Unit S2 Research Articles

Morphology of ash aggregates from wet pyroclastic surges of the 1982 eruption of El Chichón Volcano, Mexico

The detailed stratigraphic study of the pyroclastic surge units S1, IU, and S3 produced during the most violent phases of the 1982 eruption of El Chichon volcano, contains a complex succession of hydromagmatic events triggered by the interaction of different proportions of magma and external water. Component analyses of the horizons within single units reveal that almost all wet and cohesive horizons contain ash aggregates. Based on their morphology and internal structure four different types of aggregates were distinguished: (a) accretionary lapilli, (b) armored lapilli, (c) irregular aggregates, and (d) cylindrical aggregates. The first three types have been described in the volcanological literature (field and experimental studies); cylindrical forms are reported here for the first time. These hollow cylindrical aggregates consist of concentric layers of crystals and glass fragments set in a finer-grained matrix. They formed around millimeter-size foliage fragments that are locally preserved in the interior of the aggregates as scorched or completely carbonized vestiges. SEM analyses suggest different mechanisms of formation for the four types of aggregates. Irregular aggregates and armored lapilli formed nearly instantaneously, whereas accretionary lapilli and cylindrical aggregates resulted from progressive aggregation of ash in different regions of the eruptive cloud.

Lithostratigraphy, conodont biostratigraphy and depositional environment of the Middle Devonian (Givetian) to Early Carboniferous (Tournaisian) Lipak Formation in the Pin Valley of Spiti (NW India)

Bed-by-bed lithostratigraphic sections combined with sequence stratigraphy and conodont biostratigraphy provide new information on the depositional environment and age of the Lipak Formation in the Pin Valley (Spiti). The formation comprises mixed siliciclastic and calcareous sediments at lower levels, richly fossiliferous limestones with two distinct sandstone incursions at higher levels, and dark mudstones followed by a thin siltstone interval. The upper limit of the Lipak Formation is defined by the angular unconformity below the sandstones of the Permian Gechang Formation. Lithologic correlation with sections in upper Lahaul indicates that, in the Pin Valley, the formation has been truncated just below its characteristic gypsum horizon. The lower boundary of the Lipak Formation is gradational from coastal arenites of the Muth Formation; the mappable boundary is drawn at the first appearance of dark carbonaceous, argillaceous siltstone and shale. Sedimentary structures, microfacies and conodont faunas indicate a general shallow marine depositional environment of the Lipak Formation in the Pin Valley; five sequence stratigraphic units have been distinguished. Conodont data demonstrate that the lowest 33 m of the Lipak Formation of the Pin Valley is mid to late Early varcus Subzone with characteristic species of Icriodus and Bipennatus . A previously unrecognised hiatus at c. 33 m above the base, at the boundary of sequence stratigraphic units S1 and S2, represents the interval Middle varcus Subzone to at least the end of the late Famennian Early expansa Zone. Because this hiatus does not correspond to a mappable boundary, no division of the Lipak Formation into named stratigraphic units is suggested, but we refer informally to the sediments represented by cycle S1 as Lipak A, and the sediments represented by cycles S2-S5 as Lipak B. Determination of S1 as Early varcus Subzone provides a maximum age for the gradationally underlying Muth Formation. At 75 m above the base of the composite Lipak Formation section, a 58 cm black to dark grey shale interval within late Famennian fossiliferous limestones conceivably correlates with the Hangenberg Event (end-Middle praesulcata Zone). Younger conodont faunas of the Lipak Formation -dominated by species of Clydagnathus with species of Bispathodus and Pseudopolygnathus also represented- is shown to extend to the mid-Tournaisian Early crenulata Zone.

Geochemistry of the Pliocene red clay formation in the Chinese Loess Plateau and implications for its origin, source provenance and paleoclimate change

Recent field observations and sedimentological studies suggest the eolian origin of the late Tertiary red clay formation underlying the Pleistocene loess-soil sequence in the Loess Plateau, thereby extending the wind-blown record from ∼2.6 Ma down to ∼7.0 Ma in north-central China. To address the source provenance of the red clay, major and trace element concentrations were analyzed on the samples from a north-south loess transect of the Loess Plateau and a red clay section at Jiaxian. Results show that there is a good agreement of both major and trace element compositions between the last glacial-interglacial loess-soil units and the Jiaxian red clay record. The REE distribution patterns of the loess-soil and red clay samples are remarkably similar in shape, with enriched LREE and fairly flat HREE profiles and clear negative Eu anomaly. The geochemical features of the red clay are also identical to those of upper continental crust. These chemical characteristics provide further evidence for eolian sedimentation of the red clay. From the relations both between Na 2O/Al 2O 3 and K 2O/Al 2O 3 ratios and between U/Pb and Th/Pb ratios, the eolian loess and red clay materials must have been subject to thorough sedimentary differentiation and moderate chemical weathering in the source area. The chemical similarity between soil unit S1 and the red clay implies that such an environment as occurred in the last interglacial period may have already formed in northwestern China during the late Tertiary. Comparison of down-section variations in elemental ratios with field observations of pedogenic development of the red clay suggests that CaO/Al 2O 3, Sr/Rb and Sr/Ba are more sensitive to climate changes than magnetic susceptibility, and can be used to reconstruct the climate history of the late Cenozoic.

