Research On Mechanism Of The Organic Matter Enrichment And Geological Events In The Shahejie Formation,Dongying Depression,Bohai Bay Basin,China

Lacustrine organic-rich source rocks(shales and mudstones)were examined to assess the palaeo depositional environment and organic input sources of the third and fourth members of the Eocene Shahejie Formation,Bohai Bay Basin,China.Major organic contributions to the Shahejie Formation from dinoflagellates,diatoms,Botryococcus and marine Pelagophyte/Chrysophyte algae were determined by n-alkane distributions,gammacerane index and diagnostic steranes.The rare presence of higher plant biomarkers suggests that terrigenous organic inputs were limited.The pervasive presence of?-carotane,aryl isoprenoids,paleorenieratane,renieratane,isorenieratane,2?-methylhopanes and crocetane indicate significant organic contributions from bacterial communities including cyanobacteria,purple/green sulphur bacteria(Chromatiaceae/Chlorobiaceae),sulphate-reducing bacteria and anaerobic methane oxidisers(ANME).The extent of H₂S content,bacterial sulphate reduction,and photic zone euxinia(PZE)were evaluated by determining the aryl isoprenoids ratio(AIR),the presence of isorenieratene derivatives,and the fate of sulphur in palaeo-water.There was a decrease of PZE extent and duration from the northeast to the southwest of the palaeo-lake,and there was a decrease of water body salinity and bottom water anoxia from the fourth to the third member of the Shahejie Formation.Abrupt environmental perturbations in the third member of the Shahejie Formation are indicated not only by significant changes in parameters such as the pristane/phytane and?-carotane/n-C₂₀alkane ratios,the gammacerane index,the isorenieratane/C₁₈ aryl isoprenoid,and the AIR,but also by the time lag in the expansion of cyanobacterial lineages after the expansion of purple/green sulphur bacteria,as well as seawater incursions.The unexpected occurrence of intact C₃₀ 4-desmethylsteranes(24-n-propylcholestanes)in the Shahejie Formation is first reported here,after very careful checking,and provides unambiguous biological evidence for at least two marine transgression events reaching the Eocene lake.The sources and origins of the polycyclic aromatic hydrocarbons(PAHs)have been assessed in thirty eight Paleogene organic-rich shales and mudstones from the Dongying Depression,Bohai Bay Basin.The parameters for the studied samples indicate an diagenetic/petrogenic(fossil fuel)origin and possibly mixed source of combustion and diagenesis/catagenesis.The absence of larger,high temperature wildfires is due to either inadequate seasonality of humid-arid climates or limited terrigenous organic input.The thermal maturation is the major control on the relative abundance and distribution of those non-combustion derived PAHs in lacustrine sediments.An organic rich,low maturity and well laminated shale was examined to assess the heterogeneity of the hydrocarbons as proxies for differences in depositional environment and source.This work provides the firstly geochemical analyses of vertical slices at millimetre scale.The depleted?¹³C values of n-C_25-31 alkanes support the organic contributions from freshwater algae(e.g.Botryococcus braunii A race)rather than terrigenous plants.The 24-n-propylcholestane occurred in great abundance,which indicates seawater influx.The presence of isorenieratane together with varying aryl isoprenoid ratios suggest the palaeowater experienced episodic stratification and PZE.A variable depositional environment was present instead of a permanent and averaging one.The obligate anaerobes(e.g.sulphate reducing bacteria)degraded and damaged the organic matter which is initially provided by phytoplankton at the surface water layer.Also the sulphate reducing bacteria are probably responsible for the dolomites formation.Heterotrophic photosynthesiser(e.g.green sulphur bacteria)episodically thrived and reduced the population of phytoplankton.The correlations of total organic carbon with those parameters for water conditions provide a newly dynamic mechanism(at least in millimetre scale)of organic matter preservation which is mainly controlled by the interactions of different microorganisms and sulphur cycle in the water column.