Source‐to‐sink response to high‐amplitude lake level rise driven by orbital‐scale climate change: An example from the Pleistocene Lake Malawi (Nyasa) Rift, East Africa

Abstract

AbstractSedimentary systems respond to environmental forcings in dissimilar ways over different timescales. The Lake Malawi (Nyasa) Rift is an ideal natural laboratory for evaluating the overall functioning of lacustrine source‐to‐sink systems on orbital or younger scales. These closed sedimentary systems exhibited high spatiotemporal climate variability, and their responses to two late Pleistocene lake‐level stillstands are evaluated. The coarse‐grained deposits documented a dramatic transgression from 350 to 200 m below present lake‐level (BPLL) developed during an important climate transition in tropical Africa. Based on an integrated analysis of a digital elevation model and high‐resolution single/multi‐channel seismic profiles, catchment geomorphology has been linked with sediment delivery in the sink area. The coarse‐grained deposition of each source‐to‐sink system is quantified through a sediment mass calculation. A modified empirically‐derived ‘BQART’ predictor with a bedload equation to assess the sediment discharge is employed based on a Monte Carlo simulation, considering temperature lapse rate and topographic effects. The river discharges are estimated by specific empirical relationships that associate catchment area to various climate systems, developed using a global modern river database. The results show that the total sediment discharge increases from 7.53 to 9.50 Mt year−1; likewise, the preserved coarse‐grained deposits also record a significant increase in deposition rate from 350 to 200 m BPLL stage, indicating that the short length‐scale source‐to‐sink systems are sensitive to the high‐amplitude lake transgression developed from the climate shift. The volume of upstream buffered deposits may decrease within the progressively wetter climate, while the buffering degree was substantially influenced by the pre‐existing landforms. Moreover, the substantial deep‐water mud dispersal is not well‐developed, despite the relatively higher lake‐level and slightly wetter climate. This quantitative source‐to‐sink analysis with the modified sediment predictor yields preliminary constraints for system functioning in response to high‐amplitude climate change in a closed sedimentary system.