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Intern ationa l Jou rna l of Coal Geology, 12 (1989 ) 157-19 2 157E l s e v i er Sc i en c e P u b l is h e r s B .V . , A m s t e r d a m - - P r i n t e d i n T h e N e t h e r l a n d s

T h e C h a n g u i n o l a p e a t d e p o s i t o f n o r t h w e s t e r nP a n a m a : a t r o p i ca l , b a c k - b a r r i e r , p e a t ( c o a l ) -f o r m i n g e n v i r o n m e n tA R T H U R D . C O H E N 1, R O B E R T R A Y M O N D , J r . 2, A R T U R O R A M I R E Z 3, Z O R E LM O R A L E S 3 a n d F R A N C I S C O P O N C E 31Department of Geological Sciences, Un iver sity of So uth Carolina, Columbia, SC 29208, U.S.A.2Los Alam os Nation al Laboratory, Los Alamos, N M , U.S.A.3Instituto de Recursos Hidraulicos y Electrificacion, P ana ma( R e c e iv e d Apr i l 1 , 1988 ; r e v i se d a nd a c c e p te d Oc tob e r 28 , 1988)

A B S T R A C TC o h e n , A . D . , R a y m o n d , R . J r . , R a m i r e z , A . , M o r a l e s , Z. a n d P o n c e , F . , 1 98 9. T h e C h a n g u i n o l ap e a t d e p o s i t o f n o r t h w e s t e r n P a n a m a : A t r o p i c a l , b a c k - b a r r i e r , p e a t ( c o a l ) - f o r m i n g e n v i r o n -m e n t . I n : P . C . L y o n s a n d B . A l p e r n ( E d i t o r s ) , P e a t a n d C o a l : O r i g i n , F a c i e s , a n d D e p o s i t i o n a lM o de l s . I n t . J . C oa l Ge o l . , 12 : 157 - 192 .

A l a r g e ( > 8 2 k m 2 ), t h i c k ( > 8 m ) , l o w - a s h , l o w - s u l fu r p e a t d e p o s i t h a s f o r m e d o n t h e n o r t h -w e s t e r n c o a s t o f P a n a m a . I t s l o w a s h a n d l o w s u lf u r c o n t e n t s c a n b e a t t r i b u t e d p a r t l y to : ( 1 )d o m i n g ; ( 2 ) t h e o c c u r r e n c e o f a r e l a t iv e l y c o n t i n u o u s , m a r i n e - b l o c k i n g , b e a c h - b a r r i e r s h o r el i n ef e a tu r e ; a n d ( 3 ) l e a c h i n g o f t h e m i n e r a l m a t t e r . T h i s d e p o s i t is r o u g h l y r e c t a n g u l a r i n s h a p e a n do r i e n t e d w i t h i t s l o n g e s t a x i s p a r a l l e l t o t h e b e a c h - b a r r i e r s h o re l i n e. I t w o u l d p r o d u c e a n e c o n o m i cc o a l b e d t h a t i s b r i g h t a n d f i n e l y b a n d e d i n i t s t h i c k e s t c e n t r a l r eg i o n a n d m o r e c o a r s e l y b a n d e da t i t s m a r g i n s . I t s n o r t h w e s t e r n m a r g i n w o u l d b e c h a r a c t e r i z e d b y i n t e r f in g e r i n g o f c o a l w i t hs a n d s t o n e a n d s i l t s t o n e o f t h e C h a n g u i n o l a R i v e r f lo o d p l a i n , w h e r e a s i t s s o u t h e a s t e r n b o u n d a r yw o u l d b e c h a r a c te r i ze d b y t r u n c a t i o n o f t h e u p p e r p a r t o f t h e s e a m b y m a r i n e t o b r a c k i s h s h a le s ,s i l t s to n e s , a n d c a r b o n a t e s t h a t w e r e d e r i v e d f r o m A l m i r a n t e B a y .T o t a l s u l f u r w a s f o u n d t o b e v e r y l o w ( l e s s t h a n 0 .2 % d r y w t . ) e x c e p t i n a f e w p la c e s . I n t h e s er a r e c a s es , n e a r l y a ll o f t h e s u l f u r w a s p y r i t i c a n d t h e t o t a l s u l f u r c o n t e n t w a s a n o m a l o u s l y h i g h( > 1 4% ) . A l t h o u g h t h e g e n e r a l t e n d e n c y w a s f o r t o t a l s u l f u r t o i n c r e a s e w h e r e v e r t h e d e p o s i t w a si n f l u e n c e d b y m a r i n e c o n d i t i o n s , o n e c a s e w a s f o u n d w h e r e s u l f u r w a s h i g h a t t h e b a s e o f a n i n l a n df r e s h w a t e r p e a t c o r e, w h i c h s u g g e s t s e i t h e r a " h i d d e n " m a r i n e i n f l u e n c e o r t h e n e e d f o r so m e n e wm e c h a n i s m t o e x p l a i n s u lf u r e n r i c h m e n t i n s u c h a n a n o m a l o u s c a s e .D i f f e re n c e s in i n h e r e n t f i x e d c a r b o n c o n t e n t s a n d c a l o r i fi c v a l u e s d u e t o c h a n g e s i n p e a t t y p ea n d a n a l y t i c a l u n c e r t a i n t i e s t e n d e d t o m a s k s e a m w i d e t r e n d s i n t h e s e p a r a m e t e r s . H o w e v e r , f i x e dc a r b o n c o n t e n t s a n d c a l o r if i c v a l u e s s h o w e d a s l i g h t t e n d e n c y t o i n c r e a s e w i t h d e p t h ( e s p e c i al l yi f p l o t t e d f o r s i n g le p e a t t y p e s ) .V i t r i n i t e r e f l e c ta n c e w i t h i n t h e h y p o t h e t i c a l C h a n g u i n o l a c o a l b e d w o u l d b e e x p e c t e d t o i n -c r e a s e a s f ol lo w s : (1 ) t o w a r d t h e m a r g i n s o f t h e d e p o s i t ; ( 2 ) t o w a r d t h e b o t t o m a n d t o p o f t h eb e d ; a n d ( 3 ) t o w a r d s t r e a m c h a n n e l s o r t i d a l i n l e t s.

