Genesis and Paleo-ecological Interpretation of Swamp Ore Deposits at Sahara Paleo-lakes of East Niger
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In formerly vegetated flat lake-shore areas of Pleistocene and Holocene paleo-lake depressions in the Sahara of East Niger (Ténéré, Tchigai mountains and in the Erg of Bilma), ancient dune sands are covered by rampart-like or flat beds of individual or networked rhizoconcretions. The massive goethite accumulation, which partly includes an outer fringe of lepidocrocite, impregnated the ancient dune sands. Apart from Fe, P, Ca, and Mg, other heavy metals were also concentrated. The formation and morphological differentiation of these “swamp ores” were generally bound at vegetated shallow water areas of paleo-lakes in ancient dune fields. Accordingly, the swamp ores of the Ténéré, which has flat to undulating relief, display a large dissemination. In contrast, in the Erg of Bilma the high altitude and steep slopes of ancient dune ridges led to steeper shore areas of the paleo-lakes, at which beds of rhizoconcretions were unable to develop.
The oxides were formed by oxidation of Fe2 + -ions from the lake water and concentrated around the roots in the upper root zone of the swamp vegetation. The lack of oxygen in the warm lake water of the shore region, as well as the decomposition of vegetation residues, excluded high redox potentials within the deeper water near the subhydric soil surface. Hence, the formation of rhizoconcretions can only be explained by the specific physiological characteristics of the swamp vegetation, which was able to supply oxygen to the roots through an aerenchyma. The release of surplus oxygen from such roots obviously caused high redox potentials at the root surface and in the neighbouring root environment. As a result precipitation of Fe and Mn oxides occurred, which adsorbed nutrients and heavy metals from the soil solution. The redistribution of the ions from the reduced sediments of the lake basin into the root zone of the shore area resulted from diffusion and mass flow. Paleo-climatically, the swamp ore deposits denote humid periods accompanied by a stable lake water-table over a long period of time. The sequence of several swamp ore beds along the former shore, at different elevations above the lake floor, is evidence of decreasing paleo-lake water levels within a period of increasing aridity.
KeywordsLake Water Lake Basin Lacustrine Sediment High Redox Potential Shore Line
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