DELTA-C-13-DELTA-O-18 COVARIANCE AS A PALEOHYDROLOGICAL INDICATOR FORCLOSED-BASIN LAKES

The relationship between delta(18)O and delta(13)C in a closed-basin l ake is a function of hydrological change, vapor exchange, lake product ivity, and total CO2 (or carbonate alkalinity) concentration. Covarian ce of delta(18)O and delta(13)C usually occurs in lake sediments under the condition of hydrological closure for time periods of the order o f 5000 yr or longer. On shorter time scales, however, certain subtleti es in the use of the delta(13)C-delta(18)O covariance as a hydrology h istory indicator must be recognized. The covariant trend may not be fo und in hyper-alkaline lakes because of the insensitivity of delta(13)C to lake volume changes. For less alkaline closed-basin lakes, covaria nce of delta(13)C-delta(18)O Will result from relatively rapid increas e or decrease of lake volume, with the rapid-decrease condition giving rise to heavy and narrow-ranged isotopic values for both delta(13)C a nd delta(18)O due to the coupled evaporation-productivity effect. When the lake volume remains stable (as is the case for an over-flowing op en lake), poor delta(13)C-delta(18)O covariance will ensue, due to the effect of vapor exchange with the atmosphere. We have used the measur ed delta(13)C-delta(18)O covariance in carbonate sediments of Mono Lak e, California, to deduce paleo-hydrological and lake-water alkalinity (and salinity) variations and found the results in consonance with Hol ocene lake-level history derived from the delta(18)O and other informa tion. (C) 1997 Elsevier Science B.V.