REVIEWS

Precambrian organic matter LI Chao, PENG Ping'an, SHENG Guoying & FU Jiamo Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Correspondence should be addressed to Peng Ping'an

Abstract The late progress of the research on Precambrian organic matter in biomarkers, isotopes and relative analytical techniques is reviewed. The key problems in the research of Precambrian organic matter, such as indigenous test of soluble organic matter, are also discussed. At the end, the further work upon Precambrian organic matter is suggested. Keywords: Precambrian, organic matter, biomarkers, stable carbon isotope, kerogen degradation, indigenous test.

The Precambrian, one of the most important stages in the hlstory of the Earth, is less known to us mostly because of the absence of intact morphological fossils in the Precambrian strata, although much such as the research on Chinese progress has been achieved in the morphological fossils re~earch''.~], Jixian geological ~ection'~'. The research on Precambrian organic matter (RPOM), a new discipline, which studies the organic molecular fossils preserved in the Precambrian strata, could fill this blank. The RPOM could provide abundant information about the organism community, the evolution of life, the changes of paleoenvironments, the spheres interaction and the catastrophic events in the history of early Earth. Moreover, this research could contribute to the exploration of Meso- and Neoproterozoic oils. Therefore, this work has both great theoretical values and economic values. There exist considerable difficulties in RPOM. First, organic matter content is so low that it is easily affected by organic contamination from geologically younger strata or from organisms living in the vicinity of the collection sites, therefore, more carefulness and skills are needed in the preparation of samples. Second, Precambrian samples contain very little indigenous biochemical information, most of which has been lost in the long geological time. Since the early 1970s. people came to realize the difficulties in the RPOM, and gave a re-evaluation to the early work, leaving much of the early work in d o ~ b t ' ~ By * ~ ]the . 1980s, many Precambrian sediments and crude oils, which have high organic content, low maturity and appropriate geological setting have been found. The studies of them conducted reliable organic matter analysis and made considerable progress in the RPOM. Advances in instrumentation and analytical methods, especially in the GC-MS techniques and the selective chemical degradation methods of kerogen have also contributed to this progress. 1 The research of Precambrian biomarkers We are now aware that some detected compounds in the early biomarkers studies, such as amino acids, carbohydrates, fatty acids and other c ~ m ~ o u n d s ' are ~ - ~more ~ , likely to be the contaminants from the laboratories or the collection sites. Here, in the sequence of studied strata age, we review and summarize the late progress of reliable Precambrian biomarkers studies in the light of partial isotopic and microfossil results. ( i ) Archean to Paleoproterozoic. Up to date, the biomarkers preserved in Archean to Paleoproterozoic strata remain less known due to the well-known difficulties in the research. Perhaps, more reliable study is the hydrocarbon analysis of the chert of the Gunflint Iron Formation by OR6 and others in 1965[Io1.Although only were the n-alkanes C16-C32 and pristane and phytane detected because of poor analytical techniques, this result of molecular fossils study has definitely proved the existence of phototrophic organisms before 1.9 Ga. On the other hand, the discovery of cellularly preserved filamentous and colonial fossil microorganisms from Warrawoona Group (3.3-3.5 Ga) of

