Molecular Geochemical Indicators in Sediments - ACS Symposium

2 Molecular Geochemical Indicators in Sediments S. C. Brassell and G. Eglinton

Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: April 21, 1986 | doi: 10.1021/bk-1986-0305.ch002

Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom

Sediments from contemporary aquatic environments contain a diversity of compounds that provide an assessment of the sources of their organic matter. These components include lipids with structural features that are indicative of their biological origins. Thus, specific diterpenoids and triterpenoids are markers for sediment contributions from terrigenous vegetation. Similarly, among the numerous sterols recognised in sediments many are diagnostic of their algal origins, notably 4α-methylsterols derived from dinoflagellates. Several lipid types characterise contributions from bacteria; for example, acyclic isoprenoid alkanes arising from methanogens. Illustrative examples of such diagnostic lipid distributions show the possibilities for differentiating between sediments receiving allochthonous terrigenous organic matter and those dominated by autochthonous algal and bacterial contributions. Within the hydrocarbon distributions, by contrast, several features can denote non-biological sources of organic components, such as the weathering of ancient sediments, o i l seepage and o i l spillage. A further development in environmental assessment using molecular indicators stems from the recent recognition that the unsaturation of specific lipids contributed to sediments by cocco1ithophorid algae provide a measure of water temperatures. This paper concentrates on three aspects o f the application of molecular organic geochemistry to the interpretation of the biological origins of sedimentary organic matter and t h e u s e o f such information i n the evaluation of depositional environments. First, the basis

0097-6156/86/0305-0010$06.75/0 © 1986 American Chemical Society

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

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BRASSELL AND EGLN ITON

11 Molecular Geochemical Indicators


for the assignment of s p e c i f i c compounds as diagnostic markers for the contributions of particular biota to sediments i s c o n s i d e r e d and i l l u s t r a t i v e examples of such molecular indicators are given. Second, the sedimentary distributions of such components a r e d i s c u s s e d and the p o s s i b i l i t i e s for their use i n the q u a l i t a t i v e assessment of contributions from different biological sources to sediments i s addressed. T h i r d , one group of l i p i d s which occur w i d e l y i n marine sediments, namely long-chain (£37 to C ) alkenones, are considered i n terms of their potential to provide an assessment of oceanic water temperatures i n the shallow subsurface from the sedimentary molecular record. The s t a n d a r d m e t h o d o l o g y used i n i n v e s t i g a t i o n s of the organic constituents of sediments typically involves t h e i r e x t r a c t i o n , then f r a c t i o n a t i o n a c c o r d i n g to p o l a r i t y or compound class and f i n a l l y their evaluation and i d e n t i f i c a t i o n by g a s c h r o m a t o g r a p h y (gc) and c o m p u t e r i s e d gas chrornatography-ma ss s p e c t r o m e t r y (gc-ms). Details of such procedures are not given here, b u t c a n be f o u n d i n the references cited. Molecular

Indicators

in

Sediments

The development of molecular markers as indicators of biological contributions to sedimentary organic matter relies on t h e i n f o r m a t i o n from the l i p i d composition of appropriate organisms. Such information i s often scant. Therefore, existing data i s used to propose working hypotheses t h a t c a n be m o d i f i e d o r amended when additional information becomes available. The underlying rationale for this approach i s the tacit assumption that the biochemical pathways of lipid biosynthesis i n different organisms are not n e c e s s a r i l y uniform at the present time, nor have they been over geological history. Rather, i t seems that the lipid compositions of biota have been tailored throughout evolution t o meet t h e i r environmental needs. Hence, the d i s c r e p a n c i e s and s i m i l a r i t i e s between the lipids of different organisms c a n be u s e d to assess their generic relationships leading to chemotaχ onomy. Such chemotaxonomic description of organisms using l i p i d components relies on the information obtained from the analysis of both laboratory cultures and natural populations of individual species. The assignment of a given compound recognised i n sediments to a specific biological source i s based on s u c h c l a s s i f i c a t i o n s aided by information from environmental a n a l y s e s , such as the investigation of sediments thought to receive dominant inputs from a particular s p e c i e s or class of organism. Indeed, the v e r i f i c a t i o n of the b i o l o g i c a l source and significance of individual lipid components of geochemical interest i s often best served by studies designed to evaluate t h e i r o r i g i n s i n a chosen environment.


