AGE OF PRE-LATE-WISCONSIN GLACIAL-ESTUARINE SEDIMENTATION, BRISTOL-BAY, ALASKA

Pleistocene glacial-estuarine sediment deposited in an intertidal envi ronment of northeastern Bristol Bay, southwestern Alaska, was dated us ing a variety of approaches, including infrared stimulated and thermol uminescence (IRSL and TL) techniques. Analysis of modern and C-14-date d Holocene tide-pat mud demonstrates that the bulk of sediment in this environment is reset by solar radiation, thereby lending confidence t o ages obtained from similar Pleistocene deposits by luminescence tech niques. IRSL seems to be especially well suited for dating, with resol ution on time scales of < 10,000 yr. The ages of tide-flat mud of the Nushagak Formation, derived from the Ahklun Mountains to the northwest of Bristol Bay, and of Halfmoon Bay drift, derived from the Alaska Pe ninsula to the southeast, suggest contemporaneous glacial-estuarine de position related to independent glacial source areas about 75,000-80,0 00 yr ago. This age is consistent with other geochronological data tha t indicate a pre-late-Wisconsin and post-substage-5e age, including no nfinite C-14 ages, a lack of interglacial indicators, and Old Crow tep hra (similar to 140,000 yr) atop the drift, normal paleomagnetic incli nations, and amino acid (isoleucine) epimerization ratios (aIle/Ile). AIle/Ile ratios in Portlandia arctica (0.052 +/- 0.003) from a marine- lag horizon at South Naknek beach, which separates Halfmoon Bay drift above from older glacial-estuarine drift below, are only slightly high er than in Mya truncata (0.041 +/- 0.007) from last-interglacial Peluk ian deposits at Nome. As laboratory heating experiments show that the two genera epimerize at similar rates, these data imply correlation of the marine lag at South Naknek beach with Pelukian deposits. Hence, g laciers on the Alaska Peninsula experienced major pre-late-Wisconsin a dvances both before and after the last interglaciation. Shells reworke d into Halfmoon Bay drift yield aIle/Ile ratios of 0.028 +/- 0.005 for Portlandia at Second Point and 0.027 +/- 0.001 for Hiatella arctica a t Etolin Point. Together with assumptions about the postdepositional t emperature history, these ratios indicate that the shells are at least 55,000 yr, and probably closer to similar to 90,000 yr, although the uncertainty in this age estimate is broad. The amino acid and luminesc ence data converge on an age between about 75,000, and 90,000 yr, late during oxygen-isotope stage 5, for a major ice advance far beyond lat e-Wisconsin limits. (C) 1996 University of Washington.