Leg 145

The North Pacific Transect

The main objectives of Leg 145 were to collect high-resolution records of Miocene to Quaternary changes in ocean circulation, biological activity, and global climate, to recover high-latitude Paleogene and Cretaceous carbonate-bearing sediments to help decipher the oceanographic record of the old Northern Hemisphere Pacific, and to study the history of water masses on the Meiji Tongue, a North Atlantic-type drift deposit. During Leg 145, over 4 km of material was recovered from seven sites (Site 881 to Site 887), which enabled construction of long paleomagnetic reversal stratigraphies and a refined calibration of the diatom-based biostratigraphy.

In the middle Miocene, silica deposition in the North Pacific increased markedly and opal deposition in the Atlantic declined. A period of extreme deposition occurred over the latest Miocene and early Pliocene in the equatorial Pacific, Indian Ocean, and the Southern Ocean but not, to our knowledge, in the Atlantic. In most of the worldÍs oceans, the carbonate compensation depth (CCD) deepened during the last 15 m.y. but, in the North Pacific, shallowed by 1.5 km. This effect, confined to latitudes north of 40žN, probably resulted from ocean circulation changes that increased the supply of nutrients to the North Pacific.

Since 2.6 Ma, the time of onset of major Northern Hemisphere glaciation, terrigenous flux increased three- to five-fold and glacial dropstones are common. In the northwest Pacific, Siberia and the Kamchatka Peninsula are most probable source areas. In the Gulf of Alaska, dropstones are more abundant but of the same age, demonstrating simultaneous onset of glaciation in western North America and northeastern Asia. A five-fold increase in dust flux also occurs at 2.6 Ma, mineralogically matching loess deposits in China and heralding the beginning of loess deposition that occurred in conjunction with the onset of Northern Hemisphere glaciation. At almost exactly 2.6 Ma, an increase in the number and volume of ash beds signifies a significant volcanic event.

Northern provenance minerals and benthic diatoms characterize the sediments of the Meiji Drift back through the Oligocene and indicate apparently continuous depositional through the Quaternary.

Middle and upper Eocene downslope reworking and slumping of Detroit Seamount sediments increases the known latitudinal range of this reworking episode and analyses of basalts recovered at both seamounts should clarify the age and history of their respective hot spot sources. If basalt recovered near the Chinook Trough is geochemically verified as a sill, then new information about volcanic rejuvenation of the central North Pacific may be revealed.