Authigenic carbonates from the Cascadia subduction zone and their relation to gas hydrate stability

Authigenic carbonates are intercalated with massive gas hydrates in sediments of the Cascadia margin. The deposits were recovered from the uppermost 50 cm of sediments on the southern summit of the Hydrate Ridge during the RV Sonne cruise SO110. Two carbonate lithologies that differ in chemistry, mineralogy, and fabric make up these deposits. Microcrystalline high-magnesium calcite (14 to 19 mol% MgCO3) and aragonite are present in both semiconsolidated sediments and carbonate-cemented clasts. Aragonite occurs also as a pure phase without sediment impurities. It is formed by precipitation in cavities as botryoidal and isopachous aggregates within pure white, massive gas hydrate. Variations in oxygen isotope values of the carbonates reflect the mineralogical composition and define two end members: a Mg-calcite with δ18O =4.86‰ PDB and an aragonite with δ18O =3.68‰ PDB. On the basis of the ambient bottom-water temperature and accepted equations for oxygen isotope fractionation, we show that the aragonite phase formed in equilibrium with its pore-water environment, and that the Mg-calcite appears to have precipitated from pore fluids enriched in 18O. Oxygen isotope enrichment probably originates from hydrate water released during gas-hydrate destabilization

Surface Complexation Modeling of Fluoride Adsorption by Soil and the Role of Dissolved Aluminum on Adsorption

The adsorption processes of F on a natural soil as a function of varying pH and F concentration were evaluated by applying a surface complexation model (SCM) based on a single surface functional group with monodentate binding sites. A granitic soil from Tsukuba, Japan, was chosen as an example, and the SCM was developed to explain the pH dependency of F sorption isotherms on the soil. Four possible surface complexation reactions were postulated with and without including dissolved Al. Optimized constants for F surface complexation, and those for protonation and deprotonation, were used for the simulations. The SCM including dissolved Al and the adsorption of Al–F complex can simulate the experimental results, the decreasing trend of F adsorption with the increase in pH, quite successfully. Also, including dissolved Al and the adsorption of Al–F complex to the model explained the change in solution pH after F adsorption. Therefore, incorporation of dissolved Al and Al–F complex in model calculations of a soil–F system is an important improvement to predict F concentrations of soil solutions

Time-dependent dispersion coefficients for the evolution of displacement fronts in heterogeneous porous media

We present an approach for quantifying displacement fronts in heterogeneous porous media based on the concept of time-dependent apparent dispersion coefficients. The concept of constant asymptotic macrodispersion generally overestimates the area swept by a displacement front and leads to unrealistic upstream dispersion. We show that the large-scale front spreading can be captured by a one-dimensional advection–dispersion equation that is parameterized by a suitably chosen temporally evolving dispersion coefficient. For purely advective front spreading, we derive an analytical expression based on a predictive continuous time random walk approach, which applies to highly heterogeneous porous media. This analysis elucidates the variability of solute travel times as the key longitudinal spreading mechanism. It shows that the evolution of dispersion can be captured as the sum of exponentials that decay on two dominant time scales. In a particle-based picture, these scales mark the short time at which transported particles start exploring the flow variability and the large time at which the slowest particles start decorrelating their transport velocity. Based on these insights, we propose a heuristic formula that accounts for the impact of local-scale dispersion as an additional decorrelation mechanism. The heuristic expression for the longitudinal dispersion coefficient captures solute spreading for a broad range of Péclet numbers and heterogeneity variances. The proposed approach is tested against direct numerical simulations. It provides a robust and fast method for quantifying the evolution of displacement fronts in heterogeneous porous media with possible applications, for example, in groundwater contamination modelling, underground gas storage, and geothermal energy production.The Authors are grateful for the insightful comments by Aldo Fiori. ST acknowledges the financial support by JSPS KAKENHI Grant Number JP23KJ0431 (Grant-in-Aid for JSPS Fellows). MD acknowledges the financial support through Grant TED2021-129991B-C33 (GreenHUGS) funded by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR .Peer reviewe

A New Method to Determine Hydraulic Conductivity and Storage Coefficient through Simultaneous Measurements of Fluid Pressure and Strains

The concept of storage coefficient was discussed based on the theory of poroelasticity. Several different storage coefficients can be defined by different mechanical boundary conditions and assumptions on the physical properties of constitutive materials. The specific storage, which is usually used in the field of hydrogeology, is shown to be defined when the representative elementary volume is maintained in a state of zero lateral strain and constant stress perpendicular to that plane. This means that the specific storage is not measured in most laboratory pore pressure tests because the boundary condition of zero lateral strain is not satisfied. Instead, we measure a three-dimensional storage coefficient. In the latter sections of this paper, we present a new method to determine both the hydraulic conductivity and the storage coefficient through simultaneous measurements of fluid pressure and strains. In this study, a new endplug with a built-in valve was developed to accurately measure the poroelastic parameters. Our experimental assembly significantly reduces the extra volume of the system and is readily adapted to the various pore-fluid boundary conditions. The three-dimensional storage coefficient was calculated from the volumetric poroelastic parameters obtained from quasi-static strain data, and the hydraulic conductivity from the transient pore pressure diffusion data. Transient strain behavior during the pore pressure diffusion stage was used to self-check the accuracy of the parameters obtained. This technique does not require complicated inversion calculations and can be used easily for parameter identification

