5:15 PM - 6:45 PM
[BCG06-P08] Paleoecological reconstruction by molecular fossil analysis of the Mesoproterozoic sedimentary rocks from northwestern Greenland
Keywords:Mesoproterozoic, Biomarker, Bacteria, Palynomorph, Greenland
Paleontological investigation for body fossil suggested that the evolutions of marine primary producers such as cyanobacteria and eukaryotic algae have been thought to be promoted by expansion of shallow ocean environment under long-term climatic warming by volcanic activity during the Mesoproterozoic (Taira, 2007). Molecular biological investigations supported that the pronounced promotion of eukaryote evolution was possibly occurred during this era (Philippe and Adoutte, 1998). On the other hand, molecular fossil (biomarker) records indicated bacteria were predominated but eukaryotic algae were poor in the ecosystem (Brocks et al., 2023). In the present study, we performed biomarker analysis and morphological observation of the palynomorph fossils in the Mesoproterozoic sediments from northwestern Greenland to reconstruct marine paleoecosystem mainly through the marine organism records during the Mesoproterozoic.
The samples used were greyish shales of the Qaanaaq Formation belonging to the Dundas Group, collected from the Thule Basin in northwestern Greenland during July-August 2018 and 2019. These expeditions were supported by MEXT through Arctic Challenge for Sustainability (ArCS) the Arctic region research project. The kerogens were separated by HCl and HF treatments of the sediments, and observed under transmitted and fluorescent lights microscope.
In biomarker analysis, we could detect 2-methyl hopanes (cyanobacteria), 13α(n-alkyl)-tricyclic terpane (αNATT; Pseudomonas bacteria), 13β(H), 14α (H)-tricyclic terpane (TT; origin unknown bacteria), sterane (eukaryotic algae), and cadalene (eukaryotic algae?). The 2-methyl hopane index (2-MHI), which is an indicator for the relative abundances of cyanobacteria to total bacteria, were consistently high in all stratigraphic horizons, indicating predominance of cyanobacteria in the marine ecosystem. Concentrations of αNATT and TT tended to decrease with upper horizons. On the other hand, concentrations of sterane and cadalene rapidly increase in the uppermost horizon. These results suggested that cyanobacteria were main primary producer and contribution of eukaryotic algae increased in the marine ecosystem during the deposition of the Qaanaaq Formation. In palynomorph analysis, we could identify acritarch fossils such as Leiosphaeridia sp., Synsphaeridium sp., Satka sp., Navifusa majensi, Simia sp., Lophosphaeridium sp. and Schizofusa sp. These assemblage compositions varied with the horizons, which possibly reflected marine environments and ecosystem. These palynomorph records can be concordant with the biomarker records, which indicated the cyanobacteria-based marine ecosystem.
The samples used were greyish shales of the Qaanaaq Formation belonging to the Dundas Group, collected from the Thule Basin in northwestern Greenland during July-August 2018 and 2019. These expeditions were supported by MEXT through Arctic Challenge for Sustainability (ArCS) the Arctic region research project. The kerogens were separated by HCl and HF treatments of the sediments, and observed under transmitted and fluorescent lights microscope.
In biomarker analysis, we could detect 2-methyl hopanes (cyanobacteria), 13α(n-alkyl)-tricyclic terpane (αNATT; Pseudomonas bacteria), 13β(H), 14α (H)-tricyclic terpane (TT; origin unknown bacteria), sterane (eukaryotic algae), and cadalene (eukaryotic algae?). The 2-methyl hopane index (2-MHI), which is an indicator for the relative abundances of cyanobacteria to total bacteria, were consistently high in all stratigraphic horizons, indicating predominance of cyanobacteria in the marine ecosystem. Concentrations of αNATT and TT tended to decrease with upper horizons. On the other hand, concentrations of sterane and cadalene rapidly increase in the uppermost horizon. These results suggested that cyanobacteria were main primary producer and contribution of eukaryotic algae increased in the marine ecosystem during the deposition of the Qaanaaq Formation. In palynomorph analysis, we could identify acritarch fossils such as Leiosphaeridia sp., Synsphaeridium sp., Satka sp., Navifusa majensi, Simia sp., Lophosphaeridium sp. and Schizofusa sp. These assemblage compositions varied with the horizons, which possibly reflected marine environments and ecosystem. These palynomorph records can be concordant with the biomarker records, which indicated the cyanobacteria-based marine ecosystem.