3:45 PM - 4:00 PM
[BCG06-20] Paleoecological reconstruction by molecular fossil analysis of sedimentary rocks deposited across the Devonian-Carboniferous boundary in northeastern Vietnam
Keywords:Devonian-Carboniferous boundary, Hangenberg crisis, Biomarker, Vietnam
The Devonian-Carboniferous (D-C) boundary is characterized by occurrences of ocean anoxic events (OAE) and the Hangenberg Crisis, which caused the extinction event affecting more than 45% of marine and terrestrial genera (Kaiser et al., 2016). Microbial populations in the Hangenberg Crises of South China suggested that microorganisms such as cyanobacteria increased after the extinction of benthic organisms due to OAE (Hou et al., 2022). On the other hand, few studies have been conducted in Southeast Asia compared to Europe, America, and China. In the present study, we performed molecular fossil (biomarker) analysis in the D-C boundary sediments from northeastern Vietnam to reconstruct marine paleoecosystem mainly through the marine organism records.
The samples used were black shales and grey limestones of the Pho Han Formation at the Cat Co section from the Cat Ba Island in northeastern Vietnam, located in the Upper Famenian to the Lower Tournaisian, including the Hangenberg crisis. Powdered samples were extracted by solvents and then separated into fractions by silica-gel column. The aliphatic hydrocarbon fraction was separated into straight-chain and branched/cyclic fractions by urea adduction. These fractions were analyzed by GC-MS.
In biomarker analysis, we could detect steranes (eukaryotes), 13β(H), 14α (H)-tricyclic terpanes (TTs; Tasmanites green algae or Leiosphaeridia acritarch?), hopanes (bacteria), 2-methyl hopanes (cyanobacteria), and aryl isoprenoids (green sulfur bacteria). The C28/C29 sterane ratio and TT/hopane ratio, which was indicators of green algae, were high in the Hangenberg crisis, indicating predominance of green algae in the marine ecosystem. The 2-methyl hopane index (2-MHI), which is an indicator for the relative abundances of cyanobacteria to total bacteria, was very low (<0.1%) in all stratigraphic horizons. These results suggested that the cyanobacteria were absent, and the contribution of eukaryotic algae was consistently high in the Hangenberg crisis in contrast to previous studies. On the other hand, concentrations of aryl isoprenoids were high in the Hangenberg crisis, implying limited euxinic environment inhabiting green sulfur bacteria.
The samples used were black shales and grey limestones of the Pho Han Formation at the Cat Co section from the Cat Ba Island in northeastern Vietnam, located in the Upper Famenian to the Lower Tournaisian, including the Hangenberg crisis. Powdered samples were extracted by solvents and then separated into fractions by silica-gel column. The aliphatic hydrocarbon fraction was separated into straight-chain and branched/cyclic fractions by urea adduction. These fractions were analyzed by GC-MS.
In biomarker analysis, we could detect steranes (eukaryotes), 13β(H), 14α (H)-tricyclic terpanes (TTs; Tasmanites green algae or Leiosphaeridia acritarch?), hopanes (bacteria), 2-methyl hopanes (cyanobacteria), and aryl isoprenoids (green sulfur bacteria). The C28/C29 sterane ratio and TT/hopane ratio, which was indicators of green algae, were high in the Hangenberg crisis, indicating predominance of green algae in the marine ecosystem. The 2-methyl hopane index (2-MHI), which is an indicator for the relative abundances of cyanobacteria to total bacteria, was very low (<0.1%) in all stratigraphic horizons. These results suggested that the cyanobacteria were absent, and the contribution of eukaryotic algae was consistently high in the Hangenberg crisis in contrast to previous studies. On the other hand, concentrations of aryl isoprenoids were high in the Hangenberg crisis, implying limited euxinic environment inhabiting green sulfur bacteria.