Marine Chemistry in the Coastal Environment

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Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 14, 2018 | https://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch013

Origin and Distributions of Low Molecular Weight Hydrocarbons in Gulf of Mexico Coastal Waters WILLIAM M. SACKETT and JAMES M. BROOKS Department of Oceanography, Texas A&M University, College Station, Tex. 77843

The dissolved low-molecular-weight hydrocarbons, methane through the pentanes, in oceanic waters are derived from natural phenomena and/or human activities. Natural inputs have two sources; (1) biological, with methane overwhelmingly predominant and having methane to ethane plus propane ratios [CH 4 / (C2H6 + C 3 H 8 )] over 100 and (2) petrogenic, with ratios generally in the range of 1 to 30 (18). Man derived inputs, which are almost entirely petrogenic, result from offshore and onshore petroleum production, transportation and manufacturing operations. They are particularly significant in coastal waters. This paper attempts to define contemporary sources and describe the d i s t r i butions and fates of the light hydrocarbons in the ocean with particular emphasis on coastal waters of the Gulf of Mexico. Most early work by o i l industry scientists was stimulated by the potential use of light hydrocarbon concentrations in locating offshore gas seeps. Hydrocarbon seepage had been used since the beginning of o i l exploration efforts to locate commercial deposits. Link (16) reported that about one-half of the o i l re-reservesof the world were discovered by d r i l l i n g in the vicinity of seeps. The detection of offshore hydrocarbon seepage by means of concentration anomalies was described as early as 1960 by Dunlap et al. (10). During the sixties, periodic reports appeared in the commerical literature about offshore seep detection, but almost none of the large amount of work done in this subject area by the major o i l companies has been reported in the open scientific literature. Thus, the usefulness to the o i l industry of hydrocarbon concentration anomalies as an exploration tool is unknown to the general scientific community. 211

Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

MARINE CHEMISTRY


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The development of a new gas chromatographic method by Swinnerton and Linnenbom ( 2 3 ) l e d to a new surge of a c t i v i t y i n fundamental ocean s t u d i e s ( 2 ^ J L L , 2 4 , 2 6 ) and i n o i l e x p l o r a t i o n a p p l i c a t i o n s ( 2 0 ) . Recent r e p o r t s by Brooks e t a l . ( . 6 ) , Brooks and S a c k e t t (_7 ) , Lamontagne e_t al» ( 1 3 ) , Lamontagne et a l t (14) and Swinner ton and Lamontagne ( 2 5 ) p r o v i d e h y d r o c a r b o n basel i n e d a t a which g i v e a p a r t i a l u n d e r s t a n d i n g of the contemporary marine g e o c h e m i s t r y of d i s s o l v e d l i g h t hydrocarbons. An attempt i s made i n t h i s paper to o u t l i n e t h e g e n e r a l d i s t r i b u t i o n of l o w - m o l e c u l a r - w e i g h t hydrocarbons i n the ocean and p o i n t out s i g n i f i c a n t t r e n d s and problems. Procedures Our h y d r o c a r b o n a n a l y s e s a r e performed u s i n g t h r e e d i f f e r e n t p r o c e d u r e s . F o r d i s c r e t e samples, t h e methods d e s c r i b e d by Swinnerton and Linnenbom ( 2 3 ) and McA u l l i f e ( 1 7 ) a r e applied· The Swinnerton and L i n n e n bom method i s used f o r t h e a n a l y s i s of n a t u r a l l e v e l s of l i g h t hydrocarbons i n t h e open ocean· The p r o c e dure c o n s i s t s of h e l i u m - p u r g i n g one l i t e r samples and t r a p p i n g the ethane p l u s h i g h e r hydrocarbons on an a c t i v a t e d alumina column and methane on an a c t i v a t e d c h a r c o a l column, both t r a p s b e i n g i n s e r i e s and c o o l e d to d r y i c e t e m p e r a t u r e s . The t r a p s a r e i s o l a t e d a f t e r p u r g i n g and warmed to 9 0 ° C to desorb the h y d r o c a r b o n s . Each t r a p i s then s e p a r a t e l y i n j ected i n t o a gas c h r o matograph equipped w i t h a flame i o n i z a t i o n d e t e c t o r f o r s e p a r a t i o n and d e t e r m i n a t i o n of hydrocarbon c o n c e n t r a t i o n s . M c A u l i f f e ' s method c o n s i s t s of e q u i l i b r a t i n g a water sample w i t h pure h e l i u m i n a g l a s s s y r i n g e . When a 1 : 1 m i x t u r e of sea water and h e l i u m i s e q u i l i b r a t e d , more than 95% of the d i s s o l v e d hydrocarbons p a r t i t i o n i n t o the h e l i u m phase. A sample l o o p i s used to i n j e c t the h e l i u m phase i n t o t h e gas c h r o m a t o g r a p h i c stream. In our l a b o r a t o r y , d i s c r e t e samples a r e now b e i n g a n a l yzed u s i n g M c A u l i f f e ' s method f o r the d e t e r m i n a t i o n of methane and a m o d i f i c a t i o n of t h e method of Swinnerton and Linnenbom f o r t h e d e t e r m i n a t i o n of the h i g h e r hydrocarbons. The modif i c a t i o n c o n s i s t s of p u r g i n g a 1 - l i t e r sea water sample a t 2 0 0 cc/minute and u s i n g a s i n g l e t r a p of a c t i v a t e d alumina c o o l e d to l i q u i d n i t r o g e n temperature f o r q u a n t i t a t i v e l y t r a p p i n g a l l the hydrocarbons except methane· The average d e v i a t i o n determined by r e p l i c a t e a n a l y s e s of samples c o n t a i n i n g open ocean c o n c e n t r a t i o n s of methane, ethane and p r o pane i s l e s s than t e n p e r c e n t . C o n t i n u o u s d e t e r m i n a t i o n s of l i g h t hydrocarbons i n s u r f a c e water ( 3 meters below the sea s u r f a c e ) i s p e r -



