Carbonate composition and its impact on fluvial geochemistry in the NE Tibetan Plateau region
Carbonate composition and its impact on fluvial geochemistry in the NE Tibetan Plateau region
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青藏高原东北部碳酸盐岩组成及其对河流地球化学的影响
DOI：
10.1016/j.chemgeo.2015.06.009
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发表时间：
2015-09
期刊：
影响因子：
3.9
通讯作者：
Xiaoming Liu
中科院分区：
文献类型：
--
作者：
Qingquan Meng;Chengcheng Ye;Rongsheng Yang;Xiaoming Liu
基于我们的研究结果和其他已报道的数据的综合碳酸盐组成数据集表明，NE TP的基岩碳酸盐组成显示出比以往研究中报道的受限源碳酸盐端元更宽的Sr/Ca和Mg/Ca比值范围。这对晚新生代构造活跃和气候多变地区的现代风化研究和古重建具有明确的意义。基岩碳酸盐组成以浸染型碳酸盐（Sr/Ca和Mg/Ca比值较高）和沉积型碳酸盐（主要为海相）（Sr/Ca比值较低，但Mg/Ca比值变化较大）为特征。在沉积物中发现的自生碳酸盐也表现出类似的趋势，在log （Sr/Ca）与uslog （Mg/Ca）曲线上的梯度为~ 0.97-1.00，表明“方解石沉淀”过程-即：水中溶解阳离子的来源控制着它们的化学性质。根据观测和模拟，我们得出结论，自生和基岩碳酸盐端元的混合，加上基岩碳酸盐的不一致溶解，是沉积物（如黄土、砂和表土）的主要碳酸盐组成。将东北高原的基岩和沉积碳酸盐组成与已报道的河流水数据进行比较表明，导致河流Sr、Mg和Ca组成变化的主要原因是陆源沉积物中的碳酸盐风化作用，而不是基岩中的碳酸盐风化作用。研究表明，在沉积物的暴露、搬运和沉积过程中，碳酸盐与水的相互作用广泛发生，显著改变了区域碳酸盐组成和河流地球化学。
Using co-variations of Sr/Ca and Mg/Ca, we examined the carbonate compositions of various bedrocks (silicate and carbonate rocks) and sediments (eolian and fluvial sediments, sand, and topsoil) found in the NE Tibetan Plateau (TP) region. A combined carbonate composition dataset based on our results and other reported data shows that bedrock carbonate composition on the NE TP displays a much broader range of Sr/Ca and Mg/Ca ratios than restricted source carbonate endmembers reported upon in previous studies. This has clear implications for modern weathering studies in addition to paleo-reconstructions in this tectonically active and climatically variable area during the Late Cenozoic. Bedrock carbonate compositions are characterized by disseminated carbonates with higher Sr/Ca and Mg/Ca ratios, and sedimentary carbonates (mostly marine) with lower Sr/Ca, but variable Mg/Ca, ratios. The mostly authigenic carbonates found in sediments show similar trends, with a gradient ~ 0.97–1.00 in a plot of log (Sr/Ca)versuslog (Mg/Ca), suggesting that ‘calcite precipitation’ processes –i.e.the sources of the dissolved cations in the water – control their chemistry. Based on observations and modeling, we conclude that the mixing of authigenic and bedrock carbonate endmembers, plus the incongruent dissolution of bedrock carbonates, accounts for the bulk carbonate composition of sediments (e.g.loess, sand and topsoil). A comparison of bedrock and sedimentary carbonate composition with reported fluvial water data in the NE TP suggests that weathering of carbonates in terrigenous sediments, rather than in bedrock, is mostly responsible for the changes in fluvial Sr, Mg and Ca compositions. Our study suggests that interactions between carbonates and water occur widely during the exposure, transport and deposition of sediments, significantly modifying regional carbonate compositions and fluvial geochemistry.
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