Geology Weekly 5050: Dr. Steven Forman, University of Illinois at Chicago

DateFebruary 18, 2013Time3:00 - 4:30 pm
LocationBSB E231
DescriptionHolocene Lake level variability of Lake Turkana, Kenya and linkages to the Indian Ocean monsoon

The shore of Lake Turkana hosts a long and rich record of hominid evolution and associated cultural changes spanning from the late Pliocene through the Holocene. Human habitation in the Holocene is intrinsically linked to changing water level of Lake Turkana documented by the relict beaches up to 90 m above current water level. Our recent research reveals multiple 40+ m lake level variations in the past 8000 years that were previously unidentified. This scientific advance was possible through detailed mapping, sedimentologic assessment of relict beaches and careful and in tandem dating of landforms by AMS 14C analysis on individual shells and optical dating of quartz grains. The last high stand of Lake Turkana is coincident with the African Wet Period (ca. 15 to 6 ka) when fishing was a critical resource for people in the Rift Valley. There was significant drying in east African, with concomitant lake level fall by ca. 4.5 ka and expansion of pastoralist from North and to South Africa. The nature and timing of this dramatic shift in Holocene climate and associated cultural change in the Turkana Basin is not well known. Our nascent lake level record for Lake Turkana indicates three 40+ m multi-decadal to century variations in lake level between 7.5 and 4 ka. Thus, the transition to pastoralism may reflect extreme climate variability, rather than just a drought state. Recent climate assessments indicate that sustained lake level high in the East African Rift likely reflects strength of the East African Monsoon modulated by the Indian Ocean Dipole. Dry intervals may reflect atmospheric isolation of the Rift from monsoonal moisture sources and weakened Walker Circulation with cooler sea surface temperatures in the western Indian Ocean. This research indicates that Lake Turkana with relict beaches up to 90 m above the current water level provides an unparalleled record of monsoon variability in the past 8 ka. Fieldwork in 2012 has identified a new sequence of relict beaches at 120-140 m above current lake level and soil development at optical ages indicate that these relict forms were deposited during the last interglacial, ca. 130 ka; high lake stands appears to be a common response to warm climate states.
PublisherDepartment of Geology
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