Carbon-isotope records of the Early Jurassic (Toarcian) oceanic anoxic event from the Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps) sections: palaeoceanographic and stratigraphic implications

The Toarcian oceanic anoxic event ( ca 183 Ma) coincides with a global perturbation marked by enhanced organic carbon burial and a general decrease in calcium carbonate production, probably triggered by changes in the composition of marine plankton and elevated carbon dioxide levels in the atmosphere. This study is based on high-resolution sampling of two stratigraphic successions, located in Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps), Italy, which represent expressions of the Toarcian oceanic anoxic event in deep-water pelagic sediments. These successions are characterized by the occurrence of black shales showing relatively low total organic carbon concentrations (compared with coeval strata in Northern Europe), generally < 2%, and low hydrogen indices. On this basis, they are similar to other Toarcian black shales described from the Tethyan region. The positive and negative carbon-isotope records from the two localities permit a high-resolution correlation such that ammonite biostratigraphy information from Valdorbia can be transferred to those parts of the Monte Mangart section that lack these fossils. Spectral analyses of δ¹³C_org values and of CaCO₃ percentages from the sedimentary records of both the Valdorbia and Monte Mangart sections reveal a strong cyclic pattern, best interpreted as an eccentricity signal which hence implies a duration of ca 500 kyr for the negative carbon-isotope excursion. Based on the carbon-isotope curves obtained, the high-resolution correlation between the Italian successions and a section in Yorkshire (Northern Europe) confirms the supposition that the apparent mismatch between the dating of the Toarcian oceanic anoxic event in the Boreal and Tethyan realms is an artefact of biostratigraphy.     

The “slip law” of the free surface

A “slip law” connects the excess velocity or “slip” of a wind-blown water surface, relative to the motion in the middle of the mixed layer, to the wind stress, the wind-wave field, and buoyancy flux. An inner layer-outer layer model of the turbulent shear flow in the mixed layer is appropriate, as for a turbulent boundary layer or Ekman layer over a solid surface, allowing, however, for turbulent kinetic energy transfer from the air-side via breaking waves, and for Stokes drift. Asymptotic matching of the velocity distributions in inner and outer portions of the mixed layer yields a slip law of logarithmic form, akin to the drag law of a turbulent boundary layer. The dominant independent variable is the ratio of water-side roughness length to mixed layer depth or turbulent Ekman depth. Convection due to surface cooling is also an important influence, reducing surface slip. Water-side roughness length is a wind-wave property, varying with wind speed similarly to air-side roughness. Slip velocity is typically 20 times water-side friction velocity or 3% of wind speed, varying within a range of about 2 to 4.5%. A linearized model of turbulent kinetic energy distribution shows much higher values near the surface than in a wall layer. Nondimensional dissipation peaks at a value of about eight, a short distance below the surface.     

豆荚状铬铁矿豆状结构成因机制探讨--以遵化地区豆荚状铬铁矿为例

豆荚状铬铁矿是蛇绿岩套地幔构造岩中特征的矿产 ,其矿石发育丰富的岩浆 -变形结构构造类型 ,对认识大洋上地幔成因及其成矿过程具有重要意义。对遵化新太古代豆荚状铬铁矿的深入研究 ,发现它们发育了丰富的岩浆结构 (豆状结构、豆壳状结构等 ) ,还保留了大量凝缩标志和旋转特征。对比分析其各种特征结构发现豆状结构是由浸染状结构逐步演化形成的 ,并且旋转和凝缩是形成豆状结构的主要机理。豆荚状铬铁矿以其独特而稳定的物理化学特性 ,记录了大洋地幔深部岩浆活动(特别是扩张中心原始岩浆活动 )以及大洋岩石圈运动过程。因此 ,研究其成因机制能够为研究大洋上地幔动力学机制提供重要线索     

Deep‐water dunes on drowned isolated carbonate terraces (Mozambique Channel,south‐west Indian Ocean)

