Oudot, Claude

Rual, Pierre

Wauthy, Bruno

Horizon / Pleins textes

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MINERALIZATION SOUTH OF THE EQUATOR I N  RELATION WITH EQUATORIAL FERTILITY 
I 
by 
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C. OUDOT, P, RUAL and B. WAUTHY 
A l l  t h e  c r u i s e s  of t h e  R.V. Coriolis i n  t h e  Western E q u a t o r i a l  P a c i f i c  
have po in ted  ou t  a n i t r i t e  accumulat ion layer which h a s  been found a t  t h e  t o p  
of t h e  theinnocline, i n  a convergence zone between t h e  E q u a t o r i a l  Cur ren t  and 
t h e  South Equa-t;orial Counter Curren t ,  n e a r  5"s. A s p e c i a l  cruise called EPONITE 
h a s  been devoted  t o  t h e  s t u d y )  d long  t h e  167"E meridian,  of t h i s  phehomenon, 
which is of a p a r t i c u l a r  importance,  because  o f  its l o c a t i o n  j u s t  sou th  o f  t h e  
h i g h l y  fer t i le  e q u a t o r i a l  upwell ing zone, The n i t r i t e  is  an i n t e r m e d i a r y  s t e p  
i n  t h e  m i n e r a l i z a t i o n  c y c l e  of t h e  o r g a n i c  n i t rogen .  But  it can also be produced 
by  b a h t e r i a  ar by phytoplankton cells reducing  n i t r a t e .  The p resence  o f  n i t r i t e  
i n  a w e l l  oxygehated l a y e r  raises t h e  q u e s t i o n  of its o r i g i n .  ! 
The EPONITE cruise h a s  been c a r r i e d  o u t ,  wh i l e  t h e  t radewinds  blew from t h e  
eas t -no r theas t .  These winds, abnormal for t h e  season ( August-September 19701, 
c r e a t e d  i n  t h e  mixed l a y e r  : i.) n o r t h  of t h e  Equator, an h o r i z o n t a l  d ive rgence  
which produced an upwell ing i n  t h e  upper  layers, i i )  s o u t h  of t h e  Equator ,  a 
i 
convergence wi th  a f ron ta l  zone which s e p a r a t e d  t h e  warm, low s a l i n i t y  s u r f a c e  
water o f  t h e  South E q u a t o r i a l  Counter Curren t ,  from t h e  colder upwelled equato- 
r i a l  s u r f a c e  water. Underneath, i n  t h e  lower p a r t  of t h e  pycnocl ine ,  La id  t h e  
s a l i n i t y  maximum o f  t h e  South P a c i f i c  S u b t r o p i c a l  Water. 
A cross s e c t i o n  o f  t h e  n i t r a t e  c o n c e n t r a t i o n  shows a l a r g e  maximum 
(8,ug-af N03-N/1) i n  the t he rmoc l ine  between 7"s and 9 O S ,  l i m i t e d  by t h e  i sanos-  
teric s u r f a c e s  500 and 400 cl/t. The accumulat ion layer of t h e  n i t r i t e  was pre- 
s e n t  between 4"s and 8"S,  w i t h  a maximum (1-2 ug-at NO2-N/1) a t  520-530 d t ,  
between two maxima of t h e  d e n s i t y  g r a d i e n t  : t h e  upper  g r a d i e n t  i s  associated t o  
/ 
'1 
t h e  thermocl ine ,  t h e  lower one t o  t h e  s a l i n i t y  maximum. 
Y 122 ~~~~ gp3 - * Centre ORSTOM d e  Nouméa, B,-P, 4, Nouméa, Nouvelle-Calédonie, am w. s. T. O. &, 
The c o n t i n u i t y  o f  t h e  p o s i t i o n  of t h e  n i t r i t e  maximum wi th  r e g a r d  t o  t h e  
t h e r m o s t e r i c  anomaly can be t h e  r e s u l t  of an i s e n t r o p i c  mer id iona l  flow. The 
Cromwell t h e o r e t i c a l  model of t h e  mer id iona l  c i r c u l a t i o n  wi th  a n o r t h e a s t  t rade-  
wind i n d i c a t e s  t h a t  t h e  r i c h  heavy water upwelled n o r t h  of t h e  Equator, s i n k s  i n  
t h e  f r o n t a l  zone s o u t h  of t h e  Equator  and flows southward below t h e  warm homoge- 
neous layer. I n  its motion t h e  water would carry t h e  n i t r i t e  e i t h e r  as n i t r i t e ,  
