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Abstract Enhanced continental phosphorus input into the ocean has been suggested as a potential trigger for the transient oxygenation events during the mid‐Proterozoic; however, the response of phosphorus cycling to these marine oxygenations remains unclear. Here, we report the changes in phosphorus cycling associated with a ∼1.7 Ga transient oxygenation. Abundant authigenic aluminum phosphate–sulfate mineral svanbergite (SrAl3(PO4) (SO4) (OH)6; 8.02 ± 4.92 wt%) is identified within the ∼1.7 Ga Yunmengshan ironstones from the Xiong'er Basin, North China and other contemporaneous basins. This observation provides new evidence to support the suggestion that early diagenetic aluminum phosphate‐sulfate minerals could have represented a critical sink of marine phosphorus during the Proterozoic. We suggest that atmospheric oxygenation and concomitant changes in porewater redox chemistry may have enhanced the formation of early diagenetic phosphates, leading to a negative feedback on the oceanic phosphorus reservoir and atmospheric oxygen levels.

Previous studies suggest that the atmospheric and shallow seawater oxygen levels during the mid- Proterozoic may have remained persistently unbalanced due to the low atmospheric oxygen levels and that, under these conditions, the oxygen concentrations of shallow seawater would largely reflect local oxygenation caused by primary productivity. As a result, in a microbialite-rich setting, it may be difficult to differentiate oxygenation caused by local primary productivity from overall oxygenation of the surface environment. To address this issue, we conducted an integrative study of the ~1.44 Ga stromatolitic carbonates of the Fengjiawan Formation in the Xiong'er Basin, southern North China, using techniques of sedimentology, mineralogy and geochemistry. The results show that the Fengjiawan Formation is dominated by water-column precipitated carbonate mud (now dolomitized), pointing to moderately oxidized marine conditions. A significant and stable negative Cerium (Ce) anomaly (as low as 0.42) is identified in a long interval (>50 m in thickness) of this formation. This Ce anomaly is lower than the minimum values both recorded in the Great Oxidation Event (GOE; minimum 0.76) and in the Neoproterozoic Oxygenation Event (NOE; minimum 0.52), suggesting a significant oxygenation process rather than an oxygen oasis. Some intervals in this formation show I/(Ca + Mg) values higher than the Precambrian background value of 0.5 μmol/mol, providing a further support for shallow seawater oxygenation. This study, together with the enhanced oxygenation identified in the time-equivalent Tieling Formation, indicates a multi-basin oxygenation event in North China at ~1.44 Ga. [ABSTRACT FROM AUTHOR]

Oxygen availability is crucial for the evolution of eukaryotes in geological history, but the redox conditions of the mid-Proterozoic atmospheric-oceanic system remain vigorously debated. Previous geochemical studies suggested a very low mid-Proterozoic atmospheric oxygen level of probably below 0.1%-1% of the present atmospheric level (PAL) with weakly oxygenated surface ocean, which may have suppressed the early evolution of eukaryotes in the ocean. A recent study, however, revealed a pulsed oxygenation event that was recorded in shallow marine carbonate of the middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to =4% PAL based on iodine speciation. This event is associated with the earliest known multi-cellular eukaryotic fossils. To elucidate whether this pulsed oxygenation was limited to shallow local seawaters or could also reflect an increase in atmospheric oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur isotopes in the middle Gaoyuzhuang Formation were studied in two sections of the North China Platform. The results show a positive shift in authigenic Cr isotope from -0.18? to +0.66?, which was followed by a decline to -0.16?. This suggests a rise in atmospheric oxygen sufficient to initiate oxic Cr cycling and isotope fractionation (pO2 > 0.1%-1% PAL). This positive Cr isotope excursion was associated with a transient positive shift in d34SCAS from ~20? to ~50? and a subsequent decline to ~10?, which provides independent evidence of atmospheric oxygenation. This oxygenation may have enhanced oxidative terrestrial weathering and increased sulfate input to the ocean, thus stimulating bacterial sulfate reduction and pyrite burial in deep anoxic seawaters. This is broadly consistent with previous results of carbonate I/(Ca + Mg) and Ce anomaly but also reveals a lag between shallow seawater and atmospheric oxygenation. Fluctuated redox conditions and decoupled oxygenation between shallow seawater and atmosphere during the mid-Proterozoic may help to account for the current debate. [ABSTRACT FROM AUTHOR]