Petrology, magnetostratigraphy and geochronology of the Miocene volcaniclastic Tepoztlán Formation: implications for the initiation of the Transmexican Volcanic Belt (Central Mexico)

The volcaniclastic Tepoztlán Formation (TF) represents an important rock record to unravel the early evolution of the Transmexican Volcanic Belt (TMVB). Here, a depositional model together with a chronostratigraphy of this Formation is presented, based on detailed field observations together with new geochronological, paleomagnetic, and petrological data. The TF consists predominantly of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones, conglomerates and breccias, originating from fluvial and mass flow processes, respectively. Within these sediments fall deposits and lavas are sparsely intercalated. The clastic material is almost exclusively of volcanic origin, ranging in composition from andesite to rhyolite. Thick gravity-driven deposits and large-scale alluvial fan environments document the buildup of steep volcanic edifices. K-Ar and Ar-Ar dates, in addition to eight magnetostratigraphic sections and lithological correlations served to construct a chronostratigraphy for the entire Tepoztlán Formation. Correlation of the 577 m composite magnetostratigraphic section with the Cande and Kent (1995) Geomagnetic Polarity Time Scale (GPTS) suggests that this section represents the time intervall 22.8–18.8 Ma (6Bn.1n-5Er; Aquitanian-Burdigalian, Lower Miocene). This correlation implies a deposition of the TF predating the extensive effusive activity in the TMVB at 12 Ma and is therefore interpreted to represent its initial phase with predominantly explosive activity. Additionally, three subdivisions of the TF were established, according to the dominant mode of deposition: (1) the fluvial dominated Malinalco Member (22.8–22.2 Ma), (2) the volcanic dominated San Andrés Member (22.2–21.3 Ma) and (3) the mass flow dominated Tepozteco Member (21.3–18.8 Ma).     

Critical tissue copper residues for marine bivalve (Mytilus galloprovincialis) and echinoderm (Strongylocentrotus purpuratus) embryonic development: conceptual, regulatory and environmental implications

Critical tissue copper (Cu) residues associated with adverse effects on embryo-larval development were determined for the Mediterranean mussel (Mytilus galloprovincialis) and purple sea urchin (Strongylocentrotus purpuratus) following laboratory exposure to Cu-spiked seawater collected from San Diego Bay, California, USA. Whole body no-observed-effect-residues (NOER) were similar, with means of 21 and 23mugg(-1)dw, for M. galloprovincialis and S. purpuratus, respectively. Mean whole body median effect residues (ER50) were 49 and 142mugg(-1)dw for M. galloprovincialis and S. purpuratus, respectively. The difference in ER50s between species was reduced to a factor of <2 when expressed as soft tissue residues. Coefficients of variation among whole body-ER50s were 3-fold lower than median waterborne effect concentrations (EC50) for both species exposed to samples varying in water quality characteristics. This suggests that tissue concentrations were a better predictor of toxicity than water concentrations. The CBRs described herein do not differentiate between the internal Cu concentrations that are metabolically available and those that are accumulated and then detoxified. They do appear, however, to be well enough related to the level of accumulation at the site of action of toxicity that they serve as useful surrogates for the copper concentration that affects embryonic development of the species tested. Results presented have potentially important implications for a variety of monitoring and assessment strategies. These include regulatory approaches for deriving saltwater ambient water quality criteria for Cu, contributions towards the development of a saltwater biotic ligand model, the conceptual approach of using CBRs, and ecological risk assessment.     

中国早期人类分布的环境制约因素探讨

在环境因素起制约作用的背景下,随着中国古人类演化与生存能力的进步,其数量和空间分布在不同时间段呈现出不同的特征。为了更好地了解中国早期人类的时空分布特征,本研究以中国境内已发表的2110处旧石器遗址的坐标和年代信息为基础,采用统计学和空间分析的方法,对遗址的时间分布进行以蒙特卡洛模拟为基础的时间不定性模拟,对遗址的空间分布进行核密度估计和聚类分析;同时,利用环境空间数据集中的海拔、水源、气温和降水信息,对不同时期内各个遗址所在地的环境条件进行统计分析。研究结果表明,在旧石器时代,随着时间的发展,中国境内的遗址数量不断增多,分布范围也逐渐从中部、南部和东部地区向条件较恶劣的北部和西部扩张。其中,600 ka和57 ka为这一变化的两个关键时间点,600 ka之前遗址数量较少,分布也较为局限;在600 ka至57 ka之间,遗址数量缓慢增加,分布范围也在季风区内逐渐扩大;57 ka之后,遗址数量迅速增长,分布遍布中国境内的大部分地区。本文所梳理的中国境内旧石器遗址整体时空分布特征,为进一步解读中国早期人类的环境适应能力,特别是古人类的体质演化和技术进步的环境驱动机制提供参考。

