Mineralogy of southern Kerguelen Plateau sediments

Kaolinite, goethite, minor hematite, and gibbsite were found in fluvial upper Lower Cretaceous basal sediment from the Southern Kerguelen Plateau, Sites 748 and 750, 55°S latitude. This mineral assemblage, derived from the weathering of basalt, indicates near-tropical weathering conditions with high orographic rainfall, at least 100 cm per year. The climate deteriorated by the Turonian or Coniacian, as indicated by the decline in kaolinite content of this sediment. The Upper Cretaceous sediment at Site 748 consists of 200 m of millimeter-laminated, sparsely fossiliferous, wood-bearing glauconitic siltstone and clay stone with siderite concretions deposited on a shelf below wave base. Some graded and cross beds indicate that storms swept over the shelf and reworked the sediment. Overlying this unit is 300 m of intermittently partly silicified, bryozoan-inoceramid-echinoderm-rich glauconitic packstones, grainstones, and wackestones. The dominant clay mineral in both units is identical to the mineral composition of the glauconite pellets: randomly interstratified smectite-mica. The clay fraction has a higher percent of expandable layers than the mineral of the glauconite pellets, and the clay of the underlying subunit has a higher percentage of expandable layers than the clay of the carbonate subunit. Potassium levels mirror these mineral variations, with higher K levels in minerals that have a lower percentage of expandable layers. The decrease in expandability of the mineral in the upper subunit is attributed to diagenesis, the result of higher porosity.

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.

Geochemistry of Baltic Sea sediments

The Baltic Sea has experienced three major intervals of bottom water hypoxia following the intrusion of seawater ca. 8 kyrs ago. These intervals occurred during the Holocene Thermal Maximum (HTM), Medieval Climate Anomaly (MCA) and during recent decades. Here, we show that sequestration of both Fe and Mn in Baltic Sea sediments generally increases with water depth, and we attribute this to shelf-to-basin transfer ("shuttling") of Fe and Mn. Burial of Mn in slope and basin sediments was enhanced following the lake-brackish/marine transition at the beginning of the hypoxic interval during the HTM. During hypoxic intervals, shelf-to-basin transfer of Fe was generally enhanced but that of Mn was reduced. However, intensification of hypoxia within hypoxic intervals led to decreased burial of both Mn and Fe in deep basin sediments. This implies a non-linearity in shelf Fe release upon expanding hypoxia with initial enhanced Fe release relative to oxic conditions followed by increased retention in shelf sediments, likely in the form of iron sulfide minerals. For Mn, extended hypoxia leads to more limited sequestration as Mn carbonate in deep basin sediments, presumably because of more rapid reduction of Mn oxides formed after inflows and subsequent escape of dissolved Mn to the overlying water. Our Fe records suggest that modern Baltic Sea hypoxia is more widespread than in the past. Furthermore, hypoxia-driven variations in shelf-to-basin transfer of Fe may have impacted the dynamics of P and sulfide in the Baltic Sea thus providing potential feedbacks on the further development of hypoxia.

Metamorphic processes in sediments at DSDP Leg 65 Holes

Sites 482, 483, and 485 were drilled during Leg 65 on young oceanic crust south of the Tamayo Transform Fault. The Leg 65 drilling program was part of a multinational effort to study the East Pacific Rise (EPR) and complements sea bottom surveys conducted both in this area (Lewis, 1979; Cyamex Scientific Team and Pastouret, 1981) and farther south at 21 °N (Larson, 1971; Normark, 1976; Cyamex Scientific Team; Rise Project Group, 1980). These studies, which included deep-tow, Angus and submersible surveys, were recently complemented by sea-beam surveys conducted by the Jean Charcot on the Tamayo Fracture Zone and farther south along the EPR. They have led to a better understanding of the magmatic, tectonic, and sedimentary processes occurring on the East Pacific Rise between the Tamayo and Rivera fracture zones. The purpose of this chapter is to describe the metamorphic processes affecting Pliocene through Quaternary sediments found in contact, or inter layered, with basaltic units drilled during Leg 65 at the mouth of the Gulf of California.

