ATOLL RESEARCH BULLETIN

ATOLL

RESEARCH

BULLETIN

-..---

No.

53

Slicks.on. ocean surface downwind from c o r a l r e e f s by

F. R. Fosberg

Issued by

THE

PACIFIC SCIENCE BOARD

National Academy of Sciences--National Research Council Washington, D. C.

September

15,

3957

S l i c k s on Ocean Surface Downwind from Coral Reefs

2d

F. R. by Fosberg

2/

During general i n v e s t i g a t i o n s of c e r t a i n a t o l l s of t h e northern Marshall Islands i n 1952 (Fosberg, 1955) a phenomenon was noticed t h a t

seems ~~o;.thy of t h e a t t e n t i o n of oceanographers and g e o l o g i s t s , although no expl-anation i s offered f o r it. This i s t h e occurrence of narrow,

elongate s t r i p s of smooth water, r e s e m b l i x t h e e f f e c t prroduced by a f i l m of o i l on t h e surface, extending downwind from t h e c o r a l reefs.

This appearance was f i r s t noticed i n t h e lagoon of Ujae A t o l l on March

4,

1952, and was observed repeatedly t h e r e a f t e r f o r more t h a n a week along t h e windward reef i n t h e southern h a l f of t h e a t o l l , A b r i s k t r a d e wind was blowing, and t h e s u r f a c e of t h e lagoon was covered by a c l o s e

p a t t e r n of r i p p l e s , wavelets, and small waves. Perpendicular to t h e r e e f and e x a c t l y i n t h e d i r e c t i o n of t h e wind were numerous p a r a l l e l l i n e a r streaks, varying from a few centimeters t o a meter i n width, of p e r f e c t l y smooth

water. These extended s e v e r a l hundred yards downwind. I n them t h e r e was an abrupt smoothing of t h e surface disturbance of t h e water and usually a small accumulation of small bubbles and f l e c k s of foam i n a broken l i n e near t h e

c e n t e r of t h e streak. The s t r e a k s seemed t o be of d i f f e r e n t ages, t h e younger ones having very c l e a r l y defined edges, t h e older ones becoming broken up o r braided i n pattern. They tended t o become more broken up a s t h e d i s t a n c e f r o m t h e reef increased, a l s o broader with distance, On a very rough day t h e i r boundaries were not c l e a r , and t h e choppiness was not a l t o g e t h e r

smoothed out. On a day with only a very g e n t l e breeze t h e whole s t r e a k was not s t r a i g h t but a b i t i r r e g u l a r . A t about 300 m. from t h e r e e f , on t h e calmer

day t h e s t r e a k s became a s much a s

6

t o 8 m. wide. On March 9 s i m i l a r s t r e a k s were seen i n t h e open sea t o t h e leeward of t h e north end of Ujae Atoll, downwind from Bikenkar and l3nelamoj I s l e t s and t h e i r connecting r e e f . These were very w e l l developed, even d i r e c t l y i n t h e l e e of t h e i s l e t s . The l a r g e s t s e e m e d t o correspond t o l a r g e r e e n t r a n t s o r surge channels i n t h e r e e f .

On March 1 8 s i m l l a r s t r e a k s were seen on Wotho Atoll., i n t h e south p a r t of t h e lagoon i n s i d e t h e e a s t r e e f , some opposite t h e open r e e f , some inward from t h e long sand i s l e t on t h i s reef. On

arch

21 a very good opportunity was afforded t o study t h e s e s t r e a k s i n s i d e t h e n o r t h r e e f of Wotho, west o f

- Eneobnak Islet, ^. Here a r e s e v e r a l channels crossing t h e r e e f , broad toward t h e seaward s i d e of t h e r e e f , narrowing toward t h e western projection o F t h e i s l e t and sweeping westward around $is projection before e n t e r i n g t h e lagoon.

