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Measurements on air porosity of sea ice
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MEASUREMENTS ON AIR POROSITY OF SEA ICE
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Annals of Glaciology 4 1983
O International Glaciological Society
MEASUREMENTS ON AIR POROSITY OF SEA ICE
Masayoshi Nakawo*
(Geotechnical Section, Division of Building Research, National Research Council Canada, Ottawa, Ontario K 1 A OR6, Canada)
ABSTRACT
Tne a i r c o n t e n t o f sea i c e can be measured d i r e c t l y by m e l t i n g a sample and c o l l e c t i n g t h e r e l e a s e d a i r , p r o v i d e d t h e a i r s a t u r a t i o n r a t i o i n t h e m e l t w a t e r i s known. The s a t u r a t i o n r a t i o was found e x p e r i m e n t a l l y t o be a f u n c t i o n o f t h r e e para- meters: t h e t i m e a f t e r an i c e sample was melted, t h e average bubble size, and t h e a i r p o r o s i t y o f t h e sample. Since t h e l a s t parameter i s t h e term t o be determined, an i t e r a t i o n method was employed i n c a l c u - l a t i o n s of p o r o s i t y . The bubble p r e s s u r e was assumed t o be a t one atmospheric pressure. The v e r t i c a l p r o - f i l e of a i r p o r o s i t y was t h u s o b t a i n e d f o r f i r s t - y e a r sea i c e i n t h e A r c t i c . The r e s u l t s were i n good agree- ment w i t h e s t i m a t i o n s o f p o r o s i t y made from d e n s i t y valrres measured f o r t h e same samples. T h i s i n d i c a t e s t h a t t h e bubble pressure i s near one atmospheric pressure.
INTRODUCTION
Mechanical p r o p e r t i e s o f sea i c e a r e h i g h l y dependent on a i r p o r o s i t y as w e l l as b r i n e p o r o s i t y
(e.g. Weeks and Assur 1967, Schwarz and Weeks 1977). The l a t t e r has been i n v e s t i g a t e d e x t e n s i v e l y i n r e l a t i o n t o s a l i n i t y and temperature (Assur 1958, Anderson 19601, b u t s t u d i e s on a i r p o r o s i t y a r e l i m i t e d (Kusunoki 1958). R e s u l t s o f mechanical t e s t s on sea i c e have been analyzed m a i n l y t o a s c e r t a i n t h e i n f l u e n c e o f b r i n e p o r o s i t y o r b r i n e volume, n e g l e c t - i n g t h e e f f e c t s o f a i r v o i d s (e.g. Schwarz and Weeks 1977), a l t h o u g h t h e dependence o f mechanical proper- t i e s on a i r p o r o s i t y has been i n d i c a t e d (Tabata 1958).
Two d i f f e r e n t approaches have been suggested f o r d e t e r m i n i n g t h e a i r p o r o s i t y o f a sample o f sea i c e . One approach i s t o compare t h e measured d e n s i t y o f t h e sample w i t h t h e t h e o r e t i c a l d e n s i t y f o r a i r - f r e e sea i c e (Anderson 1960, Ono 1968). Another approach i s by d i r e c t measurement o f t h e volume o f a i r which i s r e l e a s e d through me1 t i n g , assuming t h a t a i r bubbles i n t h e sample a r e a t atmospheric pressure (Kusunoki 1958, Save1 'yev 1954
1.
