5:15 PM - 6:45 PM
[HQR06-P03] Toward the Application of TL Dating for Foraminifera
~Observing Fading in Response to Various Radiations and Measuring Trace Element Concentrations~
Keywords:TL dating, Carbonate, Foraminifera
Typically, quartz and feldspar are employed in thermoluminescence (TL) dating; however, carbonate minerals, although less frequently studied, have also been investigated in this context (e.g., Medline 1958, Debenham 1983). Foraminifera, some species possess carbonate shells, are widely utilized as facies and age index fossil (Toyofuku and Nagai 2019). Ages of foraminifera fossils were often estimated by the 14C dating method (Oba 2006). Given that TL dating can, in principle, measure older dates than the 14C dating method, its application to foraminifera could contribute the establishment of chronology in Quaternary period and enhancement of paleoenvironmental studies. Accurate age estimation by TL dating needs precise estimation of accumulated dose and an understanding of characteristics of luminescence signal stored by various radiations (e.g., fading behavior).
This study exposed foraminifera samples to α, β, γ, and X-rays, and thermoluminescences were measured immediately or after storage in darkness for variety of periods to study the fading behavior. The samples were collected from foraminifera
rich ooze collected during IODP Expedition 363 from the western tropical Pacific, treated with hydrogen peroxide, rinsed with distilled water, and dried at 60°C. X-ray diffraction (XRD: Rigaku Ultima IV) identified the mineral type as calcite.
Medline (1958) reported Mn2+ and Pb2+play as luminescence activators and Fe2+, Co2+, and Ni2+ as quenchers in the synthetic calcite sample. These five elements were measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS: Agilent 7500s). TL were measured in Kanazawa University (a device designed based on Ganzawa et al., 2013) for a temperature range of 100-450°C after a preheat of 80°C for 10 seconds. IRC-65L and R-60 filters were used. For normalization purposes, after the initial measurements, the samples were irradiated with an X-ray dose of 0.1 Gy/s for 1000 seconds and subsequently remeasured.241 Am,90 Sr, and 60Co sources were used for α, β, and γ radiation, respectively, and a Varian, VF-50JW unit for X-ray exposure. For α and β radiation exposure, a single disc was repeatedly measured, while multiple discs were used for γ-ray exposure, with each disc measured only once.
Measurements revealed no significant signal decay for α, β, and γ radiation over several weeks, but X-rays showed notable signal reduction within about a week. The luminescence signal in response to the dose was limited to a few dozen to several hundred counts in all measurements.
Based on these results, TLCI was conducted on samples exposed to various radiations, and the choice of filters was re-evaluated.
This study exposed foraminifera samples to α, β, γ, and X-rays, and thermoluminescences were measured immediately or after storage in darkness for variety of periods to study the fading behavior. The samples were collected from foraminifera
rich ooze collected during IODP Expedition 363 from the western tropical Pacific, treated with hydrogen peroxide, rinsed with distilled water, and dried at 60°C. X-ray diffraction (XRD: Rigaku Ultima IV) identified the mineral type as calcite.
Medline (1958) reported Mn2+ and Pb2+play as luminescence activators and Fe2+, Co2+, and Ni2+ as quenchers in the synthetic calcite sample. These five elements were measured using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS: Agilent 7500s). TL were measured in Kanazawa University (a device designed based on Ganzawa et al., 2013) for a temperature range of 100-450°C after a preheat of 80°C for 10 seconds. IRC-65L and R-60 filters were used. For normalization purposes, after the initial measurements, the samples were irradiated with an X-ray dose of 0.1 Gy/s for 1000 seconds and subsequently remeasured.241 Am,90 Sr, and 60Co sources were used for α, β, and γ radiation, respectively, and a Varian, VF-50JW unit for X-ray exposure. For α and β radiation exposure, a single disc was repeatedly measured, while multiple discs were used for γ-ray exposure, with each disc measured only once.
Measurements revealed no significant signal decay for α, β, and γ radiation over several weeks, but X-rays showed notable signal reduction within about a week. The luminescence signal in response to the dose was limited to a few dozen to several hundred counts in all measurements.
Based on these results, TLCI was conducted on samples exposed to various radiations, and the choice of filters was re-evaluated.