0 1 6 6 - 5 1 6 2 / 8 9 /$ 0 3 . 5 0 1 98 9 E l s e v i e r S c i e n c e P u b l i s h e r s B . V .

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158INTRODUCTIONObjec t ives and scope

T h e p r i m a r y o b j e c t i v e o f t h i s r e s e a r c h w a s t o a s s e s s t h e p o t e n t i a l f o r t h eC h a n g u i n o l a p e a t d e p o s i t o f P a n a m a t o b e u s e d a s a fu e l f o r a n e l e c t r ic a l p o w e rp l a n t . T h i s a s s e s s m e n t w a s c o m p l e t e d in 1 9 8 7 ( se e C o h e n e t a l. , i n p re s s , a ).A s e c o n d a r y o b j e c ti v e o f t h i s r e s e a r c h w a s t o g a t h e r i n f o r m a t i o n t h a t w o u l d b eu s e f u l in d e v e l o p i n g a m o d e l to p r e d i c t t h e c h a r a c t e r i s t i c s o f c o a l s e a m s t h a tf o r m e d in t r o p ic a l , b a c k - b a r r i e r s e t t i n g s s i m i l a r t o t h a t o f t h e C h a n g u i n o l ad e p o s i t . T h e r e s u l t s o f t h i s s e c o n d a r y o b j e c ti v e a r e r e p o r t e d i n t h i s p a p e r .P r e v i o u s w o r k a n d i ts r e le v a n ce t o th i s s t u d y

C o a l b e d s i n t e r p r e t e d a s h a v i n g b e e n f o r m e d b e h i n d b a r r i e r - b e a c h d e p o s i tsh a v e b e e n r e p o r t e d f r e q u e n t l y i n t h e l i t e r a t u r e ( e. g. , H o b d a y , 1 97 4; H o r n e e ta l. , 19 78 ; E n g l u n d e t a l. , 1 9 8 7 ). T h e s e a r e o f t e n r e f e r r e d t o a s b a c k - b a r r i e r c o a lb ed s. M o d e r n b a c k - b a rr i er p e a t - f o rm i n g e n v i r o n m e n t s a re e x t re m e l y c o m m o nt o d a y o n c o a s ta l p la i n s i n m a n y p a r t s o f t h e w o r ld . S t a u b a n d C o h e n ( 19 78 ,1 9 7 9 ), C o r v i n u s a n d C o h e n ( 1 9 8 4 ) , C o h e n ( 1 9 8 4 ) , a n d C o h e n e t al . ( 1 9 8 7 a )h a v e d e s c r i b e d i n d e ta i l t h e c h a r a c t e r i s t i c s o f s o m e w a r m - t e m p e r a t e , b a c k -b a r r i e r p e a t d e p o s it s o n t h e A t l a n t i c C o a s t a l P l a i n o f t h e U n i t e d S t a t e s ( t h eS n u g g e d y S w a m p o f S o u t h C a r o li n a a n d t h e O k e f e n ok e e S w a m p o f G e o r gi a) .T h e s e m o d e r n p e a t d e p o s i t s h av e b e e n u s e d a s a n a l o g s f o r c e rt a i n b a c k - b a r r ie rc o a ls ( e.g ., M c C a b e , 1 9 8 7 ) b u t t h e y p r o b a b l y w o u l d n o t b e a n a l o g o u s to c o a l st h a t a r e b e li e v e d t o h a v e f o r m e d i n t r o p i c a l s e t t in g s , s u c h a s t h e P o c a h o n t a sc o a l b e d s o f t h e A p p a l a c h i a n B a s i n ( E n g l u n d e t al ., 1 9 8 7 ).

U n f o r t u n a t e l y , n o t e n o u g h d e t a i le d in f o r m a t i o n o n th e g e o m e t r y a n d c o m -p o s i t i o n o f p e a t s f o r m e d in m o d e r n , w e t - t r o p i c a l , b a c k - b a r r i e r s e t t i n g s h a sb e e n p u b l i s h e d t o a l lo w t h e c o n s t r u c t i o n o f g o o d m o d e l s. S o m e i m p o r t a n t i n -f o r m a t i o n is p r o v i d e d b y C o l e m a n e t a l. ( 1 9 70 ) , w h o s t u d i e d M a l a y s i a n p e a td e p o s i t s ( s o m e o f w h i c h a r e p r o b a b l y o f t h e b a c k - b a r r i e r t y p e ); A n d e r s o n ( 1 96 4,1 97 3 ), w h o i n v e s t ig a t e d t h e g e o m e t r y a n d b o t a n y o f p e a t d e p o s i t s o f I n d o n e s i a ,a n d n u m e r o u s r e p o r t s o n t h e c o m p o s i t i o n o f p e a t s o f J a m a i c a (e .g ., R o b i n s o n ,1 98 0; B j o r k , 1 98 4 ) . A d d i t i o n a l ly , p r e l i m i n a r y r e p o r t s o f c o m p o s i t i o n a l s t u d i e so n I n d o n e s i a n p e a t s b e i n g c o n d u c t e d b y s e v e ra l r e s e a r c h e r s ( e. g. , E s t e r l e , 1 9 87 ;C e c i l e t a l. , 19 87 ; C a m e r o n e t a l. , 1 98 9 , t h i s v o l u m e ) h a v e r e c e n t l y b e e n p u b -l is h ed , as h a v e p r e l i m i n a r y s t u d i e s o n t h e p e t r o g r a p h y a n d c h e m i c a l c o m p o -s i t i o n o f p e a t s i n C o s t a R i c a ( C o h e n e t a l ., 1 9 85 , in p r e s s , b ) .

