Cenozoic vegetation, climate changes and hominid evolution in tropical Africa
Cenozoic vegetation, climate changes and hominid evolution in tropical Africa
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DOI：
10.1016/j.gloplacha.2010.01.015
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发表时间：
2010-07-01
影响因子：
3.9
通讯作者：
Bonnefille, Raymonde
中科院分区：
文献类型：
--
作者：
Bonnefille, Raymonde
本文回顾了新生代热带非洲过去植被的信息，重点关注过去 10 Ma，即中非和东非原始人类记录的时间。将在陆地地点收集的古植物学数据摘要与从大西洋和印度洋回收的两个深海核心的海洋花粉记录中记录的大陆植被带长期演化的新结果进行比较。第 2 部分包括非洲大陆植被带现代分布的摘要，以及根据时间尺度和地理位置呈现的宏观植物学（化石木材、叶子和果实）和微生物学（主要是花粉）研究结果的综合。古植物学结果强调的主要特征是：I) 早在坦桑尼亚始新世就记录了季节性植被和气候 2) 渐新世期间埃塞俄比亚西北部存在高度多样化的森林 3) 中新世期间森林组成的高度时空变化，当时埃塞俄比亚记录了落叶豆科林地，而肯尼亚西部存在湿润的常绿森林 4) 缺乏从刚果西部到东非连续的常绿森林带的证据。图 3 展示了从亚丁湾的一个长核心中恢复的新的原始花粉数据，记录了过去 11 Ma 期间东非过去大规模的植被变化。将这些结果与先前发表的尼日尔三角洲近海海洋核心的长花粉序列进行比较进行讨论。这种比较说明了大陆尺度上大型植被带随时间的地理分布变化。在第 4 节中，结合同位素研究结果和非洲原始人类进化的最新介绍，讨论了过去 10 Ma 期间记录的植被变化。海洋记录中显示了一些变化。东非 10 Ma 出现稀树草原/草地扩张，比西非 7 Ma 早 3 Ma。在大的地理范围内，海洋记录中草花粉的首次增加与森林的增加被解释为反映了整个大陆的湿润条件。事实上，在全球潮湿的条件下，稀树草原可能会蔓延到北部和东部的沙漠地区。西非 7.5 至 7 Ma 期间的森林阶段有详细记录，但东非尚未显示，主要是因为 DSDP 东非记录的分辨率分析较低，需要对该时期进行进一步调查。 6.3 到 6 Ma 之间发生了强烈的植被变化。其特点是植被的树木覆盖率显着下降，西非和东非同时出现。当时，整个热带地区都出现了非常干旱的情况，树木稀少。这发生在麦西尼亚危机爆发之前（或之时）。一般来说，干旱条件与黑猩猩/原始人类分裂的公认时间一致，以及乍得的乍得沙赫人和肯尼亚的图根人图根人的记录，尽管这些化石是在当地树木繁茂的环境下发现的。 6 至 4 Ma 期间，地猿出现，南方古猿多样化，发生在大范围植被中树木覆盖率逐渐增加的过程中，并在 3.9 Ma 期间达到顶峰，即南方古猿时期，在南方古猿阿法种首次出现之前。植被的重要变化发生在 4 Ma 和 3 Ma 之间，许多植物生态系统都适合阿法种，这种原始人类具有广泛的地理分布，并在时间气候和环境变化下持续存在于哈达尔。 2.7 Ma 时，森林花粉出现最强烈且突然的下降，同时草花粉增加，西非比东非更为明显。伴随着 C-4 草比例的显着增加，这在图尔卡纳地区得到了很好的体现，很可能是由于旱季长度的增加所致。这种显着的变化与由于北极冰盖的形成导致热带地区强烈干旱而导致的全球气候变化相对应。在西非和东非，稀树草原的扩张以牺牲雨林为代价，而亚荒漠草原则在北部的稀树草原地区扩张。山地森林沿着斜坡向下移动，更靠近大裂谷的低地。东非考古记录中的食草动物记录的动物区系同步变化、早期人属和石器的出现似乎与陆地遗址记录的当地植被变化同时发生。 (C) 2010 Elsevier B.V. 保留所有权利。
This paper reviews information on past vegetation of tropical Africa during the Cenozoic, focused upon the last 10 Ma, a time spanning hominid record in Central and East Africa. Summary of palaeobotanical data collected at terrestrial sites are compared with new results on the long term evolution of the continental vegetation zones documented from marine pollen record of two deep sea cores recovered from the Atlantic and Indian Oceans.Section 2 includes a summary of modern distribution of vegetation belts in the African continent and a synthesis of the results of both macrobotanical (fossil wood, leaves and fruits) and microbotanical (mainly pollen) studies presented according to time scale and geographical location. The main features emphasized by the palaeobotanical results are I) seasonal vegetation and climate documented as soon as the Eocene in Tanzania 2) well diversified forests existing in northern West Ethiopia during the Oligocene 3) high temporal and spatial variabilities of forests composition during the Miocene when deciduous Legume woodland was documented in Ethiopia whereas wetter evergreen forests existed in Western Kenya 4) lack of evidence for an evergreen forest belt, continuous from Western Congo to East Africa.Section 3 presents new original pollen data recovered from a long core in the Gulf of Aden documenting large scale past vegetation changes in East Africa during the last 11 Ma. These results are discussed in comparison with a summarized long pollen sequence previously