Intermittent Melting Research Articles

Multiple Overspill Flood Channels from Young Craters Require Surface Melting and Hundreds of Meters of Midlatitude Ice Late in Mars’s History

Mars’s tadpole craters are small, young craters whose crater rims are incised by one or more exit breaches but lack visible inlets. The tadpole-forming climate records the poorly understood drying of Mars since the Early Hesperian. A third of tadpole craters have multiple breaches; therefore, the climate must have been able to generate crater rim incision in multiple locations. We use HiRISE data for four multiple-breach tadpole craters to measure their crater fill, rims, and exit breaches. We compare these measurements and other data with our calculations of liquid water supply by rain, surface melting, groundwater discharge, and basal ice sheet melting to discriminate between four proposed formation hypotheses for tadpole breaches, favoring scenarios with ice-filled craters and supraglacial melting. We conclude that multiple-breach tadpole craters record hundreds of meters of midlatitude ice and climate conditions enabling intermittent melting in the Late Hesperian and Amazonian, suggesting that liquid water on Mars has only been available in association with water ice for billions of years.

Mapping the Chicxulub Impact Stratigraphy and Peak Ring Using Drilling and Seismic Data

AbstractWe integrate high‐resolution full‐waveform velocity models with seismic reflection images to map the peak ring and impactite stratigraphy at the Chicxulub structure. International Ocean Discovery Program/International Continental scientific Drilling Program Site M0077 provides ground truth for our interpretations. The peak ring is narrower (∼10 km width) where it is high relief (600–700 m below seafloor) and wider (∼15 km width) where it is lower relief (1,000–1,200 m below seafloor). Both target asymmetry and angle of impact could have contributed to observed differences in peak ring morphology. We interpret a layer of lowered velocities as a resurge layer formed from the ocean resurge, seiche, and returning tsunami flowing into the newly formed impact basin. This graded suevite layer has an average thickness of 187 ± 58 m with only local thickness differences within the annular trough, peak ring, and central basin. These observations suggest that the returning ocean was of substantial height and energetic enough to carry debris across the entire topographic peak ring. We map impact melt rock throughout the crater, with a thick impact melt sheet in the central basin (>500 m), thin intermittent melt rock capping the peak ring, and a ∼500‐m thick layer of melt rock in the annular trough near the peak ring that thins toward the crater rim. We estimate that ∼70%–75% of the melt rock volume is in the central basin. We image features above and adjacent to the central basin melt sheet that we interpret as upflow zones associated with a long‐lasting hydrothermal system.

Marked Seasonal Changes in the Microbial Production, Community Composition, and Biogeochemistry of Glacial Snowpack Ecosystems in the Maritime Antarctic

AbstractWe describe seasonal changes in the biogeochemistry, microbial community and ecosystem production of two glacial snowpacks in the maritime Antarctic during a cold summer. Frequent snowfall and low, intermittent melt on the glaciers suppressed surface photosynthesis and promoted net heterotrophy. Concentrations of autotrophic cells (algae and cyanobacteria) were therefore low (average: 150–500 cells mL−1), and short‐term estimates of primary production were almost negligible in early summer (<0.1 μg C L−1 d−1). However, order of magnitude increases in Chlorophyllaconcentrations occurred later, especially within the mid‐snowpack and ice layers below. Short‐term primary production increased to ca. 1 μg C L−1 d−1in mid‐summer, and reached 53.1 μg C L−1 d−1in a mid‐snow layer close to an active penguin colony. However, there were significantly more bacteria than autotrophs in the snow (typically 103cells mL−1, but >104cells mL−1in basal ice near the penguin colony). The ratio of bacteria to autotrophs also increased throughout the summer, and short‐term bacterial production rates (0.2–2000 μg C L−1 d−1) usually exceeded primary production, especially in basal ice (10–1400 μg C L−1 d−1). The basal ice represented the least diverse but most productive habitat, and a striking feature was its low pH (down to 3.3). Furthermore, all of the overlying snow cover became increasingly acidic as the summer season progressed, which is attributed to enhanced emissions from wet guano in the penguin colony. The study demonstrates that active microbial communities can be expected, even when snowmelt is intermittent in the Antarctic summer.

