Petrology and geochemistry of sedimentary Unit III of ODP Hole 135-840B

The sedimentary sequence recovered at Site 840, on the Tonga frontal-arc platform, is 597.3 m thick and is subdivided into three lithostratigraphic units. The lowermost, late Miocene Unit III is 336.8 m thick and consists of a sequence of volcaniclastic mass-flow deposits (predominantly turbidites) interbedded with pelagic/hemipelagic deposits. Unit III was deposited in the forearc basin of the Lau volcanic arc, probably on a slope dominated by mass flows that built eastward from the ridge front and across the forearc. Upward through the unit a thinning and fining of individual turbidites takes place, interpreted to reflect a reduced sediment supply and a change from large to smaller flows. Decreasing volcanic activity with time is inferred from a decrease in coarse-grained volcaniclastic content in the upper part of the unit. The majority of the turbidites show the typical Bouma-type divisions, although both high- and low-density turbidity currents are inferred. High-density turbidity currents were especially common in the lower part of the unit. Geochemical analyses of detrital glass lie mainly in the low-K tholeiite field with a compositional range from basalt to rhyolite. A coherent igneous trend indicates derivation from a single volcanic source. This source was probably situated on the rifted part of the Lau-Tonga Ridge, within the present Lau backarc basin. The initial opening of the Lau Basin may have been around 6.0 m.y. ago. The onset of more extensive rifting, approximately 5.6 m.y. ago, is reflected in an increase in the silica content of volcanic glass. At the boundary toward Unit II, at approximately 5.25 Ma, an influx of thicker bedded and coarser grained volcaniclastic material is interpreted to reflect increasing volcanism and tectonism during the final breakup of the Lau-Tonga Ridge.

Tab. 2: Grain-sizes of cobble on the surface of sanderwurzel

yResults of 13 field investigations between 1966 and 1990 of the southwestern to eastern margin of Kötlujökull and its proglacial area are summarized with respect to sandar and their formation. Generally, the results are based on sedimentological examinations in the field and laboratory, on analyses of aerial photographs, and investigations of the glacier slope. The methods permitted a more detailed reconstruction of sandar evolution in the proglacial area of Kötlujökull since 1945, of tendencies in development and of single data going back until the last decades of the 19th century. Accordingly, there existed special periods of "flachsander"-formations with raised coarsegrained "sanderwurzels" resultant from the outbreak of subglacial meltwater tunneloutlets and other periods with "hochsander-"formations by supraglacial drainage. At present the belts of hochsanders in front of the glacier come up to more than 4 m in thickness and 1000 m in width, therefore containing perhaps more sediment direct in front of Kötlujökull than the old belts of flachsanderwurzels. In one case the explosion-like subglacial meltwater outburst combined with the genesis of a sanderwurzel could be observed for a time and is thoroughly discussed. The event is referred to the outburst of a sub- to inglacial meltwater body being under extreme hydrostatic press ures which is combined with the genesis of a new subglacial tunneloutlet as a new flachsander. Often these outbursts led to the destruction of a morainic belt more than 1000 m in width. Presumably the whole event was finished in not more than a few days. In addition to a characteristic pear-shaped form and water-moved stones up to diameters of 1 m the wurzels possess a single "main-channel" with rectangular cross-sections as far as 4 m deep and 50 m wide just as small flat channels resembling fish bones in connection with the main channel. Presumably, they have been active only in the last stage of wurzel formation. With regard to the subglacial tunnel gates long-living L-meltwater outlets are distinguished from short-living K-meltwater outlets. These are always combined with a raised coarse-grained sanderwurzel, but its meltwater discharge is generally decreasing and ceases after some years, whereas the discharge of L-meltwater outlets continues unchanged for long times (except seasonal differences). The material of flachsanders is preponderantly composed of mugearitic and andesitic cobble extending at least for some kilometres from the glacier margin, whereas the hochsanders correspond to medium to coarse sands without clay and without alternations into the direction of flow. The hochsander fans are covered with small braidet channels. Their sedimentary structures are determined by the short time changing of supraglacial meltwater discharge and the upper flow regime combined with the development of antidunes, which rule the channel-flows during the main activity periods in summer. Unlike the subglacial drainage the supraglacial drainage led to only weak effects of erosion on the glacier foreland. So the hochsanders refilled depressions of morainic areas or grew up on older flachsanderwurzels. Whereas all large flachsanders developed in front of approximate stationary glacier margins, the evolution of coherent belts of hochsanders were combined with progressive glacier fronts. On the other hand, there was obviously no evolution at all of large sandar in front of back-melting margins of Kötlujökull. Based on examinations of the glacier surface and on analyses of aerial photographs the different types of sandar are referred to different structures of the glacier snout. Finally chances of surviving of sandar in the proglacial area of Kötlujökull are shortly discussed just as the possibility of an application of the Islandic research results on Pleistocene sandar in northern Germany.

