The oldest mid-oceanic carbonate buildup complex: Setting and lithofacies of the Vendian (Late Neoproterozoic) Baratal limestone in the Gorny Altai Mountains,Siberia

The Baratal limestone in the Gorny Altai Mountains, southern Siberia, occurs as large allochthonous blocks within a Cambrian accretionary complex that developed around the Siberia craton. Before the final accretion to Siberia in the Cambrian, the terrigenous clastic-free Baratal limestone was deposited directly upon a basaltic basement with a geochemical signature identical to that of modern oceanic plateau. The Baratal limestone with 598 ± 25 Ma (Early Vendian) Pb-Pb isochron age consists of three distinct facies; 1) massive lime mudstone with ooids and stromatolites, 2) bedded lime mudstone, and 3) limestone conglomerate/breccia dominated by ooid-bearing lime mudstone clasts. The first represents a shallow marine environment on top of an ancient oceanic plateau, while the latter two represent the deeper slope to bottom-of-slope facies of a plateau. The Vendian Baratal limestone provides the oldest example of a reconstructed shallow marine carbonate buildup complex developed on a plateau/seamount in a mid-ocean.     

Three-dimensional coupled-mode model and characteristics of low-frequency sound propagation in ocean waveguide with seamount topography

Large-scale topography, such as a seamount, substantially impacts low-frequency sound propagation in an ocean waveguide, limiting the application of low-frequency acoustic detecting techniques. A three-dimensional (3D) coupled-mode model is developed to calculate the acoustic field in an ocean waveguide with seamount topography and analyze the 3D effect. In this model, a correction is introduced in the bottom boundary, theoretically making the acoustic field satisfy the energy conservation. Furthermore, a large azimuth angle calculation range is obtained by using the operator theory and higher-order Padé approximation. Additionally, the model has advantages related to the coupling mode and parabolic equation theory. The couplings corresponding to the effects of range-dependent environment are fully considered, and the numerical implementation is kept feasible. After verifying the accuracy and reliability of the model, low-frequency sound propagation characteristics in the seamount environment are analyzed. The results indicate lateral variability in bathymetry can lead to out-of-plane effects such as the horizontal refraction phenomenon, while the coupling effect tends to restore the abnormal sound field and produces acoustic field diffraction behind the seamount. This model effectively considers the effects of the horizontal refraction and coupling, which are proportional to the scale of the seamount.     

INTERNAL LEE WAVES AND TURBULENCE MIXING OVER AN ISOLATED SEAMOUNT: RESULTS FROM TURBULENCE ENERGY MODELS

A three-dimensional, primitive equation, baroclinic numerical model incorporating a range of turbulence energy submodels is used to study the generation of internal lee waves over an isolated seamount. Attention is given to the turbulence mixing enhanced by the internal lee waves. The results show that regions of strong turbulence energy appear over the lee side of the seamount associated with the production of the lee waves. The computed vertical eddy viscosity and diffusivity using turbulence models can be as large as 1 m²s⁻¹. A comparison of the magnitude and spatial distribution of the internal lee waves does not reveal any major differences in results computed using different turbulence energy models or mixing determined from a Richards on number formulation. However, the magnitude of the vertical mixing is sensitive to the form of turbulence energy submodel. Also, a study of the relevant importance of the various terms in the turbulence energy equation shows that the term representing the advection of turbulence needs to be retained in order to accurately reproduce the mixing produced by the internal lee waves. Calculations using a range of seamount profiles show that the magnitude of near-bed turbulence is sensitive to the shape of the seamount. The magnitude and spatial distribution of the lee waves and associated flow field are affected by the parametrization of horizontal diffusion, with significant differences between the use of Laplacian and biharmonic forms of horizontal diffusion. The application of biharmonic horizontal diffusion is recommended.     

