Chemical composition of volcanic ash layers from ODP Leg 134 sites

Volcanic ash layers (1-3 cm thick) are abundant in the North Aoba Basin drill sites but less common at forearc sites. Ash deposited on the forearc slopes is liable to be redistributed as turbidites. In addition, the westerly upper winds also minimize ash-fall on the western (forearc) side of the New Hebrides Island Arc. Crystalline components in the ashes are primarily plagioclase (An90-An44), clinopyroxene (Ca46Mg49Fe5-Ca43Mg33Fe24), olivine (Fo87-Fo62), and titanomagnetite. There are also small amounts of orthopyroxene, magnetite, apatite, and quartz. Glass shards occur in most of the ashes and range in composition from basalt to rhyolite. There is often a variety of glass compositions within a single ash layer. One explanation for this is that the rate of accumulation of ash from several different eruptions or eruptive phases exceeded the background sedimentation rate: there may also have been a certain amount of reworking. The high-K and low-K trends previously recognized in volcanic rocks from the New Hebrides Island Arc are clearly represented in the Leg 134 glasses. All of the ashes investigated here are thought to have originated from the Central Chain volcanoes. The source of the high-K group was probably the Central Basin volcanoes of Santa Maria, Aoba, and Ambrym. The lower-K series includes a distinctive group of dacites and is likely to have originated from the Epi-Tongoa-Tongariki sector of the arc where major pyroclastic eruptions, associated with caldera collapse, have occurred during the Holocene, perhaps as recently as 400 yr ago.

Middle to Late Quaternary sedimentation and rock-magnetic of ODP Site 105-646

We examine rock-magnetic, carbonate, and planktonic foraminiferal fluxes to identify climatically controlled changes of terrigenous and pelagic sedimentation at Ocean Drilling Program (ODP) Site 646 (the Labrador Sea). Terrigenous sediments are brought to the site principally by bottom currents. We use a rock-magnetic parameter sensitive to changes in magnetic mineral grain size, the ratio of anhysteretic susceptibility to low-field magnetic susceptibility (XARM/X), to monitor changes in bottom-current intensity over time, with large values of XARM/X (finer-grained magnetic minerals) indicating weaker bottom currents. A second rock-magnetic parameter, magnetic mineral accumulation rate (KaT) was used to indicate variations in terrigenous flux. Planktonic foraminiferal and carbonate accumulation rates (Pfar and CaC03ar) are used as indicators of pelagic flux. Absolute age assignments are based on correlation between the planktonic foraminiferal oxygen-isotope variations for Site 646 and the SPECMAP master oxygen-isotope curve. Cross-correlation analyses of the parameters that we studied with respect to the SPECMAP curve suggest that from oxygen-isotope stages 21 to 11, sedimentation rate, KaT, X, CaCO3ar, and Pfar were at their maximums, whereas XARM/X was at its minimum during peak interglacials (i.e., 0 k.y. lag time with respect to minimum ice volume). However, all parameters we examined lag behind minimum ice volume from stages 11 to 1, indicating a change in timing of both pelagic and terrigenous fluxes at approximately 400 k.y. BP. The negative correlation coefficient between XARM/X and the SPECMAP curve further suggest that finer-grained magnetic minerals are deposited during glacial periods, which probably reflects weaker bottom currents. The shift observed in the lag times of parameters examined with respect to the SPECMAP record is attributed to a change in significance of orbital parameters. Spectral results exhibit strong power in eccentricity (about 100 k.y.) throughout the record. Kap X, CaCO3flr, and Pfar show significant power in obliquity (about 41 k.y.), whereas XARM/X shows significant power at 73 k.y. from stages 21 to 11. The 73-k.y. period in XARM/X is near the difference tone of obliquity and eccentricity: 1/43-1/102 = 1/69. Kar and XARM/X show power only in eccentricity from stages 11 to 1. X and Pfar show significant power in precession (about 18 and 22 k.y.) whereas CaC03ar has power at 34 k.y, which could be a combination of precession and obliquity. The shift in power of orbital parameters may by attributed to the effect of the about 413-k.y. signal of eccentricity.

Investigation of microfossils on sediment core CRP-2 from the Ross Sea, Antarctica

Marine diatoms are the primary biostratigraphical and paleoenvironmental tool for interpreting the upper Palaeogene and lower Neogene strata recovered during the second drilling season of the Cape Roberts Project at site CRP-2 in the western Ross Sea, Antarctica. Silicoflagellates, ebridians, and a chrysophyte cyst provide supporting biostratigraphical information. More than 100 dominantly planktic diatom taxa are recognised. Of these, more than 30 are treated informally, pending SEM examination and formal description. Many other taxa are noted only to generic level. Lower Oligocene (c. 31 Ma) through lower Miocene (c. 18.5 Ma) diatoms occur from 28 mbsf down to 565 mbsf. Below this level, to the bottom of the hole at 624.15 mbsf, diatom assemblages are poorly-preserved and many samples are barren. A biostratigraphic zonal framework, consisting of ten diatom zones, is proposed for the Antarctic continental shelf. Ages inferred from the diatom biostratigraphy correspond well with geochronological data from argon dating of volcanic materials and strontium dating of calcareous macrofossils, as well as nannofossil biochronological datums. The biochronostratigraphical record from CRP-2/2A provides an important record of diatom events and mid-Cenozoic environmental changes in the Antarctic neritic zone.