13.

S A C K E T T A N D BROOKS

Low Molecular Weight Hydrocarbons

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formed a t sea u s i n g a hydrocarbon " s n i f f e r " . The p r o cedure, d e s c r i b e d by Brooks and S a c k e t t (7^) , i n v o l v e s u s i n g a vacuum produced by the i n f l o w r e s t r i c t i o n of a 12-stage b o o s t e r pump to c o n t i n u o u s l y outgas the stream of i n t a k e water. The bubbles from the o u t g a s s i n g a r e c o l l e c t e d and passed through a sample l o o p where they are i n j e c t e d every f i v e minutes i n t o the chromatographi c stream of a Beckman I n d u s t r i a l P r o c e s s Gas Chromatograph. Because the vacuum e x t r a c t i o n procedure o n l y outgasses a p p r o x i m a t e l y 25 p e r c e n t of the t o t a l d i s s o l v e d gases i n the i n t a k e stream, r e l a t i v e v a l u e s a r e o b t a i n e d by comparison w i t h open ocean c o n c e n t r a t i o n s and c a l i b r a t i o n w i t h the q u a n t i t i v e methods d e s c r i b e d above. T h i s procedure, no doubt, i n v o l v e s more unc e r t a i n i t y than d i s c r e t e sample a n a l y s i s , but c o n s i d e r i n g the s i x o r d e r s of magnitude range i n c o n c e n t r a t i o n s t h a t have been observed i n G u l f of Mexico s u r f a c e water, the u n c e r t a i n i t i e s i n v o l v e d i n the c a l i b r a t i o n are i n s i g n i f i c a n t . Hydrocarbon " s n i f f e r " v a l u e s are n o r m a l i z e d r e l a t i v e to open ocean e q u i l i b r i u m v a l u e s . Thus r e l a t i v e v a l u e s of one f o r methane, ethane p l u s ethene and propane are equal to open ocean s u r f a c e c o n c e n t r a t i o n s of 45, 3 and 1 n a n n o l i t e r s per l i t e r , respec t i v e l y . R e s u l t s and

