Santos Felipe Holanda, da Silva Amaral Wagner, Filho Evilarde Carvalho Uchôa, de Andrade Caxito Fabrício, Souza Ana Clara Braga, Martins Douglas Teixeira, de Andrade Feitosa Brenda
Departamento de Geologia, Universidade Federal do Ceará (UFC), Fortaleza, Ceará, Brazil.
Instituto de Geociências, Universidade Estadual de Campinas (UNICAMP), Campinas, São Paulo, Brazil.
Sci Rep. 2024 Sep 11;14(1):21191. doi: 10.1038/s41598-024-71877-3.
Our study helps to unravel the complexity of the Lomagundi-Jatuli event, the largest and longest positive carbon isotope excursion ever recorded on the Earth's surface, by providing a unique view of Paleoproterozoic graphitic rocks from the Borborema province of Northeastern Brazil. Through detailed mineralogical, textural, chemical and isotopic analyses, we bring a new perspective that provide support to elevated primary productivity and large-scale organic carbon burial during the Lomagundi-Jatuli event. Graphite crystals with distinctive textural features occur in association with silicate and oxidised manganese ores, manganese quartzites, garnetites, and gneisses. The graphites were crystallised at temperatures up to 634 °C, consistent with amphibolite facies metamorphism, according to Raman thermometry. An average total carbon content of 2.1 wt%, with δC values ranging from - 15.0 to - 21.5‰, is indicated by whole-rock geochemistry and carbon isotopic composition, respectively. Based on these results, our study proposes that these graphitic rocks may represent remnants of organic matter, possibly derived from bacterial biomass associated with manganese-rich sediments, preserved under reducing environmental conditions in a redox-stratified marine setting. Biological mediation on the origin of silicates is suggested by the close relationship between reduced manganese silicates and graphite. These constraints indicate that Paleoproterozoic graphite-rich rocks represent an important but overlooked reservoir of organic carbon that was partially degassed during the metamorphism of organic-rich sequences. Overall, this research provides new insights for the enigmatic emergence of the Lomagundi-Jatuli event, highlighting the intricate interplay among organic carbon, manganese-rich rocks and Earth's evolutionary processes during this period.
我们的研究通过提供对巴西东北部博尔博雷马省古元古代石墨岩的独特视角，有助于揭示洛马贡迪 - 贾图利事件的复杂性，该事件是地球表面有记录以来规模最大、持续时间最长的正碳同位素偏移。通过详细的矿物学、结构、化学和同位素分析，我们带来了一个新的视角，为洛马贡迪 - 贾图利事件期间初级生产力的提高和大规模有机碳埋藏提供了支持。具有独特结构特征的石墨晶体与硅酸盐、氧化锰矿石、锰石英岩、石榴石岩和片麻岩共生。根据拉曼测温法，这些石墨在高达634℃的温度下结晶，这与角闪岩相变质作用一致。全岩地球化学和碳同位素组成分别表明平均总碳含量为2.1 wt%，δC值范围为 - 15.0至 - 21.5‰。基于这些结果，我们的研究提出这些石墨岩可能代表有机质的残余物，可能源自与富锰沉积物相关的细菌生物量，在氧化还原分层的海洋环境中的还原条件下保存。还原锰硅酸盐与石墨之间的密切关系表明了生物对硅酸盐起源的介导作用。这些限制表明古元古代富含石墨的岩石代表了一个重要但被忽视的有机碳库，在富含有机质序列的变质过程中部分脱气。总体而言，这项研究为洛马贡迪 - 贾图利事件的神秘出现提供了新的见解，突出了这一时期有机碳、富锰岩石和地球演化过程之间复杂的相互作用。
