Development of spatial regression models for predicting summer river temperatures from landscape characteristics: Implications for land and fisheries management

There is increasing demand for models that can accurately predict river temperature at the large spatial scales appropriate to river management. This paper combined summer water temperature data from a strategically designed, quality controlled network of 25 sites, with recently developed flexible spatial regression models, to understand and predict river temperature across a 3,000 km² river catchment. Minimum, mean and maximum temperatures were modelled as a function of nine potential landscape covariates that represented proxies for heat and water exchange processes. Generalised additive models were used to allow for flexible responses. Spatial structure in the river network data (local spatial variation) was accounted for by including river network smoothers. Minimum and mean temperatures decreased with increasing elevation, riparian woodland and channel gradient. Maximum temperatures increased with channel width. There was greater between‐river and between‐reach variability in all temperature metrics in lower‐order rivers indicating that increased monitoring effort should be focussed at these smaller scales. The combination of strategic network design and recently developed spatial statistical approaches employed in this study have not been used in previous studies of river temperature. The resulting catchment scale temperature models provide a valuable quantitative tool for understanding and predicting river temperature variability at the catchment scales relevant to land use planning and fisheries management and provide a template for future studies.     

Quantification of contemporary storm-induced boulder transport on an intertidal shore platform using radio frequency identification technology

Extreme storm events are known to produce, entrain, transport and deposit sizable boulders along rocky coastlines. However, the extent to which these processes occur under moderate, fetch-limited wave conditions is seldom considered. In this study we quantify boulder transport at a relatively sheltered location subject to high-frequency, low-magnitude storm activity. This was achieved by deploying radio frequency identification (RFID) tags within 104 intertidal limestone boulders ranging in size from fine to very coarse (intermediate axis: 0.27–2.85 m). The study was conducted over 3 years (July 2015–July 2018) and encompassed numerous storm events. Tagged boulders were relocated during 17 field surveys and their positions recorded using a differential global positioning navigation satellite system (DGNSS). On completion, we identified boulder displacement in 69% of the tagged array. The accrued boulder transport distance amounted to 233.0 m from 195 incidents of displacement, including the movement of a boulder weighing an estimated 11.9 t. Transport was not confined to autumn and winter storms alone, as displacement was also recorded during summer months (April–September), despite the seasonally reduced wave magnitude. Boulder production by wave quarrying was documented in three tagged clasts, confirming observations that the shore platform is actively eroding. Incidents of overturning during transport were also recorded, including multiple overturning of clasts weighing up to 5 t. We further identify a statistically significant difference (maximum p-value ≤ 0.03) between the transport distances attributed to constrained and unconstrained boulders, suggesting that the pre-transport morphological setting exerts considerable control over boulder transport potential. The findings establish low to moderate storm waves as a key component in the evolution of the study site. More broadly, we claim that high-frequency, low-magnitude storms regularly modify these overlooked rocky coastal locations, suggesting that the hydrodynamic capability at such sites may previously have been underestimated. © 2020 John Wiley & Sons, Ltd.     

Farewell to Wynand Kleingeld (1946 to 2020)

Mathematical Geosciences -     

Heterogeneity in ground water–surface water interactions in the hyporheic zone of a salmonid spawning stream

Spatial and temporal variability in ground water–surface water interactions in the hyporheic zone of a salmonid spawning stream was investigated. Four locations in a 150‐m reach of the stream were studied using hydrometric and hydrochemical tracing techniques. A high degree of hydrological connectivity between the riparian hillslope and the stream channel was indicated at two locations, where hydrochemical changes and hydraulic gradients indicated that the hyporheic zone was dominated by upwelling ground water. The chemistry of ground water reflected relatively long residence times and reducing conditions with high levels of alkalinity and conductivity, low dissolved oxygen (DO) and nitrate. At the other locations, connectivity was less evident and, at most times, the hyporheic zone was dominated by downwelling stream water characterized by high DO, low alkalinity and conductivity. Substantial variability in hyporheic chemistry was evident at fine (<10 m) spatial scales and changed rapidly over the course of hydrological events. The nature of the hydrochemical response varied among locations depending on the strength of local ground water influence. It is suggested that greater emphasis on spatial and temporal heterogeneity in ground water–surface water interactions in the hyporheic zone is necessary for a consideration of hydrochemical effects on many aspects of stream ecology. For example, the survival of salmonid eggs in hyporheic gravels varied considerably among the locations studied and was shown to be associated with variation in interstitial chemistry. River restoration schemes and watershed management strategies based only on the surface expression of catchment characteristics risk excluding consideration of potentially critical subsurface processes. Copyright © 2002 John Wiley & Sons, Ltd.     

