Based on foraminiferal biostratigraphy of the Neogene sedimentary sequences of Shmidta Peninsula at the northern tip and the Makarov region in southeastern Sakhalin, Russian Far East, paleoenvironment and ages are discussed. In the Makarov region, the history of environmental changes is roughly delineated, as follows: warm non-marine environment in late Early Miocene, transgression around early Middle Miocene, abrupt deepening into bathyal depth in Middle Miocene, and regression during Late Miocene and Pliocene. This succession is quite similar to those in Hokkaido and the northern Honshu in Japan. In the Pil'vo section at the west coast of Shmidta Peninsula, the similar bathymetric changes are recognized at the same time as in the Makarov region, although this section is regarded as being in the bathyal depths throughout Early to Middle Miocene.
The occurrences of some temperate-water species give evidence of the influence of warm-surface water during the Miocene period. Ammonia spp. survived in early Middle Miocene (the transition period between the MNCO and the following cooler stage in early Middle Miocene) in the Makarov region. A planktonic foraminifer, Globorotalia cf. miozea conoidea was found in the middle part of the Pili Formation (early Middle Miocene) of the Pil'vo section. This is the first discovery of keeled Globorotalia in Sakhalin, and is important for the consideration of the geohistory of the Northwest Pacific, in particular the first appearance of subarctic climate and cold Oyashio water.
