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Middle Pleistocene to recent diatoms and stratigraphy of the Magadi Basin, south Kenya Rift
Journal of Paleolimnology ( IF 2.1 ) Pub Date : 2021-01-18 , DOI: 10.1007/s10933-020-00173-7
Veronica M. Muiruri , Richard Bernhart Owen , Gijs de Cort , Robin W. Renaut , Nathan M. Rabideaux , Tim K. Lowenstein , Kennie Leet , Mark Sier , Andrew Cohen , Dan Deocampo , Christopher J. Campisano , Anne Billingsley , Anthony Mbuthia

Two cores were recovered from the Lake Magadi and Nasikie Engida Basins in the south Kenya Rift. Core MAG14-2A (194 m) contains a middle Pleistocene to Holocene record, whereas core NAS15/19 (4.36 m) covers only the late Holocene. Surficial sediments from springs and shallow-water sites were sampled in both basins. MAG14-2A rests on trachyte dated at 1.08 Ma. Diatoms are rare in the oldest sediments, but well preserved after about 545 ka, documenting a trend from less to more saline water. Core MAG14-2A contains fifteen facies, five of which are diatomaceous. In contrast, NAS15/19 is dominated by two facies, each containing well-preserved diatoms. Both sequences are distinct from others of similar age in the Kenya Rift in lacking pedogenic horizons, reflecting the location of Lake Magadi and Nasikie Engida in a tectonic sump where aquatic environments were maintained by geothermal and meteoric springs. Canonical Correspondence Analysis distinguishes three assemblages in the modern surface muds of Lake Magadi and Nasikie Engida, but with no pre-Holocene counterparts. Eleven diatom zones are recognised in MAG14-2A: Zones D2 to D10 contain rare to common diatoms dominated by Aulacoseira granulata and its varieties, Aulacoseira agassizii, Thalassiosira faurii, Thalassiosira rudolfi and Cyclotella meneghiniana. Individual samples commonly include a mixture of benthic and planktonic taxa and saline and freshwater species. These assemblages indicate waters that ranged between pH 7.4 and 11.5 with conductivities of ~ 300 to > 25,000 μS cm−1. Correlations with the neighbouring Olorgesailie and Koora Basins indicate four major environmental phases that affected the south Kenya Rift during the last million years with fresh to moderately saline water, or land surfaces, developing during Phase I (1000 to 750 ka). These environments gave way to generally wetter conditions and freshwater lakes in all basins during Phase II (~ 750 to 500 ka). Phase III (~ 500 to 325 ka) was characterised by drier conditions with paleoenvironments becoming more variable and dry during Phase IV (325 ka to present).

中文翻译:

肯尼亚裂谷南部马加迪盆地中更新世至近期硅藻和地层

在肯尼亚裂谷南部的马加迪湖和纳西基恩吉达盆地发现了两个岩心。核心 MAG14-2A (194 m) 包含中更新世到全新世的记录,而核心 NAS15/19 (4.36 m) 仅涵盖晚全新世。两个盆地都对泉水和浅水区的表层沉积物进行了采样。MAG14-2A 位于粗面岩上,年代为 1.08 Ma。硅藻在最古老的沉积物中很少见,但在大约 545 ka 后保存完好,记录了咸水从少到多的趋势。MAG14-2A 岩心包含 15 个相,其中 5 个是硅藻相。相比之下,NAS15/19 以两种相为主,每一种都含有保存完好的硅藻。两个序列都不同于肯尼亚裂谷中其他类似年龄的序列,因为缺乏成土层,反映了马加迪湖和纳西基恩吉达在构造坑中的位置,在那里水生环境由地热和流星泉维持。典型对应分析区分了现代马加迪湖和 Nasikie Engida 地表泥浆中的三个组合,但没有全新世前的对应物。在 MAG14-2A 中识别出 11 个硅藻区:D2 至 D10 区包含稀有到常见的硅藻,主要是颗粒状颗粒状颗粒藻及其变种 Aulacoseira agassizii、Thalassiosira faurii、Thalassiosira rudolfi 和 Cyclotella meneghiniana。单个样品通常包括底栖和浮游类群以及咸水和淡水物种的混合物。这些组合表明水的 pH 值介于 7.4 和 11.5 之间,电导率为 ~ 300 至 > 25,000 μS cm-1。与邻近的 Olorgesailie 和 Koora 盆地的相关性表明,在第一阶段(1000 至 750 ka)期间,在过去的百万年中影响了肯尼亚南部裂谷南部的四个主要环境阶段是新鲜至中等咸水或地表。在第二阶段(约 750 至 500 ka),这些环境让位于所有盆地的普遍湿润条件和淡水湖泊。第三阶段(~ 500 至 325 ka)的特征是干燥条件,在第四阶段(325 ka 至今)古环境变得更加多变和干燥。在第二阶段(约 750 至 500 ka),这些环境让位于所有盆地的普遍湿润条件和淡水湖泊。第三阶段(约 500 至 325 ka)的特征是干燥条件,在第四阶段(325 ka 至今)古环境变得更加多变和干燥。在第二阶段(约 750 至 500 ka),这些环境让位于所有盆地的普遍湿润条件和淡水湖泊。第三阶段(~ 500 至 325 ka)的特征是干燥条件,在第四阶段(325 ka 至今)古环境变得更加多变和干燥。
更新日期:2021-01-18
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