temperatures above 0°C is probably caused by solar radiation. Whenaccumulation events which formed the large density variation layers had occurred,surface snow temperature had been above -10°C approximately. At the accumulation
event which formed snow layers with low density variations in August 17, 22,
September 4 and September 7, surface snow temperature had been about below -10°C
approximately. It seems that relatively warm storm events form density peaks with large
variations in the accumulated snow. This correspondence between the characteristic
density layers and warm storm events support validity of the dating.

33

a

b

c

e

d

f

g

Figure 3.2: Temperature profiles at 1.5 m above snow surface (gray line) andsurface snow temperature (0-0.15 m below the surface; thinner black line) during
summer accumulation season in 2005 measured without shades. Arrows indicate
periods of accumulation events (solid line) with error ranges (break line).
Arrow b, d and g: accumulation event with large density variations; arrow a, c,e and f: accumulation event with small density variations (Fig. 3.1).

3.2. Effect of densification on detailed density profiles

Correspondence of stepwise changes in density profile and accumulation event
boundaries were confirmed in snow layers accumulated in a year, density range of
which was about 370-470 kgm-3. This range is the first stage of densification dominant
process of which is characterized by mechanical packing of snow [Maeno and Kuroda,
1986]. Density is the most variable in densification process at the range. Li and Zwally[2002] explained that annual fluctuation of density profile is induced in densification
process after snow deposition at the summit of Greenland from model result. Thisvariation of density occurred in the first stage of densification process. Gerland et al.
[1999] reported that higher density in winter layers and lower density in summer layers
at shallower part were reversed at deeper part in the ice core drilled at Barkner Island,
Antarctica. To extend the discussion to deeper part, it needs to examine whether the
stepwise feature of density profile is preserved over years in densification process or

34

not.
Figure 3.3 shows density profiles of the ice cores drilled at the Summit Calderaof Mt. Wrangell from 2003 to 2006. Shape of density profiles are similar and stepwisefeatures in density profiles are preserved in the first stage of densification process
density range of which is to 550 kgm-3. Although the 2003 core was drilled 700 m awayfrom the site of other cores, characteristic features in the density profiles were consistent.
Figure 3.4 shows density profiles of the Mt. Wrangell 2003 and 2004 cores at greater
depth. Shapes of the two profiles are very similar at 550-700 kgm-3 range like the caseof shallow parts. Therefore stepwise changes in density profiles are also kept at thisdensity range. Hence, stepwise changes in density profile is considered to reflect paststorm event boundaries not only in surface snow layers but also in deep firn layers.

2006

2005
2004

2003

Figure 3.3: Temporally corresponding part of density profiles of Mt. Wrangell
ice cores drilled in 2003, 2004, 2005 and 2006. Vertical series of boxes
indicates examples of stepwise changes that were kept for three years throughdensification process.

35

2004

2003

Figure 3.4: Temporally corresponding part of density profiles of the Mt.
Wrangell 2003 core (gray line) and 2004 core (black line).

36

4. Detailed dating of Mt. Wrangell cores4.1. Estimation of surface condition at Mt. Wrangell from weather station data

The Mt. Wrangell 2006 core was well dated by the in situ snow accumulation
measurement. However, the measurement period was limited only one year. To date theolder ice cores, we need such information for the past. Fortunately, precipitation andtemperature data are available from the early 20th century at national weather
observatories around Mt. Wrangell. To estimate the surface condition for the past,
relationship between surface condition at the ice coring site on Mt. Wrangell and theweather station’s data around Mt. Wrangell were examined for following three topics;
1)snow temperature at Mt. Wrangell was compared with temperature at the weather
stations to estimate timing of melt events; 2)seasonal snow accumulation pattern at Mt.
Wrangell was compared with seasonal precipitation pattern at the weather stations toestimate seasonal accumulation pattern for the past; 3)timing and amount of individualaccumulation events at Mt. Wrangell were compared with those of precipitation eventsat the weather stations to date individual accumulation event found in the ice core.
Data from four weather stations; Gulkana, Yakutat, Valdez and Cordova wereused for the comparison. These weather station data are from the U.S. National Weather
Service. The four stations that were selected are the closest weather stations to Mt.
Wrangell in south central Alaska (Fig. 2.1). Table 4.1 shows their locations, elevations,mean annual precipitation and mean annual temperature. Gulkana has much lessprecipitation than the three coastal stations because it is located in the precipitation
shadow of the coastal mountains and the Wrangell-St.Elias Mountains.

37

Table 4.1: Primary information of four weather stations in south central Alaska
Mean annual
Mean annual
Weather
Elevation
Locationprecipitation* [mm] temperature* [°C]station[m]
Gulkana
62°09’N, 145°27’W
481
290
-2.7
Yakutat59°31’N, 139°40’W
9
4074
4.2
Valdez61°08’N, 146°21’W
10
1712
3.5
Cordova
60°30’N, 145°30’W
13
2445
3.9
*NCDC 30 year normal (1971-2000); Alaska Climate Summaries,
http://www.wrcc.dri.edu/summary/climsmak.html

Figure 4.1 shows scatter diagram of hourly surface snow temperature at Mt.
Wrangell and hourly air temperature at Gulkana. The surface snow temperature wasmeasured by sensors which is the closest to the surface in snow buried sensors at thetime. Its depth should be 0-0.15 m from the surface. The period of data is from 19 June
to 8 December in 2005. The surface snow temperature is highly correlated with Gulkana
temperature (R=0.86) and this is the best correlation in the four stations. The author
suppose that the best correlation at Cordova in the four stations are because of theclosest location. This correlation made it possible to estimate snow surface temperatureand timing of melt event at Mt. Wrangell from the lowland temperature. Melt featuresappeared a few times in each annual layer of the ice core. These melt features were most
likely formed at the timing when the highest temperatures were recorded. The
relationship between temperatures at Mt. Wrangell and Gulkana was already pointed out
by Benson [1968]. Benson stated that warm sunny weather which affect both top of Mt.
Wrangell and Gulkana are the best time for potential melting and found excellent index
for such a weather, i. e. the warm sunny weathers come when air temperature exceed
25°C at Gulkana. The correlation shown in Figure 4.1 agreed with Benson’s index.

38

Figure 4.1: Scatter diagram of hourly air temperature at Gulkana and hourly
surface snow temperature at Mt. Wrangell in 2005 with regression curve byquadratic polynomial fitting.

Figure 4.2 shows cumulative accumulation at Mt. Wrangell and cumulative
precipitation at the four weather stations. At Mt. Wrangell, 50% of the annual
accumulation was observed in two months from the end of July to the end of September.
Weather station data at Cordova show a similar cumulative precipitation pattern.
Despite the closer location of Gulkana, the fraction of the annual precipitation received
from June to early August is much higher than at Mt. Wrangell. Valdez and Yakutat data
show a more gradual seasonal increase than Cordova. Both places had lower relative fall
precipitation and higher relative winter precipitation than Cordova and Mt. Wrangell.
The 2005-2006 year seems to have a normal seasonal precipitation distribution at
Cordova. Figure 4.3 shows the net 30 year-average of the monthly precipitation fraction
from June 1972 to May 2005 in Cordova and the fraction from June 2005 to May 2006.
Although monthly fractions were different from January to May, higher precipitationfrom late summer to fall (August to October) is common. It is therefore reasonable to39

assume that the normal precipitation pattern at Mt. Wrangell also follows this pattern.

