As human activities lower marine oxygen levels, understanding the impact on the marine nitrogen cycle is vital. The Black Sea, which became oxygen-deprived 9600 years ago, offers key insights. By studying organic compounds linked to nitrogen cycle processes, we found that, 7200 years ago, the Black Sea's nitrogen cycle significantly altered due to severe deoxygenation. This suggests that continued marine oxygen decline could similarly alter the marine nitrogen cycle, affecting vital ecosystems.