了解全新世的温度变化能为理解目前日益突出的全球变暖、评估未来全球气候变化给出重要的参考。在这项研究中,基于长江下游南漪湖沉积岩芯深度为0~450cm中161个样品的brGDGTs代用指标,对过去12.0ka的大气温度进行重建,以进一步深化对全新世温度变化的理解。发现湖泊周边土壤与湖泊沉积物brGDGTs分子组成存在显著差异:土壤以brGDGTs-Ⅰ系列为主,占到总比重的80%以上,计算得的MBT'5ME平均值为0.81;湖泊表层和柱状沉积物的brGDGTs分子组成更相似,其brGDGT-Ⅰ和brGDGT-Ⅱ分别为43%、48%和62%、35%,对应的MBT'5ME平均值分别为0.44和0.62,因此认为湖泊沉积物brGDGTs主要为自生来源,进而选用基于MBT'5ME的湖泊温度经验计算式进行古温度的重建。重建的南漪湖年均大气温度自12.0 ka B.P.以来变化范围为13.8~22.4℃,根据变化趋势,可以分为4个阶段:①阶段,早全新世(约12.0~8.2 ka B.P.),温度变化范围为15.1~20.6℃,属低温阶段;②阶段,中全新世(约8.2~6.0 ka B.P.),温度为16.8~20.0℃,为稳定高温阶段;③阶段,中晚全新世(约6.0~3.0 ka B.P.),温度为13.8~19.4℃,快速降温阶段;④阶段,晚全新世(约3.0 ka B.P.以来),温度在17.4~22.4℃,快速升温阶段。通过对比其他古气候记录,可以得到以下结论:长江下游地区在约12.0~8.2 ka B.P.时期温度变化主要受高纬度冰川残留的影响,为低温时期;在约8.2~6.0 ka B.P.时期的温度变化主要受到较强的太阳辐射量控制,属稳定高温期,对应全新世大暖期;约6.0 ka B.P.后,温度受到6.0~3.0 ka B.P.中低纬度冷事件以及上升温室气体辐射强迫共同影响,呈现先降后升的"V"型变化趋势。本研究表明长江下游地区自12.0 ka B.P.以来温度变化主要受全球温度变化控制,自晚全新世以来温室气体辐射强迫是影响其温度变化的主要因素。
Micromorphology of solonetz species with special attention to natric horizon was studied in microcatenas at the Dzhanybek Research Station (northwestern Caspian Lowland). The solonetzic (natric) horizon is easily identified, and it occurs at varying depths, which are the criteria for subdividing solonetzes into 4 species, namely, crusty, shallow, medium and deep. In this sequence, the depth of humus-accumulative horizons increases, and the upper boundary of salinity manifestations goes down. The following micromorphological features are assumed as typical for natric horizons: angular blocky microstructures with partially accommodated aggregates having sharp boundaries and narrow plane-like packing voids; b-fabric speckled in the aggregates' centers and monostriated at their peripheries merging into stress coatings; very few interpedal voids; organo-clay coatings; humus- enriched infillings; no calcite and gypsum pedofeatures. A complete set of "natric" features was found only in the crusty solonetz; the shallow solonetz lacks only illuviation coatings, while the medium and deep species have several modifications of fabric elements: blocky aggregates have a rounded shape and are penetrated by biogenic channels favoring their further biogenic reworking; plant residues became more abundant and diverse, and blackened tissues occur; illuviation clay coatings evolved into papules; stress coatings gave birth to striated bfabrics, thus maintaining a high plasma orientation. The thin sections of natric horizons made 50 and 20 years ago were examined to study the influence of environmental changes (increase in precipitation and rise of ground water table) on micropedofeatures. The following processes took place: (i) in the topsoil: humus accumulation and biogenic structurization; (ii) in the natric horizon - re-arrangement of clay coatings into micromass b-fabrics; and (iii) in the lower part of the natric horizon - development of pseudosand fabric, calcite and gypsum formation. The trends revealed are in good agreement with the environmental events. 相似文献