Cordova

80
Gulkana60
Yakutat40

Valdez20

Mt. Wrangell
1 July1 June

1 May

1 Apr.

1 Feb.
1 Mar.

1 Dec.
2006
1 Jan.

1 Nov.

1 Oct.

1 Sep.

1 Aug.

1 July02005
1 JuneCumulative precipitation /accumulation [%]

100

20

June 2005-May 2006
Net 30 year-average

15
10

MAY

APR

MAR

FEB

JAN

DEC

NOV

OCT

SEP

JUL

0

AUG

5

JUN

Fraction ofannual precipitation [%]Figure 4.2: Cumulative accumulation at Mt. Wrangell (thick black line) andcumulative precipitation at four weather stations (Gulkana: thin black line;
Yakutat: thin gray line; Valdez: thin black line; Cordova: thick gray line) insouth central Alaska, from 3 June 2005 to 22 June 2006. The values are expressedas percentage of the total amount during the time period.

Figure 4.3: Comparison of monthly precipitation in the observation year (June2005 to May 2006) with the 30-year average at Cordova. The error bars show
standard deviations. The monthly values are expressed as percentages of theannual precipitation for each year. The net 30-year average is calculated withthe data from June 1972 to May 2005. The data of 1985, 1990 and 1999 were
excluded because there are some missing values in the original precipitation
record.

40

Figure 4.4 shows daily accumulation at Mt. Wrangell that exceeded thedetection threshold and daily precipitation at the four weather stations. When Mt.
Wrangell had detectable snow accumulation events, the weather stations also show
precipitation events. This correspondence is particularly clear for Cordova and Valdez.
These two coastal weather stations seems to be located at path of storms that bringaccumulation to Mt. Wrangell. To evaluate more quantitatively, precipitation events at
Cordova and accumulation events at Mt. Wrangell were compared. Each continuous
precipitation or accumulation event was regarded as one storm event. Some events
which occurred with one day interval between them at Mt. Wrangell were counted as
single events for comparison because corresponding events at Cordova were continuous
ones. From June 3 to December 8 in 2005, there were 34 storm events at Cordova and14 of them occurred simultaneously with storm events at Mt. Wrangell (Fig. 4.4). Figure
4.5 shows a histogram of precipitation events at Cordova. Most of the storm events withless than 20 mm precipitation had no corresponding accumulation event large enough toexceed the detection threshold on Mt. Wrangell. On the other hand, all of the storm
events with more than 70 mm precipitation were associated with recorded storm eventson Mt. Wrangell. From these result, it is reasonable to consider that Mt. Wrangell has nostorm events when Cordova has no precipitation or storm events precipitation of whichis less than 20 mm. Figure 4.6 shows a scatter plot of precipitation at Cordova andaccumulation at Mt. Wrangell for each corresponding storm event. The data show apositive dependency, but poor linear correlation (R=0.29); it is therefore difficult toestimate the amount of accumulation by individual storms on Mt. Wrangell from
Cordova precipitation.

41

10

0
150
100
50
0
80
60
40
20
0
80
60
40
20
0
0.6
0.4
0.2
0.0

YakutatValdezCordova

29 Oct.
3 Nov.

29 Sep.
4 Oct.

30 Aug.
4 Sep.

31 July
5 Aug.

2005
1 June26 June1 JulyMt. Wrangell
28 Nov.
3 Dec.
8 Dec.

Snow
Daily precipitation
accumulation[mm] [mm][mm] [mm][m]

Gulkana20

Date

Figure 4.4: Daily snow accumulation rate at Mt. Wrangell and daily precipitation
rate at four weather stations in south central Alaska. Daily data at Mt.
Wrangell is unknown after 8 December. The coincident events on Mt. Wrangell andCordova are shown as black bars.

With No storm eventat Mt. Wrangell

10

Frequency

8

With storm eventat Mt. Wrangell

6
4
2

>100

90-100

80-90

70-80

60-70

50-60

40-50

30-40

20-30

10-20

0-10

0

Precipitation at Cordova [mm]Figure 4.5: Histograms of Cordova storm events from June 3 2005 to December 8
2005. The hatched bars show precipitation events which has no correspondencewith Mt. Wrangell accumulation events. The black bars show precipitation events
which occurred at same timing with Mt. Wrangell accumulation events.
Accumulation events at Mt. Wrangell always occurred at same timing with Cordova
precipitation events.

42

Figure 4.6: Plot of precipitation at Cordova and accumulation at Mt. Wrangell inindividual storm events with a regression line (gray line). The accumulation
data is converted to water equivalent by the dating (Fig. 3.1).

In a previous study that relates snow accumulation at Mt. Logan to local
climate, Rupper et al. [2004] demonstrated that only winters in a high accumulation
year have a robust connection with atmospheric circulation. The observed year at Mt.
Wrangell is an average accumulation year. However, there still appears to be a simple
relationship between snow accumulation at Mt. Wrangell and precipitation at Cordova.
The direct relationship between accumulation on Mt. Wrangell and precipitation at a
sea-level weather station, lying upwind during storms might be characteristic of Mt.
Wrangell only. Snow accumulation at Mt. Wrangell seems to reflect regional weather
systems with synoptic scale rather than local convective activity which might be
expected for a mountain site.

4.2. Detailed dating of the Mt. Wrangell ice cores
4.2.1. Method

In the Mt Wrangell ice cores, certain dates are known for several depths (Table
434.2). These were used for control points of dating. To interpolate dates between thecontrol points, the relationship between surface condition at the Summit Caldera of Mt.
Wrangell and weather station data were used. There are three important results forassuming the dates in the previous subsection;1) surface snow temperature at Mt. Wrangell is well correlated with Gulkana
temperature,2) seasonal distribution of accumulation at Mt. Wrangell is similar with that of
precipitation at Cordova,
3) timing of no accumulation period at Mt. Wrangell correspond to no precipitation or
small precipitation event at Cordova.
First, timing of melt event was estimated using the first relation. Daily maximum
temperature at Gulkana which exceed 25 °C was considered as potential index of melt
event. Melt features in ice cores were dated using this index. Ice lenses and refreezed
features of soaked firn in visual stratigraphy were distinguished and the refreezed
features of soaked firn was interpreted as surface at the melt events. The dated melt
features divide annual layers between summer seasons. Second, daily distribution of
accumulation between dated depths was presumed to be same with daily distribution of
precipitation at Cordova in corresponding period because of their similarity inseasonality. Dates from this presumption is inaccurate but the difference from theaccurate dates should not be large. At least, approximate season should be correct. This
assumption was used only as a guide of dating. Third, stepwise changes in density
profile which represent boundaries between accumulation events were dated using
estimated no accumulation period by Cordova precipitation data. The period that has no
precipitation or less than 20 mm in total amount at continuous daily precipitation at44

Cordova was considered as no accumulation period at Mt. Wrangell. Length of noaccumulation periods were taken account into. Clear stepwise changes which may beformed with wind packing were interpreted as relatively long no accumulation period.
Clear dated depths gave feedback to the first step to reduce possible dates of melt
features.
Table 4.2: Control depths in dating the Mt. Wrangell ice coresDepth[m] [m w. e.]core