Both approaches i n v o l v e several problems t h a t must be c l a r i f i e d . I n t h e former approach, t h e r e a r e un- c e r t a i n t i e s i n t h e values o f p h y s i c a l q u a n t i t i e s used f o r c a l c u l a t i n g t h e t h e o r e t i c a l d e n s i t y f o r a i r - f r e e sea i c e (e.g. d e n s i t y o f b r i n e ) . I n t h e d i r e c t measure- ment o f a i r released, t h e a i r - b u b b l e pressure i s un- c e r t a i n . A pressure h i g h e r than one atmosphere c o u l d be generated i n a i r bubbles as t h e temperature a t a g i v e n
depth decreases w i t h i n c r e a s i n g t h i c k n e s s o f an i c e cover. I n a d d i t i o n , one has t o determine t h e amount o f d i s s o l v e d a i r i n m e l t w a t e r t h a t must be added t o t h e amount of 1 ib e r a t e d a i r t o o b t a i n t h e r e a l volume o f entrapped a i r . A p p l y i n g a s i m i l a r method t o measure t h e a i r c o n t e n t o f g l a c i e r i c e , Langway (1958) found t h a t t h e m e l t w a t e r was almost s a t u r a t e d w i t h a i r . F o r sea i c e , however, t h e s a t u p a t i o n r a t i o f o r m e l t w a t e r c o u l d n o t be near complete s a t u r a t i o n , s i n c e t h e a i r c o n t e n t o f sea i c e i s expected t o be s m a l l e r than f o r g l a c i e r i c e .
I n t h i s paper, experimental r e s u l t s on t h e s a t u r - a t i o n r a t i o o f a i r i n m e l t w a t e r a r e presented, and e s t i m a t i o n s o f p o r o s i t y by t h e two methods a r e com- pared, u s i n g d a t a o b t a i n e d on f i r s t - y e a r sea i c e from t h e A r c t i c .
COLLECTION OF AIR ENTRAPPED I N AN ICE SAMPLE The a i r entrapped i n an i c e sample can be c o l l e c t e d by me1 t i n g t h e sample, t h u s r e l e a s i n g t h e a i r . As e a r l y as 1925, A r n o l ' d - A l i a b ' y e v suggested a simple apparatus f o r c o l l e c t i n g t h e a i r and measuring i t s volume: r e l e a s e d a i r i s i n t r o d u c e d i n t o a b u r e t t e through a bottomless b o t t l e ( S a v e l ' y e v 1954). T h i s t y p e o f apparatus was used by Kusunoki (1958) f o r measuring t h e amount o f a i r i n very t h i n sea i c e
(<0.2 m t h i c k ) , where a r e l a t i v e l y l a r g e amount o f a i r was expected. Although a s i m i l a r amount o f a i r i s l i k e l y i n t h e upper p o r t i o n (say above 0.2 m i n depth) o f a f i r s t - y e a r sea-ice cover i n t h e A r c t i c , t h e deeper p o r t i o n i s considered t o c o n t a i n much l e s s . The A r n o l ' d - A l i a b ' y e v system, w i t h a b u r e t t e , would probably n o t be a c c u r a t e enough t o measure t h e small amount expected. A method developed by Nakaya (1956), t h e r e f o r e , was adopted f o r c o l l e c t i n g t h e a i r r e l e a s e d by m e l t i n g and measuring i t s volume.
The apparatus i s shown s c h e m a t i c a l l y i n F i g u r e 1. An i c e sample was p u t i n t o a c o n t a i n e r o f kerosene i n a c o l d room. A watch g l a s s was placed over t h e sample and t h e whole system was b r o u g h t i n t o a warm l a b o r a - t o r y . As t h e i c e melted, a i r i n t h e sample was r e l e a s e d and trapped under t h e watch glass. The l e v e l o f kerosene was a d j u s t e d t o be c l o s e t o t h e t o p o f t h e watch g l a s s so t h a t t h e p r e s s u r e o f t h e trapped a i r would be t h e same as t h e measured a i r pressure o f t h e l a b o r a t o r y . The temperature o f t h e kerosene was measured by a thermocouple a t a p o i n t c l o s e t o t h e trapped a i r .
The c a p t u r e d a i r under t h e watch g l a s s took a form s i m i l a r t o an o b l a t e spheroid and was c i r c u l a r *Present address: Department o f A p p l i e d Physics, F a c u l t y o f Engineering, Hokkaido U n i v e r s i t y ,
Sapporo 060, Japan. 204
Nakawo: Air porosity of sea ice
travelling
~ ~ C ~ O S C O D ~ thermocou~le.
melt water
/
Fig.1. Experimental apparatus f o r d e t e r m i n i n g a i r content.
i n t h e h o r i z o n t a l plane. The diameter o f t h e c i r c l e was measured u s i n g a t r a v e l l i n g microscope, and t h e diameter converted t o volume o f a i r by means o f t h e e q u a t i o n g i v e n i n F i g u r e 2, which was o b t a i n e d by i n j e c t i n g a i r o f known volume under t h e watch glass.
