Chicxulub impactites: Geochemical clues to the precursor rocks

Abstract— The 65 Ma Chicxulub impact structure, Mexico, with a diameter of ～180 km is the focus of geoscientific research because of its link to the mass extinction event at the Cretaceous‐Tertiary (K/T) boundary. Chicxulub, now buried beneath thick post‐impact sediments, is probably one of the best‐preserved terrestrial impact structures known. Because of its inaccessibility, only limited samples on the impact lithologies from a few drill cores are available. We report major element and Sr‐, Nd‐, O‐, and C‐isotopic data for Chicxulub impact‐melt lithologies and basement clasts in impact breccias of drill cores C‐1 and Y‐6, and for melt particles in the Chicxulub ejecta horizon at the K/T boundary in Beloc, Haiti. The melt lithologies with SiO₂ ranging from 58 to ～63 wt% show significant variations in the content of Al, Ca, and the alkalies. In the melt matrix samples, δ¹³C of the calcite is about ?3%o. The δ¹⁸O values for the siliceous melt matrices of Y‐6 samples range from 9.9 to 12.4%o. Melt lithologies and the black Haitian glass have rather uniform ⁸⁷Sr/⁸⁶Sr ratios (0.7079 to 0.7094); only one lithic fragment displays ⁸⁷Sr/⁸⁶Sr of 0.7141. The Sr model ages T^SrUR for most lithologies range from 830 to 1833 Ma; unrealistic negative model ages point to an open Rb‐Sr system with loss of Rb in a hydrothermal process. The ¹⁴³Nd/¹⁴⁴Nd ratios for all samples, except one basement clast with ¹⁴³Nd/¹⁴⁴Nd of 0.5121, cluster at 0.5123 to 0.5124. In an ?_Nd‐?_Sr diagram, impactites plot in a field delimited by ?_Nd of ?2 to ?6, and ?_Sr of 55 to 69. This field is not defined by the basement lithologies described to occur as lithic clasts in impact breccias and Cretaceous sediments. At least one additional intermediate to mafic precursor component is required to explain the data.     

Magnetic Twist and Writhe of δ Active Regions

We have selected 104 active regions with a δ magnetic configuration from 1996 to 2002 to study how important a role the kink instability plays in such active regions. In this study, we employ the systematic tilt angle of each active region as a proxy for the writhe of a flux tube and the force-free parameter, α_best, as a proxy for the magnetic field twist in the flux tube. It is found that 65–67% of the active regions have the same sign of twist and writhe. About 34% (32%) of the active regions violate (follow) the Hale-Nicholson and Joy's Laws (HNJL) but follow (violate) the hemispheric helicity rule (HHR). Sixty-one (61) of the 104 active regions studied each produced more than five large flares. Active regions violating HNJL, but following HHR, have a much stronger tendency to produce X-class flares and/or strong proton events. Comparing with previous studies for active regions with well-defined (simpler) bipolar magnetic configuration, it is found that the numbers following both HNJL and HHR are significantly lower in the δ-configuration case, while numbers violating one of the laws and the rule significantly increase with the increase of the magnetic complexity of the active regions. These results support the prediction for the presence of a kink instability, that the twist and writhe of the magnetic fields exhibit the same sign for δ active regions (Linton et al., Astrophys. J. 507, 40, 1998, Astrophys. J. 522, 1205, 1999; Fan et al., Astrophys. J. 521, 460, 1999). Finally, we analyze possible origins of the twist and writhe of the magnetic fields for the active regions studied.     

Correction of inertial oscillations by assimilation of HF radar data in a model of the Ligurian Sea

This article aims at analyzing if high-frequency radar observations of surface currents allow to improve model forecasts in the Ligurian Sea, where inertial oscillations are a dominant feature. An ensemble of ROMS models covering the Ligurian Sea, and nested in the Mediterranean Forecasting System, is coupled with two WERA high-frequency radars. A sensitivity study allows to determine optimal parameters for the ensemble filter. By assimilating observations in a single point, the obtained correction shows that the forecast error covariance matrix represents the inertial oscillations, as well as large- and meso-scale processes. Furthermore, it is shown that the velocity observations can correct the phase and amplitude of the inertial oscillations. Observations are shown to have a strong effect during approximately half a day, which confirms the importance of using a high temporal observation frequency. In general, data assimilation of HF radar observations leads to a skill score of about 30% for the forecasts of surface velocity.     

Role of Sunspot and Sunspot-Group Rotation in Driving Sigmoidal Active Region Eruptions

We study active region NOAA 9684 (N06L285) which produced an X1.0/3B flare on November 4, 2001 associated with a fast CME (1810 km s⁻¹) and the largest proton event (31 700 pfu) in cycle 23. SOHO/MDI continuum image data show that a large leading sunspot rotated counter-clockwise around its umbral center for at least 4 days prior to the flare. Moreover, it is found from SOHO/MDI 96 m line-of-sight magnetograms that the systematic tilt angle of the bipolar active region, a proxy for writhe of magnetic fluxtubes, changed from a positive value to a negative one. This signifies a counter-clockwise rotation of the spot-group as a whole. Using vector magnetograms from Huairou Solar Observing Station (HSOS), we find that the twist of the active region magnetic fields is dominantly left handed (α_best = −0.03), and that the vertical current and current helicity are predominantly negative, and mostly distributed within the positive rotating sunspot. The active region exhibits a narrow inverse S-shaped H_α filament and soft X-ray sigmoid distributed along the magnetic neutral line. The portion of the filament which is most closely associated with the rotating sunspot disappeared on November 4, and the corresponding portion of the sigmoid was observed to erupt, producing the flare and initiating the fast CME and proton event. These results imply that the sunspot rotation is a primary driver of helicity production and injection into the corona. We suggest that the observed active region dynamics and subsequent filament and sigmoid eruption are driven by a kink instability which occurred due to a large amount of the helicity injection.     

