An ichnofabric approach to the depositional interpretation of the intensely burrowed Bateig Limestone, Miocene, SE Spain

The foraminiferal-rich pelagic Bateig Limestone forms several varieties of the important building stones quarried at Bateig Hill in southeastern Spain. Three principal ichnofabrics (Bichordites, mottled-Palaeophycus and mottled-Ophiomorpha) are recognized, which are present in at least two (possibly up to four) repeated successions (cycles). Each succession begins with an erosional event. The Bichordites ichnofabric represents a new type of facies, formed as thin turbidity/grain flow, stratiform units derived from sediment slips off a fault into deep water. Each slipped unit became almost completely bioturbated by infaunal echinoids, colonizing by lateral migration. Because of the thinness of the units, successive colonizations tended to truncate the underlying burrows giving rise to a pseudo-stratification. As the Bichordites ichnofabric accumulated on the fault apron, thus reducing the effective height of the fault scarp, the substrate gradually came under the influence of currents traversing the shelf. This led to a change in hydraulic regime, and to the mottled-Palaeophycus and mottled-Ophiomorpha ichnofabrics in sediment deposited under bed load transport, and associated with laminar and cross-stratified beds and local muddy intervals. Reactivation of the fault triggered erosion and channeling and a return to grain flow sedimentation, and to the Bichordites ichnofabric of the succeeding cycle. The highest unit of the Bateig Limestone is formed entirely of cross-stratified calcarenites with occasional Ophiomorpha (Ophiomorpha-primary lamination ichnofabric) and is similar to many shallow marine facies but they still bear a significant content of pelagic foraminifera. The sedimentary setting bears resemblance with that described for the Pleistocene Monte Torre Paleostrait and the modem Strait of Messina (Italy), where the narrow morphology of the depositional area enhanced tidal currents and allowed for high-energy sandy deposition in relatively deep areas. More data on the Miocene paleogeography of the Bateig area should provide further testing for this hypothesis. The ichnofacies and stacking of the Bateig Limestone differ from the classic Seilacherian model in that they reflect changes in hydraulic process and are associated with faulting and subsidence and changes in sediment supply. Recognition of the unusual ichnofabrics and their relationships provides a clear indication of the overall dynamic setting. (c) 2006 Elsevier B.V. All rights reserved.

On the steadiness of separating meandering currents

The existence of inertial steady currents that separate from a coast and meander afterward is investigated. By integrating the zonal momentum equation over a suitable area, it is shown that retroflecting currents cannot be steady in a reduced gravity or in a barotropic model of the ocean. Even friction cannot negate this conclusion. Previous literature on this subject, notably the discrepancy between several articles by Nof and Pichevin on the unsteadiness of retroflecting currents and steady solutions presented in other papers, is critically discussed. For more general separating current systems, a local analysis of the zonal momentum balance shows that given a coastal current with a specific zonal momentum structure, an inertial, steady, separating current is unlikely, and the only analytical solution provided in the literature is shown to be inconsistent. In a basin-wide view of these separating current systems, a scaling analysis reveals that steady separation is impossible when the interior flow is nondissipative (e.g., linear Sverdrup-like). These findings point to the possibility that a large part of the variability in the world’s oceans is due to the separation process rather than to instability of a free jet.

Voltage-gated potassium currents within the dorsal vagal nucleus: inhibition by BDS toxin

Voltage-gated potassium (Kv) channels are essential components of neuronal excitability. The Kv3.4 channel protein is widely distributed throughout the central nervous system (CNS), where it can form heteromeric or homomeric Kv3 channels. Electrophysiological studies reported here highlight a functional role for this channel protein within neurons of the dorsal vagal nucleus (DVN). Current clamp experiments revealed that blood depressing substance (BDS) and intracellular dialysis of an anti-Kv3.4 antibody prolonged the action potential duration. In addition, a BDS sensitive, voltage-dependent, slowly inactivating outward current was observed in voltage clamp recordings from DVN neurons. Electrical stimulation of the solitary tract evoked EPSPs and IPSPs in DVN neurons and BDS increased the average amplitude and decreased the paired pulse ratio, consistent with a presynaptic site of action. This presynaptic modulation was action potential dependent as revealed by ongoing synaptic activity. Given the role of the Kv3 proteins in shaping neuronal excitability, these data highlight a role for homomeric Kv3.4 channels in spike timing and neurotransmitter release in low frequency firing neurons of the DVN.

