Abundance and geochemical significance of C2n dialkylalkanesand highly branched C3n alkanes in diverse Meso- and

Neoproterozoic sediments.

Paul F. Greenwooda,*, Khaled R. Arourib,Graham A. Logana,c, Roger E. Summonsc,d

aGeoscience Australia, GPO Box 378, Canberra, ACT, 2601. AustraliabGeology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australia

cAustralian Centre for Astrobiology, Macquarie University, NSW 2109, AustraliadDeparment of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge MA 02139-4307, USA

Received 20 March 2003; accepted 24 October 2003

(returned to author for revision 27 June 2003)

Abstract

Several series of branched alkanes with quaternary carbon centers have been detected by GC–MS in the saturatedhydrocarbon fractions of Neoproterozoic sedimentary rocks associated with fossilised microbial mats (Officer Basin,South Australia) and of shallow water carbonates (Spitsbergen, East Greenland and Baffin Island, Canada). A series of5,5-diethylalkanes was predominant in most of these sediments and was typically accompanied by related series in

lower abundance which, based on mass spectral characteristics, are tentatively assigned as 6,6-diethylalkanes, 5-butyl,5-ethylalkanes and 6-butyl, 6-ethylalkanes. Each series displays strong odd or even carbon number preferences. Severalof the sediments contained additional series of structurally undefined C3n alkanes, notable for very negative retention

time factors which become more negative with increasing molecular weight—indicating that branching increases witheach C3 addition. The strongly sequential carbon preference of the respective C2n and C3n branched alkane series, and apredominantly light 13C signature of the 5,5-diethylalkanes (cf. co-occurring n-alkanes), are consistent with an origin

from organisms with a distinct physiology and/or carbon source. The relatively wide occurrence and high abundance ofthese branched alkanes in Proterozoic sediments containing benthic microbial mats suggests a relationship with specificpaleoenvironmental conditions, for example strong water column redox gradients, that may have been more prevalentthan in the Phanerozoic.

# 2003 Elsevier Ltd. All rights reserved.

1. Introduction

The organic matter content of Neoproterozoicsediments has revealed useful information regarding

microbial diversity (Summons and Walter, 1990) and

paleoenvironments (Logan et al., 1997, 1999, 2001) in

the period of rapid and dramatic global change leadingup to the Phanerozoic. Many of the hydrocarbonsencountered in the Neoproterozoic are the same as

those found in Phanerozoic rocks and oils and, for themost part, their precursor biolipids can be found inextant organisms, establishing a direct ‘biomarker’ link.However, some classes of compounds, for example the

C20+ mid-chain methyl alkanes that are so abundant inProterozoic oils from Oman (e.g. Hold et al., 1999) andSiberia (Fowler and Douglas, 1987), have no generally

0146-6380/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.orggeochem.2003.10.013

Organic Geochemistry 35 (2004) 331–346

www.elsevier.com/locate/orggeochem

* Corresponding author at present address: School of

Applied Chemistry, Curtin University of Technology, PO Box

U 1987, Perth, WA, 6845, Australia.

E-mail address: p.greenwood@exchange.curtin.edu.au

(P.F. Greenwood).

recognized counterparts in modern biota. Despite thesegaps in knowledge, studies of these ancient sedimentsand their molecular fossils are important because theymay provide insights into unusual sedimentary environ-

ments and microbiota in the period leading up to thefirst appearance of complex multicellular organisms.High concentrations of unusual branched alkanes

were reported to occur in Neoproterozoic sedimentsfrom the Tanana Formation of the Officer Basin, Aus-tralia (Arouri et al., 2000a,b). Whilst the exact form and

identity of these biomarkers was not established, theyappeared to be directly associated with fossilised micro-bial mat communities that occur in the same sediments.

Microbial mat communities, particularly those wherecyanobacteria are the predominant organisms, are wellknown for having distinctive patterns of short-chain(C15–C20) methyl alkanes. These same hydrocarbons

have been identified in cyanobacterial cultures (Kosteret al., 1999; Dembitsky, et al., 2001), as well as modernand ancient sediments associated with cyanobacterial

mat assemblages (Shiea et al., 1990, 1991; Robinson andEglinton, 1990; Summons and Walter, 1990; Kenig etal., 1995). Neoproterozoic sediments throughout the

Australian Centralian Superbasin are well-preservedand typically contain evidence of fossilised cyano-bacteria and protists (Schopf, 1968; Grey, 1998). Asso-

ciated bitumens are also often rich in low molecularweight organic remains of ancient algae and bacteria(Summons and Walter, 1990; Logan et al., 1997, 1999).A series of monomethyl alkanes and a series of C19–

C33 branched alkanes with a strong odd carbon num-ber preference that were originally mistakenly assignedas 3,7-dimethylalkanes on the basis of similar GC and

MS data to published literature (Mycke et al., 1988),have been reported from ancient microbial mats in the1640 Ma Barney Creek Formation in Australia, (Logan

et al., 2001). A separate series of mid-chain branchedalkanes, consisting predominantly of even carbonnumbers ranging from C22 to C36 were also found inBarney Creek sediments in association with an assem-

blage of large filamentous microfossils (Logan et al.,2001). The much broader range of carbon numbers ofthese unusually prominent methyl alkanes and their

carbon isotopic signatures suggest that they may derivefrom a non-cyanobacterial source.The present study was initiated in an attempt to

identify the unusual branched hydrocarbon productsdetected in the Tanana Formation sediments by Arouriet al. (2000a,b) through close examination of mass

spectral data and gas chromatographic elution patterns.Their distribution in co-eval sediments from the North-ern Hemisphere associated with well-documentedmicrobial organic matter types was investigated by

analysis of sediments from 700 to 800MaNeoproterozoiccarbonate successions of Svalbard and East Greenlandand in the Victor Bay Formation of Baffin Island,

Canada. Where chromatographic resolution allowed,the d13C composition of several hydrocarbons was alsomeasured.

