Hydrothermal bitumen generated from sedimentary organic matter of rift lakes – Lake Chapala, Citala Rift, western Mexico

Lake Chapala is in the Citala Rift of western Mexico, which in association with the Tepic-Zacoalco and Colima Rifts, form the well-known neotectonic Jalisco continental triple junction. The rifts are characterized by evidence for both paleo- and active hydrothermal activity. At the south shore of the lake, near the Los Gorgos sublacustrine hydrothermal field, there are two tar emanations that appear as small islands composed of solid, viscous and black bitumen. Aliquots of tar were analyzed by GC-MS and the mixtures are comprised of geologically mature biomarkers and an UCM. PAH and n-alkanes are not detectable. The biomarkers consist mainly of hopanes, gammacerane, tricyclic terpanes, carotane and its cracking products, steranes, and drimanes. The biomarker composition and bulk C isotope composition (δ¹³C = −21.4%) indicate an organic matter source from bacteria and algae, typical of lacustrine ecosystems. The overall composition of these tars indicates that they are hydrothermal petroleum formed from lacustrine organic matter in the deeper sediments of Lake Chapala exceeding 40 ka (¹⁴C) in age and then forced to the lakebed by tectonic activity. The absence of alkanes and the presence of an UCM with mature biomarkers are consistent with rapid hydrothermal oil generation and expulsion at temperatures of 200–250 °C. The occurrence of hydrothermal petroleum in continental rift systems is now well known and should be considered in future energy resource exploration in such regions.     

Multi-isotopic (Li,B, Sr,Nd) approach for geothermal reservoir characterization in the Limagne Basin (Massif Central,France)

A multi-isotopic study of thermo-mineral waters from the Limagne Basin (French Massif Central) is reported. Lithium, B, Sr and Nd isotopic signatures in thermo-mineral waters and bedrocks were combined in order to determine the origin of these fluids from a reservoir point of view. Strontium and Nd isotopic systems showed that the thermo-mineral waters are mostly derived from a granitic reservoir type, with the exception of a few water samples having Sr and Nd isotopic signatures also reflecting a volcanic contribution (basalts). In a second step, Li and B isotopes were investigated, given that Li and B isotopic systematics are potentially affected by mass dependent fractionation during water/rock interaction and could provide some information on what takes place during this process. A great diversity of thermo-mineral waters within the Aigueperse area was clearly observed, which is directly related to the origin of these thermal waters in terms of reservoirs. These different geothermal reservoirs are characterized by different geological settings and intensity of water/rock interaction (i.e. depth and temperature).     

Unstable climates: Exploring the statistical and social constructions of ‘normal’ climate

The idea of climate has both statistical and social foundations. Both of these dimensions of climate change over time: climate, as defined by meteorological statistics, changes for both natural and anthropogenic reasons; and our expectations of future climate also change, as cultures, societies and knowledge evolves. This paper explores the interactions between these different expressions of climate change by focusing on the idea of ‘normal’ climates defined by statistics. We show how this idea came into being in meteorological circles and then review how this idea of climatic normality gets entangled with cultural and psychological processes. Using data from historical and predicted climates in the UK, we illustrate the significance of choosing different baseline ‘normals’ for retrospective and prospective interpretations of climate change. Since the choice of these statistical ‘normals’ reflects cultural, political and psychological preferences and practices as much as scientific ones, we argue that expectations of the climatic future are influenced by social as well as statistical norms. Seeing climate as co-constructed between the psycho-cultural constraints of society and the physical constraints of the material world offers a different way of thinking about the instabilities of climate and the ways we adapt to them.     

Hypoxia in the northern Gulf of Mexico: Does the science support the Plan to Reduce,Mitigate, and Control Hypoxia?

We update and reevaluate the scientific information on the distribution, history, and causes of continental shelf hypoxia that supports the 2001 Action Plan for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force 2001), incorporating data, publications, and research results produced since the 1999 integrated assessment. The metric of mid-summer hypoxic area on the LouisianaTexas shelf is an adequate and suitable measure for continued efforts to reduce nutrients loads from the Mississippi River and hypoxia in the northern Gulf of Mexico as outlined in the Action Plan. More frequent measurements of simple metrics (e.g., area and volume) from late spring through late summer would ensure that the metric is representative of the system in any given year and useful in a public discourse of conditions and causes. The long-term data on hypoxia, sources of nutrients, associated biological parameters, and paleoindicators continue to verify and strengthen the relationship between the nitratenitrogen load of the Mississippi River, the extent of hypoxia, and changes in the coastal ecosystem (eutrophication and worsening hypoxia). Multiple lines of evidence, some of them representing independent data sources, are consistent with the big picture pattern of increased eutrophication as a result of long-term nutrient increases that result in excess carbon production and accumulation and, ultimately, bottom water hypoxia. The additional findings arising since 1999 strengthen the science supporting the Action Plan that focuses on reducing nutrient loads, primarily nitrogen, through multiple actions to reduce the size of the hypoxic zone in the northern Gulf of Mexico.     