Infiltration Response of Adsorbent Amended Filters for Stormwater Management under Freezing/Thawing Conditions

Coastal cold climates experience frequent intermittent melting and freezing periods over the cold period. This intermittent freezing in stormwater systems affects the infiltration capacity and hence the performance. This paper investigates the infiltration capacity of engineered filter media (composed of sand mixed with charcoal, pine bark, or olivine) under freezing temperatures in a column-based laboratory setup. Infiltration into partially frozen filter media was replicated using a climate room. The filter media in the columns were brought to −2.5 °C, and water at +2 °C was percolated through the columns with a constant head of 5 cm. Infiltration performance was assessed by observing the time until breakthrough, and the infiltration rate 24 h after breakthrough. The results were compared to the observed hydraulic conductivity for the unfrozen filter media. A novel approach combining the unfrozen water content curves with X-ray tomographic (XRT) images of the materials was adopted to better understand the thermal and infiltration processes. Breakthrough was observed between ca. 21 and 56 h in all columns. The column with homogeneously mixed filter media with sand yielded the quickest breakthrough. The infiltration rates were higher than recommendations for infiltration-based systems in cold climates, making them a suitable option in cold climates.

Selective Laser Melting of Integrated Ti6Al4V ELI Permeable Walls for Controlled Drug Delivery of Vancomycin.

Bacteria colonizing the surface of orthopedic implants are responsible for most postoperative periprosthetic joint infections. A possible alternative route for drug delivery is described in this study by utilizing the bulk of the implant itself as a reservoir. Drug release is enabled by manufacturing of integrated permeable structures possessing high porosity through application of selective laser melting technology. The concept was evaluated in two paths, with 400 μm permeable thin walls and with dense reservoirs containing an integrated 950 μm permeable wall. Components were designed and preprocessed as separate parts, allowing for allocation of different settings of laser power and scanning speed. Lowering the energy input into the selective laser melting process to induce intermittent melting of the Ti6Al4V ELI powder produced porous components through which vancomycin was released with differing profiles. Static water contact angle measurements demonstrated a significant effect on the hydrophilicity by permeable wall thickness. Relative porosities of the 400 μm structures were determined with microcomputed tomography analyses. A transition zone of 21.17% porosity was identified where release profiles change from porosity-dependent to near free diffusion. Antimicrobial activity of released vancomycin was confirmed through evaluation against Staphylococcus aureus Xen 36 in two separate agar diffusion assays. The approach is promising for incorporation into the design and manufacturing of next-generation prosthetic implants with controlled release of antibiotics in situ and the subsequent prevention of periprosthetic joint infections.

Geochemistry of the alkaline volcanicsubvolcanic rocks of the Fernando de Noronha Archipelago, southern Atlantic Ocean

<p>The Fernando de Noronha Archipelago presents, on its main island, a centrally-located stratigraphic unit, the Remédios Formation (age around 8 - 12 Ma) constituted by basal pyroclastic rocks intruded by dikes, plugs and domes of varied igneous rocks, capped by flows and pyroclastics of mafic to ultramafic rocks of the Quixaba Formation (age around 1 - 3 Ma), which is limited from the underlying unit by an extensive irregular erosion surface. A predominant sodic Remédios series (basanites, tephrites, tephriphonolites, essexite, phonolites) can be separated from a moderately potassic Remédios sequence (alkali basalts, trachyandesites, trachytes), both alkaline series showing mostly continuous geochemical trends in variation diagrams for major as well as trace elements, indicating evolution by crystal fractionation (mainly, separation of mafic minerals, including apatites and titanites). There are textural and mineralogical evidences pointing to hybrid origin of some intermediate rocks (e.g., resorbed pyroxene phenocrysts in basaltic trachyandesites, and in some lamprophyres). The primitive Quixaba rocks are mostly melanephelinites and basanites, primitive undersaturated sodic types. Geology (erosion surface), stratigraphy (two distinct units separated by a large time interval), petrography (varied Remédios Formation, more uniform Quixaba unit) and geochemistry indicate that the islands represent the activity of a protracted volcanic episode, fueled by intermittent melting of an enriched mantle, not related to asthenospheric plume activity.</p>

