Wyniki wyszukiwania - BazTech

Ograniczanie wyników

Znaleziono wyników: 2

Liczba wyników na stronie

Wyniki wyszukiwania

Sortuj według:

Ogranicz wyniki do:

240+239Pu depositional signatures as a viable geochronological tool in the Amazon Basin

Sanders L. M., Taffs K. H., Stokes D., Enrich-Prast A., Sanders Ch. J.

Geochronometria

2017

Vol. 44, no. 1

142--149

Anthropogenic radionuclide signatures associated with nuclear testing are increasingly utilized in environmental science to explore recent sedimentation. In this study, we assess the suitability of Pu radioisotope analysis in floodplain lake environments in the Amazon Basin to form geochronologies during the 20th century. The 240Pu + 239Pu ( 240+239Pu) signatures in six sediment cores indicate sediment accumulation rates in the floodplain lakes of the major rivers; Amazon (2.3 mm year–1 ), Tapajos (10.2 and 2.4 mm year–1 ) and Madeira (3.4, 4.2 and 6.2 mm year–1 ). The results from this study show that 240+239Pu fallout activities, and the well documented (240Pu/ 239Pu) atomic ratios of the above ground nuclear tests which began in the 1950’s, are sufficient and well preserved in Amazon floodplain lake sediments to infer chronologies. Lead-210 dating analyses in the same sediment cores producedv comparable sediment accumulation rates at three of the six sites. The differences between dating methods may be attributed to the different time scale these dating methods represent and/or in the solubility between Pb and Pu along the sediment column. The geochronologies derived from the 240+239Pu and 210Pb dating methods outlined in this work are of interest to identify the effects of changing sediment accumulation rates during the previous century as a result of development, including deforestation, along the Amazon Basin which increased towards the middle of the 20th century. This study shows that Pu dating provides a viable alternative geochronology tool for recent sediment accumulation (previous ~60 years) along the Amazon Basin.

The acoustic excitation of newly-formed bubbles

Deane G., Czerski H., Stokes D.

Hydroacoustics

2008

Vol. 11

71-78

Gas bubbles in water act as oscillators with a natural frequency inversely proportional to their radius and a quality factor determined by thermal, radiation, and viscous losses. Newly-formed gas bubbles are excited into breathing mode oscillations immediately after creation, causing them to radiate a pulse of sound. Although the linear dynamics of spherical gas bubbles are well-understood, the mechanism driving the sound production has not been unambiguously identified. Using bubbles released from a nozzle as a model system, it can be shown that sound production is consistant with the rapid change in volume associated with the collapse of an air neck formed immediately after bubble pinch-off. The model is able to adequately describe the production of sound by bubbles released from a nozzle, and can also explain some of the acoustic properties of bubbles fragmenting in fluid turbulence. Laboratory experiments and model calculations of the mechanism are presented. [Work supported by ONR and NSF].