AMQ abs Sechi et al SACCOM 189-192.pub Available online http://amq.aiqua.it ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, Vol. 31 (Quaternary: Past, Present, Future - AIQUA Conference, Florence, 13-14/06/2018), 189 - 192 LUMINESCENCE DATING OF A MIDDLE LATE HOLOCENE LOWER SHOREFACE, SW SARDINIA (ITALY) Daniele Sechi 1, Stefano Andreucci 2, Giovanni De Giudici 2, Vincenzo Pascucci 1 1 Università degli Studi di Sassari, Dipartimento di Architettura Design Urbanistica, Alghero, Sassari, Italy 2 Università di Cagliari, Dipartimento di Scienze Chimiche e Geologiche, Cagliari, Italy Corresponding author: D. Sechi ABSTRACT: Geochronological framework of Middle-Late Holocene coastal succession are usually based on radiocarbon dating method. Homolithic sandy bodies characterize lower shoreface deposits and shells are often the only readily available biogenic materials for radiocarbon dating. However, multiple processes of erosion and deposition frequently affect sediment-grains and biological materials. As a consequence, 14C dating performed on shells often may under or overestimate the true age of the hosting deposits. This study high- lights that luminescence dating is better suited to investigate sedimentary body formed in high energetic environments because directly dates the age of the final burial event. Moreover, combining the quartz and k-feldspar derived ages a more robust dataset and internal independent age controls can be provided. KEYWORDS: SAR-OSL, pIRIR150, geochronology, radiocarbon dating, “old shell” problem 1. INTRODUCTION In the last decade Luminescence has become one of the most used method for dating siliciclastic-rich Qua- ternary deposits (Pascucci et al., 2014). Nevertheless, its application is usually limited to the Pleistocene while Radiocarbon method is chosen for dating Holocene successions (Pascucci et al., 2014; 2018). In particular, 14C method is widely applied in shallow marine-coastal studies throughout the world but precise dating of lower shoreface deposits remains a major task. Shoreface deposits of wave-dominated system are characterized by homolithic sandy bodies transported basinward from the coastline by return flows induced by major storms. In this highly energetic environment, shells are usually the only readily available biogenic materials for radiocarbon dating. However, shells may suffer of multiple reworking processes prior the final burial event and thus 14C may overestimates the ages of storm beds producing strati- graphic discrepancies. To overcome this problem an alternative or independent geochronological approach is needed. Luminescence methods date the time elapsed since siliciclastic minerals (quartz and feldspar) were last exposed to day-light and thus directly provide the age of the final burial event (Madsen & Murray, 2009). Aim of this work is therefore to provide a lumines- cence-based chronological framework for a Middle-Late Holocene shoreface succession and compare with pub- lished radiocarbon ages provided by Romano et al. (2017). Moreover, luminescence ages will be provided using three different protocols such as SAR-OSL on quartz mineral along with IR50 and pIRIR150 on k- feldspar grains (Madsen & Murray, 2009; Reimann T. & Tsukamoto 2012). Although these protocols share some common features, they also have their own specific traits and can be considered independent from each-other and thus used for age comparisons. 2. MATERIAL AND METHODS Cala Domestica (SW Sardinia, Italy; Fig. 1) is a nar- row 100 m wide and 450 m long bay carved in the Lower Cambrian Meta sandstone and dolostones. It is charac- terized by a well-developed sandy pocket beach backed by an active dunefield system (Fig. 1). The sediments nourishing the system are mainly bioclastic materials transported onshore from sea grass meadows during storms and minor siliciclastic grains carried by NW/SE longshore current and/or by the seasonal Riu Guttu stream (Fig.1). Samples for luminescence and shells for 14C were collected from a 3-m long sediment core drilled in the shoreface (-13m below the present-day sea level). The sediment core was taken throughout opaque cylin- drical plastic tube in order to avoid exposition of sedi- ments to day-light and then opened under red subdued light. From the studied core two marine shells S1 and S5 were collected and the derived calibrated before present (BP) radiocarbon ages are already published by Romano et al. (2017). A total of six luminescence samples were collected on the same core (Fig.2). In particular ISO0 at the very top, ISO1 and ISO5 at the same position of the shells collected for 14C and the remnant samples (ISO2, ISO3, ISO4) every ~50 cm. All samples underwent chemical treatment at the Sassari Luminescence laboratory to isolate the 180-90 φm pure fraction of quartz and k- feldspar grains. Single Aliquot Regenerative protocol (SAR) for quartz and Infra-red stimulation at 50 °C (IR50) along with post Infra-red, Infra-red stimulation at low https://doi.org/10.26382/AIQUA.2018.AIQUAconference 190 Sechi D. et al. Tab.1 - Summary of quartz luminescence dating results. Table reports sample code and depth of sampling. For each mineral dosimeter chosen for luminescence dating, the luminescence protocol/signal used are reported with the estimated burial Palaeodose (De, Gy), environmental Dose Rate (Dr, Gy/ka) and calculated final burial ages expressed in years (a) from present (AD 2017). Fig.1 - A) The studied area. A) Location of Sardinia Island in the Mediterranean region where it occupies a central position. The map of Sardinia, reported the main cities (black circles). The red square highlight the position of studied area in the SW of island. B) Main bed- rock lithologies outcropping around the study area. C) Satellite view of Cala Domestica bay with drilling station. 