Radioisotope stratigraphy, sedimentology and geochemistry of Late Quaternary sediments from the Eastern Arctic Ocean

To reconstruct Recent and past sedimentary environments, marine sediments of Upper Pleistocene and Holocene ages from the eastern Arctic Ocean and especially from the Nansen-Gakkel Ridge (NGR) were investigated by means of radioisotopic, geochemical and sedimentological methods. In combination with mass physical property data and lithological analysis these investigations allow clearly to characterize the depositional environments. Age dating by using the radioisotope 230Th gives evidence that the investigated sediments from the NGR are younger than 250,000 years. Identical lithological sediment sequences within and between sediment cores from the NGR can be related to sedimentary processes which are clearly controlled by palaeoclimate. The sediments consist predominantly of siliciclastic, terrigenous ice-rafted detritus (IRD) deriving from assorted and redeposited sediments from the Siberian shelfs. By their geochemical composition the sediments are similar to mudstone, graywacke and arcose. Sea-ice as well as icebergs play a major roll in marine arctic sedimentation. In the NGR area rapid change in sedimentary conditions can be detected 128,000 years ago. This was due to drastic change in the kind of ice cover, resulting from rapid climatic change within only hundreds of years. So icebergs, deriving mostly from Siberian shelfs, vanished and sea-ice became dominant in the eastern Arctic Ocean. At least three short-period retreats of the shelf ice between 186,000 and 128,000 years are responsible for the change of coarse to fine-grained sediments in the NGR area. These warmer stages lasted between 1,000 and 3,000 years. By monitoring and comparing the distribution patterns of sedimentologic, mass physical and geochemical properties with 230Th ex activity distribution patterns in the sediment cores from the NGR, there is clear evidence that sediment dilution is responsible for high 230Th ex activity variations. Thus sedimentation rate is the controlling factor of 230Th ex activity variations. The 230Th flux density in sediments from the NGR seems to be highly dependent On topographic Position. The distribution patterns of chemical elements in sediment cores are in general governed by lithology. The derivation of a method for dry bulk density determination gave the opportunity to establish a high resolution stratigraphy on sediment cores from the eastern Arctic Ocean, based on 230Thex activity analyses. For the first time sedimentation and accumulation rates were determined for recent sediments in the eastern Arctic Ocean by 230Th ex analyses. Bulk accumulation rates are highly variable in space and time, ranging between 0.2 and 30 g/cm**2/ka. In the sediments from the NGR highly variable accumulation rates are related to the kind of ice cover. There is evidence for hydrothermal input into the sediments of the NGR. Hydrothermal activity probably also influences surficial sediments in the Sofia Basin. High contents of As are typical for surficial sediments from the NGR. In particular SL 370-20 from the bottom of the rift valley has As contents exceeding in parts 300 ppm. Hydrothermal activity can be traced back to at least 130,000 years. Recent to subrecent tectonic activity is documented by the rock debris in KAL 370 from the NGR. In four other sediment cores from the NGR rift valley area tectonically induced movements can be dated to about 130,000 years ago, related most probably to the rapid climate change. Processes of early diagenesis in sediments from the NGR caused the aobilization and redeposition of Fe, Mn and Mo. These diagenetic processes probably took place during the last 130,000 years. In sediment cores from the NGR high amounts of kaolinite are related to coarse grained siliciclastic material, probably indicating reworking and redeposition of siberian sandstones with kaolinitic binding material. In contrast to kaolinite, illite is correlated to total clay and 232Th contents. Aragonite, associated with serpentinites in the rift valley area of the NGR, was precipitated under cold bottom-water conditions. Preliminary data result in a time of formation about 60 - 80 ka ago. Manganese precipitates with high Ni contents, which can be related to the ultrabasic rocks, are of similar age.