The origin of rotational remanent magnetization

Summary. Rotational remanent magnetization, RRM, is the magnetization acquired when a sample is rotated during alternating field demagnetization. Although the existence of RRM has been well documented in different laboratories, until now no physical mechanism explaining its origin has been given. We propose that the RRM originates from thermal fluctuations biased by a precessional torque associated with the alternating field. Our theory is consistent with the observation that no directional preference exists in the experimental situation until the sample is rotated relative to the alternating field. Moreover, our theory predicts that the combined sample rotation and precession will produce a RRM that switches direction when the frequency of sample rotation increases from any value below the frequency of the alternating field to any value above that frequency as observed in experiments. Although no precise theory is given for the intensity of RRM, the model presented here can qualitatively explain previous intensity observations.     

A software tool for monitoring legal minimum length of landings: Case study of a fishery in southern Spain

The regulation of minimum legal size (MLS) of catches is a tool widely applied in the management of fisheries resources, although the MLS does not always coincide with the length at first maturity (LFM). The optimization of this management tool requires a series of quality control in fish markets and transportation. A software application has been developed to make the control of the landings of several target species easier and faster. In order to test and make this tool operational, six species of commercial interest were selected: four species of fish and two species of bivalves. It is proposed to estimate the proportion of illegal specimens in the studied lot from the proportion of illegal individuals found in the samples taken from this lot. The input data for the application are the minimum legal size (MLS) of the species and the total length (TL) of each specimen sampled. The output data is a statistical summary of the percentage of specimens of size less than the legal minimum (TL≤MLS) within different confidence intervals (90%, 95% and 99%). The software developed will serve as a fast, efficient and easy to manage tool that allows inspectors to determine the degree of compliance on MLS control and to make a decision supported by statistical proof on fishing goods.     

Effect of wave amplitude on turbulent flow in a wavy channel by direct numerical simulation

The present study numerically investigates the characteristics of three-dimensional turbulent flow in a wavy channel. For the purpose of a careful observation of the effect of the wave amplitude on the turbulent flow, numerical simulations are performed at a various range of the wave amplitude to wavelength ratio (0.01?α/λ?0.05), where the wavelength is fixed with the same value of the mean channel height (H). The immersed boundary method is used to handle the wavy surface in a rectangular grid system, using the finite volume method. The Reynolds number (Re=U_bH/ν) based on the bulk velocity (U_b) is fixed at 6760. The present computational results for a wavy surface are well compared with those of references. When α/λ=0.02, the small recirculating flow occurs near the trough at the instant, but the mean reverse flow is not observed. In the mean flow field, the reverse flow appears from α/λ=0.03 among the wave amplitude considered in this study. The domain of the mean reverse flow defined by the locations of separation and reattachment depends strongly on the wave amplitude. The pressure drag coefficient augments with increasing the wave amplitude. The friction drag coefficient shows the increase and decrease behavior according to the wave amplitude. The quantitative information about the flow variables such as the distribution of pressure and shear stress on the wavy surface is highlighted.     

Influence of microstructure of loess on triaxial shear strength

Vicksburg loess is characterized by preferred orientation of constituent grains, which on the average dip 4° toward the west (N 80–85°W). This investigation was undertaken to study, quantitatively, the relationship between natural fabric anisotropies of Vicksburg loess and the orientation of applied stress distribution Results of the study indicate the fabric anisotropies in Vicksburg loess are reflected by definite variation in triaxial shear strength of dry and moist specimens.

In two series of triaxial tests, ultimate strength of the loess is maximum where σ₁ is perpendicular to grain orientation, and it is reduced where the principal stresses are 45° to the fabric plane. In this respect, Vicksburg loess may serve as a structural model for granular earth materials in illustrating the influence of fabric on ultimate strength. Moreover, the Mohr-Coulomb fracture line consists of two line segments, with an increase in slope at higher confining pressure. This characteristic suggests that poorly-cemented sands, or sandstones, and silts, or siltstones, may undergo two failures: one at small strains where cement bonds are disrupted and the other at larger strains where internal shearing resistance of granular components is exceeded.