This article is published in:

Quaternary Geochronology (2012) Volume 9, 54-64

doi:10.1016/j.quageo.2012.01.002

TL and ESR dating of Middle Pleistocene lava flows on Lanzarote island, Canary Islands (Spain) Hans von Suchodoletza,b,*, Henrik Blanchardc, Alexandra Hilgersd, Ulrich Radtked,e, Markus Fuchsf, Michael Dietzeb, Ludwig Zöllerg a Institute of Geography, University of Leipzig, Johannisallee 19a, D - 04103 Leipzig, Germany b Institute of Geography, Technical University of Dresden, D - 01069 Dresden, Germany c Steinmann-Institute of Mineralogy-Petrology, University of Bonn, Poppelsdorfer Schloss, D - 53115 Bonn, Germany d Institute of Geography, University of Cologne, Albertus-Magnus-Platz, D - 50923 Köln, Germany e Rectorate of the University Duisburg-Essen, Universitätsstraße 2, D - 45117 Essen, Germany f Institute of Geography, University of Gießen, Senckenbergstraße 1, D - 35390 Gießen, Germany g Institute of Geography, University of Bayreuth, Universitätsstrasse 30, D - 95440 Bayreuth, Germany * corresponding author abstract: In the north of the island of Lanzarote (Canary Islands, Spain), two palaeosols from different sites were baked by overlying lava flows. Using red thermoluminescence (RTL) dating, late Middle Pleistocene ages were obtained, both varying around 170 ka thus indicating the simultaneity of the volcanic events. Blue thermoluminescence dating using different measuring techniques of the same sample material from one site (the Mála dune) yields somewhat lower ages, varying around 125 ka. However, this result is not in contradiction with the RTL age since anomalous fading was detected, expected to cause a significant underestimation of blue thermoluminescence results. Electron spin resonance dating from land snails overlying the lava flow yielded ages between 204 and 123 ka. Although this dispersion is rather large for material originating from the same stratigraphic horizon, it has to be taken into account that the amount of sample material and thus signal intensity was very low. Furthermore, small measured sample quantities are very vulnerable to dosimetric inhomogenities in the surrounding material, being a further source of uncertainty. Thus, these ESR ages bracketing the RTL age are a further support for a late Middle Pleistocene age of the lava flow. The Middle Pleistocene RTL ages of about 170 ka fill a conspicuous gap in the volcanic chronostratigraphy of Lanzarote, demonstrating that post-erosional volcanism on Lanzarote was obviously more continuous during the Middle and Late Quaternary than known before. Thus, these results demonstrate the potential of RTL and ESR dating to improve the timing of the Quaternary volcanism of the Canary Islands. 1. Introduction The Canary Islands in the Atlantic Ocean off NW Africa (Fig. 1) are one of the most intensively studied oceanic islands in the world. However, their origin remains a matter of debate until today (Carracedo et al., 1998; Anguita and Hernán, 2000; King and Ritsema, 2000; Geldmacher et al., 2001, 2005; Ancochea et al., 2006). This is because some features of Canarian volcanism are not in line with the simple hotspot theory (Holik et al., 1991) as it shows large time gaps of several million years between different volcanic phases of one island, an inconsistent spatial age progression of volcanism and a lack of subsidence of older islands (Carracedo et al., 1998; Anguita and Hernán, 2000). Further hypotheses that were proposed during the last decades include the theory of a possible link between Canarian volcanism and tectonics in the Atlas orogene or a triggering of uplift and volcanism on the Canary Islands by compressive tectonics (Anguita and Hernán, 1975; Rothe, 1974; Araña and Ortiz, 1986) (cf. Fig. 1). More recent hypotheses link a mantle plume with several other mechanisms to explain the specialities of Canarian volcanism, e.g. the theory of a hot spot on a slowly moving plate (Carracedo et al., 1998; Geldmacher et al., 2001), a fossil mantle plume influenced by compressional and tensional forces in a regional fracture system (Anguita and Hernán, 2000) or a plume interacting with edge-driven convection below the African craton (King and Ritsema, 2000; Geldmacher et al., 2005). However, these hypotheses are not definitely proven since this would require more studies about space-time magmatic and tectonic relationships of each Canarian island (Mangas, J., website: www.mantleplumes.org, Guillou et al., 2004; Ancochea et al., 2006). In spite of the continuously increasing number of numerical datings from the archipelago (e.g. Coello et al., 1992; Carracedo et al., 2003; Guillou et al., 2004; Geldmacher et al., 2005; Ancochea et al., 2006), large gaps in the volcanostratigraphies of the islands still exist. For example, the succession of post-erosional Quaternary volcanism that followed the initial shield building stage of the north-easternmost island Lanzarote is mostly unknown today (cf. Carracedo et al., 1998; Geldmacher et al.,