Oceanographic variability on the West Antarctic Peninsula during the Holocene and the influence of upper circumpolar deep water

Recent intensification of wind-driven upwelling of warm upper circumpolar deep water (UCDW) has been linked to accelerated melting of West Antarctic ice shelves and glaciers. To better assess the long term relationship between UCDW upwelling and the stability of the West Antarctic Ice Sheet, we present a multi-proxy reconstruction of surface and bottom water conditions in Marguerite Bay, West Antarctic Peninsula (WAP), through the Holocene. A combination of sedimentological, diatom and foraminiferal records are, for the first time, presented together to infer a decline in UCDW influence within Marguerite Bay through the early to mid Holocene and the dominance of cyclic forcing in the late Holocene. Extensive glacial melt, limited sea ice and enhanced primary productivity between 9.7 and 7.0 ka BP is considered to be most consistent with persistent incursions of UCDW through Marguerite Trough. From 7.0 ka BP sea ice seasons increased and productivity decreased, suggesting that UCDW influence within Marguerite Bay waned, coincident with the equatorward migration of the Southern Hemisphere Westerly Winds (SWW). UCDW influence continued through the mid Holocene, and by 4.2 ka BP lengthy sea ice seasons persisted within Marguerite Bay. Intermittent melting and reforming of this sea ice within the late Holocene may be indicative of episodic incursions of UCDW into Marguerite Bay during this period. The cyclical changes in the oceanography within Marguerite Bay during the late Holocene is consistent with enhanced sensitively to ENSO forcing as opposed to the SWW-forcing that appears to have dominated the early to mid Holocene. Current measurements of the oceanography of the WAP continental shelf suggest that the system has now returned to the early Holocene-like oceanographic configuration reported here, which in both cases has been associated with rapid deglaciation.

Consequences of open-system melting in tectonics

Partial melting and melt drainage from deep suprasolidus crust in orogens has important consequences for tectonics. Melt extraction along prograde segments of clockwise P – T paths reduces fertility and increases the density and strength of residual crust, which has implications for further melt production during decompression. Using calculated P – T phase diagrams, implications of stepwise melt loss along clockwise P – T paths for pelite and greywacke are assessed, and density of the progressively more residual source and the potential role of buoyancy in the exhumation of deep crustal rocks are evaluated. Two model P – T paths are considered: isobaric heating at 1.2 GPa followed by decompression to 0.4 GPa at 750, 820 and 890 °C, and prograde heating from the fluid-present solidus at 1.2 GPa to 860 °C at 1.8 GPa followed by isothermal decompression to 0.4 GPa. Both closed-system (undrained) and conditionally open-system (drained by intermittent melt loss) conditions are assessed. If melt is drained along clockwise P – T paths in suprasolidus crust then lower quantities of melt will be generated during decompression than sometimes inferred in tectonic models. Instead, the role of melt transfer through suprasolidus crust and melt accumulation at shallow levels in the anatectic zone should be considered rather than simply invoking the generation of large volumes of melt in decompressing crust.

