Integrated chemo- and biostratigraphic calibration of early animal evolution: Neoproterozoic-early Cambrian of southwest Mongolia

Five overlapping sections from the thick Neoproterozoic to early Cambrian sediments of western Mongolia were analysed to yield a remarkable carbon-isotope, strontium-isotope and small shellyfossil (SSF) record. Chemostratigraphy suggests that barren limestones of sequences 3 and 4, which lie above the two Maikhan Uul diamictites, are post-Sturtian but pre-Varangerian in age. Limestones and dolomites of sequence 5, with Boxonia grumulosa, have geochemical signatures consistent with a post-Varangerian (Ediacarian) age. A major negative δ13C anomaly (feature ‘W') in sequence 6 lies a shortdistance above an Anabarites trisulcatus Zone SSF asemblage with hexactinellid sponges, of probable late Ediacarian age. Anomaly ‘W' provides an anchor point for cross-correlation charts of carbon isotopes and small shelly fossils. Trace fossil assemblages with a distinctly Cambrian character first appear in sequence 8(Purella Zone), at the level of carbon isotopic feature ‘B', provisionally correlated with the upper part of cycle Z in Siberia. A paradox is found from sequence 10 to 12 in Mongolia: Tommotian-type SSFs continue to appear, accompanied by Nemakit-Daldynian/Tommotian-type 87Sr/86Sr ratios but by increasingly heavyδ13C values that cannot be matched in the Tommotian of eastern Siberia. The steady rate of generic diversification in Mongolia also contrasts markedly with the Tommotian ‘diversity explosion' in eastern Siberia, which occurs just above a major karstic emergence surface. One explanation is that sequences 10 to 12 in Mongolia preserve a pre-Tommotian portion of the fossil record that was missing or removed in easternSiberia. The Mongolian sections certainly deserve an important place in tracing the true course and timing of the ‘Cambrian radiation

Composition of phosphate containing sediments and phosphatic grains from the Namibian shelf

Recent phosphatic grains from diatomaceous oozes, as well as Pliocene grains from phosphatic sands of the Namibian shelf were studied by a scanning electron microscopy coupled with microprobe analysis. Lithification of the grains occurred together with changes in their composition and structure resulting from progressive phosphatization of biogenic material and partial crystallization of phosphatic matter, which is presented by three predominant forms: colloform, globular, and microcrystalline. Recent grains and associated nodules are authigenic. Their structure and composition related to Pliocene redeposited grains, which might be generated under similar conditions

Composition of bottom sediments and ferruginous bottom manifestations from the Baikal Lake

The investigation of the Baikal Lake bottom carried out during summer 2008 by means of the Mir deep-sea manned submersibles resulted in sampling of a series of sediments, ferruginous crusts, and peculiar mineralized tubes several centimeters high and up to 2-6 cm in diameter. According to scanning electron investigation they consist mainly of enclosing sediment particles and biogenic silica cemented by iron and minor manganese hydroxides. Chemical composition of the tubes is similar to ones of both host sediments and slightly ferruginous crusts and nodules, but the tubes and crusts are somewhat richer relative to sediments in some microelements, namely, arsenic, cadmium, and uranium. In general, structure and composition of these tubes remind one of worm tubes common in sediments of some seas. Investigation rare earth elements in some samples or ferruginous manifestations and bottom sediments revealed a positive europium anomaly, which might be related to either composition of surrounding continental magmatic rocks or to influence of hypothetical hydrothermal solutions

(Table 1) Chemical analyses of rock samples from low atolls of the Western Indian Ocean

Compositions, structures, and microstructures of different types of phosphorites and poorly phosphatized rocks from low atolls in the near-equatorial part of the Western Indian Ocean are described. The rocks were examined under optical and scanning microscopes using microprobe techniques and etching of selected samples with weak solvents as well as with the help of chemical analyses. It is proved that phosphorites have been formed owing to the uneven phosphatization of primary carbonate rocks; degree of their phosphatization ranges from traces to 40% P2O5. In the phosphorites numerous organic remains were encountered; they included fragments of plankton, debris of tortoise shells, and coccoidal and filamentous bacteria-like formations. It is suggested that the phosphorites formed due to high local biological productivity over the outer edges of coral reefs and are not related to guano accumulation or to endoupwelling

Chemical composition of rocks from the Kammu Seamount and from low atolls of the Western Indian Ocean

Devoted to studies of phosphatized rocks from the Kammu Seamount