Tidal effect on dynamics of pore water nitrate in intertidal sediment of a eutrophic Estuary   总被引：2，自引：0，他引：2  

Toshihiro Usui  Isao Koike  Norio Ogura 《Journal of Oceanography》1998,54(3):205-216

To evaluate the effect of the tidal cycle on the pore water nitrate dynamics in intertidal sediment, concentrations of inorganic nitrogen in water and sediment were monitored during tidal cycles in the mud flat of Tama Estuary, Japan. During submergence, nitrate concentration was highest in the overlying water and decreased monotonically with increasing depth in the sediment, suggesting that the primary source of nitrate in the sediment was nitrate transported from the overlying water. Pore water nitrate decreased remarkably during the initial 3–4 hours after the onset of exposure. Thereafter, it was constant or slightly increased until tidal flooding.In situ accumulation of nitrate at the end of exposure, however, did not exceed the nitrate concentrations in the overlying water. The inhibition of nitrate reduction and the stimulation of nitrification would explain the change of nitrate concentration, both consistent with the input of oxygen into the sediment following a 10 mm drop of the water table. In Tama Estuary sediments, the effect of the tidal cycle on the removal of combined nitrogen is rather negative, because high nitrate concentrations in the overlying water canceled the positive effect of nitrate accumulation by nitrification during exposure, while tidal oxygen intrusion have an inhibitory effection sedimentary denitrification.  相似文献   

Seasonal variations in the origin of lead in snow at Dye 3, Greenland   总被引：1，自引：0，他引：1  

K.J.R Rosman  W Chisholm  C.F Boutron  J.-P Candelone  J.-L Jaffrezo  C.I Davidson 《Earth and Planetary Science Letters》1998,160(3-4):383-389

The isotopic composition and concentration of lead has been measured in fresh and slightly aged snow collected at Dye 3 in southern Greenland during one full year. The lead concentration displayed large variations ranging from 14–3016 pg/g in April (spring) to 3–6 pg/g in September (summer) while the isotopic ratios changed in regular manner during the year. The ²⁰⁶Pb/²⁰⁷Pb ratios were 1.15 from spring to mid-summer snow, and increased in late summer to early autumn, reaching 1.20 in winter. These isotopic data indicate that the lead in the autumn to winter snow originated in North America, while that in spring to mid-summer snow is from Eurasia.  相似文献   

Geomagnetic paleosecular variation at Hawaii around 3 Ma from a sequence of 107 lava flows at Kaena Point (Oahu)     

Carlo Laj  Herv Guillou  Nadia Szeremeta  Robert Coe 《Earth and Planetary Science Letters》1999,170(4):191-376

We have sampled a sequence of 107 lava flows in the Waianae series on Oahu, Hawaii, in two separate stretches. The first (51 flows) at Kaena Point extends from sea level to an altitude of 190 m, and the second (56 flows) follows the Satellite Tracking Station road from 218 m to 360 m above sea level. Thermal and af demagnetization yield very similar results, and reveal only normal polarities. K/Ar dating at five different horizons combined with the paleomagnetic results indicate that the sequence was emplaced entirely in the normal polarity interval between the Upper Mammoth and the Lower Kaena polarity transitions (3.22–3.11 Ma). Some of the flows are serially correlated. Filtering does not, however, drastically affect the final results. Inclinations are shallower than those expected from a centered dipole field. Although large (13°), the inclination anomaly is consistent with results from other sites at the same latitude. The scatter of the VGPs about the geographic pole is consistent with available data from other regions at similar latitudes. The scatter of the directions is also consistent with the predictions of recent statistical models of paleosecular variation. Therefore, these results do not support the hypothesis of a Pacific dipole window.  相似文献   

江淮异常梅雨   总被引：8，自引：1，他引：7       下载免费PDF全文

章淹  白建强 《水科学进展》1999,10(3):278-287

对江淮流域1885～1998年的特别丰、枯梅雨进行研究,重点论述近60年中4个重大丰梅年和三个空梅年的降水特点、异常变化、前期有关特征及严重后延影响等,并提出讨论意见。  相似文献   

