Stratigraphy and Sedimentology of Holocene Overbank Deposits of the Mississippi River, False River Region, Louisiana.

The Holocene flood basin along a meander belt margin of the Mississippi River includes two principal geomorphic features: a flat featureless natural levee, and a system of anastomozed, entrenched crevasses with intervening splay lobes. Levee and splay lobe deposits are identical and consist of thinly (cm) to minutely (mm) rhythmically layered sand, silt and clay which are pervasively disrupted by unlined meniscate burrows of the trace fossil, Muensteria. Pelletal fabrics develop where overlapping backfilled burrows completely obliterate primary stratification so that only traces of small ripple laminations and parallel laminations are preserved. Pelletal silt is the most extensive lithology present. A proximality trend in stratification is present relative to distance from sheet flood sources. With increasing distance from the trunk channel and crevasses, sandy rhythmites are successively replaced by finer-grained and more thinly interlayered deposits such as silty rhythmites and interlaminated silt and clay. As layering thins distally, burrowers more effectively disrupt the sediment and stratification grades into pelletal silt, pelletal mud and pelletal clay of the backswamp. Thus levee and splay lobe deposits are identical except that the same lithofacies extend further basinward on splay lobes. The crevasse fill includes mudball conglomerate, plant debris, complex clay-rich laminates, graded sands, mottled muddy sand and minor ripple laminated sand. The sedimentology reveals that both morphologic features formed in an analogous manner, by sheet flow sedimentation. This result differs from previous views that (1) levees form from overbank flooding and sheet flow and (2) crevasse splays form via the crevassing process which generates thick, sharp-based, fining-upward cross-stratified sands. The crevasse channels and splay lobes are not depositional features that formed as minor mouth bars, crevasse channels, lenticular channel sands, and bar forms migrated basinward. Rather, after crevasse entrenchment, the splay lobes accreted by overbank flooding and sheet flood sedimentation. After establishment of the meander belt (marked by a lithologic change from lake clays to subaerial pelletal silt) two episodes of levee/splay progradation (3-4 m thick) are recorded. Each represents the gradual migration of the trunk channel towards the site, and abruptly ends with a neck cut-off

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data