Organic Geochemistry 42 (2011) 409–424

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Organic Geochemistry

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New triaromatic steroids distinguish Paleozoic from Mesozoic oil

Silvana M. Barbanti a,⇑, J. Michael Moldowan b, David S. Watt c, E. Kolaczkowska d

a Petrobras Research & Development Center, Cidade Universitária, Rio de Janeiro, RJ 21941-598, Brazilb Stanford University, School of Earth Sciences, Department of Geological & Environmental Sciences, Stanford, CA 94305-2115, USAc Department of Cellular and Molecular Biochemistry, University of Kentucky, Lexington, KY 40536, USAd Warsaw University of Technology, Faculty of Chemistry, ul. Noakowskiego 3, 00-664 Warszawa, Poland

a r t i c l e i n f o

Article history:Received 26 June 2010Received in revised form 30 January 2011Accepted 18 February 2011Available online 23 February 2011

0146-6380/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.orggeochem.2011.02.007

⇑ Corresponding author at: Integrated Petroleum ExJaneiro, RJ 22280-030, Brazil. Tel.: +55 21 3622 9500;

E-mail address: silvana.barbanti@ipexco.com.br (S

a b s t r a c t

Certain biomarkers in petroleum provide information on of the geologic age of its source rock and thesecan also be used to track the evolution, radiation and relationships between taxa and as proxies for paleo-environmental reconstruction. Triaromatic 23,24-dimethylcholesteroids (TA-DMC), which undoubtedlyderive from 23,24-dimethylcholesterols in dinoflagellates, haptophytes and diatoms, provide a usefulparameter to distinguish Paleozoic from Mesozoic and younger oil and rock extracts at higher specificitythan previously proposed biomarkers. In this report, we present a study of the relative abundance of anovel series of TA-DMC in source rock extracts and crude oil from different global localities and ages.Based on their taxon specificity for important primary producers, such biogeochemicals also have impli-cations for paleoenvironmental studies, including paleoclimate and global change, such as radiations fol-lowing the Permian–Triassic boundary mass extinction.

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1. Introduction

A growing number of studies have shown the utility of steroidsas indicators of geologic age (Moldowan et al., 1985, 1996, 2001;Summons et al., 1992; McCaffrey et al., 1994; Holba et al., 1998),which is particularly useful to constrain the age for petroleum thathas not been rigorously correlated to a source rock.

Steranes correlated to specific organisms (termed taxon spe-cific, Moldowan and Jacobson, 2000) are often found in crude oilwhere their presence sometimes allows a geological age to be in-ferred (Moldowan, 2000). For example, Holba et al. (1998) reportedthat 24-norcholestanes (1, see Fig. 1 and Table 1 for numberedstructures) are more prominent in some oil samples derived fromJurassic, Cretaceous and Tertiary source rocks than oil fromolder ones, which is consistent with a possible origination fromdiatoms. 24-n-Propylcholesterols are present in members of anorder (Sarcinochrysidales) of chrysophyte algae and the relatedhydrocarbons (2) are specific indicators of marine input fromPrecambrian to present (Moldowan et al., 1985, 1990). 24-iso-Propylcholesterols are present in Demospongiae (McCaffrey et al.,1994; Love et al., 2009) and the derived taxon specific 24-iso-propylcholestanes (3) are particularly abundant in some Ediacaranto Ordovician oil and extracts (Peters et al., 2005). Dinosteranes(4, 4a,23,24-trimethylcholestanes) and the structurally related

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.M. Barbanti).

triaromatic 4,23,24-trimethylcholestanes (5, TA-dinosteroids) canbe used to distinguish Mesozoic from older petroleum (Moldowanet al., 1996, 2001). Their precursor dinosterols have been widelyreported in dinoflagellates (Withers, 1987), the fossil cysts ofwhich show a history ranging from Triassic to present. The earlyorigin of dinoflagellates has been discussed (e.g., Fensome et al.,1996) with some additional biogeochemical evidence from acri-tarch geochemical studies (Moldowan and Talyzina, 1998; Talyzinaet al., 2000). The biochemically related 23,24-dimethylcholesterolshave been found in dinoflagellates, diatoms and haptophytes(Withers, 1987; Volkman et al., 1981a, 1993) (Table 2). Dinoflagel-late and haptophyte remains (cysts and coccoliths, respectively)have similar paleontological records, Triassic to present. 23,24-Dimethylcholesteroids (6) might be expected to show a similardistribution to the dinosteroids because they also occur indinoflagellates. However, the structure of the 23,24-dimethylcho-lesteroids suggest chemistry with additional biosynthetic stepscompared to that of dinosteroids, i.e., the loss of a 4-methyl group.As such, one could speculate that the occurrence of 23,24-dimeth-ylcholesteroids might be expected to be skewed toward youngersediments. This possible age related tendency is supported by thefinding of 23,24-dimethylcholesterols in numerous diatom species(Rampen et al., 2009), which radiate in the Jurassic, whereas4,23,24-trimethylcholesterols have only one known low leveloccurrence in diatoms (Volkman et al., 1993).

This report presents a study of the relative abundance of a novelseries of triaromatic 23,24-dimethylcholesteroids (7, TA-DMC) insource rock extracts and oil samples from different global localitiesand ages.

1 2

3 4

5

7

6

8, X = H9, X = CH310, X = C2H511, X = n-C3H7

X

Fig. 1. Structures of the steroid hydrocarbons cited in text and figures.

410 S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424

2. Materials and methods

2.1. Samples

Two sets of samples were selected for this study in order to testthe correlation of TA-DMC to dinoflagellate, haptophyte and dia-tom evolution and investigate the application of TA-DMC as anage related biomarker; one set of 102 organic matter rich marinesource rocks from Proterozoic to Cretaceous core, sidewall coreand outcrop samples (Table 3) and another set of 186 oil samplesgenerated from marine and non-marine Proterozoic to Tertiarysource rocks (Table 4). A similar set of organic rich marine sourcerocks was previously studied by Moldowan et al. (1996, 2001) in

order to investigate the distribution of triaromatic 4,23,24-trim-ethylcholesteroids (5, TA-dinosteroids) through geologic time.

2.2. Analytical procedures

Aromatic hydrocarbon fractions were prepared from extracts ofrock samples using high performance liquid chromatography(HPLC) as described by Moldowan et al. (1996) and Peters et al.(2005) and from oil samples using medium pressure liquid chro-matography (n-hexane as solvent). Gas chromatography–massspectrometry (GC–MS) analyses of aromatic hydrocarbon fractionswere carried out using an HP 6890 gas chromatograph interfacedto an HP 5973 MSD system using the selective ion monitoring

Table 1Identification of compounds shown in Fig. 1.

Peaka Name Carbonnumber

Structureb

a Triaromatic 23,24-dimethylcholesteroid

28 7

b Triaromatic 23,24-dimethylcholesteroid

28 7

c Triaromatic 23,24-dimethylcholesteroid

28 7

d Triaromatic 23,24-dimethylcholesteroid

28 7

e Triaromatic 23,24-dimethylcholesteroid

28 7

f Triaromatic 23,24-dimethylcholesteroid

28 7

g Triaromatic cholesteroid 20S 26 8h Triaromatic cholesteroid 20R 26 8i Triaromatic 24-methylcholesteroid

20S27 9

j Triaromatic 24-ethylcholesteroid 20S 28 10k Triaromatic 24-methylcholesteroid

20R27 9

l Triaromatic 24-ethylcholesteroid 20S 28 10m Triaromatic 24-n-propylcholesteroid

20Sc29 11

n Triaromatic 24-n-propylcholesteroid20Sc

29 11

o Triaromatic 24-n-propylcholesteroid20R

29 11

a Peak labels used in Fig. 2.b Structures are presented in Fig. 1.c The C29 20S compound is split into a doublet (peaks m and n) consisting of 24S

and 24R epimers (Peters et al., 2005).

S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424 411

mode (ions m/z 231, 245 and 253). GC was carried out on a capil-lary column (60 m � 0.25 mm i.d.) coated with DB-1 (100% methylsilicone, 0.25 lm film thickness). GC condition: initial temperatureof 55 �C for 2 min, followed by heating at 20 �C/min to 150 �C andthen a second heating ramp at 1.5 �C/min to 320 �C for 20 min. Thesix isomers of TA-DMC were identified by co-elution with authen-tic standards (Fig. 2) using two types of capillary columns, DB-1and DB-5.

All the aromatic hydrocarbon fractions, including those from or-ganic matter rich rocks and oil samples, were analyzed under thesame analytical conditions. In addition, some aromatic hydrocar-bon fractions were analyzed by metastable reaction monitoringGC–MS (MRM-GC–MS) in order to confirm the presence or absenceof TA-DMC compounds with higher specificity. Although a hybridmass spectrometer (Autospec-Q, EBE-qQ system, Micromass) wasused the analyses were carried out by metastable reaction moni-toring mode using the field-free region (FFR-1) of the instrument.

