A systematic assessment of late Quaternary hydrological change from valley fill complexes in Namibia

纳米比亚河谷填土复合体晚第四纪水文变化的系统评估

• 批准号: 1943338
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2017
• 资助国家: 英国
• 起止时间: 2017 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-1943338
• 关键词: systematic; assessment; late; Quaternary; hydrological

Southern African climate is influenced by both tropical and temperate rainfall systems and the interplay of these into the future will strongly determine climatic changes. Our understanding of how the hydrological dynamics of southwestern Africa changed in the past, however, is not well constrained. Marine sediment records highlight significant environmental changes over southwestern Africa since the last glacial maximum, yet terrestrial records are limited, and the spatial detail of changes throughout the landscape is not recorded by offshore studies. This study will produce records of fluvial activity from two ephemeral fluvial valleys: the Huab in Northern Namibia, and one in Southern Namibia. This aims to understand individually, how local river valleys have responded to past climatic change individually, and when taken together what we can infer about past regional hydrological dynamics. To produce a record of fluvial activity, sediment deposited in valley fill regions will be dated using OSL. A number of debates persist in explaining how the climate changed over this region in the past. The focus on a southern Namibian river valley will be particularly interesting in understanding the shifting boundary between winter and summer rainfall zones. Relating to the interplay between tropical and temperate rainfall systems, the location of such boundary in the past is currently relatively unknown. The Northern Namibian fluvial record aims to address questions relating to southern Africa's response to the African Humid Period. Whilst well constrained in Northern Africa, the response of Southern Africa appears to be spatially complex and not well understood. Alongside these broad scale questions of regional hydrological dynamics, the study of both northern and southern Namibian river valleys aims to have a comparative aspect, where variations in timing of fluvial activity between the two valleys will be used to investigate within-system changes. Optically stimulated luminescence (OSL) dating will be employed to obtain a timeline of sediment deposition. OSL dating investigates the accumulation of radiation within quartz grains. When subjected to sunlight, grains are bleached and the radiation signal zeroed, but once buried a signal accumulates within the grain. A quantification of radiation within the quartz grain can be converted to an estimation of the length of time the grain has been buried for. A fluvial event which deposits sediment in these valley fill regions can therefore be dated in time. In order to understand the fluvial history from such OSL records, an understanding of the sedimentary architecture of the valley fills will need to be obtained. This will potentially involve the use of ground penetrating radar (GPR), and a comparison to existing palaeoarchives as a guide to understanding gaps and clusters in ages. OSL dating has been applied to other Namibian river valleys, but with varying conclusions based on interpretations of the valley fill sediments. Some identify sediment fills as slackwater deposits from flood events and others as river end deposits from periods of reduced river flow. A component of this study will investigate in detail the origin of such sediments. We will gain a better understanding of the composition of the sediment and how transport and bleaching influences OSL ages. This has the aim of presenting a more coherent picture of Namibian fluvial activity, with the correct interpretation of such valley fill sediments. This study will therefore have two facets: to investigate how the OSL signal relates to fluvial activity, and to produce a record of fluvial activity in the two river valleys from the OSL record. This palaeoenvironmental reconstruction aims to provide an understanding of shifts in regional hydrological dynamics, which can add to the body of knowledge vital for model evaluation in this time period.

南部非洲的气候同时受到热带和温带降雨系统的影响，它们之间的相互作用将强烈地决定未来的气候变化。然而，我们对过去非洲西南部的水文动态如何变化的理解并没有受到很好的限制。海洋沉积物记录突出了自上一次冰川盛期以来西南部非洲的重大环境变化，但陆地记录有限，近海研究没有记录整个地貌变化的空间细节。这项研究将产生两个短暂河流山谷的河流活动记录：纳米比亚北部的Huab和纳米比亚南部的一个。其目的是单独了解当地河谷如何单独响应过去的气候变化，以及当综合起来时，我们可以从过去的区域水文动态中推断出什么。为了产生河流活动的记录，沉积在山谷填充区的沉积物将使用OSL测年。许多争论一直在解释这一地区过去的气候变化。将重点放在纳米比亚南部河谷，对于理解冬季和夏季降雨区之间不断变化的边界将特别有趣。关于热带和温带降雨系统之间的相互作用，过去这种边界的位置目前相对未知。北纳米比亚河流记录旨在解决与南部非洲对非洲湿润时期的反应有关的问题。虽然在北非受到很好的限制，但南部非洲的反应似乎在空间上很复杂，而且没有得到很好的理解。除了这些广泛的区域水文动力学问题外，对纳米比亚北部和南部河谷的研究旨在进行比较，利用两个河谷之间河流活动时间的差异来调查系统内的变化。将使用光激发发光(OSL)测年来获得沉积物沉积的时间线。OSL测年研究了辐射在石英颗粒中的积累。当受到阳光照射时，谷物会被漂白，辐射信号会被归零，但一旦被掩埋，信号就会在谷物中积累。石英颗粒内辐射的量化可以转化为对颗粒被埋藏的时间长度的估计。因此，在这些山谷填充区沉积沉积物的河流事件可以确定时间。为了从这些OSL记录中了解河流的历史，需要了解山谷充填物的沉积结构。这可能涉及使用探地雷达(GPR)，并与现有的古档案进行比较，以此作为了解年龄差距和聚集区的指南。OSL测年也应用于其他纳米比亚河流域，但根据对山谷充填沉积物的解释得出了不同的结论。一些人认为泥沙填充是洪水事件造成的死水沉积，另一些人则认为是河流流量减少时期的河流末端沉积。这项研究的一个组成部分将详细调查这种沉积物的来源。我们将更好地了解沉积物的组成，以及运输和漂白如何影响OSL年龄。其目的是通过对这种山谷充填沉积物的正确解释，更连贯地展示纳米比亚河流活动的图景。因此，这项研究将有两个方面：调查OSL信号如何与河流活动有关，以及从OSL记录中产生两个河流流域的河流活动记录。这一古环境重建旨在提供对区域水文动态变化的了解，这可以增加对这一时期的模型评估至关重要的知识体系。