Chronostratigraphy of sediments from the Bahamas

Continuous cores drilled during the Bahamas Drilling Project (BDP) and the Ocean Drilling Program (ODP) Leg 166 along a transect from the top of Great Bahama Bank to the basin in the Straits of Florida provide a unique data set to test the assumption in seismic stratigraphy that seismic reflections are time lines and, thus, have a chronostratigraphic significance. Seismic reflections that are identified as seismic sequence boundaries (SSBs) were dated by means of biostratigraphy in the five ODP sites and by a combination of biostratigraphy, magnetostratigraphy and Sr isotope stratigraphy in the two BDP sites. The seismic reflection horizons are carried across a variety of facies belts from shallow-water carbonates over slope carbonates to drift deposits in the Straits of Florida. Within this system 17 SSBs were identified and dated. Despite the fact that the seismic reflections cross several facies belts, their ages remain remarkably constant. The average offset in all sites is 0.38 Myr. In no cases do the seismic reflections cut across time lines. The age differences are the combined result of the biostratigraphic sampling frequency, the spacing of marker species that required extrapolation of ages, and the resolution of the seismic data. In addition, uncertainties of age determination in the proximal sites where age-diagnostic fauna are rare add to the age differences between sites. Therefore, it can be concluded that the seismic reflections, which mark the SSBs along the Bahamas Transect, are time lines and can be used as stratigraphic markers. This finding implies that depositional surfaces are preferentially imaged by reflected seismic waves and that an impedance contrast exists across these surfaces. Facies successions across the sequence boundaries indicate that the sequence boundaries coincide with the change of deposition from times of high to low sea level. In the carbonate setting of Great Bahama Bank, sea-level changes produce changes in sediment composition, sedimentation rate and diagenesis from the platform top to the basin. The combination of these factors generates differences in sonic velocity and, thus, in impedance that cause the seismic reflection. The impedance contrasts decrease from the proximal to the distal sites, which is reflected in the seismic data by a decrease of the seismic amplitude in the basinal area.

Siliceous components of Pliocene-Holocene sediments of the South Atlantic

The Pliocene-Holocene sediments recovered on ODP Leg 114 from Holes 699A, 701C, and 704B are the subject of a detailed investigation to interpret changes in the Oceanographic environment of the South Atlantic in the vicinity of the Polar Front Zone (PFZ). The cores sample sediments at shallow (Hole 704B, 2532 m), intermediate (Hole 699A, 3716 m), and basinal (Hole 701C, 4647 m) depths. Sites 699 and 704 come under the influence of the Antarctic Circumpolar Current (ACC) and Circumpolar Deep Water. It is possible that the upper reaches of Antarctic Bottom Water (AABW) may also affect Hole 699A. Site 701 is influenced by AABW. Closely spaced samples were analyzed for grain-size distribution, sand fraction components, biosiliceous microfossils, organic carbon, and water content. PFZ migrations are traced using changes in bulk sedimentaccumulation rates and the abundance of the diatoms Actiniscus ssp. and Genus et species indet. 1 Fenner (1991), as well as changes in sediment grain size and composition. Diatomaceous sediments of Gilbert age in Hole 699A indicate that the PFZ was positioned over this site, but during the Gauss it migrated north, bringing in less productive Antarctic Surface Water. All cores document a very gradual southerly movement of the PFZ throughout the Matuyama (with some sharp fluctuations of the northen PFZ border over Site 704 between 1.45 and 1.83 m.y.). This regressive shift culminated in the late Matuyama. The latest Matuyama to earliest Brunhes record in Hole 699A has been removed by a hiatus lasting from 1.0 to 0.6 m.y., which was probably caused by intensification of the deep-reaching ACC. The corresponding interval in Hole 704B, the shallowest core, contains evidence of winnowing. Sharp fluctuations of large amplitude and high frequency in the lithology of the sediments from Hole 704B in the eastern South Atlantic, starting at about 0.75 m.y. and characterizing the whole Brunhes Epoch, record the rapid movement of the northern border of the PFZ over the site. These reflect strong glacial/interglacial alternations in climate. To a lesser extent, lithologic fluctuations in Hole 701C reflect the same phenomenon, whereas in Hole 699A the lithology does not vary as dramatically.

Concentrations and accumulation rates of chemical elements and sediment components in sediments of central equatorial Pacific, DSDP data

Accumulation rates of Mg, Al, Si, Mn, Fe, Ni, Cu, Zn, opal, and calcium carbonate have been calculated from their concentrations in samples from equatorial Deep Sea Drilling Project sites. Maps of element accumulation rates and of Q-mode factors derived from raw data indicate that the flux of trace metals to equatorial Pacific sediments has varied markedly through time and space in response to changes in the relative and absolute influence of several depositional influences: biogenic, detrital, authigenic, and hydrothermal sedimentation. Biologically derived material dominates the sediment of the equatorial Pacific. The distributions of Cu and Zn are most influenced by surface-water biological activity, but Ni, Al, Fe, and Mn are also incorporated into biological material. All of these elements have equatorial accumulation maxima similar to those of opal and calcium carbonate at times during the past 50 m.y. Detritus distributed by trade winds and equatorial surface circulation contributes Al, non-biogenic Si, Fe, and Mg to the region. Detrital sediment is most important in areas with a small supply of biogenic debris and low bulk-accumulation rates. Al accumulation generally increases toward the north and east, indicating its continental source and distribution by the northeast trade winds. Maxima in biological productivity during middle Eocene and latest Miocene to early Pliocene time and concomitant well-developed surface circulation contributed toward temporal maxima in the accumulation rates of Cu, Zn, Ni, and Al in sediments of those ages. Authigenic material is also important only where bulk-sediment accumulation rates are low. Ni, Cu, Zn, and sometimes Mn are associated with this sediment. Fe is almost entirely of hydrothermal origin. Mn is primarily hydrothermal, but some is probably scavenged from sea water by amorphous iron hydroxide floes along with other elements concentrated in hydrothermal sediments, Ni, Cu, and Zn. During the past 50 m.y. all of these elements accumulated over the East Pacific Rise at rates nearly an order of magnitude higher than those at non-rise-crest sites. In addition, factor analysis indicates that some of this material is carried substantial distances to the west of the rise crest. Accumulation rates of Fe in basal metalliferous sediments indicate that the hydrothermal activity that supplied amorphous Fe oxides to the East Pacific Rise areas was most intense during middle Eocene and late Miocene to early Pliocene time.

