Richard Lyons, Stephen Tooth, Geoff A. T. Duller, Terence McCarthy
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At all three sites, the exposed stratigraphy indicates that hillslopes and floodplains underwent net sediment accumulation during most of the late Quaternary, and that present-day deep erosion is of a magnitude unprecedented probably within at least the past 100 ka. OSL ages indicate that the onset of erosion at each site significantly pre-dates European incursion and instead was broadly coincident with abrupt climatic changes that occurred during the Medieval Climatic Anomaly (MCA, ~<span>ad</span> 900–1300) and Little Ice Age (LIA, ~\n<span>ad</span> 1300–1800). Based on correlation with palaeoclimate proxy records, we propose that erosion was triggered by abrupt hydroclimatic oscillations during the MCA, and continued during the LIA in response to climate-driven, large floods. At these sites, soil type and local base level falls exert secondary controls on the specific locations, processes, rates and depths of erosion. In other areas of South Africa, clear links between land mismanagement and soil erosion have been demonstrated, but for sites where detailed investigations have yet to be undertaken, these findings challenge an often default assumption that soil erosion is necessarily attributable to human factors. 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引用次数: 0
摘要
南非旱地的水土流失主要表现为冲积层中形成的沟壑和坏地(当地人称之为 dongas),以及沿切割河流形成的河谷填土。这种侵蚀通常被归咎于土地管理不善,尤其是自欧洲人定居以来,但也有援引自然因素的,如土壤特性、当地基面下降和气候变化。为了将人为因素和自然因素区分开来,我们在文献和考古证据的支持下,利用光激发发光(OSL)测年法,对南非内陆地区三个间距较大的地点的唐加形成时间和原因进行了限制。在所有三个地点,裸露的地层表明,山坡和洪泛平原在第四纪晚期的大部分时间里都经历了净沉积物的积累,而目前的深度侵蚀程度可能是至少在过去 100 ka 年内前所未有的。OSL 年龄显示,每个地点的侵蚀开始时间大大早于欧洲入侵时间,而是与中世纪气候异常(MCA,约公元 900-1300 年)和小冰河时期(LIA,约公元 1300-1800 年)发生的气候突变基本吻合。根据与古气候代用记录的相关性,我们认为侵蚀是由中世纪气候异常时期突然的水文气候振荡引发的,并在小冰河时期因气候驱动的大洪水而持续。在这些地点,土壤类型和当地的基面下降对侵蚀的具体位置、过程、速率和深度起着次要的控制作用。在南非的其他地区,土地管理不善与土壤侵蚀之间的明显联系已经得到证实,但对于那些尚未进行详细调查的地点,这些发现对人们通常默认的土壤侵蚀必然归因于人为因素的假设提出了挑战。我们的研究结果对土壤侵蚀控制策略和南非旱地景观应对未来气候变化的评估具有重要意义。
Are human activities or climate changes the main causes of soil erosion in the South African drylands?: A palaeo-perspective from three sites in the interior
Soil erosion across South Africa's drylands occurs widely in the form of gullies and badlands (locally termed dongas) that have developed in colluvium and in valley fills along incised rivers. This erosion has commonly been attributed to land mismanagement, particularly since European settlement, but natural factors such as soil properties, local base level fall and climate change have also been invoked. To disentangle human and natural factors, we use optically stimulated luminescence (OSL) dating, supported by documentary and archaeological evidence, to constrain the timing and causes of donga formation at three widely spaced sites across interior South Africa. At all three sites, the exposed stratigraphy indicates that hillslopes and floodplains underwent net sediment accumulation during most of the late Quaternary, and that present-day deep erosion is of a magnitude unprecedented probably within at least the past 100 ka. OSL ages indicate that the onset of erosion at each site significantly pre-dates European incursion and instead was broadly coincident with abrupt climatic changes that occurred during the Medieval Climatic Anomaly (MCA, ~ad 900–1300) and Little Ice Age (LIA, ~
ad 1300–1800). Based on correlation with palaeoclimate proxy records, we propose that erosion was triggered by abrupt hydroclimatic oscillations during the MCA, and continued during the LIA in response to climate-driven, large floods. At these sites, soil type and local base level falls exert secondary controls on the specific locations, processes, rates and depths of erosion. In other areas of South Africa, clear links between land mismanagement and soil erosion have been demonstrated, but for sites where detailed investigations have yet to be undertaken, these findings challenge an often default assumption that soil erosion is necessarily attributable to human factors. Our findings have significant implications for soil erosion control strategies and assessment of South African dryland landscape response to future climate changes.
期刊介绍:
The Journal of Quaternary Science publishes original papers on any field of Quaternary research, and aims to promote a wider appreciation and deeper understanding of the earth''s history during the last 2.58 million years. Papers from a wide range of disciplines appear in JQS including, for example, Archaeology, Botany, Climatology, Geochemistry, Geochronology, Geology, Geomorphology, Geophysics, Glaciology, Limnology, Oceanography, Palaeoceanography, Palaeoclimatology, Palaeoecology, Palaeontology, Soil Science and Zoology. The journal particularly welcomes papers reporting the results of interdisciplinary or multidisciplinary research which are of wide international interest to Quaternary scientists. Short communications and correspondence relating to views and information contained in JQS may also be considered for publication.