Subaqueous sand dunes are common bedforms on continental shelves dominated by tidal and geostrophic currents. However, much less is known about sand dunes in deep‐marine settings that are affected by strong bottom currents. In this study, dune fields were identified on drowned isolated carbonate platforms in the Mozambique Channel (south‐west Indian Ocean). The acquired data include multibeam bathymetry, multi‐channel high‐resolution seismic reflection data, sea floor imagery, a sediment sample and current measurements from a moored current meter and hull‐mounted acoustic Doppler current profiler. The dunes are located at water depths ranging from 200 to 600 m on the slope terraces of a modern atoll (Bassas da India Atoll) and within small depressions formed during tectonic deformation of drowned carbonate platforms (Sakalaves Seamount and Jaguar Bank). Dunes are composed of bioclastic medium size sand, and are large to very large, with wavelengths of 40 to 350 m and heights of 0·9 to 9·0 m. Dune migration seems to be unidirectional in each dune field, suggesting a continuous import and export of bioclastic sand, with little sand being recycled. Oceanic currents are very intense in the Mozambique Channel and may be able to erode submerged carbonates, generating carbonate sand at great depths. A mooring located at 463 m water depth on the Hall Bank (30 km west of the Jaguar Bank) showed vigorous bottom currents, with mean speeds of 14 cm sec^?1 and maximum speeds of 57 cm sec^?1, compatible with sand dune formation. The intensity of currents is highly variable and is related to tidal processes (high‐frequency variability) and to anticyclonic eddies near the seamounts (low‐frequency variability). This study contributes to a better understanding of the formation of dunes in deep‐marine settings and provides valuable information about carbonate preservation after drowning, and the impact of bottom currents on sediment distribution and sea floor morphology.     

海南岛新生代玄武质熔岩源区中再循环物质及其年龄的限定:来自Hf-Sr-Nd-Pb同位素的制约

再循环洋壳和沉积物在壳.幔循环过程中扮演着重要的角色。前人研究指出板片俯冲与海南地幔柱形成关系密切,然而地幔柱源区中是否存在再循环沉积物仍然不得而知。本文将首次报导海南岛新生代玄武质熔岩的Hf同位素,并将新的Sr-Nd-Pb同位素与前人数据相结合,限定再循环物质的组成及其年龄。与同位素地幔趋势相比,给定εNd时,海南岛玄武质熔岩的εHf值更低;而且Eu/Eu*>1、Sr/Sr*>1,与Al2O3、MgO都不存在相关性,说明Eu和Sr的正异常是继承了源区的特征,指示源区中存在再循环洋壳物质的参与。海南岛玄武质熔岩的Ce/Pb与εNd、εHf呈正相关关系,Nd/Pb与87Sr/86Sr呈负相关关系,它们都指向了沉积物端元。根据εHf-εNd模型和Pb同位素演化模型,推断源区中再循环洋壳和沉积物相对年轻,其年龄应小于1.0Ga。这些再循环洋壳和沉积物将进入深部地幔,并与地幔中的橄榄岩反应形成二阶段辉石岩,随着海南地幔柱不断上升而发生熔融,最终熔体喷发至地表形成海南岛玄武质熔岩。     

东北大兴安岭北段蛇绿岩的时空分布及与区域构造演化关系的研究

大兴安岭北段作为中亚造山带东段重要组成单元,该区域的构造背景一直悬而未决,它涉及兴安与松辽地块基底属性、额尔古纳地块亲缘性以及三者之间的拼贴位置、时限和方式等众多科学问题。蛇绿岩作为研究造山带的重要工具之一,不仅是古板块或微地块的重要边界,而且是认识地幔组成和壳幔演化的重要窗口。然而大兴安岭北段蛇绿岩却鲜有报道。根据近年来野外工作及结合前人研究成果,文章探讨了蛇绿岩形成时代及构造环境。研究表明：(1)大兴安岭北段蛇绿岩整体呈北东向展布,但时代跨越较大,主要包括新元古代、早奥陶世-志留纪、石炭纪-二叠纪;(2)大兴安岭北段蛇绿岩均有E-MORB地球化学特征,其中新元古代的蛇绿岩明显亏损高场强元素Nb、Ta,而晚古生代的蛇绿岩则相对富集高场强元素;(3)大兴安岭北段新元古代蛇绿岩可能形成于洋内岛弧环境,记录着前寒武纪陆弧碰撞信息,而早奥陶世-志留纪蛇绿岩可能与早古生代弧盆体系演化相关。尽管上述认识为大兴安岭北段古生代的地球动力学背景研究提供了新的依据,但大兴安岭北段蛇绿岩仍存在较多问题亟须解决。     