(Table 1) Sedimentary thicknesses, lithologies, ages, and interpreted paleowater depths for ODP Sites 135-840 and 135-841

The sedimentary succession drilled at Sites 840 and 841 on the Tonga forearc allows the sedimentary evolution of the active margin to be reconstructed since shortly after the initiation of subduction during the mid Eocene. Sedimentation has been dominated by submarine fan deposits, principally volcaniclastic turbidites and mass-flows derived from the volcanic arc. Volcaniclastic sedimentation occurred against a background of pelagic nannofossil sedimentation. A number of upward-fining cycles are recognized and are correlated to regional tectonic events, such as the rifting of the Lau Basin at 5.6 Ma. Episodes of sedimentation dating from 16.0 and 10.0 Ma also correlate well with major falls in eustatic sea level and may be at least partially caused by the resulting enhanced erosion of the arc edifice. The early stages of rifting of the Lau Basin are marked by the formation of a brief hiatus at Site 840 (Horizon A), probably a result of the uplift of the Tonga Platform. Controversy exists as to the degree and timing of the uplift of Site 840 before Lau Basin rifting, with estimates ranging from 2500 to 300 m. Structural information favors a lower value. Breakup of the Tonga Arc during rifting resulted in deposition of dacite-dominated, volcaniclastic mass flows, probably reflecting a maximum in arc volcanism at this time. A pelagic interval at Site 840 suggests that no volcanic arc was present adjacent to the Tonga Platform from 5.0 to 3.0 Ma. This represents the time between separation of the Lau Ridge from the Tonga Platform and the start of activity on the Tofua Arc at 3.0 Ma. The sedimentary successions at both sites provide a record of the arc volcanism despite the reworked nature of the deposits. Probe analyses of volcanic glass grains from Site 840 indicate a consistent low-K tholeiite chemistry from 7.0 Ma to the present, possibly reflecting sediment sourcing from a single volcanic center over long periods of time. Trace and rare-earth-element (REE) analyses of basaltic glass grains indicate that thinning of the arc lithosphere had begun by 7.0 Ma and was the principle cause of a progressive depletion of the high-field-strength (HFSE), REE, and large-ion-lithophile (LILE) elements within the arc magmas before rifting. Magmatic underplating of the Tofua Arc has reversed this trend since that time. Increasing fluid flux from the subducting slab since basin rifting has caused a progressive enrichment in LILEs. Subduction erosion of the underside of the forearc lithosphere has caused continuous subsidence and tilting toward the trench since 37.0 Ma. Enhanced subsidence occurred during rifting of the South Fiji and Lau basins. Collision of the Louisville Ridge with the trench has caused no change in the nature of the sedimentation, but it may have been responsible for up to 300 m of uplift at Site 840.

Concentrations and isotopic compositions of low molecular weight hydrocarbons in hydrothermal fluids from ODP Field, Middle Valley, northern Juan de Fuca Ridge

The presence of sedimentary organic matter blanketing midocean ridge crests has a potentially strong impact on metal transport in hydrothermal vent fluids. To constrain the role of organic matter in metal mobility during hydrothermal sediment alteration, we reacted organic-rich diatomaceous ooze from Guaymas Basin, Gulf of California, and organic-poor hemipelagic mud from Middle Valley, northern Juan de Fuca Ridge, with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity, at 275° to 400°C, 350 to 500 bars, and initial fluid: sediment mass ratios ranging from 1.6 to 9.8. Reaction of these fluids with both sediment types released CO2 and high concentrations of ore-forming metals (Fe, Mn, Zn, Pb) to solution. Relatively low concentrations of Cu were observed in solution and likely reflect the reducing conditions that resulted from the presence of sedimentary organic matter. Both the concentrations of CO2 and dissolved metals were lower in fluids reacted with Middle Valley sediment compared with aqueous concentrations in fluids reacted with Guaymas Basin sediment. During alteration of both sediment types, metal concentrations varied strongly as a function of temperature, increasing by up to an order of magnitude over the 75°C range of each experiment. Major element fluid chemistry and observed alteration assemblages suggest that during hydrothermal alteration of organic-lean sediment from Middle Valley a feldspar-quartz-illite mineral assemblage buffered in situ pH. In contrast, data from the experimental alteration of organic-rich Guaymas Basin sediment suggest that a calcite-plagioclase-quartz assemblage regulated in situ pH. Fluid speciation calculations suggest that in situ pH during Guaymas Basin sediment alteration was lower than during alteration of Middle Valley sediment and accounts for the substantially greater metal mobility at a given temperature and pressure during the former experiment. Comparison of our results with the results of basalt alteration experiments indicate that except for Cu, hydrothermal sediment alteration results in equal or greater concentrations of ore-forming metals at a given temperature and pressure. Accordingly, the presence of ore-forming metals in fluids currently venting from sediment-covered hydrothermal systems at concentrations substantially lower than in fluids from bare-rock systems may reflect chemical reequilibration during subsurface cooling within the sediment pile.