Early Diagenesis of Organic Carbon in Sediments from the Peruvian

Apr 21, 1986 - The diagenesis of organic matter in recent sediments from the Peruvian Upwelling Zone has been studied by solid state 1 3C NMR ...
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10 Early Diagenesis of Organic Carbon in Sediments from the Peruvian Upwelling Zone W. T. Cooper, A. S. Heiman, and R. R. Yates Department of Chemistry, Florida State University, Tallahassee, FL 32306-3006

The diagenesis of organic matter i n recent sediments from the Peruvian Upwelling Zone has been studied by solid state C NMR spectroscopy of the intact sedimentary organic carbon and by conventional GC and GC-MS analyses of sterol biomarkers i n lipid extracts of the sediments. The distribution of sterols i n surficial sediments from two sites on the continental margin indicate that the distinction between marine and terrestrial sources of organic carbon i n aquatic sediments is not as well defined as previously thought. Both sites exhibit large relative abundances of twenty-eight (C-28) and thirty (C-30) carbon atom sterols, suggesting a marine source. The C-27 and C-29 abundances do not reflect this expected marine source, however, confirming recent observations that sterol distribution patterns i n nature are extremely complex. Magic angle spinning C NMR spectra with variable cross polarization contact times were obtained on the intact, non-extracted sediments. The time-dependent spectra reveal subtle differences i n organic carbon with depth; differences not observed i n single contact experiments. Dipolar-dephased spectra of these same sediments indicate the presence of substantial amounts of substituted aromatic/olefinic carbons which are rapidly altered with depth.

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The transformations of organic matter i n young, recently deposited aquatic sediments play a unique role i n the biogeochemical cycle of organic carbon. During sedimentation and residence i n the upper sediment horizons, organic matter is subject to a number of chemical, biological and physical processes which alter i t i n various ways. While the nature and extent of these transformations depend on both the type and amount of organic material and the depositional environment at the time of burial, i t is clear that the composition of sedimentary organic matter is significantly altered, and i t s fate during subsequent geochemical/geophysical evolution i s largely dependent on these i n i t i a l transformations. 0097-6156/ 86/ 0305-0158506.00/ 0 © 1986 American Chemical Society

Sohn; Organic Marine Geochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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The p r o c e s s by w h i c h b i o p o l y m e r s , the remnants o f l i v i n g organ­ isms b u r i e d i n s e d i m e n t s , a r e degraded and r e a r r a n g e d i n t o i n s o l u b l e geopolymers i s u s u a l l y r e f e r r e d t o as d i a g e n e s i s . One t h e o r y (1) holds that d i a g e n e s i s i n c l u d e s m i c r o b i a l degradation of b i o l o g i c a l macromolecules i n t o s m a l l e r components; c o n d e n s a t i o n o f t h e s e s m a l l , h i g h l y f u n c t i o n a l i z e d compounds i n t o geopolymers such as humic a c i d s , f u l v i c a c i d s and l e s s f u n c t i o n a l i z e d humin r e s i d u e s ; and i n s o l u b i l i z a t i o n o f t h e s e condensed s t r u c t u r e s v i a e l i m i n a t i o n of h y d r o p h i l i c f u n c t i o n a l groups t o form i n s o l u b l e kerogen. W h i l e o t h e r s c e n a r i o s have been proposed f o r t h e f o r m a t i o n o f kerogen ( 2 ) , i t i s g e n e r a l l y c o n s i d e r e d the main s o u r c e o f p e t r o l e u m and i s t h e p r i m a r y o r g a n i c m a t e r i a l found i n a n c i e n t sediments. D e g r a d a t i o n o c c u r s r a p i d l y i n the o v e r a l l e v o l u t i o n o f o r g a n i c m a t t e r ; u s u a l l y i n t h e f i r s t 1-10 m i n t e r v a l o f a sediment. Insolu b i l i z a t i o n i s much s l o w e r , n o r m a l l y o c c u r r i n g i n the 10-100 m range. A t t h i s p o i n t , temperature and p r e s s u r e become i m p o r t a n t , and the second phase o f the e v o l u t i o n of o r g a n i c m a t t e r commences; c a t a ­ g e n e s i s , t h e r m a l a l t e r a t i o n and the r e s u l t i n g f o r m a t i o n o f o i l and gas. In t h i s paper we p r e s e n t the i n i t i a l r e s u l t s o f s t u d i e s o f e a r l y d i a g e n e s i s of o r g a n i c c a r b o n i n t h e upper h o r i z o n s (0-30 cm) of marine sediments. S t u d i e s o f t h i s s o r t a r e c o m p l i c a t e d by t h e i n h e r e n t c o m p l e x i t y o f s e d i m e n t a r y carbon and t h e presence of l a r g e geopolymers. These geopolymers a r e i n g e n e r a l i n s o l u b l e and e x t r a c t a b l e o n l y by extreme t r e a t m e n t s ( 3 ) . These t r e a t m e n t s a r e such t h a t t h e r e i s r e a l doubt about whether t h e e x t r a c t i s t r u l y r e p r e s e n t a ­ t i v e of t h e o r g a n i c m a t e r i a l as i t e x i s t s i n the s e d i m e n t a r y e n v i ­ ronment (4_) . Our approach i s s i g n i f i c a n t l y d i f f e r e n t . We have attempted t o o b t a i n i n f o r m a t i o n we f e e l i s r e p r e s e n t a t i v e o f t h e i n - s i t u c h a r ­ a c t e r o f sedimentary o r g a n i c m a t t e r . T h i s approach has i n v o l v e d two d i s t i n c t phases o f sediment e v a l u a t i o n : - a n a l y s i s of biomarkers, or geochemical f o s s i l s , i n the l i p i d f r a c t i o n o f the s e d i m e n t a r y o r g a n i c m a t t e r , and - s o l i d s t a t e NMR s t u d i e s o f i n t a c t s e d i m e n t s , where t h e o r g a n i c f r a c t i o n i s viewed i n - s i t u . Lipid

