| 其他摘要 | The weathering of silicate mineral and pedogenic processes consume carbon dioxide, release the fundamental elements (such as Si) that essential to plant，sustain the terrestrial and marine ecosystems and change the flux of atmospheric carbon dioxide and carbon cycle. Part of the product of rock weathering is transported into ocean as solute or particle and change the composition of sea water in geological time scale. This process is one of the key step in evolution of matter in earth. Meanwhile, the rest of product pile up to form soil which supply the basic material to the development of global ecosystem and human society. Weathering is an important part of global material and energy circulation and the basic process need to be understand in critical zone research. In macro time scale, the weathering of silicate mineral generate net carbon sink, and granite is one of the main sources for silicate mineral. Denudation rate is the combine of chemical weathering and physical erosion. Thus, quantify the denudation rate of granite in different climate zone and confirm the effect of internal and external factors on weathering process can provide background value to estimate how climate affect surface erosion and water loss and how much do the weathering process contribute to the climate change. The denudation rate is indicative to long-term carbon cycle, global climate change, and modern agriculture and hydrology management. In the view of Earth System Science, quantifying the surface denudation rate is the important content in Earth System Science and critical to understand geological process. China has massive land，cross about 50 latitudes and 60 longitudes. There are complicated landform and large difference in elevation which lead to many kinds of climate types. By the comparation of weathering rates in different climate zones，the knowledge of climate effect on weathering will be deepen. Since 1980s，the development of Accelerator Mass Spectrometry technology make the cosmogenic nuclides widely applicated on earth surface process. Cosmogenic nuclides are produced by the nuclear reaction between comic ray and surface rock or soil. Familiar ones are 3He, 10Be, 14C, 26Al, 36Cl and so on. As the common nuclides pair， the 26Al, 10Be in quartz is suited to quantify the surface denudation rate and exposure age. The concentration of cosmogenic nuclides decreased exponentially with increasing of soil depth, the concentration is the function of denudation rate ε and exposure time t. in this paper, we focused on the granitic regolith and collected 6 classic granitic regolith profile in five different climate zones (Cold temperate zone, Temperate zone, Warm temperate zone, Subtropical zone and Tropical zone.) form July, 2001 to April, 2013. Then, we established the pretreatment process for cosmogenic nuclides 10Be, 26Al. the conditional experiments proved that the process has met the sample requirement for AMS test. The background value in blank samples is very low and reached 10-15 grade. After that, the samples are etched by diluted hydrofluoric acid to remove feldspar and the meteoric cosmogenic nuclides absorbed on quartz surface. The chemical extraction process is based on the procedures manual in Scottish Universities Environmental Research Centre and operated in our clean room. The production rates of 10Be, 26Al in different site were calculated by the elevation and latitude. The concentration of 26Al, 10Be and the insoluble elements were computed to get the denudation rate by Chi-square best-fitting method in depth profile and divide the denudation rate into chemical weathering rate and physical erosion rate. Several important conclusions have been drawn as follows: (1) Bioturbation in soil or soil creeping may homogenize the top layer in profile and lower the cosmogenic nuclides concentration. This phenomenon is obvious in some profiles, especially in steep sites (such as JLN-S1). The denudation rate deduced from river sediment 10Be which derive from soil surface maybe overestimated. The impact extent of Bioturbation may have something to do with land cover, local biomass and landform. Moreover, the uncertainty of density in semi regolith may cause additional error; and the sample with greater depth than ~3m may increase the uncertainty to the result due to the low nuclides concentrations and the enlargement of muon capture contribution to nuclides production. (2) The distribution of 10Be/9Be、26Al/27Al ratios are 0.40?6.59×10-13, 1.20?20.65×10-13. The background value is on 1015 grade. The concentration of in-situ cosmogenic nuclides 26Al, 10Be and 26Al/10Be ratios in the samples ranges 2.27?42.1×104 atoms/g, 13.2?281.75×104atoms/g, 6.06?7.50, respectively. The results determinated from Chi-square method show that: the denudation rates in profiles NMG, CBS, QHD, LZ, MY, JLN-S1, JLN-S2, JLN-S3, JLN-S4 and HN are 31±3 m/Ma, 33±3 m/Ma, 14±2 m/Ma, 58±6 m/Ma, 25±3 m/Ma, 19±3 m/Ma, 41±4 m/Ma, 47±4 m/Ma, 22±3 m/Ma and 59±5 m/Ma, respectively. The denudation rates of each profile in different climate zones has a little upward trend from north to south. (3) The results of exposure time shows that: the exposure time are ~0.3 Ma, >0.2 Ma, ~0.5 Ma, 0.25 Ma, ~0.04 Ma, ~0.3 Ma and >0.5 Ma for profiles NMG, CBS, QHD, LZ, MY, JLN-S1 and HN, respectively. (4) The denudation rate is 17±2 m/Ma in the MY outcrop, which is lower than the near granitic regolith profile. For the important role of water that plays on weathering process, the fast loss of water on outcrop surface may lead to lower denudation rate. The 10Be concentration in sediment samples which is from Laoshan in Qingdao city, Shandong province is obtained, Wuyixiang 12.0±0.37×104 atoms/g, Shuyuan Reservoir 14.7±0.34×104 atoms/g, Shangshuiyu 12.6±0.39×104 atoms/g. The denudation rates are 47±3 m/Ma, 40±3 m/Ma, 50±4 m/Ma, respectively. The variance may due to the difference in drainage area, mean slope gradient and human activity. The bioturbation or parent rock may explain the higher rate than granitic regolith. (5) Compared with other method about measuring weathering rate, the cosmogenic nuclide depth profile method is more reliable in the study of long term denudation rate. As a key external factor, Climate didn’t have obvious effect on long term granitic regolith denudation rate. The landform and tectonic activities may be the main factor which control the evolution on granitic regolith. The slope gradient is the external expression of landform and tectonic activities and has a higher correlation with denudation rate. In additional, compared with modern denudation rate, the human activity has been the most external factor and accelerate the surface denudation process. In the future, Multidisciplinary approach(such as biology, soil science, hydrology, geochemistry, remote sensing) and multi-method combination (10Be in sediment and outcrops, cosmogenic nuclides depth profile, U-series, 36Cl in carbonate, isotopes ratio, model research etc.) will be the trend to comprehensive study weathering process and critical zone. These objects and method has their advantages and can inspect and verify each other to profound understand the material cycle and interaction among weathering process and atmosphere, hydrosphere, lithosphere, pedosphere and biosphere in multiple time scales. |
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