Sharp Termination Research Articles

On the relationship between paleomagnetic secular variation and excursions – Records from MIS 11 and 12 – ODP Leg 172, western North Atlantic Ocean

This study has developed paleomagnetic secular variation (PSV) records from Sites 1060, 1061, 1062, and 1063 (ODP Leg 172) from the western North Atlantic Ocean during MIS 11–12 (374–478 ka). We have identified 46 inclination features and 49 declination features that can be correlated among the records. We have also developed relative paleointensity records and identified 13 paleointensity features that can be correlated among them. These features can also be dated using the oxygen-isotope dated Global relative-paleointensity record PISO-1500 of Channell et al. (2009). There is one excursion located in these four records. We use the name Levantine/Bermuda Excursion developed elsewhere by Ryan (1972) and Channell et al. (2017). The Excursion occurred at 408 ± 4 ka. The excursion is recorded at Sites 1061, 1062, and 1063. The most detailed excursion records occur at Site 1062 where we have both shipboard PSV records and one u-channel PSV record of the excursion. The excursion has two intervals of excursional directions, one short interval (∼I ky) followed by a long interval (∼5 ky). Both intervals have counter-clockwise looping of the excursional directions and the excursion is considered to be a Class I Excursion (Lund et al., 2005). Statistical study of the PSV records after removal of the excursion directions has identified two characteristics of the ‘normal’ PSV. The first feature is a long-term (>104 yrs) deviation in inclination and declination averages from the overall site averages. These deviations suggest some type on long-term memory in the regional dynamo process. The second feature is the variation in PSV angular dispersion (a measure of directional variability). The angular dispersion is quite low (∼12°) for most of the 374–478 ka interval. But, there is a short (∼8 ky) interval with angular dispersion more than double the amplitude (∼25°-30°). This interval has a sharp (<3 ky) onset and termination, This interval contains the Levantine/Bermuda Excursion. Such high-angular dispersion intervals with associated excursions and low paleointensity appear to be regular distinctive features of the Brunhes Chron PSV.

Epitaxial Growth: Out‐Of‐Plane Metal Coordination for a True Solvent‐Free Building with Molecular Bricks: Dodging the Surface Ligand Effect for On‐Surface Vacuum Self‐Assembly (Adv. Funct. Mater. 20/2021)

In article number 2011008, Alberto Bossi, Gianlorenzo Bussetti, and co-workers present a coherent layered film grown on an archetypal Fe metal electrode. The ingredients for the controlled in-vacuum self assembly are chemical decoupling of the contact interface, lateral (2D) ordering of the molecules, rigid molecule-to-molecule coupling along the third dimension and, sharp (chemical) termination of the layered film. (Graphic by Elisabetta Borseti; building: the new G. Feltrinelli Foundation headquarter in Milan is acknowledged).

Out‐Of‐Plane Metal Coordination for a True Solvent‐Free Building with Molecular Bricks: Dodging the Surface Ligand Effect for On‐Surface Vacuum Self‐Assembly

AbstractThe desirable self‐assembly (SA) of repeated 2D stacked layers requires a “holistic analysis” of three interconnected components: the electrode, the interface, and the molecular component; among them, the contact interface bears the largest burden responsibilities. Epitaxial growth (EG) of coherent 2D+n stacked heterojunction by solvent‐free deposition holds great promise, although the feasibility has never been demonstrated given multiple drawbacks (e.g., surface‐ligand effect, SLE). Here, it is demonstrated how a coherent 2D+n (n = 3) layered heterorganic film is grown on an archetypal Fe metal electrode. The groundbreaking achievement is the result of the in‐vacuum integration of: i) chemical decoupling of the basal organic layer (a ZnII‐tetraphenylporphyrine, ZnTPP) from the metal electrode, ii) 2D‐ordering of the ZnTPP commensurate to the substrate, iii) rigid, stoichiometric, and orthogonally arranged, the molecule‐to‐molecule coupling between ZnTPP and a ditopic linear bridging ligand (i.e., DPNDI) guided by SA coordination chemistry, and iv) sharp (chemical) termination of the layered film.