Surface level at the ice
coring in 2006

0.00

0.00

2006-1

2005/6/3

Surface level at the ice
coring in 2005

6.48
0.00

2.66
0.00

2006-12005b

2004/5/13

Surface level at the ice
coring in 2004

12.56
7.57
0.00

5.82
3.25
0.00

2006-2
2005b
2004

8.25
5.66
2.42
0.00
8.43
5.66

2006-2
2005b
2004
2003
2004
2003

31.0
26.87

2004
2003

Date

Descriptions

2006/6/22

2003/6/20

Surface level at the ice
coring in 2003

2001/4/13

Remarkable spring dust
event*

17.00
12.18
5.99
0.00
17.20
12.31

1992/9/17

Tephra deposition event by a
Mt. Spurr eruption*

52.6
47.12

*Yasunari et al., 2007

Figure 4.7 shows an example of dating for an annual layer from summer 2004
to summer 2005. There are two control points at the both ends. First, two melt features
seems to correspond high temperature events in late June to mid July and early August
respectably. Second, cumulative precipitation at Cordova between dated points was fit
to depth scale of ice core in water equivalent as a guide of dating. Third, depths of45

stepwise changes in detailed density profile was compared with position of precipitation
event boundaries in the cumulative precipitation pattern. In this case, stepwise change at
3.5 m w. e. in the 2006 core was remarkable and seemed to fit no precipitation period
which indicated in Figure 4.7 at Cordova. The date of this no precipitation event gave
date to the Mt. Wrangell 2006 core. This case has tough connection between events in
the ice core and at Cordova because both events are remarkable and has no comparable
event each other. Other stepwise features in the density profile were also dated by thesame way. However, most of the stepwise features and the no precipitation events has a
few comparable events each other. In such cases, events in an ice core and Cordova
precipitation data were fitted to minimize the number of events that has no comparable
events each other.

46

May 24Apr Mar
Jun 1 May 12-17 Apr 18-22 5-10 27-31

Feb 24 - Mar 14

FebJan
19-22 Feb 13-17 Feb 2-11 19-24 Jan 2 - 52005 Jun 3

Dec 9 - 23

Nov 22 - Dec 3

Sep 19 - Oct 7

Nov
15-19 Nov 6-13 Oct 23 - Nov 4

Sep
1-3

Aug
26-28

Oct 10 - 14 Sep 19 - Oct 7

Jul May 23 - Jun 2
Jul 25 - 30 20-22

2004 May 13

Figure 4.7: Comparison of cumulative precipitation at Cordova and density
profile of the Mt. Wrangell ice core for dating. Upper chart: Cumulative
precipitation at Cordova from May 2004 to June 2005 (blocks with date) and no
precipitation event period (vertical bars). Daily maximum temperatu re at Gulkana
are also shown in same horizontal scale (rhombic plot). Lower chart: Detailed
density profile (black solid line) of the Mt. Wrangell 2006-1 core (gray line)
and 2006-2 core (black line). Date at two depth are known (vertical break line;
see table 4.2 for detail). Stepwise changes in the density p rofile that are
estimated to be formed in no accumulation periods at Mt. Wrangell are connectedwith no precipitation event at Cordova which is estimated to occur same timingwith the events at Mt. Wrangell by arrows. Melt features are connected with hightemperature event in the upper chart also by arrows.

47

For missing values in precipitation data at Cordova, data at Valdez was used
after correction using a regression line shown in Figure 4.8. This displacement isreasonable because precipitation data at the two sites is well correlated (R=0.80) in
daily data (Fig. 4.8) and both Cordova and Valdez shows correspondence in timing of
precipitation events with timing of accumulation events at the Summit of Mt. Wrangell.

Figure 4.8: Plot of daily precipitation at Valdez and Cordova with a regression
line (gray line) and the equation. The plots shows data from 1991 to 2006.

4.2.2. Result and errors

In addition to 5 control points, 30 melt features and 228 accumulation event
boundaries appeared as stepwise changes in the detailed density profile were dated for
Mt. Wrangell ice cores that cover period from 1991 to 2005. In total, 263 points weredated [See Appendix C for dating points and its descriptions]. It gives 18 dating points

48

per year in average, which is monthly scale dating. Twenty five events in 30 melt
features and 37 events in 228 accumulation event boundaries are reliable in dating
because there is no other comparable events in weather station data. Figure 4.9 shows
number of melt features in each month. The melt events occurred mainly in June andJuly. This made dating of summer season reliable. Table 4.2 shows the number of dating
points in each year. Although melt events are concentrated in summer, other dating
points dispersed in each annual layers and all annual layers have several reliable dating
points. Therefore, systematic errors which exceed a few months is hard to occur. The
most possible error is caused by mistaking corresponding event at Cordova to adjacent
events in unreliable dating points. Figure 4.10 shows spans between adjacent dating
points. The most frequent span is 10-15 days and most of spans are within a month.
Therefore error in dating is considered to be within a month.

Number of melt events

12

1992-2005
n=30
Most likely
Reliable

10
8
6
4
2
0
MAY

JUN

JUL

AUG

SEP

Figure 4.9: Number of melt events that were found in the Mt. Wrangell ice coresfrom 1992 to 2005 in each month. The event with reliable dating is shown by
black bars and with less reliable dating is shown by white bars.

49

>60

55-60

50-55

45-50

40-45

35-40

30-35

25-30

20-25

15-20

10-15

20
10
0

5-10

60
50
40
30

0-5

Frequency

8070

Period between dating points [day]Figure 4.10: Frequency of periods between dating points.

4.3. Monthly variation of parameters in ice core data
4.3.1. Method

The monthly scale dating made it possible to date profile of other parameterssuch as accumulation rate, stable isotope ratio or tritium concentration. The dated resultsof these parameters are shown in this subsection. Time axis of ice core parameters was
supposed to be liner with water equivalent depth of the core between the dating points.
Hydrogen isotope ratio (δD) was analyzed using a mass spectrometer (JASCO
International Co. Ltd., Isoprime PyrOH). The estimated measurement error was lessthan 0.1‰. The sample was taken every 5 cm for the Mt. Wrangell 2006 core and every
10 cm for the other Mt. Wrangell cores (See table 2.1 for ice core information). Tritium
was analyzed by a liquid scintillation counter [Yasunari et al., 2007] for the Mt.
Wrangell 2003 core.

50

4.3.2. Result and discussion

Figure 4.11 shows monthly accumulation rate time series from 1992 to 2003 at
Mt. Wrangell which is reconstructed using the Mt. Wrangell 2003 core. Likewise themeasured result from 2005 to 2006, the highest accumulation rate was seen in latesummer to fall in most of years. However, it shows some inter-annual variations. For
example, the highest season shift later in1997 and 2000 and earlier in 1998. It is
important to note that a few storm dominate accumulation in a month. In theaccumulation measurement (Fig. 4.4), accumulation in 6 months were dominated by 17
storm event. Snow accumulation were detected only on 21 days. Therefore, monthly
accumulation rate is highly dependent on number of storm events. This is supposed to
be a reason of the inter-annual fluctuation.

51

Figure 4.11: Monthly accumulation time series from 1992 to 2003 which isreconstructed from the Mt. Wrangell 2003 core.

