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121.
Dorthe Klitgaard Kristensen Tine L. Rasmussen Nalan Koç 《Boreas: An International Journal of Quaternary Research》2013,42(3):798-813
The sediment core NP05‐71GC, retrieved from 360 m water depth south of Kvitøya, northwestern Barents Sea, was investigated for the distribution of benthic and planktic foraminifera, stable isotopes and sedimentological parameters to reconstruct palaeoceanographic changes and the growth and retreat of the Svalbard–Barents Sea Ice Sheet during the last ~16 000 years. The purpose is to gain better insight into the timing and variability of ocean circulation, climatic changes and ice‐sheet behaviour during the deglaciation and the Holocene. The results show that glaciomarine sedimentation commenced c. 16 000 a BP, indicating that the ice sheet had retreated from its maximum position at the shelf edge around Svalbard before that time. A strong subsurface influx of Atlantic‐derived bottom water occurred from 14 600 a BP during the Bølling and Allerød interstadials and lasted until the onset of the Younger Dryas cooling. In the Younger Dryas cold interval, the sea surface was covered by near‐permanent sea ice. The early Holocene, 11 700–11 000 a BP, was influenced by meltwater, followed by a strong inflow of highly saline and chilled Atlantic Water until c. 8600 a BP. From 8600 to 7600 a BP, faunal and isotopic evidence indicates cooling and a weaker flow of the Atlantic Water followed by a stronger influence of Atlantic Water until c. 6000 a BP. Thereafter, the environment generally deteriorated. Our results imply that (i) the deglaciation occurred earlier in this area than previously thought, and (ii) the Younger Dryas ice sheet was smaller than indicated by previous reconstructions. 相似文献
122.
Patrycja Jernas Dorthe Klitgaard Kristensen Katrine Husum Lindsay Wilson Nalan Koç 《Boreas: An International Journal of Quaternary Research》2013,42(1):236-255
The western and northern Svalbard continental margins (European Arctic) are environmentally sensitive areas that are dependent on the northward flow of Atlantic Water, the largest heat source of the Arctic Ocean. Two marine sediment records from the Svalbard shelf: Kongsfjorden Trough and Hinlopen Trough, were analysed with regard to the benthic foraminiferal content and lithology to assess the palaeoceanographic evolution during the past two millennia with decadal to multi‐decadal temporal resolution. In both records, an overall gradual decrease of E. excavatum f. clavata during the past two millennia reflects a change towards generally warmer and less glacially influenced conditions, presumably related to enhanced inflow of Atlantic Water (AW). The influence of AW also varied on centennial time scales, as evidenced by faunal and sedimentary shifts occurring almost synchronously at both locations. The period from AD 700 to 1200 was characterized by enhanced inflow of AW, followed by the development of highly productive oceanographic fronts at both localities from AD 1200 to 1500. In contrast, the subsequent interval (AD 1500–1900) shows particularly harsh conditions in the Hinlopen Trough, with significantly reduced foraminiferal flux and sediment input related to perennial sea ice cover. In Kongsfjorden, less severe conditions were observed, indicating that the AW advection continued. The synchronicity of changes in both records demonstrates the effect of the variability in inflow of AW to the Svalbard region during the past 2000 years. Moreover, the records seem to follow climate anomalies, for example the Little Ice Age and Medieval Warm Period, found in the North Atlantic realm. 相似文献
123.
Quantification of the saturation degree of nonaqueous phase liquid in the vadose zone using hydrocarbon partitioning tracers 总被引:1,自引:1,他引:0
Partitioning tracer tests, as an alternative to the core sampling method, were conducted to quantify the degree of saturation
of water and nonaqueous phase liquids (NAPL) in the vadose zone. Hydrocarbon gases, which have less effect on global warming
than conventional tracers, were used as partitioning tracers. Column tests using CH4, C3H5, and C4H10 as non-partitioning and partitioning tracers were performed to determine the retardation factor and partition coefficient
of the tracer into water and NAPL. The retardation factors of these tracers were estimated to be in the range of 1.0–7.0 based
on breakthrough curves of the tracers. The partition coefficient of C3H5 to water and diesel phase was calculated to be 0.57 and 8.45, respectively. For a heavier tracer, C4H10, the partition coefficient to the water and diesel phases was 1.2 and 40.5, respectively. The average value of water and
diesel saturation estimated from column tests agreed well with known values in unsaturated soil. A residence time longer than
7.5 h within soil pores was found to provide local equilibrium partitioning of the tracer to the diesel phase. The concentration
of tracer had no effect on the partitioning process. 相似文献
124.