Rare earth and other element contents in surface layer bottom sediments and ferromanganese nodules along the Transatlantic profile

Behavior of rare earth elements (REE) and Th is studied along the Transatlantic transect at 22°N. It is shown that both REE and Th contents relative to Al (the most lithogenic element) increase toward the pelagic region. The increasing trend becomes more complicated due to variations in content of biogenic calcium carbonate that acts as a diluting component in sediments. REE composition varies symmetrically relative to the Mid-Atlantic Ridge (MAR) emphasizing weak hydrothermal influence on sediments of the ridge axis, although the well-known criteria for hydrothermal contribution, such as Al/(Al+Mn+Fe) and (Fe+Mn)/Ti, do not reach critical values. Variations in REE content and composition allowed to distinguish the following five sediment zones in the transect: (I) terrigenous sediments of the Nares abyssal plain; (II) pelagic sediments of the North American Basin; (III) carbonate ooze of the MAR axis; (IV) pelagic sediments of the Canary Basin; and (V) terrigenous clay and calcareous mud of the African continental slope and slope base. Ferromanganese nodules of the hydrogenous type with extremely high Ce (up to 1801 ppm) and Th (up to 138 ppm) contents occur in pelagic sediments. It is ascertained that P, REE, and Th contents depend on Fe content in Atlantic sediments. Therefore, one can suggest that only minor amount of phosphorus is bound with bone debris. Low concentration of bone debris phosphorus is a result of relatively high sedimentation rates in the Atlantic Ocean, as compared with those in pelagic regions of the Pacific Ocean.

Composition and properties of rocks and sediments from the Cape Verde fracture zone

The book presents results of comprehensive geological and geophysical studies, carried out in the Cape Verde fault zone in the 3-rd cruise of R/V Akademik Nikolaj Strakhov (1986). Detailed characterization of bottom relief, thickness and structure of the sedimentary cover, magnetic field, crust structure, lithology and stratigraphy of sediments, petrography and geochemistry of magmatic rocks. Conclusions about tectonic layering of the crust and upper mantle in the fault zone, and about a concurrent structural section of large mantle inhomogeneities have been done. The book is the first monographic description of a major fault structure of the ocean floor.

Bulk mineralogy of Cretaceous to Quaternary sediments from the Kerguelen Plateau

Carbonate-free portions of Upper Cretaceous to Holocene sediment samples from the Kerguelen Plateau in the southern Indian Ocean were investigated by X-ray diffraction. Downhole variations in the content of opal-A, opal-CT, quartz, feldspar, barite, and clinoptilolite were studied at Site 737 on the northern Kerguelen Plateau and at Sites 744 and 738 on the southern Kerguelen Plateau. The variation of these components reflects temporal changes in the depositional history of the Kerguelen Plateau as well as major differences in the sedimentary evolution between the northern plateau and the southern plateau. Carbonate is the dominant component in the pelagic sediments on the Kerguelen Plateau. In addition, biogenic opal sedimentation plays an important role throughout most of the sequence. A major increase in opal accumulation is documented at all sites in late Miocene time, which is in accordance with the well-known increase in silica productivity probably caused by a major cooling step. Because of its position near the Polar Frontal Zone, sediments from Site 737 show a more extensive opal deposition than at Sites 744 and 738. An earlier productivity pulse is documented at Site 744 on the southern plateau within the early Oligocene, following the initial phase of intense East Antarctic glaciation. This cooling event resulted in higher amounts of ice-rafted terrigenous quartz and, to a lesser extent, feldspar. With the exception of the Site 744 sediments, opal deposition in Paleogene and older sediments can be reconstructed only from the diagenetic transformation products of opal-CT and probably clinoptilolite. In contrast to the southern sequence, on the northern Kerguelen Plateau higher amounts of clinoptilolite and no opal-CT were found. These major differences in the diagenetic environments may be due to extensive volcanism in the northern area. The volcanic influence at Site 737 is well recorded by the higher feldspar content and higher amounts of volcanic glass shards.

