Isotopic composition of individual hydrobiidae gastropods from neotropical lakes Esmeralda and Chichancanab in the Maya Cochuah region, Mexico: implications for palaeolimnological research.

Most lake-carbonate-isotope records from the Maya Lowlands are based on picking fossil carbonate ostracods and gastropods, often using the species Pyrgophorus coronatus, and then combining individual specimens from a single core layer to provide a composite sample. What has been left relatively unexplored is the variability between individual shells and the impact this might have on the values obtained from such composite samples. Here, the stable carbon and oxygen isotope signatures of modern P. coronatus, a littoral and detritivore species, and other Hydrobiidae taxa (Aroapyrgus sp. and Tryonia sp.) found in Lake Esmeralda and its much larger sister lake Chichancanab, in the Northern Maya Lowlands (also called Mayab or Yucatan Peninsula), were analysed to explore some of the environmental variables that might affect isotope composition. In addition, the carbon and oxygen isotope values derived from individual downcore gastropod shells were compared with those from bulk fine-fraction carbonate sediment from a core from Lake Esmeralda covering the last c. 6600 years. The results show that the median oxygen isotope value of a set of shells collected for a single stratigraphic layer tends to be about the same as the bulk sediment, even though their individual values can present a bimodal distribution, which might represent changes between dry and rainy seasons across the sample’s period. In addition, no significant difference in the oxygen isotope values was found in using a particular Hydrobiidae taxon over another, indicating a lack of significant vital effects. The carbon isotopic signature of modern shells varies with location within the lake, and shows relatively little variability down core, suggesting non-climatic factors may be more dominant in controlling shell carbonate δ13C values in these systems.