The impact of particle shape on the angle of internal friction and the implications for sediment dynamics at a steep, mixed sand–gravel beach

The impact of particle shape on the angle of internal friction, and the resulting impact on beach sediment dynamics, is still poorly understood. In areas characterized by sediments of specific shape, particularly non-rounded particles, this can lead to large departures from the expected sediment dynamics. The steep slope (1 : 10) of the mixed sand–gravel beach at Advocate Harbour is stable in large-scale morphology over decades, despite a high tidal range of 10 m or more, and intense shore-break action during storms. The Advocate sand (<i>d</i> < 2 mm) was found to have an elliptic, plate-like shape (Corey Shape Index, CSI ≈ 0.2–0.6). High angles of internal friction of this material were determined using direct shear, ranging from &phi; ≈ 41 to 49°, while the round to angular gravel was characterized as &phi; = 33°. The addition of 25% of the elliptic plate-like sand-sized material to the gravel led to an immediate increase in friction angle to &phi; = 38°. Furthermore, re-organization of the particles occurred during shearing, characterized by a short phase of settling and compaction, followed by a pronounced strong dilatory behavior and an accompanying strong increase of resistance to shear and, thus, shear stress. Long-term shearing (24 h) using a ring shear apparatus led to destruction of the particles without re-compaction. Finally, submerged particle mobilization was simulated using a tilted tray submerged in a water-filled tank. Despite a smooth tray surface, particle motion was not initiated until reaching tray tilt angles of 31° and more, being &geq;7° steeper than for motion initiation of the gravel mixtures. In conclusion, geotechnical laboratory experiments quantified the important impact of the elliptic, plate-like shape of Advocate Beach sand on the angles of internal friction of both pure sand and sand–gravel mixtures. The resulting effect on initiation of particle motion was confirmed in tilting tray experiments. This makes it a vivid example of how particle shape can contribute to the stabilization of the beach face

High-frequency variability of CO₂ in Grand Passage, Bay of Fundy, Nova Scotia

Assessing changes in the marine carbon cycle arising from anthropogenic CO2 emissions requires a detailed understanding of the carbonate system's natural variability. Coastal ecosystems vary over short spatial and temporal scales, so their dynamics are not well described by long-term and broad regional averages. A year-long time series of pCO2, temperature, salinity, and currents is used to quantify the high-frequency variability of the carbonate system at the mouth of the Bay of Fundy, Nova Scotia. The seasonal cycle of pCO2 is modulated by a diel cycle that is larger in summer than in winter and a tidal contribution that is primarily M2, with amplitude roughly half that of the diel cycle throughout the year. The interaction between tidal currents and carbonate system variables leads to lateral transport by tidal pumping, which moves alkalinity and dissolved inorganic carbon (DIC) out of the bay, opposite to the mean flow in the region, and constitutes a new feature of how this strongly tidal region connects to the larger Gulf of Maine and northwest Atlantic carbon system. These results suggest that tidal pumping could substantially modulate the coastal ocean's response to global ocean acidification in any region with large tides and spatial variation in biological activity, requiring that high-frequency variability be accounted for in assessments of carbon budgets of coastal regions.</p

Free radical scavengers from Cymbopogon citratus (DC.) stapf plants cultivated in bioreactors by the temporary immersion (TIS) principle

Guillermo Schmeda-Hirschmann,Alejandro Tapia, Jose Cheel, Instituto de Quımica de Recursos Naturales, Universidad de Talca, Casilla 747, Talca, Chile. E-mail: [email protected] Cristina Theodulozb, Jaime Rodrıguez, Facultad de Ciencias de la Salud, Departamento de Ciencias Basicas Biomedicas, Universidad de Talca, ChileThe biomass production of Cymbopogon citratus shoots cultivated in bioreactors according to the temporary immersion (TIS) principle was assessed under different growth conditions. The effect of gassing with CO2-enriched air, reduced immersion frequency, vessel size and culture time on total phenolic and flavonoid content and free radical scavenging effect of the methanolic extracts was measured. From the TIS-culture of C. citratus, seven compounds were isolated and identified as caffeic acid (1), chlorogenic acid (2), neochlorogenic acid (3), p-hydroxybenzoic acid (4), p-hydroxybenzoic acid 3-O- -d-glucoside (5), glutamic acid (6) and luteolin 6-C-fucopyranoside (7). The occurrence of compounds 1Ð7 and their variability in C. citratus grown under different TIS conditions was determined by HPLC. The free radical scavenging effect of the methanolic extract and compounds was measured by the discoloration of the free radical 1,1- diphenyl-2-picrylhydrazyl (DPPH). The main metabolites in 6- and 8-week-old cultures, both in 5 and 10 l vessels, were chlorogenic acid (2) (100Ð113 mg%) and neochlorogenic acid (3) (80Ð 119 mg%), while in the cultures with CO2-enriched air and reduced immersion frequency the main compound detected in the extracts was glutamic acid (6) (400 and 670 mg% for the green and white biomass and 619 and 630 mg% for the green and white biomass, respectively). The most active compounds, as free radical scavengers, in the DPPH discoloration assay were caffeic acid (1), chlorogenic acid (2), neochlorogenic acid (3) and the flavonoid luteolin 6-C-fucopyranoside (7)