The Nonesuch Formation microbiota provide a window into ca. 1075 Ma life within the interior of ancient North America. The Nonesuch water body formed following the cessation of widespread volcanism within the Midcontinent Rift as the basin continued to subside. In northern Michigan and Wisconsin, USA, the Copper Harbor Conglomerate records terrestrial alluvial fan and fluvial plain environments that transitioned into subaqueous lacustrine deposition of the Nonesuch Formation. These units thin toward a paleotopographic high associated with the Brownstone Falls angular unconformity. Due to these “Brownstone Highlands,” we were able to explore the paleoenvironment laterally at different depths in contemporaneous deposits. Rock magnetic data constrain that when the lake was shallow, it was oxygenated as evidenced by an oxidized mineral assemblage. Oxygen levels were lower at greater depth—in the deepest portions of the water body, anoxic conditions are recorded. An intermediate facies in depth and redox between these endmembers preserves detrital magnetite and hematite, which can be present in high abundance due to the proximal volcanic highlands. This magnetic facies enabled the development of a paleomagnetic pole based on both detrital magnetite and hematite that constrains the paleolatitude of the lake to 7.1 ± 2.8°N. Sediments of the intermediate facies preserve exquisite organic-walled microfossils, with microfossils being less diverse to absent in the anoxic facies where amorphous organic matter is more likely to be preserved. The assemblage of cyanobacteria and eukaryotes (both photoautotrophs and heterotrophs) lived within the oxygenated waters of this tropical Mesoproterozoic water body.

中文翻译：

---

重建中元古代含氧海岸线的古环境及其生命记录

Nonesuch地层微生物群提供了一个了解约的窗口。1075 年，马在古代北美内陆生活。随着盆地持续下沉，中大陆裂谷内广泛的火山活动停止后，Nonesuch水体就形成了。在美国密歇根州北部和威斯康星州，铜港砾岩记录了陆地冲积扇和河流平原环境，这些环境转变为 Nonesuch 地层的水下湖相沉积。这些单位向与布朗斯通瀑布角不整合相关的古地形高地变薄。由于这些“褐石高地”，我们能够横向探索同时期沉积物不同深度的古环境。岩石磁性数据表明，当湖水较浅时，它被氧化，氧化矿物组合证明了这一点。深度越深，氧气含量越低——在水体最深处，记录到缺氧条件。这些端元之间的深度和氧化还原中间相保留了碎屑磁铁矿和赤铁矿，由于邻近的火山高地，它们可能大量存在。这种磁相使得基于碎屑磁铁矿和赤铁矿的古磁极得以发展，将湖泊的古纬度限制在 7.1 ± 2.8°N。中间相的沉积物保存了精美的有机壁微化石，而在缺氧相中，微化石的多样性较低甚至不存在，而缺氧相更有可能保存无定形有机质。蓝细菌和真核生物（光合自养生物和异养生物）的组合生活在这个热带中元古代水体的含氧水中。