Dynamic river–groundwater exchange in the presence of a saline,semi‐confined aquifer

Understanding groundwater–surface water exchange in river banks is crucial for effective water management and a range of scientific disciplines. While there has been much research on bank storage, many studies assume idealized aquifer systems. This paper presents a field‐based study of the Tambo Catchment (southeast Australia) where the Tambo River interacts with both an unconfined aquifer containing relatively young and fresh groundwater (<500 μS/cm and <100 years old) and a semi‐confined artesian aquifer containing old and saline groundwater (electrical conductivity > 2500 μS/cm and >10 000 years old). Continuous groundwater elevation and electrical conductivity monitoring within the different aquifers and the river suggest that the degree of mixing between the two aquifers and the river varies significantly in response to changing hydrological conditions. Numerical modelling using MODFLOW and the solute transport package MT3DMS indicates that saline water in the river bank moves away from the river during flooding as hydraulic gradients reverse. This water then returns during flood recession as baseflow hydraulic gradients are re‐established. Modelling also indicates that the concentration of a simulated conservative groundwater solute can increase for up to ~34 days at distances of 20 and 40 m from the river in response to flood events approximately 10 m in height. For the same flood event, simulated solute concentrations within 10 m of the river increase for only ~15 days as the infiltrating low‐salinity river water drives groundwater dilution. Average groundwater fluxes to the river stretch estimated using Darcy's law were 7 m³/m/day compared with 26 and 3 m³/m/day for the same periods via mass balance using Radon (²²²Rn) and chloride (Cl), respectively. The study shows that by coupling numerical modelling with continuous groundwater–surface water monitoring, the transient nature of bank storage can be evaluated, leading to a better understanding of the hydrological system and better interpretation of hydrochemical data. Copyright © 2015 John Wiley & Sons, Ltd.     

On the pore‐scale mechanisms leading to brittle and ductile deformation behavior of crystalline rocks

Deformation mechanisms at the pore scale are responsible for producing large strains in porous rocks. They include cataclastic flow, dislocation creep, dynamic recrystallization, diffusive mass transfer, and grain boundary sliding, among others. In this paper, we focus on two dominant pore‐scale mechanisms resulting from purely mechanical, isothermal loading: crystal plasticity and crofracturing. We examine the contributions of each mechanism to the overall behavior at a scale larger than the grains but smaller than the specimen, which is commonly referred to as the mesoscale. Crystal plasticity is assumed to occur as dislocations along the many crystallographic slip planes, whereas microfracturing entails slip and frictional sliding on microcracks. It is observed that under combined shear and tensile loading, microfracturing generates a softer response compared with crystal plasticity alone, which is attributed to slip weakening where the shear stress drops to a residual level determined by the frictional strength. For compressive loading, however, microfracturing produces a stiffer response than crystal plasticity because of the presence of frictional resistance on the slip surface. Behaviors under tensile, compressive, and shear loading invariably show that porosity plays a critical role in the initiation of the deformation mechanisms. Both crystal plasticity and microfracturing are observed to initiate at the peripheries of the pores, consistent with results of experimental studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Higher tritium concentrations measured in permafrost thaw lakes in northern Alberta

Tritium concentrations were measured in a survey of 24 lakes, 15 wetlands, and 133 groundwaters in the oil sands region of northeastern Alberta and compared with both recent precipitation and precipitation sampled during the 1960s tritium peak caused by atmospheric thermonuclear weapons testing. Water samples from lakes included a group of 14 thaw lakes that had higher runoff attributed to melting of permafrost in peat plateaus within their watersheds. While tritium in all lakes was found to be intermediate between recent and 1960s concentrations, the thaw lakes were found to be significantly enriched in tritium compared with other lakes, as were unfrozen wetlands characterized by a thick sequence of low‐hydraulic conductivity peat. The results provide further evidence of different water sources to the thaw lakes and may indicate that melting of modern permafrost in part formed since the 1950s is occurring in these systems. Copyright © 2015 John Wiley & Sons, Ltd.     

