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Lake Level Fluctuations in Lake Victoria, East Africa

Lake Level Fluctuations in Lake Victoria, East Africa. Implications for Cichlid Evolution. Shannon Greene Geology and Planetary Sciences California Institute of Technology November 27, 2006. Objectives. Methods for constructing climate history Factors driving African climate change

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Lake Level Fluctuations in Lake Victoria, East Africa

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  1. Lake Level Fluctuations in Lake Victoria, East Africa Implications for Cichlid Evolution Shannon Greene Geology and Planetary Sciences California Institute of Technology November 27, 2006

  2. Objectives • Methods for constructing climate history • Factors driving African climate change • Lake level history of Lake Victoria • Implications for radiative evolution

  3. Methods • Lake, groundwater, speleothem records • Lake – geomorphology, sedimentology, stable isotopes, biological remains • Ground water – paleoclimate conditions during recharge of aquifers (element composition), stable isotopes, nobel gas content • Speleothem records – precipitation and evaporation balance; vegetation cover; paleotemperatures and isotopic composition (texture, stable isotope content of calcite crystals, fluid inclusions) • Dating • Uranium-Thorium • 14C dating • Temperature, Lake Level • 18O isotope record • Biostratigraphy • 18O data from calcareous components (carbonates or ostracods) • Aquatic cellulose

  4. African Climate Variation • Long term climatic variation attributed to Milankovich cycles • ~23,000 yr monsoon cycle = orbital procession cycle • Late pleistocene variations • Series of abrupt events • Unexplanable by monsoon cycle • Driven by interactions of orbital forcing, atmospheric, ocean, and land surface conditions

  5. (Gasse 2000)

  6. Modern Lake Victoria

  7. Modern Lake Victoria • Third largest lake in the world • Drains north to the Nile • Input from Kagera and Katonga Rivers • ~90% annual water input due to 1470mm/yr precipitation • Large surface area (68,800 km2) • Generates 70-80% of its own rainfall • Present precipitation avg. δ18O=-2.91‰ • Lowest values during Oct-Nov monsoonal rainfall (Beuning et al 1997; Beuning et al 2000)

  8. Last Glacial Maximum • 23-18 ka • Dry conditions due to lower tropical land and sea surface temperatures • Models predict cooling and reduced summer precipitation • Weaker hydrological cycle • Temperatures -4.2ºC colder • Decreased precipitation by 13% • Decreased sea surface temperature by a few degrees • Reduces evaporation • Intensifies cooling through an atmospheric decrease in water vapor

  9. Desiccation of Lake Victoria • Lake Victoria completely desiccated 20-15 ka. • Use total organic carbon (TOC) and hydrogen content (HI) to identify periods of low water cover or desiccation • Both measures decrease as surface exposure is approached • Evidence of lake refilling ~15 ka, two paleosols separated by minor transgression (30 m deep) • First occurred 20.5 to 17.9 ka • Second ended 15.3 ka • Basin filled rapidly • In between, have an arid to humid transition, coicident with warming in Antarctica (recorded in Vostok ice cores) • Caused widespread glacier collapse in South America, New Zealand in 16.5 ka (Talbot and Livingstone 1989, Johnson et al 1996, Talbot and Lærdal 2000, Gasse 2000)

  10. Examined 3 piston cores from Lake Victoria • Lake-wide discontinuity marked by vertisol • Sedimentary sequence: • low TOC, HI (abundant terrestrial plant material) • Thin muddy layer with higher HI and water content indicates humid period (few hundred years, depth ~35 m) • Vertisol layer • 15.2 ka lake bed flooded; TOC and HI increase (deep water conditions) • 13.8-13.6 ka; period of deep mixing marked by decrease in δ13C • 10-4ka; water column more stable; max TOC, HI values • Gradual decrease in values; more seasonal climate (Talbot and Lærdal 2000)

  11. Seismic Profiles

  12. Lake Victoria Cores B A (Beuning et al 2000)

  13. (Beuning et al 1997)

  14. Lake overflow dated to ca. 13 ka • Biogenic silica production decreases • Decrease in δ18O indicates increased precipitation, lower evaporation, and large flushing rates • δ18O rises to present values ~3.5 ‰ (Beuning et al 2000)

  15. Diatom Record (Stager and Johnson 2000)

  16. Geophysical data suggests desiccation of Lake Victoria from 13,200 to 12,400 14C years BP (14.6 ka) Dated paleosol layers from lake core sediments Lake contains 500 to 1000 endemic cichlid species Most rapidly evolved (known) assemblage of animal species To constrain rates of adaptive radiation, evolutionary biologists look to the geological community to constrain the East African Rift Valley lake histories. Rate of cichlid evolution in lake victoria is most debated topic in evolutionary biology today Was the lake completely or partially desiccated? For how long? Cichlid Evolution (Seehausen 2002, Fryer 2004, Johnson et al 1997)