Geochemistry of terrigenous material in deep-sea sediments of the equatorial Pacific

Biological productivity in the modern equatorial Pacific Ocean, a region with high nutrients and low chlorophyll, is currently limited by the micronutrient Fe. In order to test whether Fe was limiting in the past and to identify potential pathways of Fe delivery that could drive Fe fertilization (i.e., dust delivery from eolian inputs vs. Fe supplied by the Equatorial Undercurrent), we chemically isolated the terrigenous material from sediment along a cross-equatorial transect in the central equatorial Pacific at 140°W and at Ocean Drilling Program Site 850 in the eastern equatorial Pacific. We quantified the contribution from each potential Fe-bearing terrigenous source using a suite of chemical- and isotopic discrimination strategies as well as multivariate statistical techniques. We find that the distribution of the terrigenous sources (i.e., Asian loess, South American ash, Papua New Guinea, and ocean island basalt) varies through time, latitude, and climate. Regardless of which method is used to determine accumulation rate, there also is no relationship between flux of any particular Fe source and climate. Moreover, there is no connection between a particular Fe source or pathway (eolian vs. Undercurrent) to total productivity during the Last Glacial Maximum, Pleistocene glacial episodes, and the Miocene "Biogenic Bloom". This would suggest an alternative process, such as an interoceanic reorganization of nutrient inventories, may be responsible for past changes in total export in the open ocean, rather than simply Fe supply from dust and/or Equatorial Undercurrent processes. Additionally, perhaps a change in Fe source or flux is related to a change in a particular component of the total productivity (e.g., the production of organic matter, calcium carbonate, or biogenic opal).

Composition and distribution of silica in sediments of the Broken Ridge

Within a dipping sequence of middle Cretaceous to Eocene sediments on Broken Ridge, opal-A, opal-CT, and quartz occur as minor constituents in carbonate and ash-rich sediments. Biogenic opal-A is mainly derived from diatoms and radiolarians. Opal-A and almost all siliceous microfossils disappear within a narrow (<20-m-thick) transition zone below which authigenic opal-CT and quartz are present. These latter silica polymorphs occur together within a 750-m-thick interval, but the ratio of quartz/opal-CT increases with increasing age and depth within the pre-rift sediment sequence. The boundary between opal-A- and opal-CT-bearing sediments is also a physical boundary at which density, P-wave velocity, and acoustic impedance change. This physical transition is probably caused by infilling of pore space by opal-CT lepispheres.

Chemical composition of surficial bottom sediments from the Chukchi Sea and adjacent part of the Arctic Ocean

This paper presents data on chemical composition of bottom sediments from the Chukchi Sea and the adjacent Arctic Ocean. Multivariate statistical techniques were used for analysis of the data set and revealed that grain size fractionation of the original terrigenous component during sedimentation was the major factor of clustering of the samples in study. Secondary factors include accumulation of biogenic siliceous and carbonate material and chemogenic or biochemical accumulation of iron, manganese, and some trace elements. The latter factor was significant in areas of tectonic activity within the graben-rift system of the Chukchi Sea.

Geochemistry and mineralogy of sediments from the Atlantis II Fracture Zone

Thirteen sediment samples, including calcareous ooze, sandy clay, volcanic sand, gravel, and volcanic breccia, from Ocean Drilling Program (ODP) Sites 732B, 734B, 734G and Conrad Cruise 27-9, Station 17, were examined. Contents of major and trace elements were determined using XRF or ICP (on samples <0.5 g). Determinations of rare earth elements (REE) were performed using ICP-MS. Mineralogy was determined using XRD. On the basis of the samples studied, the sediments accumulating in the Atlantis II Fracture Zone are characterized by generally high MgO, Cr, and Ni contents compared with other deep-sea sediments. A variety of sources are reflected in the mineralogy and geochemistry of these sediments. Serpentine, brucite, magnetite, and high MgO, Cr, and Ni contents indicate derivation from ultramafic basement. The occurrence of albite, analcime, primary mafic minerals, and smectite/chlorite in some samples, coupled with high SiO2, Al2O3, TiO2, Fe2O3, V, and Y indicate contribution from basaltic basement. A third major sediment source is characterized as biogenic material and is reflected primarily in the presence of carbonate minerals, and high CaO, Sr, Pb, and Zn in certain samples. Kaolinite, illite, quartz, and some chlorite are most likely derived from continental areas or other parts of the ocean by long-distance sediment transport in surface or other ocean currents. Proportions of source materials in the sediments reflect the thickness of the sediment cover, slope of the seafloor, and the nature of and proximity to basement lithologies. REE values are low compared to other deep-sea sediments and indicate no evidence of hydrothermal activity in the Atlantis II Fracture Zone sediments. This is supported by major- and trace-element data.