191 temperature 150 °C protocols (pIRIR150) on k-feldspars were applied for luminescence burial palaeodose (De) measurements (Madsen & Murray, 2009; Reimann T. & Tsukamoto, 2012). Enviromental Dose rate (Dr) calcula- tions corrected for cosmic rays and water content contri- butions are based on high resolution gamma spec- trometer. 3. RESULTS Table 1 reports luminescence dataset for the col- lected samples (Fig. 2). Quartz and k-feldspar signals pass all laboratory tests. These provide very good re- sults for the three main reliability check-criteria: recy- cling, recuperation and dose recovery. Quartz (OSL- SAR) and feldspar (IR50, pIRIR150) signals gave in aver- age recycling ratios within the 10 % of unity and recu- peration less than 5% of natural De. Dose recovery ratios are all close unity, 1 ±0.02 (SAR-OSL), 1.00±0.01 (IR50) and 0.96 ±0.02 (pIRIR150). These data confirm the suitability of protocols and dosimeters chosen for dating the sediment of Cala Domestica. Given that k-feldspar suffers of fading (partial loss of signal recovered throughout the time) the g-value (% of loss per decade) was tested. The IR50 and pIRIR150 show an average fading ratio respectively of 3.86 and 0.89 %/dec. and thus used for correcting final feldspar ages. Quartz SAR -OSL yield ages ranging from the bottom up of 4800±300 to 540±50 a. Feldspar IR50 and pIRIR150 show ages respectively from 4217±87 and 4347±133 a to 403±12 and 433±41 a (Tab.1). 3. DISCUSSION AND CONCLUSION Three meters of Holocene core collected on shallow marine (lower shoreface) deposits, far off Cala Domes- tica bay, is dated using Luminescence and 14C methods. This study sheds a light on strenghths and weaknesses of these two approaches. Figure 2 compares the three sets of luminescence dating (SAR, IR50 and pIRIR150) with the radiocarbon ages. Estimated Luminescence ages using the two min- eral dosimeter (quartz and feldspar) and different lumi- nescence signals are in stratigraphic order downward the core. The reliability of estimated palaeodose and ages is supported by the pretest results. The uppermost sample (ISO0) yield null or small residual palaeodose for the three protocols applied. This suggests that all different luminescence signals are being well zeroed by day-light at depth of 13 m. Moreover, consistency and increasing of the ages downward confirm that mineral dosimeters well recover the palaeodose (burial time) elapsed from the last exposition and allow to exclude the presence of partial bleached inherit signal issue (incomplete zeroing). Although, SAR-OSL ages slightly overestimate the IR50 and pIRIR150 ones, these agree each-other within 1 sigma uncertainties. In particular, k- feldspar-derived ages obtained for ISO5 (bottom of core) confirm the 14C age of 4435-4290 Cal BP (Romano et al., 2017). The fading rates obtained for pIRIR150 are significantly lower than those form IR50 (Tab.1) suggesting the better stability of pIRIR150 signal over time and the potential of the protocol for dating recent to very recent sedimentary bodies. The ISO1 sample shows luminescence ages con- sistently younger than 960-860 Cal BP 14C (Fig. 2). This ca. 500 a discrepancy between luminescence and 14C ages most likely is due to the multiple reworking proc- esses that shells and sediment-grains suffering in the Luminescence dating of a middle late Holocene lower shoreface, SW Sardinia Fig. 2 - On the right the stratigraphic reconstruction of drilled core and position of radiocarbon and luminescence samples collected. The radiocarbon ages (Romano et al., 2017) are reported against the estimated SAR, IR50 pIRIR150 lumines- cence ages. On the left, the plot of geochemical Enrichment Factor (EF) along the core depth published by Romano (2017). Only element anthropogenically enriched (EF>1.5) are re- ported. shoreface environment. In particular, shells may be temporarily stored in the foreshore part of the beach system prior to be finally buried in the lower shoreface as a consequence of major storms. The anthropogenically enriched elements (Hg, Pb and Zn) measured along the studied core by Romano et al. (2017) show a strong enrichment in the first 20 cm of the core clearly referable to the recent mine activity (Fig. 2). Whereas based on the 14C available age a small peak observed at ca. 60 cm were associated as well with ore exploitations in pre-industrial times but with no possibility to refer this activity to the Phoenician, Ro- mans, Pisans or Middle Ages. However, luminescence ages point the first peak to a post-Romans ore exploita- tions better constraining the mining activity at the study area. The carried out study highlights that in highly ener- getic environments such as lower shoreface multiple reworking processes frequently affect sediment-grains and biological materials. As a consequence of this, 14C dating performed on shells might under or overestimate the final burial age of the host deposits. Therefore, lumi- nescence dating is better suited to investigate homo- lithic sedimentary body because it directly dates the mineral grains sun-exposition, transport and accumula- tion history. Moreover, using different luminescence signals and dosimeters allow to have a better knowl- edgement of sedimentary depositional history and inter- nal independent age controls. REFERENCES Madsen A.T., Murray A.S. (2009) - Optically stimulated luminescence dating of young sediments: a re- view. Geomorphology, 109, 3-16. Pascucci V., De Falco G., Del Vais C., Sanna I., Melis R.T., Andreucci S. 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