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Australia suggests that rokayotes such as cyanobacteria were well established throughout the Archean and Paleoproterozoic"'? and some were aerobic photoautotrophsf'~21. Some carbon and sulfur isotopic studies of Late Archean and Paleoproterozoic samples suggest that methanogens and methylotrophs and sulfate reducing bacteria were a~tive['~-'~]. ( ii ) Palm to Mesoproterozoic. Considerable progress has been made in Paleo to Mesoproterozoic biomarkers research. The reliable studies sites include the McArthur Basin (1.691 . 4 2 9 ~ a ) ~ ' Nonesuch ~ . ' ~ ~ , Fonnation (l.l~a)["'and Yanshan re 'on (1.4 and 1.7Ga)[18s'91. The sediments of the McArthur Basin, North Australia[15.1f and Nonesuch Formation of the North American Mid-continent Rift stern['" underwent a relatively mild thermal history, and have abundant and well-preserved indigenous organic matter. A lot of reliable biomarkers have been discovered in these sediments. The presence of steranes, hopanes and extended acyclic isoprenoids suggest the existence of eukaryote, eubacteria, and mhaebacteria respectively in Paleo and Mesoproterozoic. Particularly, the first occurrence of steranes in near 1.7Ga sediments indicate the eukaryote was present 1.7Ga ago, much earlier than 1.4 Ga, the putative time of eukaryote occurrence[m1.However, the high abundance of branched alkanes and cyclohexanes suggest a predominant prokaryote lipid input. Moreover, the similarities of hydrocarbons assemblage and distribution between the two studied sites reflects the succession and continuance in organism and environment through Palm to Mesoproterozoic. Peng et al.[19' studied the biomarkers of Tuanshanzi Formation (1.7 Ga). Jixian Section, North China, using a large amount of sample (lkg) and restrict contamination avoiding measurements. Steranes and gammacerance were detected besides other usual biomarkers. The presence of steranes supports the result of the work on McArthur asi in"^], i.e. eukaryote was present before 1.7Ga. It is also suggested that the eukaryote was present at least before 2.1 Ga from the evolving stages of the Ternplum sinica, a polynucleated green alga fossil and the filamentous eukaryotic Siphonales fossils, In 1992, Han and which were procured in the Wurnishan Formation (1.3 Ga), Jixian Runnegar reported the megascopic eukaryotic algae fossil from 2.1-billion-year-old Negaunee Iron Formation, ~ i c h i ~ a n ' ~This ' ] . discovery also supports the point above. The recognition of gammacerance which is usually thought to be the biomarker of protozoan suggests its presence 1.7 Ga ago, much earlier than the age (0.85Ga) reported by Summons et aLtnl. Notably, the evidence from biomarkers and microfossils showed the fast evolution of the organisms in Jixiail area in Precambrian. Wu et a1.[I8]unexpectedly detected two dehydroabietin isomers, i.e. 18- and 19-norabieta-8.11, 13-trienes besides other biomarkers such as steranes, hopanes, etc. in the black chert of the Gaoyuzhuang Formation (1.4Ga). Pangjiapu Section, Hebei Province, China Likewise, an^[^], Jiang et al.[241also discovered the analogous compouds in the Proterozoic strata of China. In addition, Zhang et al. [2'1 identified the retene and other dehydroditerpanecompouds in the Silurian core of the Michigan Basin, America. Moreover, our Precambrian work also shows the existence of similar compounds in the Precambrian strata (unpublished data). Therefore, it is reasonable to believe that these compounds were derived not only from some higher plants which were developed after Paleozoic, but also from some primitive organisms such as bacterias and algaes. (iii) Neoproterozoic. Several sites have been studied in more detail, they are the Walcott and the Bitter Springs Formation Member (0.85 Ga), Kwagunt Formation, Chuar Group, ~rizona[~'] (0.85 Ga) and Pertatataka Formation (0.6 Ga) of the Amadeus Basin, Central ~ u s t r a l i a ' ~and ~ ] the Xiamaling Formation (0.76 Ga), East Yanshan region, North C!hinaIn]. Wu et al.[181also studied a sample (0.76 Ga) taken from Liulou Section, North Jiangsu Province. East China. In addition to the continuous existence of archaebacteria, eubacteria and eukaryote, all of Neoproterozoic studies show the complexity in the hydrocarbons assemblage and distribution, especially for steranes among the sites studied. For instance, CX and Cn steranes are abundant in Walcott Member, whereas little of CB and C29 steranes were detected[n1. Moreover, the Cm desmethyl steranes which seem to be specific to some geological time and Cmethyl steranes (some of which are probably dinosteranes) which were derived from the acritarchs are present in abundance in Bitter Springs Formation and Pertatataka orm mat ion[^^]. Notably, a novel tricyclic terpane series with the 13a (n-alkyl) configuration as well as a regular 296

