Geochemical processes and rare earth element analysis in cold-seep pore waters of Hydrate Ridge, northeast Pacific Ocean

The concentrations of rare earth elements (REEs), sulphate, hydrogen sulphide, total alkalinity, calcium, magnesium and phosphate were measured in shallow (<12 cm below seafloor) pore waters from cold-seep sediments on the northern and southern summits of Hydrate Ridge, offshore Oregon. Downward-decreasing sulphate and coevally increasing sulphide concentrations reveal sulphate reductionas dominant early diagenetic process from ~2 cm depth downwards. A strong increase of total dissolved REE concentrations is evident immediately below the sediment-water interface, which can be related to early diagenetic release of REEs into pore water resulting from the remineralization of particulate organic matter. The highest pore water REE concentrations were measured close to the sediment-water interface at ~2 cm depth. Distinct shale normalized REE patterns point to particulate organic matter and iron oxides as main REE sources in the upper ~2-cm depth interval. In general, the pore waters have shalenormalized patterns reflecting heavy REE (HREE) enrichment, which suggests preferential complexation of HREEs with carbonate ions. Below ~2 cm depth, a downward decrease in REE correlates with a decrease in pore water calcium concentrations. At this depth, the anaerobic oxidation of methane (AOM) coupled to sulphate reduction increases carbonate alkalinity through the production of bicarbonate, which results in the precipitation of carbonate minerals. It seems therefore likely that the REEs and calcium are consumed during vast AOM-induced precipitation of carbonate in shallow Hydrate Ridge sediments. The analysis of pore waters from Hydrate Ridge shed new light on early diagenetic processes at cold seeps, corroborating the great potential of REEs to identify geochemical processes and to constrain environmental conditions.

Age model and stable isotope record of eastern equatorial Pacific sediment cores

Six deep sea cores from the eastern equatorial Pacific (EEP) were analyzed for planktonic foraminifera and stable isotopes in order to reconstruct sea surface temperatures (SST) for the last 40 ka. South of the Equatorial Front the abundance of Globorotalia inflata increased, and SST decreased by >5°C (core ODP846B), creating a stronger SST meridional gradient and advection of the Peru Current than present for the ~16-35 ka interval. A sharper SST meridional gradient forced stronger Choco jet events and a moisture increase in western Colombia, which supplied, through the San Juan River and the south-flowing equatorial and the Peru-Chile countercurrents, abundant hemipelagic quartz over the northern Peru basin (core TR163-31B). The Choco jet, and its associated mesoscale convective cells, provoked an increase in snow precipitation over the Central Cordillera of Colombia and the advance of the Murillo glacier. In synchrony with the intensified Choco jet events, the "dry island" effect over the Eastern Cordillera of Colombia intensified, and the level of Fuquene Lake dropped.

Fossil datums of Late Neogene sediments from the Southwest Pacific Ocean

The assumption of synchrony of first and last occurrences of fossil taxa can be tested using graphic correlation procedures which, by allowing measured stratigraphic sections to be compared on a common depth scale, make it possible to develop a correlation model which integrates information from a number of cores. The strategy of the test presented here is to use a graphic correlation model that is based on data from the Atlantic (Deep Sea Drilling Project (DSDP) sites 502, 516A) and north Pacific (DSDP site 577A) as a basis for determining to what extent fossil datums in the southwest Pacific are synchronous. First and last occurrences of Pliocene calcareous nannofossils and planktonic foraminifers have been compared in five DSDP cores from the southwest Pacific ocean (sites 586, 587, 588, 590A, and 592). All cores were recovered using hydraulic piston coring technology, which assures the best recovery and minimal disturbance. Most of these cores contain abundant, well-preserved foraminifers and nannofossils, as well as a partial record of many of the expected magnetic polarity reversals in this part of the section. To assure taxonomic consistency, all taxonomic identifications were made by the author. Graphic correlation of this data set suggests that several important biostratigraphic markers are highly diachronous. For example, this study confirms that Globorotalia truncatulinoides first occurs at approximately 2.4 Ma between 20° and 35° south latitude in the southwest Pacific, approximately 0.5 m.y. earlier than it is found elsewhere in the Atlantic and Pacific. Other datums, such as the last occurrence of Discoaster brouweri, are essentially synchronous. These findings suggest that biostratigraphic models based on the assumption of synchrony of first and last occurrences of fossil taxa may be incorrect. Biostratigraphic models created with the Graphic Correlation method offer an opportunity to examine the biogeographic dimensions of origination, migration, and extinction of planktonic taxa.

