IMPACTS OF SEASONAL ANOXIA ON MICROBIOTA (OSTRACODES) AND SEDIMENT CHEMISTRY AT CRYSTAL LAKE, ILLINOIS, DURING THE ANTHROPOCENE AND THE LAST GLACIAL-INTERGLACIAL TRANSITION

Seasonal anoxia currently controls profundal continental ostracode distribution in Crystal Lake (McHenry County), Illinois, favoring the species Cypria ophtalmica and Physocypria globula at water depths from 6 to 13 m. These species also thrived in the lake's profundal zone from 14,165 to 9,600 cal yr BP during the late Bølling, Allerød, and Younger Dryas chronozones, and early Holocene. Deposition of marl under relatively well-oxygenated conditions occurred during the remainder of the Holocene until the arrival of Europeans in about 1835 AD whose land-use practices indirectly reverted the lake back to seasonal anoxia. Anoxic conditions are also indicated in the lake sediment by the speciation of phosphorus, relatively high concentrations of organic carbon, as well as iron and manganese occluded to mineral grains. Initial anoxia was probably caused by the delivery of dissolved P and Fe in shallow groundwater whose chemistry was influenced by pedogenesis under a spruce forest. High delivery rates of these elements during climatic transitions have been seen in other Midwestern lakes, caused by the leakage of these elements from immature soils.The triggering may have been regionally warm conditions associated with the growth of the Two Creeks forest bed from about 14,000 to 13,000 cal yr BP. The other pertinent feature of the ostracode record from the last glacial-interglacial transition is evidence for low lake levels or deep wind-driven currents that are chronologically in step with peaks in Ca²⁺ from the NGRIP core.