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 18 O 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).