Biogeography--North Pacific Ocean

The Middle Miocene Climatic Optimum (MMCO) at ~ 16 Ma was the warmest interval during the Neogene. The peak of the MMCO in the North Pacific is marked by the appearance of the bivalve genus Kaneharaia (Bivalvia, Dosiniinae) in the high-latitude regions of Kamchatka and Alaska (55 –65°N). Specimens of Kaneharaia sp. were collected from two early middle Miocene high-latitude localities in the North Pacific – the Sea urchin Horizon of northwestern Kamchatka and the Narrow Cape Formation of Alaska. Middle Miocene Kaneharaia specimens were incrementally sampled for oxygen and carbon stable isotope records of seasonality. Results were compared with stable isotope profiles constructed for two Recent species of Dosinia from subtropical waters.

A review of the Oligocene biogeography of northern fulgorariines along the northern Pacific margin, including the western coast of North America, indicates that this subfamily had a much broader distribution during late Paleogene time than today. These data point to more favorable climatic conditions (including lower water temperatures) for dispersal of fulgorariine volutes during the Oligocene.

The large North Pacific bivalve mollusk index-fossil Fortipecten hallae (Dall, 1921) is present in a well-dated stratigraphic section of the Milky River Formation, Alaska Peninsula, southwestern Alaska. Co-occurring marine diatoms belong to the upper part of the subzone B of the Neodenticula kamtschatica diatom zone of the North Pacific diatom chronostratigraphy, with an age range of 4.8–5.1 Ma (early Pliocene). Based on coeval occurrences in northeastern Kamchatka, Russia, and synchronous changes in the two molluscan assemblages, F. hallae is a useful indicator of early Pliocene climatic warming along the high latitude North Pacific margin.