2. Experimental

2.1. Sample descriptions

2.1.1. Tanana formation

Drill core intersections of the Neoproterozoic TananaFormation in the Officer Basin of South Australia wereoriginally sampled for organic geochemical analyses of

large and well-preserved leiospheroid and acantho-morph acritarchs (Arouri et al., 1999,2000b). In addi-tion to the acritarchs, sediments from these locationsoccasionally contained large filamentous microfossils

and microbial mats in various states of preservation.Palynological examination and scanning electron

microscopy showed that the mats in these sediments

were biologically diverse (Arouri et al., 2000b). Abun-dant filamentous and coccoid microfossils, together withamorphous and finely disseminated organic matter

associated with microbially-formed fine carbonate crys-tals characterize the mat horizons. These features, toge-ther with varied biomarker patterns were used to

categorise several distinct organic facies (Arouri et al.,2000a). The prominent series of branched alkanes weredetected in Microbial Mat Type 1 and Type 2 facies.Petrographic analysis indicated the Tanana Formation

sediments had thermal alteration index 42.8 consistentwith an early oil window maturity (Arouri et al., 2000b).

2.1.2. Neoproterozoic carbonate successions ofSpitsbergen (Svalbard) and East GreenlandA suite of samples representing different formations,

lithologies and biofacies were provided by ProfessorAndrew Knoll, Harvard University and have been thesubject of detailed sedimentologic, micropaleontologicand chemostratigraphic analysis (e.g. Knoll et al.,

1986a,b,1990,1991; Green et al., 1987,1988; Swett andKnoll, 1989). Geological and palynological data of thesesamples are shown in Table 1 along with Knoll’s sample

keys and description.The Spitsbergen and East Greenland sediments from

the 700 to 800 Ma Neoproterozoic carbonate succes-

sions are renowned for their distinctive benthic micro-fossils (Green et al., 1987, 1988; Knoll et al., 1986a). Inparticular, the upper Eleonore Bay Group, which com-

prises about 4000 m of sandstones, shales and carbo-nates, contains an unusual fossil assemblage in a 1200 munit known as the Limestone-Dolomite Series, or LDS(Green et al., 1988). Within the LDS, Bed 18 consists of

oolite and pisolite beds 10–50 cm thick intercalated withother sediments and with a range of sedimentologicalfeatures suggesting a shallow, sub-tidal to intertidal

332 P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346

coastal regime. There are mat clasts, stromatolites andflake conglomerates as well as fossiliferous ooids and

pisoids. Cyanobacterial fossils represented by fila-mentous and coccoid forms dominate although thereare also protists such as Melanocyrillium present. Of

particular note are numerous epilithic, interstitial andendolithic taxa that colonised the sediments, some ofwhich (e.g. Polybessurus bipartitus) resemble forms thatcan be found today in the Bahamas (Green et al., 1987).

2.1.3. Victor Bay Formation, Baffin Island (Canada)The Victor Bay Formation occurs within the thick

(�6000 m), and relatively undeformed, sedimentarypackage of the Bylot Supergroup on Northern BaffinIsland and nearby Bylot Island (Kah et al., 1999). The

ages of these rock units are not well constrained but,based on regional correlations and recent Pb isotopegeochronology dating of sedimentary carbonates, a

depositional age of approximately 1200 Ma has beensuggested (Kah et al., 2001). The Victor Bay Formationtogether with the underlying Society Cliffs and overlyingAthole Point formations, comprise �1500 m of pre-

dominantly platform, shelf and slope carbonates. TheVictor Bay Formation itself contains a variety of car-bonate and siliclastic lithologies representing peritidal

shelf to subtidal shelf and slope environments. Unlikethe younger sediments from Svalbard and East Green-

land, the Baffin Island units are fossil-poor and havecarbonates with d13C values in a relatively narrow rangebetween +3.5 and �1.0, lacking the extreme excursions

that characterize the Neoproterozoic. Two samples,AGSO #4193 and AGSO #4194, from the Victor BayFormation were analysed (Table 1).

2.2. GC–MS analysis

Saturated hydrocarbon fractions were obtained using

standard Soxhlet extraction (i.e. 93:7 dichloro-methane:methanol, 72 hrs) and column chromato-graphy procedures (i.e. petroleum ether eluant on silica/

alumina stationary phase) and analysed by GC–MSusing full scan and selected ion recording methods. Sum-mons et al. (1995) and references therein give further

details of procedure for the isolation of the aliphatic frac-tion. The GC–MS analyses were performed using either(1) a Hewlett-Packard 5890II gas chromatograph equip-ped with a split/splitless injector and a 25 m� 0.32 mm i.d

� 0.52 mm film thickness DB5 column interfaced to anUltimaQ Autospec double focusing mass spectrometer, or(2) an Agilent 6890 GC equipped with an on-column

Table 1

Sample details for sediments from Neoproterozoic carbonate successions of Spitsbergen, East Greenland and Baffin Island samples

Sample nos.

