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The presence of the bivalve mollusks Astarte (Tridonta) borealis Schumacher and A. (T.) hopkinsi new species, in uppermost Miocene or lower Pliocene strata of the Milky River Formation on the Alaska Peninsula, southwestern Alaska, signals the earliest opening of Bering Strait. These species migrated from the Arctic Ocean into the North Pacific when the Bering Strait first flooded and, along with co-occurring marine diatoms, are primary evidence for the earliest opening of the strait, in the latest Miocene or early Pliocene. These paleogeographically important Alaskan Astarte have been cited in this context, but have not been previously illustrated or discussed.

Abstract: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. 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 generally cooler region.

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