Testing the 'megadrought' hypothesis: the timing, cause and impacts of climate change in equatorial Africa (DeepCHALLA-UK)

在过去的25000年里，东非气候对地球轨道的变化（主要是岁差）作出了反应，这些变化影响了季风降雨的长期变化和热带辐合带的迁移。但是更早时期的气候，追溯到25万年前，由于很少有连续的高分辨率陆地记录存在，人们对其知之甚少。这是我们对赤道气候的理解上的一个重大空白，赤道气候对我们自己物种的进化至关重要。非洲大干旱假说认为，在最后一次间冰期（13万至8万年前），非洲热带地区发生了持续数千年的干旱。从几个被调查的地点得到的证据表明，它们的严重程度超过了后来任何时期的干旱，并对进化过程产生了重大影响，例如智人在大陆范围内的迁徙。非洲气候历史上如此漫长而极度干燥的事件是一种意想不到的现象，其确切的时间、起源和范围尚未确定。我们假设，特大干旱的发生是由于地球绕太阳轨道形状的变化（~10万年偏心周期）引起的季风失效，放大了太阳辐射季节性分布的变化（~21,000年岁差周期）。为了充分了解这些极端气候事件，我们将探索东非赤道地区跨越两个冰期-间冰期旋回（即最近~250,000年）的气候变化，为特大干旱及其原因提供关键背景。该项目重点研究了位于乞力马扎罗山侧翼的深湖查拉湖的沉积记录。与东非的其他研究不同，查拉湖的偏东位置使其不受大西洋气候系统的直接影响，从而消除了这方面可能的强迫作用，使我们能够分离出季风变化的记录。这些湖泊沉积物具有提供长期、敏感的环境变化数据库的突出潜力，这已经通过对过去25000年的先前研究得到了证实，这导致了《自然》杂志上关于气候轨道强迫的多学科文章，《科学》杂志上关于层状沉积物及其与ENSO的关系的文章，《地质学》杂志上关于气候变化的季节性和乞力马扎罗山冰核的文章。其他数据集（发表在其他期刊上）证实，这里提出的环境代用指标和定年方法将提供高质量的湖泊记录。根据湖内沉积层的地震剖面估计了深层沉积物的年龄；需要新的绝对日期来确定基础沉积年龄和随后的沉积速率。该项目将通过放射性碳测年对新的深层岩心的沉积物年龄进行建模，通过古地磁检测确定年代的磁反转的存在，并通过Ar/Ar测年和化学对比对火山灰层进行测年。硅藻二氧化硅的碳和氧同位素数据是揭示特大干旱的极好工具，因为它们对潮湿/干旱变化的敏感性已经在过去的25000年里得到了证明。该项目是国际财团的一部分，部分由国际大陆科学钻探计划资助，从湖中回收岩心。我们的国际合作伙伴已经获得了各自国家资助委员会的支持，并将提供互补的环境指标（如花粉、有机生物标志物）以及额外的年代测定（Ar/Ar），最后，用于理解我们的结果在强迫因子方面的全球意义的气候模型（从当地水文到全球气候模型）。我们有肯尼亚和坦桑尼亚的合作者，他们也将作为渠道，确保我们的发现有助于为当前和未来的保护需求和发展战略提供信息。

Over the last 25,000 years, East African climate has responded to changes in the Earth's orbit (mainly precession) that influence long-term variations in monsoonal rainfall and the migration of the intertropical convergence zone. But climate of earlier periods, back to 250,000 years, is less well understood as few continuous high resolution terrestrial records exist. This is a major gap in our understanding of equatorial climate from a region critical to the evolution of our own species. The African megadrought hypothesis states that droughts lasting many thousands of years occurred during the last interglacial (130,000 - 80,000 years ago) across tropical Africa. Evidence from the few sites investigated indicate they were of a severity greater than any droughts of succeeding time periods, and had major implications for evolutionary processes, for example continent wide migrations in Homo sapiens. Such long and intensely dry events in the history of African climate are an unexpected phenomenon, and their precise timing, origin and extent, has yet to be established. We hypothesise that megadroughts arose due to monsoon failure caused by changes in the shape of earth's orbit around the sun (the ~100,000 year eccentricity cycle), amplifying changes in the seasonal distribution of solar radiation (the ~21,000 year precession cycle). In order to fully understand these extreme climatic events, we will explore climate changes in the East African equatorial region spanning two glacial-interglacial cycles (i.e. the last ~250,000 years) giving critical context to the megadroughts and their causes. The project focuses on the sedimentary record from Lake Challa, a deep lake on the flank of Kilimanjaro. Unlike other studies from East Africa, the easterly position of Lake Challa places it beyond the direct influence of the Atlantic climate system, thus removing this aspect as a possible forcing and allowing us to isolate a record of monsoonal variation. The outstanding potential of these lake sediments to provide a long, sensitive repository of environmental change data has been established by a prior study of the last 25,000 years, that resulted in multi-disciplinary articles in 'Nature' on orbital forcing of climate, 'Science' on the laminated sediments and relation to ENSO, and 'Geology' concerning the seasonality of climate variations and the Kilimanjaro ice core. Additional data sets (published in other journals) confirm that the environmental proxies and dating methods proposed here will deliver a high quality record from this lake. The age of the deeper sediments has been estimated from seismic profiles of the sedimentary layers in the lake; new, absolute dates are required to identify the basal sediment age and rates of subsequent sedimentation. This project will lead in modelling of sediment ages from the new, deep cores through radiocarbon dates, palaeomagnetism to detect the presence of well-dated magnetic reversals and dating of volcanic ash layers through Ar/Ar dating and chemical correlation. Carbon and oxygen isotope data from diatom silica are excellent tools to reveal the megadroughts due to their sensitivity to humid/arid shifts as already demonstrated during the last 25,000 years.This project is part of an international consortium, partly funded by the International Continental scientific Drilling Program to recover cores from the lake. Our international partners have already gained support from their own national funding councils and will contribute complimentary environmental proxies (e.g. pollen, organic biomarkers) as well as additional dating (Ar/Ar) and, finally, climate modelling (ranging from local hydrology to global climate modelling) used to understand the global significance of our results in terms of forcing factors. We have Kenyan and Tanzanian collaborators, who will also act as conduits to ensure our discoveries help inform ongoing and future conservation needs and development strategies.