Petrography, minerals and isotopic composition at DSDP Leg 66 Holes

We collected 20 carbonate nodules from the inner trench slope deposits of the Middle America Trench area off Mexico. Carbonate nodules are found only within the methane-rich layer beneath the mixed layer of methane and hydrogen sulfide. They have been investigated by microscopic, scanning electron microscopic (SEM), X-ray diffraction, and stable isotopic analytical methods. Calcite, magnesian calcite, dolomite, and rhodochrosite were recognized as carbonate minerals. Each carbonate nodule is usually represented by single species of carbonate minerals. Carbonate nodules are subdivided into micrite nodules and recrystallized nodules according to textural features. The carbonate crystallites in each micrite nodule are equidimensional. Their sizes range from several to 30 µm, as revealed by SEM micrographs. The chemical composition of calcite is changed from pure calcite to high magnesian calcite, as shown by the shift of the (104) reflection in X-ray diffraction patterns. Fe substitution for Ca in dolomite was also observed. Carbon isotopic composition shows an unusually wide range - from -42.9 to +13.5 per mil - in PDB scale, whereas oxygen isotopic compositions of almost all the carbonate nodules are constantly enriched in 18O from +3.4 to +7.60 per mil in PDB scale. These wide variations in carbon isotopic composition indicate several sources for the carbon in carbonate nodules. Carbon with a negative d13C value was derived from biochemical oxidation of methane with a negative d13C value. On the other hand, carbon with positive d13C value was probably formed during methane production in an anoxic condition.

Preliminary lipid analyses of two Quaternary sediments from DSDP Holes 66-487 and 66-491

We investigated the solvent-extractable hydrocarbons, ketones, alcohols, and carboxylic acids of two Quaternary sediments from the Middle America Trench (Sections 487-2-3 and 491-1-5). These lipids are derived from a mixed input of autochthonous and allochthonous materials, with minor contributions from thermally mature sources. Their compositions are typical of those of immature Quaternary marine sediments, and their lipid distributions show many similarities to those of Japan Trench sediments.

Late Miocene development of the western Pacific warm pool: Planktonic foraminifer and oxygen isotopic evidence

The disappearance at ~10 Ma of the deep dwelling planktonic foraminifer Globoquadrina dehiscens from the western Pacific including the South China Sea was about 3 Myr earlier than its final extinction elsewhere. Accompanying this event at ~10 Ma was a series of faunal turnover characterized by increase in mixed layer, warm-water species and decrease to a minimum in deepwater species. Paleobiological and isotopic evidence indicates sea surface warming and a deepened local thermocline that we interpret as related to the development of an early western Pacific warm pool. The stepwise decline of G. dehiscens and other deep dwelling species from the NW and SW Pacific suggests more intensive warm water pileup than equatorial localities where surface bypass flow through the narrowing Indonesia seaway appears to remain efficient during the late Miocene. Planktonic delta18O values from the South China Sea consistently lighter than the tropical western Pacific during the Miocene also suggest, similar to today, more variable hydrologic conditions along the periphery than in the core of the warm pool. Stronger hydrologic variability affected mainly by monsoons and increased thermal gradient along the western margin of the late Miocene warm pool may have contributed to the decline of deep dwelling planktonic species including the early extinction of G. dehiscens from the South China Sea region. The late Miocene warm pool became influential and paleobiologically detectable from ~10 Ma, but the modern warm pool did not appear until about 4 Ma, in the middle Pliocene.

Physical properties and geochemical pore-water analysis of sediment core GeoB16423-1

Several studies indicate that the 2011 Tohoku-Oki earthquake (Mw 9.0) off the Pacific coast of Japan has induced slip to the trench and triggered landslides in the Japan Trench. In order to better understand these processes, detailed mapping and shallow-coring landslides at the trench as well as Integrated Ocean Drilling Program (IODP) deep drilling to recover the plate boundary décollement (Japan Trench Fast Earthquake Drilling Project, JFAST) have been conducted. In this study we report sediment core data from the rapid response R/V SONNE cruise (SO219A) to the Japan Trench, evidencing a Mass Transport Deposit (MTD) in the uppermost section later drilled at this JFAST-site during IODP Expedition 343. A 8.7 m long gravity core (GeoB16423-1) recovered from ~7,000 m water depth reveals a 8 m sequence of semi-consolidated mud clast breccias embedded in a distorted chaotic sediment matrix. The MTD is covered by a thin veneer of 50 cm hemipelagic, bioturbated diatomaceous mud. This stratigraphic boundary can be clearly distinguished by using physical properties data from Multi Sensor Core Logging and from fall-cone penetrometer shear strength measurements. The geochemical analysis of the pore-water shows undisturbed linear profiles measured from the seafloor downcore across the stratigraphic contact between overlying younger background-sediment and MTD below. This indicates that the investigated section has not been affected by a recent sediment destabilization in the course of the giant Tohoku-Oki earthquake event. Instead, we report an older landslide which occurred between 700 and 10,000 years ago, implying that submarine mass movements are dominant processes along the Japan Trench. However, they occur on local sites and not during each megathrust earthquake.

