Unusual carbon and oxygen isotropic ratios of ostracodal calcite from last interglacial (Sangamon episode) lacustrine sediment in Raymond Basin, Illinois, USA

The stable isotopic records of ostracode valves deposited during the last interglaciation in Raymond Basin, Illinois, have δ¹³C and δ¹⁸O values as high as +16.5‰ and +9.2‰ respectively, the highest values yet reported from continental ostracodal calcite. Located in south-central Illinois, Raymond, Pittsburgh, Bald Knob, and Hopwood Farm basins collectively have yielded important long pollen and ostracode records that date from about 130 000 years ago to the present. Although fossils from the present-day interglaciation are not well preserved, these records constitute the only described, conformable, fossiliferous successions of this age from the interior of glaciated North America. The high δ¹³C values from Raymond Basin are attributed to the residual effects of methane loss either by ebullition or by emission through the stems of senescent emergent aquatic vegetation. A mass balance model suggests that an increase in δ¹³C of dissolved inorganic carbon on the order of +15‰ is possible within a few hours given modest rates of methanogenesis of about 0.02 mol m^-2 d^-1. The δ¹³C records from other studies of ostracode valves have values approaching, but not exceeding about +14‰ suggesting a limiting value to ‰¹³C enrichment due to simultaneous inputs and outputs of dissolved inorganic carbon. Values of δ¹⁸O in ostracodal calcite are quite variable (-4 to +9‰) in sediment from the late Sangamon subepisode. A model of isotopic enrichment in a desiccating water body implies that a reduction in reservoir volume of 20% could produce this range of isotopic values. High humidity and evaporation probably account for most of the δ¹⁸O variability.