Biomarker analysis of aliphatic fractions focused on three groups of compounds, n-alkanes, steranes and sterenes, and pentacyclic triterpanes and their derivatives. The results based on integration of peaks in various ion chromatograms are summarized in Tables T2 and T3.Table T2 presents biomarker parameters that describe the thermal maturity and conditions of organic matter preservation based on selected bio-marker ratios. Table T3 focuses on biomarkers ratios indicative of their biological sources and their relative distributions in the samples.
n-Alkanes and isoprenoid hydrocarbons were identified from their mass spectra in the total ion chromatogram (TIC) and the m/z 57 ion chromatogram, which was also used for peak integration (Fig. F1A, F1B). n-Alkanes range from n-C13 to n-C35, homologs less than C16 are not observed in most samples, probably because of the loss of these more volatile compounds during sample processing. Isoprenoids were typically identified from their elevated m/z 183 fragment in the mass spectra and their molecular ion (M+). Pristane (C19H40 ; M+ = 268), phytane (C20H42 ; M+ = 282), and lycopane (C40H82 ; M+ = 562) were present in all samples, and farnesane (C15H32 ; M+ = 212) was found in some samples.
Lycopane coelutes with n-C35 but can be identified from elevated m/z 113, 253, and 183 fragments (Kimble et al., 1974). The contribution of lycopane to the peak in the m/z 57 ion trace can be inferred from the prominence of the three diagnostic fragments above, ions attributed to n-alkyl fragments (fig. 1 in Sinninghe Damsté et al., 2003). On this basis, the coeluting peaks of lycopane and n-C35 in all samples consist almost exclusively of lycopane. Lycopane was found in high abundance in all samples; in three it was the most abundant compound (MAC) in the m/z 57 trace (Table T3).
The most abundant compound in the aquatic range of n-alkanes between C16 and C18 is presented in Table T3 (Cmax aquatic). The sources of these n-alkane homologs are typically aquatic algae or cyanobacteria (e.g., Blumer et al., 1971; Gelpi et al., 1970; Han and Calvin, 1969; review in Brassell et al., 1978), although the samples differ with respect to their most abundant compound. Phytane predominates in two cases (Table T3). n-Alkanes with 23–33 carbon atoms, and especially these from n-C27 to n-C33, originate from waxes typical of higher land plants. Exclusively for immature organic matter, the predominance of odd- vs. even-numbered n-alkanes can be employed as a measure of the proportion of terrestrially derived components. The carbon preference index (CPI) (Bray and Evans, 1961) is a mathematical expression of the odd over even predominance between n-C24 and n-C34. According to Table T3, the samples show greater uniformity in terms of their dominant compound in the plant wax–derived n-alkane range (Cmax waxes); for most samples this is n-C31.
Steranes and sterenes are derived from the sterols of cell membranes of eukaryotes, mainly algae and higher plants (overview in Mackenzie et al., 1982). A wide variety of their stereoisomers is present. The occurrence of unsaturated (sterenes) or rearranged compounds (diasterenes; m/z 257) is largely a function of the thermal maturity of the organic matter. The 5,14,17(H)-20R (5-R) isomers of three regular steranes, cholestane (C27 ; M+ = 372), 24-methylcholestane (C28 ; M+ = 386), and 24-ethylcholestane (C29 ; M+ = 400) are clearly the dominating compounds in the m/z 217 ion chromatogram and in the TIC (see peaks 4, 8, and 13 in Fig. F2; Table T4). Their relative abundances are given in Table T3 as a percentage, which shows that C27 and C29 steranes are always more abundant than those of C28. No 14,17(H)- or 20S-isomers were found, but minor amounts of C27–C29 steranes with 5,14,17(H)-20R configuration were detected (peaks labeled 2, 6, and 11 in Fig. F2; Table T4).
