ATOLL RESEARCH BULWTIM
NO.
89
Geophysical Observations on Christmas Island by
John Uorthrop
Issued by
TKE PACIFIC SC DIJCE B O W D
National Academy of Sciences--National Research Council Slashington, D. C.
Dscc?. l ?r 1 J , L :3-
Geophysical Observations on Christmas I s l a n d
Y
by
John Northrop
During October and Fovember of 1961, t h e Vhrine Physical
Laboratory of t h e Scripns I n s t i t u t i o n of Oceanography, i n cooperation with the B r i t i s h Atoiic >leapons Research Establishment, sponsored a s e r i e s of geophysical measurements on Christmas Island. Continuous recordings of t h e e a r t h ' s magnetic f i e l d were made simultaneously a t various p o i n t s on the i s l a n d under the d i r e c t i o n of Ronald G. Mason
2/
and John Northrop of the I h r i n e Physical Laboratory of t h e Univer- s i t y of C a l i f o r n i a , San Diego. In a d d i t i o n t o these recordings, d a i l y t r i p s by land rover and small boat t o various p o i n t s on the i s l a n d and i n the lagoon were made f o r t h e purpose of completing g r a v i t y and magnetic measurements with p o r t a b l e equipment. These measurements show a sharp p o s i t i v e g r a v i t y anomaly near Motu Tabu, towards t h e north end of t h e i s l a n d , i n d i c a t i n g a shallow depth t o basement i n t h a t area.
I/
This work was p a r t i a l l y supported by Contract Nonr 2216 ( 0 5 ) with the Office of Naval Research.2/
Also of Imperial College, London, England- 2 -
A t lo 52.7' ~ 1 1 5 7 ' 23.9 ' X, an unusually hot lagoon
1/4
mile long was found t o have a bottom temperature of 102' F arid a surface temper- a t u r e of 8G0 F. The surface l a y e r , about 1/2 inch thick, has a s p e c i f i c g r a v i t y of 1 . 0 and t h e bottom water1.4.
Since t h i s narrow lagoon i sonly about
3
f e e t deep, a marked inverse temperature gradient i s p r e s - e n t . A t t h e suggestion of V a l t e r I h k of Scripps I n s t i t u t i o n ofOceanography, t h e l e v e l of t h e lagoon surface was measured and found t o be about one and one-half f e e t below sea l e v e l , and about 4 f t l o v e r than t h e l e v e l of t h e ia.goon t o t h e S. W. about 100 yards away.
There w a s c l e a r l y seepage of f r e s h water i n t o our lagoon a t t h e end-- P h i l i p Helfrich noticed an " o i l i n e s s " where t h e water w a s coming i n . It i s very l i k e l y t h e r e f o r e , t h a t t h e e f f e c t e x i s t s only because of constant renewal by t h e incoming cold f r e s h water. We t h i n k t h a t t h e e f f e c t probably d i e s out as. one goes away from t h e end of t h e lagoon where we made our measurements. Thus, anomalous temperature s t r u c t u r e
i s thought t o be due t o a very t h i n f r e s h water "blanket" t h a t i s renewed from seeps along t h e walls of t h e lagoon and prevents h e a t i n t h e bottom l a y e r from r e r a d i a t i n g . To produce such a marked temp- e r a t u r e change, t h i s process must have been i n operation f o r a. good
many years. Samples of t h i s water have been s e n t t o P h i l i p Helfrich of t h e University of H a w a i i f o r f u r t h e r a n a l y s i s .
