Tsunami-induced large-scale scour-and-fill structures in Late Albian to Cenomanian deposits of the Grajaú Basin, northern Brazil

Late Albian to Cenomanian upper shoreface deposits from the Grajaú Basin, northern Brazil, consist of well‐sorted, very fine‐ to fine‐grained sandstones with swaley, trough, tabular and minor hummocky cross‐stratification. A striking feature of these deposits is the abundance of large‐scale scour‐and‐fill structures, which consist of regularly spaced, repetitive, very shallow swales with either symmetrical or asymmetrical profiles, arranged along an undulose surface or as a succession of superimposed troughs. The sediment filling these scours is characterized by very fine‐grained sandstone with gently undulose, near‐parallel lamination to very low‐angle dipping cross‐stratification intergraded with swaley and hummocky cross‐stratification. The nature of the scours and the sedimentary structures of their fills reveal the action of combined flows, which are hydrodynamically similar to those developed during storms. However, it is speculated that the combined flows responsible for the genesis of these structures were formed by tsunami waves enhanced by tsunami‐induced ebb currents and/or tidal currents. This interpretation is proposed on the basis of several lines of reasoning: (1) palaeogeographic reconstructions of the study area during the late Cretaceous show that it was outside the belt favourable for the development of storms; (2) comparison of the scour‐and‐fill structures with stratigraphically correlatable deposits exposed north of the study area, where similar features occur in association with abundant seismically induced, soft‐sediment deformation structures; and (3) the presence of several styles of soft‐sediment deformation features (i.e. convolute lamination, bed collapse, large‐scale folds, massive bedding, sand‐filled fractures and diastasis cracks) are suggestive of synsedimentary seismic activity in Cretaceous deposits located in and near to the study area. This study proposes that episodic, high‐amplitude tsunami waves, enhanced by tsunami‐induced ebb currents, develop powerful flows capable of producing complex patterns of erosion and sedimentation, which may be represented by scour‐and‐fill structures similar to those described here.     

Polychaete Communities of Greece: An Ecological Overview

Abstract. Benthic samples were collected from 188 stations distributed over the Aegean and Ionian Seas (Greece, Eastern Mediterranean). Sampled substrata represented a variety of biotopes in soft bottoms with depths ranging from 3 to 380 m. The samples yielded a total of 398 species of polychaetes. Detrended Correspondence Analysis was applied to the whole set of data as well as to three bathymetric groups into which the data was divided. The results showed that in each bathymetric group, different factors determine the spatial distribution of species. Over the whole data set the type of substrata proved to be the major factor, whereas depth is the second most important. Other controlling factors were the substratum type and exposure to hydrodynamism in the shallow group, depth and substratum in the intermediate, and sediment granulometry and depth in the deep one. Diversity, species number and density tended to decrease with depth, while diversity and species number are favoured by coarse material in the sediment.     

A strong-motion database from the Central American subduction zone

Subduction earthquakes along the Pacific Coast of Central America generate considerable seismic risk in the region. The quantification of the hazard due to these events requires the development of appropriate ground-motion prediction equations, for which purpose a database of recordings from subduction events in the region is indispensable. This paper describes the compilation of a comprehensive database of strong ground-motion recordings obtained during subduction-zone events in Central America, focusing on the region from 8 to 14° N and 83 to 92° W, including Guatemala, El Salvador, Nicaragua and Costa Rica. More than 400 accelerograms recorded by the networks operating across Central America during the last decades have been added to data collected by NORSAR in two regional projects for the reduction of natural disasters. The final database consists of 554 triaxial ground-motion recordings from events of moment magnitudes between 5.0 and 7.7, including 22 interface and 58 intraslab-type events for the time period 1976–2006. Although the database presented in this study is not sufficiently complete in terms of magnitude–distance distribution to serve as a basis for the derivation of predictive equations for interface and intraslab events in Central America, it considerably expands the Central American subduction data compiled in previous studies and used in early ground-motion modelling studies for subduction events in this region. Additionally, the compiled database will allow the assessment of the existing predictive models for subduction-type events in terms of their applicability for the Central American region, which is essential for an adequate estimation of the hazard due to subduction earthquakes in this region.     