DISSOLVED AIR I N MELTWATER
The d i s s o l v e d a i r i n me1 t w a t e r was e s t i m a t e d as f o l l o w s . B l o c k s o f a i r - f r e e columnar-grained i c e were grown i n t h e l a b o r a t o r y (Gold 1972). Specimens o f 40 t o 70 g i n w e i g h t ( t h e same range as f o r t h e sample f o r p o r o s i t y measurements, described i n t h e n e x t s e c t i o n ) were prepared and h o l e s were made w i t h a d r i l l from one s i d e o f each block. A p i e c e o f i c e was f r o z e n t o t h i s s i d e a f t e r d r i l l i n g . The volume o f t h e c a v i t i e s t h u s pre- pared was determined by measuring t h e i r dimensions w i t h a t r a v e l l i n g microscope. F o r some blocks, t h e p o r o s i t y ( t h e volume o f t h e c a v i t i e s ) was e s t i m a t e d by measuring t h e d e n s i t y a c c u r a t e l y w i t h a procedure described by Nakawo (1980). These b l o c k s w i t h c a v i t i e s o f known volume were m e l t e d i n kerosene as described i n t h e p r e v i o u s section. The volume o f a i r r e l e a s e d from t h e b l o c k s and c a p t u r e d under t h e watch g l a s s was always s m a l l e r t h a n t h e pre-determined volume o f t h e c a v i - t i e s . The d i f f e r e n c e was considered t o be t h e amount o f a i r d i s s o l v e d i n t h e me1 twater.
D u r i n g t h e experiment, t h e volume o f t h e c a p t u r e d a i r ( h o r i z o n t a l diameter o f t h e a i r ) was found t o de- crease c o n t i n u o u s l y w i t h time. T h i s decrease i n d i c a t e s t h a t a i r can be absorbed by t h e kerosene, even though t h e s o l u b i l i t y o f a i r i n kerosene i s very small. The c a p t u r e d a i r under t h e watch glass, t h e r e f o r e , can d i f f u s e toward t h e m e l t w a t e r o r toward t h e atmosphere. The l a t t e r c o u l d t a k e p l a c e because t h e c o n c e n t r a t i o n o f a i r i n kerosene would be l a r g e r near t h e trapped
a i r t h a n a t t h e p l a n a r t o p s u r f a c e o f kerosene because o f t h e e f f e c t o f curvature. When a i r was i n j e c t e d under t h e watch g l a s s when no w a t e r was underneath, however, no decrease i n t h e volume o f t h e trapped a i r was observed f o r a couple o f days. I t i s considered, t h e r e f o r e , t h a t t h e amount o f a i r escaping t o t h e atmosphere i s n e g l i g i b l e , and t h a t t h e movement i s p r i m a r i l y t o t h e me1 t w a t e r , i n c r e a s i n g i t s s a t u r a t i o n r a t i o .
With t h i s assumption, t h e a i r c o n t e n t o f t h e me1 t-
w a t e r was c a l c u l a t e d from t h e d i f f e r e n c e i n volume between t h e c a v i t i e s i n an i c e b l o c k b e f o r e me1 t i n g and t h e c a p t u r e d a i r under t h e w a t t h g l a s s a f t e r m e l t i n g . Because t h e volume o f t h e captured a i r k e p t decreasing as described above, t h e a i r c o n t e n t o f t h e m e l t w a t e r was assumed t o i n c r e a s e w i t h time.
