ESR-BASED ESTIMATES OF THE ABSOLUTE AGE OF THE PEORIA LOESS: PROBLEMS AND PROGRESS

Absolute ages based on the presence of trapped charge defects in loess can provide dates with a resolution not possible using the irregularly distributed 14C. We analyzed the ESR signal from the Peoria Loess in the northern Mississippi Embayment as well as from pure silt-sized quartz, K-feldspar, and Na-feldspar. We conclude: 1) neither K-feldspar nor Na-feldspar showed ESR peaks for any grain size at any irradiation level, thus the ESR signal of loess is entirely due to quartz, assuming removal of clay and carbonate minerals before analysis, 2) the room temperature ESR peak of loess is the E' peak of quartz, 3) an irreducible zero ESR peak is present in loess, 4) optical annealing does not remove the zero ESR peak, 5) the Peoria Loess (and sub-Peoria loess) received a dose of 0.0165 Gy/a (1.657 rads/a), 6) up to160 krads of radiation did not change the ESR peak of quartz but the peak size increased systematically in 8 and 16 mm quartz with 2 to 128 Mrads doses, 7) the optimum grain size for ESR investigations of loess is 8mm to 16mm, and 8) peak area is the best method for estimating ESR peak size. The E' ESR peak of quartz is not useable for absolute age dates of loess, in future work we must isolate the optically sensitive Ge peak.