2.3. Preparation of the standard

2.3.1. Synthesis of (20S,23f,24f)- and (20f,23f,24f)-23,24-dimethyl-5a-cholestane

The synthesis of (20S,23f,24f)-23,24-dimethyl-5a-cholestane12A and (20f,23f,24f)-23,24-dimethyl-5a-cholestane 12B utilized3b-hydroxy-5a-pregnan-20-one 13 as a starting material and em-ployed an ester-alkylation sequence (Stoilov et al., 1992, 1993,1994; Shetty et al., 1994) to construct the side chain. As shownin Fig. 3, the starting material 13 was deoxygenated at C-3 usinga tri-n-butyltin hydride reduction of the corresponding xanthate14 (Barton and McCombie, 1975; Robins et al., 1983). A Wittigreaction of 15 and hydroboration–oxidation of the resulting alkene16 provided the alcohols 17 as a mixture of C-20 diastereomersthat were separable by chromatography. The C-20S diastereomerof 17 was used to synthesize the target 12A and the mixture of

diastereomers 17 was used to synthesize 12B. For the preparationof either 12A or 12B, the chemistry from this point forward was thesame and therefore only the synthesis of 12B will be described indetail. The alcohols 17 were converted to the correspondingiodides 18 in a standard reaction and the alkylation of 18 withracemic methyl 3,4-dimethylpentanoate (Stoilov et al., 1993)secured the steroidal ester 19. A standard sequence involvingreduction of the ester to the alcohol 20, mesylation and furtherreduction of the mesylate afforded the target sterane, (20f,23f,24f)-23,24-dimethyl-5a-cholestane 12B.

2.3.2. Synthesis of C29 cholestanes2.3.2.1. 3b-Hydroxy-5a-pregnan-20-one phenyl thiocarbonate(14). See Barton and McCombie (1975) and Robins et al. (1983).A solution of 8.97 g (28.2 mmol) of 5a-pregnan-3b-ol-20-one(13) and 7.29 g (42.2 mmol, 5.85 ml) of phenyl chlorothioformatein 9.1 ml of anhydrous pyridine and 165 ml of anhydrous CH2Cl2

afforded 10.2 g (80%) of 14 as pale yellow plates: mp 189–190 �C; IR (CHCl3) 1690 (C@O), 1590 (C@O), 1485, 1290, 1190,1015 cm�1; 1H NMR (CDCl3) d 0.61 (s, 3, C-18 CH3), 0.87 (s, 3, C-19 CH3), 2.12 (s, 3, C-21 CH3), 5.10–5.30 (m, 1, CHOC(S)OPh),7.08–7.5 (m, 5, ArH); 13C NMR (CDCl3) d 11.9, 13.2, 20.9, 22.5,24.2, 26.4, 28.3, 31.3, 31.7, 32.8, 35.2, 35.3, 36.4, 38.8, 44.1, 44.4,53.9, 56.5, 63.7, 84.2, 122.2, 126.6, 129.6, 153.5, 194.8, 210.1.

Anal. Calcd. for C28H38SO3: C, 73.96; H, 8.42; S, 7.05. Found: C,74.04; H, 8.42; S, 7.12.

2.3.2.2. 5a-Pregnan-20-one (15). A solution of 27 g (92.8 mmol) of(n-Bu)3SnH in 200 ml of anhydrous benzene was added dropwiseover 5 h to a refluxing solution of 12.8 (28.1 mmol) of 14 and0.46 g (2.81 mmol) of 2,20-azobisisobutyronitrile (AIBN) in 735 mlof anhydrous benzene. The mixture was refluxed an additional6 h and cooled. The solution was washed successively with 1 MHCl solution, water, 5% NaOH solution, brine and dried over anhy-drous MgSO4. The crude yellow oil was chromatographed on silicagel with 1:3 EtOAc: hexane, and the product was recrystallizedfrom EtOH to give 6.13 g (72%) of 15: mp 134–135 �C. See Janotet al. (1960): mp 136 �C.

2.3.2.3. 20-Methylene-5a-pregnane (16). To a stirred suspension of7.9 g (22.3 mmol) of methyltriphenylphosphonium bromide (driedover P2O5) in 10 ml of anhydrous benzene under a N2 atmospherewere added 13.4 ml (22.3 mmol) of 1.66 M potassium tert-amylatein benzene. The yellow mixture was refluxed for 1 h, and 1.0 g(3.3 mmol) of 15 in 17 ml of anhydrous benzene was added. Theorange mixture was refluxed for 4.5 h and stirred for 12 h at25 �C. The mixture was poured into water and extracted withEt2O. The combined organic layers were washed successively with10% HCl solution, saturated NaHCO3 solution and brine and driedover anhydrous MgSO4. The crude product was chromatographedtwice on silica gel using hexane to give 0.70 g (70%) of 16: mp129–131 �C; IR (CHCl3) 1640 cm�1; 1H NMR (CDCl3) d 0.55 (s, 3,C-18 CH3), 0.78 (s, 3, C-19 CH3), 1.75 (s, 3, C-21 CH3), 4.68–4.72and 4.82–4.86 (m, 2, C-22 vinylic H); 13C NMR (CDCl3) d 12.0,12.7, 20.7, 22.0, 24.0, 24.5, 25.2, 26.7, 28.9, 32.0, 35.7, 36.1, 38.6,38.8, 43.2, 47.0, 54.8, 56.3, 57.3, 110.6, 146.1 (21 lines).

Anal. Calcd. for C22H36: C, 87.93; H, 12.07. Found: C, 87.85; H,12.02.

2.3.2.4. (20R) and (20S)-20-Hydroxymethyl-5a-pregnane (17). To asuspension of 1.83 g (6.1 mmol) of 16 in 6.1 ml of anhydrous THFat 0 �C under a N2 atmosphere were added 2.43 ml (2.43 mmol)of 1 M borane-THF solution via a syringe over a 10 min period.The ice bath was removed and the clear solution was stirred at25 �C for 1 h. The mixture was again cooled to 0 �C and the excessborane was destroyed with 100 ll of H2O. To this solution were

Table 2Occurrence of 23,24-dimethylcholesteroids in organisms and sediments.

23,24-Dimethyl-cholesteroids

Dinoflagellate Other algae Other organisms Sediments

5,22-dien-3b-ol (1) Gonyaulax polygramma (3) marine alga – FCRG 51 (18) soft coral (31) diatomaceous oozea

(2, 4, 6, 20, 39)Zooxanthellae

(12) diatom – Biddulphia sinensis (19) soft coral – coelenterates:alcyonarians – Sinularia polydactyla,Sarcophyta elegansb

(13, 32, 33, 34, 35, 36, 37,40, 42, 46)

(5, 45) Pyrocystis lunula (13, 14) diatom (4, 20) soft coral, gorgonians,coelenterates

(7, 8) Prorocentrum sp. (15, 43) Prymnesiophyceae(Haptophyceae)

(21) sponge – Axinella cannabina

(16) various cultured diatoms (22, 23) sponge – Cinachyra alloclada,Cinachyra kuekenthali

(41) Gymnodinium simplex (48) diatom – Fragilaria pinnata (24) holothurian – Stichopus japonicusSelenka

(44, 45) Symbiodiniummicro-adriaticum

(25, 26, 47) mollusc

(45) Coolia sp.,Prorocentrum sp.,Scrippsiella sp.,Alexandrium andersoni

(27) mollusc – Tridacna gigasc

5,23-dien-3b-ol (19) soft coral – Sarcophyta elegans

5a,7,22-dien-3b-ol (28) Asteroidea

7,22-dien-3b-ol (8) Prorocentrum sp.

5a,22-en-3b-ol (1) Gonyaulax polygramma (3) marine alga – FCRG 51 (24) holothurian – Stichopus japonicusSelenka

(13, 32d, 33, 35e, 36, 37, 40,46)

(4, 39) Zooxanthellae (16) diatom – Ditylum brightwellii(5, 45) Pyrocystis lunula(7) Prorocentrum cordatum(8, 45) Prorocentrum sp.(9) Thoracosphaera heimii(10) Ceratium furcoides,freshwater(44) Gymnodiniumsanguineum andFragilidium sp.(45) Coolia sp.

5-en-3b-ol (1) Gonyaulaxpolygramma

(16) diatom – Haslea ostrearia (47) mollusc (13, 34, 35, 36, 37, 40, 46)

(8) Prorocentrum sp.(41) Gymnodinium simplex

5a,3b-ol (1) Gonyaulax polygramma (38) diatom – Haslea ostrearia (11) bryozoan – Bugula neritinac,f (33, 36, 37, 40)(11) Zooxanthellae –Orbulina universa(6) Zooxanthellae(8) Prorocentrum sp.

5a,3a-ol (11) sponge – Stelletta conulosa

4,22-dien-3-one (5) Pyrocystis lunula

5,22-dien-3b-sulphate (17) haptophyte – Hymenomonas sp.