An example of t h e r e s u l t s o f t h i s experiment i s shown i n F i g u r e 3, i n which the. temperature i s a l s o presented. The abscissa i n t h e f i g u r e i s t h e t i m e when t h e who1 e system was brought from t h e c o l d room
t o t h e warn1 l a b o r a t o r y ; to and te in d i c a t e respec-
t i v e l y when t h e i c e b l o c k m e l t e d completely and when
' 0
0
5 10 15
m
25TIME, h
Fig.3. Time v a r i a t i o n o f a i r s a t u r a t i o n r a t i o i n m e l t w a t e r and kerosene temperature.
t = 0: t h e system was brought t o a warm l a b o r a t o r y .
t = to: an i c e sample was me1 t e d completely.
t = te: temperature reached room temperature.
t h e kerosene temperature a t a p o i n t c l o s e t o t h e captured a i r came t o room temperature. The kerosene temperature was 15°C even b e f o r e t h e i c e had m e l t e d completely, and subsequently reached t h e e q u i l i b r i u m v a l u e v e r y r a p i d l y . I t took a l o n g e r time, however, f o r t h e s a t u r a t i o n r a t i o t o reach e q u i l i b r i u m (com- p l e t e s a t u r a t i o n ) . The a i r c o n t e n t o f m e l t w a t e r was o n l y about 20 and 30% by volume o f i t s s a t u r a t i o n
v a l u e a t to and te, r e s p e c t i v e l y , i n t h i s example.
When t h e t o t a l s u r f a c e area o f t h e a i r c a v i t i e s i s l a r g e , t h e r e c o u l d be a g r e a t e r p o s s i b i l i t y d u r i n g m e l t i n g f o r a i r t o d i s s o l v e i n t o t h e meltwater. The degree o f a i r c o n t e n t o f t h e m e l t w a t e r a t a g i v e n time, t h e r e f o r e , c o u l d be a f u n c t i o n o f t h e t o t a l
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~d.6.00 rnrn I 0 I 2 3 AIR POROSITY, %Fig.2. The r e l a t i o n between diameter $I and volume v Fig.4. Dependence o f a i r s a t u r a t i o n on a i r p o r o s i t y
Naka~o: A i r porosity of sea i c e
s u r f a c e area o f t h e c a v i t i e s , and hence a f u n c t i o n of c a v i t y s i z e and b u l k p o r o s i t y . I n o r d e r t o examine t h i s , 23 b l o c k s having d i f f e r e n t p o r o s i t i e s were prepared u s i n g d i f f e r e n t s i z e d r i l l s. F o l l owing t h e above procedure, a i r s a t u r a t i o n a t to and t, was c a l c u l a t e d f o r these blocks.
The r e s u l t s a r e s h m n i n F i g u r e 4. Two d a t a p o i n t s connected by a v e r t i c a l b a r correspond t o each block, and t h e lower p o i n t represents t h e value a t to and t h e h i g h e r p o i n t a t t e r n D r i l l s i z e i s d i f f e r e n t i a t e d by d i f f e r e n t symbols. I t can be seen t h a t a i r s a t u r - a t i o n increased w i t h i n c r e a s i n g p o r o s i t y f o r t h e same s i r e o f c a v i t y . Blocks w i t h f i n e r c a v i t i e s prepared by small e r d r i 11 s r e s u l t e d i n a 1 a r g e r degree o f a i r s a t u r a t i o n i n t h e me1 t w a t e r i n comparison w i t h those b l o c k s having l a r g e c a v i t i e s w i t h t h e same p o r o s i t y . The two f i n e s o l i d l i n e s i n F i g u r e 4 are "eye f i t " curves f o r t h e s m a l l e r two d r i l l s i z e s (0.65 and 0.95 m) and t h e l a r g e r s i z e s (3.90 and 6.00
mn).
The small d i f f e r e n c e i n d r i l l s i z e d i d n o t g i v e a s i g n i - f i c a n t d i f f e r e n c e i n t h e a i r s a t u r a t i o n , although any change i n h o l e s i z e changes t h e s u r f a c e area o f t h e c a v i t i e s . The s c a t t e r o f t h e data f o r t h e curves i s about 2 5% by volume. which would cause an uncer-t a i n t y o f about 0.15% i n p o r o s i t y .
