Elsevier

Organic Geochemistry

Volume 87, October 2015, Pages 68-77
Organic Geochemistry

Compound-specific carbon isotope study on the hydrocarbon biomarkers in lacustrine source rocks from Songliao Basin

https://doi.org/10.1016/j.orggeochem.2015.07.011Get rights and content

Highlights

  • δ13Corg and molecular carbon isotopes in an U. Cretaceous lacustrine basin in China were studied.

  • δ13CGa and δ13Chop indicate the depth of the chemocline and the water conditions.

  • The paleoenvironment controls OM abundance and affects the microbial diversity.

  • δ13CGa and δ13C values of 4-methyl sterane can reflect variations in water salinity.

Abstract

Stable carbon isotopic composition of organic matter (δ13Corg) and compound-specific δ13C values of biomarkers from 15 lacustrine source rocks were analyzed to identify the original paleoenvironment and source organisms. The δ13C values of hopanes (δ13Chop) ranged from −68.7‰ to −32‰ and exhibit strongly 13C-depleted values in the lower part of Member 1 of the Nenjiang Formation (K2n1, up to −68.7‰), suggesting an origin from predominantly methanotrophic bacteria. 13C-enriched δ13CGa values and significantly 13C-depleted δ13Chop in K2n1, which coincide with water stratification and an intermittent anoxic photic zone, represents a shallow chemocline. The presence of an intermittent anoxic photic zone, which means that the anoxia expanded into the euphotic zone, is beneficial for OM preservation and results in high values of TOC and HI in this section. However, the absence of gammacerane and 13C-enrichment of δ13Chop in Member 2 of Nenjiang Formation (K2n2) reflect a deeper chemocline, corresponding to relatively oxidizing conditions and low values of TOC and HI. Moreover, the negative correlation of TOC vs δ13Corg and HI vs δ13Corg reflects the control of OM formation by sedimentary environments rather than productivity in the water column. Thus, the depth of the chemocline not only controls the abundance of OM but also affects the development of the microbial community, such as chemoautotrophic bacteria in the deep chemocline and chemoautotrophic and methanotrophic bacteria in the shallow chemocline. Moreover, δ13CGa and δ13C values for 4-methyl steranes are related to water salinity, with a higher salinity accompanied by 13C-enrichment in gammacerane and 4-methyl steranes.

Introduction

The Songliao Basin is the largest Mesozoic continental sedimentary basin in China. It contains a continuous series of Cretaceous lacustrine strata and therefore preserves valuable information related to the paleontology, paleoclimate and paleoenvironmental conditions of the basin, which are very important to understanding the geology of Cretaceous continental strata and global climate change.

The depositional environments and source of organic matter (OM) in Songliao source rocks has been extensively studied (Yang et al., 1981, Huang et al., 1998, Feng et al., 2009, Feng et al., 2011, Chamberlain et al., 2013). Prior studies have shown that the Qiangshankou and Nenjiang Formations, the two major sets of OM-rich source rocks, were deposited in a deep freshwater lake, but under eutrophic and anoxic conditions (Wang et al., 2011, Bechtel et al., 2012, Song et al., 2013). Nevertheless, Hou et al. (2000) suggested that the massive hydrocarbon source rocks were developed under brackish–saline conditions and may be associated with marine transgressions. Recently, new lines of evidence regarding marine transgressions from fossils and sulfur geochemistry have been reported (Xi et al., 2011, Huang et al., 2013). Existing investigations reveal that the sedimentary environments are more complex and spatially heterogeneous (Wang et al., 2011, Bechtel et al., 2012). However, we still do not clearly understand the relationship between sedimentary environments and OM sources during the period of the deposition of the massive lacustrine source rocks in the Songliao Basin, such as the water stratification stability, depth of chemocline, and aqueous CO2 circulation as well as the effects of microorganisms. Besides, there are few papers involving the relationship between redox conditions and biological disturbance in the lacustrine sedimentary.

Biomarker distributions, together with their carbon isotopic compositions, can provide insights into the composition of the biota in the original depositional environments (Hayes et al., 1990, Grice et al., 1998, Koch, 1998, Sepúlveda et al., 2009). The present work complements previous geochemical studies by using a stable carbon isotopic approach at the molecular level. In this study, we analyzed the δ13Corg and molecular carbon isotope compositions of the biomarkers based on a closely sampled profile from the Nenjiang Formation in the Songliao Basin. Here we aim to unravel the relationship between the depositional paleoenvironment and original organisms using molecular carbon isotopes of biomarkers.

Section snippets

Geological setting and experimental procedures

The Songliao Basin, located in the northeast of China, is a continental sag basin with late Paleozoic and early Mesozoic consolidated crust as the basement. There have been several expansions and contractions of the Songliao paleo-lake (Feng et al., 2010). There were at least five large lacustrine transgressions during basin development, and each time the lacustrine transgression formed a massive dark mudstone or oil shale. Specifically, during the deposition of K2n1 and K2n2, a wide semi-deep

Basic geochemical parameters

The Tmax values in K2n1+2 are lower than 440 °C, Ts/Tm are approximately 0.2, and 22S/(22S + 22R) of C31 hopane ratios are < 0.5, indicating an immature to low mature stage in these samples. These parameters vary slightly over the whole profile, but indicate a similar maturity for each sample. In other words, the variation of the biomarker parameters and the carbon isotopic values in profile are not due to thermal maturity.

According to the depth variation of total organic matter (TOC), hydrogen

Conclusions

The δ13C variation of OM, steranes and hopanoids in the sediments of K2n1 + 2 are mainly controlled by their parent material inputs and depositional environments. Through the distribution and carbon isotopic composition of each biomarker, we inferred the spatial heterogeneity of aquatic organisms in the Songliao paleo-lake. Photosynthetic algae and bacteria are dominant in the upper oxidizing water, submerged aquatic organisms grow in the upper part of the photic aerobic zone, and anoxic

Acknowledgements

We thank Dr. Jiangtao Guo for assistance with GC–ir–MS and GC–MS analyses. We also thank Andrew P. Murray and two anonymous reviewers for their comments, which significantly improved the paper. This research was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 41321002 and No. 4150020804).

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