88 research outputs found
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, 300â160 ka (MIS8, MIS7 and MIS6); T5, 136â75 ka (MIS5); T6, 62â30 ka (MIS3);
colluviumand aeolian sands, 30â14 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
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
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 â€1.5 Gy ka-1) to investigate this relationship. Single and multi-grain pIRIR measurements and ÎŒ-XRF analyses are made on Na- and K-rich extracts; ÎŒ-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
Towards an improvement of optically stimulated luminescence (OSL) age uncertainties: modelling OSL ages with systematic errors, stratigraphic constraints and radiocarbon ages using the R package BayLum
Statistical analysis has become increasingly important in optically
stimulated luminescence (OSL) dating since it has become possible to measure
signals at the single-grain scale. The accuracy of large chronological
datasets can benefit from the inclusion, in chronological modelling, of
stratigraphic constraints and shared systematic errors. Recently, a number
of Bayesian models have been developed for OSL age calculation; the R
package âBayLumâ presented herein allows different models of this type to be implemented,
particularly for samples in stratigraphic order which share systematic
errors. We first show how to introduce stratigraphic constraints in
BayLum; then, we focus on the construction, based on measurement
uncertainties, of dose covariance matrices to account for systematic errors
specific to OSL dating. The nature (systematic versus random) of errors
affecting OSL ages is discussed, based â as an example â on the dose rate
determination procedure at the IRAMAT-CRP2A laboratory (Bordeaux). The
effects of the stratigraphic constraints and dose covariance matrices are
illustrated on example datasets. In particular, the benefit of combining
the modelling of systematic errors with independent ages, unaffected by
these errors, is demonstrated. Finally, we discuss other common ways of
estimating dose rates and how they may be taken into account in the
covariance matrix by other potential users and laboratories. Test datasets
are provided as a Supplement to the reader, together with an R
markdown tutorial allowing the reproduction of all calculations and figures
presented in this study.</p
On the importance of grain size in luminescence dating using quartz
There are two major problems commonly encountered when applying Optically Stimulated Luminescence (OSL) dating in the high dose range: (i) age discrepancy between different grain sizes, and (ii) age underestimation. A marked and systematic discrepancy between fine-grain (4â11 ÎŒm) and coarse-grain (63â90 ÎŒm) quartz single aliquot regeneration protocol (SAR) ages has been reported previously for Romanian and Serbian loess >40 ka (De of âŒ100 Gy), generally with fine-grain ages underestimating the depositional age. In this paper, we show a similar age pattern for two grain size fractions from Chinese loess, thus pointing to a potential worldwide phenomenon. While age underestimation is often attributed to signal saturation problems, this is not the case for fine grain material, which saturates at higher doses than coarse grains, yet begins to underestimate true ages earlier. Here we examine the dose response curves of quartz from different sedimentary contexts around the world, using a range of grain sizes (diameters of 4â11 ÎŒm, 11â30 ÎŒm, 35â50 ÎŒm, 63â90 ÎŒm, 90â125 ÎŒm, 125â180 ÎŒm, and 180â250 ÎŒm). All dose response curves can be adequately described by a sum of two saturating exponential functions, whose saturation characteristics (D0 values) are clearly anticorrelated with grain diameter (Ï) through an inverse square root relationship, D0 = A/âÏ, where A is a scaling factor. While the mechanism behind this grain-size dependency of saturation characteristics still needs to be understood, our results show that the observation of an extended SAR laboratory dose response curve does not necessarily enable high doses to be recorded accurately, or provide a corresponding extended age range
- âŠ