Tracking the momentum flux of a CME and quantifying its influence on geomagnetically induced currents at Earth

We investigate a coronal mass ejection (CME) propagating toward Earth on 29 March 2011. This event is specifically chosen for its predominately northward directed magnetic field, so that the influence from the momentum flux onto Earth can be isolated. We focus our study on understanding how a small Earth-directed segment propagates. Mass images are created from the white-light cameras onboard STEREO which are also converted into mass height-time maps (mass J-maps). The mass tracks on these J-maps correspond to the sheath region between the CME and its associated shock front as detected by in situ measurements at L1. A time series of mass measurements from the STEREO COR-2A instrument is made along the Earth propagation direction. Qualitatively, this mass time series shows a remarkable resemblance to the L1 in situ density series. The in situ measurements are used as inputs into a three-dimensional (3-D) magnetospheric space weather simulation from the Community Coordinated Modeling Center. These simulations display a sudden compression of the magnetosphere from the large momentum flux at the leading edge of the CME, and predictions are made for the time derivative of the magnetic field (dB/dt) on the ground. The predicted dB/dt values were then compared with the observations from specific equatorially located ground stations and showed notable similarity. This study of the momentum of a CME from the Sun down to its influence on magnetic ground stations on Earth is presented as a preliminary proof of concept, such that future attempts may try to use remote sensing to create density and velocity time series as inputs to magnetospheric simulations.

Cusp ion steps, field-aligned currents and poleward moving auroral forms

We predict the field-aligned currents around cusp ion steps produced by pulsed reconnection between the geomagnetic field and an interplanetary magnetic field (IMF) with a B-Y component that is large in magnitude. For B-Y > 0, patches of newly opened flux move westward and eastward in the Northern and Southern Hemispheres, respectively, under the influence of the magnetic curvature force. These flow directions are reversed for B-Y < 0. The speed of this longitudinal motion initially grows with elapsed time since reconnection, but then decays as the newly opened field lines straighten. We predict sheets of field-aligned current on the boundaries between the patches produced by successive reconnection pulses, associated with the difference in the speeds of their longitudinal motion. For low elapsed times since reconnection, near the equatorward edge of the cusp region where the field lines are accelerating, the field-aligned current sheets will be downward or upward in both hemispheres for positive or negative IMF B-Y, respectively. At larger elapsed times since reconnection, as events slow and evolve from the cusp into the mantle region, these field-aligned current directions will be reversed. Observations by the Polar spacecraft on August 26,1998, show the predicted upward current sheets at steps seen in the mantle region for IMF B-Y > 0. Mapped into the ionosphere, the steps coincide with poleward moving events seen by the CUTLASS HF radar. The mapped location of the largest step also coincides with a poleward moving arc seen by the UVI imager on Polar. We show that the arc is consistent with a region of upward field-aligned current that has become unstable, such that a potential drop of about 1 kV formed below the spacecraft. The importance of these observations is that they confirm that the poleward moving events, as seen by the HF radar and the UV imager, are due to pulsed magnetopause reconnection. Milan et al. [2000] noted that the great longitudinal extent of these events means that the required reconnection pulses would have contributed almost all the voltage placed across the magnetosphere at this time. The observations also show that auroral arcs can form on open field lines in response to the pulsed application of voltage at the magnetopause.

Coherent EISCAT Svalbard Radar spectra from the dayside cusp/cleft and their implications for transient field-aligned currents

Naturally enhanced incoherent scatter spectra from the vicinity of the dayside cusp/cleft, interpreted as being due to plasma turbulence driven by short bursts of intense field-aligned current, are compared with high-resolution narrow-angle auroral images and meridian scanning photometer data. Enhanced spectra have been observed on many occasions in association with nightside aurora, but there has been only one report of such spectra seen in the cusp/cleft region. Narrow-angle images show considerable change in the aurora on timescales shorter than the 10-s radar integration period, which could explain spectra observed with both ion lines simultaneously enhanced. Enhanced radar spectra are generally seen inside or beside regions of 630-nm auroral emission, indicative of sharp F region conductivity gradients, but there appears also to be a correlation with dynamic, small-scale auroral forms of order 100 m and less in width.

DE-2 observations of filamentary currents at ionospheric altitudes

Conjunctive measurements made by the Dynamics Explorer 1 and 2 spacecraft on October 22, 1981, under conditions of southward IMF, suggest the existence of a cusp ion injection from a region at the magnetopause with a scale size of ∼ 1/2 to 1 R E . Current signatures observed by the LAPI and MAGB instruments on board DE-2 indicate the existence of a rotation in the magnetic field that is consistent with a filamentary current system. The observed current structure can be interpreted as the ionospheric signature of a flux transfer event (FTE). In addition to this large-scale current structure there exist three small-scale filamentary current pairs. These current pairs close locally and thus, if our interpretation of this event as an FTE is correct, represent the first reported observations of FTE interior structure at low-altitudes.