(Table 2) Chemical composition of bottom sediments from the Equatorial East Pacific

Fe, Mn, Cu, Ni, and Co contents in bottom sediment samples from the Clarion Clipperton fracture zone and Guatemala Basin were studied; maps of their distribution in the upper layer of sediments were prepared. At some stations contents of these elements were also measured in Pleistocene and Oligocene sediments. Elevated contents of five ore elements (except for Zn) were found at the East Pacific Rise and in the Clarion-Clipperton province; and of Mn, Ni, and Cu in the Guatemala basin. Increased zinc contents occur only in sediments of the East Pacific Rise and Guatemala Basin. Enrichment of sediments in these elements results from under¬water hydrothermal activity and high biological productivity.

Late Cenozoic stable isotope record of benthic and planktonic foraminifera from the Pacific Ocean

Stable isotopic analyses of Middle Miocene to Quaternary foraminiferal calcite from east equatorial and central north Pacific DSDP cores have provided much new informatlon on the paleoceanography of the Pacific Neogene The history of delta18O change in planktonic foraminifera reflects the changing Isotopic composition and temperature of seawater at the time of test formation. Changes in the isotopic composition of benthonic foraminifera largely reflect changes m the volume of continental ice. Isotopic data from these cores indicates the following sequence of events related to continental glaciation (1) A permanent Antarctic ice sheet developed late in the Middle Miocene (about 13 to 11.5 m.y. ago) (2) The Late Miocene (about 11.5 to 5 m.y. ago) is marked by significant variation in delta18O of about 0.5? throughout, indicating instability of Antarctic ice cap size or bottom-water temperatures (3) The early Pliocene (5 to about 3 m.y. ago) was a time of relative stability in ice volume and bottom-water temperature (4) Growth of permanent Northern Hemisphere ice sheets is referred to have begun about 3 m.y. ago (5) The late Pliocene (3 to about 1.8 m.y. ago) is marked by one major glaciation or bottom-water cooling dated between about 2.1 to 2.3 m.y. (6) There is some evidence that the frequency of glacial-interglacial cycles increased at about 0.9 m.y. There is significant variation in delta13C at these sites but no geochemical interpretation is offered in this paper. The most outstanding feature of delta13C results is a permanent shift of about -0.8? found at about 6.5 m.y. in east equatorial and central north Pacific benthonic foraminifera. This benthonic carbon shift may form a useful marker in deep-sea cores recovering Late Miocene carbonates.

Conventional and 40Ar/39Ar K-Ar ages of volcanic rocks at DSDP Leg 55 Holes

Conventional K-Ar, 40Ar/39Ar total fusion, and 40Ar/39Ar incremental heating data on hawaiite and tholeiitic basalt samples from Ojin (Site 430), alkalic basalt samples from Nintoku (Site 432), and alkalic and tholeiitic basalt samples from Suiko (Site 433) seamounts in the Emperor Seamount chain give the following best ages for these volcanoes: Ojin = 55.2 ± 0.7 m.y., Nintoku = 56.2 ± 0.6 m.y., and Suiko = 64.7 ± 1.1 m.y. These new data bring to 27 the number of dated volcanoes in the Hawaiian-Emperor volcanic chain. The new dates prove that the age progression from Kilauea Volcano on Hawaii (0 m.y.) through the Hawaiian-Emperor bend (- 43 m.y.) to Koko Seamount (48.1 m.y.) in the southernmost Emperor Seamounts continues more than halfway up the Emperor chain to Suiko Seamount. The age versus distance data for the Hawaiian-Emperor chain are consistent with the kinematic hot-spot hypothesis, which predicts that the volcanoes are progressively older west and north away from the active volcanoes of Kilauea and Mauna Loa. The data are consistent with an average volcanic propagation velocity of either 8 cm/year from Suiko to Kilauea or of 6 cm/year from Suiko to Midway followed by a velocity of 9 cm/year from Midway to Kilauea, but it appears that the change in direction that formed the Hawaiian- Emperor bend probably was not accompanied by a major change in velocity.

(Table 1) Strontium concentration and isotope ratios of carbonates from DSDP Holes 69-504B as well as 83-504B and fish debris samples from Indian and Pacific Ocean sediments

Recent revisions of the geological time scale by Kent and Gradstein (in press) suggest that, on the average, Cretaceous magnetic anomalies are approximately 10 m.y. older than in Larson and Hilde's (1975) previous time scale. These revised basement ages change estimates for the duration of alteration in the ocean crust, based on the difference between secondary-mineral isochron ages and magnetic isochron-crustal ages, from 3 to approximately 13 m.y. In addition to the revised time scale, Burke et al.'s (1982) new data on the temporal variation of 87Sr/86Sr in seawater allow a better understanding of the timing of alteration and more realistic determinations of water/rock ratios during seawater-basalt interaction. Carbonates from all DSDP sites which reached Layer 2 of Atlantic crust (Sites 105, 332, 417, and 418) are deposited within 10-15 m.y. of crustal formation from solutions with 87Sr/86Sr ratios identical to unaltered or contemporaneous seawater. Comparisons of the revised seawater curve with the 87Sr/86Sr of basement carbonates is consistent with a duration of approximately 10-15 m.y. for alteration in the ocean crust. Our preliminary Sr and 87Sr/86Sr data for carbonates from Hole 504B, on 5.9-m.y.-old crust south of the Costa Rica Rift, suggest that hydrous solutions from which carbonates precipitated contained substantial amounts of basaltic Sr. For this reason, carbonate 87Sr/86Sr cannot be used to estimate the duration of alteration at this site. A basalt-dominated alteration environment at Hole 504B is consistent with heat-flow evidence which indicates rapid sediment burial of crust at the Costa Rica Rift, sealing it from access by seawater and resulting in unusually low water/rock ratios during alteration.