Evidence from Nd isotopes for complex paleogeography and paleocirculation patterns of the Late Triassic Panthalassa.

The Triassic Panthalassa Ocean, a pivotal component of the Earth's paleoceanographic system, has long been understudied due to the scarcity of preserved archives from the open ocean. The previous attempts in reconstructing the Panthalassa paleocirculation from Nd isotopic data were solely based on localities from the eastern continental margins, leaving a significant knowledge gap. This study presents unique Nd isotopic data from the Panthalassa and isotopic variations reveal the existence of distinct oceanic masses in the Triassic Panthalassa Ocean and hence constrain models of regional and global paleogeography. The εNd values for water masses associated with the eastern Panthalassa margin, at around -8 during the Late Triassic, are most likely influenced by the continental sources. Conversely, the Panthalassa εNd signatures obtained from the Japan and Northern California Triassic terranes are in marked contrast to the eastern Panthalassa margin with values ranging between -2.2 to -0.5 ε-units. Those results indicate a Nd isotopes composition of the water masse influenced by mantle-derived mid-ocean ridge basalts. Our data support the paleogeographic PANALESIS model showing that the Panthalassa Ocean is not a wide ocean free of land, but rather presents isolated continental structures in its northern hemisphere. In such scenario, the Japan and Northern California Triassic terranes are thus representative of the open ocean Panthalassa realm. We suggest that the oceanic circulation of the Panthalassa during Late Triassic should have been complex with the possible existence of at least two oceanic cells in the northern hemisphere geographically separated by emerged continental island arcs. Such an oceanic configuration will isolate open ocean water masse from the Panthalassa eastern margin resulting in a rather homogeneous and stable Nd isotopic open ocean signature controlled by mantle-derived products while the margin water masse will remain influenced by the continental sources which are shown to vary throughout the Triassic.