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Bimodality and growth of the spectra of typhoon-generated waves in northern South China Sea
Dongxue MO, Yahao LIU, Yijun HOU, Ze LIU
Buoy-based observations of wave spectra during the passage of three typhoons in the northern South China Sea are examined. Though most spectra of mature typhoon-generated waves are unimodal, double-peaked spectra account for a significant proportion during the growing and decaying stages. This is due either to the superposition of swells on local wind waves or to the mechanism of nonlinear interaction between different wave components. The growth rate of energy density is an effective way to predict spectrum variation. The dominant wave direction depends on the location of the typhoon center to the site, but the direction spread shows no regularity in distant regions. In this study, a new six-parameter spectral formula is proposed to represent double-peaked spectra and is shown to provide a better fit than previous models. The theoretical relationship between shape parameter and spectral width is still applicable to each peak. The characteristics of the variations of spectral parameters are analyzed. It is demonstrated that the spectral parameters are not only related to the typhoon intensity and typhoon track, but also have strong intercorrelations. Moreover, the growth relation between significant wave height and significant wave period is obtained to fit the typhoon-generated waves.
key words: double peak, South China Sea, spectral parameters, typhoon, wave spectrum
Sedimentary geochemical proxies for methane seepage at Site C14 in the Qiongdongnan Basin in the northern South China Sea
Tiantian SUN, Daidai WU, Fei YANG, Lihua LIU, Xuegang CHEN, Ying YE
doi: 10.1007/s13131-019-1460-6
Recent studies have shown that specific geochemical characteristics of sediments can be used to reconstruct past methane seepage events. In this work, the correlation between the Sr/Ca and Mg/Ca ratios of sediment samples is analyzed and the sulfate concentration profile in Site C14 from cold-seep sediments in the Qiongdongnan Basin in northern South China Sea is obtained. The results confirmed that, sulfate at 0–247 cm below sea floor (Unit I) is mainly consumed by organic matter sulfate reduction (OSR), while sulfate at 247–655 cm (Unit II) is consumed by both the OSR and the anaerobic oxidation of methane (AOM). In addition, the bottom sediment layer is affected by weak methane seepage. The Mo and U enrichment factors also exhibit similar trends in their respective depth profiles. The responses of trace elements, including Co/Al, Ni/Al, Cr/Al and Zn/Al ratios to methane seepage allowed the study of depositional conditions and methane seepage events. Based on the results, it is speculated that the depositional conditions of Unit II changed with depth from moderate conditions of sulfidic and oxic conditions to locally anoxic conditions, and finally to suboxic conditions due to methane fluid leakage. The stable isotope values of chromium-reducible sulfide produced by AOM and those of sulfide formed by OSR in the early diagenetic environment suffered serious depletion of 34S. This was probably due to weak methane leakage, which caused the slower upward diffusion and the effect of early diagenesis on the samples. It is necessary to consider the effects of depositional environments and diagenesis on these geochemical parameters.
key words: Qiongdongnan Basin, trace elements, stable isotope values, anaerobic oxidation of methane, methane activity

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