Crustal attenuation beneath western North America: Implications for slab subduction, terrane accretion and arc magmatism of the Cascades

The western margin of the North American continent is a composite of diverse crustal domains featuring ancestral cratons, the Cordilleran terranes and the subducting Cascadia slab. Their existence and spatial extent have been largely inferred from seismic imaging, which is highly sensitive to variations in both crustal seismic velocity and attenuation structures. In this study, we improve upon existing, yet relatively limited, knowledge of crustal attenuation by developing a new Lg-wave attenuation (Q0) model. Benefiting from a comprehensive data set from dense seismic networks in Canada and the United States, our model reveals a clear transition from the low-attenuation (Q0>450) Canadian Shield to the highly-attenuated (Q0<150) crust near the Cascade arc. In the former region, the low-attenuation structures agree well with the distribution of thick, high-velocity crust in the Cordilleran foreland, suggesting dominant effects of elastic properties such as scattering and geometry of the crustal waveguide on the attenuation characteristics of stable cratons. On the other hand, we observe strong crustal attenuations in magmatically active areas including the Cascade arc and the Yellowstone-Snake River Plain volcanic province. The excellent spatial correlation between the high-attenuation Cascades and the surface projection of the subducting slabs further suggests a possible role of slab dehydration in increasing the fluid content of the overlying continental crust and modifying seismic attenuation. The sharp termination of the highly-attenuated crust forms a structural boundary in the Cascade back-arc that likely marks the eastern subsurface limit of the Siletzia terrane. The close spatial affinity of this boundary to the isotopic ratio lineament of 87Sr/86Sr = 0.706, which is the suggested ancestral continental margin, highlights the distinctive crustal types and/or heterogeneous structures between young accreted terranes and ancient cratons.

From Slab Coupling to Slab Pull: Stress Segmentation in the Subducting Nazca Plate

AbstractWe investigate the stress field that the Nazca slab experiences during subduction beneath the South American plate by determining the focal mechanisms of moderate subduction‐related earthquakes continuously from 20‐ to 120‐km depth and inverting for the stress directions of four slab regions. Our results show the sharp termination of the coupling zone, which is characterized by compressional stresses, uplift of the overlying mountain range, and likely the activation of preexisting slab structures. Beyond and below this zone slab pull is the dominant stress. Near the slab surface, we also find signatures of the activation of inherited structures. Deeper in the slab, fault orientations are more likely controlled by the stress field alone. Along the subduction pathway, we find indication for an increase of the absolute slab pull component of the stress field that correlates with an increase in event rate and the occurrence possibility of M &gt; 7 intermediate depth earthquakes.

A STEP fault in Central Betics, associated with lateral lithospheric tearing at the northern edge of the Gibraltar arc subduction system

We study the crustal and lithospheric mantle structure under central Betics in the westernmost Mediterranean region by migrating P-receiver functions along a dense seismic profile (∼2 km interstation distance). The profile, North–South oriented, probes the crustal structure of different geological units, from the Alboran domain in the south with metamorphic rocks, through the External Zones with sedimentary rocks to the Variscan terrains of the Iberian Massif in the north. From north to south, the Moho depth increases from ∼30 km to ∼46 km underneath the Guadix basin, due to the underthrusting of the Iberian crust below the Alboran crust, and suddenly shallows to ∼30 km underneath the Internal Zones with a step of 17 km. This sharp Moho step correlates well with a lithospheric step of ∼40 km, where the thickness of the lithosphere changes abruptly from ∼100 km in the north to ∼50 km in the south. We interpret this sharp and prominent lithospheric step as the termination of the Iberian lithosphere caused by a near-vertical STEP (Subduction-Transform-Edge-Propagator) fault that continues towards the surface as a positive flower tectonic structure of crustal scale. This STEP fault is located at the northern edge of the narrow Westernmost Mediterranean subduction system facilitating the slab rollback motion towards the west. The sharp termination of the Iberian lithosphere occurs under the contact between the Alpujarride and the Nevado-Filabride complexes of the Alboran domain in an ENE-WSW right-lateral transpressive shear zone. The thickest crust and lithosphere do not correlate with the highest topography along the profile suggesting that this high topography is a combined effect of the positive flower structure, and the push up of the asthenosphere produced by the removal of the Iberian lithosphere.