T h e C h a n g u i n o l a p e a t d e p o s i t d e s c r i b e d i n th i s p a p e r is o f s p e c ia l s ig n i fi -c a n c e w i t h r e g a r d t o o u r u n d e r s t a n d i n g o f e c o n o m i c c oa l b e d s b e c a u s e i t is n o to n l y f o r m e d i n a b a c k - b a r r i e r s e t t i n g b u t i s a l s o la r g e ( g r e a t e r t h a n 8 2 k m 2 i na r e a ) , t h i c k ( a v e r a g i n g m o r e t h a n 8 m ) , v e r y lo w i n a s h ( a v e r a g i n g le ss t h a n

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4 % o n a d ry w e i g h t b a s i s o v e r t h e e n t i r e d e p o s i t a n d l e s s t h a n 2 . 5% a s h t h r o u g hi t s c e n t r a l r e g i o n ) , a n d v e r y l o w in s u l f u r ( l e s s t h a n 0 .2 % o v e r m o s t o f t h ed e p o s i t e x c e p t f o r i t s se a w a r d e d g e s ) .Geom orphologic and geologic setting

T h e C h a n g u i n o l a p e a t d e p o s i t is lo c a t ed o n t h e n o r t h w e s t e r n c o a s t o f P a n -a m a n e a r i t s b or d e r w i t h C o s t a R i c a a n d i s j u s t s o u t h e a s t o f th e R i o C h a n g u i -

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161nola (Fig. 1 ). The peat has formed on top of recent unconsolidated sedimentsbehind (and roughly parallel to) a beach-barrier that extends continuously tothe southeast for about 10 km (Fig. 1 ). Bordering the deposit on the southwestare the foothills of a mountain range composed of Upper Cretaceous to Eocenesandstone, shale, and volcanic rocks (Fig. 2 ). Miocene rocks of the Gatun For-mation outcrop in the low hills closest to the peat deposit and may extendbeneath the unconsolidated Quaternary clastics on which the peat deposit hasdeveloped. These Miocene rocks consist of shale, mudstone, sandstone, coals,conglomerate, and pyroclastic rocks (Fig. 2 ).Bahia Almirante (Almirante Bay) borders the deposit to the southeast. Pre-liminary sampling in the bay near the deposit suggests that remnants of thepeat deposit may have been buried in places beneath the bay sediments by atransgression of the sea after the peat had formed.The vegetational and drainage patterns displayed on the aerial photographssuggest that this deposit is domed in the center. However, the precise surfacegeometry has not yet been determined by surveying.METHODSField methods

Core samples were collected at each site using a hand-operated MacaulayPeat Sampler. This device allowed collection of core samples in 30- or 50-cmincrements (depending on sampler head size) from the top to the bottom ofthe deposit at each site. Each core increment was described in the field, in-serted in a labeled, double plastic bag, and placed in back packs (out of directsunlight) for later transportation back to our laboratory. All samples werepacked in rigid cardboard boxes to prevent damage or compaction.Laboratory methods

The following tests or analyses were conducted: (1) petrographic/botanicalanalysis; (2) proximate analysis (moisture, ash, fixed carbon, and volatilematter ); (3) calorific value (Btu); (4) total sulfur content; (5) mineralogicalanalysis of the ash; and (6) pH.The petrographic/botanical analysis is a microscopic analysis of the peatusing microtome thin sections. These sections were prepared from paraffin-embedded peat by the methods described in Cohen (1982). The parametersmeasured by this method include: (1) anatomically identifiable plant types;(2) relative degree of decomposition (fiber content ); (3) relative amour~t ofoxidation (darkening of tissues); and (4) various microscopically significantcompositional characteristics (such as presence or types of fecal pellets, plantphytoliths, fungal remains, and minerals).For proximate analysis, calorific value, sulfur content, and mineralogical