F o r e s t i m a t i n g t h e a i r s a t u r a t i o n r a t i o i n m e l t - water by u s i n g t h e c a l i b r a t i o n curve given i n F i g u r e 4, one has t o know t h e average s i z e o f a i r bubbles and b u l k p o r o s i t y o f t h e sample. The former can be obtained by observation. Since t h e l a t t e r i s t h e term t h a t one i s t r y i n g t o determine, an i t e r a t i o n method has t o be employed. Assuming t h e t e n t a t i v e p o r o s i t y value t o be g i v e n o n l y by t h e volume o f c o l l e c t e d a i r , t h e a i r s a t u r a t i o n r a t i o i n meltwater i s o b t a i n e d u s i n g a curve i n F i g u r e 4. By adding t h e amount o f d i s s o l v e d a i r thus obtained t o t h e volume o f c o l l e c t e d a i r , t h e second t e n t a t i v e value f o r a i r p o r o s i t y o f t h e sample i s obtained. T h i s process can be repeated u n t i l t h e v a l u e f o r t h e p o r o s i t y converges. I t i s assumed, i n t h e above c a l c u l a t i o n , t h a t a i r bubbles i n an i c e sample a r e a t one atmospheric pressure.
SALINITY, DENSITY, AND AIR POROSITY Sample d e s c r i p t i o n
Two c o r e samples, 75 mm i n diameter, were r e - covered from f i r s t - y e a r sea i c e a t E c l i p s e Sound, near Pond I n l e t , B a f f i n I s l a n d , Canada. The c o r i n g was done on 31 March 1980, when t h e a i r temperature was about -23°C. The recovered cores were 0.740 and 0.795 m i n l e n g t h . They were brought back t o t h e main camp a t Pond I n l e t , where t h e s h o r t e r c o r e ( c o r e 1 ) was analyzed t h e f o l l o w i n g n i g h t . The l o n g e r c o r e ( c o r e 2 ) was packed w i t h d r y i c e and shipped t o Ottawa f o r t h e l a t e r analyses, which were c a r r i e d o u t a f t e r storage f o r s i x months a t a temperature o f - 4 0 " ~ .
I n t h e w i n t e r o f 1979-80, t h e freeze-up d a t e was 16 October 1979. The growth o f t h e i c e i n t h e e a r l y p a r t o f t h e season was s i m i l a r t o t h a t o f o t h e r w i n t e r seasons: t h e growth r a t e o f sea i c e was i n t h e o r d e r of 0.2 um s - l (1.7 cm d - l ) w i t h r a t e decreasing w i t h t i m e I n e a r l y November, however, when t h e thickness o f t h e i c e had reached about 0.3 m, snow o f about 0.3 m depth was deposited on t h e ice, and stayed u n t i l t h e m e l t i n g season s t a r t e d . The presence o f t h e t h i c k snow l a y e r throughout t h e w i n t e r , except f o r t h e e a r l y period, c o n t r a s t e d w i t h t h e s i t u a t i o n i n o t h e r w i n t e r s . The growth r a t e o f t h e i c e decreased d r a m a t i c a l l y t o about 0.05 pm s-1 (0.4 cm d - l ) a f t e r t h e snow came, i.e. f o r i c e below 0.3 m i n depth. The growth r a t e was almost c o n s t a n t around t h e lower value (0.05 p s-I)
during t h e w i n t e r season. The thermal c o n d u c t i v i t y o f snow r s smaller than t h a t o f i c e by an o r d e r o f magni- tude, and a snow l a y e r o f 0.3 m t h i c k n e s s i s equi- v a l e n t i n
theraal
i n s u l a t i o n t o sea i c e o f about 3 wthickness. I t i s considered t h a t t h e almost c o n s t a n t growth r a t e i n t h e w i n t e r i s due t o t h e f a c t t h a t t h e i n s u l a t i o n e f f e c t o f sea i c e , which increased w i t h time, i s very small i n comparison w i t h t h e l a r g e
i n s u l a t i o n e f f e c t o f t h e t h i c k snow l a y e r , which d i d n o t vary s i g n i f i c a n t l y w i t h time.
A continuous columnar-grained i c e was found below about 0.2 m depth l e v e l .