Prolonged plume volcanism in the Caribbean Large Igneous Province: New insights from Curaçao and Haiti

We present 36 new 40Ar‐39Ar incremental heating age determinations from the Caribbean Large Igneous Province (CLIP) providing evidence for extended periods of volcanic activity and suggest a new tectonomagmatic model for the province's timing and construction. These new 40Ar‐39Ar ages for the Curaçao Lava Formation (CLF) and Haiti's Dumisseau Formation show evidence for active CLIP volcanism from 94 to 63 Ma. No clear changes in geochemical character are evident over this period. The CLF has trace element signatures (e.g., Zr/Nb = 10–20) and flat rare earth element (REE) trends consistent with plume volcanism. The Dumisseau Formation also has plume‐like geochemistry and steeper REE trends similar to ocean island basalts. Volcanism in the Dumisseau Formation appears to have largely ceased by 83 Ma while at Curaçao it continued until 63 Ma. A rapidly surfacing and melting plume head alone does not fit this age distribution. Instead, we propose that the residual Galapagos plume head, following initial ocean plateau construction, was advected eastward by asthenospheric flow induced by subducting oceanic lithosphere. Slab rollback at the Lesser Antilles and Central America subduction zones created an extensional regime within the Caribbean plate. Mixing of plume with upwelling asthenospheric mantle provided a source for intermittent melting and eruption through the original plateau over a ∼30 Ma period.

Spatial distribution of stable water isotopes in alpine snow cover

<p><strong>Abstract.</strong> The aim of this study was to analyse and predict the mean stable water isotopic composition of the snow cover at specific geographic locations and altitudes. In addition, the dependence of the isotopic composition of the entire snow cover on altitude was analysed. Snow in four Swiss catchments was sampled at the end of the accumulation period in April 2010 and a second time during snowmelt in May 2010 and analysed for stable isotope composition of <sup>2</sup>H and <sup>18</sup>O. The sampling was conducted at both south-facing and north-facing slopes at elevation differences of 100 m, for a total altitude difference of approximately 1000 m. The observed variability of isotopic composition of the snow cover was analysed with stepwise multiple linear regression models. The analysis indicated that there is only a limited altitude effect on the isotopic composition when considering all samples. This is due to the high variability of the isotopic composition of the precipitation during the winter months and, in particular in the case of south-facing slopes, an enrichment of heavy isotopes due to intermittent melting processes. This enrichment effect could clearly be observed in the samples which were taken later in the year. A small altitudinal gradient of the isotopic composition could only be observed at some north-facing slopes. However, the dependence of snow depth and the day of the year were significant predictor variables in all models. This study indicates the necessity to further study the variability of water isotopes in the snow cover to increase prediction for isotopic composition of snowmelt and hence increase model performance of residence time models for alpine areas in order to better understand the accumulation processes and the sources of water in the snow cover of high mountains.</p>

Spatial distribution of stable water isotopes in alpine snow cover

<strong class="journal-contentHeaderColor">Abstract.</strong> The aim of this study was to analyse and predict the mean stable water isotopic composition of the snow cover at specific geographic locations and altitudes. In addition, the dependence of the isotopic composition of the entire snow cover on altitude was analysed. Snow in four Swiss catchments was sampled at the end of the accumulation period in April 2010 and a second time during snowmelt in May 2010 and analysed for stable isotope composition of ²H and ¹⁸O. The sampling was conducted at both south-facing and north-facing slopes at elevation differences of 100 m, for a total altitude difference of approximately 1000 m. The observed variability of isotopic composition of the snow cover was analysed with stepwise multiple linear regression models. The analysis indicated that there is only a limited altitude effect on the isotopic composition when considering all samples. This is due to the high variability of the isotopic composition of the precipitation during the winter months and, in particular in the case of south-facing slopes, an enrichment of heavy isotopes due to intermittent melting processes. This enrichment effect could clearly be observed in the samples which were taken later in the year. A small altitudinal gradient of the isotopic composition could only be observed at some north-facing slopes. However, the dependence of snow depth and the day of the year were significant predictor variables in all models. This study indicates the necessity to further study the variability of water isotopes in the snow cover to increase prediction for isotopic composition of snowmelt and hence increase model performance of residence time models for alpine areas in order to better understand the accumulation processes and the sources of water in the snow cover of high mountains.

