Luminescence dating of aeolian sands and silts: a performance study of different techniques and protocols

The overall aim of this thesis was to investigate the usefulness of luminescence dating as a tool to establish reliable absolute chronologies for those sediments which provide a record of the earth’s climate, especially during cold and dry periods. This research focused on the sandy infill of frost wedge structures in Flanders and on loess from China. In the first part of this thesis quartz-based optical dating was used to establish an absolute chronology for these two sediment types. The second part of this thesis focused on testing the ability of two alternative luminescence techniques to date loess samples beyond the quartz optical dating range. The potential of applying quartz SAR OSL dating to the sandy infills of relict sand wedges and composite-wedge pseudomorphs in Flanders (Belgium) was first investigated. Only those wedges that showed evidence of having been, either exclusively or to a very large degree, filled with sand (primary aeolian infill) were selected for optical dating. Based on the luminescence chronology it was concluded that thermal contraction cracking and infilling with aeolian sediment appears to have been commonplace in Flanders during the Late Pleniglacial (MIS 2); more specifically during the Last Glacial Maximum (LGM, ~23-18 ka) and the transition period between the LGM and the start of the Late Glacial (~14 ka). This study also revealed the presence of two significantly older wedge levels, with a younger (MIS 3) wedge inset into an older (MIS 6) wedge; this clearly illustrates the often complex lithostratigraphy of periglacial sediments in the Belgium lowland and highlights the risks of misinterpretations of past periglacial processes in the absence of an absolute chronology. High resolution quartz SAR OSL dating was then applied to three sites (Zhongjiacai, Le Du and Tuxiangdao) in the western part of the Chinese Loess Plateau. In total, sixty-two optical dates were obtained ranging from ~12 to ~75 ka. The age-depth patterns at all three sites revealed that loess accumulation in the western part of the CLP had an episodic character and large variations in sedimentation rate within and between sites have occurred. Moreover, a convincing sedimentary hiatus between ~20 and ~30 ka was identified at the Tuxiangdao site; this was undetected in previous proxy-record and dating studies and clearly illustrates the importance of high-resolution absolute dating studies for palaeoclimatic research on Chinese loess. Unfortunately, due to the relatively high dose rates (~3 Gy/ka) of this material, it would appear that the quartz OSL age range is rather limited, probably up to only ~40-50 ka (~120-150 Gy). Another method was thus needed to date older loess succesfully. Two other approaches to luminescence dating, one based on quartz and the other on feldspar, were tested to investigate their potential to extend the age range of the luminescence dating of (Chinese) loess. The reliability of a different single aliquot protocol was tested; this used isothermal thermoluminescence (ITL) signals from quartz. Despite the fact that the shape of the growth curve suggests that much higher doses could be measured than with OSL, it seems that the De values obtained with ITL are overestimates. The protocol employed also failed the dose recovery test, overestimating the dose given prior to any heating; this was explained by a sensitivity change which occurred when the first heat treatment was applied to measure the natural signal. It was shown that one way to circumvent this sensitivity change is to use a multiple aliquot approach (the single aliquot regeneration and added (SARA) dose procedure) in which doses are given prior to the measurement of the ITL signal. The origins and the dosimetric characteristics of the ITL signal were also investigated into more detail for quartz extracted from several other sediments and it was concluded that some initial sensitivity change is common in quartz. The accuracy and precision of IRSL dating using sand-sized K-feldspar grain was tested on an Eemian coastal marine deposit in Denmark, with well-known independent age control (~125-130 ka). The uncorrected feldspar ages severely underestimated (by ~30%) both the quartz ages and the independent age control. Using a site-averaged fading rate (g value = 3.66±0.09%/decade) to correct the optical ages of all samples provided good agreement between the average fading-corrected K-feldspar age (120±3 ka random uncertainty; ±6 ka total) and the independent age control (~125-130 ka); Nevertheless, this result is not considered significantly different from the quartz age (114±4 ka; ±7 ka total). Because of these encouraging results on a known-age site, it was decided to apply IRSL dating with anomalous fading correction to old (> ~70 ka) Chinese loess samples from two sites (Luochuan and Dongchuan) for which quartz OSL was thought to be inappropriate (see above). We showed that anomalous fading of the IRSL signal from polymineral fine-grains extracted from Chinese loess is ubiquitous and an overall average g value for these samples of 3.10±0.13%/decade was obtained. At both sites, the quartz OSL ages are always lower than the fading-corrected IRSL ages; the latter are also in better agreement with the pedostratigraphic age control (~75 and ~130 ka). Based on a comparison of the quartz OSL ages with the pedostratigraphic age control and the fading-corrected IRSL ages it is concluded that quartz OSL dating of Chinese loess from these sites should be restricted to samples not exceeding ~40-50 ka (~120-150 Gy). For IRSL dating using polymineral fine-grains with anomalous fading correction comparison with pedostratigraphic age control would suggest an upper dating limit of probably ~100-120 ka (up to ~300 Gy uncorrected dose). Ages older than this should be considered minimum ages

