Some comparisons between the middle atmosphere dynamics of the Southern and Northern Hemispheres

Comparisons are drawn between certain middle atmosphere dynamical processes in the Southern Hemisphere and the Northern Hemisphere. Attention is focused on the zonal-mean climatological state, stationary waves, transient waves of various types, stratospheric sudden warmings and polar ozone minima. Observations of the similarities and differences between the hemispheres are mentioned, and ways in which these comparisons may be used to enhance our dynamical knowledge of the whole middle atmosphere are discussed.     

Multiple deglaciations of the Hudson Bay Lowlands,Canada, since deposition of the Missinaibi (Last-integlacial?) formation

The stratigraphic record in the James and Hudson Bay Lowlands indicates that the sequence of glacial events at the geographical center of the 12.6 × 10⁶ km² Laurentide Ice Sheet may have been more complex than hitherto imagined. Isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups. Two alternative explanations of the data are offered, of which we strongly favor the first. Hypothesis 1: Setting the age of the “last interglacial” marine incursion, the Bell Sea, at 130,000 yr B.P. results in a long-term average diagenetic temperature for the lowlands of +0.6°C. Using this temperature enables us to predict the age of shells intermediate in age between the “last interglaciation” and the incursion of the Tyrrell Sea 8000 yr ago. Between these two interglacial marine inundations, Hudson Bay is predicted to have been free of ice along its southern shore about 35,000, 75,000, and 105,000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments. These results suggest (1) that traditional concepts of ice-sheet build-up and decay must be reexamined; (2) that “high” sea levels may have occurred during the Wisconsin Glaciation; and (3) that a critical reappraisal is required of the open ocean δ¹⁸O record as a simple indicator of global ice volume. An alternative, Hypothesis 2, is also examined. It is based on the assumption that the 35,000-yr-old deposits calculated on the basis of Hypothesis 1 date from the “last interglaciation”; this, in effect, indicates that the Missinaibi Formation, commonly accepted as sediments of the “last interglaciation,” are about 500,000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130,000 yr has been close to ?6°C. We argue for rejection of this alternative hypothesis.     

Groundwater-pumping optimization for land-subsidence control in Beijing plain,China

Hydrogeology Journal - Beijing, in the North China plain, is one of the few megacities that uses groundwater as its main source of water supply. Groundwater accounts for about two-thirds of the...     

40Ar/39Ar ages and zircon petrochronology for the rear arc of the Izu-Bonin-Marianas intra-oceanic subduction zone

Long-lived intra-oceanic arcs of Izu-Bonin-Marianas (IBM)-type are built on thick, granodioritic crust formed in the absence of pre-existing continental crust. International Ocean Discovery Program Expedition 350, Site U1437, explored the IBM rear arc to better understand continental crust formation in arcs. Detailed petrochronological (U–Pb geochronology combined with trace elements, oxygen and hafnium isotopes) characterizations of zircon from Site U1437 were carried out, taking care to exclude potential contaminants by (1) comparison of zircon ages with ship-board palaeomagnetic and biostratigraphic ages and ⁴⁰Ar/³⁹Ar geochronology, (2) analysing zircon from drill muds for comparison, (3) selectively carrying out in situ analysis in petrographic thin sections, and (4) minimizing potential laboratory contamination through using pristine equipment during mineral separation. The youngest zircon ages in Site U1437 are consistent with ⁴⁰Ar/³⁹Ar and shipboard ages to a depth of ~1390 m below sea floor (mbsf) where Igneous Unit Ig 1 yielded an ⁴⁰Ar/³⁹Ar age of 12.9 ± 0.3 Ma (all errors 2σ). One single zircon (age 15.4 ± 1.0 Ma) was recovered from the deepest lithostratigraphic unit drilled, Unit VII (1459.80–1806.5 mbsf). Site U1437 zircon trace element compositions are distinct from those of oceanic and continental arc environments and differ from those generated in thick oceanic crust (Iceland-type) where low-δ¹⁸O evolved melts are produced via re-melting of hydrothermally altered mafic rocks. Ti-in-zircon model temperatures are lower than for mid-ocean ridge rocks, in agreement with low zircon saturation temperatures, suggestive of low-temperature, hydrous melt sources. Zircon oxygen (δ¹⁸O = 3.3–6.0‰) and hafnium (εHf = + 10–+16) isotopic compositions indicate asthenospheric mantle sources. Trace element and isotopic differences between zircon from Site U1437 rear-arc rocks and the Hadean detrital zircon population suggest that preserved Hadean zircon crystals were probably generated in an environment different from modern oceanic convergent margins underlain by depleted mantle.     

Repeated recharge,assimilation, and hybridization in magmas erupted from El Chichón as recorded by plagioclase and amphibole phenocrysts

Compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from El Chichón record multiple cycles of country rock assimilation, magma injection, hybridization, and mixing. Laser ablation ICP-MS and electron microprobe analyses of plagioclase crystals from 7 eruptions spanning 3100 years reveal four types of zoning. These compositional and isotopic zones are often associated with textural changes observed in the crystals in thin section (e.g. sieved or patchy regions). Amphiboles are frequently zoned in Al and Si, and, in two magmas, may have clinopyroxene rims. Interestingly, most plagioclase show multiple and repeated zoning patterns. Moreover, all magmas contain all zoning patterns and textures, and crystals with substantially different sequences of zones occur within mm of one another. The most reasonable explanation for the origin of these textures is a frequently recharged chamber. Plagioclase zones with increasing anorthite contents (An) and decreasing ⁸⁷Sr/⁸⁶Sr ratios record injection by a hotter, possibly wetter, and more primitive magma (lower ⁸⁷Sr/⁸⁶Sr ratio). Zones with decreasing An and increasing ⁸⁷Sr/⁸⁶Sr ratios record assimilation of country rock and/or hybridization of the host and injected magmas; injection of hot magma may provide the heat for country rock assimilation. Changes in An without corresponding changes in ⁸⁷Sr/⁸⁶Sr ratio likely record slight variations in pressure or temperature during crystallization, or the far-field thermal effects of magma injection. Variations in ⁸⁷Sr/⁸⁶Sr ratio unaccompanied by Anzoning record assimilation of country rock. Amphibole zoning patterns also record periodic heating events; amphibole with clinopyroxene rims record episodes where the magma was heated beyond the amphibole stability field. Bulk compositional homogeneity and the juxtaposition of many crystals with disparate zoning patterns in single pumice require the magmatic system to be well mixed. Strontium diffusion rates indicate that the plagioclase zoning patterns cannot have persisted at magmatic pressures and temperatures for more than ~ 500 years, thus cycles of injection and assimilation occur on timescales equal to or shorter than the eruption recurrence interval. Long-term compositional and isotopic homogeneity indicate that there is a balance between recharge, assimilation, and crystallization.     

A lumped discrete population balance model for shear flocculation-model development

In the shear flocculation process, flocculation of ultrafine (size < 10 μm) hydrophobic particle occurs in the presence of a high shear field. Hence, it is necessary to consider both processes of coalescence and breakage, while modelling shear flocculation.In this paper, a lumped discrete population balance model is presented considering both processes of coalescence and breakage of flocs. A method is also proposed to convert discrete kernels to lumped discrete kernels by satisfying the constraints on them. The numerical solution of the model is compared with some special analytical solutions of the continuous population balance model, with which it is observed to be in agreement. The model equations are derived in such a way that it predicts correctly both the rate of change of number and the total volume of the flocs. The size distribution of a flocculating suspension can be predicted by this model.     

Channel response to sediment release: insights from a paired analysis of dam removal

Dam removals with unmanaged sediment releases are good opportunities to learn about channel response to abruptly increased bed material supply. Understanding these events is important because they affect aquatic habitats and human uses of floodplains. A longstanding paradigm in geomorphology holds that response rates to landscape disturbance exponentially decay through time. However, a previous study of the Merrimack Village Dam (MVD) removal on the Souhegan River in New Hampshire, USA, showed that an exponential function poorly described the early geomorphic response. Erosion of impounded sediments there was two‐phased. We had an opportunity to quantitatively test the two‐phase response model proposed for MVD by extending the record there and comparing it with data from the Simkins Dam removal on the Patapsco River in Maryland, USA. The watershed sizes are the same order of magnitude (10² km²), and at both sites low‐head dams were removed (~3–4 m) and ~65 000 m³ of sand‐sized sediments were discharged to low‐gradient reaches. Analyzing four years of repeat morphometry and sediment surveys at the Simkins site, as well as continuous discharge and turbidity data, we observed the two‐phase erosion response described for MVD. In the early phase, approximately 50% of the impounded sediment at Simkins was eroded rapidly during modest flows. After incision to base level and widening, a second phase began when further erosion depended on floods large enough to go over bank and access impounded sediments more distant from the newly‐formed channel. Fitting functional forms to the data for both sites, we found that two‐phase exponential models with changing decay constants fit the erosion data better than single‐phase models. Valley width influences the two‐phase erosion responses upstream, but downstream responses appear more closely related to local gradient, sediment re‐supply from the upstream impoundments, and base flows. Copyright © 2017 John Wiley & Sons, Ltd.     

Groundwater Complexity in Urban Catchments: Shenzhen,Southern China

Groundwater interactions with surface water and sewers in an urban setting are complex, and classic hydrogeological approaches must be combined with anthropogenic elements to characterize them. The level of detail needed to understand these interactions is illustrated by the analysis of an urban subcatchment in the megacity of Shenzhen in southern China that has had a drastic urban expansion in the last 40 years. The study area is characterized by the Yanshanian granite that is widespread across southern-eastern China. The urban setting is studied using multitemporal analysis of satellite images, borehole investigations and field surveys. Given the local hydrostratigraphy, a conceptual model was developed to identify the physical and anthropogenic factors that regulate the urban groundwater system. Based on the conceptual model and the data collected from the field or compiled from the literature, the average annual effective recharge is estimated to be 290 mm/year, after the urbanization process. From rural to urban conditions, it is estimated that the effective recharge increased by 170% and sewers intercept at least 23% of the effective recharge. Groundwater captured by sewers reduces river flows and increases the required capacity and costs for waste water treatment plants.     

Processes of alluvial fan sedimentation,basal rozel conglomerate formation,la têcte des hougues,Jersey, Channel Islands

The Rozel Conglomerate Formation, a coarse alluvial fan deposit, overlies the Bouley Rhyolite Formation beneath the headland called La Tete des Hougues, on the north coast of Jersey, Channel Islands. Here the rhyolites form an exhumed palaeotopography which in places is smoothly, but irregularly, eroded. Locally however, there is preservation of a deep Lower Palaeozoic weathering profile. Breccia is preserved spalled from these weathered profiles as a fossil scree deposit, which has subsequently been hydraulically reworked in a palaeotopographic low. Overlying the breccia is a prograding succession of alluvial fan sediments. A 4m thick, upwards-coarsening sequence of silty sandstones and fine pebble conglomerates is succeeded by the incision of a 3m deep channel. This is overlain by a 28m thick, upwards-fining sequence of coarse debris flow and streamflood conglomerates. The lower unit is interpreted as a prograding fan sector of reworked fan material, whilst the upper unit is thought to reflect the avulsive initiation and gradual abandonment of a fan lobe sourced from a northern hinterland.