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162

a n a l y s is , w e u s e d s u b s a m p l e s t h a t h a d p r e v i o u s l y b e e n d r i e d a t 5 0 C i n a n o v e na n d c r u s h e d i n a m i l l t o m i n u s 6 0 m e s h ( le s s t h a n 2 5 0 / ~ m ) . P r o x i m a t e a n a l y s i sw a s p e r f o r m e d w i t h a n a u t o m a t e d F i s h e r C o a l A n a ly z e r, w i t h r e s i d u a l m o i s-t u r e b e i n g d e t e r m i n e d a t 1 07 C , v o l a ti l e m a t t e r a t 9 5 0 C i n a c o v e r e d q u a r t zc r u c i b l e, a s h c o n t e n t a t 7 5 0 ~ C f o r 3 .5 h o u r s , a n d f i x e d c a r b o n b y d i f f e r e n c e .C a l or if ic v a lu e s w e r e d e t e r m i n e d o n a D i g it a l D a t a S y s t e m s C P - 5 0 0 a u t o m a t i cc a l o r i m e t e r , w h i c h u s e s a w a t e r l e ss i s o t h e r m a l s y s t e m o f a n a l y s is . T o t a l s u l f u rw a s d e t e r m i n e d w i t h a L e c o S C - 1 3 2 S u l f u r A n a l y z e r . T h e L e c o S C - 1 3 2 c o m -b u s t s t h e s a m p l e u n d e r a n o x y g e n j e t a t 1 37 0 F ( 74 3 C ) a n d u s e s a n i n f r a r e dd e t e c t o r t o m e a s u r e e n s u i n g s u l f a t e p r o d u c t i o n . F o r m i n e r a l o g i c a l a n a l y s i s ,p e a t a s h w a s o b t a i n e d u s i n g a lo w - t e m p e r a t u r e a sh e r ( L T A ) . T h e L T A u t i li z esa n R F g e n e r a t o r t o e x c it e o x y g e n w i t h i n c l o s e d v e s s e ls c o n t a i n i n g t h e p e a ts a m p l e s . A l l c a r b o n i s o x i d i z e d t o C O 2 a n d t h e r e b y r e m o v e d , w h i l e s a m p l et e m p e r a t u r e r e m a i n s u n d e r 1 5 0 C ( R u s se l l a n d R i m m e r , 1 9 79 ). M o s t o f t h em i n e r a l m a t t e r w i t h i n t h e s a m p l e is p r e se r v e d in a n o r g a n ic - f re e a n d u n a l t e r e ds t a te , r e a d y f o r a n a l y s is . T h e L T A s p l i ts o f t h e s a m p l e s w e r e a n a l y z e d f o r t h e i rm i n e r a l o g ic c o n t e n t u s i n g a S i e m e n s X - r a y p o w d e r d i f f r a c t o m e t e r ( X R D ) .RESULTS AND DISCUSSION

Pe at deposi t geo m etry an d sur face vegetat ionF i g u r e 3 is a n i s o p a c h o u s m a p o f t h e C h a n g u i n o l a d e p o s i t. B e c a u s e n o t o -

p o g r a p h i c m a p o f t h e p e a t s u r f a c e h a s y e t b e e n m a d e , i t w a s d i f f i c u lt f o r u s t ok n o w f o r c e r t a i n w h e t h e r t h e u n d e r l y i n g r o ck s u r f ac e w a s f l at o r w h e t h e r t h ep e a t f o r m e d i n a d e p r e s s i o n . A s o f t h i s w r i t i n g , a t o p o g r a p h i c s u r v e y i s b e i n gc o n d u c t e d ; o u r n e x t r e p o r t w i ll c o n t a i n t h i s i n f o r m a t i o n . H o w e v e r , f r o m v i su a li n s p e c t i o n o f t h e s u r f a c e , w e p r o p o s e t h e h y p o t h e t i c a l c r o s s s e c t io n i l l u s t r a t e db y F i g u re 4 to e x p l a i n t h e p e a t t h i c k n e s s e s . N o t e t h a t a f l a t te n e d d o m e w o u l da c c o u n t b e s t f o r t h e v e g e t a t i o n a l p a t t e r n o b s e r v e d ( F ig . 5 ) . D e n s e t r o p i c a lf o r e s t v e g e t a t i o n o c c u r s a r o u n d t h e e d g e s, w h e r e t h e p e a t i s r e l a t iv e l y t h i n . O nt h e o t h e r h a n d , t h e c e n t r a l a r e a s u p p o r t s p r im a r i l y l ow h e r b a c e o u s p l a n t s s u c ha s se d g e s, g r a ss e s , p e a t m o s s , f e r n s, a n d o t h e r p l a n t s t h a t c a n e x i s t i n w e t ( b u tn o t p o n d - l i k e ) , n u t r i e n t - p o o r c o n d i t io n s . S c a t t e r e d w e t l a n d s h r u b s a n d s h r u bh a m m o c k s a re a l s o p r e s e n t i n t h i s c e n t r a l a r ea , a s a r e f l o a t in g a q u a ti c p l a n t si n s c a t t e r e d s m a l l s h a ll o w p o n d s .F i g u re 6 is a s c h e m a t i c l o n g i t u d i n a l s e c t io n ( B - B ' ) o f t h e C h a n g u i n o l a p e a td e p o s i t o r ie n t e d r o u g h l y p ar a ll e l t o t h e C a r i b b e a n s h o r e li n e . N o t e t h a t t h e p e a ti n t e r fi n g e r s a t i ts n o r t h w e s t e r n b o u n d a r y w i t h f r e s h w a t e r s a n d s a n d c l ay s f r o mt h e R i o C h a n g u i n o la . T h e n o n p e a t i s la n d s h o w n b e tw e e n B D T - 1 4 a n d B D T -1 3 i s p r e d i c t e d t o b e p r e s e n t b y s u r f a c e v e g e t a t i o n b u t h a s n o t y e t b e e n v e r i f ie db y c o ri n g . S e d i m e n t s f r o m B a h i a A l m i r a n t e , a b r a c k is h - w a t e r b a y , c o v e r a n

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AN T E

F i g. 3 . I s o p a c h o u s m a p o f h e p e a t d e p o s i t a t C h a n g u i n o l a , P a n a m a . P e a t t h i c k n e s s ( c m ) .

