Oxygen isotope data from surface-dwelling calcareous marine organisms in the climatically sensitive high-latitudes is critical to evaluate the modeled role of meridional oceanic heat transport that creates "equable" meridional thermal distribution during past episodes of climatic warmth. The early middle Miocene global "climatic optimum" (MMCO; ca. 14.5 approximately 17 Ma) was the warmest episode of the entire Neogene as suggested by the northward incursion of warmer-water molluscan faunas in the North Pacific. However, due to rare occurrence of planktic foraminifer, no estimates of d18O-based Miocene sea-surface temperatures is available for the high-latitude North Pacific.We have established a Miocene record of fine-fraction (<63 mu m) carbonate, primarily calcareous nannofossils, oxygen isotopes supplemented by planktic foraminifer d18O data at ODP Site 883 (51 degrees 12'N 167 degrees 46'E) encompassing the MMCO. Although interpretation of isotope values may be complicated by floral-specific disequilibrium fractionation factors, the magnitude of fine-fraction isotope variation is useful where shallow-dwelling planktonic foraminifers are not present. The Site 883 fine-fraction d18O record exhibits a distinct negative excursion by about 1.1 (from 1.3 to 0.2) during MMCO time. Published d18O data for shallow-dwelling planktic foraminifers at the equatorial Pacific Site 289 (0 approximately 29'S 158 approximately 30'E) change only slightly (by *0.4) at that time. The MMCO d18O negative excursion at Site 883 is followed by a rapid positive shift to 1.8 at approximately 14 Ma, which may coincide with the global late middle Miocene benthic positive d18O shift representing growth of the East Antarctic ice sheet. The differences between fine-fraction d18O records at Site 883 and those of planktic foraminifers at Site 289 suggest that the latitudinal thermal gradient was reduced significantly during MMCO. In the absence of CO2 forcing of middle Miocene climate change, as argued by Pagani et al. (1999), some combination of increased poleward oceanic heat transport and reduced global albedo may be the most plausible mechanism that warmed the high-latitude North Pacific region during the MMCO.