Energy partitioning over a semi‐arid shrubland in northern China

During the last decade, the widely distributed shrublands in northern China have shown significant signs of recovery from desertification, the result of widespread conservation practices. However, to support the current efforts in conservation, more knowledge is needed on surface energy partitioning and its biophysical controls. Using eddy‐covariance measurements made over a semi‐arid shrubland in northwest China in 2012, we examined how surface energy‐balance components vary on diurnal and seasonal scales, and how biophysical factors control bulk surface parameters and energy exchange. Sensible heat flux (H) exceeded latent heat flux (λE) during most of the year, resulting in an annual Bowen ratio (β, i.e. H/λE) of 2.0. λE exceeded H only in mid‐summer when frequent rainfall co‐occurred with the seasonal peak in leaf area index (LAI). Evapotranspiration reached a daily maximum of 3.3 mm day^?1, and summed to 283 mm yr^?1. The evaporative fraction (EF, i.e. λE/R_n), Priestley–Taylor coefficient (α), surface conductance (g_s) and decoupling coefficient (Ω) were all positively correlated with soil water content (SWC) and LAI. The direct enhancement of λE by high vapour pressure deficit (VPD) was buffered by a concurrent suppression of g_s. The g_s played a direct role in controlling EF and α by mediating the effects of LAI, SWC and VPD. Our results highlight the importance of adaptive plant responses to water scarcity in regulating ecosystem energy partitioning, and suggest an important role for revegetation in the reversal of desertification in semi‐arid areas. Copyright © 2015 John Wiley & Sons, Ltd.     

A gradual transition to plate tectonics on Earth between 3.2 to 2.7 billion years ago

Zircon crystals precipitated from granitoid magmas contain a robust record of the age and chemistry of continental magmatism spanning some 4.375 Ga of Earth history, a record that charts initiation of plate tectonics. However, constraining when exactly plate tectonics began to dominate crustal growth processes is challenging as the geochemical signatures of individual rocks may reflect local subduction processes rather than global plate tectonics. Here we apply counting statistics to a global database of coupled U–Pb and Hf isotope analyses on magmatic zircon grains from continental igneous and sedimentary rocks to quantify changes in the compositions of their source rocks. The analysis reveals a globally significant change in the sources of granitoid magmas between 3.2 and 2.7 Ga. These secular changes in zircon chemistry are driven by a coupling of the deep (depleted mantle) and shallow (crustal) Earth reservoirs, consistent with a geodynamic regime dominated by Wilson cycle style plate tectonics.     

The impact of rural land management changes on soil hydraulic properties and runoff processes: results from experimental plots in upland UK

To develop an evidence base to help predict the impacts of land management change on flood generation, four experimental sites were established on improved grassland used for sheep grazing at the Pontbren catchment in upland Wales, UK. At each site, three plots were established where surface runoff was measured, supplemented by measurements of soil infiltration rates and soil and vegetation physical properties. Following baseline monitoring, treatments were applied to two of the plots: exclusion of sheep (ungrazed) and exclusion of sheep and planting with native broadleaf tree species (tree planted), with the third plot acting as a control (grazed pasture). Due to a particularly dry summer that occurred pre‐treatment, the soil hydrological responses were initially impacted by the effects of the climate on soil structure. Nevertheless, treatments did have a clear influence on soil hydrological response. On average, post‐treatment runoff volumes were reduced by 48% and 78% in ungrazed and tree‐planted plots relative to the control, although all results varied greatly over the sites. Five years following treatment application, near‐surface soil bulk density was reduced and median soil infiltration rates were 67 times greater in plots planted with trees compared to grazed pasture. The results illustrate the potential use of upland land management for ameliorating local‐scale flood generation but emphasise the need for long‐term monitoring to more clearly separate the effects of land management from those of climatic variability. Copyright © 2013 John Wiley & Sons, Ltd.     

New P–T constraints on the Tamayen glaucophane‐bearing rocks,eastern Taiwan: Perple_X modelling results and geodynamic implications

New pseudosection modelling was applied to better constrain the P–T conditions and evolution of glaucophane‐bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane‐bearing high‐P rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode): amphibole + quartz + epidote + garnet + chlorite + rutile/titanite (Type‐I), phengite + amphibole + quartz + garnet + chlorite + epidote + titanite + biotite + magnetite (Type‐II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type‐III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane → pargasitic amphibole → actinolite (Type‐I), barroisite → Mg‐katophorite/taramite → Fe‐glaucophane (Type‐II), glaucophane → winchite (Type‐III). Using petrographic data, mineral compositions and Perple_X modelling (pseudosections and superimposed isopleths), peak P–T conditions were determined as 13 ± 1 kbar and 550 ± 40 °C for Type‐I, 10.5 ± 0.5 kbar and 560 ± 30 °C for Type‐II (thermal peak) and 11 ± 1 kbar and 530 ± 30 °C for Type‐III. The calculations yield higher pressures and temperatures than previously thought; the difference is ~1–6 kbar and 50–200 °C. The three rock types record similar P–T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near‐isothermal conditions (Type‐I and Type‐III), with the probable exception of Type‐II where decompression followed colder geotherms. The P–T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of ~45 km within a subduction channel along a relative cold geothermal gradient of ~11–14 °C km^?1.