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tricyclic terpane series with 13P(H), 14a(H)-configuration were detected in the samples of Xiamaling Formation by Wang et aLfnl. (iv) Latest Proterozoic sediments and oils. Many latest Proterozoic sediments and oils have been studied for biomarkers, including East European platform (Riphean and ~ e n d i a n ) [ ~Rodda ], Beds ', Platform (< 0.65 (Ungoolya Group) of Easter Officer Basin in South Australia (0.6 ~ a ) ' ~Siberian ~ a ) ' ~ and ~ " ]Oman oils ( 4 . 6 5 ~a)['~-"]. The composition and distribution of biomarkers in the latest Proterozoic sediments and oils suggest the complexity of organism assemblage in this period. For example, the occurrence of 12- and 13-monomethyl-substitutedC24-Ca alkanes in organic matter of East European ~ l a t f o r t n which ~~~, seems to be specific for both Precambrian oils and sediments of Neoproterozoic, might be associated with the development of certain organism specific for Precambrian. Moreover, the unusual uniform distribution of C27-C29 steranes might also be attributed to the input of certain organism such as metazoan in the latest Proterozoic Eon although the effect of lithology could not be excluded. 18a(H)-Neohopanes in Rodda Beds of Officer asi in'^^] are present in equal or greater abundance than the 17a(H)-2lP(H)-hopanes, and these compounds might be the products of groups of bacteria more prominent in the Proterozoic than in later times. It is reasonable t believe that great changes have been taken place in the latest Proterozoic organisms from the biomarkers evidence, some were deteriorated or extinct, some have boomed and gotten an enormous radiation such as metazoan and plankton. The main feature of the Precambrian biomarkers is summarized in table 1. Sites studied East European Phtf~rm Oman Huqf

Apx' aae/Ma

-

800 -600

450

Grwp oils

Siberian platform Oils

450

Rodda Beds

600

Jiudingshan

700

Fm Xiamaling Pm.

760

-

Table 1 Summary table of ReCambrian biomarker studies Rock Biomarker assemblage ~ ~grWP by~ Wenviraunent inhoce terrigenoudde Me-branched alkanes c ~ o t h e r a 7 c 2 7 -CB &lams alecn-C31 hopcarbonate,evap/ Me-branched alkanes mtricted marine?? C a steranes (major) Ca-Cia steranes (minor) C n - C a h~ carbonatecvspl Me-branched alkanes restricted marine?? C a steranes (major) C a - C a Bteranes (minor) C n - C a hopanes 28.U)-bisnomopaOe aryl i q n m o i d C Z Isteranes (major) shales/ marine wter shelf C a -C a ste(minor) C Z I - C a hopanes Cr,-C u neohopanes shales/ Me-branched alkanes marine wter shelf C a -Cm S-t Cz7 -C3s hopanes -a'cy4-methylBteranes??? dinoflageUates77? black chert Czi -C a steranes wkaryotes (al~ae) Ca-Cm ~ O ~ B O ~ S eubackMcyanokteria dehydroabietstin unidentified hactedakae7?7 bituminous bicyclic sesquiteqmnes unidentified pkaryoks 7 .

sandstone

Bitter Springs Rn (Oillen Mbr.

850

carbonate.evap1 restricted marine

KwaguntFm. (WalmtI Mbr.)

850

carbonatepvap? and mudstones/ marine??

13u(n-alkyl) tricyclic terpanes Me-branched alkanes C a -Cm steranes C n -C3s 4-methylsteranes??? >Cm isopremids Mebranched alkanes Cn-Ca

-yC euLaryotes (4w+h'-s?) eubacbia/cyanobacteria dinofl-777 a

aRekrence l

[%I

[18.251

[271

1x361

[221

cyane&acmklcyanobacmia

(To bc ro3lrinucd on the next page)

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.--

(Continued Sites studied

t:zi

we,"vkLment OC

inference group by

Biomarker assemblage

Reference

CxC27 steranes (major) CZS-CZS steranes (tr.)