  17. Cichlid Fishes

  18. Cichlid Diversification (Kocher 2004)

  19. Desiccation Actually, rates do not vary so much between cichlids and other fishes, but between groups of cichlids Rates consistent with speciation intervals in other young lakes (< 200,000 yrs) No desiccation Fryer (2001) Non-cichlid species don’t display same speed of speciation Geophysical contradictions Estimates of lake depth vary The Debate (Seehausen 2002, Fryer 2001)

  20. (Sturmbauer et al 2001)

  21. Human Impact (Verschuren et al 2002)

  22. Human Impact (Verschuren et al 2002)

  23. Conclusions • Geological time scales • Climate controlled by orbital forcing effects • Shorter times scales • Sea surface temperature • Antarctic glaciers • P/E balance • Lake water balance • Not only affects environment type, but the local ecology • Recent desiccation lead to amazing adaptive radiation in cichlid fishes • Human activity effects sedimentation, local and global climate, and lake ecology

  24. References • Bergonzini, L., 1997, Paleoevaporation and Paleoprecipitation in the Tanganyika Basin at 18,000 Years BP Inferred from Hydrologic and Vegetation Proxies: Quaternary research, v. 47, p. 295. • Beuning, K., Kelts, K., Russell, J., and Wolfe, B.B., 2002, Reassessment of Lake Victoria-Upper Nile River paleohydrology from oxygen isotope records of lake-sediment cellulose: Geology, v. 30, p. 559-562. • Beuning, K.R.M., 1997, Paleohydrology of Lake Victoria, East Africa, inferred from 18 O/16 O ratios in sediment cellulose: Geology, v. 25, p. 1083. • Fryer, G., 2004, Speciation rates in lakes and the enigma of Lake Victoria: Hydrobiologia, v. 519, p. 167. • Fryer, G.G., 2001, On the age and origin of the species flock of haplochromine cichlid fishes of Lake Victoria: Proceedings of the Royal Society B: Biological Sciences, v. 268, p. 1147-52. • Gasse, F., 1989, Water-level fluctuations of Lake Tanganyika in phase with oceanic changes during the last glaciation and deglaciation: Nature, v. 342, p. 57. • —, 2000, Hydrological changes in the African tropics since the Last Glacial Maximum: Quaternary science reviews, v. 19, p. 189. • Johnson, T.C., Scholz, C.A., Talbot, M.R., Kelts, K., Ricketts, R.D., Ngobi, G., Beuning, K., Ssemmanda, I., and McGill, J.W., 1996, Late Pleistocene Desiccation of Lake Victoria and Rapid Evolution of Cichlid Fishes: Science, v. 273, p. 1091-3. • Jolly, D., and Haxeltine, A., 1997, Effect of Low Glacial Atmospheric CO2 on Tropical African Montane Vegetation: Science, v. 276, p. 786-788. • Kaufman, L.S., 1997, Evolution in fast forward: haplochromine fishes of the Lake Victoria region: Endeavour, v. 21, p. 23. • Kocher, T.G., 2004, Adaptive evolution and explosive speciation: the cichlid fish model: Nature reviews. Genetics, v. 5, p. 288. • Seehausen, O.O., 2002, Patterns in fish radiation are compatible with Pleistocene desiccation of Lake Victoria and 14,600 year history for its cichlid species flock: Proceedings of the Royal Society B: Biological Sciences, v. 269, p. 491-7. • Stager, J.C., 2000, A 12,400 14c yr Offshore Diatom Record From East Central Lake Victoria, East Africa: Journal of paleolimnology, v. 23, p. 373. • Sturmbauer, C., Baric, S., Salzburger, W., Ruber, L., and Verheyen, E., 2001, Lake level flucuations synchronize genetic divergence of cichlid fishes in african lakes: Molecular Biology and Evolution, v. 18, p. 144-154. • Talbot, M.R., 2000, The Late Pleistocene-Holocene palaeolimnology of Lake Victoria, East Africa, based upon elemental and isotopic analyses of sedimentary organic matter: Journal of paleolimnology, v. 23, p. 141. • Talbot, M.R., and Livingstone, D.A., 1989, Hydrogen index and carbon isotopes of lactustrine organic matter as lake-level indicators.: Paleogeography, Paleoclimatology, Paleoecology, v. 80, p. 283-300. • Verschuren, D., Johnson, T.C., Kling, H.J., Edgington, D.N., Leavitt, P.R., Brown, E.T., Talbot, M.R., and Hecky, R.E., 2002, History and timing of human impact on Lake Victoria, East Africa: Proceedings of the Royal Society B: Biological Sciences, v. 269, p. 289-94.

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