SPATIAL AND TEMPORAL VARIABILITY OF STABLE ISOTOPES IN AN UPPER BED II TUFA, OLDUVAI GORGE, TANZANIA

Olduvai Gorge, Tanzania is a part of the East Africa Rift System and has preserved 2 million years of paleontology and archaeology. An extensive tufa deposit (>1.5 km wide and 1 meter thick) was uncovered during paleoanthropological excavations in Upper Bed II. Tufas are interpreted as freshwater carbonate deposits associated with springs, wetlands and/or lakes; they are rare in arid regions. The wetland was located on the margin of a small saline-alkaline lake at the distal end of a fluvial plain (extending from Mt. Lemagurut). The topographic high created a rain shadow for Olduvai basin and trapped precipitation on the summit. The rainfall then entered the groundwater that flowed into the basin and supplied the wetland. At least one spring was identified, but seeps appear to be the main supply to the wetland. Based on carbonate sedimentary structures, the spring head may have been artesian. The δ¹⁸O and δ¹³C signatures of the tufa record the processes of formation in this paleowetland, as well as any spatial and temporal variations in the hydrology.

Previous work on rhizoliths from nearby areas indicates that carbonates precipitated from meteoric rainfall (δ¹⁸O_water = -4‰) record values of ~ -6 ‰. d¹⁸O values from BK are significantly higher, ranging from -4 to -1‰ with a mean of -2.5‰. δ¹³C values are also much higher than the regional input values of -6‰, recording values from -2.6 to -0.1‰,. There is little covariance between δ¹⁸O and δ¹³C values, which indicates either the BK region was in the highly evaporative phase with high total CO₂ which buffered the d¹³C values or the BK wetland was in an open system. Variation of the δ¹⁸O of tufa samples with distance shows higher δ¹⁸O (-1 to 0) values near the spring head and lower values (-3 to -4) in the main wetland. This implies that at the spring head, the flow rate is greater and evaporation occurred more quickly than in the main body of the wetland, where water likely entered slowly through seeps. Analysis of samples through vertical succession (temporal variability) display a change of δ¹⁸O values from -3.2 to -2.6 indicating increase of evaporative conditions through time. Alternatively, the spring feeding the BK region had different initial d¹⁸O and d¹³C values. The overall spatial and temporal variations of the oxygen isotopes track changes in the hydrologic budget of the paleowetland.