Stable isotope ratio fluctuate seasonally in this region and annual maximum has
been thought to come summer or warm season [Yasunari et al., 2007; Wake et al., 2002]
but calendar date of that had been unknown. Figure 4.12 shows hydrogen isotope ratio(δD) time series from 1992 to 2006. Calendar dates of annual maxima appeared from
July to December. Maybe the December peak in 2002 is exceptional because it was anonly case that annual maximum appeared after November in the 13 year-record. On theother hand, annual minima were scattered. Cold season which is from November toApril seems the range of annual minimum but some minimum peaks appeared in June,
July and October. It is important that stable isotope ratio fluctuate largely. It is very
often that value of a summer layer is lower than a winter value. Figure 4.13 shows52correlation coefficient between monthly δD and monthly temperature at four weather
station shown in table (4stations) from 1991 to 2003. Three month running means werecompared because ice core dating may have one month error. The correlations are
relatively high from July to November in all four weather station data. Stable isotope
data seems to reflect regional temperature from summer to early winter. The relation is
unclear in the opposite seasons. Because accumulation in a month is dominated by a
few storm events, monthly value in δD represents wet deposition in several days. The
higher correlation from summer to early winter might be relate with higher
accumulation rate at the season which provide better temporal representation for theatmospheric information.

Figure 4.12: Hydrogen isotope ratio time series at Mt. Wrangell. 1992 to 2003:
data from the Mt. Wrangell 2003 core [Yasunari et al., 2007], January 2004 to
May 2005: data from the Mt. Wrangell 2005a core, June 2005 to June 2006: datafrom the Mt. Wrangell 2006-1 core.

53

Figure 4.13: Correlation coefficients between th ree month running mean hydrogenisotopes in ice core data and three month running mean temperature at the four
weather stations in south central Alaska (Table 4.1). The correlation
coefficients were calculated for each month from 1992 to 2003. Significant
correlations are shown with filled marks.

Figure 4.14 shows tritium time series from 1992 to 2003. Annual maximum
appeared from March to June each year. Yasunari et al [2007] found annual periodicityof tritium profile and assumed tritium maxima as early spring from relative position ofother key parameters. This result agree with the assumption and it is more periodic thanaccumulation rate distribution or stable isotope ratio. The agreement support validity ofdating methods each other.

54

Figure 4.14: Tritium concentration time series at Mt. Wrangell from 1992 to 2003.
The original data is presented by Yasunari et al. [2007].

55

5. Effect of snow drift at alpine ice core sites

Generally, alpine ice core sites suffer from strong wind. In such conditions,precipitated snow is easily removed by snow drifting and/or wind scouring. Fisher et al.
[1983] pointed out effect of wind scouring in past climate reconstruction by ice cores in
the case of Arctic Canada. Snow drift can remove all of annual snow fall in some cases(e.g. bare ice field in Antarctica [Takahashi et al., 1988]). Rugged topography enhances
local gales and redistribution of snow. Hence, relatively gentle topography is preferable
for the ice coring to avoid the effect of snow drift. The Summit Caldera of Mt. Wrangell
is one of ideal sites at this point because of its wide spread gentle snow field. However,such places are rare in mountains and many ice cores are obtained at saddle locations
such as King Col, Mt. Logan because saddle locations tend to have relative thick ice andflat surface, which are essential points to retrieve older ice with less effect of glacierflow and limited on alpine glaciers. In this chapter, the effect of snow erosion was
examined by comparison of density profiles and accumulation rate at the King Col, Mt.
Logan with those obtained from the Summit Caldera of Mt. Wrangell.

5.1. Comparison of annual accumulation rates

The King Col core is dated using tritium peaks by nuclear test in early 1960s(Fig. 5.1a) and annual layer counting on multi-parameters (Fig. 5.1b) [Goto-Azuma,
unpublished data]. Profiles of oxygen isotope ratio and methanesulfonic acid (MSA) aremainly used to determine annual layers.

56

Figure 5.1: Ice core parameters and assumed dates at certain depths of the KingCol core. (a)Tritium profile. The peaks by nuclear tests determine the dates as
1962 and 1963. (b)Oxygen isotope ratio (thick gray line), MSA (thin black line)
and assumed year of accumulation. Annual minima of oxygen isotope ratio aretaken as annual layer boundaries (vertical break line) [Goto-Azuma et al.,unpublished data].

Annual minima of oxygen isotope were taken as annual layer boundaries. Annual
accumulation time series for 30 years at King Col and that of Mt. Wrangell 2003 coresite determined by annual isotope minima [Yasunari et al., 2007] are compared (Fig.
5.2a). The effect of ice deformation is negligible because the corresponding layers are
firn layers. Hence annual layer thicknesses are considered as annual net balance, which
is close to annual accumulation rate at those sites. Averaged annual net balance is 1.1 m
w. e. a-1 at King Col and 2.5 m w. e. a-1 at the Mt. Wrangell 2003 core site. Standard
deviation of the annual accumulation time series is 0.33 m w. e., 36% of the average forthe King Col data and is 0.47 m w. e., 19% of the average for the Mt. Wrangell data.
The dating method by stable isotope ratio made inter-annual variation of the Mt.
Wrangell ice core larger than the accumulation rate presented in Fig. 4.11 (Figure 5.2b).
However, it is much smaller than the King Col data in fraction to the average.
57

Figure 5.2: (a)Annual accumulation time series at the King Col (black bars) andthe Mt. Wrangell 2003 core site (gray bars). Each annual layer is determined byannual minimum peak of stable isotope ratio. (b)Annual accumulation time series
at the Mt. Wrangell 2003 core site determined by stable isotope peaks (gray bars)
and calendar date (black bars with white lines) that is presented in Figure 4.11.

Annual layer thicknesses are especially thin in 1977 (0.32 m w. e.) and 1988 (0.40 m w.
e.) in the King Col data. The density profile is compared with seasonal fluctuation of theoxygen isotope ratio and MSA around 1977 (Fig. 5.3) and 1988 (Fig. 5.4). Stepwisechange with rectangular shape in density profile implies successive erosion and theseries of such features have a high proportion in the 1977 and 1988 layers. In addition,
MSA profile has no peak in the 1977 layer but oxygen isotope minimum has a peak at
the same depth in the 1988 layer. Peak of MSA appears in summer between two
successive stable isotope minima at King Col in usual year (Fig. 5.1). These
correspondences in the ice core parameters suggest that the annual layer counting isreasonable and large fraction of annual snow disappeared in 1977 and 1988 by erosion
events. Larger variation of annual accumulation rate in King Col than Mt. Wrangell is
58

not because of variation in snow precipitations but of irregular erosion events.

*

Figure 5.3: Parameters of the King Col core around 1977. (a)Density profile.
Marked range by an asterisk indicate part of relative high density associatedwith stepwise change, which implies erosion event. (b)Oxygen isotope (thick line)
and MSA (thin line) profiles with annual layer boundaries (vertical break line)
which was determined by annual isotope minima.

59

*Figure 5.4: Parameters of the King Col core around 1988. (a)Density profile.
Marked range by an asterisk indicate part of relative high density associatedwith stepwise change, which implies erosion event. (b)Oxygen isotope (thick line)
and MSA (thin line) profiles with annual layer boundaries (vertical break line)
which was determined by annual isotope minima.

