42-6 A Record of Windiness in Tropical Africa Inferred from Coarse Grained Sediment Deposition in Lake Tanganyika

Session: Integrating 20 Years of Scientific Drilling in the East African-Syrian Rift: A Session In Honor of Andrew Cohen, Part II

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Abstract:

Lake Tanganyika, one of the world’s largest lakes, lies in the East African Rift Zone, and its unique geology and bathymetry preserve sediments that are crucial to understanding past tropical climate. Lake Tanganyika experiences strong seasonal winds and dry conditions that alternate with a prolonged wet season; variations in these processes control the lake’s hydroclimate over time. Previous core-based studies have used diatoms and sedimentology along with other geochemical and biological proxies to document decadal- to millennial-scale climate shifts. Here we use a detailed analysis of grain size of a core collected from a bathymetric ridge in central Lake Tanganyika to interpret climate variation, specifically windiness, over the last ~6 kyr BP.

We conducted grain size analysis at the cm-scale on a 106 cm long core, once chemically removing organic matter, diatoms, and carbonate. After measuring grain size using a laser particle size analyzer, we applied an end-member model to the grain size distribution and found that the data can be modeled by two silt-sized modes; however, some samples contained a third, coarse mode (> 100 µm). The coarse mode varied from ~0 to >50% in any sample of the core. Focusing on these layers, we isolated the coarse grains by sieving and analyzed them using both a petrographic microscope and SEM to determine their texture, composition, and provenance. Through SEM imaging of both the whole grains and polished rounds of epoxied grains, we observed both clay aggregates, which we interpret as parna, and quartz grains encased in clay.

Given the location of the core on a bathymetric ridge, we suggest it is unlikely that the coarse sediment was deposited via mass flow. The core was also taken far enough from shore that it is improbable that coarse grains could be suspended in river sediment plumes long enough to reach the ridge. Therefore, we suggest the coarse fraction is likely blown to the middle of the lake due to strong, seasonal winds. With this understanding, we can analyze the core to infer periods of drier, windier climates since the end of the African Humid Period (~5 kyr BP). Preliminary time series analysis of the end-members suggests periodicity in the accumulation of coarse-grained fraction, which may reflect variations in both windiness and aridity conditions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7301

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