Chronological and paleoceanographic constraints of M iocene to P liocene ‘mud sea’ in the R yukyu I slands (southwestern J apan) based on calcareous nannofossil assemblages

The C enozoic sedimentary succession in O kinawa‐jima, including the upper M iocene to P leistocene siliciclastic deposits ( S himajiri G roup) and the P leistocene reef to shelf deposits ( R yukyu G roup), suggests a drastic paleoceanographic change from a ‘mud sea’ to a ‘coral sea.’ To delineate the paleoceanographic evolution of the mud sea, we quantified the stratigraphic distribution of the calcareous nannofossil assemblages from the S himajiri G roup in a 2119.49 m‐deep well ( N anjo R 1 E xploratory W ell) drilled in southern O kinawa‐jima ( R yukyu I slands, southwestern J apan). Four late M iocene and P liocene datum planes were found in the studied interval: the first occurrence of A maurolithus spp. (7.42 Ma), the last occurrence of D iscoaster quinqueramus (5.59 Ma), the first occurrence of C eratolithus rugosus (5.12 Ma), and the last occurrence of R eticulofenestra pseudoumbilicus (3.70 Ma). The calcareous nannofossil assemblages from the T omigusuku F ormation and the lower part of the Y onabaru F ormation are characterized by a lower total number of coccoliths and abundant S phenolithus abies that is associated, at times, with common D iscoaster spp. Overall, these suggest the existence of oligotrophic conditions between 5.3 and >8.3 Ma. The total number of coccoliths increased and small R eticulofenestra spp. became more common in the middle part of the Y onabaru Formation, suggesting that eutrophic conditions were present between 3.5 and 5.3 Ma. The rare occurrence of calcareous nannofossils in the upper part of the Y onabaru F ormation indicates a return to oligotrophic conditions at 3.5 Ma. Micropaleontological evidence suggests that these oceanographic changes were likely caused by local tectonic movement (shallowing of the sedimentary basin in which the S himajiri G roup was deposited).