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Abstract

Lake Tanganyika, in the East African Rift System, provides an important modern analog for source-rock deposition in tropical lacustrine environments. In order to test models for source-rock deposition and to improve our understanding of critical processes, we have applied a computer model to simulate deposition of organic carbon in Lake Tanganyika that combines a water-body circulation model with an ecosystem model.

Lake Tanganyika is deep, highly elongated, and subjected to strong southerly winds along the axis of the lake during the dry season, May through August. Our simulations show that deep-water upwelling in the southern lake results from northward wind-driven surface flow during May through August, which causes nutrient enrichment in surface waters and results in strong phytoplankton blooming. During the wet season, weak upwelling, nutrient enrichment, and phytoplankton blooming in the northern lake result from oscillation of the lake. Our simulations suggest that although surface phytoplankton productivity is higher in the southern lake, flux of particulate organic carbon (POC) to the lake bottom is lower than in northern lake. Because the mixing depth is deeper in the southern lake, more oxygen is supplied to the deep part of the lake, which promotes the decomposition of organic matter. In addition, advection of organic matter to the deeper part of the lake was identified as an important process.

Organic-matter flux is relatively insensitive to variation in seasonal wind direction. It is highly sensitive to variation in wind velocity, which affects the strength of lake water circulation, with weaker winds resulting in decreased POC flux to the lake bottom and stronger winds resulting in increased POC flux.

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