Middle Eocene to early Oligocene planktonic and benthic foraminifers from ODP Hole 125-786A

Drilling at Site 786, located in the center of the Izu-Bonin forearc basin, penetrated an apparently continuous section of middle Eocene/lower Oligocene volcaniclastic breccias and nannofossil oozes. Planktonic foraminiferal faunas underwent a gradual transition from relatively high-diversity middle Eocene through late Eocene tropical or warm-water assemblages to a cooler-water, less diverse assemblage during the early Oligocene. In the cosmopolitan benthic foraminiferal faunas, the major transition occurred during the early late Eocene. Middle Eocene benthic assemblages resembling the bathyal 'Lenticulina' fauna (characterized by Osangularia mexicana, Cibicidoides eocaenus, and several buliminid species) changed to an upper Eocene abyssal 'Globocassidulina subglobosa' fauna (characterized by Cibicidoides praemundulus, Globocassidulina subglobosa, Gyroidinoides girardanus, Oridorsalis umbonatus, and Siphonodosaria aculeata). Even though no large, abrupt faunal changes appear to have been associated with the assumed Eocene/Oligocene boundary, benthic species turnover continued through the late Eocene and into the early Oligocene. This resulted in a slightly lower diversity early Oligocene fauna dominated by three species: Laevidentalina sp., Bulimina jarvisi, and Gyroidinoides girardanus. The progression from a middle Eocene bathyal 'Lenticulina' fauna, rather than an abyssal 'Nuttallides truempyi' fauna, to an abyssal 'Globocassidulina subglobosa' fauna during the early late Eocene, suggests that a bathymetric deepening occurred at Site 786. Increased water depths may have resulted from tectonic subsidence.

Geochemistry of Ontong Java Plateau basalts

Basement rocks from the Ontong Java Plateau are tholeiitic basalts that appear to record very high degrees of partial melting, much like those found today in the vicinity of Iceland. They display a limited range of incompatible element and isotopic variation, but small differences are apparent between sampled sites and between upper and lower groups of flows at Ocean Drilling Program Site 807.40Ar-39Ar ages of lavas from Site 807 and Deep Sea Drilling Project Site 289 are indistinguishable about an early Aptian mean of 122 Ma (as are preliminary data for the island of Malaita at the southern edge of the plateau), indicating that plateau-building eruptions ended more or less simultaneously at widely separated locations. Pb-Nd-Sr isotopes for lavas from Sites 289, 803, and 807, as well as southern Malaita, reflect a hotspot-like source with epsilon-Nd(T) = +4.0 to +6.3, (87Sr/86Sr)T = 0.70423-0.70339, and 206Pb/204Pb = 18.245-18.709 and possessing consistently greater 208Pb/204Pb for a given 206Pb/204Pb than Pacific MORB. The combination of hotspot-like mantle source, very high degrees of melting, and lack of a discernible age progression is best explained if the bulk of the plateau was constructed rapidly above a surfacing plume head, possibly that of the Louisville hotspot. Basalt and feldspar separates indicate a substantially younger age of ~90 Ma for basement at Site 803; in addition, volcaniclastic layers of mid-Cenomanian through Coniacian age occur at DSDP Site 288, and beds of late Aptian-Albian age are found at Site 289. Therefore, at least some volcanism continued on the plateau for 30 m.y. or more. The basalts at Site 803 are chemically and isotopically very similar to those at the ~122 Ma sites, suggesting that hot plume-type mantle was present beneath the plateau for an extended period or at two different times. Surviving seamounts of the Louisville Ridge formed between 70 and 0 Ma have much higher 206Pb/204Pb than any of the plateau basalts. Thus, assuming the Louisville hotspot was the source of the plateau lavas, a change in the hotspot's isotopic composition may have occurred between roughly 70 and 90 Ma; such a change may have accompanied the plume-head to plume-tail transition. Similar shifts from early, lower 206Pb/204Pb to subsequently higher 206Pb/204Pb values are found in several other oceanic plateau-hotspot and continental flood basalt-hotspot systems, and could reflect either a reduction in the supply of low 206Pb/204Pb mantle or an inability of some off-ridge plume-tails to melt refractory low 206Pb/204Pb material.