Biomarkers

L i p i d s i n c o r p o r a t e d i n t o a q u a t i c sediments behave somewhat d i f f e r ­ e n t l y than most o t h e r forms o f o r g a n i c d e b r i s . By l i p i d , we a r e u s i n g t h e geochemist's d e f i n i t i o n ; t h o s e compounds e x t r a c t a b l e w i t h o r g a n i c s o l v e n t s . C o n t a i n i n g l e s s f u n c t i o n a l groups and thus more h y d r o p h o b i c , l i p i d s g e n e r a l l y a r e much l e s s r e a c t i v e and p e r s i s t i n d i s c r e t e c h e m i c a l forms l o n g e r than p r o t e i n s , p o l y s a c c h a r i d e s , e t c . . Indeed, a number o f d i f f e r e n t l i p i d m o l e c u l e s o f known b i o l o g i c a l o r i g i n a r e r o u t i n e l y found i n p e t r o l e u m , c o a l and a n c i e n t sediments. M o l e c u l e s w h i c h p e r s i s t i n sediments o v e r g e o l o g i c time spans i n an u n a l t e r e d o r s l i g h t l y a l t e r e d s t a t e and w h i c h can be a t t r i b u t e d d i r e c t l y or i n d i r e c t l y t o l i v i n g organisms a r e f r e q u e n t l y r e f e r r e d to as " b i o m a r k e r s " , " m o l e c u l a r f o s s i l s " , o r "geochemical f o s s i l s " . A number o f g e o c h e m i c a l f o s s i l s have been i d e n t i f i e d and t h e i r presence i n s e d i m e n t a r y o r g a n i c m a t t e r u t i l i z e d f o r v a r i o u s

Sohn; Organic Marine Geochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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g e o c h e m i c a l purposes ( 5 ) . These i n c l u d e : n - a l k a n e s , 2-methyl ( i s o ) and 3-methyl ( a n t e i s o ) branched a l k a n e s ; n - a l k a n o i c a c i d s ; a c y c l i c i s o p r e n o i d - t y p e m o l e c u l e s ; s t e r o i d s ; and a v a r i e t y o f c y c l i c t r i t e r ­ penoids. One o f t h e most v a l u a b l e groups o f g e o c h e m i c a l f o s s i l s i s t h e s t e r o i d a l a l c o h o l s , or s t e r o l s . Regulators of metabolic processes and major components o f e u c a r y o t i c c e l l membranes, s t e r o l s t r u c t u r e s appear t o be s t r o n g l y r e l a t e d t o t h e organisms t h a t produce them. M a r i n e organisms produce p r i m a r i l y twenty-seven carbon atom s t e r o l s (C-27) ( 6 - 8 ) , w h i l e t e r r e s t r i a l p l a n t s produce p r i m a r i l y t w e n t y - n i n e carbon atom s t e r o l s (C-29) ( 9 ) . T h i s s p e c i f i c i t y has been used by a number o f i n v e s t i g a t o r s t o c h a r a c t e r i z e t h e e c o l o g i c a l h i s t o r y o f water columns. G a s k e l l and E g l i n t o n ( 1 0 ) , f o r example, a t t r i b u t e d v a r i a t i o n s i n t h e C-27/C-29 r a t i o w i t h depth i n a r e c e n t l a c u s t r i n e sediment t o a change i n t h e l i p i d s o u r c e o f t h e o r g a n i c carbon i n the sediment. I n a d d i t i o n t o t h i s s o u r c e s p e c i f i c i t y , s t e r o l s undergo a v a r i e t y of r e a c t i o n s which a r e of p a r t i c u l a r v a l u e i n s t u d i e s of e a r l y d i a g e n e s i s . These r e a c t i o n s a r e n o t o n l y i n d i c a t i v e o f t h e sedimentary environment, b u t a l s o r e s u l t i n p r o d u c t s w h i c h a r e s i m i l a r enough t o p r e c u r s o r m o l e c u l e s t o be e a s i l y r e l a t e d t o them. One o f t h e most i n t e n s e l y i n v e s t i g a t e d r e a c t i o n s has been t h e h y d r o ­ génation w h i c h c o n v e r t s Δ , Δ and Δ ** u n s a t u r a t e d s t e r o l s ( s t e n o l s ) i n t o t h e i r s a t u r a t e d analogues ( s t a n d s ) (11-13). Other r e a c t i o n s i d e n t i f i e d include b a c t e r i a l d e a l k y l a t i o n of the s t e r o l side chain ( 1 4 ) , and a r o m a t i z a t i o n t o form phenanthrene homologues ( 1 5 ) . Sedimentary r e a c t i o n s o f s t e r o l s w h i c h have been i d e n t i f i e d o r suggested a r e summarized i n F i g u r e 1. 5