Effects of terrigenic He components on tritium–helium dating: A case study of shallow groundwater in the Saijo Basin

Dating using a combination of 3H and 3He is believed to be the most practical method for estimating the short residence time of shallow groundwater. However, this method must estimate tritiogenic 3He alone and tends to overestimate the residence time of groundwater, if terrigenic 3He from the mantle cannot be excluded from the total dissolved 3He. We demonstrate the exclusion of terrigenic 3He in the Saijo Basin, where mantle He is easily released along the major active fault, Median Tectonic Line. The 3He/4He ratios suggest that the west bank of the Kamo River, which lies within the basin, has experienced greater emanations of mantle He than the east bank. We estimate the residence times to be 1.1–96 years by the proposed exclusion method

Groundwater Flooding on Atolls Caused by Storm Surges: Effects of the Dual-Aquifer Configuration

Storm surges associated with tropical cyclones endanger atolls through groundwater flooding, where groundwater is discharged from the land surface as the sea level rises. Atolls are characterized by a “dual-aquifer” configuration, where recent Holocene sediments unconformably overlie highly permeable Pleistocene limestone, creating an interface called a “Thurber discontinuity.” This study aimed to quantitatively analyze how the dual-aquifer configuration of atolls controls the temporal dynamics of groundwater flooding caused by storm surge. To this end, we ran surface-subsurface coupled synthetic numerical simulations using HydroGeoSphere and compared 12 scenarios with different Thurber discontinuity elevations and hydraulic conductivities of the Pleistocene aquifer (KP). The results showed that the shallower the Thurber discontinuity and the higher the KP value, the higher the maximum water depth in the freshwater swamp on the atoll during the storm surge and the longer the flooding duration. Despite the effects of the different dual-aquifer configurations, the initial water table elevation and salinity distribution were almost identical in all the simulation cases. These findings suggest that accurate information on the dual-aquifer configuration is necessary to evaluate the potential risk of groundwater flooding on atolls accompanying storm surges. Furthermore, the results indicate that groundwater flooding caused by storm surges substantially contributes to cyclone-driven flooding on atolls, and hence, it should not be neglected in flood predictions to avoid underestimation.The authors are grateful to the three anonymous reviewers for their insightful comments and invaluable suggestions. This study was financially supported by a JSPS Bilateral Joint Research Project (JPJSBP-120219908). S. Tajima thanks the financial support of the World-Leading Innovative Graduate Study Program in Proactive Environmental Studies (WINGS-PES) at the University of Tokyo.Peer reviewe

Integral Approach to Environmental Leadership Education : An Exploration in the Heihe River Basin, Northwestern China

An education program called Asian Program for Incubation of Environmental Leaders (APIEL) was established in 2008 by the University of Tokyo. The need arises to equip the students with a wide knowledge base and practical skills related to global and regional environmental issues. This paper explores an integral framework for environmental leadership education, and show how this framework has been used to conduct one of the APIEL field exercises in the Heihe River basin, Northwestern Arid China. The framework was developed by modifying Ken Wilber’s 4-quadrant approach. Our conclusion is that the use of an integral approach is effective in not only understanding environmental issues of complexity, but also development and management of environmental leadership education programs

Integral Approach to Environmental Leadership Education : An Exploration in the Heihe River Basin, Northwestern China

An education program called Asian Program for Incubation of Environmental Leaders (APIEL) was established in 2008 by the University of Tokyo. The need arises to equip the students with a wide knowledge base and practical skills related to global and regional environmental issues. This paper explores an integral framework for environmental leadership education, and show how this framework has been used to conduct one of the APIEL field exercises in the Heihe River basin, Northwestern Arid China. The framework was developed by modifying Ken Wilber’s 4-quadrant approach. Our conclusion is that the use of an integral approach is effective in not only understanding environmental issues of complexity, but also development and management of environmental leadership education programs

Stress state in the Cuu Long and Nam Con Son basins, offshore Vietnam

The results presented in this paper are the first published estimates of the complete stress tensor in the Cuu Long and Nam Con Son basins, offshore Vietnam. We analysed in situ stress and pore pressure fields in the sedimentary formations using data from petroleum exploration and production wells to evaluate the stress state in these basins. The data were obtained from the seafloor to 4300 m burial depth and include both hydrostatic and overpressured sections. Minimum horizontal stresses were obtained from extended leak-off tests and mini-fracture tests. Maximum horizontal stresses were estimated from drilling-induced fracture parameters and borehole breakout widths in twelve wells using rock properties measured in the laboratory or estimated empirically from wireline logs. Seven data points are located in sediments, and seventeen data points in igneous basement rocks at depths greater than 3000 m. The estimated magnitudes of sH are 70-110% of the sv magnitudes. Considering the errors in the stress magnitude estimates, their relative magnitudes suggest that a borderline normal/strike-slip stress regime presently exists in normally pressured sequences of the Nam Con Son and Cuu Long basins. Of the twenty-four data points, twenty have effective stress ratios at a critical stress state for faulting on the assumption that there are faults present that are optimally oriented for failure with friction coefficients of 0.5. The results suggest that the stress state in these basins is generally critical