Becker et al. (1) presented geochemical evidence that suggests that the largest mass extinction in Earth history, at the PermianTriassic boundary (PTB) 250 million years ago (Ma), coincided with an extraterrestrial impact comparable in size to the one that likely caused the end-Cretaceous extinctions 65 Ma (2). Although Becker et al. analyzed material from sections in Hungary, Japan, and China,...

In this study, the distribution of channel-bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high-intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel...

Field studies of forest gravel-bed rivers in northwestern Washington and southeastern Alaska demonstrate that bed-surface grain size is responsive to hydraulic roughness caused by bank irregularities, bars, and wood debris. We evaluate textural response by comparing reach-average median grain size (D50) to that predicted from the total bank-full boundary shear stress (t0bf), representing a hypo...

We investigate erosion patterns observed in a horizontal granular bed resulting from seepage of water motivated by observation of beach rills and channel growth in larger scale land forms. Our experimental apparatus consists of a wide rectangular box filled with glass beads with a narrow opening in one of the side walls from which eroded grains can exit. Quantitative data on the shape of the pa...

Replicate velocity observations using conventional equipment under typical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measurements. The bed shear velocity u, can be estimated within 3% using the depth-averaged velocity in the vertically averaged logarithmic velocity profile. To be accurate, this method is lim...

A series of laboratory experiments has been carried out in which parallel-laminated deposits were produced from an upper-regime plane bed. The laminae had thicknesses of a few mm and could be traced continuously over distances up to the length and width of the depositional area (0.3m by 1.5m). Fluctuations in bed elevation were measured both during deposition and at equilibrium; much of the bed...

In the present paper simulation of tidal currents on three-dimensional geometry of the Persian Gulf is performed by the solution of the depth averaged hydrodynamics equations. The numerical solution was applied on two types of discritized simulation domain (Persian Gulf); with and without major islands. The hydrodynamic model utilized in this work is formed by equations of continuity and motion...