The >250-kyr Lake Chala record: A tephrostratotype correlating archaeological, palaeoenvironmental and volcanic sequences across eastern Africa
The >250-kyr Lake Chala record: A tephrostratotype correlating archaeological, palaeoenvironmental and volcanic sequences across eastern Africa
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DOI：
10.1016/j.quascirev.2023.108476
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发表时间：
2024-02
影响因子：
4
通讯作者：
C. Martin-Jones;C. Lane;Maarten Blaauw;Darren F. Mark;Dirk Verschuren;T. Van der Meeren;M. Van Daele;Hannah Wynton;Nick Blegen;Mary Kisaka;Melanie J. Leng;Philip A. Barker
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作者：
C. Martin-Jones;C. Lane;Maarten Blaauw;Darren F. Mark;Dirk Verschuren;T. Van der Meeren;M. Van Daele;Hannah Wynton;Nick Blegen;Mary Kisaka;Melanie J. Leng;Philip A. Barker
区域温地层格架通过共享的火山灰层将保存在土壤或湖泊沉积物中的古气候、考古和火山学记录联系起来。在东非，地质考古序列之间的tephra等时线追踪是一种成熟的年代地层学方法。然而，迄今为止，从湖泊沉积物等连续沉积层序的地点发现的长tephra记录很少，这些记录提供了在区域尺度上连接局部和不连续层序的潜力。长湖沉积物序列也可以捕捉到尚未得到充分研究的火山中心的更完整的喷发历史。在这里，我们提出了从Chala湖提取的>25万年（>250-kyr）连续沉积物序列的地层记录和日期。Chala湖是位于乞力马扎罗山附近的肯尼亚-坦桑尼亚边境的火山口湖。单粒玻璃对可见层和6个隐毯岩层进行了主、次元素分析，揭示了其成分范围从基性软橄榄岩和玄武岩到更进化的毯岩-空纹岩、空纹岩、粗面岩和单一流纹岩。其中，9个与邻近的乞力马扎罗山或北部约60公里的楚鲁火山场的火山锥喷发有关；其中7个与西面约100公里的Meru山的声母火山喷发有关；以及位于北部约350公里处的肯尼亚中部裂谷（CKR）火山的四次至大规模的粗裂性喷发。唯一的流纹岩温层是一个隐温层，与73.7 ka BP（以前，以1950 CE为标准）的苏门答腊新多巴凝灰岩（YTT）有关。两个CKR温层提供了与肯尼亚东部维多利亚湖盆地中石器时代考古相关的陆相序列的直接联系。通过直接40ar /39Ar测年获得的10个tephra层的绝对年龄估计，结合过去25 kyr的6个210pb和16214C测年以及YTT的已知年龄，建立了完整的Chala湖沉积物序列的第一个绝对年代学。不间断的bbb250 -kyr Chala湖沉积档案代表了东非独特的地热层型序列，优化了地热对比在区域古环境、考古和火山学研究中的年代学价值。进一步研究查拉湖层序中的隐毯层，并对附近和更远的古代火山喷发进行进一步的地球化学表征和定年，可能最终为赤道东部非洲提供一个区域性联系和详细的毯层地层格架。
Regional tephrostratigraphic frameworks connect palaeoclimate, archaeological and volcanological records preserved in soils or lake sediments via shared volcanic ash (tephra) layers. In eastern Africa, tracing of tephra isochrons between geoarchaeological sequences is an established chronostratigraphic approach. However, to date, few long tephra records exist from sites with continuous depositional sequences, such as lake sediments, which offer the potential to connect local and discontinuous sequences at the regional scale. Long lake sediment sequences may also capture more complete eruptive histories of understudied volcanic centres. Here, we present and date the tephrostratigraphic record of a >250,000-year (>250-kyr) continuous sediment sequence extracted from Lake Chala, a crater lake on the Kenya-Tanzania border near Mt Kilimanjaro. Single-grain glass major and minor element analyses of visible and six cryptotephra layers reveal compositions ranging from mafic foidites and basanites to more evolved tephri-phonolites, phonolites, trachytes and a single rhyolite. Of these, nine are correlated to scoria cone eruptions of neighbouring Mt Kilimanjaro or the Chyulu volcanic field ∼60 km to the north; seven are correlated to phonolitic eruptions of Mt Meru, ∼100 km to the west; and four to voluminous trachytic eruptions of Central Kenyan Rift (CKR) volcanoes located ∼350 km to the north. The only rhyolitic tephra layer, a cryptotephra, correlates to the 73.7-ka BP (before present, taken as 1950 CE) Younger Toba Tuff (YTT) from Sumatra. Two of the CKR tephra layers provide direct ties with terrestrial sequences relevant to Middle Stone Age archaeology of the eastern Lake Victoria basin in Kenya. Absolute age estimates obtained by direct40Ar/39Ar dating of 10 tephra layers are combined with six210Pb and 16214C dates covering the last 25-kyr and the well-constrained known age of the YTT to build a first absolute chronology for the full Lake Chala sediment sequence. The uninterrupted >250-kyr Lake Chala sedimentary archive represents a unique tephrostratotype sequence for eastern Africa, optimising the chronological value of tephra correlations in regional palaeoenvironmental, archaeological and volcanological research. Further study of cryptotephra in the Lake Chala sequence and additional geochemical characterisation and dating of ancient volcanic eruptions from nearby and further afield may eventually produce a regionally connected and detailed tephrostratigraphic framework for eastern equatorial Africa.