Assessing the potential of a modified post-isothermal IRSL (pIt-IR) protocol to circumvent the problems posed by anomalous fading in polymineral fine grains

Abstract

Attempts to find a ‘non-fading’ infrared stimulated luminescence (IRSL) signal have been ongoing in the past two decades, due to complications regarding measurement and application of anomalous fading as well as higher age uncertainty introduced by fading correction. In this research, the potential of the post-isothermal IRSL (pIt-IR) protocol to generate ages without the need for fading correction is investigated. Polymineral fine grain samples were collected from a sediment core from Lake Chew Bahir in Ethiopia which is supported by independent age control. The equivalent dose (D_e) values determined using the pIR₅₀IR₂₂₅ pIt-IR protocol are in agreement with the fading corrected pIRIR₂₂₅ D_e values for the upper ∼40 mcd of the core (∼200 Gy), below which the pIR₅₀IR₂₂₅ pIt-IR D_e values underestimate the fading corrected pIRIR₂₂₅ D_e values. This underestimation appears to be a consequence of the IR₅₀ signal approaching field saturation since pIt-IR D_e determination depends on both IR₅₀ and pIRIR₂₂₅ signals. Increasing the measurement temperature of the first IR signal in the pIRIR₂₂₅ pIt-IR protocol (from 50 to 150 °C, i.e. pIR₁₅₀IR₂₂₅ pIt-IR) appears to extend the upper limit of the pIt-IR protocol by producing D_e values that are consistent both with the fading corrected pIRIR₂₂₅ D_e values (within 1σ uncertainties), and also in agreement with the ages of two known-age tephra (within 2σ uncertainties) down to ∼90 mcd of the core (∼450 Gy, ∼180 ka). This demonstrates that the pIt-IR protocol is capable of producing D_e values and ages that do not require fading correction as long as the first IRSL signal in the pIRIR₂₂₅ pIt-IR protocol is not in field saturation. However, verification of the accuracy of the D_e values and ages generated using the pIt-IR protocol is provided by comparison of the fading corrected IR₅₀ and corrected pIRIR₂₂₅ D_e values/ages, which could otherwise be collected directly through more conventional pIRIR protocols that are shorter in duration than pIt-IR measurement sequences. This raises questions regarding the benefit of using a pIt-IR protocol rather than a pIRIR protocol for dating, the primary advantage of which remains circumvention of the problems posed by anomalous fading for sediments that are younger than the field saturation values of the first IR signal measured in a pIt-IR protocol.