湘西北黑色泥灰岩风化剖面重金属富集的地球化学机制

通过对湘西北地区黑色泥灰岩风化剖面（DM剖面）中Cr、Ni、Cu、Zn、Cd、Pb、Th、U 8种重金属元素地球化学特征的研究，并与区域上发育于黑色页岩的2条风化剖面的对比分析，初步揭示了黑色泥灰岩风化剖面中重金属富集的地球化学机制。结果表明：（1）与黑色页岩相比，黑色泥灰岩风化成土过程中重金属元素普遍呈现出低背景、高富集的地球化学特征。（2）黑色泥灰岩风化剖面的形成，是在基岩中碳酸盐快速溶蚀的基础上（基岩的酸溶相含量为78.57%），由酸不溶相进一步演化的结果。在此过程中，对于无明显外源输入的元素Cr、Ni、Cu、Zn、Pb、Th、U，一方面，赋存在酸溶相的部分随碳酸盐的淋失而亏损甚至强烈亏损（其中Th相对惰性）；另一方面，残余在酸不溶相的部分因基岩中大量碳酸盐的淋失而显著富集，且随风化剖面的演化未显示出进一步的亏损。因此，这些元素在风化剖面中的富集是绝对（强烈）亏损和显著相对富集的综合结果。对于Cd，存在明显的外源输入，可能主要以水溶态形式带入并大量淀积在剖面下部。（3）在黑色泥灰岩风化剖面，有机质（OM）不是重金属元素的主要赋存介质，其对重金属元素地球化学行为的影响有限。（4）黑色泥灰岩发育的土壤中，Cu和Cd的平均含量分别为54.58 mg/kg和0.58 mg/kg，高于农用地土壤污染风险筛选值，特别是Cd，对土壤生态环境可能形成不利影响，应引起关注。

Geochemical Mechanism for the Enrichment of Heavy Metals in a Weathering Profile of Black Marlstone in the Northwestern Hunan Province, China

To understand the underlying geochemical mechanism of the heavy metal element (HME) enrichment in weathering profiles of black marlstone, this study characterized 8 kinds of HME, including Cr, Ni, Cu, Zn, Cd, Pb, Th and U, along a weathering profile of black marlstone (DM profile) in the northwestern Hunan Province, China, and compared with two weathering profiles of black shales (XT and HZ profiles) from the same region. The results showed that:(1) Geochemical characteristics of HMEs in processes of black marlstone weathering and pedogenesis could be summarized as relatively low background values and high enrichment, compared to black shale cases. (2) The weathering profile of black marlstone was the outcome of further evolution of acid-insoluble material after the rapid dissolution of carbonate bedrock, in which the acid-soluble content occupied 78.57%. In this process, HEM without exogenous inputs, such as Cr, Ni, Cu, Zn, Pb, Th and U, would be leached or even strongly leached if they existed in the acid-soluble component of the bedrock (among them, Th is relatively inert), or would be enriched remarkably due to large amounts leaching of carbonates from the bedrock if they existed in the acid-insoluble component, therefore, the enrichment of these elements in the profile was a comprehensive result of both their absolute (strong) loss and significant relative enrichment during weathering and pedogenesis of black marlstone. Cd, the exogenous origin element, might be entered as water-soluble form and deposited in the lower part of the profile. (3) In the weathering profile of black marlstone, organic matter (OM) was not the main medium associated heavy metals, therefore its impact on the geochemical behaviors of HME might be limited. (4) The average contents of Cu and Cd in soils developed from black marlstone were 54.58 mg/kg and 0.58 mg/kg, respectively, which are higher than the risk screening values for soil contamination in agricultural lands, especially Cd, which has adverse effects on soil ecological environment and should be paid more attention.