Modeling Snow–Canopy Processes on an Idealized Mountain

Abstract Snow interception in a coniferous forest canopy is an important hydrological feature, producing complex mass and energy exchanges with the surrounding atmosphere and the snowpack below. Subcanopy snowpack accumulation and ablation depends on the effects of canopy architecture on meteorological conditions and on interception storage by stems, branches, and needles. Mountain forests are primarily composed of evergreen conifer species that retain their needles throughout the year and hence intercept snow efficiently during winter. Canopy-intercepted snow can melt, fall to the ground, and/or sublimate into the air masses above and within the canopy. To improve the understanding of snow–canopy interception processes and the associated influences on the snowpack below, a series of model experiments using a detailed, physically based snow–canopy and snowpack evolution model [Alpine Multiscale Numerical Distributed Simulation Engine (AMUNDSEN)] driven with observed meteorological forcing was conducted. A cone-shaped idealized mountain covered with a geometrically regular pattern of coniferous forest stands and clearings was constructed. The model was applied for three winter seasons with different snowfall intensities and distributions. Results show the effects of snow–canopy processes and interactions on the pattern of ground snow cover, its duration, and the amount of meltwater release, in addition to showing under what conditions the protective effect of a forest canopy overbalances the reduced accumulation of snow on the ground. The simulations show considerable amounts of canopy-intercepted snowfall can sublimate, leading to reduced snow accumulation beneath the forest canopy. In addition, the canopy produces a shadowing effect beneath the trees that leads to reduced radiative energy reaching the ground, reduced below-canopy snowmelt rates, and increased snow-cover duration relative to nonforested areas. During snow-rich winters, the shadowing effect of the canopy dominates and snow lasts longer inside the forest than in the open, but during winters with little snow, snow sublimation losses dominate and snow lasts longer in the open areas than inside the forest. Because of the strong solar radiation influence on snowmelt rates, the details of these relationships vary for northern and southern radiation exposures and time of year. In early and high winter, the radiation protection effect of shadowing by the canopy is small. If little snow is available, an intermittent melt out of the snow cover inside the forest can occur. In late winter and spring, the shadowing effect becomes more efficient and snowmelt is delayed relative to nonforested areas.

High speed imaging technique Part 2 – High speed imaging of power beam welding phenomena

The high speed imaging technique is an attractive tool to elucidate welding phenomena. In this paper, applications of this technique are reviewed in order to understand the power beam welding phenomenon. Monochromatic imaging technique, which can take pictures of plasma with any specific spectrum wavelength, is used to analyse state of the laser induced plasma and distribution of some species, such as ions and atoms of gases and metals. During electron beam welding, intermittent melting process and spiking phenomenon can be clearly recorded by a pinhole X‐ray streak camera. The in situ X‐ray transmission imaging system is useful to observe the dynamic keyhole and fluid flow behaviour, which cannot be directly observed by any other method. High speed observations of the solidification front clearly show the solidification cracking process. In the present paper, methods to analyse the solidification cracking using high speed imaging are also described.

Viscous flow lobes in central Taylor Valley, Antarctica: Origin as remnant buried glacial ice