Rhodoph~ceae algae hetemtrophic protist??? gammacerane unidentified eubacteria CB-C~J nwhopanes tricyclic m a n e s unidentified prokaryotes Nonesuch Fm. 1OOO shales cyanobacteria [17,36-411 Me-branched alkanes eubacteridcyanobacteria C27-Css hopanes eukatyotes (algae+h'tmphs?) c26-c30 steranes unidentified eubacteria C~9+neohopanes unidentified eubactuia 28.30-bisnorhopane unidentified prokaryotes tricyclic terpanes photosynthetic organisms FQrphyrins Gaoyuzhuang 1 400 black chert C27 -C29 steranes eukaryotes (algae) [18,251 Fm. eubacteridcyanobacteria C~g-C31hopan~s unidentifieded bdalalgae??? McMinn Fm. 1 429 shales/ cyanobacteria 1161 Me-branched alkanes marine inner shelf eubactedcyanobacteria C27-C35 hopanes eukaryotes (algae+h'trophs?) CX-C~Osteranes cyanobacteria I161 Velkerri Fm. 1 429 shales/ Me-branched alkanes eubactcria/cyanobacteria marine outer shelf c27-c35 hopanes (tr.) eukaryotes (algae+h'trophs?) C27 -C29 steranes (tr.) archaebacteria [I61 carbonate,shald >Cm isoprenoids CZOisoprenoids cyanobacteria and shales/ Me-branched alkanes eubacte-dcyanobacteria lacustrine, lagoonal? C27-C3~ hopanes eukaryotes {algae+h'trophs?) C27 -C29 steranes unidentified eubacteaia C29+neohopanes unidentified eubacteria 28.30-bisnorphopane unidentified prokaryotes tricyclic terpanes cyanobacteria/others? 1191 Tuanshanzi 1 700 Argillaceous C14-C19 mid-Me-br-alkanes Fm. eukaryotes (algae+h'trophs?) dolomitelmarine C27 -C29 steranes CZ~.CZ~-C hopancs ~S eubacteria/cy~obacte& C27, C29 nwhopanes unidentified eubacteria tricyclic tcrpanes unidentified prokaryotes gammacerane bacterialciliates > I 900 chert Cl6-C32 n-alkanes bacterialalgae [lo] Gunflint Iron Fm. pristane, phytane photosynthetic organisms a) This table was made after ~ummons'"'. b) evap=evaporate;Fm., formation; h'trophs. heterotmpk, Mbr.. membrr; tr., m e . -

-

2 Stable carbon isotopic research of Precambrian organic matter ( i ) Average carbon isotopic research of Precambrian organic matter. In RPOM, it is a very useful tool for analyzing the carbon isotopic compositions of both sedimentary organic matter and its counterpart, i.e. carbonate because a lot of information about biological evolution and paleoenvironment changes on the early Earth can be taken from it. Time-related changes in the Precambrian sedimentary isotopic record of organic carbon and carbonate are consistent with the stepwise oxidation of the Precambrian environment (fig. 1).This trend has been attributed to the global formation of carbonate rocks in Precambrian and the burial and accumulation of reduced carbon in the sediment. Several time intervals during which the rates of organic burial increased prominently have been discovered in the organic burial history (fig. 2). These prominent increases in the rate of organic burial have now been linked to global tectonic processes, which tectonically enhanced the rates of weathering and erosion, and delivered more nutrients (such as N and P) to biomass, and synchronously rapid burial, rather than biological eruptive

Figs. 1 and 2 are the plots of thermal corrected s'~c,values and the fractions of carbon buried in organic form (f,,) against age of purified kerogens respectively, which were done by Des Marais et 298

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t

2,'5

I

I

I

2.0 1.5 1.O 0.5 AgeIGa s ) @c,, (circles) against age of Fig. 1. 8 3 ~ c ~ ( c r o s s eand purified kerogens. Original a3c, values obtained by Strauss et al!421, S ' 3 values ~ are corrected for postdepositional thennal alteration. The solid lines depict running averages of the data, and the rectangle along the right margin of the figure represents Phanerozoic values. Adopted from Des Marais et al.'"'.

Age/Ga Fig. 2. Fraction of carbon buried as organic matter (fd against age of puriiled kerogens. Rectangles along the bottom margin depict time intervals of enhanced global rifting and orogeny. Adopted from Des Marais et al!"'.

