THE LAST GLACIAL MAXIMUM AND DEGLACIATION EVENTS BASED ON BE-10 AND AL-26 EXPOSURE AGES FROM THE MT. TRIKORA REGION, IRIAN JAYA, INDONESIA

We report paired 10Be and 26Al exposure ages from the equatorial high altitude region of Mt. Trikora, central Irian Jaya (~3500 m, latitude of 4 S). Purmooree Valley, one of many moraine dammed valleys that drain north from the mountain crest, is ~8 km long, 2 km wide, and is delimited downvalley by broad moraines with relief of >100 m. Five major moraine systems in the lower valley section were sampled. Individual mean boulder ages range from 14ka to 27ka, exhibit clear continuos exposure without complex burial patterns and, more importantly, group into two age brackets – younger proximal ages and older distal ages. Production corrections due to cosmic ray shielding, sample thickness and paleo-geomagnetic variations are included. Published scaling algorithms generate age differences of ~5%. Age errors include all analytical errors and a 7% error in production rate. Final errors are typically 8-14%. The single sample from the defined outer or oldest moraines (P4, P5) had a mean age of 27.0 ± 1.9 ka; three samples from the crest of an intermediate moriane (P3) with a mean age 18.0 ±1.1 ka and three others, strewn within the valley floor between P2 and P3, 21.5 ± 1.4 ka. Five samples relating to the inner moraines, P1 and P2, have a mean age of 15.2 ± 1.2ka – distinctly younger than those of the P3 system. Based on this, we suggest that the LGM in Irian Jaya commenced at least 21.5 ka ago reaching peak intensity at ~18 ka, concurrent with the average global LGM age. The LGM ice front probably remained close to its maximum extent (defined by P3) until at most 15ka. The inner moraine system (P1,P2) formed at 15.2±1.2ka during the last deglaciation and represents the youngest glacial feature in this region. This glacial readvance phase appears not to be consistent with the Younger Dryas cold period that is so prominent in the GISP 2 record. Interestingly, this conclusion is reminiscent of cosmogenic exposure ages from sequences of re-advance moraine features in New Zealand and Tasmania by Fink et al (2002). D. Fink et al, Int. AMS-9 Conf, abstract O11A-1, p92-93, Japan, Sept . 2002.