Retrieving volume FeO and TiO_2 abundances of lunar regolith with CE-2 CELMS data using BPNN method

The volume FeO and TiO_2 abundances(FTAs) of lunar regolith can be more important for understanding the geological evolution of the Moon compared to the optical and gamma-ray results. In this paper, the volume FTAs are retrieved with microwave sounder(CELMS) data from the Chang'E-2 satellite using the back propagation neural network(BPNN) method. Firstly, a three-layered BPNN network with five-dimensional input is constructed by taking nonlinearity into account. Then, the brightness temperature(TB) and surface slope are set as the inputs and the volume FTAs are set as the outputs of the BPNN network.Thereafter, the BPNN network is trained with the corresponding parameters collected from Apollo, Luna,and Surveyor missions. Finally, the volume FTAs are retrieved with the trained BPNN network using the four-channel TBderived from the CELMS data and the surface slope estimated from Lunar Orbiter Laser Altimeter(LOLA) data. The rationality of the retrieved FTAs is verified by comparing with the Clementine UV-VIS results and Lunar Prospector(LP) GRS results. The retrieved volume FTAs enable us to re-evaluate the geological features of the lunar surface. Several important results are as follows. Firstly, very-low-Ti(<1.5 wt.%) basalts are the most spatially abundant, and the surfaces with TiO_2> 5 wt.% constitute less than 10% of the maria. Also, two linear relationships occur between the FeO abundance(FA) and the TiO_2 abundance before and after the threshold, 16 wt.% for FA. Secondly, a new perspective on mare volcanism is derived with the volume FTAs in several important mare basins, although this conclusion should be verified with more sources of data. Thirdly, FTAs in the lunar regolith change with depth to the uppermost surface,and the change is complex over the lunar surface. Finally, the distribution of volume FTAs hints that the highlands crust is probably homogeneous, at least in terms of the microwave thermophysical parameters.     

The Geography of Italian Pasta

Pasta is as much an institution as a food in Italy, where it has made a significant contribution to national culture. Its historical geography is one of strong regional variations based on climate, social factors, and diffusion patterns. These are considered herein; a taxonomy of pasta types is presented and illustrated in a series of maps that show regional variations. The classification scheme divides pasta into eight classes based on morphology and, where appropriate, filling. These include the spaghetti and tubular families, pasta shells, ribbon forms, short pasta, very small or “micropasta” types, the ravioli family of filled pasta, and the dumpling family, which includes gnocchi. Three patterns of diffusion of pasta types are identified: by sea, usually from the Mezzogiorno and Sicily, locally through adjacent regions, and outwards from the main centers of adoption. Many dry pasta forms are native to the south and center of Italy, while filled pasta of the ravioli family predominates north of the Apennines. Changes in the geography of pasta are reviewed and analyzed in terms of the modern duality of culture and commercialism.     

Transient Electric Field in the Mesosphere above a Γ-shape Lightning Stroke

We describe the space–time distribution of the pulsed electric field in the middle atmosphere above a positive Γ-shaped lightning stroke. The channel of such a discharge contains a vertical and a horizontal section. The current wave moves initially vertically and then turns horizontally so that radiation appears from the vertical electric dipole followed by that from the horizontal dipole. Combined with reflection from the perfectly conducting ground, the source provides three subsequent pulses in the atmosphere, with the lag being determined by the finite velocity of the current wave in the Γ-shaped stroke. The pulses are reproduced by reflections from the air-ground and the air-ionosphere interfaces and the waveform resembles the M-component, which is often noted in the negative strokes (e.g. Yashunin et al., J Geophys Res 112:D10109, 2007). The non-stationary fine structure appears in the spatial distribution of electric field, which persists for 2 ms or even more and exceeds the runaway electron threshold. Estimates support the idea of free electron bunching in the mesosphere by the pulsed electric field. Focusing may occur about 10 km away from the point of electron- field interaction; it is delayed by a few ms from the moment of interaction. The data presented might be helpful in realistic modeling of the red sprite formation.     

The 1.10- and 1.18-μm nightside windows of Venus observed by SPICAV-IR aboard Venus Express

Observations of the 1.10- and 1.18-μm nightside windows by the SPICAV-IR instrument aboard Venus Express were analyzed to characterize the various sources of gaseous opacity and determine the H₂O mole fraction in the lower atmosphere of Venus. We showed that the line profile model of Afanasenko and Rodin (Afanasenko, T.S., Rodin, A.V. [2007]. Astron. Lett. 33, 203–210) underestimates the CO₂ absorption in the high-wavelength wing of the 1.18-μm window and we derived an empirical lineshape that matches this wing well. An additional continuum opacity is required to reproduce the variation of the 1.10- and 1.18-μm radiances with surface elevation as observed by the VIRTIS-M instrument aboard Venus Express. A constant absorption coefficient of 0.7 ± 0.2 × 10⁻⁹ cm⁻¹ am⁻² best reproduces the observed variation. We compared spectra calculated with different CO₂ and H₂O line lists. We found that the CDSD line list lacks the 5ν₁ + ν₃ series of CO₂ bands, which provide significant opacity in Venus’ deep atmosphere, and we have constructed a composite line list that best reproduces the observations. We also showed for the first time that HDO brings significant absorption at 1140–1190 nm. Using the best representation of the atmospheric opacity we could reach, we retrieved a water vapor mole fraction of ppmv, pertaining to the altitude range 5–25 km. Combined with previous measurements in the 1.74- and 2.3-μm windows, this result provides strong evidence for a uniform H₂O profile below 40 km, in agreement with chemical models.