EXTENSIVE TIME-AVERAGING IN LACUSTRINE GASTROPOD ASSEMBLAGES FROM SHADOW LAKE, WAUPACA, WISCONSIN

In order to make accurate paleoenvironmental and paleoecological interpretations based on preserved fossil and subfossil assemblages, it is important to understand the degree of time-averaging that occurs within a geologic stratum. Fossils within the same sedimentary layer are generally assumed to have co-occurred when living, however, several recent studies have shown that there can be a large amount of time-averaging within a single stratum, especially in marine environments. Less emphasis has been put on quantifying the time-averaging in continental assemblages, especially in lacustrine settings. In many cases, lake sediments show distinct sedimentary layers, indicating little bioturbation or sediment reworking, and, therefore, presumably little time-averaging of preserved fossils, however, this assumption should be tested by dating individual fossil specimens to quantify time-averaging. We used amino acid racemization calibrated with 10 radiocarbon dates to date a total of 70 shells of the freshwater gastropod Promenetus umbilicatellus; 10 from surface samples and the remaining 60 from regular increments down to 95 cm sediment depth from a single core collected in Shadow Lake, Waupaca, Wisconsin. We used the rapid AMS technique, which uses carbonate targets rather than graphite to maximize the number of shells dated with our available project budget; radiocarbon dates were then used to calibrate aspartic acid LD for the remaining shells. Dates indicate that shells in the upper 20 cm of the core are time averaged to ~1000 years (inter-quartile age range) and inter-quartile age range of shells located between 40-100 cm is ~300 years in individual 10 cm-increments, despite no apparent mixing from visual observation of the core. This compares to a sedimentation rate of 2mm/year calculated for the upper 20 cm of a companion core based on cesium-137. These results indicate that not all lake sediments may provide the high-resolution records that are normally associated with lacustrine environments. Paleoecological and paleoenvironmental work on lacustrine systems could become an important component of conservation paleoecology, as paleontologists seek to make meaningful contributions to conservation questions. Therefore, we encourage further study on the time-averaging of lacustrine fossil and death assemblages to aid paleoecological interpretation.