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

南部非洲的气候受到热带和温带降雨系统的影响，这些降雨系统的相互作用将在很大程度上决定未来的气候变化。然而，我们对过去西南非洲水文动态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末次盛冰期以来非洲西南部的重大环境变化，但陆地记录有限，近海研究没有记录整个景观变化的空间细节。这项研究将从两个短暂的河流流域的河流活动的记录：在北方纳米比亚的华布，和一个在纳米比亚南部。这旨在单独了解，当地的河谷如何单独响应过去的气候变化，以及当我们结合在一起时，我们可以推断出过去的区域水文动态。为了记录河流活动，将使用OSL对山谷填充区沉积的沉积物进行测年。在解释该地区过去的气候变化方面存在许多争论。对纳米比亚南部河谷的关注将特别有助于了解冬季和夏季降雨区之间的变化边界。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。北方纳米比亚河流记录的目的是解决有关南部非洲的非洲潮湿期的反应问题。虽然在北方非洲受到很好的限制，但南部非洲的反应似乎在空间上很复杂，人们还不太了解。除了这些广泛的区域水文动态问题，纳米比亚北方和南部河谷的研究旨在有一个比较方面，在两个山谷之间的河流活动的时间变化将被用来调查系统内的变化。将采用光释光测年法获得沉积物沉积的时间轴。光释光测年法研究石英颗粒内辐射的积累。当受到阳光照射时，谷物被漂白，辐射信号归零，但一旦被掩埋，信号就会在谷物内积累。石英颗粒内的辐射的量化可以转换为颗粒被掩埋的时间长度的估计。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记录中了解河流历史，需要了解山谷填充物的沉积结构。这将可能涉及使用探地雷达，并与现有的古档案进行比较，作为了解年龄差距和年龄集群的指南。光释光测年已应用于其他纳米比亚河谷，但不同的结论的基础上解释的山谷填充沉积物。一些人认为沉积物填充物是洪水事件产生的滞水沉积物，另一些人认为是河流流量减少时期产生的河流末端沉积物。这项研究的一个组成部分将详细调查这种沉积物的起源。我们将更好地了解沉积物的成分，以及运输和漂白如何影响光释光年龄。其目的是提供一个更连贯的画面纳米比亚河流活动，正确的解释，这样的山谷填充沉积物。因此，这项研究将有两个方面：调查如何OSL信号与河流活动，并产生一个记录的河流活动在两个河谷的OSL记录。这种古环境重建的目的是提供一个了解区域水文动力学的变化，这可以添加到知识体系至关重要的模型评估在这段时间内。

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.