2001), although its chronology is crucial for a better understanding of Canarian volcanism. This is due to the fact that the hotspot theory predicts only irregular and faint activity for this volcanic stage of this relatively old island (Carracedo et al., 2003). Fúster et al. (1968) classified volcanic activity into four distinct series, whereas a more recent model attributes former series I to the initial shield building stage, and former series IIeIV to the post-erosional volcanic stage now (Coello et al.,1992; Carracedo et al.,1998; Instituto Tecnológico y Geominero de España, 2005) (Fig. 2). However, the limited number of 20 numerical datings of Quaternary volcanism on Lanzarote, among them 13 < 1 Ma (Carracedo et al., 2003; Instituto Tecnológico y Geominero de España, 2005, Fig. 3), and the consequent use of geomorphological features for geochronological classifications does not allow a precise recognition of temporal and spatial patterns of the post-erosional stage. For instance, existing numerical dating results suggest a volcanic gap between 600 and 400 ka (Fig. 3), but without further chronological confirmation it is not clear if this is simply an artefact of the scarcity of numerical datings (Instituto Tecnológico y Geominero de España, 2005). General problems occurring when only applying geomorphological features for geochronological classifications are clearly demonstrated by the Corona volcano in the north of the island (former series IVA, Fúster et al., 1968): Geomorphologic observations placed its formation into the Mid-Holocene (Zazo et al., 1997, 2002), whereas Ar/Ar datings of volcanites shifted this event to the Last Glacial maximum (Carracedo et al., 2003). This example demonstrates that more numerical dating results are needed for an understanding of the post-erosional volcanism of Lanzarote, but also of the volcanic activity of the other Canary Islands. With optically stimulated luminescence (OSL), thermoluminescence (TL) and electron spin resonance dating (ESR), the family of trapped charge dating techniques offers powerful tools for solving various chronological questions, in the case of volcanic activity by dating sediments situated above and below volcanic material. Trapped charge dating methods are based on the process of a time dependent accumulation of charge at structural defects in the crystal lattice of common minerals such as quartz or feldspars. Charge transfer is induced by ionising radiation resulting from naturally occurring radioactive processes. Release of the stored charge from the crystal lattice and thus the point in time which is dated, is achieved either by heating or exposure of the mineral to sunlight in case of luminescence dating. The released charge is measured in the laboratory as photons. This luminescence emission is in proportion to the radiation dose absorbed by the mineral since the last event of signal resetting and termed equivalent dose (De). To make this luminescence intensity usable for age calculations it is translated into dose values through calibration of the response signals against known doses of radiation in the laboratory. In order to determine the time elapsed since the last resetting, the De is divided by the effective dose rate of ionizing radiation released by the decay of naturally

occurring radionuclides such as 238U, 235U, 232Th and 40K (dose rate, .

D ) (Aitken, 1985; Singhvi and Krbetschek, 1996). One method that was developed during the last years for dating volcanic and heated material is the red thermoluminescence method of quartz (RTL) (Fattahi and Stokes, 2000; Nakata et al., 2007; Hashimoto, 2008). Advantages of the RTL over the more commonly used blue-emitting thermoluminescence (BTL) for dating heated material are that (i) lifetime of luminescence traps is in the order of up to 1 Ma or even more at ambient temperatures (ii), the dose response curve shows a good linearity, (iii) no significant sensitivity changes are observed, and (iv) signal saturation is very high, allowing dating in the range of some 100 ka (Fattahi and Stokes, 2000; Hashimoto, 2008). Since allochthonous quartz is found on the island due to the input of Saharan dust (Mizota and Matsuhisa, 1995; Menéndez et al., 2007), a dating of volcanic phases by measuring RTL from lava-heated quartz in underlying soils should be possible. The Electron Spin Resonance dating method (ESR) is used to date carbonates such as corals, speleothems, mollusc or snail shells, where the date of mineralization is determined (e.g. Grün, 1989; Schellmann et al., 2008). This method is based on the radiation-induced accumulation of charge (electrons, free radicals) at charge-deficit sites or defects, respectively, in the crystal lattice of the mineral. The number of trapped electrons is presented via the ESR microwave absorption signal and is a relative measure of the radiation that the mineral has received over time (De). As for luminescence

dating, this number is set into relation to the dose rate (.

D ) of the surrounding sediment as well as to the U-content of the organism itself. In this paper, we test the use of the RTL dating method on the eastern Canary Islands by dating soil material heated by two different lava flows originating from a volcanic chain in the northeast of Lanzarote. In order to check the reliability of the obtained ages, at one of these sites the RTL dating

was paralleled by conventional BTL dating of the same heated horizon as well as by ESR dating of land snail shells from sediments directly overlying the lava flow. 2. Study area Lanzarote is the northeastern most of the Canary Islands, stretching NNE-SSW with a maximal length of 57 km and a maximal width of 20 km. Shape and orientation of the island are linked to its location on a ridge roughly parallel to the African coast (Coello et al., 1992). Two large volcanic buildings in the south (Los Ajaches Massif) and in the north (Famara Massif) are the oldest parts of the island and originate from the Miocene (Coello et al., 1992), formerly attributed to series I (Fúster et al., 1968). Separated by a long erosional gap of some Ma, volcanic activity resumed as post-erosion volcanism and comprises former series II to IV (Fúster et al., 1968). This activity continues until the historic period (Coello et al., 1992; Instituto Tecnológico y Geominero de España, 2005). Based on geomorphological features and a limited number of datings, former series II is attributed to the Pliocene-Early Pleistocene, series III to the Early-Late Pleistocene and series IV to the Late Pleistocene-Holocene (Rothe, 1996; Carracedo et al., 2003). The volcanic chain “Las Calderetas de Guatiza” is located in the northeastern part of the island and oriented in the general NNE-SSW direction of post-erosional volcanism (Carracedo et al., 2003) (Fig. 2). It was attributed to series III by Fúster et al. (1968). Based on its geomorphological properties, the chain is suggested to represent the youngest prehistoric activity phase of the island occurring during the Latest Pleistocene and Early Holocene (Instituto Tecnológico y Geominero de España, 2005). Volcanic buildings are typical cones of basaltic tephra. Basaltic lava flows extend generally north- and eastwards, forming large volcanogenic surfaces (malpaís) and reaching the eastern coast of the island (Instituto Tecnológico y Geominero de España, 2005). 3. Studied sites 3.1. Sand pit of Mála A sand pit east of the village of Mála outcrops dunes composed of calcareous arenites that are intercalated with several silty-loamy palaeosols. Sedimentologic and magnetic properties of the outcrop were intensively investigated during former studies (Williamson et al., 2004; Damnati, 2005; Ortiz et al., 2006). At the southern wall of the sand pit (29°05’37’’N; 13°27’42’’W), a lava flow is found, overlying a palaeosol (palaeosol between samples LMA 5 and LMA 6 of Ortiz et al. (2006)) (Fig. 4a). As seen by geomorphologic features, the lava flow originates from “Montaña del Mojón”, the northernmost cone of the volcanic chain “Las Calderetas the Guatiza” located about 700 m to the southwest (cf. Meco, 2008). The flow has a thickness of ca. 1 m and is overlain by about 1.5 m of sandy sediments. The lava baked the underlying silty soil, transforming its uppermost 10 cm into a hard crust. A block of baked soil material was taken for thermoluminescence dating (MA), as well as a piece of the overlying lava for environmental dose rate estimation. Overlying sandy sediments contain land snails of the family Helicidae (Ortiz et al., 2006). Three samples for ESR dating were collected in a distance of about 30 cm above the lava flow, each consisting of about 10 specimens and being 0.5 m apart from each other, as well as surrounding sediment for environmental dose rate estimation. 3.2. Coastal plain south of Los Cocoteros Following Instituto Tecnológico y Geominero de España (2005), the lavas of the coastal plain south of the village of Los Cocoteros are associated with Lower Pleistocene volcanism of the Guanapay volcano in the northern center of the island and were ejected about 1.2 Ma ago. At the shoreline, the coastal plain is cut by a scarp with a height of about 7 m. This scarp outcrops a palaeosol of ca. 1 m thickness developed in carbonate arenites intercalated between two generations of lava flows (29°02’33’’N; 13°27’59’’W) (Fig. 4b). The overlying lava flow has a thickness of about 4 m, and a desert pavement with an underlying stone-free Bw horizon of several dm thickness is developed on its surface. The upper part of the palaeosol was baked by the lava flow and shows a brick-red colour. A blocky sample of this baked palaeosol was taken for luminescence dating (subsequently called CP), as well as a piece of the overlying lava for dose rate determination. 4. Methods 4.1. Thermoluminescence dating

