Diatom assemblage and concentration data for HSPDP-BTB13-1A (Baringo Basin, Kenya)

Kenya’s Baringo-Tugen Hills-Barsemoi drill site is one of six localities across Kenya and Ethiopia from which the Hominin Sites and Paleolakes Drilling Project has obtained sediment cores in an effort to investigate the role of environmental forcing in shaping human evolution. The Baringo Basin site features extensive exposures of the Chemeron Formation, which contains >100 fossil vertebrate localities including five hominin sites. The 227-m drill core, dating from ~3.29 to 2.56 Ma, is characterized by fluvio-lacustrine sediments, including multiple diatomites, with evidence of variable degrees of later pedogenic modification. In the lower part of the core (~3.29–3.04 Ma), diatoms were preserved only in very low abundance, consistent with predominantly fluvial or lake marginal environments. In contrast, five diatomites and two additional diatom-rich intervals were deposited after ~3.04 Ma, reflecting a major shift in the basin hydrology. Planktonic freshwater species dominated these diatom-rich intervals, whereas periphytic taxa were present in proportions less than 2%, suggesting that these intervals represent open-water deposition during lake highstands. Littoral or saline assemblages are largely absent throughout the core. Instead, we observed a pattern of increasing diatom frustule dissolution at the tops and bottoms of diatomite units, indicating increased alkalinity during the transgressive/regressive phases. A Na-bearing zeolite (analcime) indicative of saline waters precipitated in clastic-dominated intervals between diatomites, suggesting extreme environmental variability between lake highstands and lowstands. Diatom assemblages were consistently dominated by a few species belonging to the genera Aulacoseira or Stephanodiscus, which were at times co-dominant. We infer that assemblages dominated by Aulacoseira represent a well-mixed lake with abundant supply of silica. When Stephanodiscus was dominant, which occurred more frequently in the later freshwater phases, we infer incomplete mixing and reduced silica flux to the epilimnion (upper water layer).肯尼亚巴林戈-图根丘陵-巴塞莫伊钻探点是肯尼亚与埃塞俄比亚境内6处研究点之一，古人类遗址与古湖泊钻探项目（Hominin Sites and Paleolakes Drilling Project）在此获取了沉积物岩芯，旨在探究环境强迫作用对人类演化的塑造机制。巴林戈盆地点位出露大面积切梅龙组（Chemeron Formation）地层，该地层包含100余处脊椎动物化石点，其中包括5处古人类遗址。该227米长的钻探岩芯年代跨度约为3.29 Ma至2.56 Ma，以河流-湖泊相沉积物为主要特征，包含多层硅藻土，且存在不同程度的成土改造迹象。在岩芯下部（约3.29–3.04 Ma），硅藻的保存丰度极低，这与当时以河流或湖岸边缘环境为主的沉积背景一致。与之相反，约3.04 Ma之后沉积了5层硅藻土与2处额外的富硅藻层段，反映出盆地水文系统发生了重大转变。这些富硅藻层段以浮游淡水硅藻类群为主，而底栖附生分类单元的占比不足2%，表明这些层段代表了高湖面期的开阔水体沉积。整个岩芯中基本不存在滨岸或盐生硅藻组合。相反，我们观察到硅藻土单元的顶部与底部出现硅藻壳瓣溶解程度升高的现象，这指示湖进/湖退阶段水体碱度升高。一种赋存于硅藻土之间碎屑占优势沉积层段中的含钠沸石（方沸石，analcime），指示当时曾有盐水沉积，表明高湖面期与低湖面期之间存在极端的环境波动。硅藻组合始终以直链藻属（Aulacoseira）或小环藻属（Stephanodiscus）的少数物种为主，二者有时会成为共优势种。我们推断，以直链藻属为主的组合代表了混合充分且硅供应充足的湖泊。而当小环藻属占据优势时（该情况在后期淡水阶段更为频繁），则指示水体混合不充分，且到达表水层（上层水体）的硅通量降低。