Discussion

N a t u r a l Sources and D i s t r i b u t i o n s . The primary p r o c e s s e s f o r the p r o d u c t i o n of l i g h t hydrocarbons i n nature are: (1) b a c t e r i a l c a t a l y s i s , i n v o l v i n g the r e d u c t i o n of CC^ or f e r m e n t a t i o n of a c e t i c a c i d or methanol i n a n o x i c environments and y i e l d i n g p r i n c i p a l l y methane and (2) c r a c k i n g , e i t h e r thermal or c a t a l y t i c , y i e l d i n g a l a r g e spectrum of s a t u r a t e d and unsaturated products. S i n c e n e i t h e r of these p r o c e s s e s are n o r m a l l y o p e r a t i v e i n a e r o b i c environments the l i g h t hydrocarbons i n the ocean must e n t e r c h i e f l y from man-related s o u r c e s or a c r o s s the s e a - a i r or seasediment i n t e r f a c e . Sediment g e n e r a t i o n i s a l s o r e s p o n s i b l e f o r the b i o l o g i c a l l y r e a c t i v e gases such as methane, produced i n r i v e r i n e and e s t u a r i n e e n v i r o n ments and found i n h i g h c o n c e n t r a t i o n s i n r u n o f f . The c o n c e n t r a t i o n of a d i s s o l v e d n o n - r e a c t i v e gas; f o r example, argon, i n sea water i s determined, a c c o r d i n g to Henry's Law, by i t s p a r t i a l p r e s s u r e i n the atmosphere and i t s s o l u b i l i t y c o e f f i c i e n t at the temp e r a t u r e and s a l i n i t y d u r i n g water mass f o r m a t i o n . As a water mass s i n k s and spreads from p o l a r r e g i o n s i t becomes a few degrees warmer due to geothermal h e a t i n g . T h i s warming r e s u l t s i n a s l i g h t s u p e r s a t u r a t i o n of



