NEOTROPICAL NON-MARINE OSTRACODES AS INDICATORS OF PAST LAKE LEVEL AND WATER CHEMICAL COMPOSITION

Non-marine ostracodes are common microcrustaceans that inhabit a variety of aquatic ecosystems. They possess carbonate shells and various taxa have been shown to display different optima and tolerances with respect to lake water level, conductivity, temperature, pH, dissolved oxygen, and associated biological variables such as vegetation cover. We analyzed water and surface sediment samples from 63 aquatic ecosystems on the Yucatán Peninsula (Mexico), in surrounding areas of Guatemala and Belize, and from 29 lakes in central Mexico, to explore relations between ostracode assemblages and environmental variables. Results indicate that neotropical ostracodes are excellent indicators of lake water level and conductivity. Additionally, their calcite shells preserve a record of lakewater δ¹⁸O, thereby providing insight into the ratio between evaporation and precipitation, and temperature, at the time they lived. This makes ostracodes in dated sediment cores powerful proxies for past climate change. Long sediment cores were recovered in 2006 from Lago Petén Itzá (Guatemala), the deepest lake (~165 m) in the northern lowland Neotropics. A ~70-m-long core was retrieved from site PI-6, in 71 m of water. Late Glacial climate conditions were inferred using core lithology, ostracode shell δ¹⁸O values, and water-depth and conductivity transfer functions applied to fossil ostracode species assemblages. Lithologic data showed the Late Glacial Maximum (LGM) was wet, and combined lithologic and oxygen isotope data indicated alternating dry/wet conditions during the deglaciation. Data on ostracode autecology, as well as δ¹⁸O measurements on modern shells of multiple ostracode species (benthic/nektobenthic, males/females, adults/juveniles), provide new information for interpretation of the lithologic and oxygen isotope record from core PI-6. A new ostracode training set from central Mexico (n=29) and planned drilling of Lago Chalco will yield information on the late Pleistocene climate in the Basin of Mexico, and will provide insights into the moisture source for the Petén lowlands during the LGM.