     

Saturn's rings: II. Condensations of light and optical thickness of Cassini's division

“Condensations” of light have been observed when Saturn's rings are seen almost edge on, and the Sun and the Earth are on opposite sides of the ring plane. These condensations are associated with ring C and Cassini's division. If the relative brightness between the two condensations and the optical thickness of ring C are known, we can calculate the optical thickness of Cassini's division, τ_CASS. Using Barnard's and Sekiguchi's measurements, we have obtained 0.01 ? τ_CASS ? 0.05. A brightness profile of the condensations which agrees well with visual observations is also presented.We are able to set an upper limit of about 0.01 for the optical thickness of any hypothetical outer ring. This rules out a ring observed by C. Cragg in 1954, but does not eliminate the D′ ring observed by Feibelman in 1967.It is known that the outer edge of ring B is almost at the position of the 1/2 resonance with Mimas. Franklin, Colombo, and Cook explained this fact in 1971, postulating a total mass of ring B of 10^?6M_SATURN. We have derived a formula for the mass of the rings, which is a linear function of the mean particle size. We find that 10^?6M_SATURN implies large particles (～70m). If the particles are small (～10cm), as currently believed, the total mass of ring B is not enough to shift the outer edge. We conclude that the above explanation and current size estimates are inconsistent.     

Overpassing and armouring phenomena on gravel beaches

Armouring phenomena are common in river beds, gravel beaches and spits. The gravel segregation recognized at depth in three different beaches of Patagonia and Tierra del Fuego, Argentina, allows us to propose a mechanism for formation.

A mixed population of gravel set into motion by waves is deposited progressively. Granules and fine gravels with higher pivoting angles in the beach slope are easily trapped within the bed, while rounded pebbles continue rolling over (overpassing) it. Finally, decreasing flow velocity allows the deposition of the larger pebbles, thus armouring the beach.

A carpet of round clasts of uniform size is more stable than the same clasts in a mixed population armouring the bed and covering a uniform layer of rounded granules and fine gravels.     

A quadrature-free method for simulation and inversion of 1.5D direct current (DC) borehole measurements

Resistivity inverse problems are routinely solved in order to characterize hydrocarbon bearing formations. They often require a large number of forward problems simulations. When considering a one dimensional (1D) planarly layered media, semi-analytical methods can be employed in order to solve a single forward problem in a fraction of a second. However, in some situations, a large number of (over one million) simulations is required, preventing this method to be used as a real time (logging) alternative. In this paper, we propose a novel semi-analytical method that dramatically reduces the total computational time, so it can be employed for real time inversion. In our proposed method, we select an ad hoc basis representation for the spectral solution such that its inverse Hankel transform can be computed analytically. The proposed method requires a pre-process that is expensive when compared with a single evaluation in classical semi-analytical methods. However, subsequent evaluations can be rapidly obtained, decreasing thus the total computational time by orders of magnitude when the number of required forward simulations is large.     

SHIA_Landslide: a distributed conceptual and physically based model to forecast the temporal and spatial occurrence of shallow landslides triggered by rainfall in tropical and mountainous basins

Landslides are a main cause of human and economic losses worldwide. For this reason, landslide hazard assessment and the capacity to predict this phenomenon have been topics of great interest within the scientific community for the implementation of early warning systems. Although several models have been proposed to forecast shallow landslides triggered by rainfall, few models have incorporated geotechnical factors into a complete hydrological model of a basin that can simulate the storage and movement of rainwater through the soil profile. These basin and full hydrological models have adopted a physically based approach. This paper develops a conceptual and physically based model called open and distributed hydrological simulation and landslides—SHIA_Landslide (Simulación HIdrológica Abierta, or SHIA, in Spanish)—that is supported by geotechnical and hydrological features occurring on a basin-wide scale in tropical and mountainous terrains. SHIA_Landslide is an original and significant contribution that offers a new perspective with which to analyse shallow landslide processes by incorporating a comprehensive distributed hydrological tank model that includes water storage in the soil coupled with a classical analysis of infinite slope stability under saturated conditions. SHIA_Landslide can be distinguished by the following: (i) its capacity to capture surface topography and effects concerning the subsurface flow; (ii) its use of digital terrain model (DTM) to establish the relationships among cells, geomorphological parameters, slope angle, direction, etc.; (iii) its continuous simulation of rainfall data over long periods and event simulations of specific storms; (iv) its consideration of the effects of horizontal and vertical flow; and (vi) its inclusion of a hydrologically complete water process that allows for hydrological calibration. SHIA_Landslide can be combined with real-time rainfall data and implemented in early warning systems.