Lake Malawi's response to “megadrought” terminations: Sedimentary records of flooding, weathering and erosion

Sediment records from the northern basin of Lake Malawi provide a means of evaluating the lake basin's response to climate change over the past 75 ky, notably to increased precipitation at the terminations of droughts. Transitions from drier to wetter conditions provide an opportunity to evaluate the system's response to climate shifts. Upon termination of drought episodes at 62 and 72 ka, enhanced precipitation and an associated increase in streampower led to enhanced physical erosion and landscapes were flooded by rising lake waters. These processes appear to have left their mark in the sedimentary record, bringing about a spike of deposition of organic matter (probably of terrestrial origin) at times of increased rainfall. This was immediately followed by a period of deposition of chemically-weathered material that had been retained on the landscape during arid times and mobilized in response to increased precipitation. After this altered material was removed (perhaps a thousand years after the transition to wetter conditions), fresher material, richer in soluble elements including nutrients, was exposed to chemical weathering, leading to substantial diatom blooms. The lag between the onset of wetter conditions and the diatom blooms is inconsistent with significant storage of bioavailable silica in soils in this system. However, biological cycling of silica, including formation and dissolution of phytoliths, may have played a role in mobilization of the silica necessary for the diatom productivity.