A paleobathymetric transition during the mid-Pliocene warm period: Ostracode evidence from Japan

We reconstructed a paleoenvironment reflecting the mid-Pliocene warm period (MPWP) off the coast of southwest Japan on the basis of high-resolution statistical analysis of fossil ostracodes. A total of 107 ostracode taxa were obtained from the upper part of the mid-Pliocene Sadowara Formation (~ 3.25–3.12 Ma), Shintomi Town, Miyazaki Prefecture, southwest Japan. Most taxa such as Acanthocythereis munechikai, Argilloecia spp., Bradleya albatrossia, Cytheropteron mediotumidum, and Krithe spp. are extant species distributed in the lower sublittoral to upper bathyal zones. These species have been reported in the Pacific coast of southwest Japan, the Tsushima Strait, and off the coast of China. We estimated the paleobathymetry of the Sadowara Formation using Q-mode factor analysis. The results of the factor analysis showed that a regression occurred during deposition of the upper part of the Sadowara Formation. The lower part of the studied sequence was deposited at water depths greater than 300 m. The water depth decreased upward and reached around 250 m in the upper parts of the formation. The timing of the regression corresponds with the peak of the marine isotope stage, KM2. The large regression estimated to have occurred during deposition of the upper part of the Sadowara Formation could have been related to global cooling events during the MPWP.