A comparative study of sand- and silt-sized quartz fractions for MAR-VSL dating using loess-palaeosol deposits in southern Germany

Multiple-aliquot regenerative-dose violet stimulated luminescence (MAR-VSL) dating studies of the Chinese loess-palaeosol sequence in Luochuan using sand- and silt-sized quartz have previously produced inconsistent results; the VSL ages were in agreement with their independent ages up to ∼900 ka for sand-sized quartz, whereas the silt-sized VSL ages underestimated the independent chronology beyond ∼100 ka. Here we therefore evaluate the VSL dose response pattern of sand- (63–100 μm) and silt-sized (4–11 μm) quartz grains from the loess-palaeosol sequence in southern Germany in high resolution but with a limited age range up to ∼160 ka. All the samples studied benefit from good age control provided by reliable quartz optically stimulated luminescence (OSL) ages and fading corrected feldspar post-infrared infrared stimulated luminescence at 225 °C (pIRIR₂₂₅) ages, which can be used for assessing the validity of the estimated VSL ages. The comparison of the MAR standardised dose response curve (DRC) using regeneration doses up to ∼1000 Gy for both grain size fractions demonstrates that they are almost similar in shape with comparable characteristic saturation doses. The comparison of the natural and laboratory generated DRCs of each grain size reveals that they broadly overlap in the low dose range for both fractions, while in the high dose range the deviation between natural and laboratory DRCs is higher for the silt-sized quartz fraction. It is also shown that the magnitude of the characteristic saturation dose is dependent upon the size of the maximum given dose, especially for the silt-sized quartz. The constructed laboratory standardised DRCs to very high doses (up to ∼6000 Gy) showed continuous signal growth at high doses, particularly in the case of silt-sized quartz grains, thereby confirming our previous observation. The sand-sized quartz has a much less pronounced linear growth component and can therefore be considered more suitable for dating samples with equivalent doses falling on the high dose region of the DRC.     

Potential of establishing a ‘global standardised growth curve’ (gSGC) for optical dating of quartz from sediments

We report investigations of the optically stimulated luminescence (OSL) signals of sedimentary quartz from different regions of Asia, Africa, Europe and North America using a single-aliquot regenerative-dose (SAR) procedure. We show that variations in the shape of dose response curves (DRCs), or growth curves, of the test dose sensitivity-corrected OSL signals among single aliquots composed of multiple grains can be greatly reduced by normalising the DRCs using one of the regenerative dose OSL signals. We refer to this regenerative-dose normalisation procedure as ‘re-normalisation’. We find a common re-normalised DRC extends to doses of ∼250 Gy for samples that differ significantly in terms of geological provenance, sedimentary context and depositional age. This feature permits the development of a ‘global standardised growth curve’ (gSGC) for OSL signals from single aliquots of quartz. The equivalent dose (D_e) of an aliquot can be estimated from the natural signal, one regenerative dose signal and their corresponding test dose signals. For the variety of samples investigated, we find that D_e estimates obtained from the gSGC are consistent with those obtained using full SAR procedures for doses of up to ∼250 Gy. Use of the gSGC for single aliquots would greatly reduce the time required to estimate the D_e values of older samples and for a large number of aliquots.     

The build-up of the laboratory-generated dose-response curve and underestimation of equivalent dose for quartz OSL in the high dose region: A critical modelling study

The application of a regeneration procedure for optically stimulated luminescence (OSL) dating requires that the dose-response curve (DRC) of a natural sample is the same as that of a laboratory-generated one. However, the build-up of the laboratory-generated DRCs of quartz has been widely reported in the literature, i.e., the laboratory-generated DRCs are significantly higher than the natural counterparts in the high dose region (above 150 or 200 Gy). This results in severe underestimation of equivalent dose (D_e) for quartz OSL in the high dose region during the application of a single-aliquot regenerative-dose (SAR) protocol. However, the potential mechanism governing the build-up of the laboratory-generated DRC is still unclear. In this study, we performed a comprehensive investigation of the natural and laboratory-generated OSL signals and DRCs using a kinetic model for quartz. We compared the differences in charge concentrations between natural and laboratory-irradiated aliquots following irradiation and monitored the competition for holes and electrons during preheat and stimulation, for the natural, regenerative, and test dose cycles. In the course of the modelling, we could see the build-up of laboratory-generated DRCs, the underestimation of D_e, and a double exponential saturation characteristic of the DRCs. We demonstrated a discrepancy in competition for electrons in the deep electron trap and recombination centres during stimulation between the natural, regenerative, and test dose cycles. The simulation results are directly relevant to quartz OSL D_e determination using the SAR protocol and reveal the mechanisms responsible for the experimentally observed different behaviours between natural and laboratory-generated DRCs.     

