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

Cynthia M. Liutkus, James D. Wright, Gail M. Ashley and Nancy E. Sikes
Paleoenvironmental interpretation of lake-margin deposits using delta (super 13) C and delta (super 18) O results from early Pleistocene carbonate rhizoliths, Olduvai Gorge, Tanzania
Geology (Boulder) (May 2005) 33 (5): 377-380

Abstract

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


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 33
Serial Issue: 5
Title: Paleoenvironmental interpretation of lake-margin deposits using delta (super 13) C and delta (super 18) O results from early Pleistocene carbonate rhizoliths, Olduvai Gorge, Tanzania
Affiliation: Rutgers University, Department of Geological Sciences, Piscataway, NJ, United States
Pages: 377-380
Published: 200505
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 26
Accession Number: 2005-034352
Categories: Quaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2005069
Illustration Description: illus. incl. sketch map
S03°04'60" - S02°55'00", E35°19'60" - E35°25'00"
Secondary Affiliation: National Museum of Natural History, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200519
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