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the i n e r t d i s s o l v e d g a s e s . N i t r o g e n behaves i n a s i m i l a r manner e x c e p t t h a t p r o c e s s e s s u c h a s n i t r o g e n f i x a t i o n a n d d e n i t r i f i c a t i o n may r e s u l t i n s l i g h t v a r i a t i o n s of l e s s t h a n one p e r c e n t f r o m t h e c o n c e n t r a t i o n s e x p e c t e d b y s o l u t i o n o f a i r (j4) . On t h e o t h e r h a n d , d i s s o l v e d o x y g e n i s u t i l i z e d by o x i d a t i v e processes d u r i n g w a t e r mass movement u n t i l i n t h e n o r t h e q u a t o r i a l P a c i f i c i t i s almost completely d e p l e t e d . Methane and o t h e r d i s s o l v e d g a s e o u s h y d r o c a r b o n s e x h i b i t a b e h a v i o r s i m i l a r to oxygen i n t h a t e q u i l i b r i u m v a l u e s a r e o b s e r v e d i n s u r f a c e water and t h e r e i s a d e p l e t i o n i n c o n c e n t r a t i o n w i t h age of t h e w a t e r m a s s . To c a l c u l a t e the e q u i l i b r i u m c o n c e n t r a t i o n of a h y d r o c a r b o n i n s e a w a t e r , i t i s n e c e s s a r y t o know t h e a t m o s p h e r i c p a r t i a l p r e s s u r e and t h e s o l u b i l i t y c o e f f i c i e n t . Howe v e r , except f o r methane, l i t t l e i n f o r m a t i o n i s a v a i l a b l e about atmospheric h y d r o c a r b o n c o n c e n t r a t i o n s or solubility coefficients. For methane, almost a l l atmop h e r i c c o n c e n t r a t i o n s a r e i n t h e r a n g e o f 1 . 4 + 0 . 1 ppmv. V a l u e s have been o b t a i n e d f o r near s u r f a c e atmosphere s a m p l e s o v e r G r e e n l a n d and Norway ( 1 5 ) , tropical Atlant i c and P a c i f i c O c e a n s (14) a n d t h e A n t a r c t i c 03 ) . A l s o , t h i r t e e n s e p a r a t e s a m p l e s c o l l e c t e d i n May 1974 o v e r t h e n o r t h e r n G u l f of M e x i c o a l o n g t h e edge of t h e c o n t i n e n t a l s h e l f h a d v a l u e s o f a b o u t 1.4 ppmv. Thus, methane a p p e a r s to have a n e a r l y c o n s t a n t g l o b a l a t m o spheric concentration. Assuming a p a r t i a l p r e s s u r e of 1.4 ppmv a n d t h e s o l u b i l i t i e s r e p o r t e d b y A t k i n s o n and R i c h a r d s (2^) , c a l c u l a t e d e q u i l i b r i u m c o n c e n t r a t i o n s i n s u r f a c e w a t e r a r e 65 a n d 35 n a n n o l i t e r s p e r l i t e r a t - 2 C and 25 C , r e s p e c t i v e l y . These l e v e l s are a p p r o x i m a t e l y 10 n a n n o l i t e r s p e r l i t e r l o w e r t h a n m e a s u r e d s u r f a c e c o n c e n t r a t i o n s at s i m i l a r temperatures i n the R o s s S e a , A n t a r c t i c a and t h e G u l f of M e x i c o . Profiles i n t h e s e two a r e a s a r e shown i n F i g u r e 1. The v a l u e s higher than e q u i l i b r i u m c o n c e n t r a t i o n s at these s t a t i o n s a n d r e p o r t e d e l s e w h e r e (14) a r e a p p a r e n t l y due to the u b i q u i t o u s b i o l o g i c a l a c t i v i t y i n the s u r f a c e layer. The deep w a t e r i n t h e R o s s Sea had c o n c e n t r a t i o n s very near c a l c u l a t e d atmospheric e q u i l i b r i u m v a l u e s , s u g g e s t i n g t h a t h i g h s u r f a c e v a l u e s may b e a s e a s o n a l phenomenon. A l t h o u g h m e t h a n e i s n o t known t o be f o r m e d i n a e r o b i c e n v i r o n m e n t s , i t i s f o u n d i n t h e d i g e s t i v e t r a c t s of f i s h and o t h e r m a r i n e o r g a n i s m s and t h e r e f o r e must be added t o n e a r s u r f a c e w a t e r by b i o logical activity. C o o l i n g of s u r f a c e w a t e r and c o n v e c t i o n i n r e g i o n s of deep water f o r m a t i o n r e s u l t i n i s o t h e r m a l w a t e r c o l umns a s shown i n F i g u r e 1 f o r t h e R o s s S e a . Similar processes i n the North A t l a n t i c produce dense water



15 T*C

20

4

25 30

(nl/llttr)

I

I 60

I I 70 80

BOTTOM ~ 3600 m

e

114

-3

-2

10

e

A

0H

I \ \

ι I

*

-I Τ ·0

J 20

4

+1

I I L 30 40 50 CH (nl/liter)

