Scientific drilling in the high-latitude North Pacific during ODP Leg 145 has resulted in a new and detailed insight into the paleoceanographic and paleoclimatic record of this important region. Many of these accomplishments were made possible by the development of a new, much more aggressive, piston coring technique. This new technique resulted in very long APC cores which, in turn, allowed the construction of long and continuous magnetic reversal stratigraphies. Results fall into four categories, events of the Plio-Pleistocene, the Neogene, the Paleogene, and a special events category. The first category is the important changes at 2.6 Ma, the time of the onset of major Northern Hemisphere glaciation (timescale of Cande and Kent, 1992). Dropstones appear in abundance at 2.6 Ma and document sources in both Siberia and Alaska. The input of continentally derived, fine-grained elastics into the deep sea increases several fold at this time, and in the northwestern Pacific abyssal reworking of bottom sediments also begins at about 2.6 Ma and continues to the present. Finally, volcanic ash layers suddenly become abundant in sediments younger than 2.6 Ma all across the North Pacific, a volcanic event that dwarfs anything found earlier in the Cenozoic record. One of the important objectives of Leg 145 was to define better the onset and character of silica sedimentation in the North Pacific. We were able to document that opal fluxes began to increase about 12 Ma, with a pronounced maximum between about 6 and 3 Ma. Also starting in the early portion of the middle Miocene the calcite compensation depth becomes 1.5 km shallower, in marked contrast to a deepening of the CCD since the early Miocene found everywhere else in the world. Leg 145 recovered Oligocene and Eocene carbonate sediments at Detroit Seamount. The middle and upper Eocene sediments are characterized by episodes of downslope transport, reworking and slumping, the timing of which matches that of similar events in the central Pacific Basin. Several Eocene ash horizons occur, adding further definition to the poorly known Eocene volcanism history. The Meiji sediment tongue on the northeast flank of Detroit Seamount was an important drilling target. Leg 145 was able to show that the Meiji tongue is an Atlantic-type drift deposit that has been forming continuously since the early Oligocene. Many paleoceanographic implications follow this important discovery, the most important of which is the continual presence of southward thermohaline flow from the Bering Sea to the North Pacific for the past 35 million years. Leg 145

Ocean Drilling Program Leg 145 crossed the North Pacific Ocean from Japan to Canada in the summer of 1992, the first deep ocean drilling in the Subarctic Pacific since the 1971 cruises of Deep Sea Drilling Project Legs 18 and 19. All of the Cenozoic paleoceanographic objectives of the cruise were accomplished. We determined the history of silica deposition in the North Pacific and resolved the timing of the late Miocene "silica switch" when the locus of silica deposition changed from the North Atlantic to the North Pacific. A depth transect drilled on the flanks of Detroit Seamount in the far northwestern Pacific allowed the reconstruction of the calcite compensation depth for the North Pacific over the past 70 Ma. The Meiji sediment tongue was shown to be a drift deposit, identical in nature to those better known from the North Atlantic. The Meiji Drift contains northern-source diatoms and minerals from the Bering Sea region and has been accumulating since the early Oligocene. This means that southward-flowing bottom water has been exiting the Bering Sea to the North Pacific basin for the past 35 million years. Presently, water exits the Bering Sea at the same rate, 10 to 20 Sv (1 Sv = I0 6 m 3 /s), as North Atlantic Deep Water leaves the northwesternmost Atlantic. Late Cenozoic cooling was determined to have been underway by the end of Miocene time when ice-rafted debris first occurs at the Leg 145 drill sites. A middle Pliocene warm interval interrupted the high-latitude cooling for about a million years. Large-scale Northern Hemisphere glaciation is shown to have begun rather abruptly at 2.6 Ma by a rapid increase in the delivery of terrigenous hemipelagic and ice-rafted debris sediment to the North Pacific. At this same time abyssal circulation begins to modify sediment depositional patterns in the northwestern Pacific deep basin. The sequence of ash layers recovered downwind from both the Kamchatka Peninsula (the world's most active Pliocene-Pleistocene volcanic field) and the Aleutian arc allows the geochemical evolution of these volcanic centers to be determined. An order of magnitude increase in the number and thickness of ash layers occurred at 2.6 Ma. High-resolution studies of the past 4.0 Ma show that biogenic sediment fluxes are generally higher during interglacial periods. This observation indicates that more rapidly circulating deep ocean waters during interglacial episodes is the dominant source of nutrients to the Subarctic Pacific rather than those supplied by regional runoff or dust input.

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