(SW) (NE)

Tropical Sedg es, Grasses , Sugi//mia, Ferns, SphagnumH ard wo od F o r e s t s c a t t e r e d hrubs & shrub hammocks. i i i i , iii ...~.. ~ R a p h ia Palms: . . . ~ 0 . . ~ & T ro pic al S w am p~ _ ~ ~ r , . . . t . , ~ v e g e t a t i o n

o " : . ; " . ; : ' .~ V , : :: " o ' . - . ; . . : 7 - ~ . " . ~ - ~ . . , , . . . ~ . - ~ : . . . . . . . . . . . . . . . . . . .. " . . , : . ~ B A R R E R , . . ., o ' ,, ', " ,,

A A'Fig. 4. Schemat ic diagram illustrating cross sectional geometry and surface vegetation, Changu i-nola peat deposit , Panama.

undetermined portion of the peat deposit to the southeast. Contrary to mostof the soils and vegetation maps of Panama, the only mangrove vegetation

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F i g . 6 . S c h e m a t i c l o n g i tu d i n a l s e c t i o n ( B - B ' ) o f t h e C h a n g u i n o l a p e a t d e p o s i t ( n o t t o s c a le ) .covering this wetland region occurs where this primarily freshwater wetlandcontacts Bahia Almirante (Fig. 6).History of developm ent of the Changuinola peat deposit

One hypothetical explanation for the development of the Changuinola peatdeposit and its associated inorganic sediments is shown in Figure 7. The un-consolidated clastic sediments directly beneath the peat deposit probablyformed when the Caribbean shoreline was as shown in Figure 7A. The Chan-guinola Delta would have developedduring a time of relative sea-level stabilityor regression. Later, marine transgression might have eroded the delta andbegun formation of the spit (Fig. 7B ). Continued developmentof the spit (Fig.7C) and eventual restriction of the opening into Bahia Almirante (Fig. 7D)would result in freshwater peat formation as shown in Figure 7E. More rapidmarine transgression would reopen (or widen) the inlet between the mainlandand Isla Colon, erode some of the previously formed peat, and widen the bay(Fig. 7F).Petrographic~botanical com position and fiber con tent

Figure 8 shows the microscopicallydefinedpeat types determined from coresrepresenting a cross section (A-A') of the deposit from the southwest to thenortheast and a longitudinal section (B-B') from the northwest to the south-east. More detailed petrographic descriptions are given in Cohen et al. (inpress, a). The peat samples are grouped into six types based on their botanicalcomposition: (1) freshwater swamp-forest peat; (2) sedge-grass-fern peat; (3)Sagittaria et al. peat; (4) Nymphaea-Sagittaria peat; (5) marine Rhizophora(mangrove) peat; and (6) transitional brackish-water peat. The Rhizophora(mangrove) and brackish transitional peats were found only at the base of core

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R . , Z O P H O R A T R A N S ,. ,O . A S A . DM A N G R O V E B R A C K IS H O RP E A T P E A T C L A Y

Fig . 8 . S e c t ion a l p r o fi le s A - A ' an d B - B ' t h r ou gh C h an gu in o la p e a t d e p os i t sh ow in g p e a t t yp e s .

B D T - 3 , w h i c h o c c u r s n e a r t h e e d g e o f t h e d e p o s i t a t i ts b o u n d a r y w i t h B a h i aA l m i r a n t e .

T h e f r e sh w a t e r s w a m p - f o r e s t p e a t w a s f o u n d i n a l l a re a s a n d w a s e s p e c ia l lyp e r s i s t e n t i n m i n e r a l - r i c h p l a c e s s u c h a s t h e b a s e o f m o s t c o r e s a n d t o w a r d t h e

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Fig.0.etrographic/botanicalharacteristicsfeatsromhehanguinolaeposit,anama.

A.phagnum-dominatedpeat(BDT-13,500-550cm).

B.nsectmite)xoskeleton(BDT-14,0-50m).

C.ecalelletsmongsedgeoots(BDT-18,0-50m).

D.reshwaterspongepiculeshowingilicaissolutionBDT-12,780-805m).

E.harcoalandineral-richoneearheaseforeDT-22(780-805m).

FMinaczaboceB199cmshwnscmnameapeFScyas

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172

e d g e s o f t h e d e p o s i t ( F ig . 8 ) . N o t e t h e i n t e r f i n g e r i n g o f t h i s p e a t t y p e w i t h t h eR i o C h a n g u i n o l a f l o o d - p la i n s e d i m e n t s a t t h e n o r t h w e s t e r n e d g e o f t h e d e p o s i ta n d a l s o i t s p r e s e n c e a s a s u r f a c e l a y e r a t t h e n o r t h e a s t e r n a n d s o u t h e a s t e r ne d g e s o f t h e C h a n g u i n o l a p e a t d e p o s i t . T h i s p e a t t y p e i s a d i v e rs e m i x t u r e o ff r e s h w a t e r s w a m p t r e e s , s h r u b s , a n d v i n e s w i t h f e r n s a n d t r o p i c a l b r o a d l e a fh e r b a c e o u s p l a n t s a s c o m m o n a ss o c ia t es . P a l m d e b r is ( p r o b a b ly Raphia) w a sa l so e n c o u n t e r e d o c c a s i o n a l ly in t h i s p e a t ( b u t o n l y n e a r t h e s u r f a c e ).

