OSL CONFIRMATION OF WIGGLE-MATCHED U-SERIES-TUNED δ13C SPELEOTHEM RECORD WITH POLLEN ASSEMBLAGES OF THE LAST INTERGLACIAL TO GLACIAL TRANSITION, CENTRAL ILLINOIS, USA

Collective records of pollen and ostracodes from central Illinois (Pittsburg, Raymond, and Bald Knob basins) provide evidence of consistent environmental change spanning the end of the penultimate glaciation (MIS 6) to the onset of the last glaciation (MIS 3). The records highlight initial tundra-like conditions transitioning to wet/cool dense spruce forest during the late Illinois Episode, a tripartite interglacial (Sangamon Episode) succession, overlain by fossil assemblages that suggest an abrupt shift to continental conditions. The early Wisconsin Episode is marked by return of a lacustrine environment, and upward abundance of spruce pollen and cryophilic ostracodes.

The fossil assemblages pre-date reliable radiocarbon dating; establishing the chronology of dramatic changes summarized above has been daunting. Researchers have taken various approaches to this problem, such as invoking similarities with the European pollen record, or predicted vegetation biome response based on changing orbital parameters. The most satisfying and intriguing chronology is based on wiggle-matching of normalized data U-series-tuned δ¹³C record from Crevice Cave, Missouri, with stratigraphic detrended correspondence analysis (DCA) of the pollen record from Raymond Basin. The wiggles were initially tuned by visually matching maximum rates of change; the best matches result in R² values of 0.75. Among the intriguing results of the wiggle-matched chronology is that the abrupt change in warm/wet to non-analog continental conditions occurred between 71 – 70 ka in concert with the onset of MIS 4.

Independent vetting of the wiggle-matching results has been attempted for more than 15 years using OSL dating. The results have been inconsistent and vexing, in part because many samples of interest are near or beyond the practical limit of OSL dating with quartz. The latest ages are IRSL ages on feldspar. The results are consistent with wiggle-matched ages in the part of the record from 90 to 40 ka, vetting the important correlation at ca. 71 ka. Age reversals and other problems are observed in ages that are ostensibly late MIS 5 (> 135 ka). At this juncture, OSL (IRSL) ages from these sediments are best judged as “greater than” dates.