On t h e reef t h e y run bet,wean P l a t s , of r e e f conglonierate and rubble t r a c t s l y i n g on t h e reef f l a t . A f a i r l y st,rong current was fl.owing inward from t h e s e a t o t h e lagoon. In t h e s e channels t h e s l i c k s described above were

Publication authorized by t h e Director, U. S. Geological Survey.

2J Botanist, U. S. Geological Survey.

very prominent, following t h e d i r e c t i o n of t h e wind r a t h e r than that of t h e current, though t h e s e d i r e c t i o n s were only a few degrees a p a r t . Although previous observations were a l l made from small boats, here it was possible t o examine t h e s t r e a k s much more c l o s e l y while wading i n waist-deep water.

Careful examination revealed no iridescence, whatever. The boundaries were extremely sharp. There was a suggestion of a d i f f e r e n c e i n t h e f e e l of t h e water in t h e s t r e a k s f r o m t h a t nearby, but nothing very t a n g i b l e o r describable. There was no d i f f e r e n c e i n t a s t e o r f l a v o r . An attempt was made t o follow t h e s t r e a k s t o t h e i r sources, which seemed t o be i n t h e

rubble t r a c t s t o windward, but t h i s was unsuccessful, a s t h e streaksgradually became i n v i s i b l e i n t h e very q u i e t water a s t h e rubble t r a c t was approached.

There was nothing i n t h e rubble t r a c t t o suggest an o r i g i n f o r t h i s phe- nomenon. There seemed l i t t l e chance t h a t o i l y m a t e r i a l from t h e open s e a could be responsible f o r t h e phenomenon here, though such an o r i g i n could not b e excluded.

Gn March

24

s i m i l a r s t r e a k s were seen i n t h e open sea t o t h e leeward of t h e south half of Bikar Atoll. lkey were q u i t e numerous, b u t where t h e observations were made, from t h e deck of t h e ship a q u a r t e r t o half a mile from t h e r e e f , t h e y were r a t h e r braided and broken up, but some were s t i l l f a i r l y strong. On March 25, a t Pokak Atoll, t h e northernmost of the Marshalls, such s l i c k s were conspicuous a t sea on t h e leeward s i d e o f t h e leeward r e e f , e s p e c i a l l y near t h e s i n g l e channel through t h e r e e f . They extended f o r s e v e r a l miles downwind, becoming very d i f f u s e a t t h a t distance. Here they were only observed from t h e deck of t h e ship.

A 1 1 of t h e above observations were made i n t h e spring, i n a season of strong t r a d e winds. Another v i s i t was made t o Pokak and Bikar i n July and August, i n much calmer weather. A s p e c i a l attempt was made t o observe t h i s phenomenon f u r t h e r , but t h e n it was much more d i f f i c u l t t o see the s l i c k s clearly. What appeared t o be the same type of s t r e a k s were seen s e v e r a l times; however only once, on August 9, were unequivocal observations made, This was i n t h e lagoon of B i k a r A t o l l where a few n o t especially c l e a r o r sharp s t r e a k s were seen i n s i d e t h e windward reef between Almeni and Jaboero I s l e t s . There seems no doubt t h a t t k e phenomenon occurs during t h e s e calmer months, but because o f t h e weakness of s u r f a c e disturbance t h e r e i s s o

l i t t l e c o n t r a s t t h a t t h e s l i c k s do not show up very well.

The above observations form a l l of t h e f a c t u a l d a t a t o be considered.

There seems l i t t l e doubt t h a t t h e phenomenon i s a r e a l one t h a t i s reasonably constantly associated with c o r a l r e e f s , a t l e a s t i n t h e northern Marshalls area. That i t was not seen near any of t h e eight o t h e r a t o l l s v i s i t e d e a r l i e r i n t h e same expedition i s probably simply t h e r e s u l t of a t t e n t i o n not having been called t o it previous.ly. The expedition d i d n o t p r i m a r i l y concern i t s e l f with t h e ocean but with r e e f s and i s l e t s . A f t e r t h e s l i c k s were first noticed t h e y could usually b e found i n any s i m i l a r s i t u a t i o n t o t h e leeward of r e e f s i n b r i s k weather.

