A 500-year record of paleoclimate and paleoenvironment from the Lacandon Forest, southern Mexico

Tropical karst Lakes Metzabok (550 masl, surface area = ~77 ha, z_max = 25 m) and Nahá (832 masl, surface area = ~57 ha, z_max = 36 m) are located in the Lacandon Forest, in the state of Chiapas, southern Mexico. The region is characterized by high aquatic and terrestrial biodiversity. We generated high-resolution paleoclimate and paleoenvironmental records that span the last ~500 years, using invertebrate remains (ostracodes and gastropods) and geochemical variables (elemental and mineralogical contents) in sediment cores from the two water bodies. We collected a short sediment core from each lake (Metzabok = 46 cm, Nahá = 60 cm) and analyses were carried out at 1-cm intervals. Uppermost sediments in the cores were dated using ²¹⁰Pb and ¹³⁷Cs, and deeper deposits were dated by ¹⁴C. Ostracodes and snails were identified to species level and their abundances were quantified. Concentrations of Ti and Fe were determined by portable XRF. The geochemical record reveals information about past climate variability, human-mediated erosion and transport of terrigenous elements. Changes in ostracode and gastropod assemblages mainly reflect past lake level fluctuations and changes in water conductivity. The dominant ostracode species was Cytheridella ilosvayi and the most common gastropod is Aroapyrgus sp., indicative of water depths ≤40 m and low water conductivity, respectively. Analysis of the ecological distances between samples suggests that Lake Metzabok is unstable, with frequent ecological changes equal to or greater than 50% of the community. These changes may have resulted from dramatic environmental differences associated with hydrological dynamics during dry and rainy seasons. The Nahá record presents two environmental conditions, i.e. periods of high stability and periods of change, when the system was in transition from a dry period to a wetter one, or vice versa. Despite differences between the two lakes with respect to elevation, size, depth, and seasonal dynamics, both records contain evidence of droughts ca. 300 and 200 yr BP, during the Little Ice Age. Whereas both records show a long-term tendency towards higher moisture conditions, the high-resolution of our study enabled us to detect fluctuations between dry and wet periods over the last 500 years that previous studies failed to recognize.