T h e m o s t c o m m o n l y o c c u r ri n g p e a t t y p e s ( s e d g e -g r a s s- f e rn a n d Sagittariae t al. ) r e p r e s e n t f r e s h w a t e r , o p e n - m a r s h s e t t i n g s c h a r a c t e r i z e d b y v e r y s h a l l o ww a t e r a n d e p h e m e r a l d r y p e r i o d s ( se e F ig . 5C ) . T h e Sagittaria-dominatedp e a tt y p e r e p r e s e n t s s l i g h t l y w e t t e r c o n d i t i o n s t h a n d o e s t h e s e d g e - g r a s s - f e r n t y p e .

E v e n w e t t e r c o n d i t i o n s ( i.e ., a r e a s o f p o n d e d w a t e r ) a r e r e p r e s e n t e d b y t h ep r e s e n c e o f a Nymphaea-dominated p e a t t y p e . N o t e t h a t t h i s p e a t t y p e n e v e ro c c u r s n e a r t h e t o p s o f a n y c o r e s ( w i t h i n l e s s t h a n 1 .5 m o f t h e s u r f a c e ) , w h i c hs u g g e s t s t h a t p e r s i s t e n t d e e p w a t e r c o n d i t i o n s ( p o n d s ) h a v e b e e n r a re i n t h i sr e g i o n i n r e c e n t t i m e s . B e c a u s e w e h a v e n o r a d i o c a r b o n d a t e s f r o m t h i s a r e aa t p r e s e n t , w e d o n o t k n o w t h e l e n g t h o f t i m e r e p r e s e n t e d b y 1 .5 m o f p e a t .H o w e v e r , r a d i o c a r b o n d a t e s f r o m a p e a t d e p o s i t in C o s t a R i c a ( C o h e n e t al .,1 9 8 5) i n d i c a t e a n a p p r o x i m a t e r a t e o f p e a t s e d i m e n t a t i o n o f 1 m p e r 8 0 0 y r .,w h i c h s u g g e s t s t h a t p e r s i s t e n t p o n d s h a v e p r o b a b l y n o t e x i s t e d w i t h i n t h eC h a n g u i n o l a p e a t - f o r m i n g a r e a f or t h e l a s t 1 2 0 0 y e a r s. R a d i o c a r b o n d a t e s fo rt h e C h a n g u i n o l a p e a t s w i ll b e o b t a i n e d i n f u t u r e y e ar s.

N o t e t h a t , e x c e p t f o r t h e l a y e r o f s e d g e - g r a s s - f e r n p e a t a t t h e t o p o f c o r e s2 2, 2 0, 1 2, a n d 1 4 ( c e n t r a l r e g i o n ) a n d t h e p e r s i s t e n t ( b u t o f t e n t h i n ) l a y e r o ff r e s h w a t e r s w a m p - f o r e s t p e a t a t t h e b a s e o f m o s t a l l c o r es , c o r r e l a t i o n o f p e a tt y p e s b e t w e e n c o r e s i s v i r t u a l l y i m p o s s i b l e a t t h i s c o r e sp a c i n g ( r o u g h l y 2 - 2 . 5k m a p a r t ) . H o w e v e r , t h e v a r i a b i l i t y o f t h e s e s e q u e n c e s w i t h i n e a c h c o r e in d i -c a t es t h a t t h e v e g e t a t i o n a l c o m m u n i t y t h a t p r o d u c e d e a c h p e a t l ay e r d id n o tc o v e r a v e r y l a r g e a r e a n o r d i d i t l a s t f o r v e r y l o n g i n a n y o n e a r e a .T y p i c a l m i c r o s c o p i c c h a r a c t e r i s t i c s o f e a c h p e a t t y p e a r e s h o w n i n F i g u r e s9 a n d 1 0. A d d i t i o n a l p e t r o g r a p h i c c h a r a c t e r i s t i c s a r e i l l u s t r a t e d g r a p h i c a l l y i nF i g u r e s 1 1 t o 1 3. F i g u r e 1 1 s h o w s t h e f r e q u e n c y a t w h i c h s a m p l e s o f e a c h p e a tt y p e f a ll i n t o o n e o f t h e t h r e e f i b er c a t e g o r ie s o f t h e A S T M c l a s si f i c at i o n s y s-t e m ( i. e. , " f i b r ic " , " h e m i c " , o r " s a p r i c " , A S T M , 1 9 8 7 ). T h e s e f i b e r c a t e g o r ie sg e n e ra l ly r e p r e s e n t t h e d e g re e o f d e c o m p o s i t i o n o f p l a n t t i ss u e s c o m p o s i n g ap e a t ( t h a t is , t h e m o r e f ib e r s, t h e l e s s d e c o m p o s i t i o n ) . B e c a u s e t o o f e w s a m p l e so f m a n g r o v e o r t r a n s i t i o n a l p e a t w e r e e n c o u n t e r e d , t h e s e t y p e s w e r e n o t i n -c l u d e d o n t h e g r a p h s.T h e t h r e e p e a t t y p e s a r e s h o w n f r o m l e f t t o r i g h t i n o r d e r o f i n c r e a s i n g p e r -s i s t e n c e o f s t a n d i n g w a t e r ( F ig . 1 1 ) . N o t e t h a t f ib r ic p e a t i n c r e a s e s f r o m l e ftt o r i g h t a n d s a p r i c p e a t d e c r e a s e s f r o m l e f t t o r i g h t . T h u s , s w a m p - f o r e s t p e a ts a m p l e s ( w h i c h f o r m u n d e r d r i e r c o n d i t i o n s ) u s u a l l y a r e m o r e d e c o m p o s e dt h a n Nymphaea-Sagittaria p e a t s ( w h i c h fo r m u n d e r w e t t e r c o n d i t i o n s ) .

