博士 / 國立成功大學 / 地球科學系 / 102 / This study presents major and trace element data, as well as uranium (U), strontium (Sr) and lithium (Li) isotopes for the waters collected from upstream to the estuary of the Gaoping River catchment in the southwest Taiwan. A fast and simple analytical procedure using MC-ICP-MS was developed for the precise determination of U isotopic compositions in low concentration natural samples. The results for three international standards with different matrices (IAPSO, IRMM-3184 and CRM-U010) are in close agreement with the certified values. The long-term reproducibility of (234U/238U) and 238U/235U is 0.970 ± 0.002 and 137.56 ± 0.09; 1.144 ± 0.004 and 137.72 ± 0.13, respectively, for in-house U solution and IAPSO. The approach provides a fast method to measure (234U/238U) and 238U/235U ratios in sample matrices commonly encountered in studies of chemical weathering, oceanography and paleoclimatology.
In the Gaoping River, based on the variations of major ions in the river water, the silicate weathering mainly affects the river water chemistry and minor contributions may input from local hot springs or anthropogenic sources. The climatic variations may influence the cation denudation rate due to chemical weathering. The average U activity ratio in Gaoping River is highest among the worldwide rivers. The physical weathering acts an important role to control the U activity ratios whereas (234U/238U) can reflect relative level of physical weathering to chemical erosion, and the fractionation of riverine δ7Li can be used to reflect the degree of chemical weathering in the Gaoping River. A scenario is proposed to explain the chemical and isotopic observations in the Gaoping River basin. The high physical denudation is dominated at the upstream stations where primary intensive weathering to cause high (234U/238U) and low δ7Li with relatively low U and high Li concentrations. Subsequently more enhanced chemical weathering has occurred toward the downstream stations to cause low (234U/238U) and high δ7Li, as well as high U and low Li concentrations. However, the contributions of groundwater and deep weathering cannot be neglect. But it still needs systematic sampling of groundwater and detail analytical data to resolve the relationship between river water and groundwater in the Gaoping River basin.
To gain a better understanding of geochemical behavior of trace elements in Gaoping River estuary and to examine seasonal variations in associated chemical fluxes, estuarine water samples were collected in typical dry and wet seasons for analyzing the dissolved major and trace elements and Sr isotopes. The results show that dissolved Na, Mg, Ca and Cl behave conservatively along the salinity gradient and display significantly larger fluxes in the wet seasons. Vertical profiles of the major ions and trace elements of B and Sr reveal mainly two end-member mixing between riverine freshwater and seawater. Dissolved Ba and Mn were affected by uptake/release processes involving groundwater, benthic flux and water/sediment interactions. 87Sr/86Sr ratios also support a scenario of mixing between a more radiogenic continental source and seawater. It appears that the wet season samples have higher trace element concentrations due to inputs from topsoils and atmospheric dusts. This implies that chemical compositions in river waters respond sensitively to regional climatic changes. The observed high fluxes of B and Sr in the Gaoping River emphasize the potential impact of mountainous rivers on the global oceanic mass balance of these constituents.
The estuary water samples were also analyzed for U isotopes to further define the hydro-geochemical circulation in estuary. The dissolved U and Sr concentrations, as well as (234U/238U) and 87Sr/86Sr, vary significantly and show strongly seasonal variability. Three characteristic source waters can be identified in the Gaoping River estuary by the U and Sr contents and their distinctive isotopic ratios - seawater (SW), river water (RW), and groundwater (GW or evolved RW). The GW is most clearly identified in the vertical profile of the near estuary stations. These high (234U/238U) values in RW and GW reflect the outcome of intense weathering in the upper catchment, efficient leaching of α-recoil products in soils. Dissolved Sr and 87Sr/86Sr data confirm similar U mixing scenarios, but the index is more sensitive to water source variations. Estimated fluxes of submarine groundwater discharge (SGD) in the Gaoping River estuary reach a maximum value of 456–2107 × 1011 L yr−1, implying important contribution to the chemical budget in coastal regions. This study reveals that U and Sr isotopes have great potential for monitoring SGD in estuaries.
| Identifer | oai:union.ndltd.org:TW/102NCKU5135001 |
| Date | January 2014 |
| Creators | Ruo-MeiWang, 王若梅 |
| Contributors | Chen-Feng You, 游鎮烽 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 140 |
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