Local remineralization patterns in the mesopelagic zone of the Eastern North Atlantic, off the NW Iberian Peninsula

The short-time-scale variability of the remineralization patterns in the domain of Eastern North Atlantic Central Waters (ENACW) off the NW Iberian Peninsula is studied based on biogeochemical data (oxygen, nutrient salts, total alkalinity, pH, dissolved organic matter and fluorescence of dissolved humic substances) collected weekly between May 2001 and April 2002. The temporal variability of inorganic variables points to an intensification of remineralization during the summer and autumn, with an increase of nutrients, total inorganic carbon and fluorescence and a decrease of oxygen. During the subsequent winter mixing, there is a biogeochemical reset of the system, with lower nutrients, total inorganic carbon and fluorescence and higher oxygen. In contrast to inorganic variables, the levels of dissolved organic matter in the ENACW seem to respond to short-term events probably associated with fast sinking particles, where solubilisation of organic matter prevails over remineralization. Applying a previously published stoichiometric model, we observed a vertical fractionation of organic-matter remineralization. Although there is a preferential remineralization of proteins and P compounds in the entire domain of ENACW, the percentage was higher in the upper ENACW (σ<27.10 kg/m³) than in the lower; the percentage of N and P compounds in the oxidised organic matter was >80% for the upper ENACW and 63% for the lower. Likewise, the redissolution of calcareous structures contributes about 6% and 13% to the carbon regenerated in the upper and lower layers of ENACW, respectively.     

Photoionized Hβ emission in NGC 5548: it breathes!

Emission-line regions in active galactic nuclei (AGNs) and other photoionized nebulae should become larger in size when the ionizing luminosity increases. This 'breathing' effect is observed for the Hβ emission in NGC 5548 by using Hβ and optical continuum light curves from the 13-yr (1989–2001) AGN Watch monitoring campaign. To model the breathing, we use two methods to fit the observed light curves in detail: (i) parametrized models and, (ii) the memecho reverberation-mapping code. Our models assume that optical continuum variations track the ionizing radiation, and that the Hβ variations respond with time-delays τ due to light travel-time. By fitting the data using a delay-map  Ψ(τ, F _c)  that is allowed to change with continuum flux F _c, we find that the strength of the Hβ response decreases and the time-delay increases with ionizing luminosity. The parametrized breathing models allow the time-delay and the Hβ flux to depend on the continuum flux so that,  τ∝ F ^β_c  and   F _Hβ∝ F ^α_c  . Our fits give  0.1 < β < 0.46  and  0.57 < α < 0.66. α  is consistent with previous work by Gilbert and Peterson, and Goad, Korista and Knigge. Although we find β to be flatter than previously determined by Peterson et al. using cross-correlation methods, it is closer to the predicted values from recent theoretical work by Korista and Goad.     

Thresholds for post-wildfire debris flows: Insights from the Pinal Fire,Arizona, USA

Wildfire significantly alters the hydrologic properties of a burned area, leading to increases in overland flow, erosion, and the potential for runoff-generated debris flows. The initiation of debris flows in recently burned areas is well characterized by rainfall intensity-duration (ID) thresholds. However, there is currently a paucity of data quantifying the rainfall intensities required to trigger post-wildfire debris flows, which limits our understanding of how and why rainfall ID thresholds vary in different climatic and geologic settings. In this study, we monitored debris-flow activity following the Pinal Fire in central Arizona, which differs from both a climatic and hydrogeomorphic perspective from other regions in the western United States where ID thresholds for post-wildfire debris flows are well established, namely the Transverse Ranges of southern California. Since the peak rainfall intensity within a rainstorm may exceed the rainfall intensity required to trigger a debris flow, the development of robust rainfall ID thresholds requires knowledge of the timing of debris flows within rainstorms. Existing post-wildfire debris-flow studies in Arizona only constrain the peak rainfall intensity within debris-flow-producing storms, which may far exceed the intensity that actually triggered the observed debris flow. In this study, we used pressure transducers within five burned drainage basins to constrain the timing of debris flows within rainstorms. Rainfall ID thresholds derived here from triggering rainfall intensities are, on average, 22 mm h⁻¹ lower than ID thresholds derived under the assumption that the triggering intensity is equal to the maximum rainfall intensity recorded during a rainstorm. We then use a hydrologic model to demonstrate that the magnitude of the 15-min rainfall ID threshold at the Pinal Fire site is associated with the rainfall intensity required to exceed a recently proposed dimensionless discharge threshold for debris-flow initiation. Model results further suggest that previously observed differences in regional ID thresholds between Arizona and the San Gabriel Mountains of southern California may be attributed, in large part, to differences in the hydraulic properties of burned soils. © 2019 John Wiley & Sons, Ltd.     

Low-frequency current variability observed at the shelfbreak in the northeastern Gulf of Mexico: November 2004–May 2005

Fourteen acoustic Doppler current profilers (ADCPs) were deployed on the shelf and slope for 1 year just west of the DeSoto Canyon in the Northeastern Gulf of Mexico by the Naval Research Laboratory (NRL) as part of its Slope to Shelf Energetics and Exchange Dynamics (SEED) project. The winter and spring observations are discussed here in regards to the low-frequency current variability and its relation to wind and eddy forcing. Empirical orthogonal function (EOF) analyses showed that two modes described most of the current variability. Wind-forced variability of the along-shelf flow was the main contributor in Mode 1 while eddies contributed much of the variability in Mode 2. Wind-stress controlled currents on the shelf and slope at time scales of about a week. On longer time scales, variations in the currents on both the outer shelf and slope appear to be related to seasonal variations in the time-cumulated wind stress curl. Winds were dominant in driving the along-shelf transports, particularly along the slope. However, the effective wind stress component was found to be aligned with the west Florida shelf direction rather than the local shelf direction. Eddy intrusions, which were more numerous in winter and spring than in summer and fall, and winds were found to contribute significantly to cross-shelf exchange processes.     