5.2. Effect of erosions to ice core parameters

Fisher and Koerner [1994] explained that difference of oxygen isotopes in themultiple ice cores drilled on the Agassiz Ice Cap was caused by snow-drifting andwind-scouring of winter snow. There are similar examples in the profile of King Col
core. From 1979 to 1982, annual minima of oxygen isotope ratio are higher than otheryears (Fig. 5.1b). At the corresponding part, the density is relatively higher and it
implies occurrences of erosion events (Fig. 5.5). Although stable isotope ratio is
considered to reflect regional air temperature, there is no tendency that temperatures
were particularly higher than other years at the period at Yakutat, the closest weather
station from Mt. Logan. (Fig. 5.6). Therefore it is reasonable to speculate that there was60

no temperature shift at the King Col in the period. Hence the higher annual minima ofoxygen isotope ratio than usual years are the indication of winter erosion. Density
profile is thus useful proxy for the past erosion events to test validity of other ice coreparameters such as stable isotope ratio.

*

*

*

*

*

*

Figure 5.5: Parameters of the King Col core from 1978 to1984. (a)Density profile.
Marked range by asterisk indicate relative high density associated with stepwisechange occurred with annual oxygen isotope minimum. (b)Oxygen isotope (thickline) and MSA (thin line) profiles with annual layer boundaries (vertical breakline) which was determined by annual isotope minima.

61

Figure 5.6: Annual mean temperature (white square) and mean three months
temperature from December to February (black circle) at Yakutat for 30 years.
Arrows indicate years with relatively high annual minima in oxygen isotope ratio
shown in Figure 5.5.

5.3. Bending in depth-density profile

The bending in the density profile of the King Col core (Fig. 2.11) ischaracterized by relatively high density layers associated with step wise changes.
Standard deviation of density decreases with increasing depth under normal condition.
Mt. Wrangell is a typical case of that (Fig. 5.7a and 5.7b). Except the melt features,standard deviation of density decreased toward depth. On the other hand, the standard
deviation of density has second peak at 25 m w. e. in the King Col profile (Fig. 5.7d).
The depth correspond to the depth of bending in the density curve (Fig 2.11 and Fig.
5.7c). The bending and second peak of the standard deviation seems to be caused bysudden increase in density profile associated with series of stepwise changes (Fig. 5.7e
and 5.7f). Remarkable erosion events were expected at the depths. Such bending feature
in density curve is supposed to be an indicator of remarkable erosion events.

62

*

*

*

Figure 5.7: Depth-density profiles in water equivalent depth and 0.2 m w. e.
-running standard deviation of density. (a) and (b) Mt. Wrangell 2003 core; (c)and (d) King Col core; (e) and (f) King Col core with scaling up. Asterisks
shows series of stepwise changes in the density profile which forms remarkably
denser layer than the shallower part.

5.4. Redistribution of accumulated snow at the Summit Caldera of Mt. Wrangell

In the Summit Caldera of Mt. Wrangell, the 1982 core site at north side hasmuch less accumulation than the center (Fig. 2.3). Figure 5.8 shows picture of the63Summit Caldera from the summit just after a windy storm event in June 2006. A seriesof dune was formed by wine over the saddle between the summit and the North crater(Fig. 5.8a). This patterned surface was most clearly seen at the 1982 core site (Fig. 5.8b).
However, the intensity of this pattern becomes small toward the center of the caldera
and the surface becomes smooth at the 2006 camp/core site (Fig. 5.9c). The distribution
pattern of dunes suggests that accumulated snow is eroded then redistributed toward thecenter of the caldera, which is well agreed with actual accumulation distribution.

64

(a) From the Summit; 29 June 2006

2004-2006 core site

1982 core site(b) 2006 camp/core site; 29 June 2006(c) The 1982 core site; 28 June 2006

Figure 5.8: Pictures of the Summit Caldera, Mt. Wrangell on June 2006. (a)view
from the summit of Mt. Wrangell. (b)camp site. The ice core sites in 2004, 2005,
2006 are located within several tens meters. (c)The 1982 core site close to thenorth rim.

65

6. Conclusions
6.1. Interpretation of detailed density profiles as past surface condition proxydata

An 8.33 m-depth firn core, which correspond to more than a full year was
drilled at the Summit Caldera of Mt. Wrangell in June 2006. In situ temperature andsnow accumulation measurement was carried out from June 2005 to June 2006 was
successful. Detailed density profile and stratigraphy of the core was compared with thesnow accumulation history. As the result, stepwise changes in the density profile wereinterpreted as accumulation free periods and rectangular shapes of the density profile
were interpreted as erosion events at the surface. These kinds of characteristic shapes indensity profiles were maintained in the ice cores drilled in 2003, 2004, 2005 and 2006 at
the Summit Caldera of Mt. Wrangell density ranges of which were less than 700 kgm-3.

6.2. Detailed dating using density profiles

The results of in situ temperature and snow accumulation at the Summit
Caldera of Mt. Wrangell for one year was correlated with regional weather stations inSouth Central Alaska. 1)Hourly surface snow temperature at Mt. Wrangell was
correlated with hourly air temperature at Gulkana. This made it possible to date timingof surface melt event at Mt. Wrangell from air temperature data at Gulkana for the past.
2)Timing of precipitation free periods at Cordova were same as timing of accumulation
free period or erosion event at Mt. Wrangell. This made it possible to date stepwisechanges and rectangular shapes of density profiles for the past. 3)Seasonal precipitation
distribution at Cordova was similar with seasonal accumulation distribution at Mt.
Wrangell. This made it possible to estimate seasonal accumulation distribution for the66

past. As the results, Mt. Wrangell ice cores were dated with monthly temporal scalefrom 1991 to 2005. Averaged number of dating points were 18 in an annual layer.
According to the monthly scale dating, variation of accumulation rate, stable
isotope ratio and tritium became clear. Although there were inter annual variations,accumulation rate were higher in fall season. Annual maxima of stable isotope peaks
appeared from July to December. Annual tritium peaks were appeared from March toJune.

6.3. Accumulation environment of alpine ice core sites

Detailed density profile of the King Col core was examined and compared withthat of the Mt. Wrangell core. Seasonal variations of stable isotope ratio and MSA
profile of the King Col core were unclear at certain depths. Frequent rectangular shapes
in the density profile suggest that it was because of erosion events. The frequent
rectangular shape also made a bending in the depth-density curve in larger view. These
results and relative large variation of accumulation rate to the Mt. Wrangell implies
saddle locations in alpine ice core sites are affected by erosions. This effect should betaken into account for interpretations.

6.4. Meaning of this study

The most important result of this study is monthly scale dating of the ice core.
Probably, this is the highest time resolution of ice core dating in the current ice corestudies. Although this dating method is limited to the period that weather station data isavailable, which is past 100 years, it is still very important result because alpine ice core
records what sea level weather station never records in the past such as past transported
67

materials in atmospheric circulation or past climate proxy at free troposphere. This
study made it possible to connect such important information from the alpine ice core toother observed record and/or model results in future study.

