用多种方法确定热带阿达马瓦地区(喀麦隆中部)迪比沉陷湖近期沉积物的特征:对古环境重建的影响

IF 3.5 2区 环境科学与生态学 Q2 ENVIRONMENTAL SCIENCES
Agathe Catherine N. Ngo Ndje , Chavom Bachirou Mfayakouo , Soureiyatou Fadil-Djenabou , Paul-Desire Ndjigui
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引用次数: 0

摘要

由于缺乏表征土壤和沉积物物理化学组成的定量技术,人们对各种热带生态系统的演变和功能知之甚少。本文通过从淤积的迪比沉降湖(喀麦隆中部)提取的岩芯,采用多种方法对岩性变化进行了详细研究。为了揭示沉积物、矿物成分和有机物质来源的出处,研究人员使用了一系列工具,包括分光光度计、激光粒度仪、Rock-Eval 热解仪、X 射线衍射仪 (XRD)、便携式 X 射线荧光 (XRF) 光谱仪和电感耦合等离子体原子发射光谱 (ICP-AES) 仪器。所研究的岩心由底部浅色沉积物(第 1 单元)和深色腐殖质沉积物(第 2 单元)组成。沉积物的分级非常好(分级指数 Sd < 0.35Ф),表明沉积物从底部悬浮到滚动迁移。第 1 单元的总有机碳(TOC:2%-5%)含量相对较低,而深色的第 2 单元总有机碳含量较高(10%-33%)。岩心显示出较高的氧指数(OI)值(平均 = 207 毫克二氧化碳(CO2)/克 TOC)和较低的氢指数(HI)值(平均 = 107 毫克碳氢化合物(HC)/克 TOC)。这表明有机物(OM)发生了很大变化,很可能来自陆生植物的分解。叶绿素-α指纹(峰值约为 675 纳米)也显示了有机物的自生初级生产。第 2 单元的矿物组合包括高岭石、石英、长石、蒙脱石、钛铁矿、菱铁矿、辉绿岩、金红石和锐钛矿,而第 1 单元的矿物组合与第 2 单元相似,但存在伊利石,不存在辉绿岩。较高的二氧化钛(TiO2)和锆(Zr)含量证实了源岩也是由花岗岩和片麻岩组成,不含玄武岩和梯田岩等火山岩。利用一些参数,如 ln(Al2O3/Na2O)、K2O/Rb、K/Al 和风化指数(其中 Al2O3 表示氧化铝,Na2O 表示氧化钠,K2O 表示氧化钾,Rb 表示铷,K 表示钾,Al 表示铝),对源岩进行了强化风化。氧化还原敏感痕量元素的富集因子(EF)及其与总有机碳的关系,与三种元素比值(铜/锌(Cu/Zn)、钍/铀(Th/U)和钒/铬(V/Cr))相关的锰/铁比值(Mn∗)为负值,证明在沉积物沉积过程中,迪比湖底层水主要处于亚缺氧至缺氧状态。数据还显示,岩心底部的初级生产力较低,随后向顶部增加,这与风化加剧有关,潮湿的气候条件通常会增加陆地输入和古生产力。岩石学数据显示,古气候总体上温暖湿润。这种气候的变化促成了热带草原的大面积扩展和森林的消失。这项研究揭示了阿达马瓦地区晚更新世到全新世的古环境演变过程,可以将其与热带古环境从更新世到现在的区域演变过程结合起来。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Multi-method characterization of the recent sediment from the Dibi subsidence lake in the tropical Adamawa region (central Cameroon): Implications for the palaeoenvironmental reconstruction

The evolution and functioning of various tropical ecosystems is poorly understood due to a lack of quantitative techniques to characterize the physicochemical composition of soils and sediment. Here, a detailed multi-method study of lithological changes is provided from a core taken from the silted-up Dibi subsidence lake (central Cameroon). To unravel the provenance of sediment, mineral constituents, and organic matter sources, an array of tools are applied including a spectrophotometer, a laser particle sizer, Rock-Eval pyrolysis, X-ray diffractometer (XRD), a portable X-ray fluorescence (XRF) spectrometry, and an inductively coupled plasma-atomic emission spectroscopy (ICP-AES) instruments. The studied core consists of light-colored deposits at the base (Unit 1) overlain by dark humic deposits (Unit 2). The sediment is very well graded (grading index Sd < 0.35Ф), indicative of bottom suspended to rolling transport. Unit 1 is characterized by relatively low values of total organic carbon (TOC: 2%–5%) whereas the dark Unit 2 has high contents of TOC (10%–33%). The core shows a high oxygen index (OI) value (average = 207 mg carbon dioxide (CO2)/g TOC) and a low hydrogen index (HI) value (average = 107 mg hydrocarbon (HC)/g TOC). This indicates highly altered organic matter (OM), likely from decomposed terrestrial plants. The chlorophyll-α fingerprint (peak around 675 nm) also reveals an autochtonous primary production of organic matter. The mineralogical assemblage of Unit 2 comprises kaolinite, quartz, feldspars, montmorillonite, ilmenite, siderite, gibbsite, rutile, and anatase whereas that of Unit 1 is similar but differs with the presence of illite and the absence of gibbsite. The high titanium dioxide (TiO2) and zirconium (Zr) contents confirm that the source rocks also are comprised of granites, and gneiss without volcanic rocks, such as basalts and trachytes. The source rocks have been intensively weathered using some parameters such as ln(Al2O3/Na2O), K2O/Rb, K/Al, and weathering indices, where Al2O3 denotes aluminum oxide, Na2O denotes sodium oxide, K2O denotes potassium oxide, Rb denotes rubidium, K denotes potassium, and Al denotes aluminum. The enrichment factors (EF) of redox-sensitive trace elements and their relation with TOC, the negative values of manganese/iron ratio (Mn∗) associated to three elemental ratios (cupper/zinc (Cu/Zn), thorium/uranium (Th/U), and vanadium/chromium (V/Cr)) attest to the predominance of sub-oxic to anoxic bottom waters in Dibi lake during sediment deposition. The data also show low primary productivity at the base of the core, followed by an increase towards the top, linked to the intensification of weathering and facilitated by humid climatic conditions, usually increase terrestrial input and palaeoproductivity. The petrological data show a generally warm and humid palaeoclimate. This climatic variation has contributed to the large spread of the savannah and disappearance of the forest. This study sheds light on the late Pleistocene to Holocene palaeoenvironment evolution of the Adamawa region which could be integrated into the context of the Pleistocene to Present regional evolution of the tropical palaeoenvironment.

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来源期刊
International Journal of Sediment Research
International Journal of Sediment Research 环境科学-环境科学
CiteScore
6.90
自引率
5.60%
发文量
88
审稿时长
74 days
期刊介绍: International Journal of Sediment Research, the Official Journal of The International Research and Training Center on Erosion and Sedimentation and The World Association for Sedimentation and Erosion Research, publishes scientific and technical papers on all aspects of erosion and sedimentation interpreted in its widest sense. The subject matter is to include not only the mechanics of sediment transport and fluvial processes, but also what is related to geography, geomorphology, soil erosion, watershed management, sedimentology, environmental and ecological impacts of sedimentation, social and economical effects of sedimentation and its assessment, etc. Special attention is paid to engineering problems related to sedimentation and erosion.
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