a~.[~ The ~ ]increase . in 613c,, values throughout the Proterozoic reflects the stepwise oxidation of the Proterozoic environment. Two high peak intervals (2.5- 1.8 Ga and 1.3-0.7 Ga) in , f curve are consistent with two main tectonic time intervals. The PrecambrianJCambrian boundary is another focus studied for the carbon isotopic analysis of sedimentary organic carbon and carbonate because it is a time of great changes in biosphere. According to the results of studies worldwide, it is now clear that several fluctuations have happened on the This result is consistent with the preserved fossil record of planktons biomass during the period[45'461. and metazoans during this time. Some scholars haveconcentrated their attention not only in Proterozoic Eon, also in Archean. For example, the works of Schoell and ~ e l l m e r [ ' and ~ ] ~ a m e r o n [suggest '~ the possible existence of some archaebacteria such as methanogens, methylotrophs and sufate reducing bacteria in Archean. ( i i ) Compound-specific carbon isotopic analysis of precambrian organic matter. n-Alkanes extractable from sedimentary rocks of Proterozoic age are often enriched in I3crelative to coexisting kerogen, but acyclic isoprenoid is isotopically depleted. These trends seem to be recognized as characteristics of Precambrian organic matter. In contrast, in Phanerozoic, it is observed that n-alkanes are more enriched in '*c than acyclic isoprenoid. Logan et al.[47"81 suggested that this reverse relationship can be linked to the different transport mechanisms of organic matter through the water column between Precambrian and Phanerozoic age. In Precambrian, only being as individual particles or aggregates did organic matter produced by autotrophs and respiring heterotrophs sink such more slowly, and was subject to severe heterotrophic reworking. Hence, the n-alkyl carbon skeletons of photosynthetic origin have been degraded and replaced by the products of late-stage heterotrophs which is of enrichment inI3c. However, the primary and refractory products such as acyclic isoprenoids which is of enrichment in I2cSurvived. A different procedure happened in Phanerozoic water column, the primary organic matter was promptly repackaged as ralatively dense, fast-sinking faecal matter by Chinese Science Bulletin

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larger organisms and in particular of coelomate metazoans, causing organic matter to reach the sediment soon and with less opportunity for degradation. Therefore, the different isotopic characteristics from those of Precambrian age have been achieved in Phanerozoic. However, the similar capability resistant to biological degradation between n-alkanes and acyclic isoprenoids lead us to believe that both of two should have been reworked by heterotrophs, and there were different extent of degradation in primary organic matter which depended on the local environment, especially organism assemblage. According to our Precambrian work, both of n-alkanes and acyclic isoprenoid extracted from samples older than 1.4 Ga are enriched in I3crelative to associated kerogen (unpublished data). 3 Advance in analytic techniques The progress in organic geochemical analytic techniques also contributed to the development of RPOM. ( i ) Kerogen degradation. Kerogen degradation plays an important role in RPOM. It is very useful to the elucidation of kerogen structure, and it is more helpful to the identification of indigenous feature of extractable hydrocarbons and the maturity of sedimentary organic matter. Generally, kerogen degradation can be divided into two groups, thermal and chemical degradation. In addition to traditional pyrolysis methods, by the 1970s, an online pyrolysis technique termed Py-GC-MS was widely used to deal with compounds with high molecular weight in various sediments, soils and coals, and many reports and review^^^^'^^^ were published. Chemical degradation of kerogen commonly includes oxidative and reductive and specific bond cleavage reactions. The specific chemical degradation is one of most valuable chemical degradation methods. Table 2 is the summary of recent progress in specific chemical degradation methods. Reactions Base hydrolysis

Sites attacked ester bonds

Acid hydrolysis Ozonolysis

amide bonds C-C multiple bonds aromatic rings

Ru01 catalytic oxidation Raney NilHz reduction Hydrogenolysis with Rh-C Trimethylsilyliodide (TMSI) BBrdBCl3 degradation Reductive alkylation Phenolized depolymerization

Table 2 Specific chemical degradation reactions Basic methods Labelling Roducts -COOH,-OH base hydrolysis: OHfree acids, alcohol crown ethers, TMSI free acids, amines -COOH.-NH2 acid hydrolysis 0 3 carhoxylic acids -COOH

and functional

(RuC13 * 3HzO+NaI04)+ (CCL-HzO-CH3CN).6-24 Th

carbon atoms sufur bonds

room temperature Raney NiM2

ether bonds

ether bonds/ ester bonds ether bonds/ ester bonds ether bonds/ aromatic rings

COz and carboxylic acids

hydrocarbons mixture lignin-derived compounds and hydrocarbons mixture TMSI, LiAIh, extracted by hydrocarbons and CHCI3 alcohols BBrdBC13, LiAlH4 hydrocarbons and alcohols K,CHsl, in THF solution a l k y w products

-COOH

deuterlated sites deuteriated sites

deuterlatcd sites (LiAID4) deutcriatcd sites (LiAlD,) -0CH3

maxture

H+.B F m S A , phenol

phenolized products

Remark simplest and mildest reaction as above less specific less used widely used

used to sulfurrich sample high yield rate

mainly used to ether bonds

mainly used to coal

phenolyl

mixture Main data from refs. [Sl -561.

