Abstract: Oxygen, and Carbon Isotopic Variation in Pleistocene Lacustrine Stromatolites from Lake Turkana, Kenya

Paul I. Abell, Stanley M. Awramik, Robert H. Osborne

Oxygen and carbon isotope geochemistry of Pleistocene lacustrine stromatolites from the famous hominid-bearing Koobi Fora Formation of east Lake Turkana (formerly Lake Rudolf), Kenya, is yielding consistent isotopic differences between dark and light carbonate laminae within the same stromatolite. Dark laminae are enriched in the heavier isotopes whereas light laminae consistently show a relative enrichment in the lighter isotopes (overall values of ^dgrO¹⁸ range from +3 to +4 and ^dgrC¹³ from -1 to +1). Our data suggest the possible interplay of seasonal changes in temperature, rainfall, and water depth coupled with iterative periods of algal growth and stromatolite formation.

The upper Cenozoic lacustrine sediments of east Lake Turkana record at least 4 major periods of stromatolite development: 2 within the Pleistocene Koobi Fora Formation, 1 within the disconformably overlying Goumde Formation (Pleistocene), and 1 within the Golana Boi beds (C¹⁴ dated at 9,360 ± 135 years B.P.). The largest stromatolites are found within the Golana Boi with isolated heads approaching 0.6 m when growth was associated with Etheria. Oncolites, bulbous (both linked and separate), and stratiform stromatolites are the most common morphologies; columnar and pseudocolumnar forms have not been found.

The Lake Turkana stromatolites can be used confidently (a) as near-shoreline indicators; (b) to help establish periods of low accumulation of terrigenous detritus; and (c) to help interpret an ancient lake chemistry which allowed for the formation and accumulation of photosynthetically precipitated calcium carbonate-forming stromatolites. The isotopic data, when tied to individual stromatolitic laminae and stromatolite-bearing horizons, provide possible paleoclimatic information as yet unobtained from lacustrine stromatolites.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma