No abstract

Seep carbonates, the product of cold seep fluids mediated by microbial activity, archive information on seabed fluid flow and microbiological processes that led to their formation. Sedimentary fabrics are well developed in the authigenic carbonate from active seep sites at Bush Hill, Green Canyon Block 185 of the Gulf of Mexico. This authigenic carbonate has much lower carbon isotope values than typical marine cements and the carbonate is concluded to be derived from the oxidation of seeping petroleum. The authigenic fabrics commonly exhibit microbial or texturally unique sedimentary structures; e.g. framboidal pyrite, clotted microfabric and peloid, botryoidal aragonite, microfilament, and rosette-like aragonite are all preserved in seep carbonate. These fabrics are indicators of biological influence during microbial oxidation of petroleum and sulfate reduction when cold fluids seep on to the seabed. The textures are therefore considered as additional evidence that can be used to identify and interpret ancient cold-fluid seeps on seafloor. The difficulty of the preservation of these biogenic fabrics is thought to be related to the redox condition and dynamic signature (e.g. rate of fluid flow) of cold seeps.

Six marine sediments from the seafloor of the Gulf of Mexico were studied in terms of the abundance, composition and distribution of organic matter as well as biomarker chemistry. The results show that there are great spatial variations in terms of organic matter abundance and hydrocarbon composition among these samples. The S-7 and S-9 samples show the characteristics of modern organic matter that has been modified by biodegradation, while the S-8, S-10 and S-11 samples are clearly contaminated with petroleum hydrocarbons from oil seepage beneath the seafloor. The lack of n-alkanes in samples S-8 and S-11 is indicative of severe biodegradation. Only sample S-1 contains methanogenic archaea-sourced lipid biomarkers, in particular 2,6,10,15,19-pentamethylicosenes containing 1-5 unsaturated double bonds. These compounds are distributed between n-C 22 and n-C 24 in the aliphatic fraction and have very depleted 13 C values ranging from )86.7& to )115.5&, compared to the d 13 C values of adjacent n-alkanes ranging from )28.4& to )34.6&. The occurrence and carbon isotopic compositions of these compounds suggest that the S-1 sample site is likely to be associated with a gas venting system or a gas hydrate setting.

No abstract

We performed a Raman scattering study of Na2Ti2As2O. We identified a symmetry breaking structural transition at around Ts = 150 K, which matches a large bump in the electrical resistivity. Several new peaks are detected below that transition. Combined with first-principles calculations, our polarization-dependent measurements suggest a charge instability driven lattice distortion along one of the Ti-O bonds that breaks the 4-fold symmetry and more than doubles the unit cell.PACS numbers: 74.25. Kc, 74.25.nd, 71.45.Lr Because their layered structure resembles both the copper oxide and the iron-based superconductors, special attention is devoted to the ATi 2 P n 2 O (A = Na 2 , Ba, P n = As, Sb, Bi) system [1], in particular regarding anomalies in their transport properties that may be caused by orders competing with superconductivity. Although the maximum T c reported among these compounds is only 6.1 K in Ba 1−x K x Ti 2 Sb 2 O (x = 0.12), their phase diagram points towards a competition between densitywave states and superconductivity [2,3]. However, the origin of the density-wave remains controversial. While some calculations attribute the anomalies of their transport properties to a spin-density-wave (SDW) transition [4][5][6], most experiments rather favor a charge-densitywave (CDW) scenario [7,8]. Moreover, nuclear quadruple resonance (NQR) experiments show that the in-plane 4-fold symmetry at the Sb site is broken below the transition temperature in BaTi 2 Sb 2 O [9]. Recently, neutron diffraction experiments suggest an "intra-unit-cell nematic charge order" in BaTi 2 As 2 O [10], which makes the titanium oxypnictides more intriguing.In this Rapid Communication we focus on Na 2 Ti 2 As 2 O, which has the highest CDW transition temperature among this series [11]. Previous optical spectroscopy measurements indicate two CDW transitions at 42 K and 320 K in this material [12], while there is only little known about the lattice dynamics. Our Raman scattering study of Na 2 Ti 2 As 2 O is supported by first-principles calculations of the vibration modes. We observe a total of 4 single-phonon modes predicted by our first-principles calculations in the high-temperature phase and 1 additional feature assigned to a doublephonon excitation. By doing temperature-dependent Raman measurements, we reveal a structural transition breaking the 4-fold symmetry at around T s = 150 K, which is consistent with a yet not understood anomaly in the electrical resistivity. Combining the results of * p.richard@iphy.ac.cn † dingh@iphy.ac.cn our experiments with calculations, we suggest that this structural transition does not rotate the high-symmetry axes but more than doubles the primitive cell.The Na 2 Ti 2 As 2 O single crystals used in our measurements were grown by the flux method [13]. Freshly grown plate-like samples with typical size of 1 × 2 × 0.06 mm 3 were prepared and electrodes were glued on them in a glove box for avoiding air contamination. The in-plane resistivity was measured using a commercial physical properties ...