34. Magnetostratigraphy of Upper Cenozoic Sediments from Leg 145, North Pacific Ocean

Paleomagnetic measurements were performed on whole cores and discrete samples from approximately 4 km of sediment recovered during Leg 145 of the Ocean Drilling Program. The obtained magnetic polarity sequences enabled the establishment of a chronostratigraphic framework from the middle Miocene (about 18.3 Ma) to the present. The quality of the magnetic record is related directly to the sedimentology: clay-rich sediments deposited since 2.6 Ma and before 5.4 Ma preserve a relatively strong and stable magnetic signal, whereas diatom-rich sediments deposited between 5.4 and 2.6 Ma generally contain a much weaker and less stable paleomagnetic signal. Despite these problems, Sites 884 and 887 have provided some of the longest continuous polarity sequences ever obtained in the history of ocean drilling, with continuous records obtained back to 12.6 Ma and 18.3 Ma, respectively. Sediments at Site 881 are stably magnetized through the upper part of the thick diatom-rich interval and a continuous magnetic polarity record was obtained to the 4.03 Ma (C3n. In; Cochiti Subchron). However, extended core barrel drilling was employed below the Cochiti Subchron, at which point the paleomagnetic data substantially diminished in quality. Rotary drilling with the extended core barrel, when employed in Leg 145, seriously compromised the fidelity of the paleomagnetic record. Sites 882 and 883 provided continuous records to just below 2.6 Ma (i.e., the Gauss/Matuyama boundary). Low remanence intensities and unstable paleomagnetic behavior prevented retrieval of a reliable magnetostratigraphy in the diatomrich interval below 2.6 Ma at these two sites. The magnetostratigraphy from the more stably magnetized sediments of Site 884 can be transferred to the diatom-rich intervals of Sites 882 and 883, however, by correlation of susceptibility records. This permits identification of the Gilbert/Gauss boundary and the normal subchrons within the Gilbert Chron, thus extending the records from Sites 882 and 883 to beyond 5 Ma. The chronological constraints provided by the magnetostratigraphy are important for studying the nature of several important events that are observed throughout the North Pacific Ocean. The diatom ooze that begins near 5.4 Ma and extends to about 2.6 Ma defines a time of extreme silica deposition in the North Pacific Ocean. This episode was observed at all Leg 145 sites and represents a short period of extreme productivity at high latitudes that is associated with an early Pliocene period of climatic amelioration. The interval beginning near the Gauss/Matuyama boundary at 2.6 Ma is marked by high fluxes of diatoms, clay, volcanic ash, ice-rafted detritus, and quartz. The consistent appearance of abundant dropstones and other terrigenous material at the Leg 145 sites near 2.6 Ma heralds the onset of major glaciation in the Northern Hemisphere. This suggests that the onset of glaciation in the source regions for this lithogenic material, Siberia and Kamchatka, was synchronous with the onset of Laurentide and Fennoscandian glaciation. Increased fluxes of terrigenous clays near 2.6 Ma are also recorded at all of the Leg 145 sites and may be associated with the resurgence of bottom-current activity. An increase, by more than an order of magnitude, of volcanogenic material near 2.6 Ma in the North Pacific Ocean has also been known for many years, however, the Leg 145 sites provide much better-dated sequences for determining the timing of the onset of the northwest Pacific portion of this oceanwide volcanic episode. The long, continuous records obtained at Sites 884 and 887 permit the first direct correlation of the North Pacific Ocean diatom and other microfossil zonations to the magnetic reversal time scale in sediments older than latest Miocene. INTRODUCTION The North Pacific Ocean contains a critically important record of Cenozoic paleoceanographic and paleoclimatic events that were relatively poorly known prior to Leg 145 of the Ocean Drilling Program (ODP). Existing sedimentary records from the North Pacific Ocean are few and are generally disturbed by rotary drilling. The major objectives of Leg 145 were to obtain high-resolution records of Miocene to Quaternary changes in ocean circulation, biological activity, and global climate. New coring techniques used during Leg 145 allowed recovery of long, essentially undisturbed, sedimentary records 1 Rea, D.K., Basov, LA., Scholl, D.W., and Allan, J.F. (Eds.), 1995. Proc. ODP, Sci. Results, 145: College Station, TX (Ocean Drilling Program). department of Geological Sciences, University of Washington, Seattle, WA 98115, U.S.A. 'Department of Geology, University of California, Davis, CA 95616, U.S.A. Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan. 'Department de Géologie, Ecole Normale Supérieur, 75231 Paris 05, France. that totaled over 4 km in length: a record approached by few cruises during the history of the ODP and the Deep Sea Drilling Project. Obtaining a high-quality magnetic stratigraphy from these records is crucial to the success of developing a chronostratigraphic framework for the history of Cenozoic paleoceanographic and paleoclimatic events in the North Pacific Ocean. Cores were drilled at seven sites during Leg 145, in a transect across the North Pacific Ocean (Fig. 1). Given the importance of obtaining a secure temporal framework for the North Pacific sedimentary sequences, we have focused on obtaining as reliable a magnetostratigraphy as possible by conducting measurements on a large number of discrete samples as a supplement to the continuous longcore measurements made during Leg 145. We present the results of these detailed studies in this paper. METHODS Paleomagnetic measurements were routinely carried out on the archive halves of the 1.5-m core sections of nearly all of the sediments recovered during Leg 145 using the shipboard three-axis 2G