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17 3

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S A M P L E S

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F H S F H S F H S F H SF i g . 11 . F i b e r f re q u e n c y o f p e a t s a m p l e s f r o m C h a n g u i n ol a , P a n a m a . F i b e r d e t e r m i n e d m i c r o -s c o p i c a l l y a s a r e a % . F = F i b r i c ( g r e a t e r t h a n 6 7% ) ; H = H e r o i c ( 3 4 - 6 7 % ) ; S = S a p r i c ( l e s s t h a n34% ).

I n c r e a s i n g p e r s i s t e n c e o f s t a n d i n g w a t e r

1 0 0 - S W A M P - G R A S S- SE D G EISAGITTARIA NYMPHAEA-90 - FOREST FERN PEAT ET AL PEAT ET ALA R E A 8o- PEAT PEAT

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S A M P L E S 5o- j4 0 - - ~30-20-10-

L M D L M D L M D _ M DF i g . 12 . D e g r e e o f d a r k n e s s ( o p a q u e n e s s ) o f t is s u e s ( - - - o x i d a t i o n ) . L = l i g h t ; M = me d i u m; D = d a r k .

Figure 12 illustrates the relative degree of darkness (representing oxidation)of tissues as viewed in transmitted light through the microscope. Tissues inswamp-forest peats (as might be expected because of drier conditions) weredark more often than those inNymphaea-Sagittariapeats. The trends shownin Figure 13 probably also reflect the relative wetness of the depositional en-vironments. In this case, the abundance of fecal pellets (produced by vario6ssurface-dwelling animals from ingested plants) was highest in peat samplesfrom the driest environment (swamp-forest) and lowest in samples from thewettest environment (Nymphaea-Sagittaria).

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i7 0 - ,

im2o -4 IH~ lllmm i I I I I1 ~

S E D G E - G R A S S - S A G IT T A R IA N Y M P H A E AE T A L P E A T P E A T

R C A R C A R C A R C AF i g . 1 3. A b u n d a n c e o f fe c a l p e l l e t s . R = R a r e - O c c a s i o n a l ; C - - c o m m o n ; A = A b u n d a n t .Prox ima te ana lys i s

Averages an d ranges for a l l pea t sam plesProximate analyses (fixed carbon, volatile matter , and ash), sulfur, and ca-

lorific value analyses for 246 samples taken within an d a round the Changui-nola area are given in Cohen et al. (in press, a). Total moisture co nten t is notshown in this table because it was deter mine d on only a few samples, and therange (92-95% moisture) was not felt to be representat ive of the entire de-posit. Based on previous experience with other tropical peats, we predict amore likely range of 85-98% moisture.

Average values (dry weight) for ash, fixed carbon, and volatile matt er con-ten ts for all peat samples are 4% ash, 34% fixed carbon, an d 62% volatile mat-ter. Ranges for these parameters are: 1-25% ash, 25-39% fixed carbon, and52-70% volatile matter.

As h co n ten t d i s tr ibu t ionFigure 14 shows the ash c onte nt of cores take n along trans ects A-A' and B-B'(Fig. 1 ). Cores BDT-14, -20, and -22 were all taken from the treeless centralportions of the deposit. The ash content of the peat in these areas is very low(generally less tha n 2.5% ). Note tha t ash co ntent increases toward the edgesof the deposit.The two small ash peaks in core BDT-22 are exactly the same distance apartas the two ash peaks in core BDT-20. Thus, it is very likely tha t these peaksare synchronous. However, the peaks in core BDT-22 are approximately 120cm higher than are the peaks in core BDT-20, probably because we do notcurrently have a topographic map of the deposit surface and have consequentlymade the surface a horizontal line. However, either a difference in the peatsurface topography or in the topography of the underlying substrate (or both)

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177g r a s s- s e d ge p h y t o l i t h s a n d s p o n g e s p i c u l e s ) , o n l y a r a re , d e g r a d e d a n d p i t t e ds i li c e o us fr a g m e n t w a s e n c o u n t e r e d . T h i s e v i d e n c e w o u l d s e e m t o s u g g e s t t h a tt h i s m i n e r a l m a t t e r w a s l e a c h e d fr o m t h e s e l o w - a s h z o ne s , b e c a u s e t h e l i v i n gp l a n t s t h a t p r o d u c e d t h e s e p e a ts a r e th e s a m e t y p e s a s t h o s e t h a t w e h a d a s h e dp r e v i o u s l y f r o m o t h e r a r e a s a n d w e r e f o u n d t o c o n t a i n m o r e t h a n 1 0% a s h . F o ra m o r e d e t a i l e d d e s c r i p ti o n o f t h e p o t e n t i a l f o r l e a c h i n g o f m i n e r a l m a t t e r f r o mp e a t s t o p r o d u c e a l o w - a s h c o a l, se e K o s t e r s a n d B a i l e y { 19 86 ) a n d C o h e n e ta l . (1987a) .

