ABSTRACT: U-Th-4He dating of carbonates: A revival? The case of the Bear Cave flowstone, Northern Yukon, Canada.

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Abstract

Bear Cave, on the Arctic Circle in the north-western Yukon Territory, hosts rare, high latitude speleothem. Isolation from glacial advance has allowed for the preservation of a 68 cm thick flowstone (BC1), likely of Tertiary age. Unique deposits such as these attract interest in their potential for paleoenvironmental and paleoclimatic reconstruction at high latitude, and their insight into the conditions in Beringia prior to the establishment of permafrost. Dating this speleothem formation is critical in order to elucidate paleoclimatic variations inland at high latitudes. Initial 230Th/234U/238U dating was unsuccessful as the system was at secular equilibrium, thus the age is greater than 400 ka. U-Th-4He has been used in the past for dating corals and was explored to obtain an absolute age on the flowstone from Bear Cave. Samples of primary unaltered calcite were crushed to a powder and gas was extracted by stepwise heating to 700°C at GEOTOP-UQAM. 4He measured was calibrated against a pure 3He spike using a quadrupole mass spectrometer at the University of Tokyo. Significant release of 4He observed between 400°C and 600°C suggests the retention of radiogenic helium in the carbonate grains. The contents of 238U and 232Th were measured on separate aliquots by TIMS at GEOTOP. Additional experiments were carried out at the MAPL Noble Gas Laboratory-University of Ottawa for comparison on a replicate section (BC1-4Base). Gas extraction was carried out by step heating crystal separates in ultrahigh vacuum from 50 °C to 600 °C, where the helium isotopes were measured on a MAP215-50 noble gas mass spectrometer. Experiments also confirmed that 4He is retained at ambient conditions. U and Th concentrations were measured on residual material by ICP-MS. Results from MAPL were similar to those reported at GEOTOP. 4He range was found to be from 2.05 × 10-8 to 3.18 × 10-7 ccSTP/g. The 238U content varies between 0.194 and 0.271 ppm and 232Th ranges between 1.35 and 7.81 ppb. Resulting U-Th-4He calculated age for BCI-4Base is 9.70±0.70 Ma (GEOTOP), and 9.35±0.52 Ma (MAPL). These ages are congruent to those derived for other sections in the profile of the flowstone. Future work will be focused on refining the procedure to improve the chronology on the BC1 flowstone. Nevertheless, preliminary data from the present study support a revival of the U-Th-4He chronology and its applications in paleoclimatic reconstructions of other Tertiary-aged speleothems.

Bibliography of Canadian Geomorphology