Tracking the Green Sahara: Assessing periods of humidity in North-West Africa using speleothem geochemistry

横跨北非的撒哈拉干旱地带是当今气候系统的重要组成部分。然而，过去气候的变化和未来潜在的变化目前还知之甚少。这在很大程度上是因为该地区缺乏日期准确的古气候记录和历史仪器数据。最近的研究证明，在全新世期间，西撒哈拉出现了一段湿润时期，而北非在晚更新世期间出现了反复的湿润时期和“绿化”。然而，在这些时候为该地区提供水分的过程并没有被很好地理解。过去湿度的变化被认为对非洲境内的早期人类迁徙产生了重要影响，未来的变化可能对生活在该地区的现代人口产生社会影响。该项目将评估过去湿润时期相对于其他气候变化的时间，以及这些时期的湿润来源。石笋是强大的、高分辨率的古气候档案，可以进行绝对年代测定，以产生每年可分辨的区域气候变化记录。该项目将有助于弥补非洲西北部洞穴群记录目前缺乏的地理空间覆盖范围，补充湖泊沉积等其他陆地古气候档案。还将对以前从摩洛哥南部收集的样本进行更多的工作，以进一步了解全新世内的重要气候事件，如非洲湿润时期期间气候变化的程度。这些样本是从以前的一次远足中收集的，可供部门进一步分析。将在摩洛哥以及可能在西撒哈拉和阿尔及利亚进行实地收集石笋的工作，同时考虑到英国外交部对这些国家的旅行建议。然后，将在实验室根据石笋的地球化学，使用稳定的同位素比率、微量元素浓度和样品岩相学来构建古气候记录。洞穴发育是水输入碳酸盐岩溶系统的结果。因此，对石笋生长期到非生长期的分析以及生长速度的变化，是区域湿度变化的一个有价值的指标。通过对整个北非石笋中稳定同位素比率的变化和生长期的时间进行整体解释，可以观察到极度干旱的撒哈拉地带的迁移和湿润时期的发生。对记录的个别分析也将有助于先前对气候的解释。例如，观察石笋18O的地理空间变化将推断过去和现在可能的水分来源和穿越非洲西北部的运输。了解北非气候变化的控制和程度，对于限制区域水文学和全球气候变化之间的关系十分重要。该项目的目的是利用洞穴模型重建阿特拉斯山脉和撒哈拉沙漠之间的气候变化，以增进我们对该地区的了解。

The arid Saharan belt running across North Africa is an important part of today's climate system. However, changes in past climate and potential future variations are currently poorly understood. This is largely due to the lack of well-dated paleoclimate records and historical instrumental data from the region. Recent work has given evidence for a period of humidity in Western Sahara during the Holocene as well as recurrent moist periods and 'greening' of North Africa during the late Pleistocene. However, the processes which provided moisture to the region at these times are not well understood. Past variations in humidity are thought to have had important influences on early human migration within Africa and future changes may have societal implications for modern populations living in the area. This project will assess the timing of past humid periods relative to other climatic changes, and the sources of the moisture during these periods. Stalagmites are powerful, high-resolution paleoclimate archives which can be absolute dated to produce annually resolved records of regional climate change. This project will contribute to the currently lacking geospatial coverage of speleothem records in NW Africa, complementing other terrestrial paleoclimate archives such as lake deposits. Additional work will also be undertaken on previously collected samples from southern Morocco to further understand the extent of climate change during important climate events within the Holocene, such as the African Humid Period. These samples were collected from a previous excursion and are available for further analysis in department. Fieldwork to collect stalagmites will be undertaken in Morocco and potentially in Western Sahara and Algeria, taking into account FCO travel advice to these countries. A record of paleoclimate will then be constructed in the lab from the geochemistry of stalagmites, using stable isotope ratios, trace element concentrations and sample petrography. Speleothems grow as a consequence of the input of water into a carbonate karst system. Analysis of growth to non-growth periods in stalagmites, as well as changes in growth speed, are therefore a valuable indicator of changes in regional humidity. By performing a holistic interpretation of changes in stable isotope ratios and the timing of growth periods in stalagmites across North Africa, the migration of the hyper-arid Saharan belt and the occurrence of periods of humidity may be observed. Individual analysis of records would also contribute to previous interpretations of climate. For example, observing geospatial changes in stalagmite 18O would infer possible moisture sources and transport across NW Africa both in the past and towards the present. An understanding of the controls and extent of climate change in North Africa are important for constraining the relationship between regional hydrology and global climate variations. This project will aim to reconstruct climate change between the Atlas Mountains and Sahara Desert using speleothems to advance our understanding of the region.