Based on the ion chromatogram for m/z 257, nine unsaturated steroid hydrocarbons were recognized. Three ster-4-enes (elevated fragment m/z 108) represent a large portion of this specific biomarker group in the TIC trace (peaks 3, 7, and 12 in Fig. F2; Table T4). Ster-5-enes, which coelute with 5,14,17(H)-20R-steranes (Table T4), can be recognized in the m/z 257 ion trace, where they elute as relatively smaller peaks directly after the ster-4-enes. Compounds whose identities could not be assigned are a pair of C29-steradienes (M+ = 396) (peak 10 in Fig. F2; Table T4) and a methyl-C30-sterane coeluting with a methyl-C30-sterene (peak 16 in Fig. F2; Table T4).
Hopanes found in the aliphatic fraction are pentacyclic triterpenoids derived from cell membranes of prokaryotes (heterotrophic bacteria and also phototrophic cyanobacteria) (e.g., Ourisson et al., 1987; Ourisson and Rohmer, 1992). This biomarker group is, like the steranes, characterized by numerous maturity-sensitive stereoisomers (Seifert and Moldowan, 1980). It was possible to identify the majority of compounds in the m/z 191 trace. C30 to C35 17,21(H)- and 17,21(H)-hopanes are prominent compounds with 22R-isomers dominating 22S-isomers but subordinate to 17,21(H)-hopanoids. Most samples contain many unsaturated or rearranged compounds, including 30-norhop-17(21)-ene, 30-norneo-13(18)-ene, and hop-17(21)-ene. Various norhopanes are also present in all samples, including 17- and -trisnorhopane (C27), 17,21(H)-30-norhopane and 17(H)21(H)-30-norhopane, and 17,21(H)-30-norhopane.
Two late-eluting peaks (at 56.6 and 60.6 min; not shown in Fig. F2) are present in nearly all samples. They were identified as 30-(2´-methylenethienyl)-17,21(H)- and -17,21(H)-hopane (hopanoid thiophenes: M+ = 508; elevated m/z 97 fragment for both; elevated m/z 287 fragment for the latter compound) (Valisolalao et al., 1984). 2-Methylhopanoids (m/z 367, 383, and 205), which are indicative of contributions from cyanobacteria (e.g., Summons et al., 1999), were not detected.
The aromatic fractions were searched specifically for isorenieratane and related isorenieratene derivatives (m/z 133). Isorenieratene is a carotenoid pigment exclusively synthesized by photoautotrophic green sulfur bacteria (Chlorobiaceae) and therefore is indicative for euxinic conditions reaching into the photic zone (e.g., Sinninghe Damsté et al., 1993). The molecule isorenieratane itself (M+ = 546) was identified only in Sample 207-1258B-46R-2, 56–57 cm, coeluting with a hopanoid thiophene at 56.6 min (Table T2). In this sample, a variety of the S-containing isorenieratene derivatives previously described by Koopmans et al. (1996b) are present, together with several isomers of isorenieratane thianes (M+ = 576) and thiophenes (M+ = 572) and other isorenieratene derivatives (e.g., diaryl isoprenoids; M+ = 538). Isorenieratane thianes, but none of the other derivatives of isorenieratene, were abundant in four other samples (Table T2).
Source-specific long-chain alkenones—known to be derived from marine Prymnesiophyta (e.g., Marlowe et al., 1990)—were not found in our investigation. They have been recently reported for even older organic matter–rich sediments from the Aptian OAE 1a obtained during Leg 198 on Shatsky Rise in the Pacific Ocean (Sites 1207 and 1213) (Bralower, Premoli Silva, Malone, et al., 2002). Our preliminary results do not necessarily confirm the absence of these compounds at Sites 1257 and 1258 because our investigation was hampered by the low-response detection problem of the shipboard GC-MSD for masses greater than m/z 500. In this respect, reanalysis of the samples on a more sensitive instrument would be advisable.
Fluorescent samples, especially Sample 207-1258C-31R-2, 26–27 cm (Table T1), contain higher amounts of the pentacyclic aromatic hydrocarbon perylene (M+ = 252) than nonfluorescing samples. This molecule is the single most abundant polycyclic aromatic hydrocarbon found in the black shales sampled in this study.