Well/formation L ithology and biofacies

AGSO #3912

86-P-13 Draken T idal flat carbonates, muds, microbial mats with occasional thin shales. Mats deposited

during relatively quiet periods have been ripped up and washed by storms into adjacent

lagoons and then silicified (Knoll and Swett, 1990; Knoll et al., 1991). Diverse and

well-preserved microflora

AGSO #3913

86-P-152 Elbobreen B lack shale near lower boundary of formation. Chuaria compressions and vase-shaped

microfossils. This is a relatively deep basinal facies associated with maximum flooding.

AGSO #3914

86-P-105

Backlundtoppen

P

d

artially silicified laminated dolomicrite, within predominantly oolitic unit from a storm

ominated shelf environment (Knoll and Swett, 1990)

AGSO #3915

86-P-9 Draken B lack chert nodules in dolomitic micrite. part of peritidal facies package

AGSO #3916

86-M-43 Elbobreen B lack shale, part of shallowing upward series above basal flooding event (86-P-152, above);

still below fairweather wave base

AGSO #3917

86-F-5 Kingsbreen B lack shale in siliciclastic, storm-dominated shelf succession

AGSO #3919

86-G-68 Svanbergfjellet T he sample is a silcified tidal flat microbialite from the lower part of the formation

AGSO #3920

86-P-42 Backlundtoppen L ower half of the formation in oolite dominated, upper half mostly stromatolites, with

flooding by siliciclastics at top

AGSO #3921

83-E-19-5 Bed 18, B ed 18 of Limestone–Dolomite Series (Green et al., 1987). Silicified dolosiltite—

interbedded with fossiliferous oolites

AGSO # 3922

86-P-66 Grusdievbreen B lack calcisiltite/micrite in carbonate dominated inner shelf succession

AGSO #3923

86-G-13 Svanbergfjellet S ilicified black microbially laminated limestone—peritidal to inner shelf in the lower

Svanbergfjellet Fm

AGSO #3924

86-ST-5 Tokammane B lack shale, near maximum flooding above phosphatic condensed interval—mid-shelf

environment, with an abundance of acritarchs (Knoll and Swett, 1987)

AGSO #4185

P-6450 Elbobreen C alcareous black shale just below oolitic dolostone, near top of shallowing upward

Elbobreen sequence, noted above

AGSO #4193

87-SS-172 Victor Bay B oth samples are thinly intercalated black shales and flat, mechanically laminated micrites

and calcisiltites deposited in deepening upward subtidal shelf environment (Kah et al., 1999)

AGSO #4194 87-SS-225 Victor Bay

P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346 333

injector and a 60 m � 0.32 mm i.d. � 0.25 mm filmthickness DB1 column interfaced to an Agilent 5973mass selective detector (MSD). A typical gas chromato-graph oven temperature program was an initial 40 �C

(held for 2 min) raised at 2–6 �C/min to 300 �C (held for25 min). Mass spectral parameters included a full scanmass range of 50–550 Da or selected ion data for 14n+1

Da, where n=6–14 (i.e., 85, 99. . . 197 Da), an electronenergy of 70 eV and scan speeds 41 s/decade.

3. Results and discussion

At least two different types of methyl alkane serieswere detected in the Neoproterozoic-aged sedimentsfrom each of the represented formations. The totalion chromatogram (TIC) and several reconstructed ion

chromatograms of the saturate fraction of a TananaFormation sample (1234 m deep) from the Munta-1 wellare shown in Fig. 1. Microscopic and molecular

analyses distinguished three distinct organic facies inTanana sediments (Arouri et al., 2000a), and the 1234m, Munta-1 sample is representative of Microbial Mat

Facies 2 (MMF2). The TIC comprises a large unre-solved complex mixture (UCM) from which resolvedC16–C20 n-alkanes are only partially distinct from the

baseline (Fig. 1a) due to their relatively low abundance.The major resolved compounds in the TIC form twodistinctive series which progress by addition of C3 units,from C18 to at least C30. These unusual C3n hydro-

carbons (series E and F) elute at much earlier retentiontimes than n-alkanes of equivalent carbon number. Sev-eral methyl alkane series with strong even or odd carbon

number predominance (i.e., C2n series A–D) were alsodetected and resolved by subtraction of related ion pairs(Fig. 1b–e).

Hydrocarbons of series A–E, but not F, were detectedwith varied relative abundances in the Spitsbergen,E. Greenland and Baffin Island sediments.

