(Table 1) H2O and CO2 contents in submarine basaltic glasses from Ontong Java Plateau

The Ontong Java Plateau in the western Pacific is anomalous compared to other oceanic large igneous provinces in that it appears to have never formed a large subaerial plateau. Paleoeruption depths (at 122 Ma) estimated from dissolved H2O and CO2 in submarine basaltic glass pillow rims vary from ~1100 m below sea level (mbsl) on the central part of the plateau to 2200-3000 mbsl on the northeastern edge. Our results suggest maximum initial uplift for the plateau of 2500-3600 m above the surrounding seafloor and 1500+/-400 m of postemplacement subsidence since 122 Ma. Our estimates of uplift and subsidence for the plateau are significantly less than predictions from thermal models of oceanic lithosphere, and thus our results are inconsistent with formation of the plateau by a high-temperature mantle plume. Two controversial possibilities to explain the anomalous uplift and subsidence are that the plateau (1) formed as a result of a giant bolide impact, or (2) formed from a mantle plume but has a lower crust of dense garnet granulite and/or eclogite; neither of these possibilities is fully consistent with all available geological, geophysical, and geochemical data. The origin of the largest magmatic event on Earth in the past 200 m.y. thus remains an enigma.

Late Holocene pollen and organic walled dinoflagellate cysts analyses of sedimet cores from the Java Sea

The pollen, spore and organic walled dinoflagelletas cyst associations of two marine sediment cores from the Java Sea off the mouths of Jelai River (South Kalimantan) and Solo River (East Java) reflect environment and vegetation changes during the last ca 3500 years in the region. A decline in primary forest taxa (e.g. Agathis, Allophylus, Dacrycarpus, Dacrydium, Dipterocarpaceae, Phyllocladus, and Podocarpus) suggest that the major change in vegetation is caused by the forest canopy opening that can be related to human activity. The successively increase of pollen of pioneer canopy and herb taxa (e.g. Acalypha, Ficus, Macaranga/Mallotus, Trema, Pandanus) indicate the development of a secondary vegetation. In Java these changes started much earlier (ca at 2950 cal yr BP) then in Kalimantan (ca at 910 cal yr BP) and seem to be more severe. Changes in the marine realm, reflected by the dinoflagellate cyst association correspond to changes in vegetation on land. They reflect a gradual change from relatively well ventilated to more hypoxic bottom/pore water conditions in a more eutrophic environment. Near the coast of Java, the shift of the water trophic status took place between ca 820 and 500 cal yrs BP, while near the coast of Kalimantan it occurred as late as at the beginning of the 20th century. We observe an increasing amount of the cyst of Polykrikos schwarzii, cyst of P. kofoidii, Lingulodinium machaerophorum, Nematosphaeropsis labyrinthus and Selenopemphix nephroides at times of secondary vegetation development on land, suggesting that these species react strongly on human induced changes in the marine environment, probably related to increased pollution and eutrophication.

Physical, geochemical and magnetic properties of several ODP Leg 192 sites, Ontong Java Plateau

In this manuscript, we present the results of a physical properties investigation carried out on basaltic cores recovered from the four Leg 192 basement sites, focusing on the relationship between physical properties and alteration in basalts. Variations in physical properties in the Leg 192 basement sites closely resemble each other and reflect the amount of alteration and vein formation in the basement basalts. P-wave velocities, magnetic susceptibilities, and densities for the dense massive basalts are higher than those of more altered and heavily veined basalts. Porosity-dependent alteration is observed at Leg 192 basement sites: P-wave velocity displays a general decrease with increasing loss on ignition and potassium content. These trends are consistent with trends documented for typical alteration of oceanic crust and suggest that basalt alteration is largely responsible for the variation of the physical properties exhibited by rocks at Leg 192 basement sites. Our physical property data support the conclusion that only low-temperature seawater-mediated alteration occurred in the lava flows of the Ontong Java Plateau (OJP). This lack of higher-temperature hydrothermal alteration is consistent with the idea that the OJP basement sites are far from their eruptive vents.

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.