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Rend. Online Soc. Geol. It., Vol. 17 (2011) - (DOI 10.3301/ROL.2011.12)

Testing the reliability of the Optically Stimulated Luminescence method in a coastal system on Sardinia

S. Andreucci (*), J.P. Buylaert (**),(***), L. Panzeri (****), D. Sechi (*), L. Sanna (*) & V. Pascucci (*)

 

(*) Dipartimento di scienze Botaniche, Ecologiche e Geologiche, Università di Sassari, 07100 Sassari, Italia.
sandreucci@uniss.it
(**) Nordic Laboratory for Luminescence dating, Department of Earth Sciences, University of Aarhus, Risø DTU, 4000-Roskilde, Denmark.
(***) Radiation Research Division, Risø DTU, 4000-Roskilde, Denmark.
(****) Centro Universitario per le Datazioni di Milano-Bicocca (CUDaM), Dip. di Scienza dei Materiali, Università di Milano-Bicocca, 20126 Milano, Italia.


Abstract 

Although worldwide successful applications of the OSL methodology on coastal deposits has been long reported in the literature, the scientific community have not fully accepted yet this dating technique. Thus, aims of this study are to apply the OSL protocol on coastal deposits of Sardinia to test the reliability of this method and to compare the OSL results with data obtained by other techniques.
Luminescence dating is based on the ability of quartz and potassium-rich feldspar grains to retain charges, produced by the interaction with radiation originating from the naturally occurring radioactivity present in all sediments, in meta-stable traps for long periods. During the grains transportation by wind or water, charge in the crystal is reduced (ideally) to zero by exposure to sunlight (bleaching) but it builds up after sediment deposition and burial. Therefore, the OSL method gives the age of the last sediment accumulation and burial event.
In this paper, we have applied single aliquot regenerative (SAR) method on quartz grains to measure the equivalent dose (De) of several coastal samples. However, the natural quartz OSL signal of some samples is too close to saturation to be reliable (i.e. De>2*D0). Thus, we have also used SAR post-infrared stimulation (pIRIR290°) at high temperature protocol on K-rich feldspar grains.
Generally feldspar grains are affected by anomalous fading which causes a decrease of the IRSL signal with time faster than expected from thermal stability measurements.
Thus, normally, IRSL ages tend to underestimate the burial event and a fading rate has to be calculated to correct the fi nal ages. However, high temperature SAR post-IR IRSL protocol should reduce the age correction for anomalous fading up to 5% of the fi nal age that can be considered negligible for Pleistocene samples.
Coastal samples from four study areas have been selected along the North Sardinia coast (Italy): modern Alghero beach-ridge system, Argentiera pocket beach, south Alghero coast (NW Sardinia) and Bue Marino cave (Orosei Gulf; NE Sardinia).
A set of Late Holocene to modern samples was selected from a modern beach-ridge system backing Alghero bay which was human stabilized by plantation during the 1950s. The quartz OSL ages of 3 samples from the dune system span from 2500±150 years to 60±20 years ago, consistent with the known dunefield stabilization.
Preliminary pIRIR290 ages on k-feldspar grains indicate an age offset of 1610±140 years, confirming the unreliability of the pIRIR290 when applied to Holocene samples. Finally a sample from the modern berm deposit gives an age of 0.009 ± 0.014 year (zero age) confirming that during the transportation sediment is well bleached.
The sedimentary succession at Argentiera pocket beach includes a 8 m-thick sandy wind-blown unit interpreted as a coastal dunefi eld system. Five quartz luminescence
samples have been collected on the aeolian unit giving an average age of about 7500±1000 years. A terrestrial shell of Helix pomatia collected carving into the aeolianties were dated using radiocarbon method. The analysis gives an age range between 8060 and 7675 y-BP (calibrated ± 1σ error) confirming the reliability of the quartz OSL methodology for Holocene aeolian deposits.
Along the south Alghero coast a 10-metres thick lithified transgressive dunefield system is widely recognised in the literature to represent the beginning of the last glacial phase (post 80 ka). A single block cut from this deposit was dated using both quartz and K-feldspar grains. The natural quartz OSL signal lies below 2xD0 and the sample passed all the standard laboratory tests (purity test, dominant fast component, dose recovery test (1.02±0.02, n=3) suggesting that the resulting age of 76±6 ka can be considered reliable.
The fading-uncorrected post-IR IRSL290 age of 73±5 ka is in good agreement with the quartz result.

The sedimentary succession at Bue Marino cave includes a 10 m-thick sandy wind-blown unit at the cave entrance, sandwiched between two 50 cm thick calcareous crusts; U-series dates of these crusts constrain the formation of the aeolianite unit between ~125 and ~65 ka. Four luminescence samples were collected from the aeolianites. Due to saturation of the dose response curve, standard quartz SAR-OSL dating only gives minimum ages of between 70 ka and 50 ka. In contrast, fading-uncorrected pIRIR290 ages on K-feldspar extracts point to a formation between ~100 and ~80 ka, in good agreement with the U-series data.
From the results of this study, we conclude that the OSL technique seems to be a reliable method to date the coastal deposits over the last 100.000 years. Moreover, pIRIR290 method on K-feldspars shows great promise for samples at or beyond the quartz OSL age limit.

 

KEY WORDS: OSL, coastal systems, Holocene, Sardinia.

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