Samples for thermoluminescence dating were prepared in the Bayreuth luminescence laboratory under subdued red light (wavelength 640 ± 20 nm). From sample blocks, the outer 2 cm were removed in order to obtain unbleached material. This material was wet sieved, and subsequently the fractions 63-200 µm and < 63 µm were treated with HCl and H2O2 to dissolve carbonate and organic matter. Subsequently, the material was held in an ultrasonic bath for 30 min to destroy aggregates. 4.1.1. Red thermoluminescence dating (RTL) of samples MA and CP From the 63-200 µm fractions, heavy minerals (density > 2.75 g/cm3) and feldspars (density < 2.62 g/cm3) were separated in a lithium heteropolytungstate solution (LST). The remaining material was subsequently etched in 40% HF for 45 min in order to remove any remaining feldspar as well as the alpha-irradiated outer layer of the remaining quartz grains. The etched material was given a test dose of about 5 Gy and was subsequently exposed to infrared stimulation light in order to check for feldspar contamination. Since no IRSL signals were recorded, the obtained pure quartz material was fixed on aluminium cups (diameter 12 mm) using silicone oil. Measurements of the RTL were carried out on a Daybreak 1150 TL/IRSL reader equipped with an EMI 9586Q photomultiplier using an orange-red filter combination (Oriel D630/60 and D620/75) with a maximum transmission between 600 and 660 nm (based on manufacturer’s data). Irradiation was performed using a 90Sr/90Y b-source delivering 8.81 Gy/min. To obtain the equivalent dose of a sample, a multiple aliquot regenerative protocol was used (Aitken, 1985). RTL glow curves were recorded from 240 to 450°C with a ramp rate of 2 K/s. Initially, the preheat temperature was 280°C for 15 s for both samples. In order to remove an instable RTL-peak (see below) observed between 240 and 300°C in sample CP, it was remeasured applying a preheat temperature of 300°C for 10 s. The background signal of each aliquot was recorded after taking the RTL curve in a second ramp (dual ramp method), and was subtracted thereafter to obtain the net signal. In order to obtain the natural signal and to normalize luminescence intensities of different aliquots of a sample, the natural RTL of all discs was recorded. Subsequently, the mean signal of all aliquots was used to obtain the natural RTL. Afterwards, the ratio of the individual RTL signal of an aliquot and the mean natural RTL signal was used to normalize individual regenerated aliquots in further data analysis. Due to a generally low luminescence intensity of the MA sample, 30 aliquots were used here forming 8 dose groups (0, 22.5, 45, 90, 180, 220, 360, 450 Gy), each containing between 3 and 7 aliquots. From the CP sample, 18 aliquots forming 6 dose groups (0, 45, 90, 180, 360, 720 Gy) each containing 3 discs were used. Data analysis was performed using the software TL applic 4.5. 4.1.2. Blue thermoluminescence dating (BTL) of sample MA Atterberg settling-tubes were used to enrich the fraction 4-11 µm from the sieved material < 63 µm. This material was subsequently pipetted on 9.6-mm aluminium discs. The sample was measured on a Risø-reader TL/OSL-DA-15 combined with a Thorn-EMI 9235QA-photomultiplier (Bøtter-Jensen et al., 1999) using a filter combination (BG39, GG400, 2*BG3) with a maximum transmission of 390-450 nm. β-irradiation was performed using an external 90Sr/90Y-source (7.85 Gy/min.) and α-irradiation on an external Littlemore-241Am-source (1.26 Gy/min.). To obtain the equivalent dose, a multiple aliquot regenerative and a multiple aliquot additive protocol (cf. Aitken, 1998) were used in parallel. Samples were preheated with 260°C for 60 s, and subsequently BTL glow curves were recorded up to 450°C with a ramp rate of 5 K/s. The background signal of each aliquot was automatically recorded after taking the RTL curve in a second ramp (dual ramp method), and was subtracted thereafter to obtain the net signal. 21 natural discs were measured. Subsequently, 15 of these discs were used for the multiple aliquot regenerative protocol (three dose groups each containing 5 discs: 77, 154, 307 Gy β-irradiation). The six remaining discs were used to determine the a-value (cf. Aitken, 1998) applying an alpha dose of 2933 Gy and comparing their luminescence signal with the regenerative growth curve. For the multiple aliquot additive protocol, 25 discs were used (five dose groups each containing 5 discs: 77, 154, 307, 460, 614 Gy β-irradiation). After irradiation, the discs were stored for one week at 70°C. Anomalous fading was tested by irradiating 5 discs with the highest additive β-dose of 614 Gy, and measuring them after 5 months of storage at 70°C. Data analysis was performed using the software Analyst 3.07b.