5,17(20)-dien-3b-ol (45) Alexandrium andersoni (29) soft coral – Sarcophytum glaucum

5,17(20)-diene-3b,25-diol (30) soft coral – Sinularia mayi

5f,22-en-3b-ol (41) Gymnodinium simplex

(1) Volkman et al. (1984). (2) Withers (1987). (3) Nichols et al. (1983). (4) Kokke et al. (1981). (5) Kokke et al. (1982). (6) Withers et al. (1982). (7) Nichols et al. (1984). (8)Volkman et al. (1999). (9) Jones et al. (1983). (10) Robinson et al. (1987). (11) Zielinski et al. (1983). (12) Volkman et al. (1980). (13) Volkman et al. (1981b). (14) Volkman et al.(1993). (15) Volkman et al. (1981a). (16) Rampen et al. (2009). (17) Kobayashi et al. (1989). (18) Kanazawa et al. (1974). (19) Kanazawa et al. (1977). (20) Kelecom et al.(1980). (21) Itoh et al. (1983). (22) Barnathan et al. (1992). (23) Barnathan et al. (2003). (24) Kalinovskaya et al. (1983). (25) Ando et al. (1979). (26) Kerr and Baker (1991).(27) Kerr and Southgate (unpublished results). (28) Sato et al. (1980). (29) Kobayashi et al. (1979). (30) Kobayashi (1994). (31) Wardroper et al. (1978). (32) Cranwell (1982).(33) Shaw and Johns (1985). (34) Shaw and Johns (1986). (35) Wilson et al. (2001). (36) Lee et al. (1980). (37) Volkman (1986). (38) Véron et al. (1998). (39) Bohlin et al.(1981). (40) de Leeuw et al. (1983). (41) Goad and Withers (1982). (42) Fernandes et al. (1999). (43) Marlowe et al. (1984). (44) Mansour et al. (1999). (45) Leblond andChapman (2002). (46) Wakeham (1995). (47) Murphy et al. (2002). (48) Barrett et al. (1995).

a Possibly input from coelenterates.b Kobayashi et al. (1979) reported that the identification of 23,24-dimethylcholesta-5,22-dien-3b-ol is incorrect. They proposed based on PMR spectral properties that the

compound is in fact sarcosterol (23,24-dimethylcholest-5,17(20)-dien-3b-ol).c Unpublished results.d Trace.e Summer season only.f As minor component.

412 S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424

added 4.1 ml (12.2 mmol) of 3 M NaOH solution followed by 4.1 ml(12.2 mmol) of a 30% H2O2 solution. The mixture was initially

viscous and was heated for 3.5 h at 53–54 �C. The solution wascooled and the phases were separated. The aqueous layer was

S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424 413

extracted with Et2O. The combined organic layers were washedwith brine and dried over anhydrous MgSO4. The product waschromatographed on silica gel using 1:5 EtOAc: hexane to give1.43 g (74%) of 17 as a mixture of 20R and 20S diastereomers:mp 158–160 �C; IR (CHCl3) 3615 cm�1; 1H NMR (CDCl3) d 0.67 (s,3, C-18 CH3), 0.78 (s, 3, C-19 CH3), 0.95 and 1.03 (two d, 3,J = 6.6 Hz, C-21R and C-21S CH3), 3.30–3.80 (m, 2, CH2OH).

The 20R and 20S-isomers were separated by MPLC (mediumpressure liquid chromatography) on silica gel using 1:5 EtOAc:hexane. The isomers had the following spectral and physical data:

The 20S-isomer: mp 129.5–131 �C; IR (CHCl3) 3615, 3000–2810,1435, 1370 cm�1; 1H NMR (CDCl3) d 0.67 (s, 3, C-18 CH3), 0.78 (s, 3,C-19 CH3), 0.95 (d, 3, J = 6.6 Hz, C-21 CH3), 3.40–3.80 (m, 2,CH2OH); 13C NMR (CDCl3) d 12.0, 12.2, 16.5, 20.5, 22.0, 23.8, 26.6,27.6, 28.9, 32.0, 35.3, 36.1, 37.8, 38.5, 39.4, 42.2, 46.9, 52.4, 54.6,56.4, 66.8 (21 lines); mass spectrum m/z (rel. intensity) 318 (55),303 (36), 218 (44), 217 (100), 149 (43), 109 (34), 95 (37), 81(39), 67 (35), 55 (41), 41 (33); exact mass calcd. for 318: found318.2924.

Anal. Calcd. for C22H36O: C, 82.95; H, 12.02. Found: C, 82.72; H,12.00.

The 20R-isomer: mp 161–164 �C; IR (CHCl3) 3615 cm�1; 1HNMR (CDCl3) d 0.67 (s, 3, C-18 CH3), 0.78 (s, 3, C-19 CH3), 1.03 (d,3, J = 6.6 Hz, C-21 CH3), 3.30–3.68 (m, 2, CH2OH); 13C NMR (CDCl3)d 11.9, 12.0, 16.5, 20.5, 22.0, 24.1, 26.6, 27.5, 28.9, 32.0, 35.4, 36.1,38.5, 38.6, 39.8, 42.5, 46.9, 52.4, 54.6, 56.3, 68.0 (21 lines); massspectrum m/z (rel. intensity) 318 (30), 303 (29), 218 (45), 217(100), 149 (52), 109 (42), 95 (35), 81 (34), 67 (28), 55 (33), 41(23); exact mass calcd. for 318: found 318.2925.

2.3.2.5. (20f)-20-Iodomethyl-5a-pregnane (18). See Crossland andServis (1970). A solution of 517 mg (1.62 mmol) of (20f)-17 dis-solved in 22 ml of anhydrous CH2Cl2, 247 mg (2.44 mmol, 340 ll,1.5 equiv) of Et3N and 204 mg (1.79 mmol, 138 ll, 1.1 equiv) ofCH3SO2Cl afforded, after stirring at�5 �C for 45 min and employinga standard isolation procedure, the crude mesylate as a mixture ofC-20 diastereomers that was used directly in the next reaction: mp109.5–113 �C; 1H NMR (CDCl3) d 0.67 (s, 3, C-18 CH3), 0.78 (s, 3, C-19 CH3), 0.99 and 1.08 (two d, 3, J = 6.6 Hz, C-21 CH3), 3.00 (s, 3,CH2OSO2CH3), 3.91–4.41 (m, 2, C-22, CH2OSO2CH3).

See Partridge et al. (1974). A solution of 630 mg (1.59 mmol) ofcrude mesylate and 2.38 g (15.9 mmol) of NaI in 31.5 ml of acetoneat the reflux temperature for 8 h afforded, after chromatography onsilica gel using 1:3 EtOAc-hexane, 507 mg (75%) of 18: mp 95–103 �C; IR (CHCl3) 1460, 1445, 1380, 1190, 960 cm�1; 1H NMR(CDCl3) d 0.66 and 0.70 (two s, 3, C-18 CH3), 0.77 (s, 3, C-19 CH3),0.95 and 1.01 (two d, 3, J = 6 Hz, C-21 CH3), 3.11–3.53 (m, 2, CH2I).

Anal. Calcd. for C22H37I: C, 61.67; H, 8.70. Found: C, 61.77; H,8.74.

2.3.2.6. Methyl (20f,23f,24f)-24-methyl-5a-cholestane-23-carboxyl-ate (19). To a solution of 2.7 g (26.7 mmol) of diisopropylamine in5.5 ml of anhydrous THF at �20 �C were added 5.34 ml(13.4 mmol) of 2.5 M n-BuLi in hexane. The solution was stirredat �20 �C for 20 min and cooled to �78 �C. To this solution wereadded 1.92 g (13.4 mmol) of racemic methyl 3,4-dimethylpentano-ate in 2.4 ml of THF over a 15 min period. The mixture was stirredat �78 �C for 40 min, and 572 mg (1.33 mmol) of 18 in 7.2 ml ofTHF and 2 ml of ((CH3)2N)3PO were added dropwise. The mixturewas stirred at �78 �C for 7 h, allowed to warm to 25 �C, and stirredfor an additional 12 h. The mixture was diluted with Et2O andpoured into cold H2O. The aqueous layer was extracted withEt2O. The combined organic extracts were washed successivelywith saturated NH4Cl solution and brine and dried over anhydrousMgSO4. The crude product was chromatographed on silica gelusing a gradient of hexane and 1:20 EtOAc: hexane. A second

chromatography on silica gel using 1:50 EtOAc: hexane afforded445 mg (75%) of 19: IR (CHCl3) 1730 (C@O) cm�1; 1H NMR (CDCl3)d 0.58–0.94 (m, 21, CH3 signals at 0.61, 0.63, 0.72, 0.77, 0.79, 0.83,0.85, 0.88, 0.92), 3.64 and 3.66 (two s, 3, OCH3).