Viscous flow lobes are common throughout the McMurdo Dry Valleys (MDV) of Antarctica. These features have been described as rock glaciers, gelifluction lobes, solifluction lobes, talus mobilized by pore ice and/or segregation ice, and debris-covered glaciers. We investigate the origin, modification, and flow of a 2-km-long lobe (East Stocking Lobe or ESL) along the north wall of central Taylor Valley using field mapping techniques, shallow seismic surveys, time-dependent displacement surveys, and isotopic analyses of buried-ice samples. On the basis of these integrated analyses, we show that the ESL is cored with remnant glacier ice, most probably derived from an advance of nearby Stocking Glacier ∼ 130 kyr BP. Seismic data, coupled with results from ice-flow modeling assuming plastic flow of clean ice, suggest that the buried core of glacier ice is ∼ 14- to 30-m thick. Near its terminus, the ESL flows at a rate of ∼ 2.4 to 6.7 mm a − 1 . The loose drift that caps the buried ice (typically < 1 m thick) is composed of moderately stratified sand- and gravel-sized clasts; it is dry (1–3% soil gravimetric water content; GWC), except near ephemeral stream channels and the margins of melting snow banks (6–25% GWC). Stable isotopic analyses of samples from the upper 30 cm of the ice lie on a slope of ∼ 5.8 (when plotted on a δD vs. δ 18O graph), well below the local meteoric water line of 7.75, suggesting modification by freeze/thaw processes and evaporation/sublimation. Measured air and soil temperatures show that intermittent melting is most likely possible during summer months where buried ice is ≤ 35 cm below the ground surface. Morphological comparisons with ice-cored deposits in upland regions of the Dry Valleys, e.g., Mullins and Beacon Valleys (30 km inland and ∼ 500 m higher in elevation), and near the coast (40 km distant and ∼ 500 m lower) reveal marked contrasts in the style of near-surface ice degradation and cryoturbation. From these morphological comparisons, we infer that buried-ice deposits in the stable upland zone have not experienced the relatively warm climate conditions now found at the ESL and at lower elevations in the Dry Valleys region (e.g. sustained summertime temperatures of ≥−4 °C) for the last several million years.

An evaluation of applying existing bioretention sizing methods to cold climates with snow storage conditions

Eight of the current sizing and design methods proposed for bioretention facilities were evaluated for rainfall runoff and snow storage volumes for a costal cold climate in Trondheim, Norway. The RECARGA bioretention infiltration model was used to compare the performance of the methods using 30 months of observed data from a pilot scale bioretention box. The surface areas, total ponding time, number and duration of overflow events, and snow storage volumes were compared. It was found that even in a costal cold climate with several intermittent melt cycles die snow storage requirements were an important design parameter, and if more than 25% of the total snow volume should stored this became the deciding design parameter.

ステンレス鋼の高出力ファイバーレーザ溶接とその特徴的な溶接部の形成機構の解明

A high-power fiber laser can produce an ultra-high peak power density of MW/mm2 level corresponding to a focused electron beam, and is promising as one of the desirable heat sources for high-speed and deep-penetration welding. In this study, therefore, fiber laser welding and simultaneous observation with high-speed video cameras and X-ray transmission real-time imaging system were performed with the objective of obtaining a fundamental knowledge of welding phenomena and defect formation mechanisms in producing deep and narrow weld beads in Type 304 stainless steel. A 6 kW fiber laser beam through φ 100 μm feeding fiber or φ 300 μm process fiber could be focused at 1.1 MW/mm2 or 120 kW/mm2 peak power density, respectively. Consequently, deeply penetrated weld beads with narrow width were formed, and fully penetrated welds in 8 mm thick plate could be produced at less than 2 and 4.5 m/min for 120 kW/mm2 and 1.1 MW/mm2, respectively. In the case of the ultra-high power density, humping weld beads were formed at more than 6 m/min. This reason was attributed to the intermittent melt ejection from a small keyhole inlet and the subsequent melt flows on the narrow molten pool surface. On the other hand, in the case of the high power density, weld fusion zones with porosity, sound partial penetration welds and underfilled weld beads with large spatters were obtained at the welding speeds below 3 m/min, between 4.5 to 8 m/min and above 10 m/min, respectively. These formation mechanisms were interpreted by considering bubbles formation from a keyhole tip leading to pores captured by the solidifying front, the formation of a long molten pool suppressing and accommodating spattering, and the production of a narrow, short molten pool causing easy generation of spattering, respectively.