e i t h e r  a s  n i t rogenous  o r g a n i c  matter, away f r o m  t h e  e q u a t o r i a l  a r e a  p a r t i c u l a r l y  
r i c h  i n  d i s s o l v e d  and p a r t i c u l a t e  o r g a n i c  matter- The n i t r i t e  found n e a r  5OS 
could  t h u s  be o n l y  an i n t e r m e d i a t e  s t e p ,  i n  time and space , in : the- -minera l i ta~~on 
cycle ofithe-nitrogsnons=osganic matter. The n i t r a t e  maximum, s o u t h  of t h e  n i t r i -  
te  one, would be  a f u r t h e r  s t ep .  The s p a t i a l  l i m i t a t i o n  of t h e  n i t k i t e  maxima 
c o n c e n t r a t i o n s  to  t h e  SOS a r e a  is  due t o  opt ima c o n d i t i o n s  for t h e  n i t r i t e  appea- 
r a n c e  a t  t h i s  p lace ,  t h a t  is to say,  t h e  ba l ance  between t h e  ammonia oxyda t ion  
i n t o  t h e  n i t r i t e  and t h e  n i t r i t e  oxydat ion  i n t o  t h e  n i t r a t e ,  
. 
With such a d i s t r i b u t i o n  it is d i f f i c u l t  t o  f i n d  a v e r t i c a l  r e l a t i o n s h i p  
between t h e  n i t r i t e  maximum and t h e  c o n t e n t  o f  t h e  o v e r l a y i n g  waters .  The n i t r i t e  
maximum is below t h e  ch lo rophy l l amax imum,  which lies a t  t h e  bottom o f  t h e  mixed 
l a y e r  and above t h e  phaeo-pigments maximum. The mixed l a y e r  h a s  a v e r y  low pri-  
mary product ion ,  and t h e  n i t r i t e  maximum is l o c a t e d  undernea th  t h e  first d e n s i t y  
g r a d i e n t  which a c t s  as a "en between t h e  mixed l a y e r  and t h e  n i t r i t e  maximum. 
Fur thermore j  bo th  s i d e s  of t h e  n i t r i t e  m a x i m u m ,  t h e  d e n s i t y  g r a d i e n t  and t h e  
n i t r i t e  g r a d i e n t ,  are closely r e l a t e d ,  so t h a t  t h e  n i t r i t e  seems t o  mix as a 
c o n s e r v a t i v e  proper ty ;  i f  t h e r e  is a b i o l o g i c a l  o r  chemical  p rocess  i t  is  con- 
f i n e d  to t h e  n i t r i t e  maximum laye r .  Thus t h e  n i t r i t e  maximum i s  formed by a n o t h e r  
mecanism than  t h e  v e r t i c a l  mixing process wi th  t h e  mixed l a y e r .  
I n  case of a swi t ch  of t h e  wind t o  t h e  s o u t h e a s t ,  t h e  h o r i z o n t a l  d ive rgence  
and upwel l ing  s o u t h  of t h e  Equator  would upwell  t h e  n u t r i e n t s  accumulated a long  
t h e  p rev ious  convergence zone and s u p p o r t  t h e  e q u a t o r i a l  f e r t i l i t y ,  Thus t h e  
s e a s o n a l  swinging of t h e  t radewinds  cou ld  r e i n f o r c e  t h e  f e r t i l i z i n g  effect of 
t h e  winds on t h e  s u r f a c e  waters of t h e  e q u a t o r i a l  r e g i o n  by favour ing  a l a t e r a l  
s u b s u r f a c e  s t o r a g e  of n u t r i e n t s  under  v a r i o u s  forms and t h e i r  subsequent  v e r t i c a l  
t r a n s p o r k  to t h e  surface. The o r g a n i c  material, which is  s t o r e d  a long  t h e  conver- 
gences  n o r t h  and s o u t h  of t h e  Equatox, can be comple te ly  mine ra l i zed  o r  can  feed  
zooplankton popu la t ions ,  which grow f a r  from t h e  en r i ched  e q u a t o r i a l  area, 
I t  must n o t  be f o r g o t t e n  t h a t  a l l  t h e  mer id iona l  motions a r e  invo lved  i n  a 
main zonal c i r c u l a t i o n ,  and it is d i f f i c u l t  t o  s ta te  a c o n t i n u i t y  between t h e  
Equator  and the convergence zone n e a r  SOS, a long  a meridian.  


Mineralization South of the Equator in relation with equatorial fertility

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