Massive barite deposits and carbonate mineralization in the Derugin Basin, Sea of Okhotsk: precipitation processes at cold seep sites

An area of massive barite precipitations was studied at a tectonic horst in 1500 m water depth in the Derugin Basin, Sea of Okhotsk. Seafloor observations and dredge samples showed irregular, block- to column-shaped barite build-ups up to 10 m high which were scattered over the seafloor along an observation track 3.5 km long. High methane concentrations in the water column show that methane expulsion and probably carbonate precipitation is a recently active process. Small fields of chemoautotrophic clams (Calyptogena sp., Acharax sp.) at the seafloor provide additional evidence for active fluid venting. The white to yellow barites show a very porous and often layered internal fabric, and are typically covered by dark-brown Mn-rich sediment; electron microprobe spectroscopy measurements of barite sub-samples show a Ba substitution of up to 10.5 mol% of Sr. Rare idiomorphic pyrite crystals (∼1%) in the barite fabric imply the presence of H₂S. This was confirmed by clusters of living chemoautotrophic tube worms (1 mm in diameter) found in pores and channels within the barite. Microscopic examination showed that micritic aragonite and Mg-calcite aggregates or crusts are common authigenic precipitations within the barite fabric. Equivalent micritic carbonates and barite carbonate cemented worm tubes were recovered from sediment cores taken in the vicinity of the barite build-up area. Negative δ¹³C values of these carbonates (>−43.5‰ PDB) indicate methane as major carbon source; δ¹⁸O values between 4.04 and 5.88‰ PDB correspond to formation temperatures, which are certainly below 5°C. One core also contained shells of Calyptogena sp. at different core depths with ¹⁴C-ages ranging from 20?680 to >49?080 yr. Pore water analyses revealed that fluids also contain high amounts of Ba; they also show decreasing SO₄^2- concentrations and a parallel increase of H₂S with depth. Additionally, S and O isotope data of barite sulfate (δ³⁴S: 21.0-38.6‰ CDT; δ¹⁸O: 9.0-17.6‰ SMOW) strongly point to biological sulfate reduction processes. The isotope ranges of both S and O can be exclusively explained as the result of a mixture of residual sulfate after a biological sulfate reduction and isotopic fractionation with ‘normal’ seawater sulfate. While massive barite deposits are commonly assumed to be of hydrothermal origin, the assemblage of cheomautotrophic clams, methane-derived carbonates, and non-thermally equilibrated barite sulfate strongly implies that these barites have formed at ambient bottom water temperatures and form the features of a Giant Cold Seep setting that has been active for at least 49?000 yr.     

Evidence of non extensivity in the evolution of seismicity along the San Andreas Fault,California, USA: An approach based on Tsallis statistical physics

We examine the nature of the seismogenetic system along the San Andreas Fault (SAF), California, USA, by searching for evidence of complexity and non-extensivity in the earthquake record. We use accurate, complete and homogeneous earthquake catalogues in which aftershocks are included (raw catalogues), or have been removed by a stochastic declustering procedure (declustered catalogues). On the basis of Non-Extensive Statistical Physics (NESP), which generalizes the Boltzmann–Gibbs formalism to non-equilibrating (complex) systems, we investigate whether earthquakes are generated by an extensive self-excited Poisson process or by a non-extensive complex process. We examine bivariate cumulative frequency distributions of earthquake magnitudes and interevent times and determine the size and time dependence of the respective magnitude and temporal entropic indices, which indicate the level on non-equilibrium (correlation). It is shown that the magnitude entropic index is very stable and corresponds to proxy b-values that are remarkably consistent with the b-values computed by conventional means. The temporal entropic index computed from the raw catalogues indicate moderately to highly correlated states during the aftershock sequences of large earthquakes, progressing to quasi-uncorrelated states as these die out and before the next large event. Conversely, the analysis of the declustered catalogues shows that background seismicity exhibits moderate to high correlation that varies significantly albeit smoothly with time. This indicates a persistent sub-extensive seismogenetic system. The degree of correlation is generally higher in the southern SAF segment, which is consistent with the observation of shorter return periods for large earthquakes. A plausible explanation is that because aftershock sequences are localized in space and time, their efficient removal unveils long-range background interactions which are obscured by their presence! Our results indicate complexity in the expression of background seismicity along the San Andreas Fault, with criticality being a very likely mechanism as a consequence of the persistent non-equilibrium inferred from the temporal entropic index. However, definite conclusions cannot be drawn until the earthquake record is exhaustively studied in all its forms.     

Respiration and denitrification in permeable continental shelf deposits on the South Atlantic Bight: Rates of carbon and nitrogen cycling from sediment column experiments

Nitrogen (N) cycling and respiration rates were measured in sediment columns packed with southeastern United States continental shelf sands, with high permeability (4.66×10⁻¹¹ m²) and low organic carbon (0.05%) and nitrogen (0.008%). To simulate porewater advection, natural shelf seawater was pumped through columns of different lengths to achieve fluid residence times of approximately 3, 6, and 12 h. Experiments were conducted seasonally at in situ temperature. Fluid flow was uniform in nearly all columns, with minimal dead zones and channeling. Significant respiration (O₂ consumption and ∑CO₂ production) occurred in all columns, with highest respiration rates in summer. Most (78–100%) remineralized N was released as N₂ in the majority of cases, including columns with oxic porewater throughout, with only a small fraction released as NO₃⁻ from some oxic columns. A rate of 0.84–4.83×10¹⁰ mol N yr⁻¹, equivalent to 1.06–6.09×10⁻⁶ mmol N cm⁻² h⁻¹, was calculated for benthic N₂ production in the South Atlantic Bight, which can account for a large fraction of new N inputs to this shelf region. Metal and sulfate reduction occurred in long residence time columns with anoxic outflow in summer and fall, when respiration rates were highest. Because permeable sediments dominate continental shelves, N₂ production in high permeability coastal sediments may play an important role in the global N cycle.     

西北太平洋热带气旋活动影响区域大尺度环流的数值模拟研究

本文利用"模式手术"方法研究了西北太平洋热带气旋（TC）对东亚-西北太平洋区域大尺度环流的影响.结果表明,夏季频繁的西北太平洋TC活动导致东亚夏季风增强,季风槽加深;西太平洋副热带高压东退,位置偏北;东亚副热带高空急流强度增强,北太平洋（东亚大陆）上急流轴偏北（偏南）;热带地区（副热带地区）的对流层中低层出现异常上升气流（下沉气流）,并且从低纬向高纬呈现异常上升气流和异常下沉气流交替分布特征.在中国东南沿海,TC降水导致夏季降水量明显增加;而在长江中下游和华北地区,TC活动引起的异常下沉气流使夏季降水量显著减少.因此,夏季西北太平洋TC活动对东亚-西北太平洋区域气候有显著影响.