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Lipid Geochemistry of Recent Sediments from the Great Marsh, Lewes, Delaware PAUL J. SWETLAND* and JOHN F. WEHMILLER Department of Geology, University of Delaware, Newark, Del. 19711
Understanding the significance of ancient sedimentary organic matter requires knowledge of the organic distributions common to modern environments. Organic geochemical studies of recent sediments seek to determine: 1) the biological sources from which organic compounds in modern sediments were derived, 2) the organic distributions in different modern depositional environments for comparison with ancient analogs , and 3) the early diagenetic alterations of recent sedimentary organic matter. This study was designed to investigate the composition and early diagenesis of organic matter deposited in a modern salt marsh environment. Salt marshes are generally highly productive environments that contribute large amounts of herbaceous organic matter to sediments. Herbaceous, or leafy, land plants are characterized by long chain hydrocarbons and fatty acids which serve as a waxy protective coating (1). Because of the abundance of this waxy material in higher land plants, high-wax petroleums are thought to be derived from coastal environments where inputs of land plant organic matter are great (2,3). Since sediments of salt marshes contain about ten times more l i p i d mater i a l than those of marine-lagoon or open-shelf environments, salt marshes are considered probable source environments for high-wax petroleum (4). The main emphasis of this study was placed on the identification of sediment l i p i d (n-alkane and n-fatty acid) distribution patterns as primary source material or as products of diagenesis. Correlations of sediment l i p i d distributions to specific biological sources are suggested. The nature and possible rates of various diagenetic reactions responsible for alteration of the *Pres. Address: U. S. Geological Survey, F e d e r a l C e n t e r , Denver, CO 80225 285
Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
MARINE CHEMISTRY
286
lipid
d i s t r i b u t i o n s are a l s o d i s c u s s e d .
Study A r e a and Sample C o l l e c t i o n The G r e a t Marsh o c c u p i e s an a r e a o f a p p r o x i m a t e l y f i v e square m i l e s a l o n g Delaware Bay n o r t h w e s t o f the town o f Lewes, DE ( F i g u r e 1 ) . I t i s subjected to a t i d a l range o f about 30 cm w i t h s a l i n i t i e s v a r y i n g from 5 t o 30 o/oo. P r e s e n t marsh v e g e t a t i o n i s domin a t e d by the h a l o p h y t i c g r a s s e s S p a r t i n a a l t e r n i f l o r a , S. p a t e n s , and D i s t i c h l i s s p i c a t a . Other p l a n t s p e c i e s i n c l u d e S a l i c o r n i a sp., I v a f r u t e s c e n s , B a c c h a r i s h a l i m i f o l i a , and Phragmites communis, as w e l l as a v a r i e t y o f marine a l g a e . The c o m b i n a t i o n o f h i g h o r g a n i c p r o d u c t i v i t y and r a p i d s e d i m e n t a t i o n p r o v i d e s a s t r o n g l y r e d u c i n g environment f a v o r a b l e f o r t h e p r e s e r v a t i o n o f o r g a n i c matter. T y p i c a l Eh and pH v a l u e s f o r s u b s u r f a c e samp l e s i n t h i s a r e a a r e -200 mv and 7.1, r e s p e c t i v e l y (_5) . Average s e d i m e n t a t i o n r a t e s a r e on the o r d e r o f 10-15 cm/100 y e a r s (6) based upon r a d i o c a r b o n d a t e s on a v a r i e t y o f p e a t and s h e l l samples found a t v a r i o u s depths i n the g e n e r a l a r e a . The g e o l o g i c a l h i s t o r y o f t h e G r e a t Marsh i s c l o s e l y a s s o c i a t e d w i t h t h e l a t e Holocene marine t r a n s g r e s s i o n (7). A l a r g e p a r t o f t h e p r e s e n t G r e a t Marsh a r e a was once a s h a l l o w l a g o o n which began f i l l i n g about 3000 y r s . B. P. ( 7 ) . F r i n g i n g marshes a d j a c e n t t o h e a d l a n d a r e a s g r a d u a l l y a c c r e t e d upward w h i l e abundant f i n e - g r a i n e d m a t e r i a l accumulated i n n e a r - s h o r e l a g o o n a l environments. L a t e r stages i n t h i s h i s t o r y have i n v o l v e d the development o f a broad marsh o v e r b o t h the f r i n g i n g marsh and l a g o o n a l d e p o s i t s (7). The samples o f i n t e r e s t i n t h i s work come from one c o r e and one s u r f a c e sample t a k e n f o r comparative purposes a t a nearby l o c a t i o n (see F i g u r e 1 ) . Both l o c a t i o n s a r e w i t h i n an a r e a t h a t i s dominated by S p a r t i n a a l t e r n i f l o r a (8) . A c o r e o f a p p r o x i m a t e l y f o u r meters l e n g t h was t a k e n by pounding a 6.35 cm i . d . p l a s t i c p i p e i n t o t h e marsh sediment u s i n g t h e p r o c e dures d e s c r i b e d by K r a f t (9). Samples were immediately r e t u r n e d t o the l a b o r a t o r y and were f r o z e n a t -20 C. u n t i l analyzed. Core p i p e s were s p l i t l e n g t h w i s e u s i n g a handh e l d r o t a r y saw and d e s c r i p t i o n s were made on wet samples. Two d i s t i n c t l i t h o l o g i e zones a r e r e c o g n i z e d i n the c o r e ( F i g u r e 2) and can be c o r r e l a t e d t o o t h e r s t r a t i g r a p h i e s e c t i o n s i n the immediate a r e a t h a t have r a d i o c a r b o n age c o n t r o l (6). Coarse sands occupy t h e lower 1.5 meters o f t h e core? o v e r l y i n g t h e s e a r e 2.5
Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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SWETLAND
AND WEHMiLLER
Lipid Geochemistry of Sediments
287
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NEW JERSEY
DELAWARE
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MARSH
GREAT MARSH Figure 1.
Study area and sample locations DESCRIPTION Dusky yellow brown rootrrat Olive pray s i l t y clay, plant, fragments Brownish black s i l t y clay, large plant fragments Dusky yellow brown s i l t y clay, l a r ^ e plant fraçments Dark yellow brown s i l t y c l a y , few plants Olive sray s i l t y clay, lare;e plant fragments Black peat
Olive çray ccarse sand, pebbles, few plants
Inediun
