Palaeohydrology, vegetation, and climate since the late Illinois Episode (similar to 130 ka) in south-central Illinois

Our interpretation of pollen and ostracode successions from four basins in south-central Illinois provides a new synthesis of palaeovegetation, palaeo hydrology, and palaeoclimate for the period from the late Illinois Episode (about 130,000 years ago) to near the end of the Wisconsin Episode (about 2 5,000 years ago). Correlations of pollen biozones between Raymond, Pittsbur g, and Bald Knob basins are the basis for identifying the late Illinois gla cial, Sangamon interglacial, Wisconsin glacial, and Hudson interglacial epi sodes. Glacial episodes were identified primarily by the presence of Picea pollen and the ostracode Limnocythere friabilis, whereas interglacial episo des were identified by Quercus pollen and by the ostracodes Candona caudata and Heterocypris punctata. Within interglacial and glacial episodes, polle n and ostracode assemblages varied with changes in moisture balance. Local palaeohydrology was assessed primarily on the basis of environmental tolera nce indices of ostracodes and the stable isotope (C,O) stratigraphy of ostr acodal calcite. Regional moisture balance was assessed from multivariate an alyses of the pollen successions. Three climatic regimes occurred during th e Sangamon Episode. (1) One regime was characterised by precipitation excee ding evaporation that promoted basin overflow. This climate was inferred fr om the high percentages (generally >80%) of deciduous-forest pollen. Peaks in the abundance of Liquidambar and Fagus pollen indicate that winters may have been slightly warmer, and effective moisture slightly greater, than at present. (2) The second climatic regime was continental, similar to the pr esent climate of Illinois in which precipitation is equal to or just less t han evaporation. This climate is inferred from abundant Ambrosia pollen (40 to 60%) and abundant nektic (swimming) ostracode valves which suggest a sh allow lake. These conditions probably developed in association with a 'heat -low' over the interior of North America during marine oxygen isotope stage s 5e and 5c, Associated with the transition between the first two climates are fossils of the subtropical ostracode Heterocypris punctata and the gian t tortoise Geochelone crassiscutata that suggest short periods in winter wh en polar low-pressure systems did not extend into Illinois as they do today . (3) The third climatic regime occur-red during the transition from the Sa ngamon interglacial episode to the Wisconsin glacial episode. A severely co ntinental climate is indicated by the heat-tolerant ostracode Pelocypris tu berculatum, variable delta(18)O values of ostracode valves, and high enviro nmental tolerance index values for the ostracode assemblages. The weedy Che nopodiaceae and Amaranthaceae families grew on exposed mudflats, The tree p ollen associated with this type of climate included low percentages of Pice a and Liquidambar, an assemblage that has no modem analogue, We suggest tha t this transitional climatic regime was associated with the large-scale cha nges in the climate system during marine oxygen isotope stage 4. (C) 2000 E lsevier Science B.V. All rights reserved.