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Domes of the Sumisu volcanic complex (western Pacific), having summits at ocean depths of 1100, 600, 245, and 95 m, are mantled with compositionally identical rhyolitic pumice that has similar vesicularity, but that varies systematically in distribution, size, and surface texture--suggesting that facies and morphology can serve as useful indicators of eruption depth. At depths >500 m, the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant (meters to tens of meters) polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin. Vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace. At doi: 10.1130/G30500.1

We compare two intraplate, Pliocene-Pleistocene volcanic fields in different tectonic settings--the central Basin and Range and the northwest Pacific Ocean. Both fields are characterized by widely scattered, small-volume, alkali basaltic volcanoes; within the fields, each volcano apparently originates from a separate, volatile-enriched parental melt from the upper mantle. There is no evidence at either field for locally anomalous heat flow or ongoing introduction of new fluids into the upper mantle such as might occur above a subducting slab. We conclude that the volcanic fields reflect deformation-driven collection of already existing partial melts in a heterogeneous upper mantle. Deformation-driven melt collection may be an important mechanism for other diffuse intraplate volcanic fields, and this is consistent with a tectonically controlled, low-flux end member for intraplate fields where magmatism is a passive response to regional deformation. Differences in the degree of fractionation and contamination between the two fields are inferred to be related to flexure-induced vertical variations in the orientation of principal stresses in the northwest Pacific Ocean, which cause stalling of ascending dikes in the lithosphere. doi: 10.1130/G30427.1

Explosive eruptions from subaerial arc volcanoes can have significant environmental impact because of the discharge of ash and volatiles directly into the atmosphere and oceans. The link between climate cooling and voluminous volcanic eruptions has remained speculative due to a lack of supporting evidence. A compilation of 2814 K-Ar and [sup.40]Ar/[sup.39]Ar age determinations from four circum-Pacific arcs indicates that periods of high volcanic output (i.e., flare-ups) have been episodic and, in some cases, synchronous. Peak periods and subsequent lulls in arc magmatism over the past 50 m.y. have occurred coeval with major fluctuations in global climate, including the Eocene-Oligocene transition, one of the most prominent global climate reorganizations in the Cenozoic. Hundreds of intermediate to silicic eruptions occurred during an extremely vigorous period of circum-Pacific volcanism beginning in the late Eocene, which likely led to the production of sulfur aerosols in the stratosphere and fertilization of surface waters of the Pacific Ocean. We provide a mechanism that may have been partly responsible for the climatic preconditioning that must have preceded and ultimately promoted Antarctic ice sheet growth ~34 m.y. ago.

We use geochemical data from a sediment core in the shallow-silled and intermittently dysoxic Kau Bay in Halmahera (Indonesia, lat 1[degrees]N, long 127.5[degrees]E) to reconstruct century-scale climate variability within the Western Pacific Warm Pool over the past ~3500 yr. Downcore variations in bulk sedimentary [[delta].sup.15]N appear to reflect century-scale variability in basin ventilation, attributed to changes in oceanographic conditions related to century-scale fluctuations in El Nino Southern Oscillation (ENSO). We infer an increase in century-scale El Nino activity beginning ca. 1700 yr B.P. with peaks in El Nino activity ca. 1500 yr B.P., 1150 yr B.P., and ca. 700 yr B.P. The Kau Bay results suggest that there was diminished ENSO amplitude or frequency, or a departure from El Nino--like conditions during the Medieval Warm Period, and distinctive, but steadily decreasing, El Nino activity during and after the Little Ice Age. Keywords: climate, El Nino Southern Oscillation, Western Pacific Warm Pool, Indonesia.

