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世纪下半叶经历了降水模式的剧烈波动，在20世纪70年代和80年代的干旱期间对粮食安全造成了毁灭性影响。展望未来，对萨赫勒地区未来气候变化的预测结果并不一致，一些气候模型预测21世纪的气候会明显变湿，而另一些模型则预测会变干，这给气候变化对这一敏感地区的影响留下了很大的不确定性。该项目旨在通过利用位于西非海岸的深海沉积物重建过去100万年的区域气候变化，提高对北非气候控制的理解。这种气候变率的长期采样将揭示该地区在多个冰期周期中的反应，包括北半球高纬度地区比现在更温暖和更冷的时期，以及北非比现在更潮湿和更干燥的时期。通过系统地记录北非在这一气候条件范围内的反应，结果将为测试气候模型准确代表该地区过去气候变化的能力提供重要的新机会。该项目将使用最近开发的工具来重建撒哈拉沙漠的风吹尘埃排放（风和干旱的示踪剂），沉积物中化石叶子分子的碳同位素组成（大陆植被的示踪剂），以及相同叶子分子的氢同位素组成（季风强度的示踪剂）。该数据集将使人们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宣传项目，提供有关气候变化对北非影响的教育，并将为一名女性博士后研究员和多名本科研究人员提供培训和职业发展。目前对上新世-更新世期间非洲气候演变的理解，在很大程度上是基于一组海洋风沙积累率的基准记录，这些记录提供了一些跨越千万年至百万年时间尺度的唯一连续数据。现在，在这些记录最初发表25年后，水文气候重建的最新进展为新一代了解北非气候提供了机会。恒定通量代理（230Th, 3He）通过最小化横向平流和年龄模式不确定性的影响，从根本上提高了通量的精度；因此，尘埃通量可以更有把握地通过时间和空间进行比较。此外，陆生叶蜡（正构烷烃）的碳和氢同位素组成已被确定为水文代用物，可以为季风降水和植被提供独立的见解。目前，这些方法仅被有限地应用于末次冰期之前的时期，妨碍了对更新世北非气候变化的驱动因素、时间和幅度的严格探索。该项目将使用尘通量测量（由230和3he标准化确定）和叶蜡同位素（dD, d13C）来重建过去1.1 Ma非洲气候的平均状态和变异的变化。这一区间包括轨道参数、冰量和高纬度温度有较大变化的时期，包括所谓的“超级间冰期”期间高纬度温度最高，“微暖”间冰期期间低幅度冰期旋回，以及离心率特别高和特别低的时期。通过将高保真的尘埃通量与反映土地覆盖和水文的补充数据相结合，该项目将提供对这一水资源紧张的季风区过去变化性质的详细见解，并将测试与高纬度温度和当地日照在推动过去水文变化中的作用有关的特定假设。该项目将提供北非气候对各种强迫和边界条件响应的可靠记录，从而为测试用于预测该地区未来变化的气候模型提供新的机会。该数据集还将改进非洲气候的平均状态和变异与人类进化有关的假设的检验。

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.