Stable isotope values (δ18O & δ13C) of multiple ostracode species in a large Neotropical lake as indicators of past changes in hydrology

Modern lake hydrodynamics, ostracode species autecology, stable isotopes (delta O-18 and delta C-13) of multiple ostracode species, ostracode taphonomy and sediment geochemistry were studied to improve interpretation of the late Pleistocene-early Holocene (similar to 24-10 ka) stable isotope record of ostracodes in sediment core PI-6 from Lago Peten Itza, northern Guatemala. Oxygen and carbon stable isotopes in modern and fossil species assemblages of Lago Peten Itza were used as indicators of changes in the balance between evaporation and precipitation, past lake level and carbon source. Ostracode taphonomy was used to detect past periods of strong currents, high-energy environments, and possible partial or full mixing of the lake. The modern lake water isotopic composition displays clear seasonal differences that are independent of lake level fluctuations. Modern benthic species displayed lower delta O-18 and delta C-13 values than nektobenthic species, with differences of 3.0 parts per thousand and 5.3 parts per thousand, respectively. Valves of nektobenthic species display higher values of delta C-13 because these ostracodes live in shallower environments among abundant algae and aquatic plants, where productivity is high. The benthic species Limnocythere opesta Brehm, 1939 displayed the smallest average offset from delta O-18 water (+0.3 parts per thousand) and the largest offset from delta C-13(DIC) values (-4.1 parts per thousand) among studied ostracode species. Nektobenthic species Heterocypris punctata Keyser, 1975 displayed the smallest difference relative to the delta C-13(DIC) values (-0.1 parts per thousand). Late Pleistocene-early Holocene climate conditions and water levels in Lago Peten Itza can be summarized as follows: 1) high lake levels and cold conditions (Last Glacial Maximum [LGM], similar to 24-19 ka), 2) fluctuating lake levels and cold conditions (Heinrich Stadial 1 [HS1], similar to 19-15 ka), 3) high lake levels and warm and wetter conditions (Bolling-Allerod [BA], similar to 15-13 ka), 4) low lake levels and dry conditions (Younger Dryas [YD], similar to 13-11.5 ka) and 5) high lake levels and warm and wetter conditions (early Holocene, similar to 11.5-10.0 ka). Average lake level fluctuation in Lago Peten Itza during the late Pleistocene-early Holocene was as much as similar to 25 m. Ostracode analyses suggest that the LGM was characterized by relatively low delta O-18 (+4.7 to +6.0 parts per thousand), and delta C-13 values (-7.1 to -6.4 parts per thousand) in ostracode valves, high inferred water depths and high percentages of broken adult and juvenile valves (>66%), suggesting a high-energy environment, strong currents, partial to full mixing, downslope transport, colder water temperatures and wetter conditions. An increase in the relative abundance of the benthic species L. opesta and higher numbers of broken valves suggest heavy precipitation events during the LGM (similar to 23.7, 21.7, 20.8 and 20.1 ka). HS1 was predominantly dry, but we identified times when lake levels were slightly higher, at the onset of the deglacial and a brief period (similar to 17-16 ka) between HS1b and HS1a. All studied climate proxies indicate wetter and warmer conditions and lake system stability during the BA. Lake levels dropped during the YD and gradually increased during the Preboreal and early Holocene. We demonstrate that modern and fossil ostracode isotopic signatures, species assemblages and taphonomy can be used together with physical and geochemical variables in Lago Peten Itza sediments to make high-resolution inferences about late Pleistocene-early Holocene environmental changes in the lowland Neotropics. (C) 2012 Elsevier Ltd. All rights reserved.