The appearance of t h e s e s l i c k s s t r o n g l y suggests t h e presence of minute q u a n t i t i e s , perhaps monomolecular f i l m s , of some substance t h a t

s t r o n g l y a l t e r s t h e s u r f a c e tension of t h e s e a water. The smoothing out of r i p p l e s i s abrupt, l o c a l i z e d , and of considerable duration, not s h i f t i n g and inconstant a s would b e expected of phenomena due s o l e l y t o wind i n t e r - a c t i n g with t h e s u r f a c e of t h e water (see Langmuir, 1938). The dependence of s l i c k formation on t h e presence of films o f organic matter on the surface of the water has been s t r e s s e d by most authors who have discussed such

phenomena i n recent years (Dietz and LaFond, 1950; Ewing, 195Oa, 1950b).

The combined influence of organic films and e f f e c t s of strong winds, as suggested by Ewing (1950a, b) should not be disregarded a s a p o s s i b i l i t y , though t h e "braidedt1 appearance of old s l i c k s contrasted with t h e sharpness of new ones p a r a l l e l and simultaneous with them i s hard t o reconcile with any wind-induced phenomenon. The nature of t h e p a t t e r n obsemed i n t h e Narsha11 Islands does not a t a l l resemble t h e p a t t e r n of l i g h t end dark bands i n t h e Gulf of Panama discussed by Woodcock and Wyman (1947) a s shown i n t h e i r photographs i n p l a t e s 6 t o 8 .

The arrangemnt of t h e s t r e a k s , a t i r r e g u l a r i n t e r v a l s running down wind fr.s.ri t h e r e e f , suggests t h e p o s s i b i l i t y of frequent emission of minute quanl-,itj.es o.? some o i l y m a t e r i a l from t h e reef o r from some organism

connected w3.h t h e ?:eef. F. S. NacNeil ( i n conversation a t t h e time) reported seeing small emissions of bubbles coming out of r e e f s i n places. The

p o s s i b i l i t y has not been overlooked. t h a t t h e s e s l i c k s may be due t o waste o i l (from steamships) t h a t may be washed over t h e r e e f from t h e windward

side. This, howevw, seems precluded by t h e observation of t h e same phenomencn t o leeward of long i s l e t s where n o water could possibly come over from t h e windwad s i 6 e . These i s l a n d s are, a l s o , f a r from l a n e s of heavy ocean t r a f f i c a t t h e prescxt time. There seems l i t t l e doubt t h a t t h e s e s t r e a k s o r i g i n a t e on t h e reef i t s e l f .

.

.. .

These f a c t s a r e placed on record i n t h e hope t h a t e i t h e r a convincing s o l u t i o n t o t h e pFoblem of t h e o r i g i n of t h e s l i c k s may occur t o someone e l s e o r t h a t f u r t h e r i n v e s t i g a t i o n may be deemed worth while.

LITERATURE CITED

Diet?;, R. S., and Laf ond,

E.

C.

Natural s l i c k s on t h e ocean.

Journal of Marine Research

9:

69-76, 1950.

Ewing, G. C.

Relation between band s l i c k s a t t h e s u r f a c e and i n t e r n a l waves i n t h e sea.

Science 111 (2874): 91-94, 1950a.

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S l i c k s , s u r f a c e films and i n t e r n a l waves.

Journal of Marine Research 9: 6 - 1 8 7 1950b.

Fosberg, F. R.

Northern Marshall Islands Expedition, 1951-1952. Narrative.

A t o l l Research Bulletin 38: 1-36, 1955.

Langmuir, I.

Surface motion of water induced by wind.

Science 87: 119-123, 1938.

Woodcoclc, A. H., and l.lyman,

J.

Convective motion i n a i r over t h e s e a .

Annals of t h e New York Academy of Sciences 48(8),: 749-782, 1947.