U–Pb dating of zircon in hydrothermally altered rocks as a correlation tool: Application to the Mangakino geothermal field,New Zealand

Recognition and correlation of rock units within geothermal fields is often hampered by high degrees of alteration that obscure primary mineralogies and lithological boundaries, and preclude direct dating by radiometric techniques. Magmatic zircons are commonly present in silicic volcanic rocks, where zircon saturation was achieved and zircons crystallized up to the point of eruption. Young zircons are highly resistant to hydrothermal alteration and can yield a record of their crystallization ages in otherwise heavily altered rocks. Zircon crystallization-age spectra have been obtained by SIMS techniques (SHRIMP-RG) from three samples of cuttings and a core sample from ignimbrite penetrated in 3 drillholes up to ~ 3.2 km deep at the Mangakino geothermal field in New Zealand. The crystallization ages are similar between the drillcore and cutting samples, indicating that downhole mixing of cuttings has not been important, and showing collectively that volcanic units of closely similar ages are represented between ~ 1.4 and ~ 3.2 km depth. This is despite apparent changes in the inferred primary volcanic lithology that had led to earlier inferences that multiple ignimbrites of contrasting age were present in this depth interval. Comparisons of zircon crystallization-age spectra and inferred primary mineralogical characteristics from the drillhole samples with surficial ignimbrites that crop out west of Mangakino suggest that the boreholes have entered a > 1.8-km-thick intracaldera fill of ignimbrite generated in the closely-spaced Kidnappers and Rocky Hill eruptions at ~ 1 Ma.     

Residence time of bedform-driven hyporheic exchange

Biogeochemical and ecological transformations in hyporheic zones are dependent on the timing of hyporheic exchange. We show through linked modeling of open channel turbulent flow, groundwater flow, and solute transport that the residence time distributions of solutes advected by hyporheic flow induced by current–bedform interaction follow power-laws. This tailing behavior of solutes exiting the sediments is explained by the presence of multiple path lengths coupled with very large variability in Darcy flow velocity, both occurring without heterogeneity in sediment permeability. Hyporheic exchange through bedforms will result in short-time fractal scaling of stream water chemistry.     

Quantifying River‐Groundwater Interactions of New Zealand's Gravel‐Bed Rivers: The Wairau Plain

New Zealand's gravel‐bed rivers have deposited coarse, highly conductive gravel aquifers that are predominantly fed by river water. Managing their groundwater resources is challenging because the recharge mechanisms in these rivers are poorly understood and recharge rates are difficult to predict, particularly under a more variable future climate. To understand the river‐groundwater exchange processes in gravel‐bed rivers, we investigate the Wairau Plain Aquifer using a three‐dimensional groundwater flow model which was calibrated using targeted field observations, “soft” information from experts of the local water authority, parameter regularization techniques, and the model‐independent parameter estimation software PEST. The uncertainty of simulated river‐aquifer exchange flows, groundwater heads, spring flows, and mean transit times were evaluated using Null‐space Monte‐Carlo methods. Our analysis suggests that the river is hydraulically perched (losing) above the regional water table in its upper reaches and is gaining downstream where marine sediments overlay unconfined gravels. River recharge rates are on average 7.3 m³/s, but are highly dynamic in time and variable in space. Although the river discharge regularly hits 1000 m³/s, the net exchange flow rarely exceeds 12 m³/s and seems to be limited by the physical constraints of unit‐gradient flux under disconnected rivers. An important finding for the management of the aquifer is that changes in aquifer storage are mainly affected by the frequency and duration of low‐flow periods in the river. We hypothesize that the new insights into the river‐groundwater exchange mechanisms of the presented case study are transferable to other rivers with similar characteristics.     

The role of soil pipe and pipeflow in headcut migration processes in loessic soils

Headcut formation and migration was sometimes mistaken as the result of overland flow, without realizing that the headcut was formed and being influenced by flow through soil pipes into the headcut. To determine the effects of the soil pipe and flow through a soil pipe on headcut migration in loessic soils, laboratory experiments were conducted under free drainage conditions and conditions of a perched water table. Soil beds with a 3-cm deep initial headcut were formed in a flume with a 1.5-cm diameter soil pipe 15 cm below the bed surface. Overland flow and flow into the soil pipe was applied at a constant rate of 68 and 1 l min⁻¹ at the upper end of the flume. The headcut migration rate and sediment concentrations in both surface (channel) and subsurface (soil pipe) flows were measured with time. The typical response was the formation of a headcut that extended in depth until an equilibrium scour hole was established, at which time the headcut migrated upslope. Pipeflow caused erosion inside the soil pipe at the same time that runoff was causing a scour hole to deepen and migrate. When the headcut extended to the depth of the soil pipe, surface runoff entering the scour hole interacted with flow from the soil pipe also entering the scour hole. This interaction dramatically altered the headcut processes and greatly accelerated the headcut migration rates and sediment concentrations. Conditions in which a perched water table provided seepage into the soil pipe, in addition to pipeflow, increased the sediment concentration by 42% and the headcut migration rate by 47% compared with pipeflow under free drainage conditions. The time that overland flow converged with subsurface flow was advanced under seepage conditions by 2.3 and 5.0 min compared with free drainage conditions. This study confirmed that pipeflow dramatically accelerates headcut migration, especially under conditions of shallow perched water tables, and highlights the importance of understanding these processes in headcut migration processes. © 2020 John Wiley & Sons, Ltd.     