BOTTOM ~768 m 60

70

ELTANIN 51 STA. 18 FEB. 1972 77 IO'S, I72°06'E

ROSS SEA

Figure 1. Methane concentrations in the Gulf of Mexico and the Ross Sea

10

CH

J

e

23 46'N, 92 39'W JUNE 1971

•

β

v

2 3 4 6 ' Ν , 96°58'W JAN. 1972

-

Ο

OPEN GULF OF MEXICO

4

WELL MIXED ATMOSPHERE [CH ] ~ 1.4 ppmv

80 9C



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which spreads l a t e r a l l y a t depth and e v e n t u a l l y f i n d s i t s way i n t o the G u l f of Mexico. T h i s c o l d dense water b e g i n s i t s j o u r n e y w i t h methane l e v e l s of about 70 n a n n o l i t e r s per l i t e r and when found i n the G u l f , the c o n c e n t r a t i o n s have d e c r e a s e d to about 10 n a n n o l i t e r s per l i t e r . T h i s remarkable d e p l e t i o n a p p a r e n t l y cont i n u e s to the p o i n t where o n l y 3 n a n n o l i t e r s per l i t e r are found i n 4200 meter water i n the P a c i f i c ( 1 3 ) . The e x p l a n a t i o n f o r t h i s d e c r e a s e i n methane c o n c e n t r a t i o n s i s unknown but may be due to b a c t e r i a l u t i l i z a t i o n , p a r t i t i o n i n g i n t o the l i p i d s of marine organisms and/or some o t h e r process· The second major n a t u r a l i n p u t of hydrocarbons to the ocean i s a c r o s s the sea-sediment i n t e r f a c e . Again, t h e r e a r e two major s o u r c e s of s e d i m e n t a r y generated hydrocarbons ; b i o l o g i c a l a c t i v i t y and p e t r o l e u m . Methane i s , by f a r , the preponderant b i o g e n i c hyd r o c a r b o n a l t h o u g h u n s a t u r a t e d t y p e s such as ethene and propene a r e produced i n the e u p h o t i c zone of the ocean (J7, 2J5). However, t h e s e hydrocarbons have a s h o r t e r l i f e t i m e i n sea-water than methane b e f o r e deg r a d i n g to non-hydrocarbon p r o d u c t s . They a r e found o n l y i n t r a c e amounts below the e u p h o t i c zone. C l a y p o o l and Kaplan (9) have shown t h a t methane i s formed i n a n a e r o b i c marine sediments d u r i n g b a c t e r i a l r e s p i r a t i o n p r o c e s s e s p r i n c i p a l l y below the zone of s u l f a t e r e d u c t i o n where methane f o r m a t i o n i s thermod y n a m i c a l l y u n f a v o r e d (1.9). A l s o , C l a y p o o l and Kaplan (9) suggest t h a t H£S i s t o x i c to methane b a c t e r i a . Thus, methane i s formed deep i n the s u i f a t e - f r e e s e d i ment column (>5 meters) and e v e n t u a l l y may d i f f u s e and bubble upwards i n t o the o v e r l y i n g water column. I n t u i t i v e l y , the magnitude of t h i s p r o c e s s s h o u l d be p r o p o r t i o n a l to the amount of o r g a n i c s u b s t r a t e a v a i l a b l e to methane p r o d u c i n g b a c t e r i a . A n o x i c b a s i n s such as the C a r i a c o Trench a r e env i r o n m e n t s which have h i g h amounts of o r g a n i c carbon (^5% i n sediments)· The minimal exchange of t h e s e b a s i n waters w i t h the open ocean a l l o w s f o r a b u i l d u p of h i g h c o n c e n t r a t i o n s of ^ S , produced i n the s u l f a t e r e d u c i n g zone near the sediment-water i n t e r f a c e , and CH^, produced i n the sediments below the s u l f a t e r e d u c i n g zone. Thus, e x t r e m e l y h i g h methane l e v e l s (up to 0.2 m l g p / l i t e r ) a r e observed i n C a r i a c o Trench B a s i n waters (2) . The f l u x of methane from the few a n o x i c b a s i n s to the open ocean i s seemingly s m a l l compared to the add i t i o n from r i v e r i n e , e s t u a r i n e and c o n t i n e n t a l s h e l f sediment s around the w o r l d . C o a s t a l and e s t u a r i n e sediments u s u a l l y have o n l y moderate amounts of o r g a n i c T