125.
Jonghan Ko Lajpat R. Ahuja S. A. Saseendran Timothy R. Green Liwang Ma David C. Nielsen Charles L. Walthall 《Climatic change》2012,111(2):445-472
Agricultural systems models are essential tools to assess potential climate change (CC) impacts on crop production and help
guide policy decisions. In this study, impacts of projected CC on dryland crop rotations of wheat-fallow (WF), wheat-corn-fallow
(WCF), and wheat-corn-millet (WCM) in the U.S. Central Great Plains (Akron, Colorado) were simulated using the CERES V4.0
crop modules in RZWQM2. The CC scenarios for CO2, temperature and precipitation were based on a synthesis of Intergovernmental Panel on Climate Change (IPCC 2007) projections for Colorado. The CC for years 2025, 2050, 2075, and 2100 (CC projection years) were super-imposed on measured
baseline climate data for 15–17 years collected during the long-term WF and WCF (1992–2008), and WCM (1994–2008) experiments
at the location to provide inter-annual variability. For all the CC projection years, a decline in simulated wheat yield and
an increase in actual transpiration were observed, but compared to the baseline these changes were not significant (p > 0.05) in all cases but one. However, corn and proso millet yields in all rotations and projection years declined significantly
(p < 0.05), which resulted in decreased transpiration. Overall, the projected negative effects of rising temperatures on crop
production dominated over any positive impacts of atmospheric CO2 increases in these dryland cropping systems. Simulated adaptation via changes in planting dates did not mitigate the yield
losses of the crops significantly. However, the no-tillage maintained higher wheat yields than the conventional tillage in
the WF rotation to year 2075. Possible effects of historical CO2 increases during the past century (from 300 to 380 ppm) on crop yields were also simulated using 96 years of measured climate
data (1912–2008) at the location. On average the CO2 increase enhanced wheat yields by about 30%, and millet yields by about 17%, with no significant changes in corn yields. 相似文献
126.
Asian Dust effects on Total Suspended Particulate (TSP) compositions at Gosan in Jeju Island, Korea 总被引:2,自引:0,他引:2
The compositions of TSP between AD and NAD storm periods were compared to study their long-term variations and chemical characteristics. TSP samples were collected at Gosan site in Jeju Island of Korea from February to May of 1992–2004. The major ionic and elemental species of TSP aerosols were analyzed. During AD periods, the concentrations of crust components (nss-Ca2+, Al, Fe, Ca, Mg, Ba, Sr, Ti) increased remarkably, and the concentrations of anthropogenic components (nss-SO42−, NO3−, S, Zn, Pb, Cr, Ni, Cd), with the exception of NH4+, increased weakly. The concentration ratios of all major components between AD and NAD periods showed ranges from 1.2 to 8.5, except for NH4+. The slope of the linear regression indicated that the contribution of CO32− may have comprised up to 17% of the total anions. Our results suggested that the AD storm greatly influenced TSP compositions. Linear regression analyses indicated that NH4+ was not correlated with NO3−, but highly correlated with nss-SO42− during both periods. The nss-SO42− was also correlated with NH4+, K+, nss-Mg2+, and nss-Ca2+ during both periods. Interestingly, NO3− was associated with nss-Ca2+ and nss-Mg2+ during AD periods. Of the metal elements, Fe, Ca, Mg, Ti, Mn, Ba, Sr, V, and Co were highly correlated with Al during both periods, signifying that these metals were mostly originated from soils. 相似文献
127.
A numerical and experimental study on the sloshing behaviours of cylindrical and rectangular liquid tanks is addressed. A three‐dimensional boundary element method for space with the second‐order Taylor series expansion in time is established to simulate the sloshing phenomenon and its related physical quantities inside a liquid tank subjected to horizontal harmonic oscillations or recorded earthquake excitations. The small‐scale model experiments are carried out to verify some results of numerical methods in this study. The comparisons between numerical and experimental results show that the numerical method is reliable for both kinds of ground excitations. Finally, the water wave and the base shear force of a rectangular tank due to harmonic excitation are also presented at different frequencies. A huge cylindrical water tank subjected to a recorded earthquake excitation is used for application and discussion. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
128.