Discovering geothermal supercritical fluids: a new frontier for seismic exploration

Exploiting supercritical geothermal resources represents a frontier for the next generation of geothermal electrical power plant, as the heat capacity of supercritical fluids (SCF),which directly impacts on energy production, is much higher than that of fluids at subcritical conditions. Reconnaissance and location of intensively permeable and productive horizons at depth is the present limit for the development of SCF geothermal plants. We use, for the first time, teleseismic converted waves (i.e. receiver function) for discovering those horizons in the crust. Thanks to the capability of receiver function to map buried anisotropic materials, the SCF-bearing horizon is seen as the 4km-depth abrupt termination of a shallow, thick, ultra-high (>30%) anisotropic rock volume, in the center of the Larderello geothermal field. The SCF-bearing horizon develops within the granites of the geothermal field, bounding at depth the vapor-filled heavily-fractured rock matrix that hosts the shallow steam-dominated geothermal reservoirs. The sharp termination at depth of the anisotropic behavior of granites, coinciding with a 2 km-thick stripe of seismicity and diffuse fracturing, points out the sudden change in compressibility of the fluid filling the fractures and is a key-evidence of deep fluids that locally traversed the supercritical conditions. The presence of SCF and fracture permeability in nominally ductile granitic rocks open new scenarios for the understanding of magmatic systems and for geothermal exploitation.

The ability of stratospheric climate engineering in stabilizing global mean temperatures and an assessment of possible side effects

AbstractA coupled atmosphere–ocean general circulation model is used to assess the ability of stratospheric climate engineering (CE) in stabilizing global mean temperatures at the threshold level +2 °C compared to pre‐industrial conditions. CE impact by solar radiation management (SRM) method is applied, when temperature threshold (+2 °C) is reached. Injection intensity is adjusted by the model so that calculated global temperature is remained close to the threshold value. We demonstrate that CE temperature stabilization during the 21st century is possible within the range of +2 ± 0.11 °C. Changes of precipitation intensity as well as effect of sharp termination of CE are investigated.

On Solar Flares and Cycle 23

The anomalous character of Solar Cycle 23, which ended in the Summer of 2009, has been pointed out by many authors. It has even been proposed that the solar dynamo is undergoing a transition from a state of “grand maximum” to one of “regular oscillations”. We analyze the temporal distribution of the number and energy of solar flares, and the duration of intervals between them, over Cycles 21 to 23. We consider 32 355 flares of class C2 and larger (C2+) from the GOES catalogue. Daily values of X-ray flux (wavelengths 1 to 8 A) have been computed by summing the energy proxies of the events. The series of daily numbers of C2+ solar flares are strongly correlated with their daily energy flux. The long duration of Cycle 23 (12.8 years based on sunspots, 13.2 years based on flares) and the long interval with no C2+ flare between the end of Cycle 23, and the start of Cycle 24 (466 days) are remarkable compared to the two earlier cycles. The amplitudes of extreme flares increase when the mean flux decreases. We have calculated running averages of energy flux over intervals going from 7 to 365 days. The singular shape of Cycle 23 is increasingly striking with increasing interval: in the first ≈ 70% of the cycle (displayed on a logarithmic scale) we see linearly rising maxima, whereas minima are aligned along a descending slope for the latter part of the cycle. The energy flux oscillates between these and takes the shape of a bifurcation, starting near 2002 (a time when it is suggested that photospheric fields were abruptly reduced). Inter-event intervals between successive C2+ flares undergo quasi-periodic (≈ 11 years) oscillations between two distinct states, which we call “active” and “quiet”, with extremely sharp onset and termination. The ratio of time spent in the active vs. quiet states ranges from 1.8 to 1.4 for Cycles 21 to 23, Cycle 23 having the longest quiet period. It has been proposed that anomalous Cycle 23 resembles Cycle 4, which was followed by reduced Cycles 5 and 6 at the time of the Dalton minimum in solar activity, often associated with a cooler global climate. It will be interesting to monitor the evolution of solar flares in Cycle 24, in order to further our understanding of solar activity during a sequence of possibly weak and decreasing cycles, but also of its potential relation to climate change.

Structure of the InAlAs/InP interface by atomically resolved energy dispersive spectroscopy

The structure of epitaxially grown InAlAs/InP interfaces was studied using atomically resolved x-ray energy dispersive spectroscopy in scanning transmission electron microscopy. As and P sublattices show sharp termination on the interface. The In sublattice is continuous across the interface. The study has shown the depletion of the Al concentration at the interface; at the last atomic columns of the InAlAs, In occupancy is close to 100%, while Al occupancy is almost zero. A monolayer of InAs at the interface is consistent with substitution of As for P at the surface preceding growth.