CHARACTERISTICS OF 10-DAY MEAN SURFACE SENSIBLE HEAT FLUX VARIATIONS IN THE SCS MONSOON REGION AND POSSIBLE CONNECTIONS WITH THE SCS SUMMER MONSOON ONSET^*     

Zhang Yaocun  Qian Yongfu 《Acta Meteorologica Sinica》1999,13(1):11-20

The time and space variations of the ten-day mean surface sensible heat flux have been analyzed in this paper based on the data of NCEP/NCAR from January of 1979 to December of 1995 in the South China Sea (SCS) monsoon region.It is found that large variations of the surface sensible heat flux standard deviations exist in the northwestern Indochina Peninsula and the Indian Peninsula regions,and their locations and strength change significantly during the onset period of SCS monsoon.The negative deviations appear evidently earlier in the Indocbina Peninsula than in the Indian Peninsula but the deviation strength in the Indian Peninsula is stronger than that in the Indochina Peninsula.The appearance of the zonal negative mean deviations in the southern part of the Indochina Peninsula corresponds to the date of the SCS summer monsoon onset,while the occurrence of the deviation decrease corresponds to the date of the South Asian monsoon onset.The sensible heat flux increases dekad by dekad before the onset of the summer monsoon in the Indian Peninsula and the Indochina Peninsula and decreases after the monsoon onset.Therefore,the surface sensible heat flux changes in the Indochina and the Indian Peninsula regions maybe have some connections with the SCS monsoon onset and the Indian monsoon onset,and the Indochina Peninsula maybe becomes the sensitive or key region to the SCS monsoon onset and the land maybe plays an important role in triggering summer monsoon onset.  相似文献   

CHARACTERISTICS OF 10-DAY MEAN SURFACE LATENT HEAT FLUX VARIATIONS IN THE SCS MONSOON REGION AND POSSIBLE CONNECTIONS WITH THE SCS SUMMER MONSOON ONSET   总被引：1，自引：0，他引：1  

张耀存  钱永甫 《Acta Meteorologica Sinica》1999,(4)

The temporal and spatial variations of the ten-day mean surface latent heat flux (TMLH) havebeen analyzed in this paper based on the data of NCEP from January of 1979 to December of 1995in the South China Sea (SCS) monsoon region. It is found that there exist maximum centers ofTMLH standard deviation in the northwest Indochina and the Indian Peninsula as well as thewestern Pacific, SCS, the Indian Ocean and the Bay of Bengal, and their locations and strengthschange significantly during the period of SCS monsoon onset. A positive zonal deviation of TMLHoccurs first in the Indochina Peninsula, apparently earlier than that in the Indian Peninsula. Theappearance of maximum positive zonal deviations of TMLH approximately coincides with thesummer monsoon onset. Over the Indochina and Indian Peninsulas, the TMLH increases graduallywith a small amplitude of variation before the onset of summer monsoon, and the rate of increase issignificantly enhanced after the onset of the monsoon; whereas over the ocean, TMLH decreasesbefore the monsoon onset, varies little during the period of monsoon and increases gradually afterthe ending of monsoon. Therefore, it seems that the surface latent heat flux plays an importantrole in the maintenance of the summer monsoon, and its variation is an phenomenon accompanyingthe onset of summer monsoon.  相似文献   

TOVS卫星反演资料变分分析的试验研究与应用   总被引：8，自引：2，他引：6  

张菊芳  沈树勤 《气象科学》1999,19(2):206-212

为弥补海洋、高原等地带常规资料的不足,分析使用非常规卫星资料得到越来越多的重视,其中TOVS资料的精度及与常规资料的误差减少的研究尤为重要。本文采用变分分析技术对TOVS卫星反演资料进行同化分析研究,结果表明,变分结果分析场能充分体现常规与非常规资料的优势,使得天气系统能得到更精确的表示。  相似文献   