The Cretaceous has long been recognized as a time when greenhouse conditions were fueled by elevated atmospheric C[O.sub.2] and accompanied by perturbations of the global carbon cycle described as oceanic anoxic events (OAEs). Yet, the magnitude and frequency of temperature change during this interval of warm and equable climate are poorly constrained. Here we present a high-resolution record of sea-surface temperatures (SSTs) reconstructed using the [TEX.sub.86] paleothermometer for a sequence of early Aptian organic-rich sediments deposited during the first Cretaceous OAE (OAE1a) at Shatsky Rise in the tropical Pacific. SSTs range from ~30 to ~36[degrees]C and include two prominent cooling episodes of ~4 [degrees]C. The cooler temperatures reflect significant temperature instability in the tropics likely triggered by changes in carbon cycling induced by enhanced burial of organic matter. SST instability recorded during the early Aptian in the Pacific is comparable to that reported for the late AIbian-early Cenomanian in the Atlantic, suggesting that such climate perturbations may have recurred during the Cretaceous with concomitant consequences for biota and the marine environment. Keywords: [TEX.sub.86] index, oceanic anoxic events, Cretaceous sediments, carbon isotopes.

Export of pelagic carbonate tests from surface waters and their deposition at the seafloor plays a significant role in the C[O.sub.2] cycle and ability of the oceans to absorb atmospheric C[O.sub.2]. Sediment [sup.230]Th and [sup.3]He measurements have been interpreted as evidence that significant lateral advection of pelagic material occurs in the water column, leading to marked spatial variations in deposition rates and, in particular, to significant focusing of deposits on the Pacific equator. We report spatially continuous stratigraphy from two 1000 km seismic lines that show evidence of depositional anomalies near the equator. Accumulation rates were apparently enhanced locally by a factor of two, similar to the proposed modern sediment-focusing factors, but the anomalies are surprisingly patchy over the 20 m.y. period analyzed--they are not confined to an equatorial region, and they are not necessarily found on adjacent seismic profiles. These intermediate-scale anomalies are >~100 km across and represent areas of seafloor that received more deposits for one period, less in following periods, and vice versa. Variogram analysis was used to determine how the spatial scales of deposition changed over the Neogene. The period when the spatial scale of depositional variability was largest correlates with hiatuses in drill cores, a correlation that we interpret as caused by enhanced and spatially heterogeneous carbonate dissolution at that time. The study suggests that seismic stratigraphy has the potential to reveal spatial patterns related to unsteady bottom-water flow and chemistry. Keywords: pelagic carbonate seismic stratigraphy, sediment focusing, pelagic carbonate dissolution, Pacific Ocean circulation, variogram analysis.

Planktonic foraminifera in the southwest Pacific Ocean suggest the sea surface temperature was warmer during the Pleistocene than the Holocene. Change was relatively rapid, suggesting global-scale mechanisms.

We describe a model for episodic, open-ocean triple-junction migration based on observations of actual triple-junction evolutions. Migration progresses by repeated episodes of rift propagation, microplate formation, and microplate accretion to an adjacent, larger plate. These episodes may be highly variable in space and time depending on triple-junction geometry, velocity triangle, and other factors affecting local thermal and rheological conditions. Resulting tectonic features may include an abandoned transform fault, straight and potentially curving pseudofaults, sheared and potentially rotated abyssal-hill fabric, and a paleomicroplate with no associated failed rift. This model, developed mainly from the evolution of the Pacific-Antarctic-Nazca triple junction, may be relevant for other types of triple junctions such as the Bouvet and Azores triple junctions and ridge-ridge-ridge triple junctions in the Indian and Pacific Oceans. Episodic migration is found to occur even when the triple junction is kinematically stable. This model highlights the difference between predicted kinematic stability of triple junctions and observations of their true tectonic history.

We consider two hypotheses for the origin of a linear trough extending [approximately] 800 km eastward from the intersection of the Louisville seamount chain with the Tonga-Kermadec trench. An extinct spreading center origin is suggested by the trough's eastward termination at a depth discontinuity that resembles a fracture zone. The morphology of the trough, however, is inconsistent with the morphology of other extinct spreading centers, and these two features do not appear to be connected to a larger paleo-plate boundary on their eastern end. We consider an alternative hypothesis in which the trough may be a recent rift in the Pacific plate. Coupling between a large subducting seamount and the overriding Australian plate may serve as a mechanism to focus stresses and nucleate a rift in the subducting Pacific plate. In this model, the seamount would act as a rigid indenter producing tangential extension in the subducting plate about the region where it is coupled to the overriding plate. Impeded subduction is suggested by a prominent seismicity gap around the subducting seamount. The graben morphology of the trough is also consistent with an extensional origin. If hotspot volcanism can produce obstructions and local weaknesses conducive to plate coupling, stress localization, and nucleation of fractures, it may provide a mechanism for subduction-induced plate reorganizations.

