Strontium isotopic ratios in selected microfossils from ODP Holes 119-738C and 120-747A

The Maastrichtian and Danian intervals of Ocean Drilling Program (ODP) Hole 738C contain numerous microfossils above the level of their putative extinction, suggesting either (1) persistence of local communities long after species turnover occurred across the rest of the globe or (2) large-scale reworking. These interpretations have very different paleoenvironmental implications, but discriminating between them has proved difficult. To test the competing hypotheses, we measured the 87Sr/86Sr ratios of taxon-specific separates from a number of samples and compared these values both to each other and to expected seawater values at the time of deposition. Our results indicate extensive and pervasive reworking throughout Maastrichtian and lower Danian strata in ODP Hole 738C. We estimate that up to 30% of the mass of foraminifers in any sample can be contributed by individuals that have been reworked.

Geochemistry at DSDP Legs 56-57 Holes

About 200 volcanic ash layers were recovered during DSDP Leg 57. The volcanic glass in some of these layers was investigated petrographically and chemically in this study. Volcanic glass is mainly rhyolitic and/or rhyodacitic in chemical composition, and its refractive index ranges from 1.496 to 1.529. Some volcanic ash layers consist of multiple grains of different chemical compositions. All the volcanic glass belongs to the tholeiitic and the calc-alkalic volcanic rock series, in SiO2-(Na2O + K2O) diagram and FeO*/MgO-SiO2 diagram. We correlated successfully three volcanic ash layers from the standpoint of chemical composition and biostratigraphy. Hydration of volcanic glass from Leg 57 is less intense than in other DSDP cores.

Chemical element concentrations in sediments from DSDP Holes 60-458 and 60-459

The development of laser ablation-inductively coupled plasma-mass spectrometry has revolutionized the analysis of tephras by providing (1) an efficient and precise method for determining abundances of a wide variety of trace elements at low concentrations in individual glass shards and (2) assessment of geochemical heterogeneities within individual ash horizons. This development is important for petrogenetic studies of intraoceanic arc systems, where tephras provide the most complete temporal record of magmatism. Results from the Izu-Bonin and Mariana arc systems indicate that despite close geographical proximity and similar tectonic evolution, they contrast strongly in terms of geochemical evolution since 35 Ma. Whereas the Mariana tephras have exceptional compositional diversity, ranging from low-K (Oligocene), to high-K (Miocene), and subsequently medium-K compositions (Pliocene-Quaternary), the Izu-Bonin arc has been dominated by low-K compositions throughout. The Mariana increases in K are paralleled by increases in abundances of incompatible trace elements and by increased values of diagnostic ratios (e.g., Nb/yb and Th/yb) regarded as monitors of potential mantle-source fertility. The relative uniformity of Nb/yb and Nb/Zr ratios in Izu-Bonin tephras indicates that cyclic processes of backarc basin development and mantle depletion do not necessarily induce large-scale temporal geochemical variations in the associated arc. Temporal variability within the Mariana arc, and its divergence from the Izu-Bonin arc ca. 13 Ma, can be traced to a major injection of subducted sediment in the Mariana system at this time.

Helium isotope ratios and sedimentation rate models of ODP holes

A continuous age model for the brief climate excursion at the Paleocene-Eocene boundary has been constructed by assuming a constant flux of extraterrestrial 3He (3He[ET]) to the seafloor. 3He[ET] measurements from ODP Site 690 provide quantitative evidence for the rapid onset (

Velocity-density systematics for basalts at DSPD Leg 54 Holes

Precise velocity and density measurements at atmospheric and elevated pressures have been obtained on basalts drilled in 13 holes during Leg 54. The vp-sigma data show systematics which are controlled by the degree of crystallinity (or conversely, glassiness), microstructure, and original mineralogy and chemistry. Extensively fractionated basalts with marked iron enrichment produce anomalously low measured velocities at densities above 2.90 g/cm**3. Also, the effective in situ pressure acting on Leg 54 basalts is less than hydrostatic, and perhaps close to zero. At zero effective pressure, the measured velocities average 2.5 km/s higher than East Pacific Rise upper crustal velocities determined by seismic refraction. This implies that the in situ velocities are undoubtedly a result of the highly fragmented nature of East Pacific Rise crust.

Magnetic remanence acquisition in DSDP Leg 73 Holes

The major magnetic mineral in the turbidites and slumped sediments recovered at Leg 73 drill sites was near to magnetite in composition and in the form of small multidomain particles. There was no variation in magnetic mineralogy with the lithology. The variations in the intensities and directions of the natural remanent magnetization could be explained in terms of postdepositional grain rotations within the wet sediment. In the sands realignment was partial, whereas in some of the slumps the entire remanent magnetization was reset. Fine-particle magnetite was also the main magnetic constituent of the red clays. A significant proportion of a higher-coercivity mineral was also present. The magnetic characteristics of the red clays are explained as a combination of concentration and grain rotation effects. The implications to the assessment of the reliability of paleomagnetic data are discussed. Note: Conversion factors are as follows: 1 Am**2/kg = 1 emu/g, and 80 A/m about 1 Oe.