ESCADARIAS DE TERRAÇOS MARINHOS EM PORTUGAL CENTROOCIDENTAL – RELEVÂNCIA COMO INDICADORES DE SOERGUIMENTO CRUSTAL STAIRCASES OF WAVE-CUT PLATFORMS IN WESTERN CENTRAL PORTUGAL – RELEVANCE AS INDICATORS OF CRUSTAL UPLIFT

This study addresses the coastal terrace staircases of western central Portugal (Cape Mondego to Cape Espichel). Below the culminant marine unit that records the last episode of aggradation before the ongoing stage of incision, several terrace levels ar identified. The following methodology was used: a) production of geomorphological maps, combining analysis of detailed MDT’s and aerial photos, with field surveys calibrated with GPS; b) stratigraphic and sedimentological study of the sedimentary deposits associated with the marine platforms; c) luminescence dating. On each terrace staircase, the number of platforms and their elevations are different, indicating differential uplift. Quartz OSL provided age estimates up ca. 150 kyr and post-IR IRSL on K-feldspar up to ca. 700 kyr for the marine terraces under study. Using the culminant platform as reference and assuming that it is ca. 3.6 Ma old, uplift rates were estimated as ranging spatially from 0.071 m/ky to 0.019 m/ky. The ongoing luminescence dating will provide uplift rates estimated for the lower and middle marineterraces, clarifying if an acceleration of the crustal uplift is going on

Violet stimulated luminescence dating of quartz from Luochuan (Chinese loess plateau): Agreement with independent chronology up to ∼600 ka

Luminescence dating at the Luochuan loess type (China) section is at present limited to ∼0.1 Ma using quartz blue light stimulated luminescence (BLSL), but can be extended back in time to ∼0.5 Ma by resorting to the more developmental post-infrared infrared stimulated luminescence (post-IR IRSL) and thermally transferred OSL (TT-OSL) signals. Since both the latter are associated with systematic uncertainties due to the potential (a)-thermal instability of these signals, a search continues for alternative, and demonstrably stable luminescence signals that can cover the entire Quaternary timescale. Here we explore the violet stimulated luminescence (VSL) signal at the Luochuan section, which provides a continuous archive of homogenous sediment with favourable luminescence characteristics and a solid independent age framework. By testing several VSL protocols and their associated performance, we demonstrate that the Multi-Aliquot Additive-Dose (MAAD) protocol produces a VSL chronology at Luochuan which is in agreement with independent ages up to ∼0.6 Ma. For a more representative environmental dose rate of ∼2 Gy/ka (∼35% lower than at Luochuan), the documented range of MAAD-VSL sensitivity (200-1800 Gy) would correspond to the ability to date sediment up to ∼1 Ma back in time, offering a remarkable advance over existing methods.</p

Geoarchaeology of Pleistocene open-air sites in the Vila Nova da Barquinha-Santa Cita area (Lower Tejo River basin, central Portugal)