Climate instability during the penultimate glaciation: Evidence from two high‐resolution loess records, China

The Xinzhuangyuan and Lijiayuan loess sections in the northwestern part of the Chinese Loess Plateau were studied down to paleosol unit S2, which probably formed during marine oxygen isotopic stage 7, ∼200,000 years B.P. The thickness of the loess‐soil sequence above S2 is ∼63 m at Xinzhuangyuan and 43 m at Lijiayuan. A 2 cm sample spacing yields a mean depositional resolution of 50–80 years for the last and penultimate glacial loess units (L1 and L2) and ∼200 years for the last interglacial soil unit (Sl), thus enabling us to reconstruct high‐resolution climatic changes during the last two glacial‐interglacial cycles. The grain size records of the two sections are regarded as a proxy for changes in the monsoon‐desert system over northern China. Results show that frequent, large‐amplitude climatic oscillations on millennial timescales occurred during the penultimate glaciation in a manner similar to that during the last glaciation, suggesting that suborbital‐scale climatic variations may be a common feature of the climate system during glacial periods of the Pleistocene. The last interglacial soil (S1) is composed of three individual soils and two thin intervening loess horizons in both of the sections. Short‐term variations in grain size are not prominent within the soil complex, implying the absence of strong millennial‐scale climatic oscillations during the last interglaciation in northern China.

Foraminiferal Constraints on Very High-Resolution Seismic Stratigraphy and Late Quaternary Glacial History, New Jersey Continental Shelf

Internal stratigraphy of a Late Quaternary sediment wedge on the New Jersey outer continental shelf has been investigated using regional and 3-D seismic surveys, sediment cores and foraminiferal biofacies analysis. Results from the study help constrain Late Wisconsinan glacialldeglacial history in this area. Three major reflectors (channels, S and R) define 5 stratigraphic units: Si -modern sediments; S2-sands overlying a channelized surface (Channels reflector); S3-muds, clays and sandy muds below the channelized surface and above reflector S; S4-an unsampled unit between reflectors S and R; and S5-sands underlying reflector R, the latter surface defining the base of the sediment wedge. Multivariate quantitative analysis delineates four major faunal groups characterizing these stratigraphic units. Faunal patterns are interpreted in terms of modern foraminiferal distributions, planktoniclbenthic ratios and faunal abundance patterns. Group A dominates units S2 and S5 and is characterized by Cibicides lobatulus and Cassidulina islandica. The fauna indicates middle neritic water depths, moderate to high current energy, and moderate accumulation rates. Group B dominates unit S3 (muddy unit) and is characterized by Elphidium excavatum s.l., Buccella frigida, Fursenkoina fusiformis and miliolid spp. This fauna represents an environment with no modern analogue on the New Jersey continental shelf, reflecting middle neritic, low-energy conditions and the accumulation of predominantly muddy sediments. Group C dominates the modern sediment veneer (unit Si), and its diverse fauna is consistent with modern faunal distributions and environmental conditions. Group D consists of a low-diversity fauna dominated by Fursenkoina fusiformis and Nonionella sp., which characterizes a few samples in unit S3. Environmental conditions are probably similar to those described for faunal group B. The distribution and interpretation of these faunas, combined with preliminary C-14 AMS dates, indicate that the Quaternary history of the outer-shelf sedi-

Degradation of 4-aminobenzenesulfonate by a two-species bacterial coculture. Physiological interactions between Hydrogenophaga palleronii S1 and Agrobacterium radiobacter S2.

The mutualistic interactions in a 4-aminobenzenesulfonate (sulfanilate) degrading mixed bacterial culture were studied. This coculture consisted of Hydrogenophaga palleronii strain S1 and Agrobacterium radiobacter strain S2. In this coculture only strain S1 desaminated sulfanilate to catechol-4-sulfonate, which did not accumulate in the medium but served as growth substrate for strain S2. During growth in batch culture with sulfanilate as sole source of carbon, energy, nitrogen and sulfur, the relative cell numbers (colony forming units) of both strains were almost constant. None of the strains reached a cell number which was more than threefold higher than the cell number of the second strain. A mineral medium with sulfanilate was inoculated with different relative cell numbers of both strains (relative number of colony forming units S1:S2 2200:1 to 1:500). In all cases, growth was found and the proportion of both strains moved towards an about equal value of about 3:1 (strain S1:strain S2). In contrast to the coculture, strain S1 did not grow in a mineral medium in axenic culture with 4-aminobenzenesulfonate or any other simple organic compound tested. A sterile culture supernatant from strain S2 enabled strain S1 to grow with 4-aminobenzenesulfonate. The same growth promoting effect was found after the addition of a combination of 4-aminobenzoate, biotin and vitamin B12. Strain S1 grew with 4-aminobenzenesulfonate plus the three vitamins with about the same growth rate as the mixed culture in a mineral medium. When (resting) cells of strain S1 were incubated in a pure mineral medium with sulfanilate, up to 30% of the oxidized sulfanilate accumulated as catechol-4-sulfonate in the culture medium. In contrast, only minor amounts of catechol-4-sulfonate accumulated when strain S1 was grown with 4ABS in the presence of the vitamins.

Ditertiary phosphine rhodium and iridium complexes containing the molecular unit S2 or Se2 as a ligand

Cyclo-octasulphur and cyclo-octaselenium react with [Ir(dppe)2]Cl and [Rh(dmpe)2]Cl (dppe = Ph2P·CH2·CH2·PPh2 and dmpe = Me2P·CH2·CH2·PMe2) to form the complexes [Ir(S2)(dppe)2]Cl (1), [Ir(Se2)(dppe)2]Cl (2), and [Rh(S2)(dmpe)2]Cl (3).

Biscyclopentadienylniobium complexes containing the molecular unit S2 as a ligand

Biscyclopentadienylniobium complexes containing the molecular unit S2 as a ligand