Palaeoproterozoic oxygenated oceans following the Lomagundi-Jatuli Event

The approximately 2,220-2,060 million years old Lomagundi-Jatuli Event was the longest positive carbon isotope excursion in Earth history and is traditionally interpreted to reflect an increased organic carbon burial and a transient rise in atmospheric O₂. However, it is widely held that O₂ levels collapsed for more than a billion years after this. Here we show that black shales postdating the Lomagundi-Jatuli Event from the approximately 2,000 million years old Zaonega Formation contain the highest redox-sensitive trace metal concentrations reported in sediments deposited before the Neoproterozoic (maximum concentrations of Mo = 1,009 μg g^-1, U = 238 μg g^-1 and Re = 516 ng g^-1). This unit also contains the most positive Precambrian shale U isotope values measured to date (maximum ²³⁸U/²³⁵U ratio of 0.79‰), which provides novel evidence that there was a transition to modern-like biogeochemical cycling during the Palaeoproterozoic. Although these records do not preclude a return to anoxia during the Palaeoproterozoic, they uniquely suggest that the oceans remained well-oxygenated millions of years after the termination of the Lomagundi-Jatuli Event.