Abstract 3888: Ionizing radiation destabilizes inverted ALU repeats linked to LOH

Abstract Ionizing radiation is a potent inducer of Loss of Heterozygosity (LOH), driven primarily by DNA double strand breaks. Such a process impacts both tumorigenesis through deletion of linked tumor suppressor genes and, similarly, the potential development of radiation resistant tumor clones during radiation therapy. The human mucoepidermoid cell line, H292, was exposed to either 4 or 8 Gy and surviving clones developed from two rounds of cloning analyzed for LOH using polymorphic microsatellite markers. A high level of LOH was observed within individual clones extending from 11q21 to 11q24 (6/49 after 4 Gy and 8/50 after 8Gy). More detailed analysis using a SNP screen on selected LOH positive clones indicated a sharp telomeric termination of LOH adjacent to a common SNP location. The telomeric extent of LOH observed co-mapped to a region of elevated recombination instability documented previously using linkage analysis. Further, SNP analysis showed a region of approximately 1 Mbp LOH that included the region of recombination instability identified through linkage analysis, in some clones isolated either with or without irradiation. The consistency of the telomeric boundary of LOH observed in the irradiated clones permitted a targeted analysis of DNA double strand breaks at this location, using LM-PCR, both before and after 4Gy irradiation of the same cell line. Using a six primer set screen covering a 9 kbp region at the telomeric extent of radiation-induced LOH, a single primer set showed an elevation in DNA breaks from 2-8h after irradiation. The timing and discrete distribution of such breaks indicated that these were not directly induced by the radiation itself. Cells that were unirradiated were not affected. Further inspection of the location demonstrating selective radiation-linked fragmentation showed the presence of an ALU inverted repeat, separated by approximately 1kbp, where the DNA breaks determined by LM-PCR were clustered within the 5’ ALU. Others have shown previously that inverted ALU repeats separated by &amp;lt;20bp are recombinationally unstable and are underrepresented within the human genome. It is proposed that, within the H292 cell line, otherwise stable ALU inverted repeats are capable of forming a transient hairpin conformation, perhaps facilitated by their own transcription via RNA polymerase III, which is unstable within a genome undergoing DNA damage and repair. The combination of such inverted ALU repeats and genotoxic exposure, such as during radiation therapy, may therefore destabilize specific regions of the genome and contribute to the development of aberrant cell clones. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3888. doi:10.1158/1538-7445.AM2011-3888

Philipsburgite from the Yamato mine, Yamaguchi Prefecture, Japan

Philipsburgite, (Cu,Zn)5Zn(AsO4,PO4)2(OH)6·H2O, an As analogue of kipushite, was discovered in the Yamato mine, Yamaguchi Prefecture, Japan. Philipsburgite occurs in the form of spherical aggregates, 0.5 mm in diameter, with bright green in color. It is associated with malachite and cornwallite in cavities of the oxidized rocks, which consist of quartz and goethite. Spherical philipsburgite consists of platy crystals, 100 μm in length and 1 μm in thickness, with sharp termination. The empirical formula is (Cu4.91,Zn0.09)Σ5.00Zn1.00[(AsO4)1.63(PO4)0.37]Σ2.00(OH)6·1.60H2O on the basis of O = 11 in the anhydrous part per formula unit and the water content was measured by thermogravimetric analysis. The composition is closest to the As end member thus far reported. The unit cell parameters from the X-ray diffraction data are a = 12.359(8), b = 9.247(7), c = 10.734(5) A, β = 97.22(5)° and V = 1216.9(9) A3.

The Rockall Bank Mass Flow: Collapse of a moated contourite drift onlapping the eastern flank of Rockall Bank, west of Ireland

The Rockall Bank Mass Flow (RBMF) is a large, multi-phase submarine slope failure and mass flow complex. It is located in an area where the Feni Drift impinges upon the eastern flank of the Rockall Bank in the NE Atlantic. A 6100 km 2 region of slope failure scarps, extending over a wide water depth range and with individual scarps reaching up to 22 km long and 150 m high, lies upslope of a series of mass flow lobes that cover at least 18,000 km 2 of the base of slope and floor of the Rockall Trough. The downslope lobe complex has a negative topographic relief along much of its northern boundary, being inset below the level of the undisplaced contourite drift at the base of slope. The southern margin is topographically more subtle but is marked by the sharp termination of sediment waves outside the lobe. Within the lobe complex the southern margin of the largest lobe shows a positive relief along its southern margin. The initial failure is suggested to have occurred along coherent layer-parallel detachment surfaces at depths of up to 100 m and this promoted initial downslope block sliding which in turn transformed into debris flows which moved out into the basin. The remains of a deep erosional moat linked to the onlapping contourite complex bisects the region of failed slope, and post-failure thermohaline currents have continued to modify the mass flow in this area. Differential sedimentation and erosion associated with the moat may have promoted slope instability. Following the major failure phase, continuous readjustments of the slope occurred and resulted in small-volume turbidites found in shallow gravity cores collected on the lobes. The short term trigger for the failure remains uncertain but earthquake events associated with a deep-seated tectonic lineament to the north of the mass flow may have been important. A Late Pleistocene age for the slope failure is likely. The RBMF is unusual in that it records large-scale collapse of a contourite body that impinged on a sediment-undersupplied slope system. Unlike many other large slope failure complexes along the NE Atlantic margin, the RBMF occurs in a region where there was little overloading by glacial sediment.

