摘要
网纹土地层包括网纹黄(色)土和网纹红(色)土,形成于低山、丘陵地理景观和热带、亚热带湿热气候环境中,是温暖湿润的气候环境下形成的第四纪重要地层单元。但其网纹化的机制及其第四纪古气候、古环境的意义一直比较模糊。对长江中游三地不同发育程度的网纹红土地层中网纹与基质分离测试研究表明:网纹中,细黏粒颗粒(<0.2μm)相对少、含水量高,土体中发生了以Fe、Mn、Co、Ni、Cr、V等铁族元素为主的过渡金属元素的强烈淋失和Zr、Hf等稳定元素及SiO2的强烈富集;较低的pH、较低的氧化还原电位及较低的Fe^3+/Fe^2+比值,表明网纹具有较强的酸性和还原性。而基质中,游离铁(主要是赤铁矿)强烈富集,较高的Fe^3+/Fe^2+比值、较高的pH与氧化还原电位,指示土体处于偏氧化环境。网纹与基质中这种酸碱特点与氧化―还原环境的差异,有利于过渡金属元素呈低价态随水渗滤到基质中转化成高价态而沉淀积累。只要土体环境反复干湿交替,在网纹与基质之间就会形成以铁元素为主的过渡金属元素的淋溶―沉淀;元素物质的淋溶―沉淀,增加了网纹与基质之间孔隙度的对比,从而也增加了土体中网纹―基质之间的含水量差异,进一步促进了元素物质的淋溶―沉淀。这一过程,在网纹与基质之间,激发了一个正反馈作用,这是网纹形成与发育的环境动力机制。本文研究比较了中国南方网纹地层的地质年代与冬、夏季风在第四纪的代用指标记录,初步认为中国南方网纹红土是东亚冬、夏季风在早更新世晚期达到一定强度,形成显著的干湿交替的气候环境特点后,而在较为温暖湿润的热带―亚热带气候环境下形成的一种特有地层。
Pleistocene reticulated soil including reticulated yellow clay and reticulated red clay, located in low mountains and hills in tropical and sub-tropical climate regime, is the result of difference of free iron in different parts of soil buried in a certain depth, and is a unique strata formed under warm humid monsoon climate conditions. However, a broad controversy about on the formation of mottles and its relationship with Quaternary paleoclimate are still existing. This paper, by testing some proxies of the segregated mottles and matrix of samples from three sections in the middle and lower reaches of Yangtze River, initial results show that: 1) compared with the matrix parts, the mottles contain higher water content, less fine clay, less transitional elements(Fe, Mn, Co, Ni, Cr, V, Ti), but Zr, Hf, Si elements enriched strongly in soil with acid and reducing conditions and the lower ration of Fe^3+/Fe2;2) the matrix parts, however, have relatively higher ration of Fe^3+/Fe^2+, pH and oxidation-reduction potential, and also are completely contrary to the mottle parts in others. Theoretically, the difference of physico-chemical properties between matrix and mottle in reticulated red clay should be favorable to fine clay particles(< 0.2 μm) and transition elements, such as Fe, Mn, Co, Ni, Cr, V, Ti loss to matrix parts from mottle parts by water remove, which results in a sharper contrast of porosity, and then water content between matrix and mottle. Therefore, this process must arouse a positive feedback between matrix and mottle for water loss. So long as soil is in alternative drying-wetting state annually, we deduced, this positive feedback should be existing, which further promoted materials loss from mottle to matrix. By comparing the stratigraphical chronology of reticulated red clay in southern China, climate change of Quaternary East Asia winter and summer monsoon recorded by loess-paleosol sequences at Luochuan, and carbon isotopic change recorded by planktonic foraminifera of the core 1143 in South China Sea, this paper suggests that the reticulated red clay is just a regional soil in warm-humid tropical and subtropical climate regime, and during Quaternary, when the East Asian winter monsoon and summer monsoon intensified to reach a certain critical point in late Early Pleistocene and typical monsoon climate regime with alternate drying-wetting characteristics came into being significantly;then, the reticulated process was aroused. So reticulated(red) clay is a kind of paleosol, the reticulating process in some of which are still progressively at present.
作者
张智
凌超豪
贾玉连
廖富强
王昕梅
彭学敏
蒋梅鑫
ZHANG Zhi;LING Chao-hao;JIA Yu-lian;LIAO Fu-qiang;WANG Xin-mei;PENG Xue-min;JIANG Mei-xin(School of Geography and Environment,Key Laboratory of Poyang Lake Wetland and Watershed Research,Ministry of Education,Jiangxi Normal University Nanchang 330022;State Key L aboratory of Lake Science and Environment,Nanjing Institute of Geography and Limnology,Chinese Academy of Sciences(NIG LAS),Nanjing 210008;University of Chinese Academy of Sciences,Beijing 100049)
出处
《地层学杂志》
CSCD
北大核心
2020年第1期95-103,共9页
Journal of Stratigraphy
基金
江西省重大安全生态问题监控协同创新项目(JXS-EW-00)
鄱阳湖湿地与流域研究教育部重点实验室开放基金(PK2018004)
江西省教育厅科技项目(8884)
江西师范大学研究生创新基金项目(YJS2018069)联合资助.
关键词
网纹土
第四纪
干湿交替
冬、夏季风
正反馈
plinthitic horizon
Pleistocene
alternation of wet and dry period
winter and summer monsoon
positive feedback