The influence of topographic features and density variations upon the internal tides in shelf edge regions

A brief outline of the development of a three‐dimensional primitive equation baroclinic model for computing the internal tides is presented. A series of calculations are performed to examine the internal tide in the shelf edge region off the west coast of Scotland, with the aim of determining the influence upon the internal tide in the region of topographic features, namely large‐scale features, such as seamounts, and small‐scale features, such as a detailed specification of the shelf slope, which is possible using results from a recent survey rather than topographic data taken from a coarse grid model using depths interpolated from charts. The influence upon the solution of small variations in the density field and seasonal variations is examined. Also, the influence of a well‐mixed bottom boundary layer at the base of the shelf slope, produced possibly by cascading of dense water off the shelf during a major wind event, upon the internal tide is also examined. In the calculations described here the model is used in cross‐shelf form. A comparison of internal tides computed with and without the inclusion of an offshore seamount shows that internal tides are produced along the sides of the seamount, and can propagate towards the shelf edge, and thereby influence the internal tidal signal in that region. A comparison of internal tides computed using previously available bottom topography extracted from charts with those computed using a detailed topographic data set from a recent survey shows the importance of accurate topography in the shelf edge region for internal tide calculations. Simulations show that the internal tide is influenced by variations in the density field, suggesting that an accurately measured density field is essential for internal tide determination. Calculations also show that if a well‐mixed sea bed boundary layer occurs at the bottom of the shelf slope, the internal tide can be reflected by this feature giving rise to significant internal tides in the ocean at larger distances from the shelf edge than in the absence of this water mass. Copyright © 1999 John Wiley & Sons, Ltd.     

中太平洋M海山富钴结壳稀土元素地球化学

用ICP-AES和重量法测试了中太平洋M海山不同水深范围内(1300~3000 m)25个富钴结壳样品的稀土元素和常量元素。结果显示M海山富钴结壳为水成结壳,稀土元素总量平均为2084.69μg.g-1,LREE/HREE平均为4.84,具有正的Ce异常。ΣREE(不含Ce)随水深增加而增加,而Ce和LREE/HREE比值则以文石溶跃面为界(水深2000 m)呈现不同的变化,稀土元素这些变化除了与海洋背景颗粒的吸附清扫有关外,还与稀土元素在海水中的行为有关。富钴结壳Ce虽随水深变化,但与水深关系不明显,可能反映其通量恒定。富钴结壳中Ce基本全为Ce(Ⅳ),表明氧化富集过程,这一过程受动力学因素控制。     

Three-dimensional mode coupling around a seamount

A recently developed three-dimensional normal-mode model is adopted to investigate mode coupling around a seamount in a deep water environment. As indicated by the theoretical analysis and verified by the numerical results, strong mode coupling occurs at the edge of a seamount under certain conditions. Therefore, mode coupling is critical for the investigation of the acoustic field in the presence of a seamount. In addition, the issue regarding the number of sectors assuring convergence is also presented. T...     

Guadalupian (Middle Permian) giant bivalve Alatoconchidae from a mid-Panthalassan paleo-atoll complex in Kyushu,Japan: A unique community associated with Tethyan fusulines and corals

Unique new fossil assemblages containing the large bivalve family Alatoconchidae are recorded from the Guadalupian (Middle Permian) shallow marine limestone in Kamura, Kyushu. The large bivalves occur in the Neoschwagerina Zone and Lepidolina Zone. This discovery establishes that the biostratigraphic range of the family Alatoconchidae extends up to the top of the Lepidolina Zone (upper Capitanian of upper Guadalupian) i.e., to the end-Guadalupian extinction level. The largest Alatoconchidae in Kamura occurs in the Neoschwagerina Zone, the size of which is up to 50 cm long and 5 cm thick. Although details are still unknown, their morphology with a wing-like side projection of their valves appears very similar to that of Alatoconchidae that includes the well-known genus Shikamaia Ozaki. The bivalve-bearing Iwato Formation was derived from a mid-oceanic shallow marine carbonate build-up formed on a mid-oceanic paleo-seamount. The close association among the Alatoconchidae, typical Tethyan fusulines (Verbeekinidae) and rugose corals (Waagenophyllidae), plus their common extinction pattern suggests that the Alatoconchidae flourished in warm, shallow (photic) marine environments in low latitude areas in Panthalassa as well as Tethys. The extra-large size and double-layered shell with a translucent outer layer composed of prismatic calcite suggests that these bivalves may have hosted abundant photosynthetic algal symbionts to support their large-body metabolism.