Shaping of silicon crystals for channelling experiments through anisotropic chemical etching

We present an idea and its development to realize crystals for channelling experiments, which result in significant improvement of the crystal quality with respect to the traditional methods of fabrication. The technique relies on non-conventional usage of the established technique of the anisotropic etching of silicon in micro-machining. Morphological and structural analyses were carried out through electron and scanning-probe microscopy to show that the crystal exhibited flat surfaces with atomically sharp termination, i.e. no appreciable surface damage was induced by the preparation. Thereby, the crystal meets the stringent requirements that are demanded for operation with high-energy beams and in particular for halo collimation in modern hadron colliders.

In 2O 3 nanowires for gas sensors: morphology and sensing characterisation

Thin and densely packed In 2O 3 nanowires have been synthesised on alumina substrates via transport and condensation method, starting from nanoparticles of indium or palladium as catalysts for the condensation process. Indium catalyst promoted wires growth according to vapour–solid (VS) mechanism, while palladium catalyst leads to wires formation based on vapour–liquid–solid (VLS) condensation. Electron microscopy and related diffraction analysis demonstrated that the wires are monocrystalline, with atomically sharp termination of the lateral sides, and are free from extended defects. The sensing properties of nanowires bundles have been tested to acetone using the flow through technique in the temperature range between 100 and 500 °C.

Yarkovsky footprints in the Eos family

The Eos asteroid family is the third most populous, after Themis and Koronis, and one of the largest non-random groups of asteroids in the main belt. It has been known and studied for decades, but its structure and history still presented difficulties to understand. We first revise the Eos family identification as a statistical cluster in the space of proper elements. Using the most to-date catalogue of proper elements we determine a nominal Eos family, defined by us using the hierarchical-clustering method with the cut-off velocity of 55 m/s, contains some 4400 members. This unforeseen increase in known Eos asteroids allows us to perform a much more detailed study than was possible so far. We show, in particular, that most of the previously thought peculiar features are explained within the following model: (i) collisional disruption of the parent body leads to formation of a compact family in the proper element space (with characteristic escape velocities of the observed asteroids of tens of meters per second, compatible with hydrocode simulations), and (ii) as time goes, the family dynamically evolves due to a combination of the thermal effects and planetary perturbations. This model allows us to explain sharp termination of the family at the J7/3 mean motion resonance with Jupiter, uneven distribution of family members about the J9/4 mean motion resonance with Jupiter, semimajor axis distribution of large vs small members in the family and anomalous residence of Eos members inside the high-order secular resonance z 1 . Our dynamical method also allows us to estimate Eos family age to 1.3 −0.2 +0.15 Gyr . Several formal members of the Eos family are in conflict with our model and these are suspected interlopers. We use spectroscopic observations, whose results are also reported here, and results of 5-color wide-band Sloan Digital Sky Survey photometry to prove some of them are indeed spectrally incompatible with the family.

Seismic detection of sublithospheric plume head residue beneath the Pitcairn hot-spot chain

We study array seismograms of French nuclear explosions from two islands in French Polynesia and use these to constrain the structure in the upper mantle beneath the islands. Seismograms of nuclear explosions on the hot-spot-related Fangataufa atoll show discrete, large-amplitude P-coda phases which are not observed in recordings of explosions on Mururoa, an atoll ∼40 km to the north. The source for these P-coda phases is located beneath the Fangataufa atoll, indicating that structural heterogeneities are present in the oceanic upper mantle in this region on very small scale lengths. Synthetic seismograms for models of the upper mantle beneath Fangataufa require a layer with P-wave velocity elevated by ∼10% between depths of 51 and 85 km with a sharp termination in the north, possibly at the Austral Fracture Zone, to match these P-coda phases. Few mineralogic scenarios exist that can explain this structure, and the properties of this layer imply that extensive enrichment in garnet occurs in this depth range. The garnet-enriched layer is likely of similar origin to the well-known xenolithic ‘garnet megacryst’ suite found in kimberlitic regions. We propose a model for the formation of the Fangataufa and Mururoa atolls involving garnet-enriched zones being generated at depth through magmatic processes at the Pitcairn plume head. Thus, the initiation of hot-spots could produce complex geochemical and structural heterogeneities at depth in the suboceanic mantle.