We present new seismic images of Cretaceous crust formed at a fast-spreading center in the Pacific. The high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast-spreading centers are not seismically detectable. Subhorizontal reflections can be traced at 600-800 ms two-way time below the top of basement for tens of kilometres, suggesting the presence of a widespread seismic boundary, possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes. Lower-crustal reflections, dipping dominantly toward the paleo-spreading center, may represent mafic-ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections.

The Southern Ocean south of the Great Australian Bight hosts a globally anomalous region of mid-ocean ridge, the Australian-Antarctic Discordance. In addition to its recognized morphological and geophysical anomalies, the discordance represents the current on-axis location of a proposed isotopic boundary between Indian Ocean and Pacific Ocean upper mantle convective regimes, previously defined from analyses of [less than or equal to]4 Ma mid-ocean ridge basalts (MORB). Pb, Sr, and Nd isotope analyses of [approximately]36-66 Ma seafloor dredged from either side of the northward extrapolation of the Australian-Antarctic Discordance show that [greater than or equal to]36 Ma seafloor to the east has an Indian Ocean MORB isotopic signature. This indicates that the Indian-Pacific isotopic boundary does not extend directly north of the ridge toward the southern margin of Australia. Progressive westward migration of an arcuate-shaped front of Pacific Ocean upper mantle therefore appears to be a consequence of Australian-Antarctic rifting and Southern Ocean opening, suggesting that the current location of the isotopic boundary within the discordance may be entirely coincidental.

Surface productivity is correlated with the rate of accumulation of benthic foraminifera on the deep-sea floor. As a rule of thumb, for each 1 mg of organic carbon arriving at the sea floor, one benthic foram shell > 150 [micrometer] is deposited. The correlation can be used to reconstruct organic flux to the sea floor in the past, and hence the productivity of past oceans. Applying the appropriate equations to box core data from Ontong Java Plateau in the western equatorial Pacific, we found that productivity during the last glacial maximum exceeded present productivity by a factor of between 1.5 and 2.0, with intermediate values for the mid-transition period. Accumulation of benthic foraminifera was depressed on top of the plateau during the glacial and transitional period, presumably because increased winnowing removed part of the food supply.

New biostratigraphic data based on radiolarians recovered from deep within the oceanic crustal section of Ocean Drilling Program Hole 801C in the western Pacific, along with existing radiometric information, date this oceanic crust as late Bajocian-early Bathonian (170-165 Ma). The overlying basal sediments at Hole 801C are essentially identical in age (middle Bathonian, 164-162 Ma) to the basal sediments at Deep Sea Drilling Program Hole 534A in the central Atlantic. We estimate the time of formation of the Pacific plate as 175-170 Ma and the time of initial separation of the Pangea supercontinent in the central Atlantic as 190-180 Ma. We also identify a time of extensive subduction-zone magmatism (175-159 Ma) at the eastern and western edges of Pangea. We suggest that the initial plate separation of Pangea increased subduction rates at its outer margins and altered the plate boundaries in the Pacific superocean, leading to formation of the Pacific plate. Keywords: Jurassic, Ocean Drilling Program, ocean crust, Pacific plate, Pangea, Radiolaria.