We performed nitrogen and argon isotopic analyses in single 200-μm-sized ilmenite grains of lunar regolith samples 71501, 79035 and 79135. Cosmogenic and trapped components were discriminated using stepwise heating with a power-controlled CO2 laser. Cosmogenic 15N and 38Ar correlate among different ilmenite grains, yielding a mean 15Nc/38Arc production ratio of 14.4±1.0 atoms/atom. This yields a 15N production rate in bulk lunar samples of 3.8-5.6 pg (g rock)−1 Ma−1, which agrees well with previous estimates. The trapped δ15N values show large variations (up to 300‰) among different grains of a given soil, reflecting complex histories of mixing between different end-members. The 36Ar/14N ratio, which is expected to increase with increasing contribution of solar ions, varies from 0.007 to 0.44 times the solar abundance ratio. The trapped δ15N values correlate roughly with the 36Ar/14N ratios from a non-solar end-member characterized by a 36Ar/14N ratio close to 0 and variable but generally positive δ15N values, to lower δ15N values accompanied by increasing 36Ar/14N ratios, supporting the claim of Hashizume et al. (2000) that solar nitrogen is largely depleted in 15N relative to meteoritic or terrestrial nitrogen. Nevertheless, the 36Ar/14N ratio of the 15N-depleted (solar) end-member is lower than the solar abundance ratio by a factor of 2.5-5. We explain this by a reprocessing of implanted solar wind atoms, during which part of the chemically inert rare gases were lost. We estimate that the flux of non-solar N necessary to account for the observed δ15N values is comparable to the flux of micrometeorites and interplanetary dust particles estimated for the Earth. Hence we propose that the variations in δ15N values observed in lunar regolith can be simply explained by mixing between solar wind contributions and micrometeoritic ones infalling on the Moon. Temporal variations of δ15N values among samples of different antiquities could be due to changes in the micrometeoritic flux through time, in which case such flux has increased by up to an order of magnitude during the last 0.5 Ga. 相似文献
129.
The potential application of tuned liquid column damper (TLCD) for suppressing wind-induced vibration of long span bridges
is explored in this paper. By installing the TLCD in the bridge deck, a mathematical model for the bridge-TLCD system is established.
The governing equations of the system are developed by considering all three displacement components of the deck in vertical,
lateral, and torsional vibrations, in which the interactions between the bridge deck, the TLCD, the aeroelastic forces, and
the aerodynamic forces are fully reflected. Both buffeting and flutter analyses are carried out. The buffeting analysis is
performed through random vibration approach, and a critical flutter condition is identified from flutter analysis. A numerical
example is presented to demonstrate the control effectiveness of the damper and it is shown that the TLCD can be an effective
device for suppressing wind-induced vibration of long span bridges, either for reducing the buffeting response or increasing
the critical flutter wind velocity of the bridge. 相似文献
130.
D. E. Dietrich C. A. Lin Dr. A. Mestas-Nunez D. S. Ko 《Meteorology and Atmospheric Physics》1997,64(3-4):187-201
Summary The Gulf of Mexico (GOM) circulation is simulated using the DieCAST ocean model, with a horizontal resolution of 1/12° and 20 vertical layers. The results compare well with observations of both large and small scale features, including Loop Current frontal occlusions associated with frontal eddies. The simulation is carried out without any data assimilation. The frontal eddies tend to be spaced at about 90° intervals around the Loop Current, leading to a Loop Current head shaped like a square with rounded corners. The pattern rotates as the eddies circle the Loop, and frontal eddies elongate as they squeeze through the Florida Strait. Major warm core eddies separate regularly from the Loop Current and propagate to the western GOM. Old warm core eddies in the western Gulf dissipate through bottom drag effects, which also generate cyclonic parasitic eddies. Newly arrived warm core eddies merge with old ones in the western GOM. Recently separated elongated Loop Current eddies can rotate and reattach temporarily to the Loop Current. The barotropic flow component develops eddies between the main separated warm core eddy and the Loop Current due to eastward dispersion, as the main eddy itself propagates westward into the Gulf.With 10 Figures 相似文献