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NMR S t u d i e s o f Sedimentary O r g a n i c M a t t e r 1 3

Recent developments i n C n u c l e a r magnetic resonance s p e c t r o s c o p y have made i t p o s s i b l e t o probe t h e c h e m i c a l environment o f carbon atoms i n r i g i d and s e m i - r i g i d systems. U s i n g t h e t e c h n i q u e s o f c r o s s p o l a r i z a t i o n (CP) (16) and magic a n g l e s p i n n i n g (MAS) ( 1 7 ) , the p r i m a r y o b s t a c l e s t o o b t a i n i n g NMR s p e c t r a o f s o l i d s w i t h r e a ­ sonable l i n e widths (chemical s h i f t a n i s o t r o p y , a n i s o t r o p i c d i p o l e d i p o l e i n t e r a c t i o n s ( d i p o l e c o u p l i n g ) , and l o n g s p i n - l a t t i c e r e ­ l a x a t i o n t i m e s ) c a n now be c i r c u m v e n t e d . S i n c e i t s i n t r o d u c t i o n by S c h a e f e r and S t e j s k a l ( 1 7 ) , t h e C NMR t e c h n i q u e combining c r o s s p o l a r i z a t i o n and magic a n g l e s p i n n i n g (CP-MAS-NMR) has been a p p l i e d t o a wide v a r i e t y o f i n s o l u b l e o r g a n i c complexes such a s f o s s i l f u e l s , p o l y m e r s , c o a l , kerogen and h u m i c / f u l v i c a c i d s ( 1 8 ) . These a p p l i c a t i o n s r e p r e s e n t what M a c i e l (18) d e s c r i b e s as t h e " f i r s t g e n e r a t i o n " o f NMR s o l i d s a p p l i c a t i o n s . I t i s t h e "second g e n e r a t i o n " o f s o l i d s t a t e t e c h n i q u e s , i n w h i c h t h e onset o f r e l a x a t i o n o f m a g n e t i z a t i o n i s observed i n t h e time domain, w h i c h o f f e r promise f o r t h e d e t a i l e d , i n - s i t u i n v e s t i ­ g a t i o n o f e a r l y d i a g e n e t i c p r o c e s s e s . F o r example, Hagaman and Woody (19) showed t h a t t h e time dependence o f c r o s s p o l a r i z a t i o n c o u l d be used t o improve t h e r e s o l u t i o n o f complex s p e c t r a o f c o a l , s i n c e d i f f e r e n t c a r b o n types c r o s s p o l a r i z e a t d i f f e r e n t r a t e s . W i l s o n (20-21) has used d i p o l a r dephasing t o d i s t i n g u i s h carbon types i n s o i l s and r e s i n s based on r e l a x a t i o n r a t e s . We have used b o t h time dependent c r o s s p o l a r i z a t i o n and d i p o l a r 1 3

Sohn; Organic Marine Geochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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dephasing t e c h n i q u e s t o v i e w carbon atoms i n r e c e n t marine sediments. V a r i a t i o n s i n c r o s s p o l a r i z a t i o n c o n t a c t times have been used t o improve t h e r e s o l u t i o n o f a d j a c e n t resonances w h i c h would o t h e r w i s e o v e r l a p . D i p o l a r dephasing experiments were employed t o d i s t i n g u i s h p r o t o n a t e d and nonprotonated carbon atoms, p a r t i c u l a r l y i n t h e aromatic r e g i o n .

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Experimental Sampling. Sediment samples from t h e c o a s t o f P e r u were c o l l e c t e d d u r i n g C r u i s e 23-06 o f t h e R/V Robert Conrad, June - J u l y , 1982. Sampling l o c a t i o n s a r e i n d i c a t e d i n F i g u r e 2. A s p e c i a l l y d e s i g n e d box c o r e r (22) was used i n o r d e r t o o b t a i n u n d i s t u r b e d s u r f a c e s e d i ­ ments (