Collaborative Research: Insights into North African climate variability over the last 1.1 million years from dust fluxes and leaf wax isotopes

北非萨赫勒地区在20世纪后半叶经历了降水模式的剧烈波动，在1970年代和1980年代的干旱期间对粮食安全造成了毁灭性的影响。展望未来，对萨赫勒地区未来气候变化的预测几乎没有达成一致，一些气候模型预测21世纪的气候条件将明显变得更加潮湿，另一些模型则表明气候将变得干燥，这给气候变化对这一敏感地区的影响留下了极大的不确定性。该项目旨在通过利用西非海岸外的深海沉积物重建过去100万年来的区域气候变化，提高对北非气候控制的了解。这种气候变异性的长期采样将揭示该区域在多个冰期周期中的反应，包括北半球高纬度地区比现在明显温暖和寒冷的时期，以及北非比现在潮湿和干燥得多的时期。通过系统地记录北非在这一气候条件下的反应，结果将提供重要的新机会，以测试气候模型准确描述该地区过去气候变化的能力。该项目将使用最近开发的工具来重建撒哈拉沙漠的风尘排放(风和干旱的示踪)、沉积物中化石叶子分子的碳同位素组成(大陆植被的示踪)以及相同叶子分子的氢同位素组成(季风强度的示踪)。这一数据集将使我们能够更好地了解过去是什么驱动了气候变化，以及这对北非未来的变化意味着什么。该项目还将包括公共外联项目，以提供关于北非气候变化影响的教育，并将为一名女性博士后研究人员和多名本科生研究人员提供培训和职业发展。目前对非洲上更新世气候演变的认识在很大程度上是以一套海洋风尘累积速度的基准记录为基础的，这些记录提供了一些跨越千年至百万年时间尺度的唯一连续数据。现在，在这些记录最初公布25年后，水气候重建的最新进展为新一代人对北非气候的洞察提供了机会。恒定通量指标(230，3He)通过最大限度地减少横向平流和年龄模型不确定性的影响，从根本上提高了通量的准确性；因此，可以更有把握地跨时间和空间比较尘埃通量。此外，陆地叶蜡(正构烷烃)的碳和氢同位素组成已被确定为可提供对季风降水和植被的独立见解的水文指标。目前，这些方法仅在上一次冰川周期之前的时期得到了有限的应用，无法对更新世北非气候变化的驱动因素、时间和幅度进行严格的探索。该项目将使用尘埃通量的测量(由230和3He归一化确定)和叶蜡同位素(Dd，d13C)来重建过去1.1 Ma期间非洲气候的平均状态和可变性的变化。这一间隔包括轨道参数、冰量和高纬度温度有很大变化的时期，包括所谓的“超间冰期”期间的最高高纬度温暖时期，“卢克-温暖”间冰期期间的低幅度冰期周期，以及特别高偏心率和低偏心率时期。通过将高保真尘埃通量与反映土地覆盖和水文的补充数据配对，该项目将对这一缺水季风区过去变化的性质提供详细的见解，并将检验与高纬度温度和当地日照在推动过去水文变化方面的作用有关的具体假设。该项目将通过提供北非气候对各种强迫和边界条件的反应的可靠记录，为测试用于预测该区域未来变化的气候模型提供新的机会。这一数据集还将改进对非洲气候平均状态和可变性的变化与人类进化相关的假设的测试。

The Sahel region of North Africa experienced dramatic swings in precipitation patterns over the latter half of the 20th century, with devastating impacts on food security during the droughts of the 1970s and 1980s. Looking ahead, projections of future climate change in the Sahel show little agreement, with some climate models predicting significantly wetter conditions in the 21st century and others suggesting drying, leaving great uncertainty as to the impacts of climate change on this sensitive region. This project aims to improve understanding of controls on North African climate by reconstructing regional climate changes over the last 1 million years using deep-sea sediments located off the west African coast. This long-term sampling of climate variability will reveal the region's response during multiple ice age cycles, including periods when the high latitudes of the Northern Hemisphere were substantially warmer and colder than at present and periods when North Africa was much wetter and drier than at present. By systematically documenting North Africa's response during this range of climatic conditions, the results will provide important new opportunities to test climate models' ability to accurately represent past climate variability in the region. The project will employ recently developed tools to reconstruct windblown dust emissions from the Sahara desert (a tracer of winds and aridity), the carbon isotopic composition of fossil leaf molecules in the sediment (a tracer of vegetation on the continent), and the hydrogen isotopic composition of the same leaf molecules (a tracer of monsoon strength.) This dataset will enable a better understanding of what has driven climate change in the past, and what this will mean for future change in North Africa. The project will also include public outreach projects to offer education about the impact of climate change in North Africa, and it will provide training and career development for a female postdoctoral researcher and multiple undergraduate researchers. The present understanding of African climate evolution over the Plio-Pleistocene is based in large part on a benchmark set of marine records of the accumulation rate of windblown dust that provide some of the only continuous data spanning thousand to million year timescales. Now, 25 years after the initial publication of these records, recent advances in hydroclimate reconstruction offer the opportunity for a new generation of insights into North African climate. Constant flux proxies (230Th, 3He) allow fundamental improvements in the accuracy of fluxes by minimizing the effects of lateral advection and age model uncertainties; as a result, dust fluxes can be compared through time and space with much greater confidence. Additionally, the carbon and hydrogen isotope composition of terrestrial leaf waxes (n-alkanes) have been established as hydrological proxies that can offer independent insights into monsoon precipitation and vegetation. At present, these methods have received only limited application to periods prior to the last glacial cycle, precluding rigorous exploration of the drivers, timing and amplitude of North African climate change over the Pleistocene. This project will use measurements of dust flux (determined by 230Th- and 3He-normalization) and leaf wax isotopes (dD, d13C) to reconstruct changes in the mean state and variability of African climate over the last 1.1 Ma. This interval includes periods with broad variations in orbital parameters, ice volume, and high-latitude temperatures, including maximum high latitude warmth during so-called "super-interglacials", low-amplitude ice age cycles during the "luke-warm" interglacials, and times of especially high and low eccentricity. By pairing high-fidelity dust fluxes with complementary data reflecting land cover and hydrology, the project will offer detailed insights into the nature of past changes in this water-stressed monsoonal region and will test specific hypotheses relating to the roles of high-latitude temperatures and local insolation in driving past hydrological changes. The project will offer new opportunities for testing climate models used to forecast future changes in the region by providing robust records of the response of North African climate to a wide range of forcings and boundary conditions. This dataset will also enable improved testing of hypotheses relating changes in the mean state and variability of African climate to human evolution.