Dynamics of nutrient cycles in the Permian–Triassic oceans

Marine biochemical cycles underwent profound changes across the Permian–Triassic (P–T) transition, coinciding with Phanerozoic’s most devastating mass extinction. This review endeavours to untangle the complexity of marine biochemical cycles at this time, focusing on key components of the oceanic nutrient cycles, namely the nitrogen, phosphorus, iron, and molybdenum cycles.

The oceanic nitrogenous nutrient structure saw the shift from nitrate to ammonium dominance in warm and anoxic P–T waters. Nitrogen isotope evidence suggests enhanced denitrification began in the latest Permian, followed by augmented N₂ fixation in the Early Triassic. As nitrification was inhibited by prolonged and widespread anoxia while denitrification enhanced in the same conditions, nitrate was probably depleted while ammonium accumulated. Thus, the lost oceanic fixed-N should have been compensated by enhanced N₂-fixation if the oceanic nutrient-N inventory was in balance. Such changes altered microbial respiration efficiency, promoted algal blooms, and possibly caused ammonium toxication.

A phosphorus burial anomaly is registered in the P–T marine sediments, featuring reduced burial of biogenic apatite and organic phosphorus, a phosphorite gap in continental margins, and unusual diagenetic phosphate replacement in calcitic and aragonitic fossils. This suggests decreased reactive phosphorus availability in shallow waters, conflicting with the expected increase from riverine inputs. This discrepancy points to P sequestration in shelf seas and deep waters, resulting in reactive P deficiency in open surface water. The delivery of riverine nutrients to the open ocean was difficult because of the largely dry Pangaea interiors, enlarged coastal areas, and strong sediments trapping and nutrient uptakes by primary producers in epicontinental seas. This probably led to a general lack of detrital nutrients in Panthalassa.

Iron (Fe) dynamics were equally complex, primarily influenced by atmospheric deposition and oceanic redox conditions. Fe availability in the P–T oceans depended not only on Pangaea’s configuration but, more significantly, on the oceanic redox evolution. As anoxia mobilises sedimentary Fe and facilitates lateral Fe transportation, Fe limitation was more likely to occur in the Permian ocean than in the anoxic Early Triassic ocean. The development of the Lower Triassic ammonitico rosso facies in Neotethys also points to replete Fe supply to the open water.

Molybdenum (Mo) likely became a bio-limiting nutrient in the P–T oceans, due to strong Mo removal in anoxic environments. With a small input into a large sink, Mo scarcities could have been prominent in the open ocean. Even in epicontinental seas, Mo depletion is indicated by low nitrogen isotope values that are suggestive of an absence of Mo-Fe nitrogenase.

Changes in the nutrient cycle impacted the P–T marine productivity, which is faithfully documented in the marine sedimentary record. The observed gaps in chert and phosphorite deposits, alongside reductions in sedimentary organic carbon and phosphorus content, indicate a productivity collapse across the boundary beds, aligned with the phytoplankton shift towards a prokaryote and prasinophyte dominance. These reflect catastrophic environmental changes, and the nuanced interplay of nutrient limitations (e.g., P, N, and Mo) exacerbated by ocean stratification and deoxygenation. Despite theories suggesting anoxia-driven eutrophication feedback, such dynamics might not have been universally predominant across the P–T oceans.