68

Reference

Akitaya, E., and T. Yamada (1991), Snow survey (in Japanese), in Survey methods forsnow and ice (in Japanese), edited by Japanese Society of Snow and Ice, pp.
29-45, Hokkaido University Press, Sapporo.
Bader, H. (1954), Sorge's Law of densification of snow on high polar glaciers, Journalof Glaciology, 2(15), 319-323.
Benson, C. S. (1962), Stratigraphic studies in the snow and firn of the Greenland Ice
Sheet, SIPRE Research Report, 70.
Benson, C., S. (1968), Glaciological studies on Mount Wrangell, Alaska, 1961, Arctic,
21(3), 127-152.
Benson, C. S., and R. J. Motyka (1978), Glacier-volcano interactions on Mount
Wrangell, Alaska, Annual Report, Geophysical Institute, University of Alaska,
pp. 1-25.
Benson, C., S. (1984), Ice core drilling at Mt. Wrangell, Alaska. 2nd International
Workshop/Symposium on Ice Drilling Technology, CRREL Special report,
84(34), 61-68.
Benson, C., and A. Follett (1986), Application of photogrammetry to the study of
volcano-glacier Interactions on Mount Wrangell, Alaska, PhotogrammetricEngineering And Remote Sensing, 52(6), 813-827.
Benson, C., et al. (2007), Glacier-volcano interactions in the North Crater of Mt
Wrangell, Alaska, Annals of Glaciology, 45, 48-57.
Clarke, G., et al. (1989), Radar imaging of glaciovolcanic stratigraphy, Mount
Wrangell caldera, Alaska - Interpretation model and results, Journal ofGeophysical Research-Solid Earth and Planets, 94(B6), 7237-7249.
Dansgaard, W., et al. (1993), Evidence for general instability of past climate from a
250-kyr ice-core record, Nature, 364(6434), 218-220.
Devaux, J. (1933), L'économie radio-thermique des champs de neige et des glaciers,
Annales de physique. Ser. 10, 20, 5-67.
Fisher, D. A., et al. (1983), Effect of wind scouring on climatic records from ice-core
oxygen-isotope profiles, Nature, 301(5897), 205-209.
Fisher, D. A., and R. M. Koerner (1994), Signal and noise in four ice-core records fromthe Agassiz Ice Cap, Ellesmere Island, Canada: details of the last millennium
for stable isotopes, melt and solid conductivity, The Holocene, 4(2), 113-120.
69

Fisher, D. A., et al. (2004), Stable isotope records from Mount Logan, Eclipse ice coresand nearby Jellybean Lake. Water cycle of the North Pacific over 2000 years
and over five vertical kilometres: Sudden shifts and tropical connections,Géographie physique et Quaternaire, 58(2-3), 337-352.
Gerland, S., et al. (1999), Density log of a 181 m long ice core from Berkner Island,
Antarctica, Annals of Glaciology, 29, 215-219.
Goto-Azuma, K., et al. (2003), An overview of the Japanese glaciological studies on
Mt. Logan, Yukon Territory, Canada in 2002, Bulletin of GlaciologicalResearch, 20, 65-72.
Herron, M. M., and C. C. Langway (1980), Firn densification - an empirical-model,
Journal of Glaciology, 25(93), 373-385.
Holdsworth, G. H., et al. (1992), Ice core climate signals from Mount Logan, Yukon
A.D. 1700-1987, in Climate Since A.D. 1500, edited by R. S. a. J. Bradley,
Philip D., pp. 483-504, Routledge, London.
Hori, A., et al. (1999), A detailed density profile of the Dome Fuji (Antarctica) shallow
ice core by X-ray transmission method, Annals of Glaciology, 29, 211-214.
Jun, L., and H. J. Zwally (2002), Modeled seasonal variations of firn density induced
by steady-state surface air-temperature cycle, in Annals of Glaciology, 34,
299-302Kang, S., et al. (2005), A 290-a record of atmospheric circulation over the North
Pacific from a Mt. Logan ice core, Yukon Territory, Acta Oceanologica Sinica,
24(4), 81-90.
Koerner, R. M., and S. Ohio State University. Institute of Polar (1971), A stratigraphic
method of determining the snow accumulation rate at Plateau Station,
Antarctica, and application to South Pole-Queen Maud Land traverse 2,
1965-1966, Ohio State University, Institute of Polar Studies, Columbus, OH.
Kohshima, S., et al. (2002), Ice core drilling on Southern Patagonia Icefield-Development of a new portable drill and the field expedition in 1999, Memoirsof National Institute of Polar Research. Special issue, 56, 49-58.
Maeno, N., and T. Kuroda (1986), Structures and properties of snow and ice (inJapanese), pp. 143-148, Kokon-shoin, Tokyo.
Greenland Ice-core Project Members (1993), Climate instability during the lastinterglacial period recorded in the GRIP ice core, Nature, 364(6434), 203-207.
Nakazawa, F., et al. (2005), Dating of seasonal snow/firn accumulation layers using
pollen analysis, Journal of Glaciology, 51(174), 483-490.
70

Partington, K. (1998), Discrimination of glacier facies using multi-temporal SAR data,
Journal Of Glaciology, 44(146), 42-53.
Paterson, W. S. B. (1994), The Physics of Glaciers, Third edition ed., Butterworth
Heinemann, Oxford.
Petit, J. R., et al. (1999), Climate and atmospheric history of the past 420,000 years
from the Vostok ice core, Antarctica, Nature, 399(6735), 429-436.
Rupper, S., et al. (2004), The relationship between snow accumulation at Mt. Logan,
Yukon, Canada, and climate variability in the North Pacific, Journal of Climate,17(24), 4724-4739.
Schwikowski, M., et al. (1999), A high-resolution air chemistry record from an Alpine
ice core: Fiescherhorn glacier, Swiss Alps, Journal of GeophysicalResearch-Atmospheres, 104(D11), 13709-13719.
Shiraiwa, T., et al. (2001), Characteristics of a crater glacier at Ushkovsky volcano,Kamchatka, Russia, as revealed by the physical properties of ice cores andborehole thermometry, Journal of Glaciology, 47(158), 423-432.
Shiraiwa, T., et al. (2003), Ice core drilling at King Col, Mount Logan 2002, Bulletin of
Glaciological Research, 20, 57-63.
Shiraiwa, T., et al. (2004), Shallow ice-core drilling at Mount Wrangell, Alaska,
Bulletin of Glaciological Reaserch, 21, 71-77.
Takahashi, A. (1996), Development of a new shallow ice core drill (in Japanese),
Seppyo, 58(1), 29-37.
Takahashi, S., et al. (1988), A bare ice field in East Queen Maud Land, Antarctica,caused by horizontal divergence of drifting snow, Annals of Glaciology, 11,
156-160.
Thompson, L., et al. (1984), El-Nino southern oscillation events recorded in thestratigraphy of the tropical Quelccaya ice cap, Peru, Science, 226(4670), 50-53.
Wake, C., et al. (2002), The climate signal recorded in the oxygen-isotope,
accumulation and major-ion time series from the Eclipse ice core, Yukon
Territory, Canada, Annals of Glaciology, 35, 416-422.
Watanabe, O., et al. (1999), The paleoclimate record in the ice core at Dome Fuji
station, East Antarctica, in Annals of Glaciology, 29, 176-178.
Yalcin, K., et al. (2006), A 1000-yr record of forest fire activity from Eclipse Icefield,
Yukon, Canada, The Holocene, 16(2), 200-209.
71

Yasunari, T., et al. (2007), Intra-annual variations in atmospheric dust and tritium in
the North Pacific region detected from an ice core from Mount Wrangell,
Alaska, Journal of Geophysical Research-Atmospheres, 112(D10), D10208,
DOI:10.1029/2006JD008121.
Zagorodnov, V., et al. (2005), Intermediate-depth ice coring of high-altitude and polar
glaciers with a lightweight drilling system, Journal of Glaciology, 51(174),491-501.
Zwally, H. J., and L. Jun (2002), Seasonal and interannual variations of firndensification and ice-sheet surface elevation at the Greenland summit, Journalof Glaciology, 48(161), 199-207.