3.1. GC–MS features of series A–D branched alkanes

3.1.1. Relative retention factors

Relative retention factors (RRF), described by Kissinet al. (1986), provide a numerical representation of theGC elution characteristics of branched alkanes com-

pared to n-alkanes of the same carbon number. They arecalculated by the following equation:

RRFðXnÞ ¼KFXn�nð100Þ

100

Where,

KFðXnÞ ¼ 100ðn� 1Þ þ100ðRTXn � RTCn�1Þ

RTCn � RTCn�1

RT=retention time, X=branched alkane; Cn=normalalkane with n carbons.The RRF values of series A–D as measured on a DB5

column are listed in Table 2 and plotted in Fig. 2. The

negative values reflect their early elution compared withn-alkanes of the same carbon number and this can beattributed to alkyl-branching which reduces the struc-

tural volume of the hydrocarbon and the interaction ithas with the GC column phase. The hydrocarbonswithin each A–D series show a linear trend of RRF

values indicative of homologous or pseudo-homologousseries. Series C (��1.59) and D (��1.75) have notice-ably more negative RRF values than A (��0.85) and B

(��0.95) indicating a relatively higher degree ofbranching in the former (Fig. 2).

3.1.2. Mass spectral characteristics

The mass spectra of several members of series A

are shown in Fig. 3. The carbon number of these(and series B–F) hydrocarbons was assigned on the

basis of a corresponding odd electron/even mass par-ent typically detected in very low abundance. Theseries A hydrocarbons are characterized by a major

peak at 127 Da (C9H19+). This ion is consistently

higher in abundance than the 113 Da ion, an unusualfeature for acyclic aliphatic hydrocarbons which for-

tuitously provides for selective recognition of the ser-ies via the 127–113 Da subtracted ion chromatogram(Fig. 1b).The chromatographic and mass spectral data repor-

ted here are almost identical to those published for aseries of odd carbon numbered alkanes detected insediments of various ages and paleoenvironments (see

Table 3). Largely based on mass spectral interpretation,these compounds were consistently and incorrectlyassigned as 3, 7-dimethylalkanes (Structure I, Appendix)

or 3, (o-8)-dimethylalkanes (e.g., Dill et al., 1988; Fla-viano et al., 1994). Closer consideration of their massspectra and the synthesis and analytical correlation ofone member of the series by Kenig et al. (2002, 2003)

demonstrated that the previous characterizations wereerroneous and that the series are actually 5,5-diethyl-alkanes (II).

The quaternary carbon containing fragment (C9H19+)

from the 5, 5-diethylalkanes (II) gives rise to an abun-dant 127 Da ion, and the presence of two ethyl groups

accounts exactly for the [M�29]+ ion (where M=par-ent ion MW) being twice as abundant as the [M�57]+

fragment ion (i.e. only one butyl group). Furthermore,

whilst hydrogen rearrangement can accompany frag-mentation adjacent to a quaternary carbon, it does nottypically occur to such an extent that the even mass/oddelectron fragment ion is of similar or higher abundance

to its corresponding odd mass/even electron fragmention at plus one Da (Kenig et al., 2002). The previouslyproposed 3,7-dimethylalkanes would probably yield

334 P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346

Fig. 1. (a) Total ion chromatogram; and (b–e) reconstructed chromatograms from the GCMS analysis of the 1234 m Munta-1

saturate fraction. Series of A–F branched alkanes are indicated. Relative abundances (cf. to TIC) are indicated in italics. The

numbered peaks in (a) correspond to n-alkanes and the peaks labelled as E and F are C3n branched alkanes.

P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346 335

very similar mass spectra to 5,5-diethylalkanes withlarge 127 Da, [M�57]+ and [M�29]+ fragment ionsanticipated. However, a further prominent fragment ion

would be expected at [M�99]+ from C6 to C7 cleavageadjacent to the C7-methyl group in the 3,7-dimethyl-alkanes (Kenig et al., 2003).

The 141–127 Da subtracted chromatogram shown inFig. 1c reveals another series of branched alkanes (heredenoted series B) characterized by a prominent C10H21

+

fragment ion (141 Da). This series comprises only even-carbon numbered members from C18 to C32. The massspectra of three members of this series are shown in

Table 2

Relative retention factors (RRF) for series A–F branched alkanes measured with a DB5 phase capillary column. DE=diethyl;

B,E=butyl,ethyl

Carbon nos.

Series A 5,5DE- Series B 6,6DE- Series C 5B,5E- Series D 6B,6E- Series E Series F

18

�2.79 �2.42

19

20

�1.07

21

�0.95 �1.58 �3.88 �3.45

22

�1.08 �1.75

23

�0.94 �1.60

24

�1.02 �1.74 �4.97 �4.50

25

�0.89 �1.61

26

�1.02 �1.73

27

�0.86 �1.61 �6.05 �5.57

28

�0.98 �1.76

29

�0.83 �1.58

30

�0.97 �1.78 �6.94 �6.45

31

�0.80 �1.57

32

�0.94 �1.70

33

�0.77 �1.57

34

�0.91 �1.74

35

�0.73 �1.57

36

�0.76 �1.73

Fig. 2. Relative retention factor (RRF) values (Kissin et al., 1986) of series A–F branched alkanes. HBIs=Highly branched

isoprenoids.

336 P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346

Fig. 4. The large 141 Da, [M�29]+ and [M�71]+ frag-ment ions are consistent with a 6, 6-diethylalkane struc-

ture (III). Note the [M�29]+ fragment ion derived fromethyl groups is double the abundance of the [M�71]+

fragment ion derived from the one pentyl group.

