Paleoenvironmental interpretation of lake-margin deposits using {delta}13C and {delta}18O results from early Pleistocene carbonate rhizoliths, Olduvai Gorge, Tanzania

摘要

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 18O and 13C values from rhizoliths formed in transgressive lacustrine (waxy) claystones are relatively low. Low 18O averages (–5.5) 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 13C averages (–4.1) could indicate little atmospheric exchange, high plant decay, and/or increased groundwater (low 13C) flow. Higher averages in 18O (–3.6) and 13C (–2.0) 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.