72

Appendix A
Detection of snow burial dates at the snow accumulation measurement

Appendix A shows temperature profiles on the accumulation measurement withdescriptions of detected events. Snow burial events, re-exposed events and melt eventsof temperature sensors at certain heights were detected from their temperature profiles.
Heights of each sensors are expressed as height from surface level on 3 June 2005.
June 2005

The sensor at 0.24 m-high assumed to be buried on 19 June because amplitude ofdiurnal fluctuation had been smaller after that (Fig. A.1a).

73

aFigure A.1: Temperature profiles at every 0.15 m height on the accumulation
measurement tower in June 2005. See main text for descriptions about ar rows.

July 2005

The sensor at 0.39 m-high assumed to be buried on 17 July because amplitude of74

diurnal fluctuation had been smaller after that than other exposed sensors above (Fig.
A.2a).
The sensor at 0.69 m-high assumed to be buried on 26 July because amplitude ofdiurnal fluctuation had been smaller after that than other exposed sensors above (Fig.
A.2b).
The sensors at 0.84 m-high and 0.99 m-high assumed to be buried on 31 July because
amplitude of diurnal fluctuation had been smaller after that than other exposed sensors
above (Fig. A.2c; Fig. A.3).

75

c

b

a

Figure A.2: Temperature profiles at every 0.15 m height on the accumulation
measurement tower in July 2005. See main text for descriptions about ar rows.

August 2005

The sensor at 1.14 m-high assumed to be buried on 1 August because amplitude of76

diurnal fluctuation had been smaller after that than other exposed sensors above (Fig.
A.3a).
The sensor at 1.29 m-high assumed to be buried on 4 August because amplitude ofdiurnal fluctuation had been smaller after that than other exposed sensors above (Fig.
A.3b).
The sensor at 1.44 m-high assumed to be buried on 5 August because amplitude ofdiurnal fluctuation had been smaller after that than other exposed sensors above (Fig.
A.3c).
The sensor at 1.44 m-high assumed to be exposed again on 7 August because amplitudeof diurnal fluctuation had became similar with exposed sensor above after that (Fig.
A.3d).
The sensor at 1.29 m-high assumed to be exposed again on 9 August because amplitudeof diurnal fluctuation had became similar with exposed sensor above after that (Fig.
A.3e).
The sensor at 1.14 m-high assumed to be exposed again on 10 August because the daily
maximum temperature was higher than melting point (Fig. A.3f).
The sensor at 0.99 m-high assumed to be exposed again on 11 August because the daily
maximum temperature was higher than melting point (Fig. A.3g).
Melt events occurred at daytime from 11 to 14 August Some of buried sensors hadrecorded melt points (0.99m-high:11 Aug.; 0.84 m-high:12 Aug.-14 Aug.; 0.69m-high:Aug.12).
The sensor at 0.84m-high assumed to be exposed again on 15 August because the daily
maximum temperature slightly exceeded melting point (Fig. A.3h).
The sensor at 0.84m-high assumed to be buried again on 17 August because amplitude77

of diurnal fluctuation had been smaller after that than exposed sensors above (Fig. A.3i).
The sensors at 0.99m-high, 1.14m-high, 1.29 m-high, 1.44m-hgih and 1.59m-highassumed to be buried on 21 August because amplitude of diurnal fluctuation had beensmaller after that than exposed sensors above (Fig. A.3j).
The sensors at 1.74m-high and 1.89m-high assumed to be buried on 23 August because
amplitude of diurnal fluctuation had been smaller after that than exposed sensors above(Fig. A.3k).
The sensor at 2.04m-high assumed to be buried on 25 August because amplitude ofdiurnal fluctuation had been smaller after that than exposed sensors above (Fig. A.3l).

78

l

c

a

b

d

j

e f g

h

k

j
i

Figure A.3: Temperature profiles at every 0.15 m height on the accumulation
measurement tower in August 2005. See main text for descriptions about arrows.

79September 2005

The sensors at 2.19m-high and 2.34m-high assumed to be buried on 3 September
because amplitude of diurnal fluctuation had been smaller after that than exposed
sensors above (Fig. A.4a).
The sensor at 2.49m-high assumed to be buried on 6 September because amplitude ofdiurnal fluctuation had been smaller after that than exposed sensors above (Fig. A.4b).
The sensor at 2.64m-high assumed to be buried on 7 September because amplitude ofdiurnal fluctuation had been smaller after that than exposed sensors above (Fig. A.4c).
The sensors at 2.94m-high and 3.09m-high assumed to be buried on 13 September
because amplitude of diurnal fluctuation had been smaller after that than exposed
sensors above (Fig. A.4d).
The sensor at 3.24m-high assumed to be buried on 19 September because amplitude ofdiurnal fluctuation had been smaller after that than exposed sensors above (Fig. A.4e).
The sensors at 3.39m-high and 3.54m-high assumed to be buried on 24 September
because amplitude of diurnal fluctuation had been smaller after that than exposed
sensors above (Fig. A.4f).

80

e
d

c

d

b c
a

Figure A.4: Temperature profiles at every 0.15 m height on the accumulation
measurement tower in September 2005. See main text for descriptions about ar rows.

October 2005

The sensor at 3.69m-high assumed to be buried on 6 October because amplitude of81

diurnal fluctuation had been smaller after that than exposed sensors above (Fig. A.5a).

aFigure A.5: Temperature profiles at every 0.15 m height on the accumulation
measurement tower in October 2005. See main text for descriptions about arrows.

82

November 2005

The sensors at 3.99m-high, 4.14m-high and 4.29m-high assumed to be buried on 15
November because amplitude of diurnal fluctuation had been smaller after that than
exposed sensors above (Fig. A.6a).

83

aFigure A.6: Temperature profiles at every 0.15 m height on the accumulation
measurement tower in November 2005. See main text for descriptions about arrows.

December 2005

The sensor at 4.44 m-high assumed to be buried on 5 December because amplitude of84

diurnal fluctuation had been smaller after that than exposed sensors above (Fig. A.7a).
The sensor at 4.74m-high and 4.89m-high assumed to be buried on December 7 because
amplitude of diurnal fluctuation had converged after that (Fig. A.7b).

85

b
a

Figure A.7: Temperature profiles at every 0.15 m height on the accumulation
measurement tower in December 2005. See main text for descriptions about arrows.

January to May 2006

Diurnal temperature fluctuations had been converged at all sensors. Fluctuations on mid

86March was interpreted as surface erosion event (Fig. A.8; See chapter 2.3.2).

Figure A.8: Temperature profiles at every 0.15 m height on the accumulation
measurement tower from Jaunuary 2006 to May 2006.

