PALEOCLIMATIC IMPLICATIONS OF U-SERIES DATING OF ARAGONITE AND CALCITE STALAGMITES FROM CENTRAL NEPAL

The calcium carbonate mineralogy of speleothems has been linked to paleoclimatic conditions. Some research has suggested that aragonite is the predominant mineral in cave formations during arid periods when dripwater Mg/Ca ratios are elevated in response to increased rock/water interactions during infiltration and/or evaporation in the cave. Calcite forms in response to lower Mg/Ca ratios during wetter periods. Stalagmites from five dolomitic caves scattered across the Pokhara Valley, central Nepal, were dated using U-Th disequilibrium mass spectrometry techniques in order to assess the synchroneity of changes carbonate mineralogy between these samples. No stalagmite analyzed in this study grew prior to 6000 yr BP and most formed during the late Holocene. Aragonite stalagmites were well-dated, containing both high U and low inherited Th abundances. Dating calcite stalagmites, however, was complicated by very low U concentrations (<100 ppb) that yielded measured 230Th/232Th ratios indistinguishable from crustal averages (5e-6). The 230Th/232Th vs 234U/232Th isochron technique failed to constrain a unique initial 230Th/232Th value that would decrease the uncertainty of TIMS dates. The temporal control on calcite stalagmites, therefore, remains poor, and precise correlations of calcite intervals between speleothems from different caves were not possible.