--> --> Abstract: Controls on Water Balance and Facies Development in a Lacustrine Rift Basin Setting: Lake Malawi, East Africa, by Robert Lyons and Christopher Scholz; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Controls on Water Balance and Facies Development in a Lacustrine Rift Basin Setting: Lake Malawi, East Africa

Robert Lyons1; Christopher Scholz1

(1) Syracuse University, Syracuse, NY.

Lake Malawi is one of the largest lakes in the world, located at the southern edge of the East African Rift System. The hydrologic budget of this tropical rift basin is primarily controlled by precipitation - evaporation balance, both which vary seasonally in a wet-dry annual cycle. Due to a sensitive hydrologic state, this lake has experienced several severe lowstand events throughout the Late Quaternary. Paleo-shorelines from these events are defined by ancient lowstand delta deposits in as much as 500 m of water today, characterized by prograding clinoform packages identified in high-resolution seismic reflection data. In conjunction with erosional truncation surfaces, the downlap surfaces of each lowstand delta define basal sequence boundaries of each lake level cycle. These sequence boundaries are tied to deepwater drill sites, where scientific drilling up to 380 m subbottom reveals a high resolution continuous record of East African climate covering the last at least 500 kyr. In the drill core, lowstands are characterized by dense, organic-poor, carbonate-rich clays and sands while highstands are associated with organic rich, finely-laminated mud. Lowstands are caused by intervals of extreme aridity that occurred during insolation minima on precession and half-precession frequencies during times of high orbital eccentricity. Highstand, relatively stable lake level conditions occur during periods of low eccentricity when the precession signal is muted and tropical climate is dominated by high latitude forcing. To estimate the atmospheric conditions required to sustain equilibrium during these lowstands, we generate a hydrologic model for the catchment to simulate paleo-lowstand conditions. We estimate historical lake level over the last ~100 years to assess model sensitivity to input parameters. Hydrologic budgets are estimated for the catchment during lowstands to determine paleo-atmospheric conditions. Paleo-precipitation estimates from the model output are induced instantaneously on the modern lake state to determine the time required for lake level to reach equilibrium with the new atmospheric conditions. The results from this study demonstrate the dramatic, frequent, and rapid variability syn-rift lakes can experience under changing atmospheric conditions.