2.3.2.7. (20f,23f,24f)-23-(Hydroxymethyl)-24-methyl-5a-cholestane(20). To a suspension of 120 mg (3.16 mmol) of LiAlH4 in 1 ml ofanhydrous THF at 0 �C under a N2 atmosphere were added445 mg (1.0 mmol) of crude 19 in 5 ml of THF over a 15 min period.The mixture was stirred at 25 �C for 3 h. The mixture was cooled to0 �C and diluted with Et2O. To this solution were added 120 ll ofwater followed by 360 ll of 15% NaOH solution and finally an addi-tional 360 ll of water. The supernatant was decanted and theremaining white precipitate was triturated with of Et2O. The com-bined organic extracts were washed with brine and dried overanhydrous MgSO4. The crude product was chromatographed on sil-ica gel using 1:10 EtOAc: hexane to afford 295 mg (71%) of 20 as amixture of C-20, C-23, and C-24 diastereomers: 1H NMR (CDCl3) d0.62–0.68 (m, 3, C-18 CH3), 0.74–0.96 (9 peaks, C-19 and side chainCH3 at 0.77, 0.81, 0.83, 0.85, 0.86, 0.88, 0.90, 0.92, 0.94), 3.25–3.75(m, 2, CH2OH).

2.3.2.8. (20f,23f,24f)-23,24-Dimethyl-5a-cholestane (12B). SeeCrossland and Servis (1970). A solution of 295 mg (0.71 mmol) of20 in 4.8 ml of anhydrous CH2Cl2, 108 mg (1.06 mmol, 1.5 equiv)of Et3N and 89.3 (0.78 mmol, 1.1 equiv) of CH3SO2Cl afforded acrude mesylate: 1H NMR (CDCl3) d 0.65 and 0.66 (two s, 3, C-18CH3), 0.72–0.98 (C-19 and side chain CH3 at 0.75, 0.77, 0.80, 0.83,0.86, 0.89, 0.91, 0.93, 0.94), 3.00 (s, 3, CH2SO2CH3), 3.75–4.30 (m,2, CH2OSO2CH3). This mesylate was used directly in the nextreaction.

To a stirred suspension of 75.3 mg (1.98 mmol) of LiAlH4 in 3 mlof anhydrous THF under an N2 atmosphere were added 329 mg(0.66 mmol) of crude mesylate in 6 ml of anhydrous THF. The mix-ture was refluxed for 5 h. The solution was cooled to 0 �C and di-luted with 5 ml of Et2O. To this solution were added 75 ll ofwater followed by 215 ll of 15% NaOH solution and finally215 ll of water. The organic layer was separated and the whiteprecipitate was triturated with Et2O. The combined organic layerswere washed with brine and dried over anhydrous MgSO4. Thecrude product was chromatographed on silica gel using hexaneto afford 218 mg (82%) of 12B: IR (CHCl3) 3000–2730, 1445,1465, 1380, 1100 cm�1; 1H NMR (CDCl3) d 0.62–0.94 (m, 21, CH3

signals at 0.64, 0.65, 0.66, 0.70, 0.74, 0.77, 0.79, 0.80, 0.83, 0.85,0.86, 0.89, 0.90, 0.94). GC–MS data confirmed that all eight isomerswere C29H52 compounds.

Anal. Calcd. for C29H52: C, 86.92; H, 13.08. Found: C, 86.94; H,13.02.

2.3.3. Preparation of triaromatic 23,24-dimethylcholesteroid standardSynthetic 23,24-dimethylcholestane (12B) was treated with 5%

Pd/C catalyst in a sealed, evacuated glass tube at 268 �C for 72 haccording to the published procedure (Seifert et al., 1983). Thepyrolysate was recovered and separated into saturated and aro-matic hydrocarbon fractions using HPLC. The yield of aromatic ste-roids was about 73% by GC–MS. The produced TA-DMC standardgave a pattern of six peaks (eight stereoisomeric compounds withtwo co-elutions, Fig. 2D).

3. Results and discussion

3.1. Triaromatic 23,24-Dimethylcholesteroids (TA-DMC)

The conversion of 23,24-dimethylsterols to triaromatic steroidhydrocarbons proceeds with the migration (i.e., C-13b ? C-17b)

Table 3Identification of organic rich marine source rocks and measurements of triaromatic steroid ratios.

SampleID

Country-Region Basin Formation Depth Period Age Biomarker ratiosa

TA-DMC

TA-dinosteroids

A565 (Australia) McArthur Velkerri 181.3 m M. Pro. Ectasian-Tonian 0.000 0.000A564 (Australia) McArthur McMinn 64.0 m M. Pro. Ectasian-Tonian n/a n/aA468 AZ (USA) Colorado Plateau Kwagunt, Chuar Surface E. L. Pro. Cryogenian 0.000 0.000A466 AZ (USA) Colorado Plateau Kwagunt, Chuar Surface E. L. Pro. Cryogenian 0.000 0.069S2 Middle East Central Arabia Huqf >1000 m L. L. Pro. L.V 0.000 0.000A476 Middle East Central Arabia Huqf 4447 m L. Pro. Ediacaran 0.000 0.699C9 MO (USA) ? Bonneterce 2337 ft. Cam. 0.000 0.000333 ND (USA) Williston ? ? Cam. ? 0.000 0.000S3 Middle East Central Arabia Huqf >1000 m E. Cam. 0.000 0.000C151 E. Siberia (Russia) Yudoma-Olenek Kuonamka E. Cam. 0.000 0.000S1 Middle East Central Arabia Huqf >2000 m E. Cam. 0.000 0.000C152 E. Siberia (Russia) Yudoma-Olenek Kuonamka Surface E. Cam. 0.000 0.000A316 E. Siberia (Russia) Yudoma-Olenek Kuonamka Surface? E. M.

Cam.? 0.000 0.000

C156 E. Siberia (Russia) ? ? Surface M. Cam. 0.000 0.000A221 (Australia) Georgina Inca 107.6 m M. Cam. L. Templeton-

Florian0.000 0.028

A220 (Australia) Georgina Currant Bush 22.18 m M. Cam. Florian-Undillian n/a n/aA197 (Sweden) ? Alum Shale 44.88 m M. Cam. 0.000 0.000A198 (Sweden) ? Alum Shale 24.21 m L. Cam. L LA200 (Sweden) ? Alum Shale 22 m E. Ord. Tremadocian 0.000 0.000A199 (Sweden) ? Alum Shale 9 m E. Ord. Tremadocian 0.000 0.000315 (Russia) ? Dictyonema <1000 m E. Ord. ? 0.000 0.000A569 (Australia) Amadeus Horn Valley 227.9 m E. Ord. Floian 0.000 0.000A568 (Australia) Amadeus Horn Valley 217.5 m E. Ord. Floian 0.000 0.000A436 (Australia) Amadeus Horn Valley 216.5 m E. Ord. Floian 0.000 0.000A382 Middle East Central Arabia Hanadir 14.829 ft. M. Ord. ? 0.055 0.363A229 (Australia) Canning Goldwyer 936 m Ord. Dapingian 0.000 0.000A333 IA (USA) ? Glenwood 752.1 ft. Ord. Sandbian 0.000 0.000A030 IA (USA) Decorah-Guttenberg 953.3 ft. Ord. Sandbian 0.000 0.000A330 NV (USA) Basin & Range

ProvinceVinini 8 ft. Ord. Sandbian n/a n/a

A437 IA (USA) ? St. Peter 761 ft. Ord. Sandbian 0.000 0.000A900 (Jordan) ? ? 2576 m Ord. Sandbian 0.000 0.000C25 NV (USA) Vinini Cr Surface M. L. Ord. 0.000 0.000A242 (Libya) Cryenacian Platform ? L. Ord. Sandbian 0.050 0.313A291 (Algeria) Ghadames Rhazziane 3214 m L. Ord. Sandbian 0.000 0.188A289 (Algeria) Illizi ? 2083 m L. Ord. Sandbian 0.000 0.000A332 IA (USA) Maquoketa Shale 461.6 ft. L. Ord. Katian-Hirnantian n/a n/aA241 (Libya) Cryenacian Platform ? E. Sil. Ruddanian-

Telychian0.000 0.280

A383 Middle East Central Arabia Qusaiba 13,505 ft. E. Sil. ? 0.000 0.189A288 (Algeria) Ghadames Oued Ali ? Sil. ? n/a n/aA449 (Bolivia) South Sub-Andean

FTB? 2191.5 m L. Sil. Gorstian-

Ludfordian0.000 0.000

A448 (Bolivia) South Sub-AndeanFTB

? 2188.9 m L. Sil. Gorstian-Ludfordian

0.000 0.000

A447 (Bolivia) Chaco Atuguis ? E. Dev. ? 0.000 0.000A376 Middle East Central Arabia Jauf 14,379 ft. E. Dev. ? n/a n/aA239 (Russia) Timan-Pechora ? 4028–

4034 mE. Dev. Emsian 0.000 0.000

A237 (Russia) Timan-Pechora ? 3700–3708 m

E. Dev. Emsian 0.000 0.000

A446 (Bolivia) ? Tita ? Dev. ? 0.000 0.000A605 (Bolivia) Sub-Andean ? ? M. Dev. L. Givetian 0.000 0.002C73-d93 (Bolivia) ? ? ? L. Dev. ? n/a n/aA606 (Bolivia) Sub-Andean ? ? L. Dev. Frasnian 0.000 0.200A604 (Bolivia) Sub-Andean ? ? L. Dev. Frasnian 0.000 0.000A602 (Bolivia) Sub-Andean ? ? L. Dev. Frasnian 0.000 0.000A949 (Bolivia) ? ? ? L. Dev. ? 0.000 0.000A940 (Bolivia) ? ? ? L. Dev. ? 0.000 0.000A445 (Bolivia) ? ? ? L. Dev. ? 0.000 0.000A227 (USA) ? Woodford Shale 8943 ft. Car-Dev. 0.000 0.000654 ND (USA) Williston Bakken (Upper Member) 10,002 ft. E. Car. Tournaisian n/a n/aA948 Manchester (UK) ? ? Surface E. Car. Serpukhovian-

