Paleosalinity significance of occurrence and distribution of methyltrimethyltridecyl chromans in the Upper Cretaceous Nenjiang Formation, Songliao Basin, China
Highlights
► Methyltrimethyltridecyl chromans were detected in an Upper Cretaceous lacustrine basin in China. First time found in Chinese sediments older than Tertiary. Their occurrence is indicative of the redox and salinity conditions in the depositional environments. Massive Cretaceous lacustrine source rocks in the Songliao Basin were deposited under a stratified water column.
Introduction
Methyltrimethyltridecyl chromans (MTTCs) are structurally similar to tocopherols (vitamin E) but with a hydrogen replacing an OH at C-6 (R2; Fig. 1). According to the combination of functional groups at the R1, R3 and R4 positions, MTTCs are termed α-MTTC, β-MTTC, γ-MTTC, δ-MTTC, ζ-MTTC and η-MTTC, although η-MTTC has never been found in a geological sample. In addition, MTTCs are classified as trimethyl-MTTC (α-MTTC), dimethyl-MTTC (β-, γ- and ζ-MTTC) and methyl-MTTC (δ-MTTC). Since the initial identification of α- and γ-MTTC in bituminous limestone from a gypsum sedimentary region (Goossens et al., 1984, Sinninghe Damsté et al., 1987), MTTCs have been reported from various modern and ancient sediments and crude oils. Sinninghe Damsté et al. (1989) indicated that δ- and γ-MTTC are dominant in hypersaline environments, while α-MTTC is the most abundant member in non-hypersaline environments associated with an absence or trace of δ-MTTC and γ-MTTC. In addition, β-MTTC is often present in both hypersaline and non-hypersaline environments, without a clear preference (Sinninghe Damsté et al., 1989). Furthermore, the α/δ MTTC ratio has been proposed as a marker for paleosalinity (Sinninghe Damsté et al., 1993, Grice et al., 1998), while a combination of Pr/Ph and MTTCI (MTTCI = α-MTTC/total MTTCs) values have been used to distinguish hypersaline, meso-saline and normal marine environments (Schwark et al., 1998, Peters et al., 2005). MTTCs have also been detected in sediments and crude oils from a wide range of depositional environments in China (Bao et al., 1987, Sheng et al., 1987, Fan and Fu, 1988, Jiang et al., 1990, Jiang et al., 2004, Zhang and Huang, 1990, Huang, 2006). The isomers detected include δ-MTTC, α-MTTC, β-MTTC, γ-MTTC, 6-MTTC and ζ-MTTC, and these are found mainly in Tertiary and younger sediments and crude oils (Zhu et al., 2003, Zhu et al., 2005); they have not been reported from pre-Tertiary sediments in China. The oldest reported occurrence globally was in Phosphoria retort shales of Permian age from the northwestern part of Montana (USA; Sinninghe Damsté et al., 1987).
Despite their wide occurrence, the origin and precursors of MTTCs are not clear and are still debated (Peters et al., 2005). Sinninghe Damsté and co-workers proposed a biological origin, possibly from eubacteria or archaea (Sinninghe Damsté et al., 1987, de Leeuw and Sinninghe Damsté, 1990, Kenig et al., 1995), while other studies suggest that they may be formed abiotically from biological precursors during diagenesis (Li et al., 1995, Lu et al., 2007). Consequently, the use of MTTCs as indicators of paleosalinity is ambiguous.
The finding of MTTCs in Upper Cretaceous sediments from a continuous core of the SK-1 southern borehole in the Songliao Basin of China provides an excellent opportunity for studying their source and significance in a lacustrine environment, including the implications for paleosalinity. In addition, we document new organic geochemical evidence for the paleoenvironmental conditions during deposition of massive lacustrine source rock formations of the Upper Cretaceous in the Songliao Basin.
Section snippets
Samples and experimental procedures
The Songliao Basin is in northeastern China and covers a vast area of 260,000 km2, with a total thickness of 10 km of Cretaceous lacustrine sediments (Fig. 2). The SK-1 boreholes are situated on the Qijia-Gulong Depression, one of the major depositional centers in the basin. During the Upper Cretaceous, the basin developed two major sedimentary settings for organic rich source rocks, associated with the K2q1+2 of the Qingshankou Formation and the K2n1+2 of the Nenjiang Formation. The core samples
Results
Five MTTC isomers were found in the aromatic fractions (Fig. 3). Their occurrence and distribution in the core profile display clear changes coinciding with chronostratigraphic unit changes. Based on the MTTC distribution and the variation in related parameters such as α-MTTC/total-MTTC (MTTCI) and pristane/phytane (Pr/Ph), the profile could be divided into five sections (I–V). Fig. 3 shows the representative summed mass chromatograms for the MTTCs from each section. The details of each section
Discussion
To enable a better understanding and discussion of the significance of the occurrence and distribution of MTTCs and their related ratios, more detailed information for all 42 samples is provided in Table 1, including burial depth, TOC content, hydrogen index (HI), MTTC ratios, gammacerane index and the assembly of algae microfossils and lithology. The related Fig. 4, Fig. 5, Fig. 6 are plotted according to the data in Table 1, which shows that the five sections are clearly distinguished from
Conclusions
- (i)
The occurrence of MTTCs in core samples from the Upper Cretaceous Nenjiang Formation of the Songliao Basin in China is geochemically significant in indicating the redox and salinity conditions of paleo-depositional environments.
- (ii)
The absence of MTTCs and the lack of detectable algal fossils in section IV of the K2n1 interval may be supportive of an algal origin for MTTCs or their precursors.
- (iii)
The δ-MTTC isomer seems to be more sensitive to redox conditions as it is often absent from oxic and low
Acknowledgements
This research was supported by NSFC fund of 40973033 and State “973” Project (2006CB701404). We thank S. Killops and an anonymous reviewer for their constructive comments.
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