87

Appendix B

Description of visual stratigraphy of the Mt. Wrangell 2006-1 core
Mt. Wrangell 2006-1 core
Depth Depth[m] [m w.e.]
1.156 0.362
1.206 0.383
1.232 0.394
1.568 0.519
1.600 0.534
1.863 0.635
2.436 0.858
2.475 0.874
2.502 0.886
2.562 0.911
2.687 0.962
2.742 0.984
2.768 0.994
2.827 1.019
2.912 1.056
2.943 1.070
2.993 1.090
2.995 1.091
3.064 1.119
3.117 1.141
3.359 1.245
3.367 1.249
3.401 1.264
3.404 1.265
3.459 1.289
3.565 1.335
3.575 1.339
3.606 1.353
3.612 1.355
3.725 1.403
3.854 1.459
3.928 1.491
3.940 1.496
3.943 1.498
3.964 1.507
3.977 1.513
4.010 1.528
4.029 1.537

Descriptions
Grains become finer below
A crust (width: 0.2mm)
A crust (width: 0.2 mm); Grains become coarser below
Grains become finer below
Grains become finer below
Grains become finer above; Grains become coarser below
Grains become coarser above; Grains become finer below
Grains become finer above; Grains become coarser below
Grains become coarser above; Grains become finer below
Grains become finer above; Grains become coarser below
Grains become coarser above
Grains become larger below
Grains become larger above
A boundary of some structures
A boundary of some structures
A boundary of some structures
Grains become coarser below
Graines become coarser above
A layer with unclear boundaries
Graines become coarser above; Grains become finer below
3.359-3.404m depth looked one layer; grains become finer and denser below
Grains become finer and denser above
Grains become finer and denser below
Grains become finer and denser above
Grains become finer below; An unclear boundary
A layer with coase grains below
An unclear boundary of some structures
An unclear boundary of some structures
A layer with coarse grains above
An unclear boundary of some structures
Grains become coarser below
Grains become finer below
Bands with coarse grains below
Bands with coarse grains above; unclear three bands below (3.943-3.964m depth)
Bands with coarse grains below
Bands with coarse grains above
Grains become coarser below
Grains become finer above88

Mt. Wrangell 2006-1 core
Depth Depth
Descriptions[m] [m w.e.]
4.090 1.563
Large grains (diameter: max. 5mm)
4.196 1.609
Grains become finer below
4.298 1.654
Large grains (diameter: max. 2mm)
4.432 1.714
Grains become coarser below
4.542 1.762
Large grains below; diameter: about 1mm
4.544 1.763
Large grains above; diameter: about 1mm/ Grains become coarser below
4.585 1.781 An unclear boundary of some structures; Coase grains below (diameter: max. 0.5mm)
4.640 1.807
Coase grains below (diameter: max. 1mm)
4.675 1.823
Grains become finer below
4.693 1.832
Grains become finer above; Coarse grains below (diameter: max. 1mm)
4.695 1.833
Coase grains above (diameter max. 1mm)
4.712 1.841
Grains become finer above; Coarse grains below (diameter: max. 1mm)
4.745 1.857
A layer consists of small heils (diameter: max. 2mm) below
4.755 1.861
A layer consists of small heils (diameter: max. 2mm) above
4.767 1.867
Grains become coarser above (diameter: max. 0.5mm)
4.823 1.892
Grains become coarser below
4.833 1.897
Grains become coarser above
4.886 1.921
Grains become coarser below
4.921 1.936
An unclear boundary of some structures; Grains become coarser below
4.944 1.947
Coarse grains (diameter: max 1mm) are included below
4.946 1.948
Coarse grains (diameter: max 1mm) are included above
5.222 2.074
An unclear boundary of some structures; Grains become coarser below
5.340 2.127
Coarse grains are included below
5.342 2.128
Coarse grains are included above
5.363 2.137
Coarse grains (diameter: max. 2mm) are included below
5.365 2.138
Coarse grains (diameter: max. 2mm) are included above
5.420 2.163
Coarse grains (diameter: max. 0.5mm) are included below
5.421 2.164
Coarse grains (diameter: max. 0.5mm) are included above
5.456 2.180
Grains become finer below
5.473 2.188
Coarse grains are included below
5.500 2.201
Coarse grains are included above
5.510 2.206
bands of coarse grains below (diamter: max. 2mm)
5.546 2.224
bands of coarse grains above (diamter: max. 2mm)
5.569 2.234
Grains become finer below
5.620 2.258
Grains become finef above; Grains becoe coarser below
5.655 2.273
Grains become coarser above; Grains become finer below
5.691 2.289
Grains become finer above89

Mt. Wrangell 2006-1 core
Depth Depth[m] [m w.e.]
5.719 2.301
5.732 2.307
5.732 2.307
5.734 2.308
5.827 2.349
5.856 2.361
5.907 2.385
5.957 2.408
5.969 2.413
5.985 2.421
5.990 2.423
6.030 2.441
6.035 2.444
6.112 2.480
6.124 2.486
6.125 2.487
6.133 2.490
6.180 2.513
6.185 2.515
6.207 2.526
6.275 2.558
6.303 2.573
6.317 2.580
6.335 2.589
6.408 2.625
6.427 2.634
6.623 2.725
6.660 2.743
6.815 2.812
6.821 2.815
6.950 2.874
6.980 2.888
8.127 3.484
8.300 3.575

Descriptions
Granular grains (diamter: max. 3mm) below
Granular grains (diamter: max. 3mm) above
An ice lense below (width: 2mm)
An ice lense above (width: 2mm)
An ice lense (max. 4mm width)
Grains become coarser above (diameter: max. 0.5mm)
Grains become coarser below
Grains become coarser above; Grains become finer below
Grains become finer above
An unclear boundary of some structures; Grains become finer below
An unclear boundary of some structures; Grains become finer above
An unclear boundary of some structures; Grains become finer below
An unclear boundary of some structures; Grains become finer above
An unclear boundary of some structures; Grains become finer below
A layer with fine grains below
A layer with fine grains above
An unclear boundary of some structures; Grains become finer above
Coarse grains (diameter: max. 2mm) are included below
Coarse grains (diameter: max. 2mm) are included above
An unclear boundary of some structures
A layer with fine grains below (A coudy band)
A layer with fine grains above (A coudy band); Grains become coarser below
Grains become coarser above; Grains become finer below
Grains become finer above
Grains become finer below
Grains become finer above
An unclear boundary of some structures
Grains become coarser below
Grains become finer below
Grains become finer above
Grains become coarser below
An unclear boundary; Grains become coarser above
A crust (width: <0.5mm)
A crust (width: <0.5mm)

90Acknowledgements

The author would like to express his gratitude to Dr. Takayuki Shiraiwa, Chairof the doctoral degree Supervisory Committee and his supervisor. Appreciation goes to
Dr. Takeo Hondoh, Dr. Ralf Greve, Dr. Kumiko Goto-Azuma and Dr. Sumito Matoba,
member of the doctoral degree Supervisory Committee.
This study was supported by the Sasakawa Scientific Research Grant from The
Japan Science Society and Grants-in-Aid for Creative Scientific Research(No.14GS0202) and Basic Research B (No.16403005) from the Japanese Ministry of
Education, Culture, Sports, Science and Technology.

91