Geochemistry of recent sediments from the Indian Ocean

During the International Indian Ocean Expedition (1964/65) sediment cores were taken on six profiles off the western coast of the Indian Subcontinent. These profiles run approximately perpendicular to the coast, from the deep-sea over the continental slope to the continental shelf. Additional samples and cores were taken in a dense pattern in front of the delta of the Indus River. This pattern of sampling covered not only marine sediments, but also river and beach sediments in Pakistan. The marine samples were obtained with piston, gravity and box corers and by a Van Veen grab sampler. The longest piston core is about 5 meters long. 1. Distribution of the elements on the sediment surface The area of maximal carbonate values (aprox. 80-100% CaCO3) essentially coincides with the continental shelf. The highest Sr values were observed largely within this area, but only in the vicinity of the Gulf of Cambay. Mainly the aragonitic coprolites are responsible for those high Sr contents. The Mg contents of the carbonates are comparatively low; surprisingly enough the highest Mg concentrations were also measured in the coprolites. The maximum contents of organic matter (Core) were found along the upper part of the continental slope. They coincide with the highest porosity and water content of the sediments. Frequently the decomposition of organic matter by oxydation is responsible for the measured Corg contents. On the other side the quantity of originally deposited organic material is less important in most cases. The enrichment of the "bauxitophile" elements Fe, Ti, Cr and V in the carbonate- and quartz-free portions of the sediments is essentially due to the influence of coarse terrigenous detritus. For the elements Mn, Ni and Cu (in per cent of the carbonateand quartz-free sediment) a strong enrichment was observed in the deep-sea realm. The strong increase in Mn toward the deep-sea is explained by authigenesis of Mn-Fe-concretions. Mn-nodules form only under oxydizing conditions which obviously are possible only at very low rates of deposition. The Mg, B and, probably also Mn contents in the clay minerals increase with increasing distance from the continent. This can be explained by the higher adsorption of those elements from sea water because of increasing duration of the clay mineral transport. The comparison of median contents of some elements in our deep-sea samples with deep-sea sediments described by TUREKIAN & WEDEPOHL (1961) shows that clear differences in concentration exist only in the case of "bauxitophile" elements Cr and Be. The Cr and Be contents show a clear increase in the Indian Ocean deep-sea samples compared to those described by TUREKIAn & WEDEPOHL (1961) which can obviously be attributed to the enrichment in the lateritic and bauxitic parent rocks. The different behaviour of the elements Fe, Ti and Mn during decomposition of the source rocks, transport to the sea and during oxydizing and reducing conditions in the marine environment can be illustrated by Ti02/Fe and MnO/Fe ratios. The different compositions of the sediments off the Indus Delta and those of the remaining part of the area investigated are characterized by a different distribution of the elements Mn and Ti. 2. Chemical inhomogenities in the sediments Most longer cores show 3 intervals defined by chemical and sedimentological differences. The top-most interval is coarse-grained, the intermedial interval is fine grained and the lower one again somewhat coarser. At the same time it is possible to observe differences from interval to interval in the organogenic and detrital constituents. During the formation of the middle interval different conditions of sedimentation from those active during the previous and subsequent periods have obviously prevailed. Looking more closely at the organogenic constituents it is remarkable that during the formation of the finer interval conditions of a more intensive oxydation have prevailed that was the case before and after: Core decreases, whereas P shows a relative increase. This may be explained by slower sedimentation rate or by a vertical migration of the oxygen rich zone of the sea-water. The modifications of the elements from minerals in detrital portion of the sediments support an explanation ascribing this fact to modifications of the conditions of denudation and transportation which can come about through a climatic change or through tectonic causes. The paleontological investigations have shown (ZOBEL, in press) that in some of the cores the middle stratum of fine sedimentation represents optimal conditions for organic life. This fact suggests also oxydizing conditions during the sedimentation of this interval. In addition to the depositional stratification an oxydation zone characterized by Mn-enrichment can be recognized. The thickness of the oxidation zone decreases towards the coast and thins out along the middle part of the continental slope. At those places, where the oxydation zone is extremely thin, enrichment of Mn has its maximum. This phenomenon can probably be attributed to the migration of Mn taking place in its dissociated form within the sediment under reducing conditions. On the other side this Mn-migration in the sediment does not take place in the deep-sea, where oxydizing conditions prevail. 3. Interstitial waters in the sediments Already at very small core depths, the interstitial waters have undergone a distinct modification compared with the overlying sea water. This distinct modification applies both to total salinity and to the individual ions. As to the beginning of diagenesis the following conclusions can be drawn: a) A strong K-increase occurs already at an early stage. It may be attributable to a diffusion barrier or to an exchange of Mg-ions on the clays. Part of this increase may also originate from the decomposition of K-containing silicates (mica and feldspars). A K-decrease owing to the formation of illite (WEAVER 1967), however, occurs only at much greater sediment depth. b) Because of an organic protective coating, the dissolution of carbonate is delayed in recent organogenic carbonates. At the same time some Ca is probably being adsorbed on clay minerals. Consequently the Ca-content of the interstitial water drops below the Ca-content of the sea water. c) Already at an early stage the Mg adsorption on the clays is completed. The adsorbed Mg is later available for diagenetic mineral formations and transformations.

Composition of aerosols collected in the Western Russian Arctic in August-September 1991

In August-September 1991 during the SPASIBA expedition (Scientific Program on the Arctic and Siberian Aquatorium) aboard R/V Yakov Smirnitzky in the Laptev Sea ten samples of aerosols were collected by nylon nets. A combined approach including various analytical techniques, such as single-particle analysis, instrumental neutron activation analysis, and atomic absorption spectrophotometry, was used to study composition of the samples. Mass concentration of coarse-grained (>0.001 mm) insoluble fraction of aerosols ranged from 80 to 460 ng/m**3. In all the samples remains of land vegetation were found to be the dominant component. Organic carbon content of the aerosols ranged from 23 to 49%. Inorganic part of the samples was represented mainly by alumosilicates and quartz. Anthropogenic ''fly ash'' particles were observed in all the samples. Temporal variations of element concentrations resulted from differences in air masses entering the studied area.

Preliminary results and documentation of sediment cores CRP2 and CRP-2A from the Ross Sea off Cape Roberts, Antarctica

