Occurrence and type of dolomite in ODP Site 124-768 samples (Table 1)

Occurrence of deep-sea dolomites has been reported from numerous settings (for discussion see Lumsden, 1988). Different authors agree that dolomite formation in the pelagic realm is a relatively early diagenetic process (e.g., Jorgensen, 1983; Shimmield and Price, 1984; Kablanow et al., 1984; Kulm et al., 1984). Baker and Burns (1985) suggest that most of the pelagic dolomites formed within a few tens of meters below the seafloor within the zone of microbial sulfate reduction. According to Fuechtbauer and Richter (1988), dolomite can form in the deep-sea at a minimum temperature of 10°C. Other deep-sea dolomites are products of fluids derived from underlying evaporites or submarine weathering of basalts (Garrison, 1981). In some cases (Mullins et al., 1985; Dix and Mullins, 1988; Mullins et al., 1988), the existence of dolomite is linked to disconformities and its formation may have resulted from circulation of seawater through the sediment during prolonged exposure (Dix and Mullins, 1988, p. 287). At Site 768 (Fig. 1), lithified carbonate layers, some containing variable amounts of dolomite, occur below 201 mbsf (Miocene). These beds alternate with unconsolidated or semi-lithified marl layers interbedded in clays and siliciclastic turbidites. The irregular depth distribution of the limestone beds and the variation in preservation and recrystallization of the calcareous microfaunas suggest that lithification of carbonates at Site 768 not only reflects burial diagenesis as described by Garrison (1981) and others, but in part may be a selective, early diagenetic process. The different types and distribution of the dolomite additionally seem to support this assumption. The purpose of this report is to document the occurrence and textural nature of the dolomite at Site 768. Methods used were analyses of stained thin sections (Alizarin S and Ferrocyanide) and studies with the scanning electron microscope. No geochemical analyses (e.g., stable isotopes) were carried out; they will be the subject of further investigations.

Chemical composition of Fe-Mn manifestations from the Laptev Sea

In sediments of the Laptev Sea unknown earlier ferromanganese manifestations have been found. On the basis of structural-textural external signs they have been divided to five groups: 1) tube- and spindle-shaped pseudomorphs after and within invertebrates; 2) nuclear and non-nuclear nodules; 3) flagellum- and tube-like skeletons of polychaetes; 4) flat and flattened crustate nodules and crusts; 5) micronodules. All types of ferromanganese manifestations have been sorted in three main genetic series: eigenferrous formations of autochthonous (polychaetes, goethite micronodules) and allochthonous (nuclear nodules) nature; ferromanganese nodules formed under mild hydro-geodynamic conditions at the sediment-seawater geochemical barrier; and ferromanganese manifestations formed under conditions of the variable physico-chemical environment. Ferromanganese manifestations of allochthonous type have signs of littoral zones. They contain both ferrous and ferric iron and have low oxidation degree of manganese in comparison with the autochthonous type manifestations. Manganese minerals with moderate oxidation degree are represented by vernadite and buserite. Such features of iron and manganese indicate different conditions of their formation and occurrence. The main distinctive feature of ferromanganese mineralisation in the Laptev Sea is the redox barrier: the oxidized water layer enriched in oxygen and reduced sediments. This barrier provides favorable conditions for bacterial formation of ferromanganese ores. Understanding of the genesis of ferromanganese manifestations should be found in a study of organic matter reworking by bacteria.

Geochemical and rock magnetic investigation of sediment cores of the western South Atlantic

Geochemical and rock magnetic investigations of sediments from three sites on the continental margin off Argentina and Uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane (AOM). The western Argentine Basin represents a suitable sedimentary environment to study nonsteady-state processes because it is characterized by highly dynamic depositional conditions. Mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals. As a typical feature of these deposits, distinct minima in magnetic susceptibility (k) are observed. Pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate/methane transition (SMT) where HS- is generated by the process of AOM. The released HS- reacts with the abundant iron (oxyhydr)oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility. Modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate (SR) which occurred during the Pleistocene/Holocene transition. We assume that the strong decrease in mean SR encountered during this glacial/interglacial transition induced a fixation of the SMT at a specific depth. The stagnation has obviously enhanced diagenetic dissolution of iron (oxyhydr)oxides within a distinct sediment interval. This assumption was further substantiated by numerical modeling in which the mean SR was decreased from 100 cm/kyr during glacial times to 5 cm/kyr in the Holocene and the methane flux from below was fixed to a constant value. To obtain the observed geochemical and magnetic patterns, the SMT must remain at a fixed position for ~9000 yrs. This calculated value closely correlates to the timing of the Pleistocene/Holocene transition. The results of the model show additionally that a constant high mean SR would cause a concave-up profile of pore water sulfate under steady state conditions.

