MIDDLE MIOCENE WARMING AT HIGH LATITUDES: BIOGEOGRAPHIC CONSEQUENCES AND ISOTOPIC SIGNATURES

Middle Miocene Climatic Optimum (MMCO) at ~ 16 Ma was the warmest interval during the Neogene. Faunal composition of high latitude molluscan assemblages in Kamchatka and Chukotka (from 56 to 66^oN) and stable isotopic profiles of selected genera were used as proxy to infer the extent of biogeoraphic provinces and marine paleotemperatures in the high latitudes North Pacific. Faunal and isotopic data were calibrated against Recent mollusks from middle and high latitudes. Peak of the climatic optimum at 62^oN is marked by the appearance of the bivalve genus Dosinia. Stratigraphic interval directly below the Dosinia occurrence is characterized by the uniform composition of molluscan assemblages from 56 to 66^oN. Both bulk d¹⁸O samples and the growth profiles of the Middle Miocene Macoma suggest the range of growth temperatures from 10.7 to 21.8^oC. High resolution ?18O profiles of Recent and Middle Miocene Macoma from the Gulf of Rekinniki suggest that growth temperatures were from 4.9 to 8.2^oC higher during the MMCO than today. Faunal composition and isotopic data indicate that modern counterparts of the Middle Miocene assemblages exist today in the southern part of the Subarctic province. Their occurrences at 66^oN in Chukotka imply approximately 18 - 21 degrees (2500 - 3000 km) latitudinal shift to the North during the MMCO which suggest that the southern part of the Subarctic province was considerably more laterally extensive in the Middle Miocene, than today. The shape of biogeographic boundaries during the Middle Miocene resembles those of today in the maximum northward extension following the distribution pattern of shallow-water assemblages.