The history of the Tsushima Warm Current since the middle Miocene: Co-evolution with the Kuroshio Current and the Western Pacific Warm Pool

The Western Pacific Warm Pool (WPWP), a vast reservoir of heat and moisture, plays a critical role in global/regional climates. The Kuroshio Current (KC) transports and distributes this heat and moisture from the WPWP to the northern mid-latitudes. Despite the KC’s importance, its long-term evolution and links to the development of the WPWP remain unclear. Here we investigate the history of the Tsushima Warm Current (TWC), a branch of the KC, to constrain the evolution of the KC and WPWP. Diatom assemblages and diatom-based paleo-temperature index from Integrated Ocean Drilling Program Sites U1425 and U1430 in the Japan Sea reveal the history of the TWC since ∼15 Ma. The early TWC entered the Japan Sea from ∼12 to 11 Ma, coinciding with the emergence of the proto-KC and WPWP attributed to the gradual constriction of the Indonesian Gateway. Presence of tropical-subtropical diatoms in the Japan Sea from 10 to 7 Ma provides new evidence for TWC's inflow, accompanied by stronger KC. The absence of warm-water diatoms in the Japan Sea from 7 to 4 Ma indicates a cessation of TWC's inflow, accompanied by weaker KC linked to late Miocene global cooling. From 4 to 3 Ma, the repeated inflow of the TWC into the Japan Sea was associated with establishment of the modern KC and modern WPWP, in response to final closure of the Panama Seaway and further restriction of the Indonesian Gateway. From 3 to 2 Ma, the intensity of TWC became weak due to Northern Hemisphere Glaciation, corresponding to weakening of the Kuroshio system and WPWP. Following the Mid-Pleistocene Climate Transition, WPWP contracted and TWC periodically flowed into Japan Sea as a result of sea-level related controls on the strength of KC and the relative restriction of Tsushima Strait.