In east‐central Brazil, the Ediacaran‐Cambrian Bambuí Basin has the potential to provide a record of unique geochemical responses of Earth's ocean and atmosphere evolution during this key time interval. From this perspective, we studied an interval of the upper Bambuí Basin using sedimentologic, stratigraphic, and chemostratigraphic tools. The lower Cambrian Jaíba Member of the uppermost Serra da Saudade Formation is an interval of up to 60 m‐thick of carbonate rocks disposed into two shallowing upward trends. Inner to outer ramp and high‐energy shoal deposits are described, in which laminated microbialites are the prevailing sedimentary facies. REE + Y data suggest contamination by iron (oxy)hydroxides that are dissociated from the riverine detritic flux. Sedimentary iron enrichment may be related to the settling of iron nanoparticles in coastal environments, diagenetic iron mobilization, or both. MREE enrichment is caused by microbial degradation of organic matter in the iron reduction zone during the anoxic early‐diagenetic stage. Chromium isotopes yielded negatively fractionated values (δ⁵³Cr = −0.69 to −0.27‰), probably resulting from biotic and abiotic reduction of dissolved Cr(VI) to light and less toxic Cr(III) within pores of microbial mats. The δ⁵³Cr data of the Jaíba microbialite are thus a product of metabolic reactions in microbial mats and do not reflect seawater signal. The isotopic offset from seawater is feasible from molecular diffusion of Cr into pore water and reduction reactions occurring deep inside the mat, although the exact mechanism and consequences are not yet fully understood due to the poor preservation of metabolic reactions in the geological record. Our study suggests that Cr isotopes can be used to reconstruct Cr and other metals cycling within ancient microbial mats, and that caution should be taken when using past microbialites to infer seawater Cr records and redox state of the atmosphere and ocean.

中文翻译：

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微生物引起的铬同位素分馏和微量元素的行为，来自巴西班贝盆地Jaíba成员的下寒武统微生物岩

在巴西中东部，Ediacaran-CambrianBambuí盆地有潜力提供在此关键时间间隔内地球海洋和大气演化的独特地球化学响应的记录。从这个角度出发，我们使用沉积学，地层学和化学地层学工具研究了班巴伊盆地上段的间隔。最高Serra da Saudade组的下寒武统Jaíba成员是一个间隔最大为60 m的碳酸盐岩层，分布成两个浅的向上趋势。描述了内部到外部的斜坡和高能浅滩沉积物，其中层状微辉石是主要的沉积相。REE + Y数据表明，从河水的有害通量中分离出的氢氧化铁被氧化。沉积铁的富集可能与沿海环境中铁纳米颗粒的沉降，成岩铁动员或两者都有关系。MREE富集是由于缺氧早成岩阶段铁还原带中有机物的微生物降解引起的。铬同位素产生负分馏值（δ⁵³ Cr = -0.69至-0.27‰），可能是由于微生物和非生物方式将微生物垫孔隙中溶解的Cr（VI）还原为轻质和毒性较小的Cr（III）所致。该δ ⁵³因此，Jaíba微恶岩的Cr数据是微生物垫中新陈代谢反应的产物，不能反映海水信号。尽管由于地质记录中新陈代谢反应的不良保存，确切的机理和后果尚未完全了解，但从Cr分子扩散到孔隙水中以及发生在垫层内部的还原反应中，可以从海水中吸收同位素。我们的研究表明，Cr同位素可用于重建古代微生物垫中的Cr和其他金属循环，并且在使用过去的微生物岩推断海水中的Cr记录以及大气和海洋的氧化还原状态时应格外小心。