5 research outputs found
The geology of the Daitari Greenstone Belt, Singhbhum Craton, India - insights into early life 3.5 Ga ago
No full textAbstract: Archaean greenstone belts offer the opportunity to study the dynamic early Earth conditions. The Singhbhum Craton (SC) of India hosts several volcano-sedimentary greenstone sequences that preserve Archaean rocks locally exceeding 3.5 Ga. The Daitari Greenstone Belt (DGB) comprises a well-perpetuated submarine volcano-sedimentary succession of mafic-ultramafic rocks, felsic volcanics dated at c. 3.51 Ga, and intercalated banded chert, shale and iron formation. Common volcanic rocks include komatiite, pillow basalt, and felsic volcaniclastic rocks. Felsic volcanic rocks dating back to c. 3.53-3.51 Ga from the Daitari and Gorumahisani greenstone belts mark a period of widespread felsic magmatic event in the SC. Sedimentary rocks of the DGB are dominated by chemical precipitates of banded black-and white-chert, thinly laminated black chert, banded iron formation and minor siliciclastic rock such as shale. The DGB hosts altered silicified/carbonated volcanic rocks and cherts, an assemblage typical for Palaeoarchaean successions...Ph.D. (Geology
Hadaean to Palaeoarchaean stagnant-lid tectonics revealed by zircon magnetism
No full textPlate tectonics is a fundamental factor in the sustained habitability of Earth, but its time of onset is unknown, with ages ranging from the Hadaean to Proterozoic eons1–3. Plate motion is a key diagnostic to distinguish between plate and stagnant-lid tectonics, but palaeomagnetic tests have been thwarted because the planet’s oldest extant rocks have been metamorphosed and/or deformed4. Herein, we report palaeointensity data from Hadaean-age to Mesoarchaean-age single detrital zircons bearing primary magnetite inclusions from the Barberton Greenstone Belt of South Africa5. These reveal a pattern of palaeointensities from the Eoarchaean (about 3.9 billion years ago (Ga)) to Mesoarchaean (about 3.3 Ga) eras that is nearly identical to that defined by primary magnetizations from the Jack Hills (JH; Western Australia)6,7, further demonstrating the recording fidelity of select detrital zircons. Moreover, palaeofield values are nearly constant between about 3.9 Ga and about 3.4 Ga. This indicates unvarying latitudes, an observation distinct from plate tectonics of the past 600 million years (Myr) but predicted by stagnant-lid convection. If life originated by the Eoarchaean8, and persisted to the occurrence of stromatolites half a billion years later9, it did so when Earth was in a stagnant-lid regime, without plate-tectonics-driven geochemical cycling
Organic geochemical evidence for life in Archean rocks identified by pyrolysis–GC–MS and supervised machine learning
No full textThroughout Earth’s history, organic molecules from both abiogenic and biogenic sources have been buried in sedimentary rocks. Most of these organic molecules have been significantly altered by geologic processes through deep time. Nonetheless, the nature and distribution of those ancient fragmentary organic remains have the potential to reveal diagnostic biomolecular information after billions of years of burial. Here, we analyzed 406 fossil, modern biological, meteoritic, and synthetic samples using pyrolysis gas chromatography and mass spectrometry. We explored these analytical data via supervised machine-learning methods to discriminate samples of biogenic vs. abiogenic origin, plant vs. animal phylogenetic affinity, and photosynthetic vs. nonphotosynthetic physiology. Dividing 272 samples with known phylogenetic affinity and physiology into 9 categories, each further divided into 75% training and 25% testing sets, our random forest models accurately predict pairwise assignments of modern vs. fossil or meteoritic organics (100% correct assignments), fossil plant tissues vs. meteoritic organics (97%), modern vs. fossil plant tissues (98%), and modern plants vs. animal tissues (95%). Pairwise comparisons between fossil biogenic samples vs. abiogenic samples resulted in 93% correct classifications, while analysis of modern and ancient photosynthetic vs. nonphotosynthetic samples also resulted in 93% correct assignments. Our analyses demonstrate that molecular biosignatures can survive in ancient fossils and allow for the identification of organismal origins and traits. Consistent with previous morphological and isotopic inferences, we present evidence for biogenic molecular assemblages in Paleoarchean rocks (3.33 Ga) and for photoautotrophy in Neoarchean rocks (2.52 Ga)