Based on an understanding of these features of themass spectra, we were able to search for other series

using subtracted ion chromatograms. The 155–141 and169–155 Da subtracted chromatograms shown in Fig. 1dand e, respectively, reveal two additional series (denoted

Fig. 3. 70 eV mass spectra of (a) C21H44, (b) C23H48 and (c) C25H52 members of series A (5,5-diethylalkanes) detected in the 1234 m

Munta-1 saturate fraction.

Table 3

Geological and biological details of samples in which the 5,5-diethylalkanes (or incorrectly assigned 3,7-dimethylalkanes) have been

previously detected

Sample details

Reference

Precambrian (2.2 Ga) Shungit Fm (USSR)

Mycke et al.. 1988

Precambrian (1.64 Ga) Barney Creek Fm (Australia)

Logan et al., 2001

Precambrian (1.5 Ga) Mt Isa (Australia)

Mycke et al., 1988

Neoproterozoic Pertatataka and Tanana Fm (Australia)

Logan et al., 1999; Arouri et al., 2000

Permian clastic and vocaniclastic sediments

Dill et al., 1988

Lower Toarcian Dortrien Formation (France)

Flaviano et al., 1994

Cenomanian–Turonian shales of Canada

Kenig et al., 2002

Holocene cyanobacterial mats

Kenig et al., 1995

Chlorella marina (resistant biopolymers from the green microalga)

Derenne et al., 1996

Norcardia opaca (heterotrophic bacterium)

Flaviano et al., 1994

Water column particulates at ODP site 1026B in the vicinity of

undersea volcanic vents of the central Juan de Fuca Ridge

Kenig et al., 2002

P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346 337

here as C and D) of branched alkanes showing strongcarbon preference. Series C is comprised of only oddC-numbers and series D only even C-numbers. The mass

spectra of the C23, C25 and C27 members of series C areshown in Fig. 5. Consistently abundant fragment ionsare observed at 155, [M�29]+ and [M�57]+ Da. Fur-thermore, the [M�57]+ fragment was twice as abundant

as the [M�29]+ fragment, consistent with this logic anda 5-ethyl-5-butylalkane structure (IV).The concentration of the series D compounds were low

and their mass spectra (not shown) of poorer quality.Nevertheless, several consistent features were still observedincluding prominent 169 Da, [M�29]+, [M�57]+ and

[M�71]+ ions of approximately equal abundances sug-gesting a 6-butyl-6-ethylalkane structure (V).The series B–D assignments are based solely on mass

spectral characteristics and remain tentative. More defi-nitive structural information awaits analytical compar-ison with isolated products or synthetically preparedanalogs. The abundance of the respective subtracted

chromatograms highlighting the C2n series was always A> B > C > D, suggesting that additional related series(e.g. characterized by prominent 183–169, 197–183 Da

chromatograms) may exist below the detection limit ofthe present analytical approach.

3.1.3. d13C values of 5,5-dimethylalkanesThe 5,5-diethylalkanes (series A) were detected in the

saturate fraction of several sediments with sufficientabundance and appropriate GC resolution to permit

compound specific isotope analysis (CSIA). The d13Cvalues of the 5,5-diethylalkanes, n-alkanes and also5-ethylalkanes, all detected in AGSO sample #3921 (Bed

18), are shown in Fig. 6. The d13C values of the C23–C295,5-diethylalkanes were generally in the range �35 to�38 % and 5 to 8 % more depleted than the corre-

sponding odd carbon numbered n-alkanes (�30 to �32%). The d13C n-alkane profile resembles a saw toothpattern with the even carbon numbers 1 to 2 % more

depleted than odd carbon numbers. Interestingly, thed13C of the 5-ethylalkanes (assigned on the basis of MSand GC characteristics) were 3–4 % more depleted thanthe odd-numbered n-alkanes and 1–4 % more enriched

than the 5,5-diethylalkanes, suggesting a trend ofincreased 13C depletion with an increased degree of ethylsubstitution.

Fig. 4. 70 eV mass spectra of (a) C22H46, (b) C24H50 and (c) C26H54 members of series B (6,6-diethylalkanes) detected in the 1234 m

Munta-1 saturate fraction.

338 P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346

3.2. Geochemical significance of series A–D diethyl- andbutyl-, ethyl-alkanes

The C2n features of odd or even carbon number pref-erence and butyl and/or ethyl substitution observed forthe A–D series of branched hydrocarbons is consistentwith the biosynthetic elongation of a specific precursor

by acetate addition as is the case with other poly-methylenic lipid classes. They may derive from thedefunctionalisation of specific precursors, for example

biolipids containing carboxyl or hydroxyl groups.Methyl substitutions at specific chain locations arecommon among bacterial lipids, fatty acids for example

having preferential methylation sites at C-2, C-3 andC-10 (e.g. Dowling et al., 1986; Pond et al., 1985; Kosteret al., 1999). A biological origin is more likely than for-

mation via geosynthetic processes (e.g., thermal crack-ing of a longer precursor molecule), chemosynthesis orcatalyzed rearrangement of n-alkenes during catagenesis(Logan et al., 1999), all of which would contribute to

random carbon number and isomer distributions.Similar distributions of 5,5-diethylalkanes and the

previously reported, but incorrectly assigned, 3,7-

dimethylalkanes have been reported in a wide variety ofsamples ranging from extant biopolymers to Paleopro-terozoic sediments known to contain microbial mats