4.2. Electron spin resonance dating About 10 specimens of Helicidae shells from one sample were put together in order to obtain sufficient material for dating. The thickness of the shells was measured before and after cleaning the shells by etching with 2% HCl (see Table 3 for average values). Subsequently, the material was gently ground in an agate mortar and the grain size fraction of 100-200 µm was finally used for dating. A multiple aliquot additive dose protocol was used for equivalent dose determination based on 9 sub-samples (each of ca. 0.05 g instead of the commonly used 0.2 g due to the limited sample amount) which were irradiated using a 60Co-Source at the Centre of Nuclear Medicine at the University of Düsseldorf (dose steps: 17.8, 71.2,111.25,142.4,178, 222.5, 311,5, 400.5, 534 Gy). Thus, the maximum irradiation dose was between four to five times the De-value, which is considered to be sufficient for accurate equivalent dose determination by the additive dose protocol (cf. Schellmann et al., 2008). The ESR measurements were carried out using a Bruker ESP 300E X-band spectrometer at a frequency of 9.8 GHz, with a microwave power of 25.3 mW (Schellmann and Kelletat, 2001; Schellmann et al., 2008). The field modulation amplitude was 0.5 G with center field at 3495 G, scan width was 40 G, scan time 21 s and 5 scans in total. We used these measurement settings following Schellmann and Kelletat (2001) who successfully demonstrated the potential of ESR dating of terrestrial mollusc shells in a similar age range as expected for our study sites. The equivalent dose (De) was derived from the analysis of the dating signal at g = 2.0006 ± 0.0001 (Schellmann and Kelletat, 2001) and determined using the program “Simplex-fit” (version 1993). Age calculations were carried out using the program “ESRData V.6” (version 1999), both written by R. Grün. 4.3. Dose rates We determined natural radioactivity of sediments (baked soils for luminescence and sandy material from Mála overlying the lava flow for ESR) and lava flows by measuring concentrations of the radioactive elements U, Th and K using dry, ground material. Two methods were alternatively used: i) U, Th and K contents were calculated using neutron activation analysis conducted at the Becquerel Laboratories Mississauga/Canada. ii) U and Th contents were determined using thick-source α-counting (42 mm) at the University of Bayreuth, whereas potassium concentrations were measured at the Bayreuth Center for Ecology and Environmental Research (BayCEER) by inductively coupled plasma source mass spectrometry (ICPMS). The latter method was also used to determine internal dose rates of ESR samples at the Institute for Geology and Mineralogy at the University of Cologne/Germany. Dose rates were calculated using the conversion factors of Adamiec and Aitken (1998). ESR samples were taken in a distance > 30 cm above the Mála-lava flow so that no γ-contribution from this material had to be considered (Aitken, 1998). In contrast, thermoluminescence samples were taken nearby to the lava flows so that a γ -contribution of 60% from the sediment, and of 40% from the lava flows was used for calculations (cf. Table 1). Cosmic dose rates were determined following Prescott and Hutton (1994). For BTL measurements, the same β- and γ-dose rates as for the RTL dating were applied. However, due to the application of fine grains a γ-attenuation of 1 was used (Mejdahl, 1979). Furthermore, the contribution of the α-dose rate had to be considered, calculated using an a-value of 0.068 ± 0.003 that was determined from this sample. The effective water contents of baked soil material and the sandy material surrounding dated land snails from Mála were determined gravimetrically: The sample weight was determined before and after drying at 105°C for 24 h. However, for the moisture content variation since deposition these values are representative only to a limited extent: (i) In Mála, the sand pit has been open for several years and thus soil and sand material should have dried out during the last years. Consequently, the water content was remeasured after soaking inwater for 24 h and drying for 30 min at air temperature, what is regarded as the maximal representative water content for this material. However, due to the general semiarid climate during the Middle and Late Pleistocene (Suchodoletz et al., 2009) and the draining character of underlying sandy material, it is unlikely that this value was reached during the past. Thus, the mean value between the measured values was taken for age calculation. (ii) In the coastal plain south of Los Cocoteros, the material is now water saturated due to sea spray, but was probably much drier during most of the Middle and Late Pleistocene when sea level was lowered by up to 120 m and sea spray did not reach the material (e.g. Bard et al., 1990). Here, for age calculation the water content was remeasured after soaking the material in water for 24 h and subsequently drying at air temperature for 24 h.

4.4. XRD and microscopic analyses To get further independent indicators of the validity of our OSL-measurements (i.e. heating temperatures) the samples were analysed by X-ray diffraction (XRD) and microscope. For XRD analyses, 500 mg of the material were air dried, ground in a corundum mortar and dry sieved with 63 µm mesh width. For textural analyses, 300 mg of material were suspended in destilled water and poured on a glass slide allowing clay minerals to orientate. Measurements were executed at Dresden University of Technology with a Siemens D 5000 diffractometer (CoKα, 40 kV, 30 mA, 5-80°2θ, step scan 4 s, step size 0.03°, mount rotation speed 30 rpm) for powder samples and a SEIFERT XRD 3000 TT diffractometer (CuKα, 40 kV, 30 mA, 2.5-30°2θ, step scan 15 s, step size 0.03°, non-rotating mount) for oriented samples. The latter were measured three times after the following preparation steps: air drying, solvation with ethylene glycol (48 h) and heat treatment (2 h at 550°C). Interpretation was done using the programme Siemens Diffracplus BASIC 4.0#1 and following standard approaches (cf. Moore and Reynolds, 1997). Microscope investigations were carried out using a Nikon SMZ2T stereo microscope with variable zoom and an attached digital camera. 5. Results 5.1. Red thermoluminescence of samples MA and CP RTL glow curves from both samples show a peak between 300 and 400°C (Fig. 5, insets). Furthermore, sample CP displayed a strong peak between 270 and 300°C, in a temperature region showing no age plateau so that the sample was remeasured using a preheat of 300°C for 15 s (Fig. 5, insets). As stated by Fattahi and Stokes (2000), the former peak is stable so that a plateau between 300 and 360°C in sample MA, and between 330 and 350°C in sample CP, respectively, were used for further analysis (Fig. 5). As observed in former studies (Fattahi and Stokes, 2002), this peak slightly shifted towards higher temperatures when applying higher regenerative doses. Both samples exhibited a thermal background of about 5000 cts/s at 350°C. The signal intensity for natural aliquots from the MA sample was about 400 cts/s and thus < 10% of background intensity. The natural signal of sample CP was about 5000 cts/s and has thus the same intensity as the background value. However, the low signal-to-background ratio did not pose a severe problem since this kind of background constitutes the near-red thermal radiation of the sample and is thus systematic and reproducible. Moreover, the problem of a low signal/noise ratio of sample MA was overcome using tightly spaced dose groups containing up to 7 discs. Using a linear fit, we obtained a RTL equivalent dose of 182 ± 7.5 Gy for sample MA (Fig. 6a). A saturating exponential fit yielded an equivalent dose of 473 ± 11 Gy for sample CP (Fig. 6b). Concentrations of radioactive elements, together with β- and γ-dose rates are listed in Table 1. Due to the soil matrix interspersed with calcareous sand generally showing low radioactivity, concentrations of radioactive elements and thus dose rates are much lower in the baked soil from Mála (MA) than those from the baked soil in the coastal plain (CP). Conversely, the concentration of radioactive elements in the overlying lava flow of MA is higher than that from the lava flow of CP. Due to removal of the outer α-irradiated rim of measured fine sand grains by HF-etching, only the contributions of β- and γ-radiation had to be considered. The contribution of β-radiation was completely taken from the baked soil material since β-radiation only penetrates a few mm into the material (Aitken, 1998). For β-attenuation, a grain diameter of 140 µm was used (Mejdahl, 1979). In contrast, γ-radiation must be considered up to a radius of 30 cm around the sample (Aitken, 1998). Based on the values given by Guibert et al. (1998) and due to the fact that the carbonate arenite is porous and thus quite permeable for gamma radiation, we assumed that 60% of the γ-contribution originated from the baked soils themselves, and a contribution of 40% was delivered from the overlying lava flows. Water contents of baked soils measured after sampling yielded δ-values of 1.12 for sample MA, and 1.30 for sample CP, respectively. The water content of sample MA remeasured after soaking in water for 24 h and drying for 30 min at air temperature yielded δ = 1.20, regarded as the maximal representative water content for this material. The mean value between the measured values (1.16) was taken for age calculation since it is regarded as the most representative for the past. Renewed measurement of water contents from sample CP after soaking in water for 24 h and subsequent drying at air temperature for 24 h yielded a value of 1.08, assumed to be close to the water content during most of the Middle and Late Pleistocene period due to a generally lower sea level with subsequent dry

