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TEMPERATURE DEPENDENCE OF THE CRYSTAL GROWTH RATE IN POLAR GLACIERS
J. Petit, P. Duval, C. Lorius
To cite this version:
J. Petit, P. Duval, C. Lorius. TEMPERATURE DEPENDENCE OF THE CRYSTAL GROWTH RATE IN POLAR GLACIERS. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-661-C1-662.
�10.1051/jphyscol:1987198�. �jpa-00226464�
JOURNAL DE PHYSIQUE
Colloque C1, suppl6ment au n o 3, Tome 48, mars 1987
TEMPERATURE DEPENDENCE OF THE CRYSTAL GROWTH RATE IN POLAR GLACIERS
J.R. PETIT, P. DUVAL and C. LORIUS
Laboratoire de Glaciologic et GBophysique de l'Environnement, B.P. 96, F-38402 St-Martin-d'HBres Cedex, France
Abstract :
In p o l a r f i r n , t h e a v e r a g e c r o s s - s e c t i o n a l a r e a o f g r a i n s i n c r e a s e s l i n e a r l y w i t h t i m e . The v a r i a t i o n o f t h e growth r a t e K w i t h t e m p e r a t u r e i s g i v e n by :
w h e r e E i s a n a p p a r e n t a c t i v a t i o n e n e r g y (si 0 . 5 4 eV) ( 1 , 2 ) . The r a t e o f i n c r e a s e i s n e a r l y t h e same i n f i r n and i c e a s long a s t h e s t r a i n e n e r g y i s s m a l l i n comparison w i t h t h e d r i v i n g f o r c e f o r g r a i n g r o w t h .
A much lower growth r a t e h a s been found i n 20000 y e a r s - o l d i c e formed d u r i n g t h e L a s t G l a c i a l Maximum i n b o t h Dome C ( 3 ) and Vostok ( 4 ) c o r e s , two s i t e s where i c e d e f o r m a t i o n i s v e r y s m a l l . M i c r o p a r t i c l e s and s o l u b l e i m p u r i t i e s c a n n o t a f f e c t g r a i n g r o w t h s i g n i f i c a n t l y a s w e l l a s v a r i a t i o n s w i t h t i m e of t h e t e m p e r a t u r e T o f i c e p a r t i c l e s d e p o s i t e d s i n c e t h e l a s t g l a c i a l a g e .
In t h e o t h e r hand, growth r a t e v a l u e s a l o n g t h e 900 m Dome C i c e c o r e a r e v e r y w e l l c o t r e l a t e d t o t h e i s o t o p i c p r o f i l e . As t h e i s o t o p i c c o n t e n t o f p o l a r snow i s p r i m a r i l y g o v e r n e d b y t h e t e m p e r a t u r e o f f o r m a t i o n o f p r e c i p i t a t i o n , we assume t h a t p o l a r i c e k e e p s t h e memory o f t h e t h e r m a l c o n d i t i o n s d u r i n g t h e r a p i d m e t a m o r p h i s m o f snow i n t h e f i r s t 10 m o f f i r n . I h e i c e s t r u c t u r e g o v e r n i n g t h e d i f f u s i o n of w a t e r m o l e c u l e s t h r o u g h t h e l a t t i c e o r g r a i n b o u n d a r i e s would b e i r r e v e r s i b l y m o d i f i e d d u r i n g t h e r a p i d t r a n s f o r m a t i o n o f snow i n t h e v e r y i n i t i a l s t a g e o f s i n t e r i n g . Such an approach i s i n a g r e e m e n t w i t h t h e m o d e l p r o p o s e d b y V a s s o i l l e e t a l . ( 5 ) t o e x p l a i n t h e anomalous e l e c t r i c a l and m e c h a n i c a l b e h a v i o r o f p o l a r i c e .
We p r o p o s e t h e c r y s t a l growth r a t e i s g i v e n by :
Ef Em
K = K e x p - - exp -,
RTs
RT
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987198
JOURNAL DE PHYSIQUE
w h e r e Ef i s t h e f o r m a t i o n e n e r g y o f d e f e c t s and E t h e a c t i v a t i o n e n e r g y o f t h e m
m i g r a t i o n o f w a t e r m o l e c u l e s . The t e m p e r a t u r e T i s t h e e f f i c i e n t t e m p e r a t u r e a t w h i c h t h e i c e s t r u c t u r e i s i r r e v e r s i b l y m o d i f i e d n e a r t h e s u r f a c e o f s n o w . T h e r e f o r e , T i s assumed e q u i v a l e n t t o t h e mean a n n u a l t e m p e r a t u r e o f t h e s i t e .
By t a k i n g i n t o a c c o u n t r e c e n t r e s u l t s g i v e n by H i g a s h i e t a l . ( 6 ) and Goto e t a l . ( 7 ) o n t h e mechanism o f s e l f d i f f u s i o n i n i c e and on t h e v a l u e of 0.16 eV f o r Em t h e f o r m a t i o n e n e r g y o f d e f e c t s Ef i s 0 . 3 8 eV. A s t h e v a r i a t i o n o f e x ~ - F r n / ~ ~ r e m a i n s s m a l l , e q u a t i o n ( 2 ) g i v e s a h i g h s e n s i t i v i t y o f t h e growth r a t e w i t h T and t h i s would e x p l a i n t h e good r e l a t i o n s h i p between t h e c r y s t a l g r o w t h r a t e and t h e p a s t c l i m a t e .
As a p a l e o c l i m a t e a p p l i c a t i o n o f t h e m o d e l , a t e m p e r a t u r e change o f a b o u t 10" C i s found between t h e L a s t G l a c i a l Maximum and t h e p r e s e n t c l i m a t e f o r c e n t r a l E a s t A n t a r c t i c a , a r e s u l t c o n s i s t e n t w i t h e s t i m a t i o n s from i s o t o p e s t u d i e s . For o t h e r s i t e s , s u c h a s Byrd i n A n t a r c t i c a o r Camp C e n t u r y and Dye 3 i n G r e e n l a n d , t h e s t r a i n e n e r g y s t o p s g r a i n growth and t h i s e f f e c t becomes s i g n i f i c a n t a l r e a d y f o r Holocene i c e . We t h e r e f o r e b e l i e v e t h a t t h e s u r f a c e t e m p e r a t u r e s i g n a l i s s c r e e n e d by s u c h an e f f e c t .
REFERENCES
( 1 ) GOW, A . J . , J o u r n a l o f G l a c i o l o g y , 8 , 5 3 , ( 1 9 6 9 ) , 241-252.
( 2 ) GOW, A.J., IAHS, Publ. N o 114, ( 1 9 7 4 ) , 25-42.
( 3 ) DUVAL, P. and LORIUS, C , E a r t h P l a n e t . S c i . L e t t . , 4 8 (19801, 59-64.
( 4 ) BARKOV, N . H . a n d LIPENKOV, V . , Academy o f S c i e n c e , S o v i e t G e o p h y s i c a l Committee, 5 2 , ( 1 9 8 5 ) , 178-186.
( 5 ) VASSOILLE, R., PEREZ, J . , TATIBOUET, J . , DUVAL, P. and MACCAGNAN, M . , Annals o f G l a c i o l o g y , 3 , ( 1 9 8 2 ) , 307-311.
( 6 ) HIGASHI, A . , FUBUDA, A., HONDOH, T . , GOTO, K. and AMAKAI, S . , D i s l o c a t o n s i n s o l i d s . Yamada S c i e n c e F o u n d a t i o n , U n i v e r s i t y o f Tokyo p r e s s , ( 1 9 8 5 1 , 511-515.
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