Isotope Geochemistry Group, Department of Geosciences, University of Tübingen, Germany; Spanish National Research Council (CSIC), Andalusian Earth Sciences Institute (IACT), 18100 Armilla, Granada, Spain.
Micropaleontology Group, Department of Geosciences, University of Tübingen, Germany.
Sci Total Environ. 2024 Nov 15;951:175082. doi: 10.1016/j.scitotenv.2024.175082. Epub 2024 Aug 7.
Lake Naivasha, Kenya's second-largest freshwater body is a wetland of international ecological importance and currently subjected to unprecedented anthropogenic influence. The study aims to chronologically reconstruct the main human activities and background weathering reactions that govern metal mobilizations into the lake and their potentially adverse effects on its ecological status. We combine extensive geochemical analyses (major, trace elements, Zn-Pb isotope ratios) in a dated lake sediment record and catchment rocks with remote sensing techniques. Downcore geochemical variations reflect natural ecosystem destabilizations occurring as early as the first half of the 20th century. These coincide with changes towards less radiogenic Pb-isotope values which persist towards the top of the core (Pb/Pb = 1.243 at core base ∼1843, to Pb/Pb = 1.225 at ∼1978). We interpret the land-clearance for agricultural purposes on the Aberdare Range and documented early aviation activities as possible vectors of this early Pb-isotope excursion. The overlapping Pb-isotope signatures between sediment sources and anthropogenic contributions challenges a straightforward deconvolution of the two. Our conservative model calculations suggest, nevertheless, that an addition of up to ∼1.8 % Pb-gasoline influx to the total Pb flux, peaking in the 1980s is able to explain the Pb distribution trend. Homogeneous Zn-isotope compositions in sediments deposited until ∼1970s (δZn = 0.216-0.225 ‰) do not follow major hydro-climatic events or anthropogenic forcing but likely reflect lake-specific natural cycling. Subsequent higher variations to both heavier (up to δZn = 0.242 ± 0.005 ‰) and lighter (down to δZn = 0.184 ± 0.003 ‰) Zn-isotope values are contemporaneous with intensification of large-scale horticultural industry in the catchment. Together with supporting indicators, the lighter Zn-isotope compositions in youngest analysed sediments (21st century) are attributable to increased biological productivity (algal blooms) and ongoing lake eutrophication. Our study demonstrates the applicability of the heavy metal isotope tool to reconstruct human influences on lake environments with complex geological settings such as the East African Rift System.
肯尼亚第二大淡水湖纳瓦沙湖是一个具有国际生态重要性的湿地，目前正受到前所未有的人为影响。本研究旨在按时间顺序重建控制金属进入湖泊并对其生态状况产生潜在不利影响的主要人类活动和背景风化反应。我们结合广泛的地球化学分析（主要元素、微量元素、Zn-Pb 同位素比值）和对有年代记录的湖泊沉积物和集水区岩石的遥感技术。岩芯中的地球化学变化反映了早在 20 世纪上半叶就发生的自然生态系统不稳定。这些变化与放射性成因 Pb 同位素值的变化一致，这种变化一直持续到岩芯顶部（核心底部的 Pb/Pb = 1.243，约为 1843 年，到核心顶部的 Pb/Pb = 1.225，约为 1978 年）。我们将为农业目的而清除奥布多雷山脉土地和有记录的早期航空活动解释为这种早期 Pb 同位素偏移的可能因素。沉积来源和人为贡献之间重叠的 Pb 同位素特征使得无法直接解卷积这两者。然而，我们保守的模型计算表明，高达约 1.8%的 Pb-汽油流入量，在 20 世纪 80 年代达到峰值，足以解释 Pb 的分布趋势。直到 20 世纪 70 年代为止，沉积的沉积物中均匀的 Zn 同位素组成（δZn = 0.216-0.225‰）不遵循主要的水文气候事件或人为强迫，但可能反映了湖泊特有的自然循环。随后，较重（高达δZn = 0.242±0.005‰）和较轻（降至δZn = 0.184±0.003‰）的 Zn 同位素值的变化与集水区大规模园艺产业的加强同时发生。与支持性指标一起，在分析的最年轻沉积物（21 世纪）中较轻的 Zn 同位素组成归因于生物生产力的提高（藻类大量繁殖）和持续的湖泊富营养化。本研究证明了重金属同位素工具在重建具有复杂地质背景的湖泊环境中的人类影响方面的适用性，例如东非大裂谷系统。
