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Geology; May 2005; v. 33; no. 5; p. 377-380; DOI: 10.1130/G21132.1
© 2005 Geological Society of America
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Paleoenvironmental interpretation of lake-margin deposits using {delta}13C and {delta}18O results from early Pleistocene carbonate rhizoliths, Olduvai Gorge, Tanzania

Cynthia M. Liutkus*,1, James D. Wright1, Gail M. Ashley1 and Nancy E. Sikes*,2

1 Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, New Jersey 08854, USA
2 Human Origins Program, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, USA

Isotope analyses of lake-margin rhizoliths from paleo–Lake Olduvai (ca. 1.75 Ma) form the basis of a new model proposed here for interpreting stable isotope values of phreatic rhizolith carbonates. Average {delta}18O and {delta}13C values from rhizoliths formed in transgressive lacustrine (waxy) claystones are relatively low. Low {delta}18O averages (–5.5{per thousand}) reflect meteoric water values consistent with increased precipitation during wet periods that would increase the outflow of fresh water from subsurface seeps and shift a brackish groundwater zone lakeward. Low {delta}13C averages (–4.1{per thousand}) could indicate little atmospheric exchange, high plant decay, and/or increased groundwater (low {delta}13C) flow. Higher averages in {delta}18O (–3.6{per thousand}) and {delta}13C (–2.0{per thousand}) occur during dry periods and lake regressions (earthy claystones), when hydraulic head is reduced, the lake recedes, and water within the wetlands is subjected to intense evaporation and gas exchange with the atmosphere. The isotope ratios of the rhizoliths from lowermost Bed II change in response to groundwater hydrology on Milankovitch time scales, but the isotopes also provide evidence of shorter-term (decadal to centennial scale) climate fluctuations. The orbitally driven climate changes are recorded faithfully by lithologic variations and stable isotope patterns.

Key Words: Olduvai Gorge • rhizolith • oxygen isotope • carbon isotope • paleoclimate






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