conditions at the sampling site. However, since a sea level as high as today existed during short periods such as the Eemian and most of the Holocene, the value of 1.08 was slightly enhanced to 1.1 for age calculation (Table 1). Resulting ages are 171.5 ± 16.6 ka for sample MA, and 167.6 ± 18.3 ka for sample CP. Despite the largely different effective dose rates for both samples, these ages agree within confidence intervals. 5.2. Blue thermoluminescence dating of sample MA Due to low natural luminescence intensity (30-40 cts/s above background), BTL glow curves show a rather diffuse peak between 150 and 300°C (Fig. 7). The low luminescence sensitivity of the material is also expressed by intensities of only 400 cts./s after additive β-irradiation of 614 Gy. However, both additive and regenerative methods yield adequate growth curves when fitted exponentially and show good luminescence plateaus between 270 and 360°C (Fig. 8). The equivalence dose determined with the multiple aliquot additive dose (MAAD) protocol is 161.7 ± 3.1 Gy, and the multiple aliquot regenerative (REGEN) protocol yields 171.6 ± 26.5 Gy (Table 2). The larger uncertainty of the regenerative protocol, especially for low doses, is explained by the low luminescence intensity. Apparent BTL ages are 135.7 ± 10.5 ka for the MAAD, and 144.0 ± 21.7 ka for the REGEN method. Anomalous fading was about 14% after storage of 5 months at 70°C. 5.3. Electron spin resonance dating As only 9 doses could be measured due to the limited sample amount, the De-Dmax plot procedure (Schellmann and Radtke, 1999) could not be applied here. For testing the reproducibility of the results, ESR-samples 2 and 3 were remeasured after ca. 12 months using the same measurement settings as used for the first measurement. Fig. 9 shows additive growth curves for ESR-samples 2 and 3 plotted for the second De-measurement. The De-values of the first and the second ESR-measurement agree within 1σ-errors with ratios 1st/2nd measurement of 0.92 and 0.98 for samples ESR-2 and -3, respectively. For both measurements only limited precisions of 10-14% (relative 1σ-error) were obtained. In order to test whether the low precision of the De-values could be improved by incorporating more dose steps to construct a more robust dose response curve, eight subsamples of ESR-samples 2 and 3 were irradiated a second time in addition to the already administered dose. This approach was necessary since no further sample material was available. The natural sub-sample was re-measured for normalisation of the second measurement. The deviation in case of sample 2 was only 4%, but 19% in case of sample 3. With only one value available for normalisation this approach is considered not to be very robust and the results obtained for the extended growth curve are regarded as rough dose estimates only. Therefore, final age calculation was based on the first dose measurement results. However, the equivalent dose values calculated for the extended growth curves (17 dose points, maximum dose 890 Gy) agree with the original De-values within the 1σ-error range: 144 ± 12 Gy instead of 122 ± 17 Gy (ESR 2) and 101 ± 11 Gy instead of 94 ± 10 Gy (ESR-3). Additionally, we calculated De-values only all even (dose step 2, 4, 6 etc.) and uneven dose steps (step 1, 3, 5 etc.), respectively. Still, all results overlap with their 1s-errors; sample ESR-2: 134 ± 16 Gy (even steps) and 150 ± 13 Gy (uneven steps), ESR-3: 104 ± 16 Gy (even steps) and 98 ± 10 Gy (uneven steps). Although the precision of the De-estimates for samples ESR-2 and -3 could not be improved by inclusion of more dose steps, these results support the accuracy of the ESR-measurements. The relevant parameters used here for age calculation are summarized in Table 3. The water content of the surrounding sandy material was measured as d = 1.09. Most pores in sand are wide pores. They have low field capacity and were thus never permanently filled with water (Scheffer and Schachtschabel, 1998). Therefore, the measured value was regarded as representative for the past. The most striking results as shown in Table 3 are the large differences in radionuclide contents and thus in dose rates amongst the three different samples, particularly bearing in mind that all of them were collected from the same sediment layer. These sediments comprise a mixture of autochthonous carbonate rich sands, volcanic ashes and allochthonous dust input from the Sahara. Determination of the effective dose rate for each individual shell is thus very imprecise. In addition, the pooling of a certain number of shells to obtain a critical sample amount for ESR measurement introduces a further source of uncertainty. A correlation of the equivalent dose to the effective dose rate is not straightforward in this study. Finally, both the equivalent dose as well as the dose rate values have to be considered as average estimates. Hence, the calculated ages should be interpreted accordingly.

However, ESR-samples 2 and 3 agree within 1σ-errors errors, thus supporting a rough age estimate for the deposit of 136 ± 16 ka (error weighted mean). 5.4. XRD and microscopic analyses X-ray diffractograms show the presence of aeolian quartz and kaolinite in both samples. Aeolian muscovite was detected in the CP sample (cf. Mizota and Matsuhisa, 1995; Menéndez et al., 2007). Volcanogenic minerals were also identified, i.e. augite in the CP sample and olivine and pyroxene in the MA sample. The latter showed an overall richer mineralogic composition including anorthite, calcite and aragonite patterns, whereas in the CP sample hematite was identified. The mineralogical composition is resembled by microscope analyses. Both samples show quartz grains with a size of about 100 µm and prominent features of thermal stress. The MA sample shows features of chemical weathering: Basaltic fragments are engulfed by pale grey, loamy aggregates formed by clay translocation. These aggregates exhibit red to dark grey, glassy coatings as a result of thermal exposure (Fig. 10a). The CP sample shows silt sized grains surrounded by an intensive reddish coating integrated into a continuous, glassy matrix, due to heating by the lava flow (Fig. 10b). The matrix shows channels and caverns of some 100 µm in diameter filled with or coated by halite crystals, partly showing clustered aggregates (Fig. 10c). 6. Discussion Heating temperatures of ca. 450°C caused by overlying lava flows were described for soils on Tenerife by Benayas et al. (1987). Similarly, for the investigated samples heating temperatures were lower than 550°C as demonstrated by the presence of kaolinite. Its mineralogic structure would have been destroyed above such temperatures (Smykatz-Kloss,1974). The presence of kaolinite prior to heating is demonstrated by the overprint of loamy aggregates by heating features in sample MA (Fig. 10a). However, RTL as well as BTL luminescence plateau tests show that temperatures overprinting the original soil structures were sufficient to reset the thermoluminescence signals (Figs. 5 and 8). Thus, the application of thermoluminescence dating techniques to the baked soils is justified. Generally, a high thermal background is a limiting factor for the application of the RTL technique to samples with young ages or weak RTL signals as that from Mála (MA) with a geological RTL signal < 10% of the thermal background (e.g., Miallier et al., 1991). However, since thermal backgrounds and geological RTL signals for individual aliqutos of this sample were well reproducible, this shows that the signals are systematic rather than the results of statistical error. Thus, this sample could be dated in spite of the low signal/background ratio. Interestingly, the Mála sample also showed a very weak BTL-signal intensity. The RTL age of 172 ± 17 ka for the baked MA soil agrees well with the RTL age of 168 ± 18 ka for the baked CP soil. However, when looking at the parallel BTL and ESR ages from Mála, both show some disagreement with the RTL age (Fig. 11): Taking the additive and regenerative BTL ages of 136 ± 11 ka and 144 ± 22 ka, respectively, they agree within error bars but underestimate the RTL age by about 25%. This underestimation can be explained by the detected anomalous fading of the BTL-feldspar signal (14%), similar to fading rates reported from other luminescence measurements of feldspar from Lanzarote (Suchodoletz et al., 2008). Thus, the BTL ages, if regarded as minimum ages, support the Middle Pleistocene RTL age. Looking at the ESR ages derived from land snail shells from the sandy horizon overlying the lava flow (204 ± 36 ka, 123 ± 18 ka and 146 ± 17 ka), they show errors up to 18% and additionally strongly deviate from each other. This is probably caused by low signal intensities because of very small sample amounts and thus a strong vulnerability to even small dosimetric inhomogenities. An obvious correlation between sediment U-content and the ESR-ages could indicate to U-mobility after the burial of the snails, causing radioactive disequilibrium (Table 3). However, the trend in U-concentrations is parallelled by K (Table 3), an element immobile after burial. This demonstrates that there was no U-mobility after burial, and instead the sediment surrounding sample ESR-1 shows a generally lower radionuclide content probably due to a lower concentration of volcanogenic minerals. Thus, all ESR-ages are not influenced by radioactive disequilibria. The ESR ages bracket the RTL age and thus support it by an independent method. Taken together, both the RTL age and ESR results thus confirm a late Middle Pleistocene age for the lava flow and thus the whole sand pit in Mála, in agreement with former observations of Meco (2008). This is much older than the upper Late Pleistocene age assumed by Ortiz et al. (2006) based on 14C-dating. The good agreement of the Mála-age with the RTL-age of 166 ± 18 ka from the coastal plain south of