電子ビーム溶接現象に関する研究 （第４報） 電子ビーム溶接時の溶融過程と溶融金属の流動挙動

This paper describes the effect of molten metal behavior on melting process and spiking phenomenon in electron beam welding. X-rays generated as a consequence of beam-metal interaction were detected at specific sites of 3 mm in length along penetration to clarify the melting process for various beam focussing conditions and welding speeds. Behavior of molten metal flow was also observed using high speed cinecamera simultaniously. JIS SM 50 A steel, Fe-10% Mn alloy and Fe-20% Mn alloy were used as base metals to investigate the effect of vapor pressure on melting process.Results obtained were as follows.(1) Spiking phenomenon was caused by an intermittent melting of base metal depending on the fluctuation in morphology of beam cavity.(2) Spiking was suppressed with an increase in Mn content involved in the base metal, since the beam cavity was stable and always clear of molten metal during welding by high pressure Mn vapor.(3) The front wall of the beam cavity was melted almost continuously for upper focussing condition, whereas it was melted intermittently for down focussing condition due to the periodical flow of the molten metal, resulting in more probable occurrence of acute spiking.(4) Spiking was more likely to occur with a decrease in the welding speed under the same penetration depth, since the electron beam could drill the bottom deeply for low welding speed.(5) The periodical flow was caused by natural vibration of the molten metal or high voltage power ripple depending on the amount of the molten metal around the beam cavity.

Water yield from forest snowpack management: Research findings in Arizona and New Mexico

Snow falling in high‐elevation forests is an important source of water for much of the arid Southwest. Snowpack conditions in Arizona and New Mexico differ from those in more northern regions, because of the variability in annual accumulations and the intermittent melting throughout the winter season. Snow management research in Arizona and New Mexico over the past 25 years has indicated the possibilities of increasing snowmelt water yields through forest management activities. The effects of management can be predicted from forest inventory data. Other research has resulted in the development of snow‐runoff forecasting procedures and computer simulation models of forest snowpack dynamics, as well as increasing the general knowledge of snow hydrology.

Effect of beam dwell time on surface changes during laser scribing

Laser scribing can be used to reduce the ferromagnetic domain size, and thus core loss, in grain-oriented electrical steels. The localized stress required for domain-size control can be produced either by shock deformation, optimally associated with beam dwell times (pulse lengths) less than 2×10−8 sec, or by thermal stressing occurring with dwell times optimally longer than 1×10−4 sec. In this paper we show that comparable core loss improvements can be obtained at greatly different dwell times, but that the associated changes in the surface condition of coated silicon steel laser scribed for maximum core loss reduction vary dramatically as dwell time varies. At the very short times characterizing shock deformation, achieved using a Q-switched solid-state laser, the coating is almost completely removed, but there is little effect on the steel itself, and recoating has been used to restore coating integrity. Under optimum dwell times for thermal expansion effects, achieved using continuous-wave radiation, there is no significant effect on the coating, and no postscribing treatment is required. At intermediate dwell times, however, significant core loss reduction is associated with extensive disruption of the coating and with melting of the steel. At dwell times of approximately 1×10−6 sec using CO2 laser radiation, there is an unstable transition between the shock (vaporization) and heating modes, with intermittent melting.

Vertical mixed layer convection in the Weddell Sea1

Abstract A pressure‐dependent numerical model of vertical mixing in the surface mixed layer is developed, and applied to the problem of Antarctic Bottom Water (AABW) formation in the Weddell Sea. The results are used to formulate a new model of AABW formation. The surface salinity of water circulating around the southern edge of the Weddell Gyre gradually increases, possibly owing to horizontal exchange with water on the continental shelf and/or freezing along the edges of ice shelves. This permits surface mixed layer convection to penetrate deeper and deeper as the water drifts westward. By the time the water reaches the southwest corner of the Weddell Sea, the combined driving of cooling and salt release during freezing, both of which are important, can drive the convection to the bottom in deep water. This forms a water mass called Deep Mixed Water (DMW: θ = ‐0.6 to ‐0.8°C, S = 34.64‰). Upward mixing of Warm Deep Water causes intermittent melting (usually followed by refreezing) at all stations; at the...