Bashk.n/a n/a

A226 OK (USA) Anadarko Big Lime 4022.2 ft. M. Car. Moscovian 0.000 0.000A225 UT (USA) Paradox Hermosa 2782 ft. M. Car. Moscovian 0.000 0.000A190 TX (USA) Palo Duso Motley Co TX 5461–

5471 ft.M.-L. Car. Pennsylvanian 0.000 0.000

A391 Middle East Central Arabia Unayzah 6196 ft. Per-Car. 0.000 0.000C73-d3 (Bolivia) ? Bolivia ? Per. n/a n/a

414 S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424

Table 3 (continued)

SampleID

Country-Region Basin Formation Depth Period Age Biomarker ratiosa

TA-DMC

TA-dinosteroids

A603 (Bolivia) Sub-Andean ? ? E. Per. Sakmarian-Artinskian

0.000 0.000

A601 (Bolivia) Sub-Andean ? ? E. Per. Sakmarian-Artinskian

0.000 0.000

A559P (Brazil) ? ? ? E. Per. Artinskian-Kungurian

n/a n/a

A367 (Brazil) Parana Irati ? E. Per. Artinskian-Kungurian

n/a n/a

A337 MT (USA) Rocky Mountain Phosphoria (Mead PeakMember)

408.5 ft. E. Per. Kungurian 0.000 0.000

A342 MT (USA) Rocky Mountain Phosphoria (Mead PeakMember)

431.5 ft. E. Per. Kungurian n/a n/a

A336 MT (USA) Rocky Mountain Phosphoria (Mead PeakMember)

207 ft. E. Per. Guadalupian 0.000 0.000

A335 MT (USA) Rocky Mountain Phosphoria (Mead PeakMember)

205 ft. L. Per. Guadalupian 0.000 0.000

C124 Svalbard (Norway) Sticky Keep ? E. Tri. n/a n/aC125 Svalbard (Norway) Sticky Keep ? E. Tri. n/a n/aA576 Alaska (USA) Surface E. Tri. 0.071 0.608A416 WY (USA) ? Dinwoody 34.1 ft. E. Tri. Induan-Olenekian 0.000 0.237A415 WY (USA) ? Dinwoody 32.1 ft. E. Tri. Induan-Olenekian 0.000 0.000A414 WY (USA) ? Dinwoody 30.9 ft. E. Tri. Induan-Olenekian 0.000 0.000C123 Svalbard (Norway) Botneheia ? n/a n/a393 (Switzerland) Southern Alps FTB Meride Shale 327–332 ft. M. Tri. Anisian-Landinian 0.232 0.901898 (Syria) Northwest Arabian ? 675–825 m Tri. ? n/a n/a280 ID (USA) Thaynes Limestone 1941 ft. Tri. ? 0.000 0.523208 Alaska (USA) North Slope Shublik 8891–

8903 ft.Tri. ? 0.040 0.355

207 Alaska (USA) North Slope Shublik 8825–8830 ft.

Tri. ? 0.126 0.548

797 (Italy) Central Apennines Dolmia Pricipale Surface L. Tri. Norian 0.113 0.663A793 (Papua New

Guinea)South Papuan FTB Koi-Lange ? M. Jur. E.-M. Callovian n/a n/a

213 Alaska (USA) North Slope Kigak Shale 9390 ft. L. Jur. ? 0.064 0.475210 Alaska (USA) North Slope Kigak Shale 8399–

8405 ft.L. Jur. ? 0.059 0.472

313 (Australia) Barrow Dampier Dingo Clayston 2835 ft. L. Jur. ? 0.024 0.416314 (Australia) Barrow Dampier Dingo Clayston 2545 ft. L. Jur. ? 0.050 0.461997 (UK) North Sea 6888 ft. M. Jur. 0.006 0.182993 (Scotland) Celtic Sea Cullaidh M. Jur. Bathonian 0.218 0.795A793 (Papua New

Guinea)South Papuan Koi-Lange M. Jur. E.-M. Callovian n/a n/a

413 Middle East Central Arabia Tuwaiq Mountain 6815 ft. L. Jur. Oxfordian 0.087 0.584412 Middle East Central Arabia Hanifa 6540 ft. L. Jur. L. Oxfordian 0.090 0.475A789P (UK) North Sea Kimmeridge Clay 5320 ft. L. Jur. Oxford.-E. Kimm. n/a n/a410 Middle East Central Arabia Hanifa 1009 ft. L. Jur. L. Oxford.-Kimm. 0.057 0.517A790 (Papua New

Guinea)South Papuan FTB Imburu Surface L. Jur. E. Kimmeridgian 0.037 0.441

A788P (UK) North Sea Kimmeridge Clay 53,201 ft. L. Jur. Kimmeridgian n/a n/aA799 (Papua New

GuineaSouth Papuan FTB Imburu 9380 ft. L. Jur. L. Kimmeridgian 0.043 0.372

A791 (Papua NewGuinea)

South Papuan FTB Imburu Surface L. Jur. E. Tithonian 0.051 0.507

A142 (Russia) West Siberia Bezhenov 2773 m L. Jur. Tithonian 0.130 0.608A798 (Papua New

Guinea)South Papuan FTB Leru 7880 ft. E. Cret. L. Berriasian 0.053 0.496

A503 (Argentina) Neuquen Vaca Muerta Surface E. Cret. Berriasian-L.Valang.

0.062 0.411

312 (Australia) Barrow Dampier Muderong Shale 1291 m E. Cret. Hauterivian 0.097 0.782209 Alaska (USA) North Slope ? 11,568 ft. E. Cret. Barremian-Albian 0.095 0.516A619 (Colombia) Upper Magdalena Paja Surface E. Cret. L. Berriasian-L.

Haut.0.067 0.615

A631P (Italy) South Alps FTB ? Surface E. Cret. Aptian 0.019 0.250A627P (Ecuador) Oriente Napo 295 m E. Cret. Aptian 0.060 0.595A617 (Colombia) Upper Magdalena Similti Surface E. Cret. L. M. Albian 0.097 0.570A629P (Australia) Eromanga Toolebuc ? E. Cret. L. Albian 0.065 0.451A616 (Colombia) Upper Magdalena Similti Surface L.-E. Cret. E. Cenomanian 0.107 0.442A539P (Angola) Lower Congo Vermelha 9500 ft. L. Cret. E. Cenomanian 0.058 0.347A630P (Italy) South Alps FTB ? Surface L. Cret. Cenomanian 0.023 0.304A533 (Angola) Lower Congo Vermelha 7088 ft. L. Cret. L. Cenomanian 0.067 0.472A600 (Bolivia) Sub-Andean Basin ? Surface L. Cret. Cenomanian-

Turonian0.098 0.197

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S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424 415

Table 3 (continued)

SampleID

Country-Region Basin Formation Depth Period Age Biomarker ratiosa

TA-DMC

TA-dinosteroids

C34 CO (USA) ? ? L. Cret. L. Turonian n/a n/aA554P (Angola) Lower Congo Vermelha 8233 ft. L. Cret. Turonian 0.096 0.424A411 (Ecuador) Oriente Napo 144 m L. Cret. Turonian n/a n/aA547P (Angola) Lower Congo Vermelha 6100 ft. L. Cret. Coniacian 0.053 0.354226 Alaska (USA) North Slope Seebee Shale 11,161 ft. L. Cret. ? 0.077 0.482A552 (Angola) Lower Congo Vermelha 7536 ft. L. Cret. Santonian 0.057 0.449A545 (Angola) Lower Congo Vermelha 5366 ft. L. Cret. Santonian 0.061 0.549583 WY (USA) Hilliard (Upper Member) 70–80 ft. L. Cret. Santonian 0.000 0.000A623 CA (USA) San Joaquin ? 14,112 ft. L. Cret. Campanian 0.051 0.434A543 Cabinda (Angola) Lower Congo Vermelha 4545 ft. L. Cret. Campanian 0.048 0.461A613 (Colombia) Upper Magdalena La Luna Surface L. Cret. L. Campanian 0.260 0.830A608P (Colombia) Upper Magdalena La Luna Surface L. Cret. L. Campanian 0.083 0.536A537 (Angola) Lower Congo Labe 9952 ft. L. Cret. L. Campanian 0.079 0.528A386 (Trinidad) Onshore Trinidad Naparima Hill Surface L. Cret. L. Campanian-

Maast.n/a n/a

A385 (Trinidad) Onshore Trinidad Naparima Hill Surface L. Cret. Maastrichtian n/a n/a

TA-DMC ratio: TA-DMC/(TA-DMC + C28 TA-steroid 20S).TA-dinosteroid ratio: TA-dinosteroids/(TA-dinosteroids + C28 TA-steroid 20S).n/a = measurement not taken.L = low biomarker concentrations.

a Refer to Fig. 1 and text for compound structures and names.

