Cretaceous–Paleogene boundary

The Cretaceous–Paleogene (K–Pg) boundary, formerly known as the Cretaceous–Tertiary (K–T) boundary, is a geological signature, usually a thin band of rock containing much more iridium than other bands. The K–Pg boundary marks the end of the Cretaceous Period, the last period of the Mesozoic Era, and marks the beginning of the Paleogene Period, the first period of the Cenozoic Era. Its age is usually estimated at around 66 million years, with radiometric dating yielding a more precise age of 66.043 ± 0.011 Ma. The K–Pg boundary is associated with the Cretaceous–Paleogene extinction event, a mass extinction which destroyed a majority of the world's Mesozoic species, including all dinosaurs except for birds. Strong evidence exists that the extinction coincided with a large meteorite impact at the Chicxulub crater and the generally accepted scientific theory is that this impact triggered the extinction event. The word "Cretaceous" is derived from the Latin "creta" (chalk). It is abbre

Biomass burning and marine aerosol processing over the southeast Atlantic Ocean: a TEM single-particle analysis

Athanasios Nenes, Lu Zhang

This study characterizes single-particle aerosol composition from filters collected during the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) and CLoud-Aerosol-Radiation Interaction and Forcing: Year 2017 (CLARIFY-2017) campaigns. In particular the study describes aged biomass burning aerosol (BBA), its interaction with the marine boundary layer and the influence of biomass burning (BB) air on marine aerosol. The study finds evidence of BBA influenced by marine boundary layer processing as well as sea salt influenced by BB air. Secondary chloride aerosols were observed in clean marine air as well as in BB-influenced air in the free troposphere. Higher-volatility organic aerosol appears to be associated with increased age of biomass burning plumes, and photolysis or oxidation may be a mechanism for the apparent increased volatility. Aqueous processing and interaction with the marine boundary layer air may be a mechanism for the presence of sodium on many aged potassium salts. By number, biomass burning potassium salts and modified sea salts are the most observed particles on filter samples. The most commonly observed BC coatings are inorganic salts. These results suggest that atmospheric processes such as photolysis, oxidation and cloud processing are key drivers in the elemental composition and morphology of aged BBA. Fresh BBA inorganic salt content, as it has an important role in the particles' ability to uptake water, may be a key driver in how aqueous processing and atmospheric aging proceed.

COPERNICUS GESELLSCHAFT MBH2022

Scaling laws for jets of single cavitation bubbles

Nicolas Dorsaz, Mohamed Farhat, Philipp Kobel, Danail Obreschkow, Outi Supponen, Marc Tinguely

Fast liquid jets, called micro-jets, are produced within cavitation bubbles experiencing an aspherical collapse. Here we review micro-jets of different origins, scales and appearances, and propose a unified framework to describe their dynamics by using an anisotropy parameter zeta >= 0, representing a dimensionless measure of the liquid momentum at the collapse point (Kelvin impulse). This parameter is rigorously defined for various jet drivers, including gravity and nearby boundaries. Combining theoretical considerations with hundreds of high-speed visualisations of bubbles collapsing near a rigid surface, near a free surface or in variable gravity, we classify the jets into three distinct regimes: weak, intermediate and strong. Weak jets (zeta < 10(-3)) hardly pierce the bubble, but remain within it throughout the collapse and rebound. Intermediate jets (10(-3) < zeta < 0.1) pierce the opposite bubble wall close to the last collapse phase and clearly emerge during the rebound. Strong jets (zeta > 0.1) pierce the bubble early during the collapse. The dynamics of the jets is analysed through key observables, such as the jet impact time, jet speed, bubble displacement, bubble volume at jet impact and vapour-jet volume. We find that, upon normalising these observables to dimensionless jet parameters, they all reduce to straightforward functions of zeta, which we can reproduce numerically using potential flow theory. An interesting consequence of this result is that a measurement of a single observable, such as the bubble displacement, suffices to estimate any other parameter, such as the jet speed. Remarkably, the dimensionless parameters of intermediate and weak jets (zeta < 0.1) depend only on zeta, not on the jet driver (i.e. gravity or boundaries). In the same regime, the jet parameters are found to be well approximated by power laws of zeta, which we explain through analytical arguments.

Cambridge University Press2016

Turbulent flow within a random array of rigid, emergent stems : Laboratory characterization of the drag coefficient

Ana Margarida Da Costa Ricardo, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

Within the bioengineering framework, designing a non-erodible channel is a complex fluid dynamics problem as it involves knowing the drag exerted on the boundary, the drag exerted on the plant stems and the overall friction slope. Most of the existing design criteria employ resistance formulas such as Manning’s, calibrated ad hoc. Moving toward physically based design criteria, progresses have been made in the characterization of 3D flows over irregular boundaries, mainly due to the application of Double-Averaging methods (D-AM), which are a particular form of upscaling. Such methods are especially pertinent for the characterization of the flow within and in the near vicinity of plant canopies.This work is aimed at the determination of the drag coefficient associated to rigid emergent stems in turbulent flows. Specific objectives are i) the detailed characterization and quantification of the flow within vegetated areas susceptible to be simulated by dense arrays of vertical emergent stems and ii) the independent quantification of the forces, per unit bed area, acting on the stems and on the bed boundary. Both objectives concur for a better knowledge of the flow resistance in wetlands and vegetated areas in general. To meet the proposed objectives, conditions similar to those found in nature were reproduced in laboratory facilities, using Particle Image Velocimetry (PIV).Since actual wetlands exhibit patchiness and spatial variability in stem density, the experiments featured a periodic distribution of stem densities with minimum and maximum values of 400 and 1600 stems/m2, respectively. The treatment of the data was done with the Double-Averaging methodology (D-AM), to account for the great spatial variability of the flow. The results reveal that the contribution of form-induced stresses, namely longitudinal and shear stresses is of the order of magnitude of the contribution of Reynolds stresses. Hence, in general, this stresses cannot be neglected. The analysis of form-induced stresses helps to explain the increase of the drag coefficient when the stem density increases.