Discovery of a single faint AGN in a large sample of z > 5 Lyman break galaxies

As part of a large spectroscopic survey of   z > 5  Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an active galactic nucleus (AGN). Out of a sample of more than 50 spectroscopically confirmed R -band dropout galaxies at   z ∼ 5  and above, only J104048.6−115550.2 at   z = 5.44  shows evidence for a high ionization potential emission line indicating the presence of a hard ionizing continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Lyα line. Uniquely within this sample of LBGs, emission from N  v is also detected, a clear signature of AGN photoionization. The object is spatially resolved in Hubble Space Telescope ( HST ) imaging. This, and the comparatively high Lyα/N  v flux ratio indicates that the majority of the Lyα (and the UV continuum longward of it) originates from stellar photoionization, a product of the ongoing starburst in the LBG. Even without the AGN emission, this object would have been photometrically selected and spectroscopically confirmed as a Lyman break in our survey. The measured optical flux  ( I _AB= 26.1)  is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN is found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.     

Crystallization of primitive basaltic magmas at crustal pressures and genesis of the calc-alkaline igneous suite: experimental evidence from St Vincent,Lesser Antilles arc

Near-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent (Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H₂O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H₂O in depressing plagioclase crystallization leads to a direct relation between the Al₂O₃ content of the residual melt and its H₂O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo_89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H₂O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H₂O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H₂O concentrations (2–5 wt%).     

Dryland salinity processes within the discharge zone of a local groundwater system,southeastern Australia

Detailed study of a localised saline discharge zone in southeastern Australia shows that the salinisation is mostly due to the shallow water table (<1–2 m from the surface). Direct evaporation, particularly in summer, leads to extremely high soil–water salinities at the surface, even though the underlying groundwater is moderately fresh. Groundwater discharge is localised at a break of slope, where the water table intersects the surface, and where the transition from permeable sands to clay-rich sediments inhibits lateral groundwater flow. Higher salt concentrations build up in the clays because of the long residence times during which soil-waters are exposed to evapotranspiration and the reduced potential for salts to be flushed from the sediments. As a result the saline discharge area does not correspond to the part of the site with the largest salt store. Results of the study demonstrate that for dryland salinisation to occur, the groundwater beneath the discharge zone need not be saline, and the presence of a large salt store does not necessarily lead to problems of dryland salinisation if, as in the clay-rich sediments at the site, the salt lies below the pasture root zone. Furthermore, mobilisation of salt stores within low permeability sediments by rising groundwater may be minor.

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Résumé L’étude détaillée d’une zone localisée d’émergence salée au sud-est de l’Australie montre que la salinisation est surtout due à la nappe phréatique (<1–2 m sous la surface). L’évaporation directe, particulièrement durant l’été, conduit à des salinités de l’eau du sol extrêmement élevées à la surface, même si l’eau souterraine sous-jacente est modérément douce. L’émergence de l’eau souterraine est localisée à la rupture de pente, là où la nappe phréatique rencontre la surface du sol et où la transition entre sables perméables et sédiments riches en argiles inhibe les écoulements d’eau souterraine latéraux. Les plus fortes concentrations en sel s’accumulent dans les argiles du fait de temps de résidence élevés, durant lesquels les eaux du sol sont exposées à l’évapotranspiration et à un lessivage réduit des sédiments. Il en résulte que l’aire d’émergence des eaux salées ne correspond pas à la partie du site rencontrant la réserve de sel la plus importante. Les résultats de l’étude démontrent que pour que la salinisation d’une zone aride devienne effective, l’eau souterraine sous la zone ne doit pas être forcément salée, et la présence d’une zone étendue de réserve de sel ne conduit pas forcément à des problèmes de salinisation de zones arides si, comme dans les sédiments argileux du site, le sel ne repose pas sous la zone de pature. De plus, la mobilisation des réserves de sel dans les sédiments peu perméables par la montée du niveau de l’eau souterraine devrait être mineure.

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Resumen El estudio detallado de una zona local de descarga salina en el sudeste de Australia, muestra que la salinización es principalmente debido a un nivel freático poco profundo (<1–2 m de la superficie). La evaporación Directa, particularmente en verano, conlleva a salinidades de suelo-agua sumamente altas en la superficie, aunque el agua subterránea subyacente es moderadamente dulce. La descarga de Agua subterránea se localiza en una interrupción de la ladera, dónde el nivel freático intercepta la superficie, y donde la transición de las arenas permeables a los sedimentos ricos en arcilla inhibe el flujo lateral del agua subterránea. Las concentraciones de sal más altas se forman en las arcillas debido a los tiempos de residencia largos durante los cuales se exponen el conjunto suelo-agua a la evapotranspiración y también por el potencial reducido para las sales de ser expulsadas de los sedimentos. Como resultado el área de la descarga salina no corresponde a la parte del sitio con el contenido de sal más grande. Los resultados del estudio demuestran que para que ocurra la salinización en terrenos secos, el agua subterránea bajo la de zona de descarga no necesita ser salina, y que la presencia de un almacenamiento de sal grande, no necesariamente lleva a los problemas de salinización en terrenos secos si, como en los sedimentos ricos en arcilla del sitio, la sal yace debajo de la zona de raíz de la pastura. Además, la movilidad de depósitos de sal dentro de los sedimentos de permeabilidad baja pueden ser menores, por causa del agua subterránea ascendente.

