Optically stimulated luminescence dating techniques and multi-proxy analysis to quantify the timing of the last two major climatic transitions, as recorded by loess-palaeosol sequences

The practice of tuning different climate proxies prevents the observation of regional response times of terrestrial archives to global changes. Thus, it is imperative to develop correlation protocols based on absolute chronologies. Loess-palaeosol (L/S) deposits are continental archives of Quaternary paleoclimates and loess is generally considered an ideal material for the application of luminescence dating.The agreement previously obtained for 10-20 ka ages using different techniques has given us confidence in using the state of the art measurement protocols for young deposits, as confirmed by comparison with independent age control.  Therefore, we propose detailed investigations of loess samples collected in close proximity to the transition to the recent soil, with the purpose of obtaining a temporal quantification of the ending of the Late Tardiglacial and the beginning of the Holocene (i.e. L1/S0 boundary).</p

Investigations on the Luminescence Properties of Quartz and Feldspars Extracted from Loess in the Canterbury Plains, New Zealand South Island

The applicability of the single-aliquot regenerative-dose (SAR) protocol, by using the optically stimulated lumi-nescence (OSL) signal of quartz as well as the post-infrared–infrared (pIRIR) signals of polymineral fine grains, namely pIRIR225 and pIRIR290, was assessed for dating loess in New Zealand South Island. OSL signals of quartz grains displayed low sensitivity. However, the application of repeated irradiation/bleaching cycles did not result in an increase in sensitivity; annealing in the 300–500°C temperature range generated the sensitisation of both the 110°C thermoluminescence (TL) peak as well as the OSL signal, likely by activation of yet unidentified luminescence centres. After heating, the quartz signal is comparable to that of ideal samples, but the annealing is precluding successful dating. On the other hand, feldspar infrared-stimulated signals displayed satisfactory properties, al-lowing estimation of ages ranging from 14 ± 1–29 ± 3 ka for the investigated deposit. It was shown that pIRIR225 and pIRIR290 methods have potential for dating loess in the South Island of New Zealand, based on the following observations: (i) Dose recovery tests were successful with recovered-to-given dose ratios with a <10% deviation from unity, (ii) constant residual values of about 4 Gy and about 10 Gy were obtained after exposures for 48 h in the case of pIRIR225 signals and 96 h in the case of pIRIR290 signals, respectively, (iii) while a slight dose-dependence of the residual was reported, and for a dose as large as 1600 Gy the residual values are ≅9 Gy and ≅19 Gy for pIRIR225 and pIRIR290 signals, respectively

Testing the potential of using fine quartz for dating loess in South Island, New Zealand

Highlights • OSL signals of coarse (˃ 63 μm) quartz from loess in New Zealand are not amenable for dating. • Fine quartz (4–11 μm) signals allowed the application of OSL dating for last glacial loess. • Differences between the two grain sizes could not be unravelled by ESR characterisation. • Ages obtained on polymineral fine grains using pIRIR overestimate the fine quartz chronology. The applicability of optically stimulated luminescence (OSL) dating on quartz from South Island, New Zealand is hampered by the poor behaviour of the targeted signals. However, most OSL dating studies have been focused on using coarse quartz fractions. Since a previous study conducted from a nearby site demonstrated that coarse quartz (63–90, 90–125, 125–180 and 180–250 μm) is not suitable for OSL dating, we attempt using fine quartz here. Therefore, the standard SAR protocol was applied on 4–11 μm quartz extracted from a loess/paleosol section. Unlike the coarser fractions, the OSL signal of fine quartz displayed satisfactory characteristics which allowed estimating ages ranging from 0.3 ± 0.04 ka to 16 ± 1 ka. In order to understand the differences between the two quartz fractions, we characterise fine (4–11 μm) as well as the usually used coarser grain sizes (˃ 63 μm) of quartz by electron spin resonance (ESR). No significant differences are reported in qualitative terms between the grain sizes investigated and calibration quartz. We report a higher abundance of intrinsic defects in the fine grain fraction; however, this is typical for quartz from other regions as well, that was amenable for OSL dating. As such, the differences between the fine quartz fraction and the coarse fraction is not yet understood. In addition, two elevated temperature post-infrared infrared protocols (pIRIR225 and pIRIR290) were applied and polymineral grains extracted from the same samples. Despite residual dose corrections being performed using a modern analogue, pIRIR ages overestimate quartz ages by 19–122% in the case of the application of the pIRIR225 protocol and by 25–217% in the case of the application of the pIRIR290 protocol. The effect could not be circumvented by the application of a test dose with a magnitude of 50% of the equivalent dose in the pIRIR290 protocol. In the case of the application of pIRIR290 protocol, dose recovery tests ratios vary from 1.07 ± 0.06 to 1.23 ± 0.05. While not ideal, these results cannot fully explain the differences reported between the ages obtained by fine quartz OSL and the polymineral fine grains pIRIR methods