Reciprocal space mapping of GaN xAs 1 −x grown by RF plasma-assisted solid source molecular beam epitaxy

We present the change in diffused scattering intensity and crystal truncation rod in X-ray reciprocal space mapping (RSM) of GaN x As 1− x grown by a radio frequency (RF) nitrogen plasma source in a solid source molecular beam epitaxy system. The RSM results are discussed in relation to variation in the low-temperature photoluminescence (PL) efficiency and peak energy. The X-ray RSM plots were recorded using a high-resolution triple-axis X-ray diffractometer on GaNAs samples annealed from 550 to 800°C. The RSM plots show higher diffused scattering around the GaNAs and GaAs (0 0 4) reflection point for the as-grown GaN x As 1− x sample with x=1.3%, indicating a high incorporation rate of N atoms and N complexes as interstitials, compared to a 5 μm-thick unintentionally doped GaAs sample. A reduction in diffused scattering around the GaN x As 1− x and GaAs (0 0 4) Bragg reflection point was observed with sharp termination of the crystal surface in the sample annealed at ∼700°C for 10 min. This is in good agreement with the observed improved PL efficiency, due to annihilation of interstitial point defects.

Achromatic optical device for generation of a broadband frequency spectrum with high-frequency stability and sharp termination

We have implemented a new optical scheme to produce a broadband frequency spectrum with customizable shape and high stability. The scheme consists of the generation of a mode structure by allowing a frequency-stabilized single-mode laser to pass through an acousto-optical modulator working in the Raman–Nath regime. Diffraction yields several diffracted beams, which are shifted in frequency (ΔνM) by acoustic waves in the acousto-optical modulator. Then the diffracted beams are merged into one beam by means of a telescope. The scheme has worked in both the visible and the UV: Frequency spectra as wide as nΔνM with n=7 have been achieved, resulting in widths of 225 and 150 MHz for the visible and the UV spectral regions, respectively. The spectra possess the same stability as the primary single-mode laser. Their profiles may exhibit a sharp termination on one side of the spectrum, a feature that allows them to be utilized for high-efficiency laser cooling of an ion beam in a storage ring.

Fast imaging in two dimensions resolves extensive sources of Ca2+ sparks in frog skeletal muscle.

Ca2+ sparks were monitored by confocal laser-scanning microscopy of fluo-3 at video rates, in fast twitch muscle fibres, stimulated by exposure to caffeine and/or low [Mg2+]. Scanning was in two spatial dimensions ('2D') or 2D plus time, at 4 ms per image frame. Sparks were identified in 2D images of normalized fluorescence by an automatic procedure, which also evaluated the event's location and morphometric parameters. Most sparks were circular, but some were elongated, especially in caffeine. Separation of the spark from circular symmetry was quantified by its eccentricity (length/width - 1). In an internal solution with 0.4 mM [Mg2+], sparks (989 events in 4 cells) had amplitude 0.73, width 1.94 microm, length 2.12 microm and eccentricity not significantly different from 0. Upon application of 1 mM caffeine, length (of 2578 events in the same cells) increased significantly (by 0.41 microm, or 19 %), width increased by 0.18 microm (9 %) and amplitude decreased slightly. The eccentricity became significantly different from 0, and the sparks' long axis predominantly oriented parallel to the plane of the Z disks. More than 10 % of the events in caffeine had length greater than 4 microm, a relatively flat top, and a sharp termination at both ends of the major axis. Additionally, there was only a weak correlation between eccentricity and amplitude. These properties suggest that the elongated events are produced by simultaneous opening of multiple channels within a junction, rather than anisotropic diffusion of Ca2+ or random overlap of round sparks. Elongated events often increased in eccentricity early in their evolution. Then, most remained elongated during their rise and decay, while others spread spatially in the plane of the Z disks. In 1-2 % of the events, the centre of mass migrated in space, over time, at approximately 0.1 microm x ms(-1). These spatio-temporal features require the involvement of multiple release channels, at spatially resolvable locations. Because sources often spread over distances greater than 1 or 2 microm, and arrays of junctional elements (couplons) are at most 1.2 microm long, it must be possible for activation of release to propagate between neighbouring couplons, especially under the influence of caffeine and/or low [Mg2+].