Changes in oxygen 18 as a measure of long-term fluctuations in tropical lake levels and molluscan populations1

In Laguna Chichancanab, the largest closed basin in northern Yucatan, large-scale changes in lake levels have been documented by “0 analyses of snail shell carbonates. A continuous 9-m series of lake sediments has been deposited during the last 8,000 years, whereas a discontinuous sedimentary record extends from 9 m to beyond 12 m in depth. At least one marked hiatus occurred in this older record during which time the lake is thought to have been seasonally dry (or very much reduced in size). The fluctuations are verified by other stratigraphic evidence including absolute numbers of shells, loss on ignition, carbon:nitrogen ratios, and major cations (calcium, magnesium, and sodium). We here consider a new approach to determine lake level stability in tropical, closed lake basins where mean annual temperatures have remained relatively stable in comparison with changes in rates of inflow and evaporation. The general goal is to document long term changes in the lake’s water balance and to relate this information to population dynamics of aquatic organisms, particularly benthic molluscs living in the littoral zone. We use “closed lake” to mean a lake which receives inflow from precipitation and its drainage basin but loses water only by evaporation. Many previous studies on lake level stability ( see Richardson 1969 for a review ) have emphasized the need to correlate several independent lines of evidence. This isotopic analysis is another direct approach which can yield unique information unavailable from other types of analyses of closed, tropical lakes. Naturally occurring isotopes of 180 and 14C can be considered tagged atoms and their movements monitored for the long term study of water balance. These iso’ Part of this research was supported by National Science Foundation Grants GB 3811, 6598, 5653, and 12895 to E. S. Dcevey and G. E. IIutchinson. The 14C and I80 work was supported by Atomic Energy Commission Contract AT (30-l ) 3204 and by National Science Poundation Grant GA-36762 to M. Stuivcr. topes have been extensively used as tracers in marine hydrologic studies during the last 25 years, but analyses of IsO in lacustrine environments are just beginning. In a closed lake basin at a latitude with relatively equable temperatures and uniform atmospheric circulation, the main variables controlling ‘so content will be the relative rates of evaporation and inflow. Vapor from evaporating water contains a proportion of 180 to I60 of about 1:500. Because the abundant ‘“0 is lighter in mass, it evaporates faster than the rarer isotope IsO. As evaporation continues, the remaining water is enriched with 180, Thus, if the sediment ages can be determined by 14C dating, variations in the 180 : I60 ratio of their carbonates can be used to determine variations with time in evaporation and inflow. Craig et al. ( 1963) have documented the relative effects of precipitation and evaporation and the significance of relative humidity, and stressed the importance of molecular exchanges with atmospheric water vapor in determining the ultimate lsO content of water bodies. Measurements of seasonal IsO changes in lake waters (Dinqer 1968; Fame et al. 1970; Gonfiantini et al. 1973; Stuivcr 1970) are in agreement with predictions from these experimental studies, and Dirqer ( 1968) concluded that overall composition is determined by the LIMNOLOGY AND OCEANOGRAPHY 682 JULY 1974, V. 19(4) Oxygen 18 in a tropical lake 683 ratio of inflow : evaporation. Using these relationships, Stuiver ( 1968, 1970) has demonstrated thermal effects in studies of modern and ancient lakes in the temperate zone. This study, extending this approach to a closed, tropical basin in the Yucatan Peninsula of Mexico, was part of a Ph.D. thesis by A.C. We thank E. S. Deevey and G. E:. Hutchinson for their encouragement and support. A.C. received support from the Cave Research Foundation, Explorer’s Club of New York, National Defense Education Act, and National Science Foundation. We especially thank A. Barrcra, P. E. Cloud, I-1. Craig, R. Hedlund, H. Konrad, C. J. McCoy, D. C. Rhoades, J. L. Richardson, and B. A. Voorhies.