The Pacific plate is usually considered to host 14 hotspots, but most of this volcanism does not seem to originate from deep-mantle plumes. To reach this conclusion, we tried to establish how many of the seamount alignments on the Pacific plate correspond to classic hotspots, i.e., long-lived hotspots linked to oceanic basaltic plateaus. We retraced the tracks of the 14 Pacific hotspots by using (1) the absolute stage poles representing the Pacific plate absolute motion since 145 Ma, (2) an updated compilation of radiometric ages of seamounts and oceanic plateaus totaling 266 seamounts or islands, (3) the detailed bathymetry of the Pacific Ocean, and (4) the present locations of the hotspots. This analysis allowed us to correlate only three hotspots with the beginning of their tracks possibly corresponding in space and time to an oceanic plateau: Easter to the eastern Mid-Pacific Mountains, Louisville to the Ontong Java plateau, and, with less confidence, Marquesas to Hess Rise and Shatsky Ridge. In addition, the Hawaii hotspot has produced long-lived volcanism. These four are the only classic hotspots on the Pacific plate. However, seven hotspots present short tracks ([is less than] 35 m.y.) that cannot be traced to an oceanic plateau and thus are not related to any deep-mantle phenomena: Foundation, Macdonald, Pitcairn, Rarotonga, Rurutu, Samoa, and Society. The two northeast Pacific hotspots, Kodiak-Bowie and Cobb, and the Caroline hotspot are unclassifiable because of close proximity to a subduction zone where the prior history of volcanism has been lost. Keywords: Pacific plate, mantle plume, hotspot track, oceanic plateau.

The clockwise bend at 99 Ma (mid-Cretaceous) in linear volcanic chains in the tropical Pacific coincides with a change from pre-99 Ma head-on Chilean-type subduction of the Pacific plate beneath eastern Gondwana to 99-43 Ma sinistral oblique Mariana-type subduction and strike-slip breakup by simple sea-floor spreading between Australia and Antarctica and by backarc spreading in the southwest Pacific. The 99 Ma breakup of Australia from Antarctica is documented by a mid-Cretaceous unconformity. This tectono-stratigraphic change founded modern Australia, with a mountain chain along an upper plate margin in the east and lowlands on the lower plate margin in the south. The counterclockwise bend at 43 Ma (mid-Eocene)--the Emperor-Hawaiian bend--coincides with the onset of structure in the Challenger Rift of New Zealand, the Eromanga-Cooper basin of central Australia, and the oil-shale grabens of coastal Queensland. Keywords: Australia, mid-Cretaceous, mid-Eocene, Pacific, hotspot bends.

SeaMARC II records show a 220 [km.sup.2] area of unusually high reflectivity on the East Pacific Rise (EPR) near lat 8[degrees]S which we interpret to be a recently erupted lava field. This is the only segment of the EPR exhibiting high reflectivity over such a large area in our continuous SeaMARC II coverage from last 8[degrees]N to 18[degrees]N and 3[degrees]S to 23[degrees]S. The lavas appear to issue from the axial neovolcanic zone between lat 8[degrees]15' and 8[degrees]17'S. The lava field extends up to 18 km off-axis and inundates scarps up to 100 m high. On the basis of measurements of scarp heights just outside areas where they are buried, we estimate the average thickness of the field to be 70 [+ or -]20 m. Thus, the volume of the lava fields is 15 [+ or -]4 [km.sup.3]. In comparison, the average annual volcanic budget for the Earth is approximately 4-5 [km.sup.3], and the largest historic basaltic eruption (from the Icelandic Laki fissure in 1783) was approximately 12.3 [km.sup.3]. When did the eruption occur? Comparative studies of acoustic reflectively and photographed bottom characteristics elsewhere along the EPR suggest that the lava field has little or no sediment cover and is very young. This is also one of the only areas on the EPR where earthquakes occur that do not appear to be caused by transform faulting. If the earthquakes are related to the lava field, then major eruptions may have occurred in 1964, 1965, and 1969. A large helium plume near lat 13[degrees]S may be caused in part by eruptions of the lat 8[degrees]S lava field. This is consistent with estimates of currents at 2-3 km depth, which are such that plume generated at lat 8[degrees]S would eventually appear off-axis at lat 13[degrees]S. Our observations suggest that this spreading segment, which is bounded by small ridge-axis discontinuities at lat 8[degrees]05'S and 8[degrees]47'S, may have been the most volcanically active segment along the EPR during the past 25 yr.