Optically stimulated luminescence and isothermal thermoluminescence dating of high sensitivity and well bleached quartz from Brazilian sediments: from Late Holocene to beyond the Quaternary?

Abstract

The development of optically stimulated luminescence (OSL) dating of sediments has led to considerable advance in the geochronology of the Quaternary. OSL dating is a well established technique to determine sediment burial ages from tens of years to few hundred thousand years. Recent studies have shown that Quaternary sediments of Brazil are dominated by quartz grains with high luminescence sensitivity, allowing the determination of precise and reliable OSL burial ages. In this paper, we show examples of OSL dating of quartz aliquots and single grains from different regions in Brazil, including young coastal-eolian Late Holocene (< 100 years) to Late Pleistocene (~ 150 ka) fluvial sediments. We discuss the OSL data and ages of sediments from carbonate and terrigenous (distributary and tributary systems) fluvial depositional contexts in Brazil. Most of the studied fluvial sediments show equivalent dose distributions with low to moderate dispersion, suggesting well bleached sediments. The comparison between aliquot and single grain data suggests that high overdispersion in equivalent dose distributions of some samples is more related with sediment mixture due to bioturbation than with incomplete bleaching during transport. Well bleached fluvial sediments contrast with the poor bleached pattern usually described for fluvial sediments in the literature. A large part of the fluvial sedimentary record in Brazil is older than the age limit for quartz OSL dating using blue light stimulation. Thus, isothermal thermoluminescence (ITL) dating protocols were tested for dating of fluvial sands from the Xingu River (eastern Amazonia). The studied sample can recover reliable equivalent doses up to 1600 Gy using the ITL 310oC signal. Therefore, this signal would be suitable to extend the age limit of quartz luminescence to the whole Quaternary or beyond (> 2 Ma) in the low dose rate (0.5 - 1.0 Gy/ka) environments typical for Brazilian sediments.

Paleoecology explains Holocene chemical changes in lakes of the Nhecolândia (Pantanal-Brazil)

“…Briefly, the OSL method is based on the single aliquot dose concept and it functions by treating quartz grains as dosimeters, which record low amounts of ionizing ambient radiation (Aitken, 1985;Murray & Wintle, 2003). Sediment samples from NM1 were collected in a dark room to avoid exposure to sunlight, then wet sieved to 63-125 lm, treated with HCl, H 2 O 2 , and HF, and subjected to heavy liquid separation in order to isolate quartz grains (Sawakuchi et al, 2016). Sample aliquots were bleached for 3-5 h, preheated to 200°C, and delivered a 25 Gy dose using a Risø TL/OSL DA-20 with Sr/Y beta radiation source, in order to determine the OSL age (using the central age model of Galbraith et al, 1999).…”

Chronostratigraphy of a 1.5±0.1 Ma composite sedimentary record from Colônia basin (SE Brazil): Bayesian modeling based on paleomagnetic, authigenic 10Be/9Be, radiocarbon and luminescence dating

“…However, previous dating studies demonstrate that a wide range of equivalent doses can be reliably estimated in quartz from southeastern Brazilian sediments using a preheat temperature of 200 o C (e.g. Sawakuchi et al, 2016). Dose recovery tests for the pIRIR 290ºC signal were performed with aliquots of sample COL17-2_15.72 mcd bleached under a solar simulator (24 or 72 hours) and for aliquots bleached on the Risø TL/OSL DA-20 reader.…”

Datação por Luminescência Opticamente Estimulada (LOE) de uma planície de cordões litorâneos do litoral norte do Rio Grande do Sul, Brasil

“…A técnica de datação por LOE determina o período de tempo decorrido entre a última exposição direta de grãos minerais (usualmente quartzo e feldspato) à luz solar e o instante da análise em laboratório (Guedes et al 2011, Sawakuchi et al 2016. Ou seja, essa técnica de geocronologia absoluta fornece a idade de deposição do sedimento.…”