( ii ) GC-MS Techniques in biomarker analysis. Low content and coelution of biomarkers are the main obstacles in biomarker GC-MS analysis. The usual full scan and multiple ion detection (MID) GC-MS mode sometimes do not meet the demand for the determination of biomarkers. A powerful GC-MS technique named selected metastable ion monitoring (SMIM) or metastable reaction monitoring (MRM) showed its potential to solve these problems"7p5n. MRM-GC-MS allows the determination of specific parent-daughter ionsTrelationshipsby monitoring those daughter ions that occur in the first field-free region of a double focusing mass spectrometer. The analytical specif~cityof

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this method eliminates most coelution problems of GC peaks, and boosts signal-to-noise ratios and sensitivities to levels that are orders of magnitude better than those obtained using conventional MID-GC-MS. Similar results yielded by MRM-GC-MS can also be accomplished by tandem mass spectrometers (GC-MS-MS). Now, GC-MS-MS with the help of computer (CAD-GC-MS-MS) allows numerous parentldaughter relationships to be monitored simultaneously, and allows resolution of several carbon numbers of the same family or of several different families of biomarkers. The results provided by CAD-GC-MS-MS are more reliable than those from other techniques because of its great resolution of both parent and daughter ionsf5*].Moreover, a new technique called isotope-ratiomonitoring gas chromatography mass s ectrometry (IRM-GC-MS) allows numerous stable carbon isotopic analysis of individual biomarkersP59.601 . 4

Key problems in RPOM

( i ) Test for indigenous soluble organic matter. Because of great age of Precambrian samples, the test for indigenous organic matter, especially the extractable material, become the key problem of this work. Peters and oldo ow an[^'] provided four principal tests to help establish whether a soluble organic matter (bitumen) is indigenous to the rock from which it has been extracted: i ) Consistency of bitumemTOC andlor production index (PI) values with the level of thermal maturity of the kerogen (e.g. T,, or vitrinite reflectance). i i ) Comparison of the thermal maturity of the bitumen (e.g. CPI or biomarker maturity ratios) with that of the kerogen. iii) Isotopic relationships between the bitumen and kerogen. iv) Comparison of biomarkers in the bitumen with those released from the kerogen by mild thermal or chemical degradation. We generally agree with Peters and Moldowan, but another point, the investigation of sample lithology and geological setting of the collection site is also important to the discussion of indigenous feature of soluble organic matter. As to third point, we think that it should also include the consistency of isotopic composition of kerogen with individual biomarker isotopic characteristics of bitumen and all kinds of product. of kerogen degradation. ( i i ) Overcoming the analytical difficulties. Most of the early organic geochemical studies of Precambrian materials were mainly hampered by inadequate preservation of the organic record and/or organic contamination of the samples. Various measurements have been adopted to avoid the contamination from laboratory. However, there is so far no an effective method to deal with the low content of the indigenous organic matters. Peng et al.['91 successfully studied the Precambrian sample with low organic content by means of biomarker concentration and a series of measurements to avoid the contamination.

5

Further work

Most work of RPOM now still concentrates on the Proterozoic strata with the age 2.5-0.55 Ga, and an overall study to Precambrian geological section over interval of long time (21.0 Ga) so far is absent. Therefore, a systematic study to the geological section over long time interval or to older strata will be the new breakthrough in RPOM. Most work of RPOM now still concentrates on the analysis of extractable organic matter, little work has been done with kerogen, especially the specific chemical degradation of kerogen. Kerogen, the main indigenous organic matter preserved in sediments, should be the important subject of this work. Most work of RPOM now still concentrates on the composition and distribution of extractable materials andlor the product of pyrolysis, little work has been done with the carbon isotopic composition of the individual compounds of various organic matter including soluble product of chemical degradation of kerogen. This point should also be stressed in the further work. In the future work, the nonuniformity of global biological evolution and its effects on the characteristics of Precambrian organic matter should be taken into account. In 1986, the mechanism of RNA self-splicing was elucidated by Cech et al.[621.This discovery Chinese Science Bulletin

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make people believe that the template of protein synthesis was RNA rather than DNA before sometime in Precambrian. However, this great change in biochemical pracess must have left some trace in the preserved organic matter, especially in nitrogen isotopic composition. Therefore, the study to this problem seems to be more valuable in RPOM. At last, the foundation of an effective experimental method to Precambrian samples will also be an important work in the future. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 49321003 and 4945003).

References

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