416 S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424

of an angular methyl group but the constitution of the side chainremains unaltered (Ludwig et al., 1981). TA-DMC analysis of aCampanian source rock (sample A613, Table 3) from the UpperMagdalena Basin, Colombia, and an authentic TA-DMC standardare illustrated (Fig. 2A, C and D). GC–MS (m/z 231) and MRM-GC–MS (transition m/z 372 ? 231) analyses of both sample andstandard were carried out by using similar analytical conditions.TA-DMC occur as a series of six peaks (Fig. 2, peaks a–f) relatedto stereoisomerization at C-20, C-23 and C-24. Eight S and R stereo-isomers are expected and therefore two of the peaks are mostlikely co-elutions. The GC–MS trace (m/z 231) of a Proterozoicsource rock from Central Arabia Basin, Middle East (sample S2, Ta-ble 3), is also illustrated (Fig. 2B).

In contrast with their saturated counterparts (23,24-dimethyl-cholestanes), TA-DMC are reliably identified by GC–MS in the aro-matic hydrocarbon fractions of most extracts and oil samples usingthe mass spectral base peak m/z 231. The predominant C29 ster-anes, 24-ethylcholestanes, completely mask the 23,24-dimethyl-cholestanes in most samples, where they are present, whenanalyzed using m/z 217 or metastable transition of the molecularion m/z 400 to the diagnostic ion m/z 217 by MRM-GC–MS.

A new ratio is proposed in order to monitor the distribution ofTA-DMC. This normalized ‘‘TA-DMC Ratio’’ refers to the second iso-mer of TA-DMC relative to 20S triaromatic 24-ethylcholesteroid(peaks b/(b + j), Fig. 2). This particular TA-DMC (peak b) was chosenfor the ratio because it elutes in a part of the m/z 231 mass chro-matogram relatively free of interference (Fig. 2A).

3.2. Organic rich rocks

Normalized ratios of TA-DMC/TA-C28 20S plotted againstTA-dinosteroids/TA-C28 20S for the set of organic rich marine sourcerocks (Table 3) exhibit values ranging from 0.006 up to 0.260 for allextracts of the Jurassic and Cretaceous rock samples, while TA-DMCwere not detected in any except two Paleozoic (Ordovician) rocks(Fig. 4). The presence of TA-DMC in those two samples fromOrdovician (A382, Middle East, Central Arabia basin and A242,Libya) was confirmed by MRM-GC–MS analysis.

In contrast, the percentage of Paleozoic samples that revealTA-dinosteroids is higher (30%) compared to those that showTA-DMC (4%). However, almost all Jurassic and younger marinerock samples in the present data set contain TA-DMC and TA-

dinosteroids with the exception of few samples that are severelydepleted in all TA-steroids due to a low steroid source componentoverall or advanced maturity. TA-DMC were undetected in 50% ofthe Triassic samples and TA-dinosteroids in 25% of them (Fig. 5).Two Triassic core samples (Wyoming, USA, Dinwoody Fm. and Ida-ho, USA, Thaynes Fm.) exhibit high TA-dinosteroid ratios, 0.237 and0.523, respectively, but lack TA-DMC. The contrast in the frequencyof TA-DMC occurrence between Proterozoic to Triassic and Jurassicand younger rocks parallels the earliest appearance of dinoflagel-lates and haptophytes in the Triassic, with strong radiation of spe-cies and, presumably, an increase in the prevalence and relativeabundance of their biomass in the Jurassic and Cretaceous andcontinuing throughout the Tertiary.

3.3. Oil samples

The oil samples originated from Proterozoic to Tertiary sourcerocks yield a result similar to that of the rock samples (Fig. 6 andTable 4). A few Paleozoic samples contain TA-DMC. The presenceof TA-DMC in Paleozoic oil samples may be related to a pre-Mesozoicchemical record of dinoflagellates or closely related forms(Moldowan et al., 2001), or haptophytes or diatoms. Therefore,TA-DMC demonstrate good potential for application as age relatedbiomarkers and they can be analyzed together with TA-dinosteroids.

Besides their application as age related biomarkers, TA-DMCalso are potential indicators to distinguish source rock depositionalenvironments (Fig. 6). The oil samples from different depositionalenvironments record systematic variations in the relative abun-dance of TA-DMC compounds. The Cretaceous oil samples fromBrazil emanate from the synrift section, where source rocks weredeposited in restricted lacustrine environments at various salini-ties. They show the highest TA-DMC and TA-dinosteroid ratios inthe data set. The related source rocks of these lacustrine oil sam-ples from Brazil have abundant dinoflagellate cysts (Arai et al.,2000), methylsterane/sterane ratios greater than 1.0 and abundantTA-dinosteroids and their saturated dinosterane counterparts(Mello et al., 1988a,b, 1989). These results favor the interpretationthat the algal population in these rift lake environments wasdominated by dinoflagellates or that dinoflagellate blooms oc-curred frequently.

Although the likely sterol precursors for both TA-DMC andTA-dinosteroids occur in dinoflagellates, diatoms and haptophytes,

Table 4Identification of oil samples and measurements of triaromatic steroid ratios.

Sample ID Country-Region Source rock age Biomarker ratiosa

TA-DMC TA-dinosteroids

Stan 434 Tarragona (Spain) Tertiary 0.088 0.598Stan 435 Tarragona (Spain) Tertiary 0.043 0.689Caspian T171 So. Caspian (Azerbaidjan) Tertiary 0.106 0.583Caspian T023 So. Caspian (Azerbaidjan) Tertiary 0.108 0.633Caspian T173 So. Caspian (Azerbaidjan) Tertiary 0.126 0.758Caspian T286 So. Caspian (Turkmen) Tertiary 0.104 0.672Caspian T299 So. Caspian (Turkmen) Tertiary 0.104 0.623Tengiz 177 So. Caspian (Azerbaidjan) Tertiary 0.120 0.735Tengiz 291 So. Caspian (Turkmen) Tertiary 0.097 0.645Stan 616 Pt. Arguello (USA) Tertiary 0.122 0.726Stan 7 San Joaquin (USA) Tertiary 0.120 0.697B-1 (Brazil) Tertiary 0.050 0.591B-2 (Brazil) Tertiary 0.071 0.554WT-2 (Vietnam) Tertiaryb 0.041 0.713WT-7 (Vietnam) Tertiary b 0.056 0.764Stan 840 Lower Magdalena (Colombia) Tertiary b 0.081 0.458ZZ005404 W. Natuna Sea (Indonesia) Tertiary 0.057 0.664NG000066? Niger Delta (Nigeria) Tertiary 0.050 0.708NG000068? Niger Delta (Nigeria) Tertiary 0.209 0.786NG000079 Niger Delta (Nigeria) Tertiary 0.081 0.683ID004313 W. Natuna Sea (Indonesia) Tertiary b 0.069 0.558ID004315 W. Natuna Sea (Indonesia) Tertiary b 0.095 0.758Stan 693 Pearl River Mouth (China) Tertiary b 0.048 0.793Stan 3 San Joaquin (USA) Tertiary 0.076 0.528Stan 1-3 UT (USA) Tertiary b 0.033 0.460Stan A90 Niger Delta (Nigeria) Tertiary 0.074 0.509Stan 307 (Central Sumatra) Tertiary b 0.021 0.412Stan 305 (Central Sumatra) Tertiary b 0.062 0.765Stan 302 (Central Sumatra) Tertiary b 0.058 0.644Stan C256 N.E. Trinidad (Trinidad) Tertiary b 0.050 0.246Stan 21 GOM – LA (USA) Tertiary 0.057 0.476B-22 (Brazil) Late Cretaceous 0.039 0.270B-23 (Brazil) Late Cretaceous 0.070 0.724B-3 (Brazil) Early Cretaceous 0.222 0.966B-4 (Brazil) Early Cretaceous 0.204 0.962B-5 (Brazil) Early Cretaceous 0.211 0.944B-6 (Brazil) Early Cretaceous 0.226 0.952B-7 (Brazil) Early Cretaceous 0.234 0.952B-8 (Brazil) Early Cretaceous 0.243 0.953B-9 (Brazil) Early Cretaceous 0.245 0.955B-10 (Brazil) Early Cretaceous 0.172 0.934B-11 (Brazil) Early Cretaceous 0.199 0.933B-12 (Brazil) Early Cretaceous 0.177 0.944B-13 (Brazil) Early Cretaceous 0.122 0.863B-14 (Brazil) Early Cretaceous 0.081 0.830B-15 (Brazil) Early Cretaceous 0.140 0.900B-16 (Brazil) Early Cretaceous 0.147 0.924B-17 (Brazil) Early Cretaceous 0.076 0.825B-18 (Brazil) Early Cretaceous 0.095 0.766B-19 (Brazil) Early Cretaceous 0.082 0.611B-20 (Brazil) Early Cretaceous 0.113 0.749B-21 (Brazil) Early Cretaceous 0.095 0.680B-24 (Brazil) Early Cretaceous 0.068 0.844B-25 (Brazil) Early Cretaceous 0.079 0.862B-26 (Brazil) Early Cretaceous 0.060 0.780B-27 (Brazil) Early Cretaceous 0.043 0.756B-28 (Brazil) Early Cretaceous 0.032 0.678B-29 (Brazil) Early Cretaceous 0.042 0.698B-30 (Brazil) Early Cretaceous 0.025 0.650B-31 (Brazil) Early Cretaceous 0.027 0.476B-32 (Brazil) Early Cretaceous 0.046 0.750B-33 (Brazil) Early Cretaceous 0.034 0.622B-34 (Brazil) Early Cretaceous 0.049 0.546B-35 (Brazil) Early Cretaceous 0.048 0.765B-36 (Brazil) Early Cretaceous 0.045 0.767B-37 (Brazil) Early Cretaceous 0.030 0.642B-38 (Brazil) Early Cretaceous 0.026 0.636B-39 (Brazil) Early Cretaceous 0.061 0.787AE001788 Middle East (Dubai) Cretaceous 0.099 0.736Stan 181 WY (USA) Cretaceous 0.074 0.557Stan 846 San Jacinto (Colombia) Cretaceous 0.135 0.774Stan C255 San Joaquin (USA) Cretaceous 0.084 0.587Stan A89 Niger Delta (Nigeria) Cretaceous 0.091 0.624

