A CHANGING LANDSCAPE IN THE SEMI-ARID TROPICS: INSIGHTS FROM PALEOSOLS, EAST AFRICA RIFT, KENYA

Documentation of changing landscapes in terrestrial settings is elusive. Depositional records are often spatially and temporally incomplete. Change in paleoenvironment is recorded indirectly by alterations in hydrologic budget (P +/- ET), vegetation response, weathering intensity, and soil development. The East African Rift valley is an ideal setting for interdisciplinary study of paleosols and a landscape approach to the study of environmental change. Rift tectonics (uplift) and volcanism provide a wide range of parent material compositions for soil development. Lakes, rivers and wetlands fluctuate with climate change and a pervasive catena (soil) records the changing landscape.

The Kapthurin Fm., Lake Baringo basin, Kenya, is a Middle Pleistocene alluvial fan-littoral plain sediment wedge (150 km2) produced by uplift of the adjacent Tugen Hills. The sequence contains dated volcanics that allow a tentative correlation to the global climate record at ~550 ka. A new proxy that synthesizes paleopedology, clay mineralogy and sedimentology was developed and can be used for correlation within the basin, documenting wet/dry cycles and tracking environmental changes through time. An 11 m section of the lacustrine facies of the Middle Silts and Gravels Member (K3') is sandwiched between a trachyte and a basalt and records two lake cycles that each begin with lake sediments and terminate with subaerial exposure and an extensive overprint of pedogenesis. The basal cycle has a geochemical signal enriched in Zr, Y, and Hf, (trachyte source), whereas the upper cycle is highly enriched in Ti and Cr (basaltic source). The lake phase is represented by green claystones of celadonitic 10-Å clay, dioctahedral Mg-smectite, zeolites and sediments enriched in Ca, Mg, and Mn. The paleosol phase contains a more Al-rich 10-Å clay consistent with illitic compositions and Al-rich smectite with sediments enriched in K, Na, Rb and Ba. Repetitive normal faulting is suggested by synchronous spring activity elsewhere in the section. This may have influenced lake levels, but the potential role of regularly timed astronomic forcing must also be considered, because the two cycles have similar sediment thicknesses (~5m), and the maturity of the paleosols suggests comparable duration.