Carbonate systems along nutrient and temperature gradients: some sedimentological and geochemical constraints

Research over the past several decades has clearly demonstrated that changes in the ocean environment have had major impacts on carbonate systems. Changes in climate, ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry, including carbonate saturation state and nutrient availability, and thereby have determined boundary conditions for the biota that form carbonate platforms. In turn, the biota determine accumulation rates and facies zonations, thus controlling platform geometry and facies dynamics. In the first section of this paper, we examine how nutrient availability influences carbonate facies associations. We first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations. Then we discuss how carbonate sedimentation can be characterized along nutrient gradients. In the second section, we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems. We highlight the variety of existing proxies and their limitations, and suggest that multiple lines of evidence are needed for valid interpretations. Our short review discusses sedimentological, biogenic, and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleoceanographic controls over the distribution of carbonate associations. However, it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs. nutrient supplies changes and facies in ancient carbonate sequences.

---

Examination of ‘global atmospheric temperature monitoring with satellite microwave measurements’: 2. Analysis of satellite data

Using Microwave Sounding Unit (MSU) channel 2 (Ch. 2, 53.74 GHz) data, Spencer and Christy (1992a) determined that the earth exhibits no temperature trend in the period 1979–90, while other authors find a temperature increase of roughly 0.1 K. Based on a theoretical analysis Prabhakara et al. (1995) showed that the information about the global atmospheric temperature deduced from MSU Ch. 2 observations has a small contamination, T ₂, as a result of the attenuation due to hydrometeors in the atmosphere. A method is developed in this study, that utilizes coincident measurements made by MSU in Ch. 1 (50.3 GHz), to estimate this T ₂ over the global oceans. The magnitude of T ₂ is found to be about 1 K over significant parts of the tropical oceanic rain belts and about 0.25 K over minor portions of the mid-latitude oceanic storm tracks. Due to events such as El Niôo, there is variability from year to year in the rain areas and rain intensity leading to significant change in the patterns of T ₂. The patterns of T ₂ derived for March 82 and March 83 reveal such a change. When averaged over the global oceans, from 50° N to 50° S, T ₂ has a value of 0.25 and 0.29 K for March 1982 and 1983, respectively. Due to these reasons the interannual temperature change derived by Spencer and Christy from MSU Ch. 2 will contain a residual hydrometeor effect. Thus in evaluating decadal trend of the global mean temperature of the order of 0.1 K from MSU Ch. 2 data one has to take into account completely the contamination due to hydrometeors.     

Compositional and mineralogic constraints on the genesis of ophiolite hosted nickel mineralization in the Pevkos area,Limassol Forest,Cyprus

Mineralization composed dominantly of primary troilite, maucherite, pentlandite, and chalcopyrite, and secondary valleriite occurs in serpentinized transition zone rocks of the Limasol Forest segment of the Troodos ophiolite complex, Cyprus. Whole-rock and electron microprobe analyses of this mineralization gives ranges of Cu/(Cu+Ni)=0.16 to 0.47, Pt/(Pt+Pd)=0.66 to 0.51, Ni/Co=6.33 to 13.4, and chondrite normalized plots with low concentrations of Rh, Pt, and Pd, but relatively high Au. Estimated distribution coefficients of nickel and iron between olivine and ore range from 0.5 to 7.4. Most of these data are unlike values from magmatic sulfide deposits and indicate either a complete alteration of a preexisting magmatic sulfide concentration or, more likely, a nonmagmatic origin for this mineralization.     

Simultaneous oxidation of arsenic and antimony at low and circumneutral pH,with and without microbial catalysis

Arsenic and Sb are common mine-water pollutants and their toxicity and fate are strongly influenced by redox processes. In this study, simultaneous Fe(II), As(III) and Sb(III) oxidation experiments were conducted to obtain rates under laboratory conditions similar to those found in the field for mine waters of both low and circumneutral pH. Additional experiments were performed under abiotic sterile conditions to determine the biotic and abiotic contributions to the oxidation processes. The results showed that under abiotic conditions in aerated Fe(III)–H₂SO₄ solutions, Sb(III) oxidizes slightly faster than As(III). The oxidation rates of both elements were accelerated by increasing As(III), Sb(III), Fe(III), and Cl⁻ concentrations in the presence of light. For unfiltered circumneutral water from the Giant Mine (Yellowknife, NWT, Canada), As(III) oxidized at 15–78 μmol/L/h whereas Sb(III) oxidized at 0.03–0.05 μmol/L/h during microbial exponential growth. In contrast, As(III) and Sb(III) oxidation rates of 0.01–0.03 and 0.01–0.02 μmol/L/h, respectively, were obtained in experiments performed with acid unfiltered mine waters from the Iberian Pyritic Belt (SW Spain). These results suggest that the Fe(III) formed from microbial oxidation abiotically oxidized As(III) and Sb(III). After sterile filtration of both mine water samples, neither As(III), Sb(III), nor Fe(II) oxidation was observed. Hence, under the experimental conditions, bacteria were catalyzing As and Sb oxidation in the Giant Mine waters and Fe oxidation in the acid waters of the Iberian Pyrite Belt.     

