Grain coarsening in contact metamorphic carbonates: effects of second-phase particles, fluid flow and thermal perturbations

Under contact metamorphic conditions, carbonate rocks in the direct vicinity of the Adamello pluton reflect a temperature‐induced grain coarsening. Despite this large‐scale trend, a considerable grain size scatter occurs on the outcrop‐scale indicating local influence of second‐order effects such as thermal perturbations, fluid flow and second‐phase particles. Second‐phase particles, whose sizes range from nano‐ to the micron‐scale, induce the most pronounced data scatter resulting in grain sizes too small by up to a factor of 10, compared with theoretical grain growth in a pure system. Such values are restricted to relatively impure samples consisting of up to 10 vol.% micron‐scale second‐phase particles, or to samples containing a large number of nano‐scale particles. The obtained data set suggests that the second phases induce a temperature‐controlled reduction on calcite grain growth. The mean calcite grain size can therefore be expressed in the form D = C₂ e^Q*/RT(d_p/f_p)^m*, where C₂ is a constant, Q* is an activation energy, T the temperature and m* the exponent of the ratio d_p/f_p, i.e. of the average size of the second phases divided by their volume fraction. However, more data are needed to obtain reliable values for C₂ and Q*. Besides variations in the average grain size, the presence of second‐phase particles generates crystal size distribution (CSD) shapes characterized by lognormal distributions, which differ from the Gaussian‐type distributions of the pure samples. In contrast, fluid‐enhanced grain growth does not change the shape of the CSDs, but due to enhanced transport properties, the average grain sizes increase by a factor of 2 and the variance of the distribution increases. Stable δ¹⁸O and δ¹³C isotope ratios in fluid‐affected zones only deviate slightly from the host rock values, suggesting low fluid/rock ratios. Grain growth modelling indicates that the fluid‐induced grain size variations can develop within several ka. As inferred from a combination of thermal and grain growth modelling, dykes with widths of up to 1 m have only a restricted influence on grain size deviations smaller than a factor of 1.1. To summarize, considerable grain size variations of up to one order of magnitude can locally result from second‐order effects. Such effects require special attention when comparing experimentally derived grain growth kinetics with field studies.     

Assessment of boundary-element method for modelling a free-floating sloped wave energy device. Part 1: Numerical modelling

The boundary-element method has been widely used as a design tool in the offshore and ship building industry for more than 30 years. Its application to wave energy conversion is, however, more recent. This paper deals with the numerical modelling of a free-floating sloped wave energy device. The power take-off mechanism of the device consists of an immersed tube with a piston sliding inside. The modelling is done using the boundary-element method package WAMIT. The model is first worked out for the case where the axis of the tube is vertical. It is then derived for the tube inclined and successfully verified against numerical benchmark data. A companion paper presents results of a detailed comparison with a physical model study.     

Influence of the vertical distribution of cyanobacteria in the water column on the remote sensing signal

An increased intensity of cyanobacterial blooms and their potentially harmful effects have attracted the attention of environmental agencies, water authorities and the general public worldwide. Reliable operational monitoring methods of coastal waters, lakes and ponds are needed. Mapping of the surface extent of cyanobacterial blooms with remote sensing is straightforward, but recognizing waters dominated by cyanobacteria throughout the water column and quantitative mapping of cyanobacterial biomass with remote sensing is more complicated. Unlike most algae, cyanobacteria can regulate their buoyancy and move vertically in the water column. We used the Hydrolight 4.2 radiative transfer model and the specific optical properties of three species of cyanobacteria to study the impact of vertical distribution of cyanobacteria on the remote sensing signal. The results show that the vertical distribution of cyanobacteria in the water column has a significant impact on the remote sensing signal. This result indicates that developing remote sensing methods for quantitative mapping of cyanobacterial biomass is much more complex than quantitative mapping of an algal biomass that is uniformly distributed in the top mixed layer of water column.     

Predicting suitable habitat for the cold-water coral Lophelia pertusa (Scleractinia)

Ecological-niche factor analysis (ENFA) was applied to the reef framework-forming cold-water coral Lophelia pertusa. The environmental tolerances of this species were assessed using readily available oceanographic data, including physical, chemical, and biological variables. L. pertusa was found at mean depths of 468 and 480 m on the regional and global scales and occupied a niche that included higher than average current speed and productivity, supporting the theory that their limited food supply is locally enhanced by currents. Most records occurred in areas with a salinity of 35, mean temperatures of 6.2–6.7  °C and dissolved oxygen levels of 6.0–6.2 ml l⁻¹. The majority of records were found in areas that were saturated with aragonite but had low concentration of nutrients (silicate, phosphate, and nitrate). Suitable habitat for L. pertusa was predicted using ENFA on a global and a regional scale that incorporated the north-east Atlantic Ocean. Regional prediction was reliable due to numerous presence points throughout the area, whereas global prediction was less reliable due to the paucity of presence data outside of the north-east Atlantic. However, the species niche was supported at each spatial scale. Predicted maps at the global scale reinforced the general consensus that the North Atlantic Ocean is a key region in the worldwide distribution of L. pertusa. Predictive modelling is an approach that can be applied to cold-water coral species to locate areas of suitable habitat for further study. It may also prove a useful tool to assist spatial planning of offshore marine protected areas. However, issues with eco-geographical datasets, including their coarse resolution and limited geographical coverage, currently restrict the scope of this approach.     

