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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 locali- ties 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 con- structed for two Recent species of Dosinia from subtropical waters. Assuming a constant seawater δ 18 O of -1.5‰ (Lear et al., 2000), including latitudinal correction (Zachos et al., 1994), oxygen isotope data yield mean annual temperatures of 19.3°C and 23.5°C and a mean annual range of temperatures of 19.8°C and 11.0°C for Alaska and Kamchatka, respectively. These temperature ranges are comparable to those of modern subtropical (40°N) mixed-layer waters, which implies significant warming episodes at high North Pacific latitudes during the peak of the MMCO. The variations in isotopic composition and inferred temperature ranges, as well as patterns of stratigraphic occurrence, imply that warm surface waters of the MMCO were not persistently present in high North Pacific latitudes, as they were at low latitudes. Instead, relatively short-term incursions of warm surface waters from the subtropical western Pacific episodically introduced warm-water mollusks into this gener- ally cooler region.

Magnitude of Middle Miocene warming in North Pacific high latitudes: stable isotope evidence from Kaneharaia (Bivalvia, Dosiniinae)