Compositional variability on the surface of 1 Ceres revealed through GRaND measurements of high‐energy gamma rays

High‐energy gamma rays (HEGRs) from Ceres’s surface were measured using Dawn's Gamma Ray and Neutron Detector (GRaND). Models of cosmic‐ray‐initiated gamma ray production predict that the HEGR flux will inversely vary with single‐layer hydrogen concentrations for Ceres‐like compositions. The measured data confirm this prediction. The hydrogen‐induced variations in HEGR rates were decoupled from the measurements by detrending the HEGR data with Ceres single‐layer hydrogen concentrations determined by GRaND neutron measurements. Models indicate that hydrogen‐detrended HEGR counting rates correlate with water‐free average atomic mass, which is denoted as <A>*. HEGR variations across Ceres’s surface are consistent with <A>* variations of ±0.5 atomic mass units. Chemical variations in the CM and CI chondrites, our closest analogs to Ceres’s surface, suggest that <A>* variations on Ceres are primarily driven by variations in the concentration of Fe, although other elements such as Mg and S could contribute. Dawn observations have shown that Ceres’s interior structure and surface composition have been modified by some combination of physical (i.e., ice‐rock fractionation) and/or chemical (i.e., alteration) processes that has led to variations in bulk surface chemistry. Locations of the highest inferred <A>* values, and thus possibly the highest Fe and least altered materials, tend to be younger, less cratered surfaces that are broadly associated with the impact ejecta of Ceres’s largest craters.     

The outflow of Windermere,Cumbria: A re-appraisal

Possible causes for the outflow of Windermere through the Leven valley instead of the Cartmel valley are discussed. An earlier seismic investigation of the problem by Coster and Gerrard (1947) is reviewed and shown to be inconclusive. Further geophysical surveying and mapping demonstrate a rock bar in the Cartmel valley, 3km southeast of Newby Bridge. This barrier is 21m above the present water level in Windermere. Blockage by glacial deposits is disproved and the probable history of the outflow discussed. It is concluded that the outflow from Windermere has used the Leven valley from an early stage.     

A Stochastic Time Series Model for Threshold Crossing Statistics of Concentration Fluctuations in non-Intermittent Plumes

A numerical stochastic model is developed for the upcrossing rate across a specified threshold concentration. The model assumes that the concentration time series at a given spatial point within a dispersing plume can be approximated as a first-order Markovian process designed to be consistent with a given time-invariant concentration probability density function (pdf). The model requires only the specification of a concentration pdf with a given mean and variance and a concentration fluctuation integral time scale. Predicted upcrossing rates are compared with atmospheric plume concentration data obtained from a point source near the ground. For this data set, a log-normal pdf is found to give better estimates of the threshold crossing rate than a gamma pdf.     

Some Distinctions Between Self-Similar and Self-Affine Estimates of Fractal Dimension with Case History

Compass, power-spectral, and roughness-length estimates of fractal dimension are widely used to evaluate the fractal characteristics of geological and geophysical variables. These techniques reveal self-similar or self-affine fractal characteristics and are uniquely suited for certain analysis. Compass measurements establish the self-similarity of profile and can be used to classify profiles based on variations of profile length with scale. Power spectral and roughness-length methods provide scale-invariant self-affine measures of relief variation and are useful in the classification of profiles based on relative variation of profile relief with scale. Profile magnification can be employed to reduce differences between the compass and power-spectral dimensions; however, the process of magnification invalidates estimates of profile length or shortening made from the results. The power-spectral estimate of fractal dimension is invariant to magnification, but is generally subject to significant error from edge effects and nonstationarity. The roughness-length estimate is also invariant to magnification and in addition is less sensitive to edge effects and nonstationarity. Analysis of structural cross sections using these methods highlight differences between self-similar and self-affine evaluations. Shortening estimates can be made from the compass walk analysis that includes shortening contributions from predicted small-scale structure. Roughness-length analysis reveals systematic structural changes that, however, cannot be easily related to strain. Power-spectral analysis failed to extract useful structural information from the sections.