An expression of Philippine Sea plate rotation: the Parece Vela and Shikoku Basins

The Philippine Sea plate, located between the Pacific, Eurasian and Australian plates, is the world's largest marginal basin plate. The motion of the Philippine Sea plate through time is poorly understood as it is almost entirely surrounded by subduction zones and hence, previous studies have relied on palaeomagnetic analysis to constrain its rotation. We present a comprehensive analysis of geophysical data within the Parece Vela and Shikoku Basins—two Oligocene to Miocene back-arc basins—which provide independent constraints on the rotational history of the Philippine Sea plate by means of their seafloor spreading record. We have created a detailed plate model for the opening of the Parece Vela and Shikoku Basins based on an analysis of all available magnetic, gravity and bathymetric data in the region. Subduction along the Izu–Bonin–Mariana trench led to trench roll-back, arc rupture and back-arc rifting in the Parece Vela and Shikoku Basins at 30 Ma. Seafloor spreading in both basins developed by chron 9o (28 Ma), and possibly by chron 10o (29 Ma), as a northward and southward propagating rift, respectively. The spreading orientation in the Parece Vela Basin was E–W as opposed to ENE–WSW in the Shikoku Basin. The spreading ridges joined by chron 6By (23 Ma) and formed a R–R–R triple junction to accommodate the difference in spreading orientations in both basins. At chron 6No (20 Ma), the spreading direction in the Parece Vela Basin changed from E–W to NE–SW. At chron 5Ey (19 Ma), the spreading direction in the Shikoku Basin changed from ENE–WSW to NE–SW. This change was accompanied by a marked decrease in spreading rate. Cessation of back-arc opening occurred at 15 Ma, a time of regional plate reorganisation in SE Asia. We interpret the dramatic change in spreading rate and direction from E–W to NE–SW at 20±1.3 Ma as an expression of Philippine Sea plate rotation and is constrained by the spacing between our magnetic anomaly identifications and the curvature of the fracture zones. This rotation was previously thought to have begun at 25 Ma as a result of a global change in plate motions. Our results suggest that the Philippine Sea plate rotated clockwise by about 4° between 20 and 15 Ma about a pole located 35°N, 84°E. This implies that the majority of the 34° clockwise rotation inferred to have occurred between 25 and 5 Ma from paleomagnetic data may have in fact been confined to the period between 15 and 5 Ma.     

Multiple Collector ICP-MS: Introduction to Instrumentation, Measurement Techniques and Analytical Capabilities

The technique of multiple collector ICP-MS (MC-ICP-MS) has recently attracted much attention, because it permits the precise measurement of isotope compositions for a wide range of elements. This review article discusses the particular advantages of this mass spectrometric technique and provides an overview of the instrumentation. The performance characteristics of MC-ICP-MS instruments and procedures for the correction of instrumental mass discrimination are covered. The strategies that are employed to avoid analytical artefacts from spectral interferences and matrix effects are also considered. A range of applications are then discussed, to provide an overview of the versatility of this mass spectrometric technique. The diversity of applications, which include the measurement of radiogenic and stable isotope ratios, the determination of trace element concentrations by isotope dilution and in situ isotopic measurements using a laser ablation system, highlight the great relevance of MC-ICP-MS to present and future research in various Earth science disciplines.     

Mineralogical Characteristics and Cationic Exchange Properties of a Zeolite Mineral from Romana, NW Sardinia, Italy

An investigation was conducted into the Tertiary ignimbrite formations in the Romana area in NW Sardinia. Scanning electron microscopy–energy-dispersive spectrometry indicated the presence of clinoptilolite and subordinate smectite, quartz, mordenite and calcite. Laboratory analyses of representative samples collected from the deposit indicated a cation exchange capacity ranging from 85 to 135 mEq per 100 g. Ion exchange experiments conducted in flasks containing different initial cation concentrations and in columns supplied with solution having constant cation concentration, showed different selectivity towards the cations examined, in the order: NH₄⁺> Pb²⁺> Cd²⁺> Cu²⁺> Zn²⁺> Mn²⁺> Ni²⁺. Compared to the untreated material, under the experimental conditions used and considering the eluate, and the Italian regulatory limits for discharge into surface waters, the zeolite treated with a 1-N NaCl solution yielded a 1.3–5-fold increase in the volume of treated water, depending on the cation examined. The experimental results obtained show that the cationic exchange properties of the zeolite rock from the Romana deposit are comparable with the average quality of zeolites currently imported into Italy from non-European countries.