Major and rare earth element contents and their speciations in ferruginous nodules and host sediments from the Barents Sea

Contents of Fe, Mn, Al, P, and rare earth elements (REE) in ferruginous nodules and host sediments of the eastern Barents Sea were studied. A direct Fe-P correlation in reactive components of the sediments and nodules was found. The nodules were shown to be formed through Fe(II) oxidation in the surface layer of sediments and cementation of terrigenous fraction of sediments by Fe(III) oxyhydroxides. The latter accumulate phosphorus due to processes of sorption - co-precipitation, by forming Fe(III) hydrophosphates. REE composition in the sediments and nodules normalized to NASC contents is characterized by increased proportion of light REE that may be caused by regional features of their sources. Due to significant share of terrigenous matter in the Fe nodules (up to 65% for Nd), REE composition of bulk samples is similar to that of host sediments. A negative cerium anomaly in composition of reactive REE may result from REE sorption from seawater. REE bulk composition of a ferruginous crust is closer to that of seawater than one of the ferruginous nodules from the sediments because of essentially lower content of diluent terrigenous matter.

Iron and carbon geochemistry of sediment core GeoB1401-4

Iron speciation was determined in hemiplegic sediments from a high productivity area to investigate systematically the early diagenetic reactivity of Fe. A combination of various leaching agents (1 M HCI, dithionite buffered in citrate/acetic acid, HF/H2SO4, acetic Cr(II)) was applied to sediment and extracted more than 80% of total Fe. Subsequent Fe species determination defined specific mineral fractions that are available for Fe reduction and fractions formed as products of Fe diagenesis. To determine the Fe speciation of (sheet) silicates we explored an extraction procedure (HF/H2SO4) and verified the procedure by application to standard rocks. Variations of Fe speciation of (sheet) silicates reflect the possible formation of Fe-bearing silicates in near surface sediments. The same fraction indicates a change in the primary input at greater depth, which is supported by other parameters. The Fe(II)/ Fe(III) -ratio of total sediment determined by extractions was compared with Mössbauer-spectroscopy ] at room temperature and showed agreement within 10%. M6ssbauer-spectroscopy indicates the occurrence of siderite in the presence of free sulfide and pyrite, supporting the importance of microenvironments during mineral formation. The occurrence of other Fe(II) bearing minerals such as ankerite (Ca-, Fe-, Mg-carbonate) can be presumed but remains speculative.

Chemical composition of basalts from DSDP Hole 46-396B

Samples of crystalline basalt from Site 396 B are all more or less altered, usually in strongly zoned patterns. Evidence has been found for several related or independent alteration stages, including (1) minor localized deuteric (amphibole and mixed clay minerals in miarolitic voids); (2) minor widespread nonoxidizing (pyrite on walls of vugs and cracks); (3) localized diffusion-controlled rug filling ("glauconite" in black halos); (4) pervasive low level oxidizing (transformation of titanomagnetite to cation-deficient titanomaghemite); (5) localized diffusion-controlled strongly oxidizing (breakdown of olivine and titanomaghemite in brown zones). Plagioclase and pyroxene are essentially unaltered. Detailed analyses of gray and brown zones in pillow basalts show that low temperature oxidation has proceeded in a step-wise fashion, with the relative stabilities of the igneous minerals controlling the steps. Secondary minerals that crystallized from pore fluids on to the walls of vugs may or may not be related to local alteration of primary phases. During the most intense stage of alteration, brown oxidation zones grew into basalt fragments behind diffusion controlled fronts. The specific reactions and products of this stage differ among the lithologic units at the site. A model is proposed whereby efficient seawater circulation through the pillow units maintains the pH and the concentrations of Mg2+ and SiO2 dissolved at low levels in pore fluids, so that olivine is replaced by hydrous ferric oxides, and Mg and SiO2 are removed from the system. In the massive basalt unit, circulation is somewhat less effective and Mg and SiO2 are retained in smectites. Deposition of authigenic minerals in the sequence saponite/Fe-Mn oxides/phillipsite/calcite in vugs and cracks may reflect the gradual closing of the systems and probably signals the end of localized oxidation in parts of the core. Mineral compositions indicate that most of these deposits formed from seawater at very low temperature.

