Paleo-Oceans Were Still Transitioning to a Modern State During the Frasnian (late Devonian)

The timing and process of the paleo-oceanic transition to a modern-like state remain in debate. The Frasnian, following the emergence of the first forests in the Givetian, represents a key stage during which geochemical proxies indicate a trend toward increasing oxygenation. However, whether the Frasnian ocean achieved a modern-style oxygenation state requires further investigation from a spatial perspective. Here, we present systematic geochemical and sedimentological analyses of three carbonate sections (two platform and one slope-basin facies) in South China, including carbonate carbon and oxygen isotopes (δ13Ccarb ,δ18Ocarb), organic carbon isotope (δ13Corg), carbonate-associated phosphate (CAP), Fe speciation, cerium anomaly (Ce/Ce*), and scanning electron microscopy (SEM). These data are used to characterize the cycling of key elements (C–P–O) and the redox conditions of both shallow and deep water masses during the Frasnian. Our results show that: (1) During the Frasnian, δ13Ccarb underwent a gradual negative excursion in shallow-water sections in contrast to a positive excursion in deep-water sections; (2) CAP content was higher in deep-water areas (mean 0.12) than in shallow-water areas (means <0.04); (3) Shallow-water facies were more frequently anoxic than deep-water facies during the early Frasnian, as shown by iron speciation and Ce/Ce* data. These results suggest the Frasnian ocean already showed some key modern features: gradually increasing phosphorus concentrations from shallow to deep waters and a shallow-water oxygen minimum zone. However, bottom and pore waters in slope settings remained anoxic, with the chemocline likely still within the lower part of the water column rather than below the sediment-water interface. Redox analyses of Frasnian marine facies representing a range of water depths not only enhance our understanding of the history of oceanic oxygenation but also potentially offer insights into controls on the Frasnian–Famennian mass extinction.