鄂尔多斯盆地塔巴庙地区上古生界源岩热演化特征

鄂尔多斯盆地北部主力气源岩太原组、山西组煤系地层热成熟史的研究对本区天然气充注过程和有利目标区预测具有重要的参考价值。在对研究区烃源岩评价和一维、二维地质建模研究的基础上。利用BasinMod盆地模拟软件对单井以及研究区内二维剖面、平面进行了煤系烃源岩热演化史模拟研究。研究结果表明：（1）该区在中三叠世进入生烃门限,中侏罗世以后,烃源岩持续埋深,早白垩世末期至最大埋深（4000m左右）,绝大多数的天然气都在这一阶段生成,早白垩世末构造抬升以后只有少量天然气生成;（2）研究区上古生界太原组和山西组煤系源岩最大累积生烃强度可达到2200×10^8m^3／km^2,对现今天然气的分布具有较强的控制作用。     

Developing a conceptual model of marine farming in New Zealand

Survey and Geographic Information System (GIS) data analysis describes the relative influence of biophysical and human variables on site choices made by marine farmers in New Zealand. Community conflicts have grown in importance in determining farm location and different government planning strategies leave distinct signature patterns. Recent legislation empowers local governments to choose among three strategies for future regional aquaculture development. This paper suggests each strategy could result in different spatial outcomes. Simulation modelling of the type described here can provide a better understanding of farmer responses to management approaches and the range of futures that could result from planning choices made today.     

Does compaction-induced subsidence control accommodation space at the top of prograding carbonate platforms? Constraints from the numerical modelling of the Triassic Esino Limestone (Southern Alps,Italy)

The demise of the high-relief, steep-slope, prograding Ladinian-Early Carnian carbonate platforms of the Esino Limestone (Central Southern Alps of Italy) is marked by subaerial exposure of the platform top associated with different erosional (mainly karst-related), depositional and diagenetic processes (Calcare Rosso). The exposure-related deposits consist of three major facies associations: 1) residual soils with thin lenses of conglomerates with black pebbles, and, locally, weathered vulcanites; 2) chaotic breccia lenses irregularly distributed in the uppermost part of the Esino Limestone carbonate platform, interpreted as collapse breccias in karstic setting: 3) inter-supratidal carbonate cycles with dissolution and development of paleosols and tepee structures.Facies distribution follows the sub-environments of the underlying Esino Limestone. Facies 1 and 2 typically characterize the core of the platform, covering the underlying inner platform facies. Facies 3 instead develops toward the edge of the platform, above reef-upper slope facies of the prograding facies of the Esino Limestone. The thickness of facies 3 decreases toward the core of the platform. Facies distribution reflects differences in the accommodation space and sedimentary processes from the rim (highest accommodation, favouring the deposition of peritidal-supratidal carbonates) to the core (reduced accommodation, causing pedogenesis and karstification) of the carbonate system.The observed thickness changes may be controlled by different factors: 1) syndepositional tectonics, 2) subsidence induced by magmatic activity or 3) differential subsidence controlled by the stratigraphic architecture of the Esino Limestone platform and adjoining basins. As evidence of tectonics was not observed and the presence of volcanic bodies is only documented tens of km away from the study area, the scenario involving the creation of accommodation space by compaction of the basinal sediments (resedimented, fine-grained calciturbidites) during the progradation of the carbonate platform is here investigated. Numerical modelling was performed to verify the compatibility of compaction-induced subsidence with the observed depositional architecture. The models were built to simulate the architectural evolution of the platform by progressively adding layers from deepest to shallowest, while compacting the underlying sediments, in order to evaluate compaction-induced subsidence (and accommodation space for the Calcare Rosso) after the deposition of the youngest platform strata. Modelling results allow us to conclude that the wedge geometry of the Calcare Rosso, deposited on top of the extinct Esino carbonate platform, can be explained by subsidence controlled by compaction of the basinal sediments present below the early-cemented, fast prograding platform slope deposits.     

Comparison of direct statistical and indirect statistical-deterministic frameworks in downscaling river low-flow indices

ABSTRACT

This work explores the ability of two methodologies in downscaling hydrological indices characterizing the low flow regime of three salmon rivers in Eastern Canada: Moisie, Romaine and Ouelle. The selected indices describe four aspects of the low flow regime of these rivers: amplitude, frequency, variability and timing. The first methodology (direct downscaling) ascertains a direct link between large-scale atmospheric variables (the predictors) and low flow indices (the predictands). The second (indirect downscaling) involves downscaling precipitation and air temperature (local climate variables) that are introduced into a hydrological model to simulate flows. Synthetic flow time series are subsequently used to calculate the low flow indices. The statistical models used for downscaling low flow hydrological indices and local climate variables are: Sparse Bayesian Learning and Multiple Linear Regression. The results showed that direct downscaling using Sparse Bayesian Learning surpassed the other approaches with respect to goodness of fit and generalization ability.

Editor D. Koutsoyiannis; Associate editor K. Hamed     

Approximating constant‐Q seismic propagation in the time domain

In this study, we investigate the accuracy of approximating constant‐Q wave propagation by series of Zener or standard linear solid (SLS) mechanisms. Modelling in viscoacoustic and viscoelastic media is implemented in the time domain using the finite‐difference (FD) method. The accuracy of numerical solutions is evaluated by comparison with the analytical solution in homogeneous media. We found that the FD solutions using three SLS relaxation mechanisms as well as a single SLS mechanism, with properly chosen relaxation times, are quite accurate for both weak and strong attenuation. Although the RMS errors of FD simulations using a single relaxation mechanism increase with increasing offset, especially for strong attenuation (Q = 20), the results are still acceptable for practical applications. The synthetic data of the Marmousi‐II model further illustrate that the single SLS mechanism, to model constant Q, is efficient and sufficiently accurate. Moreover, it benefits from less computational costs in computer time and memory.