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 40N, 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.