The site for CRP-2, 14 km east of Cape Roberts (77.006°S; 163.719°E), was selected to overlap the early Miocene strata cored in nearby CRP-1, and to sample deeper into the east-dipping strata near the western margin ofe he Victoria Land Basin to investigate Palaeogene climatic and tectonic history. CRP-2 was cored from 5 to 57 mbsf (metres below the sea floor) (core recovery 91 %), with a deviation resulting in CRP-2A being cored at the same site. CRP-2A reached down to 624mbsf (recovery 95%), and to strata with an age of c. 33-35 Ma. Drilling took place from 16 October to 25 November 1998, on 2.0-2.2 m of sea ice and through 178 m of water. Core fractures and other physical properties, such as sonic velocity, density and magnetic susceptibility, were measured throughout the core. Down-hole logs for these and other properties were run from 63 to 167 mbsf and subsequently from 200 to 623 mbsf, although density and velocity data could be obtained only to 440 mbsf because of hole collapse. Sonic velocity averages c. 2.0 km S-1 for the upper part of the hole, but there is an sharp increase to c. 3.0 km s-1 and also a slight angular unconformity, at 306 mbsf, corresponding most likely to the early/late Oligocene boundary (c. 28-30 Ma). Velocity then increases irregularly to around 3.6 km s-1 at the bottom of the hole, which is estimated to lie 120 m above the V4/V5 boundary. The higher velocities below 306 mbsf probably reflect more extensive carbonate and common pyrite cementation, in patches, nodules, bedding-parallel masses and as vein infills. Dip of the strata also increases down-hole from 3° in the upper 300 in to over 10° at the bottom. Temperature gradient is 21° k-1. Over 2 000 fractures were logged through the hole. Borehole televiewer imagery was obtained for the interval from 200 to 440 mbsf to orient the fractures for stress field analysis. Lithostratigraphical descriptions on a scale of 1:20 are presented for the full length of the core, along with core box images, as a 200 page supplement to this issue. The hole initially passed through a layer of muddy gravel to 5.5 mbsf (Lithological Sub-Unit or LSU 1.1), and then into a Quaternary diatom-bearing clast-rich diamicton to 21 mbsf (LSU 2. l), with an interval of alternating compact diamicton and loose sand, and containing a rich Pliocene foraminiferal fauna, to 27 mbsf (LSU 2.2). The unit beneath this (LSU 3.1) has similar physical properties (sonic velocity, porosity, magnetic susceptibility) and includes diamictites of similar character to those of LSU 2.1 and 2.2, but an early Miocene (c. 19 Ma) diatom assemblage at 28 mbsf (top of LSU 3.1) shows that this sub-unit is part of the older section. The strata beneath 27 mbsf, primary target for the project, extend from early Miocene to perhaps latest Eocene age, and are largely cyclic glacimarine nearshore to offshore sediments. They are described as 41 lithological sub-units and interpreted in terms of 12 recurrent lithofacies. These are 1) mudstone, 2) inter-stratified mudstone and sandstone, 3) muddy very fine to coarse sandstone, 4) well-sorted stratified fine sandstone, 5) moderately to well-sorted, medium-grained sandstone, 6) stratified diamictite, 7) massive diamictite, 8) rhythmically inter-stratified sandstone and mudstone, 9) clast-supported conglomerate, 10) matrix-supported conglomerate, 11) mudstone breccia and 12) volcaniclastic sediment. Sequence stratigraphical analysis has identified 22 unconformity-bounded depositional sequences in pre- Pliocene strata. They typically comprise a four-part architecture involving, in ascending order, 1) a sharp-based coarse-grained unit (Facies 6,7,9 or 10), 2) a fining-upward succession of sandstones (Facies 3 and 4), 3) a mudstone interval (Facies l), in some cases coarsening upward to muddy sandstones (Facies 3), and 4) a sharp-based sandstone dominated succession (mainly Facies 4). The cyclicity recorded by the strata is interpreted in terms of a glacier ice margin retreating and advancing from land to the west, and of rises and falls in sea level. Analysis of sequence periodicity awaits afirmer chronology. However, apreliminary spectral analysis of magnetic susceptibility for a deepwater mudstone within one of the sequences (from 339 to 347 mbsf) reveals ratios between hierarchical levels that are similar to those of the three Milankovitch orbital forcing periodicities. The strata contain a wide range of fossils, the most abundant being marine diatoms. These commonly form up to 5% of the sediment, though in places the core is barren (notably between 300 and 412 mbsf). Fifty samples out of 250 reviewed were studied in detail. The assemblages define ten biostratigraphical zones, some of them based on local or as yet undescribed forms. The assemblages are neritic, and largely planktonic, suggesting that the sea floor was mostly below the photic zone throughout deposition of the corcd sequence. Calcareous nannofossils, representing incursions of ocean surface waters, are much less common (72 out of 183 samples examined) and restricted to mudstone intervals a few tens of metres thick, but are important for dating. Foraminifera are also sparse (73 out of 135 samples) and represented only by calcareous benthic species. Changing assemblages indicate a shift from inshore environments in the early Oligocenc to outer shelf in the late Oligocenc, returning to inshore in the early Miocene. Marine palynomorplis yielded large numbers of well-preserved forms from most of the 116 samples examined. The new in situ assemblagc found last year in CRP-1 is extended down into the late Oligocene and a further new assemblage is found in the early Oligoccnc. Many taxa are new, and cannot us yet contribute to an improved understanding of chronology or ecology. Marine invertebrate macrofossils, mostly molluscs and serpulid tubes, are scattered throughout the core. Preservation is good in mudstones but poor in other lithologies. Climate on land is reflected in the content of terrestrial palynomorphs, which are extremely scarce down to c. 300 mbsf. Some forms are reworked, and others represent a low growing sparse tundra with at least one species of Nothofagus. Beneath this level, a significantly greater diversity and abundance suggests a milder climate and a low diversity woody vegetation in the early Oligocene, but still far short of the richness found in known Eocene strata of the region. Sedimentary facies in the oldest strata also suggest a milder climate in the oldest strata cored, with indications of substantial glacial melt-water discharges, but are typical of a coldcr climate in late Oligocene and early Miocene times. Clast analyses from diamictites reveal weak to random fabrics, suggesting either lack of ice-contact deposition or post-depositional modification, but periods when ice grounded at the drill site are inferred from thin zones of in-situ brecciated rock and soft-sediment folding. These are more common above c. 300 mbsf, perhaps reflecting more extensive glacial advances during deposition of those strata. Erosion of the adjacent Transantarctic Mountains through Jurassic basalt and dolerite-intruded Beacon strata into basement rocks beneath is recorded by petrographical studies of clast and sand grain assemblages. Core below 310 mbsf contains a dominance of fine-grained Jurassic dolerite and basalt fragments along with Beacon-derived coal debris and rounded quartz grains, whereas the strata above this level have a much higher proportion of basement derived granitoids, implying that the large areas of the adjacent mountains had been eroded to basement by the end of the early Oligocene. There is little indication of rift-related volcanism below 310 mbsf. Above this, however, basaltic and trachytic tephras are common, especially from 280 to 200 mbsf, from 150 to 46 mbsf, and in Pliocene LSU 2.2 from 21 to 27 mbsf. The largest volcanic eruptions generated layers of coarse (up to 1 cm) trachytic pumice lapilli between 97 and 114 mbsf. The thickest of these (1.2 m at 112 mbsf) may have produced an eruptive column extending tens of km into the stratosphere. A source within a few tens of km of the drill site is considered most likely. Present age estimates for the pre-Pliocene sequence are based mainly on biostratigraphy (using mainly marine diatoms and to a lesser extent calcareous nannofossils), with the age of the tephra from 112 to 114 mbsf (21.44k0.05 Ma from 84 crystals by Ar-Ar) as a key reference point. Although there are varied and well-preserved microfossil assemblages through most of the sequence (notably of diatoms and marine palynomorphs), they comprise largely taxa either known only locally or as yet undescribed. In addition, sequence stratigraphical analysis and features in the core itself indicate numerous disconformities. The present estimate from diatom assemblages is that the interval from 27 to 130 mbsf is early Miocene in age (c. 19 to 23.5 Ma), consistent with the Ar-Ar age from 112 to 114 mbsf. Diatom assemblages also indicate that the late Oligocene epoch extends from c. 130 to 307 mbsf, which is supported by late Oligocene nannofossils from 130 to 185 mbsf. Strata from 307 to 412 mbsf have no age-diagnostic assemblages, but below this early Oligocene diatoms and nannofossils have been recovered. A nannoflora at the bottom of the hole is consistent with an earliest Oligocene or latest Eocene age. Magnetostratigraphical studies based on about 1000 samples, 700 of which have so far undergone demagnetisation treatment, have provided a polarity stratigraphy of 12 pre-Pliocene magnetozones. Samples above 270 mbsf are of consistently high quality. Below this, magnetic behaviour is more variable. A preliminary age-depth plot using the Magnetic Polarity Time Scale (MPTS) and constrained by biostratigraphical data suggests that episodes of relatively rapid sedimentation took place at CRP-2 during Oligocene times (c. 100 m/My), but that more than half of the record was lost in a few major and many minor disconformities. Age estimates from Sr isotopes in shell debris and further tephra dating are expected to lead to a better comparison with the MPTS. CRP-2/2A has recorded a history of subsidence of the Victoria Land Basin margin that is similar to that found in CIROS-170 km to the south, reflecting stability in both basin and the adjacent mountains in late Cenozoic times, but with slow net accumulation in the middle Cenozoic. The climatic indicators from both drill holes show a similar correspondence, indicating polar conditions for the Quaternary but with sub-polar conditions in the early Miocene-late Oligocene and indications of warmer conditions still in the early Oligocene. Correlation between the CRP-2A core and seismic records shows that seismic units V3 and V4, both widespread in the Victoria Land Basin, represent a period of fluctuating ice margins and glacimarine sedimentation. The next drill hole, CRP-3, is expected to core deep into V5 and extend this record of climate and tectonics still further back in time.