This paper aims to provide insight into human occupation and landscape change during the Pleistocene in a central area of the Lower Tejo basin (Portugal). Detailed geomorphological mapping, coupled with lithostratigraphy, sedimentology and luminescence dating, supports the identification of a complete terrace staircase sequence. It consists of six gravely terraces located below the culminant (Pliocene) basin unit. A chronological framework for the sedimentary sequences and associated human industries is proposed and correlated with marine oxygen isotope stages (MIS): T1 terrace, not dated; T2, not dated; T3, >300 ka; T4, 300–160 ka (MIS8, MIS7 and MIS6); T5, 136–75 ka (MIS5); T6, 62–30 ka (MIS3); colluviumand aeolian sands, 30–14 ka (MIS2); valley fill deposits, 14 ka to present (MIS1). The oldest artefacts were found at the base of the T4 terrace, with the local stratigraphic level dated to 175 6 ka (Middle Pleistocene). The lithic assemblages collected from distinct stratigraphic levels (T4, T5 top, T6 terraces and colluvium) are characterized by the predominance of opportunistic technological choices, a feature that can be attributed partly to the preferential exploitation of the available raw material, dominated by local-sourced quartzites and quartz pebbles. The adaptation to local raw material (texture and volume), together with subsistence patterns and behaviours, could explain the rarity of Acheulian types (handaxes and cleavers) and picks in the T4 terraces of the Tejo tributaries; this is in contrast to the same terrace of the Tejo valley, in which these types are found. Interpretation of the environmental conditions (controlled by climate and glacio-eustatic sea-level changes) affecting the hunter-gatherer human groups is also presente

On the relationship between K concentration, grain size and dose in feldspar

Previous work has been unable to establish a relationship between K concentration and De in single-grains of feldspar. Here we use four well-bleached sediments with low external dose rate (typically ≤1.5 Gy ka-1) to investigate this relationship. Single and multi-grain pIRIR measurements and μ-XRF analyses are made on Na- and K-rich extracts; μ-XRF is directly applied to grains sitting in single-grain discs to minimise uncertainty in grain identification. Micro-XRF is shown to be sufficiently precise and accurate and luminescence instrument reproducibility is confirmed using dose recovery measurements on heated feldspar. We are again unable to establish any correlation between single-grain De and K concentration, even in feldspar grains for which the internal dose rate should dominate. We also measure highly variable Rb concentrations in these grains and are unable to detect, at the single-grain level, the correlation between K and Rb previously observed in multi-grain investigations. Nevertheless, these results are unable to explain the lack of De correlation with K. Finally, we investigate the dependence of De on grain size (isochrons). Linear correlations are observed but slopes are inconsistent with model prediction. We conclude that this surprising absence of the expected relationships between dose and K concentration and grain size does not arise from analytical precision, incomplete bleaching, sediment mixing or fading. It appears that we cannot measure feldspar doses in these samples as accurately as we thought

An exceptionally long paleoseismic record of a slow-moving fault: the Alhama de Murcia fault (Eastern Betic Shear Zone, Spain)

Most catastrophic earthquakes occur along fast-moving faults, although some of them are triggered by slow-moving ones. Long paleoseismic histories are infrequent in the latter faults. Here, an exceptionally long paleoseismic record (more than 300 k.y.) of a slow-moving structure is presented for the southern tip of the Alhama de Murcia fault (Eastern Betic shear zone), which is characterized by morphological expression of current tectonic activity and by a lack of historical seismicity. At its tip, the fault divides into a splay with two main faults bounding the Góñar fault system. At this area, the condensed sedimentation and the distribution of the deformation in several structures provided us with more opportunities to obtain a complete paleoseismic record than at other segments of the fault. The tectonic deformation of the system was studied by an integrated structural, geomorphological, and paleoseismological approach. Stratigraphic and tectonic features at six paleoseismic trenches indicate that old alluvial units have been repeatedly folded and thrusted over younger ones along the different traces of the structure. The correlation of the event timing inferred for each of these trenches and the application of an improved protocol for the infrared stimulated luminescence (IRSL) dating of K-feldspar allowed us to constrain a paleoseismic record as old as 325 ka. We identifi ed a minimum of six possible paleoearthquakes of Mw = 6-7 and a maximum mean recurrence interval of 29 k.y. This provides compelling evidence for the underestimation of the seismic hazard in the region