Decoupled lake history and regional moisture availability in the middle elevations of tropical Mexico

During the Holocene, most of the climatic variability in the tropics has manifested through changes in the precipitation regime. Although there is relatively a large amount of information on this climatic variability from the lowlands during this time period, little is known regarding the environmental variability in the middle elevations. Here we present evidence of environmental change from a lacustrine sedimentary record recovered from Lake Lacandón, a small shallow water body located in the middle elevations of Chiapas, Mexico. Through analyses of cladoceran assemblages and chemical properties of the sediments (concentrations of C, Ca, N, Rb, Sr, Ti, and Zr), we aimed to reconstruct local and regional environmental conditions during the last 4,000 years. Our results show a highly dynamic cladoceran community apparently associated with changes in the size of the lake, which although variable has been enlarging through the last 4,000 years. Geochemical evidence also suggests high variability in moisture availability overimposed on a general trend towards drier conditions. Whereas the growing lake size seems associated with decreasing insolation seasonality, the trend to drier regional climates corresponds to the well-known southward displacement of the Intertropical Convergence Zone through the Holocene. Although the regional climate has been deteriorating, the enlarging lake suggests a decreasing water deficit though time. This pattern is probably a result of increasing insolation during the dry season, causing advection of marine moisture that precipitates because of the steep elevation gradient. Our results highlight the intricate nature of the millennial scale environmental variability, and the potential role of the middle elevations for sheltering biodiversity through times of environmental hardship.