Distribution of diatoms in Oligocene to Pleistocene sediments of the tropical Pacific

According to the drilling probes of the Deep Waier Drilling Project, Neogene sediments in a tropical area of the Pacific Ocean are divided into 15 zones based on diatoms. The author shows that a unique zonation may be applied for the entire region. Identification of diatoms zones boundaries was conducted through their direct correlation with nannoplancton, radiolarian and foraminiferal zonal sceals. Their ultra-structure and morphological relationship are being analysed. The mode of siliceous accumulation within the equatorial belt differed through the western central and eastern region since the early Miocene and the difference become more evident from the end of Middle Miocene. The distribution of Neogene diatomaceous silt in the tropical area is controlled by the character of gyre-water circulation and agrees with the modern geographical zonation.

Benthic foraminiferal biofacies and stable isotopic record of St. Stephens Quarry, Alabama

We integrate upper Eocene-lower Oligocene lithostratigraphic, magnetostratigraphic, biostratigraphic, stable isotopic, benthic foraminiferal faunal, downhole log, and sequence stratigraphic studies from the Alabama St. Stephens Quarry (SSQ) core hole, linking global ice volume, sea level, and temperature changes through the greenhouse to icehouse transition of the Cenozoic. We show that the SSQ succession is dissected by hiatuses associated with sequence boundaries. Three previously reported sequence boundaries are well dated here: North Twistwood Creek-Cocoa (35.4-35.9 Ma), Mint Spring-Red Bluff (33.0 Ma), and Bucatunna-Chickasawhay (the mid-Oligocene fall, ca. 30.2 Ma). In addition, we document three previously undetected or controversial sequences: mid-Pachuta (33.9-35.0 Ma), Shubuta-Bumpnose (lowermost Oligocene, ca. 33.6 Ma), and Byram-Glendon (30.5-31.7 Ma). An ~0.9 per mil d18O increase in the SSQ core hole is correlated to the global earliest Oligocene (Oi1) event using magnetobiostratigraphy; this increase is associated with the Shubuta-Bumpnose contact, an erosional surface, and a biofacies shift in the core hole, providing a first-order correlation between ice growth and a sequence boundary that indicates a sea-level fall. The d18O increase is associated with a eustatic fall of ~55 m, indicating that ~0.4 per mil of the increase at Oi1 time was due to temperature. Maximum d18O values of Oi1 occur above the sequence boundary, requiring that deposition resumed during the lowest eustatic lowstand. A precursor d18O increase of 0.5 per mil (33.8 Ma, midchron C13r) at SSQ correlates with a 0.5 per mil increase in the deep Pacific Ocean; the lack of evidence for a sea-level change with the precursor suggests that this was primarily a cooling event, not an ice-volume event. Eocene-Oligocene shelf water temperatures of ~17-19 °C at SSQ are similar to modern values for 100 m water depth in this region. Our study establishes the relationships among ice volume, d18O, and sequences: a latest Eocene cooling event was followed by an earliest Oligocene ice volume and cooling event that lowered sea level and formed a sequence boundary during the early stages of eustatic fall.