Chemical composition and accumulation rates of chemical elements in metalliferous sediments from axial zones of the East Pacific Rise and the Mid-Atlantic Ridge

In the monograph metalliferous sediments of the East Pacific Rise near 21°S are under consideration. Distribution trends of chemical, mineral and grain size compositions of metalliferous sediments accumulated near the axis of this ultrafast spreading segment of the EPR are shown. On the basis of lithological and geochemical investigations spatial and temporal variations of hydrothermal activity are estimated. Migration rates of hydrothermal fields along the spreading axis are calculated. The model of cyclic hydrothermal process is suggested as a result of tectono-magmatic development of the spreding centre.

3He/4He and 4He/20Ne ratios in pore water and gas in Pacific deep-sea sediments

We have measured the 3He/3He and 3He/20 Ne ratios of thirty-nine pore water and gas samples in deep-sea sediments collected at twelve sites on the Pacific Ocean bottom during the cruises of Deep Sea Drilling Project Legs 87, 89, 90 and 92. The 3He/4He and 4He/20Ne ratios vary from 0.000000215 to 0.00000165 and from 0.29 to 20, respectively. He in the sample is composed of four components: (1) atmospheric He dissolved in seawater; (2) atmospheric He with mantle-derived He in Pacific bottom water; (3) in situ radiogenic He in the sediment; and (4) crustal He in the basement rock. Assuming that the 20Ne contents are constant with the value of seawater, the depth variations in the 4He/20Ne ratios at five Sites, 583D, 594, 597A, 598A and 504B, may provide useful information on 4He flux at the ocean bottom. The estimated 4He fluxes vary from 2000 to 40000 atoms cm**-2 s**-1 and are one to three orders of magnitude less than those calculated from the excess He in deep ocean water. An overall similarity between the geographical distribution of the 3He/4He ratios and heat flow data is found in the study area, between the East Pacific Rise across the Pacific Ocean and the Japanese Islands. The tendency is well explained by a conventional sea-floor spreading model.

Calcareous nannofossils in sediments of the southeast Pacific

This study provides the first detailed documentation of calcareous nannofossil assemblages in Eocene subantarctic Eltanin piston cores recovered from the southeast Pacific Ocean. These Eltanin cores are important because they have been reported to contain ice-rafted quartz. The present study confirms early and middle Eocene ages for the cores and dates them more precisely using calcareous nannofossils. Semiquantitative study of the nannoflora indicates that they are of a warmer water character than those from the higher latitudes (such as Falkland Plateau, Maud Rise, and Kerguelen Plateau). This study concludes that it is unlikely for the ice-rafted quartz in the Eocene sections to be downcore contaminants from the overlying Neogene sediment and suggests that the grains are probably the result of Eocene ice-rafting from Antarctica when the Drake Passage was closed and water circulation patterns were different from those of today.

Chemistry on manganese nodules from the Pacific Ocean

Three manganese nodules from the Pacific Ocean have been analysed for 35 elements by using mainly spectrophotometric and spectrographic methods. Cu, Co, Ni, Zn, and Pb were found in amounts approaching 1 %, which far exceeds their average concentrations in igneous rocks. On the other hand, elements having readily hydrolysable ions, such as Ga, Sc, Zr, Y, La and Ti, are present only in amounts comparable with their concentrations in igneous rocks. Sb, Bit Be, and Cr were not detected. The hydrochloric acid-insoluble fraction of nodules is practically free of the heavy metals that are characteristic of the acid-soluble fraction; it consists principally of clay minerals, together with lesser amounts of quartz, apatite, biotite and sodium and potassium felspars.

Distribution of radiolaria in Oligocene to Pleistocene sediments of the North-West Pacific

Neogene stratigraphy of the tropical and subtropical Pacific on radiolaria is studied in the book. A detailed comparison of coeval systems from tropics and subtropics is given. A possibility of use of a uniform zonal scale in these areas is proved. Magnitude of changes of complexes on borders of Neogene zones is studied in detail. Six stages in development of radiolarians are identified in the tropics in Neogene. Stratigraphic levels, where the greatest changes of fauna occurred, are natural boundaries of these stages. 72 species of radiolarians (two of which are new) are described in the book.

Physical, chemical and geological properties of sediments from the central equatorial Pacific

The solution rate of biogenic opal in near-surface sediments in the Central Equatorial Pacific is three to eight orders of magnitude lower than similar acid-cleaned samples. Iron, magnesium and calcium aluminosilicates may be the minerals which are forming on the surface of the opal and reducing its solution rate. The scale height of the system studied suggests that diffusive and not advective processes are primarily responsible for the removal of dissolved silica in sediments. Solution budget calculations for this area suggest that 90-99 per cent of the biogenic opal produced in surface waters dissolves before reaching the sediment-water interface; an additional amount dissolves within the sediment and diffuses into bottom waters leaving 0.05-0.15 per cent of the original amount of opal produced by organisms in the sedimentary record. The relative solution potential of the upper 1000 m of the water column varies by more than an order of magnitude from the Antarctic to Equator and may have a pronounced effect on the accumulation rate of biogenic opal in underlying sediments.