CARBON AND NITROGEN ISOTOPE EXCURSIONS IN PLEISTOCENE/HOLOCENE GREAT SALT LAKE SAPROPELS

During the Late Pleistocene and early Holocene, hydrologic changes during glacial-interglacial cycles resulted in relatively short duration high lake levels in the Great Basin. Shoreline deposits hundreds of meters above the modern Great Salt Lake attest to its deep-water phases. The most recent were the Little Valley Lake Cycle (ca. 150 ka) and Lake Bonneville (30-12 ka). Theses freshwater lakes were in stark contrast with modern Great Salt Lake (salinity = 50-270), which has a maximum depth of 10 m. Throughout the Pleistocene and Holocene, lake levels were typically similar to modern and hypersaline conditions persisted.

In Great Salt Lake sediments, the deep water intervals are recorded as C and N isotope excursions. Perturbation of biogeochemical cycles resulted in an average -6‰ shift in d¹³C_org and -5.5‰ shift in d¹⁵N_tot in Lake Bonneville. During the Little Valley Lake Cycle, there were similar -6.5‰ excursions for both d¹³C_org and d¹⁵N_tot. The lower d¹³C_org values near -27‰ and d¹⁵N_tot values near 4‰ are similar to values in modern large lakes; thus the excursions actually mark a shift toward “normal” conditions characterized by diverse algal inputs. The Lake Bonneville and Little Valley Lake sediment intervals are overlain by the Upper and Lower Salt and Sapropel units, respectively, both consisting of dark laminated sediments and evaporite layers of variable thickness (cm to m-thick). We did not detect pigments indicative of anoxygenic photosynthesis in these units, suggesting that these are not true sapropels and that high salinity rather than anoxia inhibited burrowing organisms. During the prevailing high-salinity lowstand intervals, Dunaliella and cyanobacteria were likely dominant in the water column, resulting in sedimentary d¹³C_org values near -20‰ and d¹⁵N_tot values from 10-18‰. The d¹⁵N_tot values are typical of closed basin lakes, where nutrients accumulate and d¹⁵N_tot values are elevated.