(Table 1) Anisotropy of magnetic susceptibility parameters of ODP Hole 134-833B

Ocean Drilling Program (ODP) Sites 832 and 833 were drilled in the intra-arc North Aoba Basin of the New Hebrides Island Arc (Vanuatu). High volcanic influxes in the intra-arc basin sediment resulting from erosion of volcanic rocks from nearby islands and from volcanic activity are associated with characteristic magnetic signals. The high magnetic susceptibility in the sediment (varying on average from 0.005 to more than 0.03 SI) is one of the most characteristic physical properties of this sedimentary depositional environment because of the high concentration of magnetites in redeposited ash flows and in coarse-grained turbidites. Susceptibility data correlate well with the high resolution electrical resistivity logs recorded by the formation microscanner (FMS) tool. Unlike the standard geophysical logs, which have low vertical resolution and therefore smooth the record of the sedimentary process, the FMS and whole-core susceptibility data provide a clearer picture of turbiditic sediment deposition. Measurements of Curie temperatures and low-temperature susceptibility behavior indicate that the principal magnetic minerals in ash beds, silt, and volcanic sandstone are Ti-poor titanomagnetite, whereas Ti-rich titanomagnetites are found in the intrusive sills at the bottom of Site 833. Apart from an increase in the concentration of magnetite in the sandstone layer, acquisition of isothermal and anhysteretic remanences does not show significant differences between sandstone and clayey silts. The determination of the anisotropy of magnetic susceptibility (AMS) in more than 400 samples show that clayey siltstone have a magnetic anisotropy up to 15%, whereas the AMS is much reduced in sandstone layers. The magnetic susceptibility fabric is dominated by the foliation plane, which is coplanar to the bedding plane. Reorientations of the samples using characteristic remanent magnetizations indicate that the bedding planes dip about 10° toward the east, in agreement with results from FMS images. Basaltic sills drilled at Site 833 have high magnetic susceptibilities (0.05 to 0.1 SI) and strong remanent magnetizations. Magnetic field anomalies up to 50 µT were measured in the sills by the general purpose inclinometer tool (GPIT). The direction of the in-situ magnetic anomaly vectors, calculated from the GPIT, is oriented toward the southeast with shallow inclinations which suggests that the sill intruded during a reversed polarity period.