(Table 3). The series B–D branched alkanes have not beenso widely reported although Simons et al. (2002) recentlyreported B and C in Cenomanian–Turonian sediments.In the Tanana Formation sediments (Arouri et al.,

2000a), the seriesA–D branched alkanes were consistentlydetected in moderate to high abundance inMicrobial MatFacies 1 (MMF1) and occasionally in minor to trace

amounts in MMF2. Other hydrocarbon markers andfeatures diagnostic of specific bacterial types were occa-sionally detected in MMF1 sediments. For example,

prominent C16–C20 monomethylalkanes and 2a- and3b-methylhopanes (Summons and Jahnke, 1992; Sum-mons et al., 1999) are indicative of input from cyano-

bacteria and/or methanotrophs. Progression beyond C20of the monomethylalkane series, an even carbon numberpreference at the high MW end and the odd carbon siteof branching are features consistent with distributions

of mid-chain branched alkanes attributed to sulfide oxi-dizing bacteria (Logan et al., 1999). Additionalmolecular evidence for algae (e.g., even carbon-

Fig. 5. 70 eV mass spectra of (a) C23H48, (b) C25H52 and (c) C27H56 members of series series C (5-butyl, 5-ethylalkanes) detected in the

1234 m Munta-1 saturate fraction.

P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346 339

numbered n-alkylcyclohexanes—Fowler et al., 1986;Gelin et al., 1994), other photoautotrophic organisms

(isoprenoids <C22) and microbial reworking (highUCM) was occasionally evident in MMF2 sediments(Arouri et al., 2000a).

These particular biomarker features, however,showed no apparent correlation with the distributionand relative abundances of the series A–D branched

alkanes. Furthermore, collectively the C2n pseudo-homologous nature of A–D, their wide carbon numberrange (i.e., > C20), distinct branching pattern and thedepleted 13C composition of A compared to co-occur-

ring n-alkanes have no direct and unambiguous matchwith known classes of lipid or hydrocarbon productsfrom other organisms.

The A–D branched alkane series were prevalentthroughout the Neoproterozoic sediments from Spits-

bergen and East Greenland as well as the older Meso-proterozoic sediments of Baffin Island. Table 4comprises a qualitative estimate of the relative abun-

dance of series A and B in the saturate fraction of thesesediments as well as a semi-quantitative measure of themost abundant member of each series as represented by

the respective ion chromatograms diagnostic of eachseries (i.e. 127 Da for A and 141 Da for B). The ionchromatograms typically showed the series to have aunimodal distribution. The largest branched alkane of

each series is expressed as a % relative to the neighbor-ing n-alkane (Table 4). The diagnostic ion of therespective branched alkane series was used to measure

Fig. 6. (a) d13C values of 5,5-diethylalkanes (series A; &) 5-ethylalkanes (~) and n-alkanes (^) from the CSIA analysis of the

saturate fraction of AGSO #3921; (b) Partial total ion chromatogram of AGSO sample #3921 (Bed 18). Peak assignments: e.g.,

22=C22 n-alkane, A23=C23 member of diethylalkane series (A); and X25=C25 member of ethylalkane series.

340 P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346

the abundance of both the branched alkane and neigh-

boring n-alkane, so are biased towards the detection ofthe branched alkane. AGSO sample #3912, for example,contains both series A and B in very high relative

abundances with the 5,5-diethylpentacosane (200% cf.C30 n-alkane based on m/z 127 data) and 6, 6-diethylte-tracosane (80% cf. C29 n-alkane based on m/z 141 data)

being the most abundant members of series A and B,respectively.The relative abundances of the A–D series hydro-

carbons detected in the Spitsbergen and Baffin Island

sediments showed no overt geological or lithologicalassociation, although there was a persistent presence oflow contents of organic matter of apparent benthic

microbial origin. Examination of the broader secularand sedimentary distribution patterns of series A

(Table 3) presents some clues. Their presence in water

expelled from a contemporary sub-sea hydrothermalvent (Axial volcano) and modern to Holocene micro-bial mats (Abu Dhabi) indicates that the source

organism(s) are prokaryotic and presently extant(Kenig et al., 2002). Based on their high relative abun-dances in some samples, the source organisms appar-ently can represent significant concentrations and

quantities of biomass in some kinds of habitat. Thepresence of series A in a deep sea hydrothermal envir-onment discounts photoautotrophs as a likely source.

Reports of their occurrence in Phanerozoic sediments

are rare whereas they do appear to be more abundantin the Proterozoic. Mesozoic occurrences are in blackshales which, in the case of the Cenomanian–Turonian

at least, are attributed to oceanic anoxia (Simons et al.,2002).It has been hypothesized that Earth’s oceans were

largely anoxic at the beginning of the Paleoproterozoic,anoxic and sulfidic by the Mesoproterozoic and largelyventilated by the end of the Neoproterozoic (Canfield,1998; Anbar and Knoll, 2002). During this period there

was a major shift in the redox chemistry of marine sul-fur and nitrogen species and there would have existedsteep redox gradients, concomitant with shifts in spe-

ciation of both S and N in the water column as there arein sediments today. Organisms having some role in thesulfur cycle, or possibly the nitrogen cycle, could have

exploited these redox gradients and been moreprevalent. This would explain an enhanced expressionof their lipids in Proterozoic sediments compared to

Phanerozoic ones where the marine water column wasonly rarely affected by anoxia. A detailed search ofmodern environments where there are steep redoxgradients supporting primary production by microbes is

a plausible approach to finding further examples ofthese lipids and, through the associated biogeochemicalprocesses, the responsible microbes.

