Centro de Pesquisas Manoel Teixeira da Costa, Instituto de Geociências, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil.
Department of Geoscience and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark.
Geobiology. 2021 Mar;19(2):125-146. doi: 10.1111/gbi.12426. Epub 2020 Dec 21.
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 (δ Cr = -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 δ Cr 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.
在巴西中东部，埃迪卡拉纪-寒武纪的班古伊盆地有可能提供地球海洋和大气演化在这一关键时间间隔独特地球化学响应的记录。从这个角度来看，我们使用沉积学、地层学和地球化学地层学工具研究了上盆地的一个间隔。最上层的 Saudade 组的下寒武统的 Jaíba 成员是一个 60 米厚的碳酸盐岩间隔，被划分为两个向上变浅的趋势。内到外斜坡和高能浅滩沉积物被描述，其中纹层微生物岩是主要的沉积相。REE+Y 数据表明，铁（氧）氢氧化物的污染是从河流碎屑通量中分离出来的。沉积铁的富集可能与铁纳米颗粒在沿海环境中的沉降、成岩铁的迁移或两者都有关。MREE 的富集是由于在缺氧早成岩阶段微生物降解铁还原带中的有机质。铬同位素产生负分馏值（δ Cr=-0.69 至-0.27‰），可能是由于生物和非生物还原溶解 Cr(VI)为光和毒性较小的 Cr(III)在微生物席的孔隙内。Jaíba 微生物岩的 δ Cr 值因此是微生物席代谢反应的产物，而不反映海水信号。由于在地质记录中代谢反应保存不佳，Cr 同位素从海水的偏移是从 Cr 通过分子扩散进入孔隙水和在席内深处发生还原反应中产生的。我们的研究表明，铬同位素可用于重建古代微生物席内铬和其他金属的循环，并且在使用过去的微生物席来推断海水 Cr 记录和大气及海洋的氧化还原状态时应谨慎行事。