Magnetostratigraphic age-depth tie points of ODP Leg 208 sites

We report the paleomagnetic and rock magnetic results from discrete sample analysis of sediments from Walvis Ridge, Leg 208 of the Ocean Drilling Program. In an effort to refine the shipboard magnetostratigraphy, alternating field and thermal demagnetization of discrete samples were carried out, predominantly on samples from Sites 1262 and 1267. Results are generally consistent with the shipboard pass-through cryomagnetometer data, though in some cases the discrete samples resolved ambiguities in the reversal record. Significantly, the C24r/C24n reversal boundary was identified at Sites 1262 and 1267, and most boundaries in the Paleocene and Upper Cretaceous sections are now identified to within 10-30 cm. Magnetic mineralogy results show that prior to the late Miocene, the predominant detrital magnetic component was coarse-grained magnetite and that after the late Miocene, titanomagnetite has also been present. This suggests a possible change in detrital source at that time.

C1-C5 hydrocarbons from core gas pockets at DSDP Leg 56 Holes

C1-C5 hydrocarbons from DSDP Legs 56 and 57 sediment gas pockets were analyzed on board ship. Results suggest that the C2-C5 hydrocarbons accompanied biogenic methane and were generated at low temperatures - less than 50° C - either by microorganisms or by low-temperature chemical reactions. Neopentane, a rare constituent of petroleum, is the major C5 component (about 80%) in much of the sediment at Site 438. This compound, which appeared in smaller amounts at Sites 434, 439, 440, and 441, seems to correlate with either fractured or coarse-grained sediments. Scatter in C4 and C5 isomer ratios and generally good correlation between C3, C4 and C5 components suggest local sources for these molecules.

Median grain size and distribution of sand in ODP Hole 108-660A (Fig. 5)

50 m of Middle Eocene pure radiolarian ooze were drilled at ODP Site 660 in the equatorial East Atlantic, 80 km northeast of the Kane Gap. The oozes comprise a 10 m high and 2 km broad mound of seismic reverberations, covered by manganese-rich sediment, and contain trace amounts of sponge spicules and diatoms, negligible organic carbon (0.15%), clay, and variable amounts of pyrite. The yellow to pale brown silty sediments are relatively coarse-grained (30-45% coarser than 6 µm), little bioturbated, and commonly massive or laminated on a cm-scale. The unlithified radiolarian ooze may indicate an interval of high oceanic productivity, probably linked to a palaeoposition of Site 660 close to the equatorial upwelling belt during Middle Eocene time. The absence of organic matter, however, and both the laminated bedding and the mound-like structure of the deposit on the lower slope of a continental rise indicate deposition by relatively intense contour currents of oxygen-rich deep water, which passed through the Kane Gap, winnowed the fine clay fraction, and prevented the preservation of organic carbon. The ooze may be either a contourite-lag deposit, or a contourite accumulation of displaced radiolarians, originating south of the Kane Gap and being deposited in its northern lee, thus documenting the passage of a strong cross-equatorial bottom-water current formed near Antarctica. These Eocene contourites may be an analogue for ancient radiolarites in the Tethyan Ocean.

Gas hydrates at DSDP Leg 84

On DSDP Leg 84, gas hydrates were found at three sites (565, 568, and 570) and were inferred, on the basis of inorganic and organic geochemical evidence, to be present at two sites (566 and 569); no evidence for gas hydrates was observed at Site 567. Recovered gas hydrates appeared as solid pieces of white, icelike material occupying fractures in mudstone or as coarse-grained sediment in which the pore space exhibited rapid outgassing. Also a 1.05-m-long core of massive gas hydrate was obtained at Site 570. Downhole logging indicated that this hydrate was actually 3 to 4 m thick. Measurements of the amount of methane released during the decomposition of these recovered samples clearly showed that gas hydrates had been found. The distribution of evolved hydrocarbon gases indicated that Structure I gas hydrates were present because of the apparent inclusion of methane and ethane and exclusion of propane and higher molecular weight gases. The water composing the gas hydrates was fresh, having chlorinities ranging from 0.5 to 3.2 per mil. At Sites 565, 568, and 570, where gas hydrates were observed, the chlorinity of pore water squeezed from the sediment decreased with sediment depth. The chlorinity profiles may indicate that gas hydrates can often occur finely dispersed in sediments but that these gas hydrates are not recovered because they do not survive the drilling and recovery process. Methane in the gas hydrates found on Leg 84 was mainly derived in situ by biogenic processes, whereas the accompanying small amounts of ethane likely resulted from low-temperature diagenetic processes. Finding gas hydrates on Leg 84 expands observations made earlier on Leg 66 and particularly Leg 67. The results of all of these legs show that gas hydrates are common in landward slope sediments of the Middle American Trench from Mexico to Costa Rica.

