排序方式： 共有35条查询结果，搜索用时 15 毫秒
31.
Sebastiaan W. Rampen Stefan Schouten Ellen C. Hopmans Ben Abbas Anna A.M. Noordeloos Jan A.J. Geenevasen J. Michael Moldowan Peter Denisevich Jaap S. Sinninghe Damsté 《Organic Geochemistry》2009,40(2):219-228
23-Methyl sterols have been reported to be synthesized by a few marine algae, but unambiguous identification of 23-methyl steroids in sediments and petroleum is lacking. We report the presence of 23-methylcholesta-5,22E-dien-3β-ol in 14 out of 106 diatom cultures, thereby showing that diatoms, together with dinoflagellates, may be an important environmental source for such steroids. Synthesis of authentic 23-methylcholestanes showed that their mass spectra are identical to those of 24-methylcholestanes, but that they elute earlier on apolar stationary phases during gas chromatography (GC) analysis. Co-injection of the authentic standards with sediment extracts revealed the presence of these compounds in the Skole unit of the Oligocene Menelite Formation, the Miocene Monterey Formation and the Messinian Vena del Gesso Formation. In addition, we tentatively identified 23,24-dimethyl-27-norcholestanes in some of these sediments. Molecular clock calculations suggest that diatoms are a possible source for 23-methyl steroids in sediments and petroleum from the late Jurassic onwards. 相似文献
32.
Water levels and flows in the Singapore coastal waters are driven by the complex interactions of the Indian and Pacific Ocean
tides, seasonal monsoon-driven contributions and also forced by local winds. The Singapore Regional Model was developed to
simulate hydrodynamics in the Strait of Singapore which produces representative sea level variation in this region. However,
resolution and alignment of the grid system of the model with respect to depth contours in some of its subregions, i.e., the
Johor Estuary area require further improvement. For this, the grid system of the model was modified and compared the simulated
results with field measurements. The computed flow velocities agreed better with field observations when the grid resolution
was increased. However, improving the alignment of the grid with the channel boundary (with a much lower increase in grid
resolution) provided a substantially larger improvement of the model performance. The grid modification greatly influenced
the computed salinity in the estuary, while water levels are slightly affected. Further analysis of model results showed a
pronounced ebb tidal asymmetry generated by the O1–K1–M2 tidal constituents in the estuary. 相似文献
33.
Detlef P. van Vuuren Sebastiaan Deetman Jasper van Vliet Maarten van den Berg Bas J. van Ruijven Barbara Koelbl 《Climatic change》2013,118(1):15-27
Limiting climate change to 2 °C with a high probability requires reducing cumulative emissions to about 1600 GtCO2 over the 2000–2100 period. This requires unprecedented rates of decarbonization even in the short-run. The availability of the option of net negative emissions, such as bio-energy with carbon capture and storage (BECCS) or reforestation/afforestation, allows to delay some of these emission reductions. In the paper, we assess the demand and potential for negative emissions in particular from BECCS. Both stylized calculations and model runs show that without the possibility of negative emissions, pathways meeting the 2 °C target with high probability need almost immediate emission reductions or simply become infeasible. The potential for negative emissions is uncertain. We show that negative emissions from BECCS are probably limited to around 0 to 10 GtCO2/year in 2050 and 0 to 20 GtCO2/year in 2100. Estimates on the potential of afforestation options are in the order of 0–4 GtCO2/year. Given the importance and the uncertainty concerning BECCS, we stress the importance of near-term assessments of its availability as today’s decisions has important consequences for climate change mitigation in the long run. 相似文献
34.
Non Okumura Sebastiaan N. Jonkman Miguel Esteban Bas Hofland Tomoya Shibayama 《Natural Hazards》2017,88(3):1451-1472
This paper presents a methodology for tsunami risk assessment, which was applied to a case study in Kamakura, Japan. This methodology was developed in order to evaluate the effectiveness of a risk-reducing system against such hazards, also aiming to demonstrate that a risk assessment is possible for these episodic events. The tsunami risk assessment follows these general steps: (1) determination of the probability of flooding, (2) calculation of flood scenarios, (3) assessment of the consequences and (4) integration into a risk number or graph. The probability of flooding was approximated based on the data provided by local institutes, and the flood scenarios were modeled in 1D using the Simulating WAves till SHore model. Results showed that a tsunami in Kamakura can result in thousands of casualties. Interventions such as improvements in evacuation systems, which would directly reduce the number of casualties, would have a large influence in risk reduction. Although this method has its limits and constraints, it illustrates the value it can add to existing tsunami risk management in Japan. 相似文献
35.
This paper describes the appearance and maintenance of crescentic dunes in high wind speed conditions on a frozen beach at Schiermonnikoog, The Netherlands. The dunes were cresentic forms with horns. They were barchanoidal in plan view, but had reverse morphologies to typical barchans: the highest and steepest slopes were upwind and led to long low slopes downwind. Slipfaces were absent. It is hypothesized that such crescentic dunes may be a stable aerodynamic form under high to very high (c. 15–20 m s−1) flow conditions. © 1997 by John Wiley & Sons, Ltd. 相似文献