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matter and p r o b a b l y become s u l f a t e - f r e e w i t h i n a few meters of the sediment-water i n t e r f a c e . It i s highly l i k e l y t h a t a l l of t h e s e w i d e l y d i s t r i b u t e d sediments a r e s o u r c e s of methane which m a n i f e s t themselves as widespread band of d i f f u s i v e seeps. These may be e i t h e r s m a l l b u b b l i n g seeps or n o n - b u b b l i n g w i t h methane b e i n g exchanged by d i f f u s i v e p r o c e s s e s i n the s e d i ments. In many r e g i o n s of the c o n t i n e n t a l s h e l f s m a l l b u b b l e s w i l l d i s s o l v e i n the o v e r l y i n g water column b e f o r e r e a c h i n g the s u r f a c e . R e g a r d l e s s of whether the seep i s b u b b l i n g or n o n - b u b b l i n g the g r e a t e s t dep o s i t i o n of hydrocarbons w i l l take p l a c e i n near bottom water. A l o n g the n o r t h e r n c o n t i n e n t a l s h e l f of the G u l f of Mexico, over one hundred a c t u a l seeps have been l o c a t e d by sonar t e c h n i q u e s (_1, JL2, 2 6 ) . These seeps a r e a s s o c i a t e d w i t h f o u r t y p e s of g e o l o g i c a l s e t t i n g s : ( 1 ) s h a l l o w n o n s t r u c t u r e d s e d i m e n t a r y beds, ( 2 ) f a u l t i n g , (3) domes, and ( 4 ) mud mounds ( 2 6 ) . The o n l y two gas samples which have been a n a l y z e d and r e p o r t e d i n the l i t e r a t u r e have m o l e c u l a r ( > 9 9 . 9 % methane) and isotopic (6^^C ^ - 6 0 vs PDB) c o m p o s i t i o n s i n d i c a t i v e of a b i o g e n i c o r i g i n (8)· However, both of t h e s e seep gases were a s s o c i a t e d w i t h banks t h a t a r e not known to be a s s o c i a t e d w i t h s a l t domes or deep f a u l t i n g . A c o n c e n t r a t i o n v e r s u s depth p r o f i l e a t one seep l o c a t i o n ( F i g u r e 2 ) shows h i g h d i s s o l v e d methane c o n c e n t r a t i o n s a t d e p t h , i n d i c a t i n g t h a t a p p r e c i a b l e amounts of the seep gases d i s s o l v e i n the o v e r l y i n g water column. The r e l a t i v e l a c k of higher concentrations of ethane and propane at depth than observed a t the s u r f a c e i s a l s o c h a r a c t e r i s t i c of a b i o g e n i c seep. I t i s h i g h l y l i k e l y , a l t h o u g h not s u b s t a n t i a t e d at the p r e s e n t time to the a u t h o r s s a t i s f a c t i o n , t h a t g e o l o g i c a l f a u l t s and s a l t domes may g i v e r i s e to p e t r o g e n i c gas seeps. As p r e v i o u s l y suggested p e t r o g e n i c seepage would be c h a r a c t e r i z e d by low C-/ i ^ + C o ) r a t i o s and ), the p i s t o n v e l o c i t y or exchange r a t e i s on the o r d e r of one to two meters per day f o r methane w i t h slower r a t e s f o r h i g h e r m o l e c u l a r weight and/or more h y d r o p h i l i c species. By e l i m i n a t i n g a l l a n t h r o p o g e n i c s o u r c e s , i t would take about one-month f o r c o a s t a l waters w i t h an i s o p y c n a l water column s t r u c t u r e , such as found f o r w i n t e r c o n d i t i o n s , to r e t u r n to e q u i l i b r i u m w i t h r e s p e c t to the atmosphere. T h i s e s t i m a t e i s based on a mean depth of about 50 meters. For summer c o n d i t i o n s when a t h e r m o c l i n e d e v e l o p s , t h i s exchange would o n l y take p l a c e r e a d i l y w i t h i n the mixed l a y e r . High h y d r o c a r b o n l e v e l s i n the deeper water such as shown on F i g u r e 2. might w e l l be l a t e r a l l y advected out i n t o the open G u l f , and thereby p r o v i d e a tag f o r s t u d y i n g the m i x i n g dynamics between c o a s t a l and open G u l f w a t e r s .



Figure 6. Relative hydrocarbon concentrations and Ci/(C + C ) ratios in the Mississippi River at the delta for 1900-2400 LMT on 14 May 1974 2

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Summary Thousands of to d i s s o l v e d hydrocarbon de t e r m i n a t i o n s have been made on water samples from the G u l f of Mexico i n o r d e r to l o c a t e and/or c h a r a c t e r i z e (1) n a t u r a l seeps and (2) man-derived s o u r c e s of pe troleum c o n t a m i n a t i o n . R i s i n g bubbles from over 100 n a t u r a l gas seeps have been l o c a t e d by v a r i o u s groups u s i n g c o n v e n t i o n a l sonar t e c h n i q u e s , but the o n l y bub b l e s a n a l y z e d thus f a r have hydrocarbon (>99% CHi*) and isotopic (

Marine Chemistry in the Coastal Environment

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