     

Bristlecone pine tree rings and volcanic eruptions over the last 5000 yr

Many years of low growth identified in a western USA regional chronology of upper forest border bristlecone pine (Pinus longaeva and Pinus aristata) over the last 5000 yr coincide with known large explosive volcanic eruptions and/or ice core signals of past eruptions. Over the last millennium the agreement between the tree-ring data and volcano/ice-core data is high: years of ring-width minima can be matched with known volcanic eruptions or ice-core volcanic signals in 86% of cases. In previous millennia, while there is substantial concurrence, the agreement decreases with increasing antiquity. Many of the bristlecone pine ring-width minima occurred at the same time as ring-width minima in high latitude trees from northwestern Siberia and/or northern Finland over the past 4000-5000 yr, suggesting climatically-effective events of at least hemispheric scale. In contrast with the ice-core records, the agreement between widely separated tree-ring records does not decrease with increasing antiquity. These data suggest specific intervals when the climate system was or was not particularly sensitive enough to volcanic forcing to affect the trees, and they augment the ice core record in a number of ways: by providing confirmation from an alternative proxy record for volcanic signals, by suggesting alternative dates for eruptions, and by adding to the list of years when volcanic events of global significance were likely, including the mid-2nd-millennium BC eruption of Thera.     

Intensified mid-Holocene Asian monsoon recorded in corals from Kikai Island, subtropical northwestern Pacific

Coupled records of Sr/Ca and oxygen isotope ratios (δ¹⁸O) of coral skeletons have been used to produce quantitative estimates of paleo-sea surface temperature (SST) and δ¹⁸O of surface seawater that can in some cases be converted to sea surface salinity (SSS). Two fossil corals from Kikai Island in the subtropical northwestern Pacific, a location affected by East Asian summer and winter monsoons, were analyzed to investigate differences between mid-Holocene and present-day SST and SSS. At 6180 cal yr BP, SSTs were roughly the same as today, both in summer and winter; δ¹⁸O_seawater and SSS values were higher both in summer (+ 0.5‰, +1.1 psu) and in winter (+ 0.2‰, + 0.6 psu) than modern values. At 7010 cal yr BP, SSTs were slightly cooler both in summer and winter (−0.8 and −0.6 °C), whereas δ¹⁸O_seawater and SSS had higher values in summer (+ 0.3‰, + 0.6 psu) and in winter (+ 0.8‰, + 1.9 psu) than present-day values. These results are consistent with other marine records for the mid-Holocene of the low and midlatitudes in the northwestern Pacific. Such regional conditions indicate that the East Asian summer and winter monsoons were more intense in the mid-Holocene, which was likely a function of the mid-Holocene insolation regime.     

An analytical study of the effect of penetration rate on piezocone tests in clay

Finite element cavity expansion analysis investigating the effect of penetration rate on piezocone tests in clay is presented. A coupled analysis was performed, in which the rate of cavity expansion was linked to the penetration rate of the cone and the cone angle, using the assumption that the deformation was wholly radial, and took place only between the cone tip and the cone shoulder. The soil was modelled using modified cam clay with two sets of parameters and varying values of overconsolidation ratio (OCR). The influence of penetration rate on the stress and pore pressure distributions was examined. For slower penetration rates, the excess pore pressure at the cone shoulder is lower since consolidation is permitted coincident with penetration. The radial profiles of post‐penetration voids ratio demonstrate that partially drained penetration is permitted by volume change in the near field, in addition to radial movement in the far field. The radial distribution of excess pore pressure after slow penetration differs from the undrained case, with a relatively low radial gradient existing at the cone face. As a result, the dissipation curves after slow penetration lag behind those following fast penetration. The cone velocity is made dimensionless by normalizing with the coefficient of consolidation and the cone diameter. ‘Backbone’ curves of normalized velocity against normalized tip resistance and excess pore pressure capturing the transition from undrained to drained penetration are derived. The normalized pore pressure backbone curve is unique, whilst the normalized tip resistance shows a small dependency on OCR. These backbone penetration curves are compared with centrifuge model piezocone tests conducted at varying rates, and subsequent dissipation tests. The numerical and experimental results suggest that the value of consolidation coefficient operative during the dissipation phase is 2–4 times higher than the virgin compression value due to changes in the operative soil stiffness, as demonstrated from the stress paths of individual soil elements. The use of multi‐rate penetration tests to deduce values of consolidation coefficient is discussed, in light of these differences. Copyright © 2005 John Wiley & Sons, Ltd.