On emission lines of hydrogen,helium and ionized calcium seen on a coronal spectrogram of the March 7, 1970 eclipse

Emission lines of the Balmer series, D₃ and H and K are reported present on a coronal spectrogram obtained at the March 7, 1970 eclipse. Arguments are presented to show that these could not have originated from scattering in the Earth's atmosphere and hence possibly have a coronal origin.     

Late Holocene dispersal and accumulation of terrigenous sediment on Poverty Shelf, New Zealand

We use coupled numerical models (HydroTrend and SedFlux) to investigate the dispersal and accumulation of sediment on Poverty Shelf, North Island, New Zealand, during the past 3 kyr. In this timeframe, we estimate that the Waipaoa River system delivered ∼10 Gt of sediment to Poverty Shelf,  5–10% of which was transported to the outer shelf and continental slope. The domain of the two-dimensional model (SedFlux) is representative of a 30 km traverse across the shelf. Comparing the model output with seismic reflection data and a core obtained from the middle shelf shows that, without extensively modifying the governing equations or imposing unrealistic conditions on the model domain, it is possible to replicate the geometry, grain size and accumulation rate of the late Holocene mud deposit. The replicate depositional record responds to naturally and anthropogenically induced vegetation disturbance, as well as to storms forced by long-period climatic events simulated entirely within the model domain. The model output also suggests that long-term fluctuations in the amount and caliber of river sediment discharge, promoted by wholesale changes in the catchment environment, may be translated directly to the shelf depositional record, whereas short-term fluctuations conditioned by event magnitude and frequency are not. Thus on Poverty Shelf, as well as in depocenters on other active continental margins which retain a much smaller proportion of the terrigeneous sediment delivered to them, flood-generated event beds are not commonplace features in the high-resolution sedimentary record. This is because the shelf sedimentary record is influenced more by the energy available to the coastal ocean which helps keep the sediment in suspension and facilitates its dispersal, than by basin hydrometeorology which determines the turbidity and velocity of the river plume.     

Parametric integral model of the wind disturbance concerted with the All-Union wave constructional standards and regulations (CSR)

In a modern statement, the concept of a parametric integral model consists of integration of the energy balance equation (or wind forcing) over the frequency. The angular distribution of the energy and the frequency spectrum are prescribed. The equation is solved by the numerical method taking into consideration the energy advection and its local pumping with the assigned wind field varying in time and space. To calculate the swell waves one can employ the well-known Longuet-Higgins theorem on the invariance of a two-dimensional spectrum. or the ray discrete technique.UDK 551.466.315Translated by Mikhail M. Trufanov.     

Quaternary evolution of Cedar Creek alluvial fan, montana

Cedar Creek alluvial fan is a textbook example of an alluvial fan because of its fan shape with smooth, concentric contours and excellent symmetry. Similar planimetric shapes have been used to infer uniform fan deposition; however, Cedar Creek alluvial fan is composed of four fan deposits of Quaternary age, Qf1 (oldest) to Qf4 (youngest), indicating that fan deposition was nonuniform in both time and space. Field studies indicate that deposition of Cedar Creek alluvial fan is related to glaciofluvial outwash activity during the Pleistocene and upper-fan entrenchment and lower-fan deposition during the Holocene.Qf1 and Qf2 deposits are sub-horizontally bedded, clast-supported sandy gravels uniformly imbricated upfan. Comparison of soil profiles developed in these deposits to radiogenically-dated chronosequences within the region indicates that Qf1 and Qf2 are correlative with Bull Lake and Pinedale-age deposits, respectively. These relationships are substantiated by physical correlation of Qf1 and Qf2 with Bull Lake and Pinedale moraines, respectively, in the Cedar Creek drainage basin. The sedimentology and timing of Qf1 and Qf2 indicate deposition in high-energy, proglacial, braided streams. Furthermore, the present morphology of Cedar Creek alluvial fan was established largely during aggradation of Qf1 and Qf2 when sediment supply to the fan was sufficient to activate 60% to greater than 90% of the total fan area. During Bull Lake glaciation, the apex of Qf1 deposition formed the apex of Cedar Creek alluvial fan as Qf1 covered more than 90% of the present fan area. During Pinedale glaciation, Qf2 deposition shifted downfan; Qf2 is inset into Qf1 above the intersection point, but below the intersection point it eroded and/or buried Qf1 as it activated as much as 60% of the fan area.Qf3 and Qf4, comprising 21% of the fan area, are inset into Qf2 in the lower fan area. Soil development in Qf3 and Qf4 deposits indicate episodic deposition and entrenchment beginning in early Holocene and continuing to present. A post-glacial decrease in sediment supply to Cedar Creek alluvial fan is indicated by sediment storage within the Cedar Creek drainage basin. Decreased sediment supply to the fan resulted in upper-fan entrenchment of Qf2 and deposition of Qf3 and Qf4 in the lower-fan area.     

Authigenic rhodochrosite from a gas hydrate-bearing structure in Lake Baikal

International Journal of Earth Sciences - Early diagenetic carbonates are rare in Lake Baikal. Siderite (Fe carbonate) concretions in the sediments were discovered only recently. Here, we discuss...