Paleomagnetic of ODP Leg 133 holes

Paleomagnetic and rock-magnetic analyses from discrete samples of carbonate sites on the Queensland Plateau were used to determine magnetic polarity reversal stratigraphy and the nature of magnetization in these sediments. Magnetic polarity zones were correlated with the geomagnetic polarity time scale in the upper portions of cores at Sites 812 through 814, usually back to a late Pliocene age. Loss of reliable directional data was coincidental with a major decrease in magnetic intensity, below which, no stable polarity zones could be identified. The intensity reduction is either an in-situ alteration of magnetic grains, or an input signal representing progressive increase in the magnetic component of Queensland Plateau sediments. Although not conclusive at this point, the geochemical conditions and differing age of intensity reduction support the former hypothesis. Rock-magnetic analysis of carbonate sediments suggests that ultrafine-grained magnetite or maghemite crystals is an important carrier of remanence and may be biogenic in origin. Application of a recently calibrated anhysteretic remanent magnetization test to assess configuration of single-domain crystal within a natural matrix indicates that cementation (ooze-chalk-limestone) may be important in post-depositional changes affecting magnetostatic grain interaction.

Contents, partition, and isotopic composition of sulphur in sediments from ODP Sites 680 and 686

We have determined (1) the abundance and isotopic composition of pyrite, monosulphide, elemental sulphur, organically bound sulphur, and dissolved sulphide; (2) the partition of ferric and ferrous iron; (3) the organic carbon contents of sediments recovered at two sites drilled on the Peru Margin during Leg 112 of the Ocean Drilling Program. Sediments at both sites are characterised by high levels of organically bound sulphur (OBS). OBS comprises up to 50% of total sedimentary sulphur and up to 1% of bulk sediment. The weight ratio of S to C in organic matter varies from 0.03 to 0.15 (mean = 0.10). Such ratios are like those measured in lithologically similar, but more deeply buried petroleum source rocks of the Monterey and Sisquoc formations in California. The sulphur content of organic matter is not limited by the availability of porewater sulphide. Isotopic data suggest that sulphur is incorporated into organic matter within a metre of the sediment surface, at least partly by reaction with polysulphides. Most inorganic Sulphur occurs as pyrite. Pyrite formation occurred within surface sediments and was limited by the availability of reactive iron. But despite highly reducing sulphidic conditions, only 35-65% of the total iron was converted to sulphide; 10-30% of the total iron still occurs as Fe(III). In surface sediments, the isotopic composition of pyrite is similar to that of both iron monosulphide and dissolved sulphide. Either pyrite, like monosulphide, formed by direct reaction between dissolved sulphide and detrital iron, and/or the sulphur species responsible for converting FeS to FeS2 is isotopically similar to dissolved sulphide. Likely stoichiometries for the reaction between ferric iron and excess sulphide imply a maximum resulting FeS2:FeS ratio of 1:1. Where pyrite dominates the pool of iron sulphides, at least some pyrite must have formed by reaction between monosulphide and elemental sulphur and/or polysulphide. Elemental sulphur (S°) is most abundant in surface sediments and probably formed by oxidation of sulphide diffusing across the sediment-water interface. In surface sediments, S° is isotopically heavier than dissolved sulphide, FeS and FeS2 and is unlikely to have been involved in the conversion of FeS to FeS2. Polysulphides are thus implicated as the link between FeS and FeS2.