published from a marine core offshore the Niger delta. This comparison illustrates variations in geographical distribution of large vegetation zone at the continental scale, through time.In Section 4, vegetation changes registered during the last 10 Ma are discussed in relation with the results of isotopic studies and an updated presentation of hominids evolution in Africa. Several changes are shown in the marine records. An expansion of savanna/grassland is shown at 10 Ma in East Africa, 3 Ma earlier than in West Africa where it is documented at 7 Ma. At large geographical scale, this first increase in grass pollen simultaneously to forest increase in the marine records is interpreting as reflecting wetter conditions over the continent. Indeed, under global humid conditions, savanna could spread over the desert areas in the Northern and Eastern directions. A forest phase is well documented in West Africa between 7.5 and 7 Ma, but has not been shown in East Africa, mainly because of low resolution analysis of the DSDP East African record which needs further investigation for that period. A strong vegetation change took place between 6.3 and 6 Ma. It was marked by a trend of important decrease tree cover of the vegetation, simultaneous in West and East Africa. At that time, very arid conditions shown by scarce tree cover occurred over the whole tropical region. This happened before (or at) the early beginning of the Messinian crisis. Generally arid conditions coincide with the accepted timing for the Chimpanzee/hominid split, and record of Sahelanthropus tchadensis in Chad and Orrorin tugenensis in Kenya, although these fossils were found under locally wooded environment. The period from 6 to 4 Ma saw the appearance of Ardipithecus and diversification of Australopithecines occurring during a progressive increased tree cover in the broad-scale vegetation that culminated at 3.9 Ma, during A. anamensis time and before the first appearance of Australopithecus afarensis. Important variations in the vegetation occurred between 4 and 3 Ma, and many plant ecosystems were available to A. afarensis, a hominid which had a wide geographical distribution and persisted at Hadar under temporal climatic and environmental variability. The strongest and abrupt decline of forest pollen accompanied by an increase in the grass pollen was found at 2.7 Ma, more pronounced in the West than in East Africa. It was accompanied by a significant increase in C-4 grass proportions, well indicated in the Turkana region and likely explained by an increase in dry season length. Such marked changes correspond to the global climate change due to the establishment of the Arctic ice sheet that induced strong aridity in the tropics. Savanna expanded at the expense of rainforest, both in West and East Africa, whereas sub-desertic steppe expanded over savanna areas in the North. Mountain forests moved down slopes, closer to lowland sites in the Rift. Simultaneous faunal changes documented by the herbivores in the fauna, the appearance of early Homo and stone tools in the archaeological record of East Africa appear contemporaneous to local vegetation changes documented at terrestrial sites. (C) 2010 Elsevier B.V. All rights reserved.