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901.
A. Veldkamp 《地球表面变化过程与地形》1992,17(5):487-500
The combined effects of climate and tectonism on general terrace stratigraphy and valley asymmetry during the last half million years in the Allier system (France) are simulated by a 3-D conceptual model (LIMTER). This model allows the formulation and evaluation of long term terrace formation scenarios for the Allier system. Simulation results suggest that terrace stratigraphy in the study area is mainly the result of internal dynamics and climatic change. Local tectonism contributed to the development of unpaired terraces while the general regional uplift played a dominant role in determining terrace formation and preservation in general. 相似文献
902.
904.
905.
Jing Zhang 《Aquatic Geochemistry》1995,1(3):241-275
Water and suspended sediment samples from the Huanghe and its delta region were analyzed to determine arsenic concentrations. Comparison with the data of previous studies and other world pristine and less disturbed rivers reveals high levels of both dissolved and particulate As in this high turbidity river. Relatively stable distribution along the river course and in delta sediment cores shows that antrhopogenic activities have not altered As concentrations by any significant magnitude since the 1940s. Therefore, high levels of As in the Huanghe should be controlled by natural weathering and lithology. In the estuary, biological removal/regeneration and abiotic events (e.g. remobilization) have been identified to cause the active (non-conservative) As distributions. With the limited data set of the present study, the abiotic and biological contributions cannot be assessed accurately, however. 相似文献
906.
Yue Li 《Aquatic Geochemistry》1995,1(2):147-156
Based on the geological background, R-mode factor statistics, and the analysis of the stability diagram for the corresponding system, five weathering reactions controlling the surface-water chemical composition in the watershed of the Changhuajiang River are deduced. In the mass balance model, the precipitation accounts for some solute input, since the rainwater is dilute without pollution. Most of the Ca2+ and HCO
3
–
ions are from the dissolution of calcite, K+, Na+, H4SiO4 and some of the Mg2+ and HCO
3
–
come from albite and biotite weathering to kaolinite. The dissolution of dolomite and gypsum controls the mass balances of Mg2+ and SO
4
2–
. The dissolution of calcite is the dominant chemical weathering reaction in the watershed because of its reactivity and high concentration. In the watershed in 1986, the chemical weathering rate was 0.073 (kg/m2 a), and the mechanical denudation rate is 0.093 (kg/m2 a). The chemical weathering mass output proportion of carbonate rocks to silicate rocks was about three to one. 相似文献
907.
华北陆块、秦岭地块和扬子陆块构造演化的古地磁证据 总被引:15,自引:0,他引:15
通过对河北井陉和湖北宜昌、兴山的39个采样点、392个样品进行的古地磁测试,以及7个采样点、71个样品中的原生剩磁以及笔者前期研究结果,系统分析了三大块体的古地磁极位置、古地磁偏角以及古纬度自寒武纪以来的变化特征,并进行了古地理复原。研究表明,三块体为不同地质构造单元,寒武纪时,华北陆块、秦岭地块以及扬子陆块自北到南顺序排列于北纬2.9°、南纬5.5°以及南纬17.0°。秦岭地块北侧和南侧分别存在一古洋盆,此时华北陆块和扬子陆块相距约1400Km左右;三块体向北漂移的过程中伴随着旋转运动;三块体开始拼合的时间为晚二叠纪至早三叠纪之间,华北陆块和秦岭地块完全拼合的时间为早三叠纪至中三叠纪之间,三块体完全拼合的时间为中三叠纪至晚白垩纪之间;晚白垩纪以后,一体化的三块体继续向北漂移,并于第四纪到达现今地理位置。 相似文献
908.
909.
Abstract Isotopic analyses of organic carbon from the mid-Cretaceous sequence in Hokkaido, Japan, revealed a 2‰ positive excursion of δ13C values at the biostratigraphically defined Cenomanian/Turonian (C/T) boundary recognized in the Yezo Group. The planktonic foraminiferal Whiteinella archaeocretacea Zone, which is known to bracket the Cenomanian/Turonian boundary elsewhere in the world, was recognized in the Oyubari area of central Hokkaido based on the distribution of commonly occurring planktonic foraminifera. In the Tappu area of northwestern Hokkaido, where diagnostic planktonic foraminifera are rare but calcareous nannoplankton occur commonly, the interval coeval with the W. archaeocretacea Zone can also be established by recognizing the conjoined last appearance levels of Corollithion kennedyi and Axopodorhabdus albianus, both calcareous nannoplankton species. Carbon isotope profiles exhibit a similar pattern with comparable peaks and troughs occurring in the same stratigraphic position in the sequences. A prominent, positive 2‰ shift of δ13C values, here called ‘δ13C spike’ occurs in the middle of the W. archaeocretacea Zone in the Oyubari area and just above the conjoined last appearances of the two above-mentioned nannoplankton taxa in the Tappu area. The Cenomanian/Turonian boundary can be drawn just above the peak position of the spike in both sections. The Rock Eval analyses and biomarker analyses of organic carbon indicate that organic carbon subjected to our isotope analyses is of terrestrial origin. Therefore, the observed 2%o shift should reflect changes in the isotopic composition of the atmospheric CO2. A unique layer composed predominantly of sand-grain sized spumellarian Radiolaria is present immediately above the δ13C spike both in the Oyubari and Tappu areas, suggesting an increasing availability of both nutrients and silica in surface waters. 相似文献
910.