Sediment subduction in Hadean revealed by machine learning.

Due to the scarcity of rock samples, the Hadean Era predating 4 billion years ago (Ga) poses challenges in understanding geological processes like subaerial weathering and plate tectonics that are critical for the evolution of life. The Jack Hills zircon from Western Australia, the primary Hadean samples available, offer valuable insights into magma sources and tectonic genesis through trace element signatures. However, a consensus on these signatures has not been reached. To address this, we developed a machine learning classifier capable of deciphering the geochemical fingerprints of zircon. This allowed us to identify the oldest detrital zircon originating from sedimentary-derived "S-type" granites. Our results indicate the presence of S-type granites as early as 4.24 Ga, persisting throughout the Hadean into the Archean. Examining global detrital zircon across Earth's history reveals consistent supercontinent-like cycles from the present back to the Hadean. These findings suggest that a significant amount of Hadean continental crust was exposed, weathered into sediments, and incorporated into the magma sources of Jack Hills zircon. Only the early operation of both subaerial weathering and plate subduction can account for the prevalence of S-type granites we observe. Additionally, the periodic evolution of S-type granite proportions implies that subduction-driven tectonic cycles were active during the Hadean, at least around 4.2 Ga. The evidence thus points toward an early Earth resembling the modern Earth in terms of active tectonics and habitable surface conditions. This suggests the potential for life to originate in environments like warm ponds rather than extreme hydrothermal settings.

Low oxygen levels with high redox heterogeneity in the late Ediacaran shallow ocean: Constraints from I/(Ca + Mg) and Ce/Ce* of the Dengying Formation, South China.

Most previous studies focused on the redox state of the deep water, leading to an incomplete understanding of the spatiotemporal evolution of the redox-stratified ocean during the Ediacaran-Cambrian transition. In order to decode the redox condition of shallow marine environments during the late Ediacaran, this study presents I/(Ca + Mg), carbon and oxygen isotope, major, trace, and rare earth element data of subtidal to peritidal dolomite from the Dengying Formation at Yangba, South China. In combination with the reported radiometric and biostratigraphic data, the Dengying Formation and coeval successions worldwide are subdivided into a positive δ13 C excursion (up to ~6‰) in the lower part (~551-547 Ma) and a stable δ13 C plateau (generally between 0‰ and 3‰) in the middle-upper part (~547-541 Ma). The overall low I/(Ca + Mg) ratios (<0.5 µmol/mol) and slightly negative to no Ce anomalies (0.80 < [Ce/Ce*]SN < 1.25), point to low-oxygen levels in shallow marine environments at Yangba. Moreover, four pulsed negative excursions in (Ce/Ce*)SN (between 0.62 and 0.8) and the associated two positive excursions in I/(Ca + Mg) ratios (up to 2.02 µmol/mol) are observed, indicative of weak oxygenations in the shallow marine environments. The comparison with other upper Ediacaran shallow water successions worldwide reveals that the (Ce/Ce*)SN and I/(Ca + Mg) values generally fall in the Precambrian range but their temporal trends differ among these successions (e.g., Ce anomaly profiles significantly different between Yangba and the Yangtze Gorge sections), which point to low oxygen levels with high redox heterogeneity in the surface ocean. This is consistent with the widespread anoxia as revealed by low δ238 U values reported by previous studies. Thus, the atmospheric oxygen concentrations during the late Ediacaran are estimated to be very low, similar to the case during the most Mesoproterozoic to early Neoproterozoic period.

Biogenic Iron-Rich Filaments in the Quartz Veins in the Uppermost Ediacaran Qigebulake Formation, Aksu Area, Northwestern Tarim Basin, China: Implications for Iron Oxidizers in Subseafloor Hydrothermal Systems.

Fe-(oxyhydr)oxide-encrusted filamentous microstructures produced by microorganisms have been widely reported in various modern and ancient extreme environments; however, the iron-dependent microorganisms preserved in hydrothermal quartz veins have not been explored in detail because of limited materials available. In this study, abundant well-preserved filamentous microstructures were observed in the hydrothermal quartz veins of the uppermost dolostones of the terminal-Ediacaran Qigebulake Formation in the Aksu area, northwestern Tarim Basin, China. These filamentous microstructures were permineralized by goethite and hematite as revealed by Raman spectroscopy and completely entombed in chalcedony and quartz cements. Microscopically, they are characterized by biogenic filamentous morphologies (commonly 20-200 µm in length and 1-5 µm in diameter) and structures (curved, tubular sheath-like, segmented, and mat-like filaments), similar to the Fe-oxidizing bacteria (FeOB) living in modern and ancient hydrothermal vent fields. A previous study revealed that quartz-barite vein swarms were subseafloor channels of low-temperature, silica-rich, diffusive hydrothermal vents in the earliest Cambrian, which contributed silica to the deposition of the overlying bedded chert of the Yurtus Formation. In this context, this study suggests that the putative filamentous FeOB preserved in the quartz veins might have thrived in the low-temperature, silica- and Fe(II)-rich hydrothermal vent channels in subseafloor mixing zones and were rapidly fossilized by subsequent higher-temperature, silica-rich hydrothermal fluids in response to waning and waxing fluctuations of diffuse hydrothermal venting. In view of the occurrence in a relatively stable passive continental margin shelf environment in Tarim Block, the silica-rich submarine hydrothermal vent system may represent a new and important geological niche favorable for FeOB colonization, which is different from their traditional habitats reported in hydrothermal vent systems at oceanic spreading centers or volcanic seamounts. Thus, these newly recognized microfossils offer a new clue to explore the biological signatures and habitat diversity of microorganisms on Earth and beyond.