(U-TH)/HE GEOCHRONOLOGY OF SPELEOTHEMS: COLD-CLIMATE FULL RETENTIVITY ACROSS GEOLOGIC TIMESCALES (104-107 YR)

Speleothems provide one of the best terrestrial archives for paleoclimate and its long-term variability, a record which is predominantly based on U-Th disequilibrium dating. We propose a new avenue to date speleothems utilizing (U-Th)/He in calcite and aragonite - a geochronometric system with no upper age limitation which could potentially extend speleothems-based paleoclimatic records beyond ~500 ka. The method can also complement and help cross-check the recently-matured U-Pb carbonate dating methodology and could potentially be easier to apply.

Our initial results indicate that radiogenic helium accumulates to measurable quantities in speleothems with common uranium content. In order to characterize and quantify the long-term helium retentivity of speleothem-derived carbonates we utilize speleothems from 7 caves with average annual temperatures which vary from 0^oC to >20^oC, stretching from Siberia at the north, to the northern margin of the Saharan-Arabian desert at the south, with independent age control provided by U-Th disequilibrium and U-Pb ages. Aragonite speleothems from caves with near freezing temperatures (~1^oC) yield (U-Th)/He ages (10 ka to 2.2 Ma) which are identical to independent geochronologic constraints and thus demonstrate full retentivity to helium across geologic timescales (10⁴-10⁷ yr). Helium diffusivity (D/a²) in aragonite at 0-1^o C is constrained to values lower than 10^-16.5 [s^-1]. Under identical temperature conditions calcite speleothems are often fully retentive but significant helium loss was encountered occasionally and is yet to be explained. Speleothems from caves with much higher average annual temperature of 20^oC display significant (~60-80%) helium loss. Initial step-heating diffusion experiments for speleothem-derived calcite delineate activation energy of 29.2-31.4 kcal/mol and a multi diffusion domain (MDD) behavior. Such experimental data can be used to predict the fractional loss of helium under given long-term temperature conditions and could also constrain near-surface paleo-temperatures where independent speleothem ages are available.