Biostratigraphy and Sr isotopic composition of sediments from DSDP Legs 12, 14, 15, 17, 20-22, and 30

We present the data used to construct the Cenozoic and Cretaceous portion of the Phanerozoic curve of seawater 87Sr/86Sr that had been given in summary form by W.H. Burke co-workers. All Cenozoic samples (128) and 22 Cretaceous samples are foram-nannofossil oozes and limestones from DSDP cores distributed among 13 sites in the Atlantic, Pacific and Indian Oceans, and the Caribbean Sea. Non-DSDP Cretaceous samples (126) include limestone, anhydrite and phosphate samples from North America, Europe and Asia. Determination of the 87Sr/86Sr value of seawater at particular times in the past is based on comparison of ratios derived from coeval marine samples from widely separated geographic areas. These samples are characterized by a wide variety of diagenetic and burial histories. The large size and cosmopolitan nature of the data set decreases the likelihood that, among coeval data, systematic error has been introduced by a similar pattern of diagenetic alteration of the ratios. There is good clustering of data points throughout the Cenozoic and Cretaceous curve. The consistency of data is illustrated by Cenozoic and Cretaceous data plots that include a separate symbol for each DSDP site and non-DSDP sample location. More than 98% of the data points are enclosed by upper and lower lines that define a narrow band. For any given time, the correct seawater ratio probably lies within this band. A line drawn within the band represents our estimate of the actual seawater ratio as a function of time. The general configuration of the Cenozoic and Cretaceous curve appears to be strongly influenced by the history of plate interactions and sea-floor spreading. Specific rises and falls in the 87Sr/86Sr of seawater, however, may be caused by a variety of factors such as variation in lithologic composition of the crust exposed to weathering, configuration and topographic relief of continents, volcanic activity, rate of sea-floor spreading, extent of continental inundation by epeiric seas, and variations in both climate and paleooceanographic conditions. Many or all of these factors are probably related to global tectonic processes, yet their combined effect on the temporal variation of seawater 87Sr/86Sr can complicate a direct platetectonic interpretation for portions of the seawater curve.

Os and Re concentrations and Os isotope ratios for DSDP and ODP metalliferous sediments from the Pacific Ocean (Table 1)

We report new 187Os/186Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield 187Os/186Os values of between 3 and 6.5 and 187Re/186Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to 187Os/186Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in 187Os/186Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater. New data for the late Cenozoic further substantiate the rapid increase in the 187Os/186Os of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and Os isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic isotopic compositions. The positive correlation between 87Sr/86Sr and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in 187Os/186Os from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and account for the periods of decoupling of the marine Sr and Os isotope records.

Latest Oligocene through early middle Miocene diatom biostratigraphy of the eastern tropical Pacific

Study of DSDP Sites 71, 77, and 495 has allowed the development of a refined diatom biostratigraphy for the latest Oligocene through early middle Miocene of the eastern tropical Pacific which is well correlated to the low-latitude zonations for planktonic foraminifers, coccoliths, and radiolarians. Six zones and 7 subzones are proposed, and correlation with high-latitude diatoms zonations for the North Pacific, the Norwegian Sea, and the Southern Ocean is suggested by the discovery of selected diatoms in these tropical sediments which were previously thought to be restricted to high latitudes. Six new species and one new variety of diatoms which are stratigraphically useful are proposed : Actinocyclus hajosiae, n. sp., A. radionovae, n. sp., Coscinodiscus blysmos, n. sp., C. praenodulifer, n. sp., Craspedodiscus rydei, n. sp., Thalassiosira bukryi, n. sp., and Coscinodiscus lewisianus var. robustus n. var.

Interstitial water chemistry at DSDP Leg 67 Holes

Interstitial water chemistry has proved to be a sensitive indicator for early diagenetic reactions, particularly those related to organic matter oxidation. Downhole chemical variations in the pore waters from Deep Sea Drilling Project Holes 496 and 497 on the Middle America Trench slope off Guatemala are anomalous because both salinity and chlorinity show strong decreases to half the values of seawater, and d18O values become positive (maximum of about +2.5% at the bottom of the holes). These observations are explained in terms of dilution of pore waters after retrieval as a result of decomposition of the gas hydrates before removal of pore waters by shipboard squeezing techniques. In all holes, except Hole 495 (drilled in pelagic sediments), decomposition of organic matter leads to rapid sulfate depletion and subsequent methane generation. Associated with methane generation are large increases in alkalinity and dissolved ammonia. The latter component causes ion exchange reactions with clay minerals, which results in maxima in magnesium and perhaps potassium. At greater depths, as yet unidentified reactions cause the removal of magnesium. Especially in the deeper Trench Sites 499 and 500, rapid variations in calcium, magnesium, and alkalinity occur in turbidite sequences.