Table 4

Relative abundance of C2n (series A and B) and C3n (series E) branched alkanes detected in saturate fractions of Spitsbergen, East

Greenland and Baffin Island sediments

Samples

Series A—5,5-diethyl

alkanes (127 Da)

Series B—6,6-diethyl

alkanes (141 Da)

Series E—C3nalkanes (155 Da)

Relativeabundancea

Largest(cf. n-alk)b

Relativeabundancea

Largest(cf. n-alk)b

Relativeabundancea

Largest(cf. n-alk)b

AGSO #3912 (86-P-13)

++++ A29 (200) +++ B28 (80) +++ E24 (110) AGSO #3913 (86-P-152) ++ A29 (40) + B30 (20) + E21 (40) AGSO #3914 (Biod.c) (86-P-105) +++ A29 (210) + B30 (100) ++ E21 (740)c

AGSO #3915 (86-P-9)

++++ A29 (210) ++ B28 (70) ++ E24 (200) AGSO #3916 (86-M-43) + A23 (20) + B28 (30) ++ E27 (140) AGSO #3917 (86-F-5) +++ A29 (320) ++ B28 (100) +++ E27 (60)

AGSO #3918 (Biod.c) (86-F-6)

+ A29 (20) ND � ++ E30 (30) AGSO #3919 (86-G-68) + A23 (10) ND � +++ E24 (90)

AGSO #3920 (Biod.c) (86-P-42)

+ A23 (3) ND � ++ E24 (690)c

AGSO #3921 (83-E-19-5)

++++ A27 (140) ++ B28 (80) + E27 (70) AGSO #3922 (86-P-66) ++++ A27 (220) ++ B28 (80) + E27 (30)

AGSO #3923 (86-G-13)

++++ A29 (320) ++ B28 (100) + E27 (60) AGSO #3924 (86-ST-5) + A25 (40) ND � + E30 (30) AGSO #4185 (P-6450) ++ A27 (50) + B28 (20) ++ E24 (60)

AGSO #4193 (87-SS-172)

++++ A25 (250) ++ B28 (160) ++++ E27 (790) AGSO #4194 (87-SS-225) ++++ A25 (220) ++ B28 (180) ++++ E27 (430)

ND=not detected.a +, ++, +++, ++++ reflects relative abundance.b % Compared with the n-alkane eluting just prior to the most abundant branched alkane. An=Series A branched alkane with n

carbon numbers.c Saturate hydrocarbon distribution shows evidence of biodegradation.

P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346 341

3.3. GC–MS of C3n series E and F branched alkanes

3.3.1. Relative retention factorsThe series E and F compounds detected in Tanana

sediments elute at very early retention times comparedto n-alkanes of corresponding carbon number indicatinga very high degree of branching. The RRF values of E

and F are extremely negative and decrease with increas-ing MW (Table 2, Fig. 2). These values are of a similarorder to those reported previously for highly branched

isoprenoids (HBIs), notwithstanding that the latter weremeasured on columns with different stationary phases(Barrick et al., 1980; Prahl et al., 1980). HBIs (e.g.,VI and

VII of Appendix) are ubiquitous in Cenozoic to modernmarine sediments and are some of the most highly methylsubstituted compounds known (Robson and Rowland,1986; Rowland and Robson, 1990). The RRF profiles of

E and F are more negative than the HBI series suggestingthey exhibit an even higher degree of branching. Thedecrease in the RRF values with increasingMW indicates

increased branching with each C-3 addition.

3.3.2. Mass spectral characteristicsThe mass spectra of the C18, C21 and C24 members of

hydrocarbon series E are shown in Fig. 7 and reflect adifferent style of branching. The series F hydrocarbons

detected in the Tanana sediments at slightly longerretention times showed almost identical mass spectra. Amolecular ion, albeit of low abundance, is evident in

each of the mass spectra allowing for carbon numberassignment. The classical odd mass [CnH2n+1]

+ frag-mentation pattern typical of normal alkanes (i.e. 57,

71, 85, 99. . . Da ) is prominent in each mass spectrum,but few other distinctive fragment ions were detected.The essentially featureless mass spectra (and very early

elution compared to n-alkanes) would be consistentwith a highly branched structure. The facile loss ofmethyl groups would give rise to a parent ion of lowabundance and a proximity of methyl groups would

impede diagnostic odd mass (even electron) and evenmass (odd electron) fragment ions (Nelson et al., 1975;Pomonis et al., 1980; Kenig et al., 1995; Koster et al.,

1999).

Fig. 7. 70 eV mass spectra of (a) C18H38, (b) C21H44 and (c) C24H50 members of series E highly methylated alkanes detected in the

1234 m Munta-1 saturate fraction.