Measurements and r e s u l t s
A s i m i l a r procedure was f o l l o w e d i n determining s a l i n i t y , density, and a i r p o r o s i t y f o r b o t h t h e - - I
f i e l d measurements ( c o r e 1 ) and t h e l a b o r a t o r y meas- urements ( c o r e 2). The cores were sectioned i n t o seg- ments o f e i t h e r 50 m ( c o r e 1 ) o r 25 mn ( c o r e 2). The r a d i a l s u r f a c e o f each segment was trimmed, r e s u l t i n g i n t h e f i n a l w e i g h t o f each sample b e i n g between 40 and 70 g. Mass, volume, and d e n s i t y were determined a c c u r a t e l y u s i n g a h y d r o s t a t i c method, i n which t h e samples were weighed b o t h i n a i r and 2,2,4-trimethyl- pentane (Nakawo 1980). Measurements were made a t about -15'C f o r core 1 and a t about -28'C f o r c o r e 2. The accuracy o f t h e d e n s i t y measurement was +- 0.4 t o t0.8 k g r3.
I n t h e e s t i m a t e o f p o r o s i t y , t h e curve f o r t h e s m a l l e r two s i z e s (0.65 and 0.95
mm)
o f d r i l l shown i n F i g u r e 4 was adopted f o r c a l i b r a t i n g t h e d i s s o l v e d a i r i n me1 twater, s i n c e most o f t h e a i r bubbles i n t h e samples were approximately 0.5 t o 1.0 mn i n diameter. Large bubbles o f 2 t o 3mm
i n diameter were found as w e l l as t h e s m a l l e r bubbles i n t h e samples from 0 t o 0.2 m depth. Using t h e curve f o r t h e smaller s i z e s o f c a v i t i e s c o u l d r e s u l t i n overestimating d i s s o l v e d a i r i n meltwater f o r these samples. The amount o f a i r c o l 1 ected from t h e s h a l l ow samples, however, was much l a r g e r than t h e amount o f d i s s o l v e d a i r , even f o r t h e c o n d i t i o n o f a i r - s a t u r a t e d meltwater. The f i n a l poro- s i t y value, t h e r e f o r e , was n o t i n f l u e n c e d s i g n i f i - c a n t l y by t h e overestirnation.A f t e r t h e measurements on p o r o s i t y , t h e r e s u l t i n g meltwater was used f o r
the
s a l i n i t y measurement by arefractometer/salinometer.
A simple experiment, which Has c a r r i e d o u t b e f o r e t h e measurements, i n d i c a t e d t h a t a small amount of kerosene i n t h e water would n o t a f f e c t t h e s a l i n i t y readings. It was d i f f i c u l t , how- ever, t o t a k e a r e a d i n g when a l a r g e r amount o f kero- sene was p r e s e n t and so c a r e was taken t o remove as much as ~ o s s i b l e from t h e me1 twater. A1 thoush i t was n o t p o s s i b l e t o e l i m i n a t e i t completely, t h e remain- i n g kerosene was n o t s i g n i f i c a n t .SALINITY. % AIR P0ROSITV;I. DENSITY, kg m-3
Fig.5. V e r t i c a l p r o f i l e s o f s a l i n i t y , d e n s i t y , and a i r p o r o s i t y f o r c o r e samples o f f i r s t - y e a r sea i c e . ( a ) : i n s i t u measurement a t -15'C ( c o r e 1 ) . ( b ) : l a b o r a t o r y measurement a t -28'C a f t e r 6 months o f storage a t -40°C ( c o r e 21.
