The oxygen content of Earth's atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth's oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth's oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest-lived positive δ 13 C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.GOE | Paleoproterozoic | marine chemistry | Mo isotope | trace metal

The depositional setting of the 2·1 Ga fill of the Franceville Basin of Gabon is important for understanding the habitat (energy and availability of light and oxygen) and taphonomy of recently discovered early macro‐organisms buried in black shales in Unit FB. The available data bearing on the stratigraphy and sedimentology of Unit FB provide new insight into processes acting on the palaeo‐sea floor. The shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands. The latter (Poubara sandstones) were emplaced during a forced regression during the terminal infill of fault‐bounded sub‐basins following a stage characterized by a ferruginous to anoxic water column. The structureless sandstones were deposited from high‐density gravity currents along with a locally strong bottom oscillation of the water column. Tuft structures preserved in cyanobacterial mats, together with the position of the macro‐organisms at the top of the sandstone beds within associated black shales, point to a water depth of less than 80 m. The relative sea‐level fall that drove deposition of the Poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows.

No abstract