摘要
目前,国内外利用碳酸钙沉积物进行古环境和古气候重建的研究,空间上主要涉及到地球化学指标的区域分布差异,时间上则将分辨率提高到了年,甚至季节尺度,但对于地球化学指标在同一区域的空间分布和日变化特征则注意不多。采用仪器自动记录、现场滴定和样品室内测试相结合的方法,对正在发生碳酸钙(钙华)沉积的云南白水台溪流和引渠中的地球化学指标的空间和日变化特征进行了研究。结果发现,在碳酸钙大量快速沉积前(方解石饱和指数SIc小于1.0),向下游方向,水的CO2分压降低,pH值和SIc升高。但当SIc大于1.0后,碳酸钙沉积开始快速产生,此后向下游方向,水的CO2分压趋于稳定,甚至略有升高;同时,水的pH值和SIc也不再升高,而呈现下降现象。与此有关,在溪流下游观测点,由于白天碳酸钙的快速沉积,水的pH值和SIc是下降的,而其CO2分压则升高。这一现象很可能是白天碳酸钙大量快速沉积时产生的CO2在水中聚集,来不及向大气释放的结果;此外,流速对碳酸钙沉积具有显著的控制作用,表现在流速快的地形陡坎部位,Ca2+和HCO-3降低更快,因而沉积速率也更大。研究还发现,富含轻碳稳定同位素12C的CO2向大气的释放是向下游方向钙华碳稳定同位素组成(δ13C)和中午水中溶解无机碳δ13C增加的主要原因,增加幅度分别可达1‰/100m和0.6‰?
So far, the study of paleoenvironmental and paleoclimatic reconstruction with carbonate deposits has mainly concentrated in regional difference and high temporal resolution (in annual or even seasonal scale) identification of geochemical indicators. However, there has been little attention to partial and diurnal variations of the geochemical indicators in the same area. A method of combining hydrochemical data auto logging and in situ titrating with measurements of samples in laboratory was used in present study to investigate the spatial and diurnal variations of geochemical indicators in calcite precipitating stream and canal at Baishuitai, Yunnan Province, SW China. It is found that before mass precipitation of calcite was produced (or SI < 1.0), CO2 partial pressure in water decreased and pH and the calcite saturation index increased downstream. After SI > 1.0, CaCO3 precipitated quickly and the CO2 partial pressure tended to stabilize, or even increased downstream, while pH and the calcite saturation index decreased. At the monitoring sites where SI > 1.0, pH and the calcite saturation index decreased at noon, while the CO2 partial pressure increased due to the quick precipitation of CaCO3, which made the build up of CO2 in water. In addition, flow velocity of water controlled the precipitation of calcite obviously. Especially at the travertine cascades, the flow velocity of water is higher, Ca2+ and decreases more quickly, which leads to the quick precipitation of calcite. It is also found that degassing of CO2 rich in 12C from water leads to the increase of δ 13C in travertine downstream (up to 1‰ /100m) and the dissolved inorganic carbon (DIC) of water at noon (the diurnal change from approximately 0.6‰ to 1.3‰ ). Kinetic fractionation of carbon isotope between DIC of water and travertine is observed. Depending on precipitation rates, the fractionation values can be from 0‰ to 3.22‰ .
出处
《地球化学》
CAS
CSCD
北大核心
2004年第3期269-278,共10页
Geochimica
基金
国家重大基础研究前期研究专项(2002CCA05200)
国家自然科学基金(40372117)
国土资源部"百人计划"项目(9806)
广西自然科学基金(桂科回0144010)
关键词
碳酸钙
钙华
水化学
碳稳定同位素
空间分布
日变化
白水台
云南省
calcite
travertine
hydrochemistry
stable carbon isotope
spatial variation
diurnal variation
Baishuitai
Yunnan