Nakawo: Air porosity of sea ice
The r e s u l t s o f t h e measurements on s a l i n i t y , a i r p o r o s i t y , and d e n s i t y a r e shown i n F i g u r e 5 ( a ) f o r c o r e 1 and i n ( b ) f o r c o r e 2. I t was found t h a t b o t h s e t s o f d a t a agree w i t h each other. The s a l i n i t y decreased g r a d u a l l y from t h e t o p t o about t h e 0.3 m depth l e v e l , stayed almost c o n s t a n t through t h e body o f t h e i c e cover, and i n c r e a s e d s h a r p l y a t t h e l o w e r surface. A s i m i l a r p r o f i l e can be seen f o r a i r poro- s i t y , i n d i c a t i n g a p o s i t i v e c o r r e l a t i o n between t h e two q u a n t i t i e s . The v e r t i c a l p r o f i l e o f d e n s i t y , on t h e o t h e r hand, appears t o e x h i b i t an i n v e r s e propor-
ti onal i t y between d e n s i t y and s a l i n i t y o r a i r poro- s i t y , i .e. t h e small d e n s i t y corresponds w i t h t h e l a r g e s a l i n i t y and a i r p o r o s i t y , and v i c e versa. DISCUSSION
From t h e d e n s i t y values measured, a i r p o r o s i t y was c a l c u l a t e d by comparing w i t h t h e t h e o r e t i c a l d e n s i t y f o r a i r - f r e e sea i c e (Anderson 1960). The v a l u e f o r t h e t h e o r e t i c a l d e n s i t y was o b t a i n e d from t h e s a l i n - i t y v a l u e f o r each segment and t h e temperature (-15°C f o r c o r e 1 and -28°C f o r c o r e 2). The a i r p o r o s i t y values t h u s o b t a i n e d a r e compared w i t h those from t h e d i r e c t measurements i n F i g u r e 6. They a r e i n good agreement, i n d i c a t i n g t h a t a i r bubbles a r e approxi- mately a t atmospheric pressure, a1 though t h e assumed t h e o r e t i c a l d e n s i t y values s t i l l cause some uncer- t a i n t y , as d e s c r i b e d i n t h e i n t r o d u c t i o n . CORE CORE
(LE
4 0.2 0.2 0.4 0.6 0.8 1 2 4 6AIR POROSITY DETERMINED BY COLLECTNG AIR, %
Fig.6. Comparison o f a i r p o r o s i t y between t h e values from d e n s i t y measurement and from t h e d i r e c t measurement o f a i r content.
I t was i m p o s s i b l e t o p r e v e n t t h e i n i t i a l b r i n e drainage completely when t h e cores were recovered from t h e i c e . A i r c o u l d have been i n t r o d u c e d i n t o some o f t h e b r i n e channels a t t h a t time, which would have t h e e f f e c t o f making t h e apparent average bubble p r e s s u r e c l o s e r t o t h e atmospheric pressure.
The measured s a l i n i t y o f about 0.3% i n samples below 0.3 m depth, f o r which t h e growth r a t e was about 0.5 pm s - l , i s s m a l l e r by 0.1% than t h e v a l u e expected f o r t h e growth r a t e from a r e g r e s s i o n c u r v e g i v e n by Nakawo and Sinha (19811, which was based on a s e r i e s o f o b s e r v a t i o n s made soon a f t e r c o r e samples Mere recovered. T h i s s a l i n i t y d i f f e r e n c e , t h e r e f o r e , c o u l d be due t o d e s a l i n a t i o n d u r i n g storage. I c e temp- e r a t u r e f o r t h e 1979-80 w i n t e r , however, was e x t r a - o r d i n a r i l y high, owing t o t h e t h i c k snow l a y e r on t h e i c e . S u b s t a n t i a l n a t u r a l d e s a l i n a t i o n , t h e r e f o r e , c o u l d have taken p l a c e b e f o r e c o r e recovery i n t h i s p a r t i c u l a r w i n t e r . A d d i t i o n a l b r i n e drainage d u r i n g s t o r a g e ( f r o m t h e c o r e recovery t o t h e t i m e o f t h e measurements) i s considered t o have been small,
because t h e p e r i o d was short, about 10 h, f o r c o r e 1
and t h e s t o r a g e temperature was low, -40°C, f o r c o r e 2.
I n a d d i t i o n t o t h e e f f e c t o f d e s a l i n a t i o n , mech- a n i c a l r e l a x a t i o n c o u l d a l s o cause decrease o f t h e bubble pressure, even i f i t were a t a h i g h e r p r e s s u r e than one atmosphere. A simple c a l c u l a t i o n i n d i c a t e s , however, t h a t i t takes l o n g e r than a y e a r f o r t h e p r e s s u r e t o decrease from several atmospheres t o near one atmosphere. Thus, t h e r e l a x a t i o n e f f e c t i s con- s i d e r e d t o be n e g l i g i b l e .
