Lacustrine Turbidites from Tropical African Lakes as Indicators of Hydrologic and Climatic Changes

• Main Author: Zhang, Xuewei
• Format: Text
• Language: unknown
• Published: SURFACE at Syracuse University 2014
• Subjects: climatic change; droughts; floods; hydrologic change; tropical African lakes; turbidite; Physical Sciences and Mathematics; North Atlantic
• Online Access: https://surface.syr.edu/etd/182; https://surface.syr.edu/cgi/viewcontent.cgi?article=1182&context=etd

Flood-sourced turbidites (i.e. hyperpycnites) preserved in lake basins are proven indicators of hydrologic changes, yet their usefulness as recorders of tropical paleoclimate variability has long been overlooked. The primary focus of this dissertation research is to investigate the late Quaternary hydroclimatic changes in tropical Africa, using hyperpycnites, dated sediment cores, and high-resolution seismic reflection profiles from Lake Kivu in the East African Rift and Lake Bosumtwi in equatorial West Africa. A secondary focus of this dissertation is to image ancient turbidite systems of the Lake Albert rift in East Africa, using 2-D and 3-D seismic reflection data, and to assess the structural controls on turbidite sedimentation. Reoccurring megaturbidites (covering >200 km2) over the past ~12 k.y. were revealed by integrating seismic reflection data and sediment core results from Lake Kivu, which is subject to potential limnic overturns and degassing events. Sedimentological evidence and seismic and lake-floor bathymetric data suggest that the turbidites were sourced by hyperpycnal river flows during exceptional floods. Time series of turbidite bed-thickness and accumulation rate were generated and compared with regional paleohydrologic records of tropical East Africa, and it is found that the temporal occurrence of the turbidites is climatically controlled. It is also suggested that extreme floods in Lake Kivu's recent history may have triggered deep mixing events, and that potential geologic hazards associated with extraordinary turbidity currents may pose a risk to the current gas-extraction efforts in the lake. To further evaluate the effectiveness of lacustrine hyperpycnites as indicators of tropical African hydroclimatic changes, a 65 k.y. record of extreme hydrologic events in equatorial West Africa was reconstructed, using flood-sourced turbidites, seismic and sedimentological lake-level indicators, and paleohydrologic proxies, including total organic carbon and carbon isotopes, from the Lake Bosumtwi impact crater. Peak turbidite sedimentation is found to correlate with intervals of high TOC and markedly negative d13C values, suggesting that the turbidites were deposited during periods of high lake-levels. The multi-proxy paleohydrologic record from Lake Bosumtwi suggests that over the past 65 k.y., millennial-scale variability of hydrologic extremes in equatorial West Africa was linked to North Atlantic climate, with exceptional rainfall events linked to Dansgaard-Oeschger interstadials and megadrought events associated with slowdowns of the North Atlantic Meridional Overturning circulation during Heinrich stadials. Finally, 2-D and 3-D seismic reflection data from the Lake Albert Rift were analyzed to assess turbidite sedimentology in tectonically active rift lakes. Large channelized turbidite systems were observed and characterized in 3-D seismic data using seismic attribute analyses. In the absence of sediment failures and related mass transport deposits from the seismic record, it is postulated that the turbidites were sourced by hyperpycnal river flows during floods, and that there was a change in the sediment source of the turbidites, caused by drainage reversals due to rift shoulder uplift. Sediment dispersal pathways, changes in depositional facies, and evolution of the sublacustrine turbidite systems are largely controlled by syndepositional tectonism in that system.