Mineral composition and major element oxides of gabbros from ODP Hole 179-1105A, southwest Indian Ridge

Hole 1105A penetrated 158 m of gabbros at a site offset 1.3 km east-northeast from Hole 735B on the Atlantis Bank near the Atlantis II Fracture Zone. A total of 118 m of dominantly medium- to coarse-grained intercalated Fe-Ti oxide gabbro and olivine gabbro was recovered from Hole 1105A that shows many petrographic features similar to those recovered from the upper part of Hole 735B. The main rock types are distinguished based on the constituent cumulus phases, with the most primitive gabbros consisting of olivine, plagioclase, and clinopyroxene. The inferred crystallization order is subsequently Fe-Ti oxides (ilmenite and titanomagnetite), followed by orthopyroxene, then apatite, and finally biotite. Orthopyroxene appears to replace olivine in a narrow middle interval. The magmatic evolution is likewise reflected in the mineral compositions. Plagioclase varies from An66 to An28. Olivine varies from Fo78 to Fo35. The gap in olivine crystallization occurs between Fo46 and Fo40 and coincides approximately with the appearance of orthopyroxene (~En50). The clinopyroxenes show large compositional variation in Mg/(Mg + Fe total) from 0.84 to 0.51. The nonquadrilateral cations of clinopyroxene similarly show large variations with Ti increasing for the olivine gabbros and decreasing for the Fe-Ti oxide gabbros with the decrease in Mg/(Mg + Fe total). The apatites are mainly flourapatites. The compositional variation in the gabbros is interpreted as a comagmatic suite resulting from fractional crystallization. Pyroxene geothermometry suggests equilibration temperatures from 1100°C and below. The coexisting Fe-Ti oxide minerals indicate subsolidus equilibration temperatures from 900°C for olivine gabbros to 700°C for the most evolved apatite-bearing gabbros. The cryptic variation in the olivine gabbros defines two or three lenses, 40 to 60 m thick, each characterized by a distinct convex zoning with a lower segment indicating upward reverse fractionation, a central maximum, and an upper segment showing normal fractionation. The Fe-Ti oxide gabbros show cryptic variations independent of the host olivine gabbros and reveal a systematic upward normal fractionation trend transgressing host olivine gabbro boundaries. Forward fractional crystallization modeling, using a likely parental magma composition from the Atlantis II Fracture Zone (MgO = 7.2 wt%; Mg/[Mg + Fe2+] = 0.62), closely matches the compositions of coexisting olivine, plagioclase, and clinopyroxene. This modeling suggests cosaturation of olivine, plagioclase, and clinopyroxene from 1155°C and the addition of Fe-Ti oxides from 1100°C. The liquid line of descent initially shows increasing FeO with moderately increasing SiO2. After saturation of Fe-Ti oxides, the liquid strongly decreases in FeO and TiO2 and increases in SiO2, reaching dacitic compositions at ~10% liquid remaining. The calculations indicate that formation of olivine gabbros can be accounted for by <65% fractionation and that only the residual 35% liquid was saturated in Fe-Ti oxides. The modeling of the solid fractionation products shows that both the olivine gabbro and the Fe-Ti oxide gabbros contain very small amounts of trapped liquid (<5%). The implications are that the gabbros represent crystal mush that originated in a recharging and tapping subaxial chamber. Compaction and upward melt migration in the crystal mush appear to have been terminated with relatively large amounts of interstitial liquid remaining in the upper parts of the cumulate mush. This termination may have been caused by tectonic disturbances, uplift, and associated withdrawal of magma into the subaxial dike and sill system. Prolonged compaction and cooling of the trapped melt in the mush formed small differentiated bodies and lenses by pressure release migration and crystallization along syntectonic channels. This resulted in differentiation products along lateral and vertical channelways in the host gabbro that vary from olivine gabbro, to Fe-Ti oxide gabbro, gabbronorite, and apatite gabbros and show large compositional variations independent of the host olivine gabbros.

Surface sediment analyses, bivalve and gastropod taxa of marine sediments from the Banc d'Arguin, Mauritania

High-nutrient tropical carbonate systems are known to produce sediments that, in terms of skeletal composition, are reminiscent of their extra-tropical counterparts. Such carbonate systems and associated carbonate grain assemblages in the tropics are rare in the present-day world. Nonetheless, it is crucial to gain a better understanding of those ecosystems, including their drivers and players because such settings potentially represent models for ancient depositional systems as well as for predicted future environmental conditions. One of the modern occurrences of eutrophic tropical carbonate systems is the northern Mauritanian Shelf. The marine environment is characterized by an eastern boundary upwelling system that pushes cool and nutrient-rich intermediate waters onto a wide epicontinental platform (Golfe d'Arguin) where the waters warm up to tropical temperatures. The resulting facies is mixed carbonate-siliciclastic with a dominant foramol association grading into bimol and barnamol grain assemblages in the shallowest areas forming the Banc d'Arguin. Besides this cool water-related heterozoan association, the carbonate sediment is characterized by tropical molluskan species, while chlorozoan biota (e.g., corals and algal symbiont-bearing foraminifers) are entirely absent. We here present a first comprehensive facies analysis of this model example of eutrophic tropical carbonates. Furthermore, we reconstruct the loci of carbonate production and provide a conclusive depositional model of the Banc d'Arguin that received little attention to date due to its poorly accessible nature.

