delta C-13-delta O-18 covariance as a paleohydrological indicator for closed-basin lakes

The relationship between delta(18)O and delta(13)C in a closed-basin lake is a function of hydrological change, vapor exchange, lake productivity, and total CO2 (or carbonate alkalinity) concentration. Covariance 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 of 5000 yr or longer. On shorter time scales, however, certain subtleties in the use of the delta(13)C-delta(18)O covariance as a hydrology history indicator must be recognized. The covariant trend may not be found in hyper-alkaline lakes because of the insensitivity of delta(13)C to lake volume changes. For less alkaline closed-basin lakes, covariance of delta(13)C-delta(18)O Will result from relatively rapid increase or decrease of lake volume, with the rapid-decrease condition giving rise to heavy and narrow-ranged isotopic values for both delta(13)C and delta(18)O due to the coupled evaporation-productivity effect. When the lake volume remains stable (as is the case for an over-flowing open 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 measured delta(13)C-delta(18)O covariance in carbonate sediments of Mono Lake, California, to deduce paleo-hydrological and lake-water alkalinity (and salinity) variations and found the results in consonance with Holocene lake-level history derived from the delta(18)O and other information. (C) 1997 Elsevier Science B.V.