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S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424 417

Table 4 (continued)

Sample ID Country-Region Source rock age Biomarker ratiosa

TA-DMC TA-dinosteroids

Stan 895 (Jordan) Cretaceous 0.183 0.752Stan 861 Maracaibo (Venezuela) Cretaceous 0.159 0.845Stan A402 Oriente (Ecuador) Cretaceous 0.099 0.749Stan 896 (Jordan) Cretaceous 0.076 0.605Stan 917 (Angola) Cretaceous b 0.103 0.845Stan 889 (Angola) Cretaceous b 0.178 0.918Stan A28 (Oman) Cretaceous 0.083 0.745Stan 179 Rocky Mtn FTB (USA) Cretaceous 0.109 0.694Stan 161 Rocky Mtn FTB (USA) Cretaceous 0.101 0.715Stan 599 (Sudan) Cretaceous b 0.073 0.807Stan 601 (Sudan) Cretaceous b 0.080 0.802Stan 494 GOM – FL (USA) Cretaceous 0.044 0.543Stan A192 Magallanes-Austral (Chile) Cretaceous 0.034 0.359TE-A-1 (Mongolia) Cretaceous b 0.015 0.418SWZB-301B (Mongolia) Cretaceous b 0.034 0.716Kuwait Central Arabia (Kuwait) Cretaceous 0.040 0.532Stan C7 (New Zealand) Cretaceous 0.123 0.656Stan 9 (Canada) Cretaceous 0.074 0.563Stan 975 North Sea (UK) Late Jurassic (Oxfordian-Tithonian) 0.136 0.753NO001158 North Sea (Norway) Late Jurassic (Oxfordian-Tithonian) 0.085 0.658NO001144 North Sea (Norway) Late Jurassic (Oxfordian-Tithonian) 0.090 0.665SK-3 Barrow-Dampier (Australia) Late Jurassic (Tithonian) 0.025 0.462Stan 93 Barrow-Dampier (Australia) Late Jurassic (Tithonian) 0.030 0.353Stan A2 South Papuan FTB (PNG) Late Jurassic (Oxfordian-Tithonian) 0.079 0.833Stan B199 Sureste (Mexico) Late Jurassic (Oxfordian) 0.054 0.681Stan 219 North Slope (USA) Late Jurassic (Oxfordian-Kimm.) 0.049 0.543Stan A27 (Oman) Late Jurassic (Oxfordian-Kimm.) 0.036 0.453Stan C144B Onshore Gulf Coast – AL (USA) Late Jurassic (Oxfordian-Kimm.) 0.069 0.734SK-21 Vulcan Graben (Australia) Middle Jurassic 0.015 0.156TH96-08 Turpan (China) Early – Middle Jurassic b 0.010 0.240TH96-05 Turpan (China) Early – Middle Jurassic b 0.000 0.154BT2 Cooper Cooper (Australia) Jurassic b 0.028 0.370A-1 (Argentina) Jurassic 0.098 0.652A-2 (Argentina) Jurassic 0.108 0.684A-3 (Argentina) Jurassic 0.133 0.812A-4 (Argentina) Jurassic 0.140 0.815A-5 (Argentina) Jurassic 0.095 0.695A-7 (Argentina) Jurassic 0.131 0.801A-8 (Argentina) Jurassic 0.136 0.808A-9 (Argentina) Jurassic 0.070 0.615A-10 (Argentina) Jurassic 0.101 0.608A-11 (Argentina) Jurassic 0.098 0.639A-12 (Argentina) Jurassic 0.104 0.681A-13 (Argentina) Jurassic 0.107 0.686A-14 (Argentina) Jurassic 0.051 0.509A-15 (Argentina) Jurassic 0.082 0.634A-16 (Argentina) Jurassic 0.055 0.496A-17 (Argentina) Jurassic 0.092 0.652A-18 (Argentina) Jurassic 0.101 0.690A-19 (Argentina) Jurassic 0.094 0.664A-20 (Argentina) Jurassic 0.090 0.667A-22 (Argentina) Jurassic 0.072 0.583A-23 (Argentina) Jurassic 0.088 0.696A-24 (Argentina) Jurassic 0.108 0.689A-26 (Argentina) Jurassic 0.090 0.639A-27 (Argentina) Jurassic 0.101 0.626A-28 (Argentina) Jurassic 0.092 0.627A-30 (Argentina) Jurassic 0.037 0.421A-31 (Argentina) Jurassic 0.058 0.522A-32 (Argentina) Jurassic 0.026 0.336A-34 (Argentina) Jurassic 0.030 0.363A-35 (Argentina) Jurassic 0.028 0.335A-40 (Argentina) Jurassic 0.096 0.640Stan 563 Central Adriatic (Italy) Triassic/Jurassic 0.022 0.600Stan 559 Southern Adriatic (Albania) Triassic/Jurassic 0.039 0.418Stan 19 (Canada) Triassic/Jurassic 0.107 0.696Stan 218 North Slope (USA) Triassic/Jurassic 0.044 0.470Tengiz 18 Manghyshlak (Kazakhstan) Triassic/Jurassicc 0.014 0.197Tengiz 12 Pricaspian (Russia) Triassic/Jurassic b 0.000 0.000Tengiz 21 Manghyshlak (Kazakhstan) Triassic/Jurassic c 0.000 0.000Caspian T018 Manghyshlak (Kazakhstan) Triassic/Jurassic c 0.007 0.089Caspian T019 Manghyshlak (Kazakhstan) Triassic/Jurassic c 0.044 0.403Caspian T021 Manghyshlak (Kazakhstan) Triassic/Jurassic c 0.000 0.000Caspian T195 Pricaspian (Kazakhstan) Devonian–Permian 0.000 0.109

418 S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424

Table 4 (continued)

Sample ID Country-Region Source rock age Biomarker ratiosa

TA-DMC TA-dinosteroids

Caspian T201 Pricaspian (Russia) Devonian–Permian 0.000 0.000Caspian T202 Pricaspian (Russia) Devonian–Permian 0.000 0.095Caspian T206 Pricaspian (Kazakhstan) Devonian–Permian 0.000 0.039Caspian T214 Pricaspian (Kazakhstan) Devonian–Permian 0.000 0.006Caspian T224 Pricaspian (Kazakhstan) Devonian–Permian 0.057 0.464Caspian T225 Pricaspian (Kazakhstan) Devonian–Permian 0.000 0.000Caspian T228 Pricaspian (Kazakhstan) Devonian–Permian 0.000 0.000Caspian T357 Pricaspian (Kazakhstan) Devonian–Permian 0.000 0.144TH96-10 (China) Paleozoic b 0.000 0.069BT4 Cooper Cooper (Australia) Paleozoic b 0.000 0.551Stan 359 Rocky Mtn FTB-WY (USA) Paleozoic 0.000 0.164Stan C215 East Middlands (UK) Paleozoic 0.000 0.266Stan 644 Moncton (Canada) Paleozoic b 0.000 0.263Stan C115 Madre de Dios (Bolivia) Paleozoic 0.023 0.280Stan 28 (Canada) Paleozoic 0.000 0.078Stan 23 (Canada) Paleozoic 0.000 0.103Stan A115 Timan-Pechora (Russia) Paleozoic 0.000 0.113Tengiz 181 Pricaspian (Kazakhstan) Paleozoic 0.000 0.051Tengiz 15 Pricaspian (Russia) Paleozoic 0.000 0.182Tengiz 1 Pricaspian (Kazakhstan) Paleozoic 0.000 0.302Tengiz 197 Pricaspian (Kazakhstan) Paleozoic 0.022 0.284Tengiz 353 Pricaspian (Kazakhstan) Paleozoic 0.000 0.079Tengiz 199 Pricaspian (Kazakhstan) Paleozoic 0.000 0.091Tengiz 206 Pricaspian (Russia) Paleozoic 0.000 0.000Tengiz 203 Pri-Caspian (Russia) Paleozoic 0.000 0.000Stan A179 NW Arabian (Turkey) Paleozoic 0.023 0.336Hawtah-1 Central Arabia (Saudi Arabia) Paleozoic 0.000 0.153980930-09 Tarim (China) Paleozoic 0.014 0.368Stan A136 Timan-Pechora (Russia) Paleozoic 0.000 0.000Stan A138 Timan-Pechora (Russia) Paleozoic 0.000 0.096Stan A137 Timan-Pechora (Russia) Paleozoic 0.000 0.000Stan 259-1 Urals (Russia) Paleozoic 0.000 0.000T466 East Siberia (Russia) Paleozoic 0.000 0.303T335 East Siberia (Russia) Paleozoic 0.000 0.500T338 East Siberia (Russia) Paleozoic 0.000 0.000T528 East Siberia (Russia) Paleozoic 0.000 0.211Stan C216 (Oman) Paleozoic 0.000 0.000Stan A24 (Oman) Paleozoic 0.000 0.000N530 Moscow (Russia) Paleozoic 0.000 0.139