The bubble pressure, t h e r e f o r e , i s considered t o be near one atmosphere i n t h e i c e , some u n c e r t a i n t y b e i n g caused by t h e i n i t i a l b r i n e drainage t h a t may have taken p l a c e when t h e cores were removed. Samples of sea i c e t o be used f o r mechanical t e s t s a r e u s u a l l y s u b j e c t t o t h e same problem: i t i s d i f f i c u l t t o a v o i d t h e i n i t i a l b r i n e drainage completely. F o r such samples, a t l e a s t , t h e a i r bubbles can be considered t o be a t atmospheric pressure. T h i s i m p l i e s t h a t a i r p o r o s i t y can be determined from e i t h e r d e n s i t y meas- urements o r d i r e c t measurement o f a i r volume.
S a l i n i t y has a s t r o n g c o r r e l a t i o n w i t h growth r a t e (Cox and Weeks 1975, Nakawo and Sinha 1981). A i r p o r o s i t y a1 so c o u l d be dependent on growth r a t e . If a r e l a t i o n between them i s known, and w i t h i n f o r m a t i o n on growth r a t e only, t h r e e i m p o r t a n t f a c t o r s t h a t a f f e c t mechanical p r o p e r t i e s can be estimated, i.e. s a l i n i ty , d e n s i t y , and a i r p o r o s i t y , s i n c e d e n s i t y i s a f u n c t i o n o f s a l i n i t y and a i r p o r o s i t y . C a r t e (1961) and B a r i and H a l l e t t (1974) have i n v e s t i g a t e d e x p e r i m e n t a l l y t h e dependence o f a i r c o n t e n t o f i c e on growth r a t e , b u t t h e i r growth r a t e s a r e t o o l a r g e t o be used f o r n a t u r a l l y grown i c e .
The p r e s e n t r e s u l t s shown i n F i g u r e 5 suggest a dependence o f a i r p o r o s i t y on growth r a t e , t h e poro- s i t y decreasing w i t h decreasing growth r a t e ( 0 t o 0.3 m), and almost c o n s t a n t a i r p o r o s i t y f o r c o n s t a n t growth r a t e (0.3 t o 0.8 m). A c o r r e l a t i o n a n a l y s i s , however, was n o t made as t h e growth process was considered t o vary w i t h t i m e near t h e s u r f a c e o f t h e i c e , as was i n d i c a t e d by t h e c r y s t a l l o g r a p h i c t e x t u r e e x h i b i t e d i n a t h i n v e r t i c a l section, and t h e r e was l i t t l e v a r i a t i o n i n growth r a t e i n t h e lower, c o n t i n - uous columnargrained s t r u c t u r e . I t would be i n t e r e s t - i n g t o o b t a i n a v e r t i c a l p r o f i l e o f a i r p o r o s i t y i n sea i c e h a v i n g a continuous columnar-grained s t r u c - t u r e f o r which t h e growth r a t e v a r i e d s i g n i f i c a n t l y . ACKNOWLEDGEMENTS
Tne c o r e recovery and t h e i n s i t u measurements were made w i t h t h e h e l p o f S t e l t n e r nevelopment and M a n u f a c t u r i n g Company L t d . The a u t h o r wishes t o thank H A R S t e l t n e r f o r a r r a n g i n g those a c t i v i t i e s , and S Koonoo, M Komangapik, and E Pewaotulook f o r t e c h n i - c a l a s s i s t a n c e i n t h e f i e l d . The a u t h o r i s a l s o i n d e b t e d t o D r N K Sinha and t o
D
W r i g h t o f t h e D i v i s i o n o f B u i l d i n g Research, N a t i o n a l Research Council o f Canada, f o r encouraging him throughout t h e work and f o r t e c h n i c a l a s s i s t a n c e i n t h e l a b o r a t o r y measurements.T h i s paper i s a c o n t r i b u t i o n from t h e D i v i s i o n o f B u i l d i n g Research, N a t i o n a l Research Council o f Canada, and i s p u b l i s h e d w i t h t h e approval o f t h e D i r e c t o r o f t h e D i v i s i o n .
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