Transcript 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
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Methods for constructing climate history
Factors driving African climate change
Lake level history of Lake Victoria
Implications for radiative evolution
Methods
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Lake, groundwater, speleothem records
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Dating
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Uranium-Thorium
14C dating
Temperature, Lake Level
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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)
18O
isotope record
Biostratigraphy
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18O
data from calcareous components (carbonates or ostracods)
Aquatic cellulose
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
(Gasse 2000)
Modern Lake Victoria
Modern Lake Victoria
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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)
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Generates 70-80% of its own
rainfall
Present precipitation avg.
δ18O=-2.91‰
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Lowest values during Oct-Nov
monsoonal rainfall
(Beuning et al 1997; Beuning et al 2000)
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
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)
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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)
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Examined 3 piston cores from Lake Victoria
Lake-wide discontinuity marked by vertisol
Sedimentary sequence:
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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)
Seismic Profiles
Lake Victoria Cores
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A
(Beuning et al 2000)
(Beuning et al 1997)
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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)
Diatom Record
(Stager and Johnson 2000)
Cichlid Evolution
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Geophysical data suggests
desiccation of Lake Victoria
from 13,200 to 12,400 14C years
BP (14.6 ka)
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Lake contains 500 to 1000
endemic cichlid species
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Dated paleosol layers from lake
core sediments
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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?
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.
(Seehausen 2002, Fryer 2004, Johnson et al 1997)
Cichlid Fishes
Cichlid Diversification
(Kocher 2004)
The Debate
• 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
(Seehausen 2002, Fryer 2001)
(Sturmbauer et al 2001)
Human Impact
(Verschuren et al 2002)
Human Impact
(Verschuren et al 2002)
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
References
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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.
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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
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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.
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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.,
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