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首页> 外文期刊>Earth-Science Reviews: The International Geological Journal Bridging the Gap between Research Articles and Textbooks >Miocene fluvial systems and palynofloras at the southwestern tip of Africa: Implications for regional and global fluctuations in climate and ecosystems
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Miocene fluvial systems and palynofloras at the southwestern tip of Africa: Implications for regional and global fluctuations in climate and ecosystems

机译：非洲西南端的中新世河流系统和植物区系：对气候和生态系统区域和全球波动的影响

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High amplitude climate fluctuations have been inferred from marine isotope data in the early Neogene, but few well documented terrestrial records exist from this era to gauge the effects of these high latitude events on continental climates and ecosystems. The extensive, three-dimensional exposures of Miocene fluvial and fluvio-lacustrine sediments in the Rondeberg clay pit near Cape Town provide a unique window on this era. Palaeomagnetic data suggests that the deposits accumulated over a period of <. 1. Ma. The presence of meso-megathermic palynoforms (Palmae, Ilex-type, Euphorb-type, Rhamnaceae) and mesothermic (Podocarpus-type) palynofloras suggests a humid subtropical/tropical climate. However, abundant charcoal, charred in situ tree stumps, overall poor preservation of organics, evidence for upward-drying lacustrine successions and an appreciable fynbos presence, point to cyclical periods of drought. We suggest that these climate fluctuations may have been influenced by the orbital pacing seen in the marine isotope record of the earlier Miocene, pointing to a high latitude link with mid-latitude terrestrial climate patterns. Earlier studies of pollen spectra from the nearby, slightly older Noordhoek deposits show cyclical alternations from tropical to cooler climates and more recent biogeochemical work has shown dramatic coincident fluctuations in depositional temperature. These vegetation changes were previously correlated with major global events embracing the entire Neogene from the Oligo-Miocene (late Oligocene to early Miocene) to the Pliocene. We offer a different interpretation, suggesting that the deposits represent a much shorter time interval in the earlier Miocene and that these climate fluctuations may have been influenced by orbital forcing evinced in the marine isotope record. Along the northern west coast, the Arrisdrift vertebrate fossil assemblage in Early-Middle Miocene terrace deposits of the Orange River indicate a tropical climate but possibly less humid than in the south, with more open vegetation patterns. The presence of pedogenic calcretes and gypcretes in the deposits suggests periodic extremes of aridity not seen in south; the current pronounced north-aridity gradient from humid temperate to hyper-arid may have had its inception in the earlier Miocene.e present steep west coast climate gradient may have earlier Miocene origins.
机译：从新近纪早期的海洋同位素数据可以推断出高振幅的气候波动，但是从这个时代开始，很少有有据可查的地面记录来衡量这些高纬度事件对大陆性气候和生态系统的影响。在开普敦附近的朗德伯格粘土坑中，中新世河床和河湖相沉积物的广泛三维接触为这一时代提供了独特的窗口。古地磁数据表明，该矿床在＜时期内积累。 1.妈中高温热的古菌形（Palmae，Ilex型，Euphorb型，鼠李科）和等温的（Podocarpus型）古生物形态表明潮湿的亚热带/热带气候。但是，大量的木炭，原木烧焦的树桩，有机物的总体保存不佳，湖相演替的向上干燥以及相当多的雌雄同株的存在，都表明干旱是周期性的。我们建议，这些气候波动可能受到中新世早期海洋同位素记录中看到的轨道起伏的影响，这表明与中纬度陆地气候模式的高纬度联系。对附近较旧的Noordhoek矿床的花粉光谱的早期研究表明，从热带到凉爽的气候周期性变化，最近的生物地球化学研究表明，沉积温度出现了剧烈的同时波动。这些植被变化以前与主要的全球性事件相关，包括从新中新世（晚渐新世到中新世早期）到上新世的整个新近纪。我们提供了不同的解释，表明在中新世早期，这些沉积物所代表的时间间隔要短得多，而且这些海洋气候波动可能已经受到海洋同位素记录中记录的轨道强迫的影响。沿着北部西北海岸，奥兰治河中新世中早期阶地沉积物的Arrisdrift脊椎动物化石组合显示为热带气候，但可能不如南部潮湿，植被开放。沉积物中存在成岩的钙质和石膏质，这表明南部没有周期性的极端干旱现象。当前从湿温带到高干旱的明显北干旱梯度可能始于中新世早期。e目前陡峭的西海岸气候梯度可能具有中新世起源。

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