Ice core dating with the 36Cl/10Be ratio

Extremely thinned layers and possible folding make the dating of the deepest sections of ice cores especially challenging. Cosmogenic radionuclides have the potential to provide independent age estimates. The ³⁶Cl/¹⁰Be ratio is largely independent of production rate changes that affect individual radionuclides and has an effective half-life of 384 kyr, making it an ideal tool for dating the new 1.5 Myr old ice core that the Beyond EPICA Oldest Ice Core project aims to retrieve at Little Dome C in East Antarctica. However, the loss of ³⁶Cl through hydrogen chloride outgassing at low accumulation sites complicates its application and the long-term decay of the ³⁶Cl/¹⁰Be ratio in ice has not been studied. Here, we show that ³⁶Cl is preserved in glacial periods and that the ³⁶Cl/¹⁰Be ratio decreases more slowly than expected from physical decay over the last 900 kyr. While the glacial ³⁶Cl flux decreases at the expected rate of physical decay within the uncertainty, the ¹⁰Be flux decreases faster, which may be linked to a post-depositional mobility of ¹⁰Be in deep ice and leads to the slower decrease of the ³⁶Cl/¹⁰Be ratio. In addition to this long-term trend, the ³⁶Cl/¹⁰Be ratio fluctuates around a fitted decay curve, which is likely caused by different climate sensitivities of the transport and deposition pathways of the individual radionuclides. Both effects need to be better understood and quantified to improve age estimates based on the ³⁶Cl/¹⁰Be ratio.