A COMPARISON OF N-ALKANE AND N-ACID ISOTOPIC COMPOSITION IN PLANTS, SOILS, AND LAKE SEDIMENT FROM THE BIG HORN MOUNTAINS IN WYOMING

Lake sediments record changes in the surrounding ecology and environment through time and can provide insight to past climatic conditions and ecosystem response. The isotopic compositions of biomarkers isolated from lake sediments discern changes in precipitation, vegetation, and hydrology. The most commonly analyzed compounds are n-alkanes and n-acids, which are derived from terrestrial and aquatic vegetation and are highly resistant to degradation. Here, we investigate the spatial resolution of biomarkers integrated into lake sediments in the Big Horn Mountains of Wyoming. Sediment cores were taken from three ponds in the Big Horn Mountains, which were chosen for their limited watershed size and surrounding vegetation cover (conifer versus sagebrush/conifer mix). In addition, pine needles and soil litter were sampled from 8 directions around each pond. N-alkanes and n-acids were isolated from total lipid extracts and analyzed for isotopic composition on a continuous flow isotope ratio mass spectrometer.

Environmental stressors and hydrologic setting can affect the deuterium and carbon isotope composition of alkanes and alkanoic acids within vegetation, litter, and sediment. However, the relationships between the source (plants), intermediary (litter), and sink (lake sediment) are not well characterized. In plants, metabolic processes determine ¹³C and ²H fractionation, which varies with needle age, canopy position, tree age, season, and year. Soil litter integrates the vegetation signal across time and space and, therefore, deuterium and carbon isotopic variation decreases compared to the variation within needle samples at each site. Finally, we compared the deuterium and carbon isotope values of the pine needles and soil litter to surface samples from the sediment cores. Although these ponds are small and their watersheds were well sampled, organic particles transported from adjacent regions or autochthonous aquatic vegetation contributions may mask the terrestrial signal. By sampling the terrestrial vegetation and soil litter, we link the sedimentary isotopic signals to the variability within the watershed and evaluate the representativeness of the sediment samples. This study demonstrates the resolution of biomarker isotope composition on local and regional scales in lake sediments.