Geochemistry of volcanivlastic sediments of ODP Sites 134-832 and 134-833

Volcaniclastic sediments of North Aoba Basin (Vanuatu) recovered during Ocean Drilling Program (ODP) Leg 134 show a mineralogical and chemical overprint of low grade hydrothermal alteration superimposed on the primary magmatic source compositions. The purpose of this study was to identify authigenic mineral phases incorporated in the volcaniclastic sediments, to distinguish authigenic chemical and mineralogical signals from the original volcaniclastic mineralogical and chemical compositions, and to determine the mechanism of authigenic minerals formation. Mineralogical, micro-chemical and bulk chemical analyses were utilized to identify and characterize authigenic phases and determine the original unaltered ash compositions. 117 volcaniclastic sediment samples from North Aoba Basin Sites 832 and 833 were analyzed. Primary volcaniclastic materials accumulated in North Aoba Basin can be divided into three types. The older basin-filling sequences show three different magmatic trends: high K, calc-alkaline, and low K series. The most recent accumulations are rhyodacitic composition and can be attributed to Santa Maria or Aoba volcanic emissions. Original depositional porosity of volcaniclastic sediments is an important factor in influencing distribution of authigenic phases. Finer-grained units are less altered and retain a bulk mineralogical and chemical composition close to the original pyroclastic rock composition. Coarser grained units (microbreccia and sandstones) are the major hosts of authigenic minerals. At both sites, authigenic minerals (including zeolites, clay minerals, Mg-carbonates, and quartz) exhibit complex zonation with depth that crosses original ash depositional boundaries and stratigraphic limits. The zeolite minerals phillipsite and analcime are ubiquitous throughout the altered intervals. At Site 832, the first zeolite minerals (phillipsite) occur in Pleistocene deposits as shallow as 146 meters below seafloor (mbsf). At Site 833 the first zeolite minerals (analcime) occur in Pleistocene deposits as shallow as 224 mbsf. The assemblage phillipsite + analcime + chabazite appears at 635 mbsf (Site 832) and at 376 mbsf (Site 833). Phillipsite + analcime + chabazite + thomsonite + heulandite are observed between 443 and 732 mbsf at Site 833. Thomsonite is no longer observed below 732 mbsf at Site 833. Heulandite is present to the base of the sections cored. The zeolite assemblages are associated with authigenic clay minerals (nontronite and saponite), calcite, and quartz. Chlorite is noticeable at Site 832 as deep as 851 mbsf. Zeolite zones are present but are less well defined at Site 832. Dolomite and rare magnesite are present below 940 m at Site 832. The coarse-grained authigenic mineral host intervals exhibit geochemical signatures that can be attributed to low grade hydrothermal alteration. The altered intervals show evidence of K2O, CaO, and rare earth elements mobilization. When compared to fine-grained, unaltered units, and to Santa Maria Island volcanics rocks, the altered zones are relatively depleted in rare earth elements, with light rare earth elements-heavy rare earth elements fractionation. Drilling at Site 833 penetrated a sill complex below 840 m. No sill was encountered at Site 832. Complex zonation of zeolite facies, authigenic smectites, carbonates and quartz, and associated geochemical signatures are present at both sites. The mineralogical and chemical alteration overprint is most pronounced in the deeper sections at Site 832. Based on mineralogical and chemical evidence at two locations less than 50 km apart, there is vertical and lateral variation in alteration of the volcaniclastic sediments of North Aoba Basin. The alteration observed may be activated by sill intrusion and associated expulsion of heated fluids into intervals of greater porosity. Such spatial variation in alteration could be attributed to the evolution of the basin axis associated with subduction processes along the New Hebrides Trench.

Messinian and pre-Messinian sediments of ODP Holes 107-652A and 107-654A

Sedimentology, mineralogy, and petrology of the pre-Pliocene sediments drilled at ODP Sites 652 and 654 in the Tyrrhenian Sea (Leg 107) have been studied with emphasis on the lower Messinian to pre-Messinian intervals. Messinian at Site 652 is essentially turbiditic and basinal in character; it was deposited during the syn-rift phase in a strongly subsiding half-graben and is correlatable with emerged coeval sequences; in part with the Laga Formation of the foredeep of Apennines, and in part with the filling of grabens dissecting that chain in the Tyrrhenian portion of Tuscany. The sequence found in Site 654 indicates an upper Tortonian to Messinian transgression accompanying crustal stretching in the western Tyrrhenian Sea and is perfectly correlatable with the so-called "Sahelian cycle" and with "postorogenic" cycles recognized in peninsular Italy and in Sicily.