TIMING, NATURE, AND EXTENT OF EARLY MIDDLE MIOCENE WARMING IN THE HIGH-LATITUDE NORTH PACIFIC

One of the most intriguing climatic developments of the Late Cenozoic was the early middle Miocene (14.5 to 17 Ma) global warming event known as the Climatic Optimum. When global warming peaked in the earliest middle Miocene, at about 16 Ma, marine and terrestrial temperatures were warmer than today and higher than at any time after the Eocene. The Climatic Optimum was confined mainly to Chrons C5C and C5B, primarily equivalent to the N8 and part of the N9 planktonic foraminiferal zones, with an age range of 14.8 to 16.4 Ma, and lasted for approximately 1.6 M.y. Surface-water warming in the high latitude North Pacific, is clearly implied by the northward incursion of warmer-water molluscan taxa as far north as northwestern Kamchatka and Alaska, up to 62oN. Modern analogs of subtropical mollusks that migrated northward in response to marine warming during the early middle Miocene now live no farther north than 44oN These warm-faunal episodes occur primarily within the Neodenticula lauta diatom zone (15.9 to 14.9 Ma), spanning approximately 1 M.y. Warm-water mollusks in the high latitude North Pacific occupy relatively thin stratigraphic intervals in the Miocene rocks of the southwestern Alaska and northwestern Kamchatka and delineate the northern extent of the climatic optimum. Preliminary evidence implies that warm surface waters of the Climatic Optimum were not persistently present in the high-latitude North Pacific, as they were at low latitudes. Instead, relatively short-term incursions of warm surface waters from the western Pacific periodically introduced warm-water mollusks and planktonic foraminifers into this cooler region. Oxygen isotopic data obtained from sampling shells of bivalves across the growth increments, both as groove samples and each growth line individually, indicate the substantial warming in the high latitudes North Pacific during the Early Middle Miocene Climatic Optimum. Analysis of isotopes across growth increments in shells from different horizons suggests that the seasonality and the intensity of warming were not identical during the different warm water incursions in the high latitudes in the early Middle Miocene