Abstract Grain-size analysis and end-member modeling of the clastic fraction of the 25-kyr sediment sequence from Lake Chala, a meromictic crater lake on the lower east slope of Mt. Kilimanjaro, reveal crucial aspects of climate-driven environmental change in equatorial East Africa since the Last Glacial Maximum. The finely laminated sediments of Lake Chala contain only up to 40% of clastic components, the rest are mainly diatom frustules and amorphous organic matter. Measured grain-size distributions were split into six statistically meaningful end members, of which four could be linked to a distinct source and transport mechanism of clastic mineral input: fine aeolian dust from distal sources (EM1), fine catchment runoff (EM2), coarser aeolian dust from proximal sources (EM5) and coarse erosive material from the upper crater slopes (EM6). The two other end members (EM3 and EM4) represented frustules of the two most common diatom taxa in Lake Chala, Afrocymbella barkeri and Nitzschia fabiennejansseniana, which had (partly) survived sample pre-treatment. Temporal variation in normalized abundance of the two dust-derived end members are valuable proxies for past changes in monsoon circulation over equatorial East Africa. During Northern Hemisphere cold periods (e.g., Last Glacial Maximum and Younger Dryas) the Inter-Tropical Convergence Zone shifted southward, enhancing northeasterly monsoon winds in the Lake Chala area and increasing advection of fine dust from the dry Horn of Africa region. Simultaneously, more modest continental heating reduced the prevalence of small-scale atmospheric turbulence, and thus the occurrence of dust devils, resulting in reduced influx of coarse dust from drylands nearby. Conversely, abrupt intensification of the southeasterly monsoon at the onset of the Holocene is recorded by an abrupt increase in the amount of coarse dust delivered to Lake Chala. Temporal variation in the end members representing catchment run-off (EM2) and erosion (EM6) mainly reflect changes in lacustrine sedimentation dynamics associated with major lake-level fluctuation, as evidenced by other paleoenvironmental proxies. Overall this study shows that subdivision of the clastic fraction of lacustrine sediments into statistically robust grain-size end members can provide multiple independent and quantitative proxies which help constrain reconstructions of a region’s multi-faceted climate history.

中文翻译：

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从粒度分布重建东非季风变异性：查拉湖富含硅藻沉积物的端元建模和源归属

摘要 25-kyr 沉积物层序碎屑颗粒的粒度分析和端元模拟，来自查拉湖（位于 Mt. 东坡下坡的一个meromictic 火山口湖）。乞力马扎罗山揭示了自末次盛冰期以来赤道东非气候驱动的环境变化的关键方面。查拉湖细层状沉积物仅含碎屑成分高达40%，其余主要为硅藻壳和无定形有机质。测量的粒度分布被分成六个具有统计意义的末端成员，其中四个可能与碎屑矿物输入的不同来源和运输机制有关：来自远端来源的细风积尘 (EM1)、细小的集水径流 (EM2)、较粗的来自近端源 (EM5) 的风尘和来自上火山口斜坡 (EM6) 的粗糙侵蚀物质。另外两个末端成员（EM3 和 EM4）代表 Chala 湖中两种最常见的硅藻类群 Afrocymbella barkeri 和 Nitzschia fabiennejansseniana 的硅藻壳，它们（部分）在样品预处理后幸存下来。两个源自尘埃的末端成员的归一化丰度的时间变化是赤道东非季风环流过去变化的有价值的代表。在北半球寒冷时期（如末次盛冰期和新仙女木），热带辐合带向南移动，加强了查拉湖地区的东北季风，并增加了干燥的非洲之角地区细尘的平流。同时，更温和的大陆加热减少了小规模大气湍流的流行，从而减少了尘暴的发生，导致从附近旱地流入的粗尘减少。相反，在全新世开始时东南季风的突然加强是通过输送到查拉湖的粗尘量的突然增加来记录的。代表流域径流 (EM2) 和侵蚀 (EM6) 的末端成员的时间变化主要反映与主要湖泊水位波动相关的湖泊沉积动力学变化，其他古环境代理证明了这一点。总体而言，这项研究表明，将湖相沉积物的碎屑部分细分为统计上稳健的粒度端部成员可以提供多个独立和定量的代理，这有助于限制对一个地区多方面气候历史的重建。在全新世开始时东南季风突然加强的记录是通过输送到查拉湖的粗尘量的突然增加来记录的。代表流域径流 (EM2) 和侵蚀 (EM6) 的末端成员的时间变化主要反映与主要湖泊水位波动相关的湖泊沉积动力学变化，其他古环境代理证明了这一点。总体而言，这项研究表明，将湖相沉积物的碎屑部分细分为统计上稳健的粒度端部成员可以提供多个独立和定量的代理，这有助于限制对一个地区多方面气候历史的重建。在全新世开始时东南季风突然加强的记录是通过输送到查拉湖的粗尘量的突然增加来记录的。代表流域径流 (EM2) 和侵蚀 (EM6) 的末端成员的时间变化主要反映与主要湖泊水位波动相关的湖泊沉积动力学变化，其他古环境代理证明了这一点。总体而言，这项研究表明，将湖相沉积物的碎屑部分细分为统计上稳健的粒度端部成员可以提供多个独立和定量的代理，这有助于限制对一个地区多方面气候历史的重建。代表流域径流 (EM2) 和侵蚀 (EM6) 的末端成员的时间变化主要反映与主要湖泊水位波动相关的湖泊沉积动力学变化，其他古环境代理证明了这一点。总的来说，这项研究表明，将湖相沉积物的碎屑部分细分为统计上稳健的粒度端部成员，可以提供多个独立和定量的代理，这有助于限制对一个地区多方面气候历史的重建。代表流域径流 (EM2) 和侵蚀 (EM6) 的末端成员的时间变化主要反映与主要湖泊水位波动相关的湖泊沉积动力学变化，其他古环境代理证明了这一点。总体而言，这项研究表明，将湖相沉积物的碎屑部分细分为统计上稳健的粒度端部成员可以提供多个独立和定量的代理，这有助于限制对一个地区多方面气候历史的重建。