TA-DMC ratio: TA-DMC/(TA-DMC + C28 TA-steroid 20S).TA-dinosteroid ratio: TA-dinosteroids/(TA-dinosteroids + C28 TA-steroid 20S).

a Refer to Fig. 1 and text for compound structures and names.b Refer to non-marine.c Refer to possible non-marine.

S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424 419

the paleontological records of these compounds are different. Onthe one hand, Moldowan et al. (1996) demonstrated the earlieroccurrences of triaromatic dinosteroids in some extracts from Pro-terozoic to Devonian rocks, with the frequency of occurrence rang-ing from �9–60% per geologic time period. On the other hand, thefrequency of occurrence of the triaromatic 23,24-dimethyl steroidsin Paleozoic rocks is very low exhibiting a frequency of 10% andonly in some Ordovician samples. Therefore, the occurrence ofTA-DMC is more specific to the Mesozoic Era as can be viewedby comparing the frequency of their occurrence against geologictime (Fig. 5).

The chemical record of TA-DMC parallels that of dinoflagellatecysts (Fig. 5), which first appear with certainty in the Triassicand are abundant in the Jurassic and Cretaceous. Haptophytesare responsible for coccolith fossils, which increase in occurrencein the Triassic and are abundant throughout the later Mesozoic.The TA-DMC could be related to any, or possibly all three taxa,but may represent a modified chemical evolutionary adaptation(i.e., the loss of a methyl group at C-4 from the sterol precursor).TA-dinosteroids occur relatively more frequently in the Proterozoicand Paleozoic, particularly during periods of high acritarch diver-sity (Ediacaran to Devonian) (Moldowan et al., 1996, 2001). Thus,they may indicate dinoflagellate ancestors (e.g., Moldowan and

Talyzina, 1998), while the TA-DMC appear to relate almost exclu-sively to post-Permian species.

4. Conclusions

Triaromatic 23,24-dimethylcholesteroids (TA-DMC) occur in allanalyzed marine Jurassic and Cretaceous and about one half of theTriassic rock samples. The TA-DMC also occur in oil samples (mar-ine and non-marine) derived from source rocks of that age rangewhere triaromatic steroids of any type are present. Proterozoic–Paleozoic samples record rare occurrences of TA-DMC, which arelimited thus far to several Ordovician rock and oil samples fromNorth Africa.

The distribution of TA-DMC in the analyzed samples (a set of al-most 300 samples ranging from Proterozoic to Tertiary) has led tothe conclusion that these biomarkers can be used together withtriaromatic dinosteroids (TA-dinosteroids) to distinguish Protero-zoic to Triassic from Jurassic and younger oil and rock samples.Both of these triaromatic steroid types appear to be taxon specificfor dinoflagellates and possibly haptophytes based on the fossilrecord. During the Triassic, the initial radiation and probable a lackof a significant contribution of the related taxa correspond tosporadic TA-DMC occurrence. TA-DMC compounds are abundant

Fig. 2. Mass chromatograms showing the distribution and identification of triaromatic 23,24-dimethylcholesteroids (chromatographic peaks a–f) among other triaromaticsteroids (peaks g–o) by GC–MS monitoring the ion m/z 231 in (A) Upper Cretaceous source rock from the Upper Magdalena Basin, Colombia, sample A613 and (B) Proterozoicsource rock from Central Arabia Basin, Middle East, sample S2; and by MRM-GC–MS monitoring the transition m/z 372 ? 231 in (C) sample A613, and (D) the authenticstandard (Table 1).

420 S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424

in source rock extracts and oil samples from Cretaceous rift basinlacustrine environments of the Brazilian margin. This suggests

depositional environments dominated by algae and dinoflagellateblooms, in accordance with the available paleontological data.

HO

O

HHH

a b

H PhOC(S)OH

O

H

O

CH2CH2 OHCH2 I c

de , f

5

2 3 4

67

20

H HH

g

CO2 CH3 CH2 OH

h e ,h

8 9 1 B

2023

24

12B2019

18 17 16

13 14 15

HO

O

HHH

a b

H PhOC(S)OH

O

H

O

CH2CH2 OHCH2 I c

de , f

5

2 3 4

67

20

H HH

g

CO2 CH3 CH2 OH

h e ,h

8 9 1 B

2023

24

12B2019

18 17 16

13 14 15

Fig. 3. Synthesis of (20f,23f,24f)-23,24-dimethyl-5a-cholestane 12B. (a) PhOC(S)Cl, Py, CH2Cl2; (b) (n-Bu)3SnH, AIBN, C6H6; (c) Ph3P@CH2, KOt-Am, benzene; (d) BH3� THF;

NaOH, H2O2; (e) Et3N, MsCl, CH2Cl2; (f) NaI, acetone; (g) LDA, (CH3)2CHCH(CH3)CH2CO2CH3; (h) LiAlH4, THF.

1.00

0.80

0.60

Proterozoic

0.40

TA-D

inos

tero

id R

atio

Cambrian

Ordovician

Silurian

0.20

Devonian

Carboniferous

Permian

Triassic

Jurassic

Cretaceous

0.000.300.200.100.00

TA-DMC Ratio

Fig. 4. Cross plot of TA-DMC versus TA-dinosteroids for rock samples. Triaromatic 23,24-dimethylcholesteroids are, with few exceptions, restricted to Mesozoic and youngerrock extracts. Triassic source rocks are split 3:4 between TA-DMC absence and occurrence, while only two of 57 Paleozoic source rocks show these compounds. This contrastswith a much wider occurrence of triaromatic dinosteroids in Paleozoic rocks. The TA-DMC ratio is calculated based on the compounds identified as peaks b and j (b/(b + j),Fig. 2). The TA-dinosteroid ratio is the sum of eight stereoisomeric (at C-20, C-23 and C-24) TA-dinosteroids (5, Fig. 1) divided by the summation of compounds 5 plus TA-24-ethylcholesteroid (20S, peak j, Fig. 2).

S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424 421

Acknowledgements

The authors thank the Center for Structural Biology ChemistryCore Facility, the NIH National Center for Research Resources

(NCRR) and the Molecular Organic Geochemistry IndustrialAffiliates (MOGIA) of Stanford University for providing the funding.The authors would also like to thank Dr. Jürgen Rullkötter and Dr.Roger E. Summons for their reviews of the manuscript and helpful

Fig. 5. Frequency of occurrence of TA-DMC in the rock samples. (a) Dinoflagellate cyst genera (adapted from MacRae et al., 1996, by Moldowan et al., 2001) and (b) acritarchgenera (adapted from Strother, 1996, by Moldowan et al., 2001) compared with (c) frequency of occurrence of TA-DMC in rock samples by percentage from a given period.

0.00

0.20

0.40

0.60

0.80

1.00

0.00 0.10 0.20 0.30

TA-D

inos

tero

id R

atio

TA-DMC Ratio

Tertiary

Cretaceous

Jurassic

Jurassic/ Triassic

Paleozoic

Cretaceous (Brazil)

Fig. 6. Cross plot of TA-DMC versus TA-dinosteroids for oil samples. Using the same parameters as Fig. 4, triaromatic 23,24-dimethylcholesteroids distinguish Mesozoic andPaleozoic oils sampled from many basins. The Cretaceous oil families of Brazil are indicated individually. Oil samples generated from the Brazilian Cretaceous lacustrinesource rocks show high TA-DMC ratios.

422 S.M. Barbanti et al. / Organic Geochemistry 42 (2011) 409–424

comments, and Frederick J. Fago of Stanford University for assis-tance in preparing the pyrolyzate.

Associate Editor—John Volkman

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