Los Cocoteros points to a common origin of both lava flows from the same phase of volcanic activity about 170 ka ago, probably associated´with the “Las Calderetas de Guatiza” volcanic chain. On the other hand, our ages demonstrate that the attribution of the CP-lava flow to the Lower Pleistocene Guanapay volcanism (Instituto Tecnológico y Geominero de España, 2005) that was about 1.2 Ma ago can not be supported. Instead, an origin of the lava from the “Las Calderetas des Guatiza” chain can be assumed (The southernmost cone of this chain, “La Caldera”, is situated about 2 km to the north). This can be supported by field observations: Several lava flows originating from the “La Caldera” volcano extend towards the south and the east, although due to a complicated topography with several overlying lava flows their extension until the sampling site can not clearly be demonstrated. A cañon (Barranco de la Espoleta) is incised up to 6 m into the lava field between the “La Caldera” cone and the dated site (Fig. 2). This points to a much faster Quaternary erosion rate at this site (about 3.5 cm/ka) than the value of 0.75-1.2 cm/ka supposed by Höllermann (2006) based on the observation of similar barrancos in the northern and eastern part of the neighbouring island Fuerteventura. Our results demonstrate on the one hand that an assumed Latest Pleistocene/Early Holocene age derived from geomorphological observations of the “Las Calderetas de Guatiza” volcanic chain (Instituto Tecnológico y Geominero de España, 2005) is obviously strongly underestimated. On the other hand, the assumed Early Pleistocene age of the lava flow in the coastal plain is strongly overestimated (Instituto Tecnológico y Geominero de España, 2005). In agreement with Carracedo et al. (2003), this demonstrates that due to the arid climate of the island and thus low weathering intensity geomorphologic features are in general not appropriate for age estimations of volcanites on the Eastern Canary Islands. Similar problems with age estimations of lava flows based on geomorphic features are also reported from the Ice Springs volcanic field in the semi arid Black Rock desert, Utah, USA (Hoover, 1974; Oviatt, 1991). Likewise, 14C-datings of mollusc shells from the sand pit of Mála are obviously strongly underestimated since they yield ages of ca. 30 ka cal. BP for the baked Middle Pleistocene soil horizon (Ortiz et al., 2006). These authors argue that a similar age difference observed in Fuerteventura between 14C ages and much older OSL-ages (Bouab and Lamothe, 1997) is caused by an incomplete bleaching of these sediments during transport. In this study, this argument can be disproved by two facts: i) the luminescence signal of baked soils was zeroed by heat instead of light. Microscopic analyses and plateau tests show that the samples were strongly heated and thus the reset of the TL signal was sufficient. ii) TL and ESR dating are based on different principles but both yield late Middle Pleistocene age estimations for the same lava flow. Thus, these results confirm the finding of Meco et al. (2002) that 14C-ages from the eastern Canary Islands beyond the Holocene period are generally not reliable. Accordingly, general problems of the 14C-dating-technique in arid regions were also reported by Singhvi and Krbetschek (1996). 7. Conclusions Although only two lava flows have been dated so far, the results of our study from the “Las Calderetas de Guatiza” volcanic chain yielding an age of ca. 170 ka fill a conspicuous gap in the volcanic chronostratigraphy of Lanzarote between about 90 and 240 ka. This shows that post-erosional volcanism on this island was apparently more continuous during the Middle and Late Quaternary than known before (Fig. 11). As seen by the high saturation level of the RTL signal in our study, dating of baked soils by red thermoluminescence has a great potential to further enlarge and refine the volcanic stratigraphy of the Canary Islands for a large part of the Quaternary (Fattahi and Stokes, 2000), and thus offers a complementary dating method to Ar/Ar or K/Ar dating. The latter often show large uncertainties for ages < 100-200 ka (cf. Carracedo et al., 2003). Thus, RTL can help to improve the timing of the Quaternary volcanism of the Canary Islands. ESR also holds a potential of dating volcanic activity by dating fossil shells in sediments bracketing volcanic products. Acknowledgements Financial support for this work was given by Deutsche Forschungsgemeinschaft (project Zo 51/29-1). Gunter Ilgen (Bayreuth Center for Ecology and Environmental Research, Bayreuth/Germany) is thanked for ICPMS analyses of K-contents.

