Pollen based vegetation and climate change records deduced from the lacustrine sediments of Kikar Tal (Lake), Central Ganga Plain, India

A. Saxena, Anjali Trivedi
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引用次数: 7

Abstract

A 2.0 m thick sediment profile from Kikar Tal, Raebareli District is studied for the pollen assemblage to infer the vegetation and climate variability scenario during the Holocene period. Four distinct palynozones were demarcated based on the palynological succession in the core sediment. However, the radiocarbon dates for the profile could not be procured; therefore, the lake profile is corroborated with the chronology of adjacent lake ‘Chaudhary–Ka–Tal’, Raebareli District. It is envisaged that during 8,000–7,200 cal yrs BP, (Pollen Zone KT–I), forest groves with Holoptelea, Acacia, Madhuca indica, Syzygium and Prosopis as major tree taxa were present in the adjoining area of the lake. The forest groves were interspersed with wider stretches of open vegetation comprising grasses, Tubuliflorae, Liguliflorae, Chenopodiaceae/Amaranthaceae, etc. and shrubs of Adhatoda vasica and Acanthaceae. This vegetation scenario reflects that the region was under a warm and humid climate during that time. The existence of lake is also envisaged by the presence of aquatic elements, Lemna and Typha. Subsequently, during 7,200–4,200 cal yrs BP (Pollen Zone KT–II), the forest groves became more diversified and dense in composition as evidenced by the expansion of most of the existing trees as well as presence of Emblica officinalis, Aegle, Adina cordifolia, Symplocos and Sapotaceae and thickets of Adhatoda vasica and Mimosa pudica. This significant change in the vegetation mosaic as a whole implies the initiation of a warm and more–humid climatic condition, owing to climate amelioration. The encounter of Cerealia and concomitant cropland weeds, viz. Cheno/Am. Cannabis sativa, Brassica, etc. suggests that the vicinity of the lake was under agrarian practice and other anthropogenic activities. The ground flora was still dominated by grasses; however, it also turned more profuse than before. The increasing trend of Cerealia coupled with concomitant cropland weeds signifies the augmentation in Cereal–based agrarian practice owing to favourable climatic condition. The lake expanded as manifested by the increase in aquatic elements, such as Typha, Potamogeton and Lemna during this period. During the time bracket of 4,200–1,800 cal yrs BP (Pollen Zone KT–III), the arboreals reveal a decrease in their frequency and diversity of major tree taxa with incursion of Moringa and Bombax ceiba suggesting warm and moderately humid climate with reduced monsoon. The last zone (Pollen Zone KT–IV), from 1,800 cal yrs BP–Present, elucidates open grassland vegetation corresponding to reduced monsoon precipitation, with the shrinkage of lake area. The presence of Cerealia and other culture pollen taxa suggests the prevalence of intense agrarian practice in the vicinity of the lake.
印度恒河平原中部Kikar Tal湖沉积物的花粉植被和气候变化记录
研究了Raebareli地区Kikar Tal地区2.0 m厚沉积物剖面的花粉组合,以推断全新世期间的植被和气候变化情景。根据岩心沉积物的孢粉演替,划分出四个不同的孢粉带。但是,无法获得剖面的放射性碳年代;因此,该湖泊剖面与Raebareli地区邻近的“chaudhari - ka - tal”湖的年代学相吻合。据推测,在距今8000 - 7200 cal年(花粉区KT-I),湖的邻近地区存在以Holoptelea、Acacia、madhua indica、Syzygium和Prosopis为主要树种的森林。林林中散布着较宽的开阔植被,包括禾草、管花科、柳花科、藜科/苋科等,以及水杨花科和刺科灌木。这种植被情景反映了该地区在那段时间处于温暖潮湿的气候下。湖泊的存在也被设想为水生元素的存在,Lemna和Typha。随后,在7200 - 4200 cal年BP(花粉带KT-II)期间,森林群落的组成变得更加多样化和密集,主要表现为大部分现存树种的扩张,以及Emblica officinalis、Aegle、Adina cordifolia、Symplocos和saptacae的存在,以及Adhatoda vasica和Mimosa pudica的灌丛。植被马赛克整体上的这一重大变化意味着由于气候改善,气候条件开始变得更加温暖和湿润。遇到的谷类和伴随的农田杂草,即Cheno/Am。大麻、芸苔属等表明湖泊附近曾有过农耕实践和其他人为活动。地面植物群仍以禾本科植物为主;然而,它也变得比以前更加丰富。由于有利的气候条件,谷物种植的增加趋势以及伴随的农田杂草表明以谷物为基础的农业实践的增加。在这一时期,湖泊的扩张表现为水生元素的增加,如Typha, Potamogeton和Lemna。在4200 ~ 1800 cal - ybp(花粉带KT-III)期间,随着辣木和木棉的入侵,主要乔木类群的出现频率和多样性均有所下降,气候温和湿润,季风减少。最后一个带(花粉带KT-IV),从1800 cal年BP-Present开始,阐明了开放草地植被与季风降水减少相对应,湖泊面积缩小。谷类植物和其他文化花粉分类群的存在表明,湖附近普遍存在密集的农业实践。
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