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ORGANC I MARN IE GEOCHEMSITRY

For some y e a r s i t has been h e l d t h a t d i n o f 1 a g e 1 l a te s are the biological source of the 4ol-methylsterols that occur, frequently as abundant components, in marine sediments (1.2.2^· a s s i g n m e n t s were i n i t i a l l y based on the widespread literature concerning 4o(-methylsterols in cultured dinof1age1 lates (£) and the abundance of these compounds in sediments known to receive major c o n t r i b u t i o n s from t h i s c l a s s of algae. The d i s t r i b u t i o n s of 4 e C - m e t h y l s t e r o l s , and a l s o 4o(-methylstanones, observed in the bottom sediments of P r i e s t Pot, a s m a l l lake i n the English Lake D i s t r i c t , were almost identical to those recognised in a bloom of the d i n o f l a g e l l a t e £Ϊ£ίΐΐίϋΐΐ1 i—EluiSLliii c o l l e c t e d f r o m t h e o v e r l y i n g w a t e r s ( 5 ) . This study demonstrated t h a t P j _ i° ™!li£!Lii t 4-me t h y l s t e r o i d s in Priest Pot sediments and, in more general terms, provided convincing evidence in support of the link between sedimentary 4oc-me t h y l s t e r o i d s and their presumed b i o l o g i c a l source, d i n o f l a g e l l a t e algae. S

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O v e r a l l , there are c l e a r i n d i c a t i o n s that particular organic c o m p o n e n t s may be specific to s i n g l e or multiple biological sources or, alternatively, may be non-diagnostic of their biological origins. In simple terms an individual component, or the distribution of a specific compound class, may be representative of contributions to sedimentary lipids from either algal, terrigenous higher plant or bacterial sources. Not a l l components, however, can be p l a c e d i n t o one of these three classes; for example, some o c c u r in a l l three, some may only occur in animals, whereas the o r i g i n s of others are unknown. In a d d i t i o n t o c o n s i d e r a t i o n s of their ultimate biological origins individual compounds in sediments can be either unaltered biosynthetic products or, alternatively, derivatives formed by diagenetic / catagenetic processes, which retain structural elements that attest to their original biological source. Most biological marker compounds o c c u r r i n g in sediments can be considered as m a r k e r s o f one of the following five groups: (i) ηon-diagnostic components (i i ) algae ( i i i ) terrigenous higher plants (iv) bacteria (ν ) un k n o w n ' E x a m p l e s o f i n d i v i d u a l c o m p o n e n t s h e l d t o be repres entative of these five categories are shown in Figures 1-5. The majority are known lipid constituents of organisms, whereas others are either diagenetic products or of an as yet undefined origin. The compounds chosen provide an i l l u s t r a t i v e , rather than a comprehensive, indication of the range of structural types in each category. Most of them have been d i s c u s s e d elsewhere in a fuller review paper (6), together with other structurally similar examples. Also, the individual compounds and compound d i s t r i b u t i o n s biοsyηthe s i s e d by, and held to be diagnostic for, algal, terrigenous higher plant and 1


