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Paleo-environment and flooding of the Limpopo River-plain, Mozambique, between c. AD 1200–2000

Sitoe, Sandra Raúl, Risberg, Jan, Norström, Elin, Snowball, Ian, Holmgren, Karin, Achimo, Mussa, Mugabe, João
Catena 2015 v.126 pp. 105-116
Bacillariophyceae, carbon, climate, floodplains, magnetic properties, microfossils, oxbow lakes, paleoecology, prediction, rivers, sand fraction, sea level, sediments, soil formation, uncertainty, Mozambique
Multi-proxy analysis was performed on a radiocarbon-dated core, collected from a relic oxbow lake in the Limpopo River-plain, Mozambique, with the aim to reconstruct paleo-environment and past flooding of the lower river system over the past c. 800years. An additional objective was to evaluate and investigate the potential use of different proxies as recorders of paleo-flooding events and paleo-environmental variability within the floodplain. The proxies applied in this study were: mineral magnetic properties, grain-size distribution, organic carbon content and diatom microfossil assemblages. We found that sediment grain-size and mineral magnetic properties of the minerogenic fraction were the most sensitive proxies in terms of detecting signals from high-intensity river-discharge events. In the 800year long sequence, variations in sand content, magnetic susceptibility and saturation isothermal remnant magnetization suggest at least four major flooding events at the site during the reconstructed period; in the mid-1200, late-1300, mid-1500AD and during the last century. The diatom proxy reflects the development of the site from an open oxbow lake to a mainly terrestrial area. The diatom assemblage indicates that open lake conditions prevailed at the site between c. AD 1200–1400, with periodic inundation by marine water, most likely due to late Holocene sea-level changes. From c. AD 1400 and onwards, diatoms were rarely deposited at the site, which indicates drier conditions. This was a result of soil formation and gradual in-filling of the lake, a process which possibly was accentuated by a regionally dry climate situation. Our study shows that oxbow lakes and the proxies used here have great potential for reconstructing flooding events, a knowledge that is crucial for potential prediction and mitigation of flooding events in Mozambique in the future. Although chronological uncertainties limit comparisons to other paleo-environmental records, it seems that the flooding events recorded at our site occurred both during regionally wet and dry periods. Our data infer however, that flooding was probably more clearly recorded during the lake-stages than during infilled stage, probably as the terrestrial environment was more exposed to erosion.