20世纪北大西洋温盐环流的年代际变化试评估

根据相对丰富的大气器测资料,综合前人对有限的海洋资料的诊断分析,从北大西洋涛动(NAO)变率、表层海温(SST)变率、格陵兰海和拉布拉多海的深对流活动长期变化等不同角度,对20世纪大洋温盐环流(Thermohaline Circulation,THC)变率进行了试评估.结果表明:(1)19世纪末以来,大西洋温盐环流的变化可分为4个时期:1900年以前的一段时期,THC较强;1904年到1930年,THC较弱;1931年到1972年,THC较强;1973年至1995年,THC较弱,目前则又有所增强.(2)与THC的变化相联系,大西洋主要气候要素的变化,相互间存在着某种协调关系,THC强,NAO弱,北大西洋北部SST升高,格陵兰海的对流活动增强,拉布拉多海的对流活动则减弱.     

北补连蛇绿岩的特征,形成环境及其构造意义

文中总结了北祁连蛇绿岩的特征，指出北祁连蛇绿岩大多具有ＭＯＲＢ的性质，有玻安岩产生，形成在弧后和岛弧环境，北祁连蛇绿岩大多侵位在岛弧增生楔或活动陆缘地体之上，蛇绿岩属于科迪勒拉型，早古生代的北祁连造山带属于科迪勒拉型造山带，部分蛇绿岩之上整合产出一套沉积一火山岩系，称为蛇绿岩的上覆岩系，指出蛇绿岩及其上覆岩系的枕状熔岩分别来自不同的源区，具有不同的构造意义，还讨论了北祁连早古生代板块构造格局，认为     

从有孔虫分析西藏南部白垩纪海平面升降

白垩纪是地质历史中的最大海侵时期。海平面的升降改变了海洋的物理、化学及生物因素,从而影响了有孔虫的演化与发展。根据对有孔虫丰度、分异度及演化类型的研究,认为西藏白垩纪最大海侵时期形成于赛诺曼期与土仑期的界线附近。此后海平面总的处于下降趋势,只是在康尼亚克期至三冬期海平面又有回升。马斯特里赫特末期有孔虫的大量灭绝反映了白垩纪与第三纪界线事件的影响。     

Short-term variabilities of upper ocean current in the warm pool region during TOGA/COARE IOP

Oceanic current data in the warm pool region of the western equatorial Pacific measured by upward-looking moored Acoustic Doppler Current Profilers at two equatorial sites (147°E and 154°E) and two off-equatorial sites (2°N and 2°S, 156°E) during TOGA/COARE Intensive Observing Period (IOP) from November 1992 to February 1993 are used to examine short-term variabilities in the upper layer above 160–240 m. In time series of the zonal and meridional currents in many layers, spectral peaks are found at periods around 2 days and 4 days in addition to high energies in a period range longer than 10 days. The signal with the period of about 2 days has significantly high energies at all sites, and its magnitude is higher for the meridional current than for the zonal one. This signal is especially active in the first half of IOP from November to December in 1992. In this period, the quasi-2-day signal in the current field is coherent between northern (2°N) and southern (2°S) stations, but it has no evident relationship with that in the surface wind field around the stations. The quasi-4-day signal with the period of about 4 days has highest energies in layers above 160 m at the southern station, and is coherent between northern and southern stations. Besides, the signal at the station of 2°S has a significantly high coherence with that in the wind at the southern station, suggesting that it is a local phenomenon.