We thank Ulrich Hambach for his help during field work, and Manfred Fischer for his help during sample preparation in the laboratory (both University of Bayreuth, Germany). Sarah Rittner is thanked for ESR-measurements. References

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Tables Table 1 Dose rates, water contents and a-value of luminescence measurements. The γ-dose rate is composed of a contribution from baked soil and from the overlying lava flow (assuming a proportion of 60:40%). Sample Baked soil Mála

(sample MA)

Lava Mála Baked soil coastal plain (sample CP)

Lava coastal plain

U-content (ppm) 0.80 ± 0.023a 1.23 ± 0.027a 1.66 ± 0.049a 0.85 ± 0.058a

Th-content (ppm) 2.76 ± 0.08a 4.20 ± 0.09a 5.7 ± 0.17a 2.9 ± 0.165a

K-content (%) 0.58 ± 0.017b 1.29 ± 0.039b 2.49 ± 0.075b 0.78 ± 0.123b

β-dose rate (Gy/ka) 0.608 ± 0.014c - 2.327 ± 0.057c -

γ-dose rate (Gy/ka) 0.363 ± 0.006 0.652 ± 0.011 1.064 ± 0.206 0.424 ± 0.032

Total γ-dose rate (Gy/ka)

0.479 ± 0.008c - 0.808 ± 0.14c -

Cosmic dose rate (Gy/ka)

0.15 ± 0.01c - 0.1075 ± 0.005c -

Water content

1.12d, 1.20e, 1.16f c - 1.3d, 1.1g c -

a-value (blue TL)

0.068 ± 0.003c - - -

a measured with thick-source α-counting. b measured with ICPMS. c values used for age calculation. d water content obtained from fresh material. e water content obtained after soaking material for 24 h in water and drying in air for 30 min. f average value of d and e. g water content obtained after soaking material for 24 h in water and subsequent drying in air for 24 h (= 1.08), taken as the average water content during dry periods. Due to some shorter periods of high sea level and thus wet conditions at the coast during the past, this value was slightly enhanced to 1.1.

Table 2 Equivalent doses and ages of luminescence measurements. Equivalent doses are marked italic, and ages in bold. Sample Baked soil Mála (sample MA)

Baked soil coastal plain (sample CP)

Red thermoluminescence

182 ± 7.5 Gy 473 ± 11 Gy

(coarse grain regenerative)

171.5 ± 16.6 ka 167.6 ± 18.3 ka

Blue thermoluminescence

161.7 ± 3.1 Gy -

(fine grain addivitive)

135.7 ± 10.5 -

Blue thermoluminescence

171.6 ± 26.5 Gy -

(fine grain regenerative)

144.0 ± 21.7 ka -

Table 3 Dose rate data, equivalent doses and resulting ESR-ages for site Mála. Sample

ESR-1 ESR-2 ESR-3

U-content sediment (ppm)

0.78 ± 0.017a 1.21 ± 0.09c 1.09 ± 0.10c

Th-content sediment (ppm)

2.66 ± 0.06a 3.80 ± 0.23c 2.80 ± 0.20c

K-content sediment (%)

0.40 ± 0.01b 0.99 ± 0.14c 0.67 ± 0.11c

U-content internal (ppm)

0.17 ± 0.01b 0.23 ± 0.01b 0.12 ± 0.01b

Alpha-efficiency

0.1 ± 0.02 0.10 ± 0.02 0.10 ± 0.02

Thickness (mm)

824 ± 100 950 ± 100 1770 ± 100

Removed thickness (mm)

224 ± 60 130 ± 60 930 ± 100

Density (g/cm3)

2.95 2.95 2.95

Cosmic dose rate (Gy/ka)

0.1914 ± 0.01 0.1914 ± 0.01 0.1914 ± 0.01

Water content

1.09 1.09 1.09

Equivalent dose (Gy)

132 ± 23 122 ± 17 94 ± 10

Dose rate (Gy/ka)

0.65 ± 0.02 0.93 ± 0.05 0.64 ± 0.03

Age (ka)

204 ± 36 123 ± 18 146 ± 17 a measured with thick-source α-counting. b measured with ICPMS. c measured with NAA.

Figures: Figure 1: Overview of the Canary Islands region with Lanzarote. The Atlas Orogene in NW-Africa is shown by broken lines. The assumed hot spot track is redrawn from Carracedo et al. (1998).

Figure 2: Major and minor volcanic eruption centers on Lanzarote. The NE-SW directed volcanic chain “Las Calderetas de Guatiza” in the NE of the island is highlighted with a frame. The inset shows a 3-D blowup of this chain with sampling sites (I = in the coastal plain (sample CP), II = Mála sand pit (sample MA)).

Figure 3 Published numerical ages of volcanic eruptions from different studies from Lanzarote for the last 1 Ma.

Figure 4 Sampling sites for luminescence and ESR dating. a) Mála (MA) b) coastal plain south of Los Cocoteros (CP).

Figure 5 RTL-equivalent dose plateaus from 240 to 450°C. Insets show typical natural RTL-glow curves from both samples (including a preheat of 240°C in Mála, and of 300°C in the sample from the coastal plain). a) Mála (MA) b) coastal plain south of Los Cocoteros (CP).

Figure 6 Regenerated RTL-growth curves. a) Mála (MA) b) coastal plain south of Los Cocoteros (CP).

Figure 7 Natural BTL-glow curve from Mála.

Figure 8 BTL-growth curves and luminescence plateaus (insets) from Mála a) additive method b) regenerative method.

Figure 9 ESR-growth curves of samples ESR-2 (a) and ESR-3 (b).

Figure 10 Microscopic images of heated soils. a) Mála (MA): red, glassy coatings on pale grey, loamy aggregates. b) Coastal plain (CP): red-coated grains on a continuous glassy matrix c) Coastal plain (CP): salt crystallisations on channels and caverns in the glassy matrix.

Figure 11 Ages of volcanic eruptions from former studies from Lanzarote (circles) compared to the results from this study (triangle). The blowup shows the temporal distribution of datings from this study with error bars (open: ESR ages, black: RTL ages, grey: BTL ages) and the corresponding 14C-age of Ortiz et al. (2006) (open diamond).

Figure 6 Regenerated RTL-growth curves. a) Mála (MA) b) coastal plain south of Los Cocoteros (CP).