F i x e d c a r b o n a n d v o l a ti le m a t t e r c o n t e n t sA s e x p e c t e d , f i xe d c a r b o n a n d v o l a t i le m a t t e r c o n t e n t s g e n e r a l l y e x h i b i t e da n i n v e r s e r e l a t io n s h i p . T h e r e f o r e , d i a g r a m s o f f i xe d c a r b o n c o n t e n t w e r e s u f-f i c ie n t t o d e p i c t t h e d i s t r i b u t i o n o f b o t h p a r a m e t e r s . A l so , b e c a u s e o f lo w a s hc o n t e n t , f ix e d c a r b o n d e t e r m i n e d o n a d r y , a s h - f re e b a s is ( d a f ) w a s v e r y s i m -i l a r t o t h a t d e t e r m i n e d o n a d r y b u t n o t a s h - f r e e b a s is . T h u s , a d i a g r a m s h o w i n gf i x ed ca r b o n o n e i t h e r b a s i s w a s s u f f i c i en t t o d ep i c t e i t h e r c a s e .

F i g u r e 1 6 s h o w s f ix e d c a r b o n ( d a f ) w i t h d e p t h a l o n g t r a n s e c t s A - A ' a n d B -B ' . W i t h t h e e x c e p t i o n o f m i n o r f l u c t u a t i o n s , f i xe d c a r b o n g e n e r a l l y i n c r e a s e sw i t h d e p t h . T h e s e m i n o r f l u c t u a t i o n s c o u l d b e c a u s e d b y a n a l y t i c a l e r r o r o r b yt h e v a r i a b i l it y o f p e a t t y p e s w i t h d e p t h ( i. e. , i n h e r e n t d i f fe r e n c e s i n c h e m i c a lc o m p o s i t i o n a n d e n v i r o n m e n t s o f d e p o s i t i o n ) . W h e n f i x e d c a r b o n ( d a f ) i sp l o t t ed ag a i n s t p ea t t y p e ( F i g . 1 7 ) , l a r g e d i f f e r en ces i n f i x ed ca r b o n can b eo b s e rv e d . H o w e v e r , i n t h i s f ig u re , e a c h b a r r e p r e s e n t s s a m p l e s f r o m m a n y d if -f e r e n t d e p t h s , m a k i n g i t d i f fi c u l t t o s e p a r a t e t h e e f f e c t s o f d e p t h f r o m i n h e r e n td i f f e r en ces i n p ea t co m p o s i t i o n . F i g u r e 1 8 , f o r ex am p l e , s h o w s f i x ed ca r b o n( d a f ) f o r a l l s am p l e s o f a s in g l e p ea t t y p e p l o t t ed ag a i n s t d ep t h r eg a r d l e s s o fc o r e lo c a t i o n. N o t e t h e t e n d e n c y t o w a r d i n c r e a s i n g f i x e d c a r b o n w i t h d e p t h .H o w e v e r , g e o g r a p h ic l o c a t i o n o f t h e c o r e f ro m w h i c h t h e s a m p l e s w e r e t a k e na l s o s eem s t o a f f ec t f ix ed ca r b o n co n t en t . R e f e r r i n g t o F i g u r e 1 6 , o n e can o b -s e r v e t h a t t h e a v e r a g e fi x e d c a r b o n c o n t e n t o f a ll p e a t s a m p l e s w i t h i n a n y c o r et e n d e d t o d e c re a s e f ro m s o u t h w e s t to n o r t h e a s t a l o n g t r a n s e c t A - A ' a n d t oi n c r e a s e fr o m n o r t h w e s t t o s o u t h e a s t a l o n g t r a n s e c t B - B ' .

W h e n f i x e d c a r b o n ( d a f ) i s p l o t t e d a g a i n s t a s h c o n t e n t f o r a s u it e o f c a r b o n -c o n t a i n i n g s a m p l e s { p e at a s w e l l a s n o n p e a t ) f r o m t h e C h a n g u i n o l a d e p o s it( F i g. 1 9 ) , f i x ed ca r b o n i n i t i a l l y i n c r ea s e s w i t h i n c r ea s i n g a s h c o n t e n t , p o s s i b l ya s a r e s u l t o f e i t h e r l o s s o f v o l a t i l e o r g an i c s b y d eco m p o s i t i o n o r ch an g es i np l a n t c o m m u n i t i e s ( o r b o t h ) . H o w e v e r , w h e n t h e a s h c o n t e n t g o e s a b o ve a b o u t2 0 % , th i s t e n d e n c y r e v e r se s ( i. e. , f i xe d c a r b o n t e n d s t o d e c r e a s e as a s h c o n t e n ti n c r e a s e s ) . T h i s p a t t e r n s u g g e st s s e v e r a l h y p o t h e s e s : ( 1 ) t h e c o m p o s i t i o n o ft h e s o u r c e o rg a n i c s m i g h t b e c h a n g i n g t o w a r d m o r e v o l a t i le - r i c h i n g r e d i e n t s( s u c h a s al g a e o r p o l l e n g r a i n s ) a s t h e a s h c o n t e n t i n c re a s e s ; ( 2 ) t h e i n c r e a s i n gi n o r g a n i c s m a y b e t r a p p i n g t h e v o l a t il e m a t t e r ; o r ( 3 ) t h e i n o r g a n i c s m a y b er e l e a s in g v o l at i le s t h a t e f fe c t t h e a c c u r a c y o f t h e a n a l y t i c a l p r o c e d u r e .

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178

DEPTH(cm)

A(SW)BDT-22 BOT-200

1 2 018 0

2 4 0

3 6 O4 2 0

5 4 06 0 06 6 O " ~720 K78o ~840 . . . . . . .25 29 33 37

BDT-12