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BRASSELL AND EGLN ITON

13 Molecular Geochemical Indicators


bacterial inputs to recent sediments are published elsewhere (3^). Within categories ( i i ) , ( i i i )and ( i v ) i n d i v i d u a l m a r k e r c o m p o u n d s c a n be u n i q u e l y r e p r e s e n t a t i v e of a particular species, genus or class of organism. Alternatively, they c a n be n o n - s p e c i f i c components which occur widely i n d i f f e r e n t , but perhaps distantly related, families. Clearly, s i m i l a r i t i e s i n the l i p i d compositions of related species or genera may r e f l e c t their ancestral l i n k s , w h e r e a s d i f f e r e n c e s may s t e m f r o m t h e d i v e r g e n c e o f their biosynthetic pathways. Non-diagnostic biological markers. The components considered t o be n o n - d i a g n o s t i c i n d i c a t o r s (Figure 1) a r e likely to be those that play a fundamental role i n biosynthetic processes. For example, essential constituents of cell structures ( e . g . membranes) or s i m i l a r p h y s i o l o g i c a l u n i t s m i g h t be e x p e c t e d t o be common to many d i f f e r e n t types of organism and t h e r e f o r e occur ubiquitously. Hexadecanoic acid, f o r example, i s a typical c o n s t i t u e n t o f t h e membranes o f numerous l a n d and aquatic plants, animals and b a c t e r i a . I t may b e p r e s e n t in the form o f wax esters or t ri acy 1 g l ycer o l s but i s generally of l i t t l e value i n differentiating between contributions to sediments from i t s various sources. Similarly squalene i s the biosynthetic p r e c u r s o r o f many triterpenoids and, thus, occurs widely i n organisms. Cholesterol, like hexadecanoic acid, i s a constituent of the cell membranes of many different families of organisms, with the notable exception of bacteria. I t acts as a ' r i g i d i f i e r ' i n cell membranes. I t i s a prominant sterol o f many algae and land plants, and i s often the only sterol component of copepods. Tocopherols are held to play an important role i n ph ο t osyn t he t ic processes. Hence, they are abundant i n many photosynthetic organisms, including higher plants, algae and c y a n o b a c t e r i a ( 7 ) ,making them n o n - d i a g n o s t i c m a r k e r s of the sources of sedimentary organic matter. Algal indicators. Individual marker compounds indicative of sediment contributions from algae (Figure 2 ) may occur in both marine and l a c u s t r i n e environments, a l t h o u g h their source species w i l l differ. Indeed, botryococcene i s the only component shown which has y e t t o be recognised i n both environmental regimes. Carotenoids are perhaps the most s o u r c e s p e c i f i c l i p i d c o n s t i t u e n t s o f o r g a n i s m s . For example, diatoxanthin occurs only i n diatoms and can therefore be regarded as a highly specific marker f o r their contributions to sediments. I t s absence i n a sediment, however, cannot be t a k e n as e v i d e n c e f o r the l a c k o f d i a t o m c o n t r i b u t i o n s t o o r g a n i c m a t t e r , due t o i t s l a b i l i t y . Indeed, this l a b i l i t y makes carotenoids of l i m i t e d use as markers of the sources of organic matter i n most s e d i m e n t a r y environments, with the notably exception of relatively s h a l l o w water systems, such as p o s t - g l a c i a l lakes ( 8 ) . Recent work (2,5) has provided convincing


14

ORGANC I MARN IE GEOCHEMSITRY

Ο )H


HEXADECANOIC ACID (PLANTS Sc ANIMALS)

SQUALENE (ALMOST UBIQUITOUS)

(WIDESPREAD,

CHOLESTEROL EXCEPT FOR

BACTERIA)

o< -TOCOPHEROL ( PHOTOSYNTHETIC ORGANISMS) Figure 1. Compound structures, names and b i o l o g i c a l origins of examples of ηo n - d i a g n o s t i c indicators of the sources o f sedimentary organic matter.


2. BRASSELL AND EGLN ITON Molecular Geochemical Indicators


DIATOXANTHIN

(DIATOMS)

0' >• 4-METHYLG0RG0STAN0L (DINOFLAGELLATES)

DINOSTERONE (DINOFLAGELLATES)

HEPTATRIACQNTA-15#22-DIEN-2-0NE

(COCCOLITHOPHORIDS)

BOTRYOCOCCENE

Molecular Geochemical Indicators in Sediments - ACS Symposium

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