(Table 1) Trace element and isotopic data for ODP Hole 145-887D basalts

Age-progressive, linear seamount chains in the northeast Pacific appear to have formed as the Pacific plate passed over a set of stationary hotspots; however, some anomalously young ages and the lack of an "enriched" isotopic signature in basalts from the seamounts do not fit the standard hotspot model. For example, published ages (28-30 Ma) for basalts dredged from the Patton-Murray seamount platform in the Gulf of Alaska are 2-4 m.y. younger than the time when the platform was above the Cobb hotspot. However, the lowermost basalt recovered by ocean drilling on Patton-Murray yielded a 40Ar-39Ar age of 33 Ma. This age exactly coincides with the time when the seamount platform was above the Cobb hotspot, consistent with a stationary, long-lived mantle plume. A 27 Ma alkalic basalt flow recovered 8 m above the 33 Ma basalt is similar in age and composition to the previously dredged basalts, and may be the alkalic capping phase typical of many hotspot volcanoes. A 17 Ma tholeiitic basalt sill recovered 5 m above the 27 Ma basalt was emplaced long after the seamount platform moved away from the hotspot, and may be associated with a period of intraplate extension. Anomalously young phases of volcanism on this and other hotspot seamounts suggest that they can be volcanically rejuvenated by nonhotspot causes, but this rejuvenation does not rule out the hotspot model as an explanation for the initial creation of the seamount platform. The lack of an "enriched" isotopic signature in any of these basalts shows that enriched compositions are not necessary characteristics of plume-related basalts. The isotopic compositions of the lower basalts are slightly more depleted than the 0-9 Ma products of the Cobb hotspot, despite the fact that the hotspot was closer to a spreading ridge at 0-9 Ma. It appears that this hotspot, like several others, has become more enriched with time.

Radiolarian assemblages of the late Neogene North Pacific

Radiolarian census and abundance data were collected from three deep-sea cores drilled by the Ocean Drilling Program Sites 884, 887 and 1151 to investigate patterns of ecologic changes in space and time during the last 16 million years for the mid-latitude to subarctic North Pacific. High concentrations of radiolarians occurred between 9.0 and 2.7 Ma. Radiolarian species richness was highest in the early middle Miocene at each site and gradually decreased up to about 7 Ma, coinciding with a well-established global cooling trend. A degree of overlap index calculated for radiolarian assemblages revealed 11 faunal change events, of which 8 corresponded to global cooling events and expansions of polar ice sheets. Three of the faunal change events were observed within the peak of radiolarian accumulation rate and were ascribed to changes in primary productivity in the North Pacific rather than global climatic changes. Our assemblage analyses revealed that north-south differentiation in radiolarian assemblages in the northwestern Pacific has existed since 16 Ma and became more distinct via major steps at 6.8 Ma and 2.7 Ma, coinciding with major glaciation events, and that east-west faunal contrasts in the subarctic region became obvious beginning at 11.7 Ma and changed to a different mode around 6.8 Ma. The observed east-west faunal differences possibly reflect east to west climate differences that were characterized by cooler temperatures in the east than the west during the late Miocene (11.7-6.8 Ma) and then by the opposite temperature trend (6.8 Ma-Recent). A severe glaciation at 2.7 Ma played a large role, particularly in temporal changes in radiolarian accumulation rate and assemblage composition.