全球变暖对长江洪水的可能影响及其前景预测     

施雅风  姜彤  王俊  张强  苏布达  秦年秀 《湖泊科学》2003,15(Z1):1-15

要长江流域近150a间发生的1870、1931、1935、1954与1998年特大洪水灾害损失严重;长江洪水是我国的心腹之患.1990年以来,长江大洪水高频发生,达6次.长江洪水的发生,除湖泊蓄洪功能减弱等因素外,与全球变暖有关.20世纪90年代为近千年中全球最暖的年代,水循环加快,长江中下游夏季降水量为近120a最多的十年,高出1961-1990平均值112mm;而降雨集中和大暴雨降水事件的增加是洪水增加的主要原因.区域气候模式模拟在CO₂倍增时,长江流域温度升高2.2℃,夏季降水增加10%-20%,气溶胶的增加可能使此值降低一些.考虑气候变暖可能促进潜在蒸发增加9%-15%的假定情景,计算在降水增加10%,蒸发增加9%条件下,最大洪峰流量在大通站将会达到8.4×10⁴ m³/s左右,己超过1998年洪峰流量;汉口站7.9×10⁴ m³/s,超过有记录以来所有的洪峰流量;而在宜昌站高达6.94×10⁴ m³/s,超过自有实测记录以来的除1896年和1981年以外所有的洪峰流量.假定情景的最高值出现在降水增加20%,蒸发增加15%时,大通站流量将达到9.45×10⁴ m³/s,超过该站近百年最大值,1954年的9.26×10⁴ m³/s;宜昌站将出现7.82×10⁴ m³/s流量,超过1882年以来所有实测记录值,但比1870年据洪痕推算的10.5×10⁴ m³/s仍有逊色.未来气候若继续变暖,降水量增加将给长江洪水防御带来巨大的压力.但上述估算是粗糙的,有一定的不确定性,需在以后的研究中进一步改进.  相似文献   

湖北长湖富营养化状况及时空变化(2012-2013年)   总被引：4，自引：1，他引：3  

何勇凤  李昊成  朱永久  杨德国 《湖泊科学》2015,27(5):853-864

为评估长湖水体富营养化程度,2012-2013年分4个季度对全湖区20个采样点的物理、化学和生物要素进行监测,在评价水质现状的基础上采用综合营养状况指数法和浮游植物细胞丰度指数法综合评价水体营养状况,并应用典型相关分析(CCA)方法揭示水体富营养化状况与湖泊理化要素之间的典型相关性.结果显示:4个季节长湖全湖区的水质均处于地表水IV类~劣V类水标准;综合营养状态指数值在49.54~82.55之间,浮游植物细胞丰度在2.88×106~61.73×106cells/L之间,均显示其处于富营养化状态;长湖富营养化状况的分布呈现一定的时空差异性;CCA分析显示,长湖理化要素变量可解释68.6%的水体富营养化状况变量的变异,影响其富营养化状况的主要理化因素有水体总磷、总氮、溶解氧、亚硝态氮、硝态氮浓度,水深和沉积物总磷、总氮含量.长湖水体富营养化主要是由于外源的磷污染,其次是氮污染,富营养化最严重的夏、秋季浮游植物的生长主要受氮营养限制,而冬、春季则部分受磷营养限制,部分属于过渡类型.因此,建议大力削减围网/围栏养殖量,同时考虑结合水生植物栽种等生态工程建设措施以降低长湖水体发生严重富营养化的风险,并进一步改善长湖的水质现状.  相似文献   

地磁绝对子夜均值年变化量空间分布特征     

孙贵成  胡秀娟  丁成  李文军  孙澎涛  张玉林  相传芳 《地震地磁观测与研究》2017,38(5):62-65

选用中国21个地磁基准台站2011年的绝对子夜均值数据,分析D、H、Z分量年变化量空间分布特征。结果显示,各分量年变化量等值线分布比较均匀,其中:D分量年变化量等值线大致沿南北向分布,零经线附近台站年变化量较小;H分量年变化量等值线大致呈U型分布,且中心点位于甘肃省;Z分量年变化量等值线沿纬线方向,随纬度减小而增大。  相似文献