Classification and organic provenance of microbialite peloids in early-Triassic

Key words: 
• Permian-Triassic /
• South China /
• microbialite /
• peloid /
• organic origin

Abstract: [Objective] After the end-Permian mass extinction, microbialites were widely distributed in South China, indicating a microbial bloom following the catastrophe. However, microbialites generally exhibit low paleo-productivity, which seems contradictory to the microbial explosion. This study analyzes the microbialites at the base of the Triassic Tianwan section in the Luodian area of Guizhou Province, focusing on the petrological characteristics of micritic pellets and their thermal metamorphic evolution and material sources of organic matter. It clarifies the respective influences of microbial-derived organic matter and seawater-trapped organic matter on pellet formation, aiming to provide empirical evidence for exploring carbon cycling mechanisms in post-extinction microbialite systems. [Methods] This study employs in situ micro-area analytical techniques including optical microscopy, fluorescence microscopy, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), and laser Raman spectroscopy to conduct a detailed analysis of various types of peloids within microbialites from the Tianwan section. These methods reveal the distribution patterns and thermal metamorphic evolutionary characteristics of internal organic matter. [Discussions] Based on morphology and infill materials, various peloids were classified into six types. Raman spectroscopy-derived metamorphic roasting temperatures provide insights into the thermal history and transformation of these peloids. The organic matters in I, II, IV, and VI (MIP, RMIP, ISP, and AMIP) types of peloids show similar thermal maturity to that of primary cyanobacteria in microbialites and shell fragments, indicating a common diagenetic history and microbial origin. Among them, peloid I and peloid II exhibit low thermal maturation temperatures, with organic matter mainly derived from the photosynthetic and metabolic processes of primary cyanobacteria and other microorganisms. Peloid IV and peloid VI show moderate thermal maturation temperatures, with their organic matter formed through the combined processes of detrital micritization and biological detritus micritization. In contrast, peloids III and V (ADP and DP) exhibit significantly higher metamorphic temperatures, suggesting multiple thermal alteration events and a potential detrital or externally derived component, possibly from reworked organic-rich sediments or transported detrital material. [Results] In summary, after the end-Permian mass extinction, microbial blooms, especially cyanobacterial photosynthesis, led to the generation of large amounts of dissolved organic carbon (DOC) in the oceans. Due to frequent fluctuations in seawater redox conditions and rapid temperature increases, DOC was typically oxidized into CO2 and released into the atmosphere, with only a small fraction being preserved as particle organic carbon (POC) and deposited on the seafloor. Raman geothermometry analysis shows that most of the organic matter in the microbialite peloids, particularly those associated with bioclastic peloidal envelopes, originates from the metabolic activity of primary cyanobacteria, representing the original organic matter within the microbialite system. A small portion of the organic matter may have been sourced from long-stored terrestrial organic matter in seawater, which underwent multiple diagenetic thermal alterations and shows higher thermal maturation temperatures. The thermal maturation of organic matter and mineral recrystallization during diagenesis had a significant impact on the preservation efficiency of organic matter, ultimately resulting in a low total organic carbon (TOC) content in the microbialites.