Elemental and stable isotopic composition of some metalliferous and pelagic sediments at DSDP Leg 70 Holes

Nontronite, the main metalliferous phase of the Galapagos mounds, occurs at subsurface depths of about 2 to 20 meters; Mn-oxide material is limited to the upper 2 meters of the mounds. The nontronite forms intervals of up to a few meters' thickness, consisting essentially of 100% nontronite granules, which alternate with intervals of normal pelagic sediment. Electron microprobe analyses of nontronite granules from different core samples indicate that: (1) there is little difference in major element composition between nontronites from varying locations within the mounds, with adjacent granules from a given sample having very similar compositions; (2) individual granules show little internal variation in composition. This indicates that the granules are composed of a single mineral of essentially constant composition, consistent with relatively uniform conditions of Eh and composition during nontronite formation. Mn-oxide crusts have very low Fe contents, a feature characteristic of rapidly deposited Mn-oxide crusts formed under hydrothermal influences. The rare-earth element (REE) abundances of the nontronites are generally extremely low, totalling less than several ppm. Two samples have the negatively Ce anomaly typical of authigenic precipitates formed relatively rapidly from seawater. A Mn-oxide crust sample has low REE contents, typical of Mn-oxide crusts formed under hydrothermal influences, but no negative Ce anomaly. A sample of unusual Mn-Fe-oxide mud has relatively high REE concentrations and a seawater-type pattern; both of these features are also found for metalliferous sediments from the East Pacific Rise. The oxygen and hydrogen isotopic composition of the nontronites define a restricted field within a d18O-dD plot. In manganiferous sediments, d18O and dD appear to decrease with increase in the Mn-oxide content of the sediment. From the d18O values of the nontronites, formation temperatures in the range of about 20-30°C have been estimated. By comparison, temperatures of up to 11.5 °C at a 9-meter depth have been directly measured within the mounds (Corliss et al., 1979), and heat-flow data suggest present basement/sediment interface temperatures of 15-25°C. In a plot of Fe + Mn vs. d18O, the Mn-oxide crust and Mn-Fe-ooze plot near the tie-lines for authigenic Mn nodules and silicate phases, implying that they have formed in isotopic equilibrium with seawater at or close to bottom-water temperatures.

40Ar/39Ar single-crystal dating of detrital muscovite and K-feldspar of ODP Holes 116-717A and 116-718C

Detrital K-feldspars and muscovites from Ocean Drilling Program Leg 116 cores that have depositional ages from 0 to 18 Ma have been dated by the 40Ar/39Ar technique. Four to thirteen individual K-feldspars have been dated from seven stratigraphic levels, each of which have a very large range, up to 1660 Ma. At each level investigated, at least one K-feldspar yielded an age minimum which is, within uncertainty, identical to the age of deposition. One to twelve single muscovite crystals from each of six levels have also been studied. The range of muscovite ages is less than that of the K-feldspars and, with one exception, reveal only a 20-Ma spread in ages. As with the K-feldspars, each level investigated contains muscovites with mineral ages essentially identical to depositional ages. These results indicate that a significant portion of the material in the Bengal Fan is first-cycle detritus derived from the Himalayas. Therefore, the significant proportion of sediment deposited in the distal fan in the early to mid Miocene can be ascribed to a significant pulse of uplift and erosion in the collision zone. Moreover, these data indicate that during the entire Neogene, some portion of the Himalayan orogen was experiencing rapid erosion (<= uplift). The lack of granulite facies rocks in the eastern Himalayas and Tibetan Plateau suggests that very rapid uplift must have been distributed in brief pulses in different places in the mountain belt. We suggest that the great majority of the crystals with young apparent ages have been derived from the southern slope of the Himalayas, predominantly from near the main central thrust zone. These data provide further evidence against tectonic models in which the Himalayas and Tibetan plateaus are uplifted either uniformly during the past 40 m.y. or mostly within the last 2 to 5 m.y.

Diatom stratigraphy of ODP Leg 150 holes

Diatoms are present in middle to lower upper Miocene sections of all holes examined during Leg 150, but are generally absent or in low abundance in Pleistocene to middle upper Miocene sediments. An exception is the alternating diatom-rich, diatom-poor intervals in upper Quaternary sediments. Five new diatom zones, covering an interval from near the lower/middle Miocene boundary to the lower upper Miocene, are proposed. Some of the taxon used to define these zones are also used in zonal schemes for the East Coast of the United States, and allow for correlations to be drawn between this region and Leg 150 sites. Lower Miocene and older levels are not included in this study. Although older Tertiary diatoms are present at some of the sites, dissolution has largely compromised their usefulness as zonal markers.