This research provides the first field-based and numerically-constrained investigation of the extent and chronology of glaciation in the Anadyr Region of western Beringia. Fieldwork during the summers of 1995 and 1996 in the Pekulney and Koryak Mountains indicates a maximum ice advance prior to late Stage 5, and likely within the middle Pleistocene, and a minimum advance during Stage 2 (Fig.1). Previous work by Russian geologists included extensive air photo interpretation of moraines, terraces and other ice marginal landforms; however, a numerical age control on the glacial history in this region of Chukotka has never been established. Glushkova (1995) suggested that the maximum extent of glaciation across Chukotka occurred during the early Late Pleistocene (Zyryan) and that this glacial advance was three times more extensive than the Late Pleistocene (Sartan) glaciation. Although Glushkova (1995) concludes that snowline was 150-300 m higher during the Sartan than the Zyryan glaciation, she states that "there is still no prevailing idea on the number of glacial epochs, character and scale of glaciation, glacial geography and age boundaries."
Cosmogenic isotope analyses (36Cl) as well as radiocarbon age estimates provide the numerical age control of the glacial episodes. Supplemental to the numerical ages are relative age studies including moraine morphometric measurements on terminal and lateral moraines, soil development studies and amino acid analyses of fossiliferous diamicts near the coast. Samples were collected from moraine crests and glacially-sculptured bedrock surfaces. Due to sample size and lithology, 36Cl was the only isotope measured in the first round of submitted samples and those dates provide only minimum age estimates. Erosion estimates were made using field observations, such as relief of surface, as well as published values for arctic areas. Age estimates calculated from 36Cl/37Cl ratios are shown in Table 1. Two recently-developed software programs (CHLOE and RICH) were used to integrate the factors affecting isotopic production and ultimately calculate minimum ages. The average value obtained from the CHLOE program for a "best estimate" erosion rate for the two oldest dates on glacially-sculptured bedrock is 88 ka (Table 1). If we assume this mean age to be a minimum age, then it requires glaciation of the site in the Tanyurer River valley during at least d18O Stage 6 or earlier. Two younger dates of 58.1 ka and 68.8 ka (using the CHLOE best estimate values) 33 km farther north and upvalley (Fig. 2) may record deglaciation of a short late Stage 5 or Stage 4 advance. In any case, the glaciations are likely much older than the minimum ages. Cosmogenic minimum age estimates of 21.3 ka and 11.3 ka suggest deglaciation in the Pekulney Mountains of a much less extensive Stage 2 advance (Sartan Glaciation).
Slope angles on terminal moraines in the Pekulney Mountains and the Tanyurer River valley range cluster in two groups. Average distal slope angles for moraines in the Pekulney Mountains are 15.5°, while terminal moraines downvalley of the Tanyurer River have shallow slopes between 1° - 5° (Mostoller et al. 1996). Mean alle/Ile ratios in Astarte of 0.032 (n=11) collected from diamicton at Cape Dionysia (Fig. 2) can be compared with similar Last Interglacial ratios from outer Chukotka (Brigham-Grette et al. in press). These supplementary data further support the idea that the Lower Tanyurer and Anadyr River Lowlands have been ice-free since the Last Interglaciation and that the maximum ice model for the Late Sartan (Wisconsinan) in this region as proposed by Grosswald (1988) is unfounded.
The interpretations presented here are based largely on 10 samples analyzed for one cosmogenic isotope with estimated erosion rates. In order to test the hypothesis of extensive mid-Pleistocene ice followed by retreat and possible short readvances to the Late Sartan limit, additional samples from this region and the northern Koryak Mountains will be analyzed for 10Be and 26Al. Implications for this research will ultimately help us to understand how the role of winter sea ice conditions in the Bering Sea and Bering Strait, the southward migration of available moisture, and the warm Last Interglacial conditions in Beringia controlled the extent of regional alpine glaciation.
