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Assessing the Stratigraphic Response to Late Quaternary Lake Level Variability, and Estimating Hydrologic Sensitivity to Atmospheric Variability in a Tropical Rift Lake Basin: Lake Malawi, East Africa

Lyons, Robert 1; Scholz, Christopher 1; Kroll, Charles 2
1 Department of Earth Sciences, Syracuse University, Syracuse, NY.
2 Department of Environmental Resources and Forest Engineering, SUNY ESF, Syracuse, NY.

Lake Malawi is one of the largest lakes in the world, located at the southern edge of the East African Rift System. Several inlets enter the lacustrine rift basin, but the outlet is shallow and low in annual discharge. Thus the hydrologic budget of this tropical rift basin is primarily controlled by precipitation - evaporation variability, where both are typically well above 1000 mm/yr. Meter-scale lake level variations can be observed even on seasonal time scales, as is typical of tropical lacustrine systems. Due to this 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 clinoform packages identified in high-resolution seismic reflection data. In conjunction with erosional truncation surfaces, the clinoform downlap surfaces define the basal sequence boundaries within each lake level cycle. Scientific drilling to 380 m subbottom reveals a high resolution continuous record of East African climate covering at least the last ~350 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 silt. We interpret lowstands to be induced by intervals of extreme drought, which occurred during insolation minima on precession and half-precession frequencies. The lowstands develop during periods of high orbital eccentricity, whereas 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 induce these severe lowstands, we generate a hydrologic budget for the catchment to simulate paleo-lowstand conditions. We simulate historical lake level conditions to assess model sensitivity to temperature, precipitation, and vegetation parameters. Paleo-precipitation and evaporation are estimated for the basin during lowstands to determine paleo-atmospheric conditions. By inducing paleo-precipitation estimates from the model output on the modern lake state, we can then assess the time required for lake level to reach equilibrium with the new atmospheric conditions.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009