Thermoluminescence and Optically Stimulated Luminescence Measured in Marine Sediments Indicate Precipitation Changes Over Northeastern Brazil

Abstract

Marine sediment cores offer a great number of proxies for reconstructions of past environmental conditions, such as ocean temperature, salinity, primary productivity, stratification of the upper water column, and continental precipitation. Up to date, continental precipitation archived in marine sediment cores is reconstructed based mainly on the hydrogen isotopic composition of plant‐wax compounds (i.e., n‐alkane δD) or on the ratio between terrigenous and marine sediments expressed as elemental ratios (e.g., ln (Fe/Ca)). Although these proxies provide reliable precipitation reconstructions, there are some inherent limitations, as plant‐wax δD application depends on the availability of n‐alkanes in marine sediments and elemental ratios can be influenced by other factors like the relative sea‐level, primary productivity, and postdepositional processes. Here we introduce new precipitation proxies based on optically stimulated luminescence and thermoluminescence signals of quartz and feldspar. The rationale is that when precipitation changes over the catchment through time, different sediment sources regarding weathering intensity and parent rock types are drained, supplying sediments with varying signals of luminescence to the ocean. We compared our new proxy records with records of well‐established proxies, for the same (ln (Fe/Ca)) and neighboring (n‐alkane δD) marine sediment cores. The comparison among all proxies as well as with a state‐of‐art transient climate model run (TraCE‐21ka) demonstrates that the new proxies accurately constrain precipitation changes over northeastern Brazil for the last 30,000 years. The main advantage of these new proxies relies on their fast response to precipitation changes over the continent. Furthermore, they are straightforward to measure and not expensive.

Optically Stimulated Luminescence Sensitivity of Quartz for Provenance Analysis

“…Experiments aimed to simplify protocols for OSL sensitivity measurements, including evaluation of heavy minerals contamination and whether thermal treatments to eliminate unstable OSL signals are necessary. Our experiments show that quartz OSL sensitivity measured without preheating and with optical stimulation at room temperature (~20 • C) can be used for provenance analysis in the same way as the OSL measured with preheating and light stimulation at 125 • C [21][22][23]28]. This expands the measurements conditions of quartz OSL sensitivity to in situ measurements in sediment cores and outcrop sedimentary successions, as well as in thermally sensitive materials hosting quartz.…”

“…When quartz OSL is measured by using preheat (160-260 • C), blue stimulation at 125 • C, and light detection in the ultraviolet band, the initial 1 s of light emission is presumably dominated by the fast OSL component [26], which was intensely characterized for dating purposes [25,35]. The extensive research to develop dating protocols based on the fast OSL component of quartz has also supported applications in sediment provenance studies, e.g., [21][22][23][24]27,28]. However, thermal treatments to isolate the fast OSL component usually require mounting samples on discs or cups for measurements in OSL readers built for dosimetry purposes.…”

“…This step is especially important for samples rich in feldspar. For the fine-grained sediments of core GeoB16206-1, sample preparation [28] included the following: (1) sample drying at 60 • C; (2) weighing of 0.5 g of each sample; (3) treatment with H 2 O 2 27% and HCl 10%, to remove organic matter and carbonate minerals; and (4) addition of acetone to the content until 5 mL. [14].…”

“…Then, the BOSL 20 • C was normalized by subtracting the average sensitivity and dividing by the standard deviation of all samples. This normalization was applied in order to compare the BOSL 20 • C with BOSL F values [28]. Table 4.…”

“…The OSL signal measured under this condition is presumably dominated by the fast OSL component [26], but quartz with low OSL sensitivity occurring in sediments around young mountain ranges can show the initial 1 s of light emission with significant contribution of medium and slow components [14]. Despite the successful use of the unstable 110 • C thermoluminescence (TL) peak of quartz in provenance analysis of coastal and marine sediments [27,28], little is known about the suitability of using unstable OSL signals for sediment-fingerprinting purposes. In this study, we performed experiments to improve measurements of quartz OSL sensitivity for provenance analysis.…”