Constraints on Pleistocene pluvial climates through stable-isotope analysis of fossil-spring tufas and associated gastropods, Kharga Oasis, Egypt

摘要

Stable-isotope analyses of fossil-spring carbonates (tufas) and freshwater gastropods (Melanoides tuberculata) from the currently hyperarid Western Desert of Egypt indicate that this region received enough precipitation to support a small perennial lake during the height of the oxygen-isotope stage 6/5e pluvial event, and a substantial volume of spring discharge during prior pluvial phases. Tufa and gastropod oxygen-isotope ratios are generally low relative to modern precipitation in the region, confirming that the majority of Pleistocene pluvial precipitation came from an Atlantic source, as determined by previous workers. Mean tufa δ~(13)C (-2‰ PDB) suggests a significant proportion of C4 vegetation in the recharge area of the tufa-depositing springs, while the paucity of tufas with 13C greater than 0‰ suggests that equilibrium between spring water and atmospheric CO_2 represented an endpoint for the evolution of spring water carbon. Though spring carbonate deposition requires a substantial increase in humidity in central Egypt relative to modern conditions, high variation in tufa δ~(18)O (up to 4‰ within individual stratigraphic units), as well as positive covariance between tufa carbon- and oxygen-isotope ratios, reflect significant effects of evaporation on spring water during tufa formation. Similarity in δ~(18)O and δ~(13)C values of tufas from different stratigraphic units indicates that climatic conditions did not vary significantly among pluvial phases recorded by spring-carbonate deposition. The lack of substantial carbonate formation at tufa localities during the Holocene pluvial phase, however, suggests that tufa deposition represents substantially wetter conditions than have existed during Holocene time.