Can the 4He clock be calibrated by 36Cl for groundwater dating?

Abstract

The ratio of dissolved excess 4He concentration to the 4He accumulation rate is equivalent to the residence time of groundwater if 4He is accumulated at a constant rate. Groundwater contains 36Cl from cosmogenic and subsurface production sources. The 4He clock can be applied by using the correlation between cosmogenic 36Cl decay or the subsurface 36Cl production and the excess 4He increase. We calibrated the 4He clock in the Great Artesian Basin (GAB), Australia, considering dissolution of chloride from the rock matrix. Cosmogenic 36Cl is dominant over 36Cl produced in the subsurface in the Coonamble Embayment of the GAB, because the quartzose sandstone aquifers have a low density of thermal neutrons. The 4He accumulation rate was estimated to be 9.6×10-11ccSTP/gH2O·y-1 in the GAB. We then applied this method to estimation of the groundwater residence time in the Tokachi Basin, Japan, where both cosmogenic and subsurface 36Cl are present in the groundwater. The longest estimated residence time was 2.3 million years, based on a 4He accumulation rate of (1.2-2.1)×10−12ccSTP/cmH2O3·y-1, assuming that ion filtration occurred as pore water was squeezed by the deeply subsiding sedimentary strata in the basin.