SPATIAL AND TEMPORAL VARIABILITY OF STABLE ISOTOPES IN AN UPPER BED II TUFA, OLDUVAI GORGE, TANZANIA
Previous work on rhizoliths from nearby areas indicates that carbonates precipitated from meteoric rainfall (δ18Owater = -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.