T03_P16

Inorganic radiocarbon dating of a moraine profile – a case study in a LGM moraine in Tianshan Mountains

Yi C^1,4, Liu X², Liu K³, Yi R^1,3

¹Beijing Niandai(dating) Company, Beijing, China, ²Institute of Heavey Ion Physics, Peking University, Beijing, China, ³Zhijiudiance (Nuclide-electron dating) company at Suzhou, Suzhou, China, ⁴Institute of Tibetan Plateau Research, Chine Academy of Sciences, Beijing, China

Organic matter in glacial moraines in humid areas is used to date the burial ages of glaciations using radiocarbon. However, biomass is rare in glacial sediments in arid and semi-arid areas. Our results show that a moraine in the source area of the Urumqi River valley, Tianshan Mountains was dated to 20790+-140 yr cal using carbonate in fine, very similar to ¹⁰Be exposure dating age. The ages of the upper part of the profile were younger than the middle part and then younger than the lower part. The ages in coarse size fraction (250-125 μm) were younger than medium size fraction (125-63 μm) and then younger than fine size fraction (< 63μm). The carbonate deposits on a roche moutonnee in front of Glacier 1 and in the matrix of a modern till were dated to modern carbon. We suggest that inorganic carbon decomposed from calcium minerals dissolving in subglacial meltwater was exchanged sufficiently with atmospheric carbon. When a glacier melted, pressure in sediment decreased and carbonate precipitated on the surface of the particles. Vertical movement of ground water dominates in arid and semi-arid areas. The carbon in the upper part of the sediment was most likely exchanged with atmospheric carbon after post deposition. The fraction with large grain size has large voids which were easily let water pass through for carbon exchange and ¹⁴C age become younger. At a depth, exchange of carbon in fine matrix with atmosphere was weak and ¹⁴C can be used for dating glacial moraine.