342 P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346

3.4. Geochemical significance of series E and F highlybranched hydrocarbons

Neither of the E and F hydrocarbons are known in

extant organisms or modern organic sediments and nobiological reason has been identified for their occurrencein ancient sediments. However, such a sequence of

compounds that are systematically elongated by C-3units is almost certainly a consequence of biosynthesis.The C3n branched alkanes were detected in only a

small number of samples associated with microbial matfacies 2 (MMF2) of the Tanana Formation. They werenot detected in completely autochthonous MMF2 sedi-

ments nor the other organic facies (Arouri et al., 2000a).This mat facies was originally deposited in shallow-water extending from the peritidal shelf to upper slope-apron zones. The rocks containing these mats can be

distinguished into light and dark colored sediments, thedarker color due to autochthonous mats together withother kerogen fragments pervading the abundant silt

laminae, but E and F were occasionally detected insediments of both colors. Some MMF2 sediments showbiomarker evidence for cyanobacteria (e.g., C16, C18 and

C20 mid-chain monomethylalkanes) and/or thermo-acidophilic bacteria (C17 and C19 alkylcyclohexanes; deRosa et al., 1972; Oshima and Ariga, 1975). Pyrite

indicative of sulfate reducing bacteria was also detectedin some darker colored sediments. Again, however,none of the potentially diagnostic molecular featuresshowed any firm correlation with the distribution of E

and F throughout the MMF2 sediments.Series E, but not F, branched alkanes were detected in

all of the Spitsbergen and Baffin Island samples (Table 4).

The relative abundances of E varied from very high(AGSO# 3912, 3917, 1919) to low (AGSO# 3913, 3921,3922, 3923, 3924) in the Spitsbergen sediments and was

very high in both Baffin Island sediments (AGSO #4193and #4194). The saturate fractions of several Spitsbergensamples (AGSO# 3914, 3918, 3920) showed evidence ofbiodegradation which enhances the abundances of low

MW series E hydrocarbons relative to the n-alkanes oflower carbon numbers eluting just prior to them (Table 4).

4. Conclusions

Pseudo-homologous series of odd carbon numberedC17–C35 5,5-diethylalkanes (series A) and 5-butyl, 5-ethyl-alkanes (C); and even carbon numbered C16–C34 6,6-

diethylalkanes (B) and 6-butyl, 6-ethylalkanes (D) areprevalent in Neoproterozoic sediments associated withbenthic microbial mats. Two additional series denoted E

and F, with a different and presently unknown style of

branching, have a more restricted distribution. The identi-fication of A as 5,5-diethylalkanes is based on the Keniget al. (2002) synthesis of the C19 homologue while the

other assignments are tentatively based on interpretationof mass spectral data and relative gas chromatographicretention times. Further work, such as comparisons withadditional authentic standards prepared by synthesis, is

needed to confirm these assignments while the structureelucidation of the E and F compounds will likely requireisolation and further spectroscopic investigation.

The presence of these hydrocarbons in very complexorganic matrices, such as with microbial mat-derivedorganic matter, makes them difficult to detect unless

specific ion profiles or subtracted ion chromatogramsare examined closely. The paucity of reports of theirexistence is probably a consequence of this and their

generally low abundance in Phanerozoic sediments. Useof the diagnostic subtracted ion chromatograms employedhere to detect the C2n series A–D hydrocarbons maylead to further examples of their natural occurrence and,

consequently, more clues about possible source organisms.Identification of the series E and F hydrocarbons is

even more problematic due to a lack of diagnostic mass

spectral features and this has almost certainly con-tributed to the infrequency of their detection.The series A–F branched hydrocarbons appear to

reflect inputs of organic matter from prokaryoticmicrobes although their exact sources remain enigmatic.The C2n distributions of A–D show no direct correlation

with the C3n distributions of E and F in any of theTanana, Spitsbergen and Baffin Island formations and,as a consequence, the various series may be proxies fordifferent kinds of physiologies.

Possible source organisms are specific bacteria thatexploit the redox gradients that prevail in hydrothermalsystems and anoxic and/or euxinic environments and

include organisms involved in either the sulfur or nitrogencycles.

Acknowledgements

The authors gratefully acknowledge the provision of

samples and contextual information by ProfessorAndrew Knoll, Harvard University. Analytical assis-tance for the extraction, purification and isotopic ana-

lysis of hydrocarbon fractions was provided by JanetHope, Geoscience Australia. Reviews by Steve Rowlandand an anonymous reviewer greatly improved an early

draft of the manuscript. PFG and GAL publish with thepermission of the CEO of Geoscience Australia.

Associate Editor — Steve Rowland

Appendix

Representative molecular structures: (I) 3, 7-dimethyl-nonadecane; (II) 5, 5-diethylheptadecane (=a member

P.F. Greenwood et al. / Organic Geochemistry 35 (2004) 331–346 343

of series A) (III) 6, 6-diethylheptadecane; (IV) 5-butyl-,

5-ethylheptadecane; (V) 6-butyl-, 6-ethylheptadecane;(VI) 2,6,10-trimethyl-7-(3-methylpentyl)-dodecane (C20

HBI); (VII) 2,6,10,14,18-pentamethyl-7-(3-methylpen-tyl)-nonadecane (C30 HBI).

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