Variability in quartz OSL signals caused by measurement uncertainties: Problems and solutions

We simulated the variability in measured quartz optically stimulated luminescence (OSL) signals and dose response curves (DRCs) caused by measurement uncertainties, including counting statistics and instrumental irreproducibility. We find that these measurement errors can give rise to large variations in the observed luminescence signal and contribute to among-aliquot or among-grain scatter in DRCs and equivalent dose (D_e) values. Different measurement systems (i.e., luminescence readers) may have different counting statistics properties and, hence, may exhibit differing extents of variation in the observed OSL signal, even for the same sample. Our simulation shows that the random measurement uncertainties may result in some grains or aliquots being ‘saturated’ (that is, the measured natural signal is consistent with, or lies above, the saturation level of the measured DRC) and that the rejection of these ‘saturated’ grains may result in a truncated D_e distribution, with D_e underestimation for samples with natural doses close to saturation (e.g., twice the characteristic saturation dose, D₀). We propose a new method to deal with this underestimation problem, in which standardised growth curves (SGCs) are established and the weighted-mean natural signal (L_n/T_n) from all measured grains is projected on to the corresponding SGCs to determine D_e. Our simulation results show that this method can produce reliable D_e estimates up to 5D₀, which is far beyond the conventional limit of ∼2D₀ using the standard SAR procedure.     

Construction of a ‘global standardised growth curve’ (gSGC) for infrared stimulated luminescence dating of K-feldspar

We investigated the infrared stimulated luminescence (IRSL) and post-infrared IRSL (pIRIR) signals emitted by K-feldspars from sedimentary samples from Asia, Europe and Africa using a single-aliquot multiple elevated temperature (MET) stimulation procedure. For separate aliquots of the same sample, we show that variation among the dose response curves (DRCs), or growth curves, constructed from the regenerative dose signal (L_x), the test dose signal (T_x, an indicator of luminescence sensitivity) and the sensitivity-corrected signal (L_x/T_x) can be largely eliminated by normalising the DRCs using one of the regenerative dose signals; we call this procedure ‘regenerative-dose normalisation’ or re-normalisation. Furthermore, for the MET-pIRIR signals measured at 250 °C, we find that different samples have re-normalised DRCs that follow the same growth function, despite the samples differing significantly in terms of their geological provenance, sedimentary context, equivalent dose (D_e) and luminescence sensitivity. This common feature offers the potential to establish a ‘global standardised growth curve’ (gSGC) for different samples of K-feldspar, and thereby enable D_e values to be estimated for a large number of single aliquots by projecting the re-normalised natural signals on to the gSGC. For the 18 samples investigated in this study, we find that D_e estimates obtained from the gSGC are consistent with those obtained using full single-aliquot regenerative dose (SAR) procedures for doses of up to ∼1600 Gy. The establishment of a gSGC would greatly reduce the time required to date older samples using K-feldspar, as regenerative doses of several hundreds to a few thousands of Gy are typically delivered to each aliquot in each SAR cycle.     

Investigation of the applicability of standardised growth curves for OSL dating of quartz from Haua Fteah cave,Libya

The establishment of standardised growth curve (SGC) for equivalent dose (D_e) determination can substantially reduce the amount of instrument time required for OSL measurements. In this study, we investigated the applicability of SGC for the optically stimulated luminescence (OSL) signal from single grains and small aliquots of sedimentary quartz from Haua Fteah cave, in Libya. The samples exhibit large inter-grain and inter-aliquot variation in the shape of their single grain and small aliquot dose response curves (DRCs) constructed from a range of sensitivity-corrected regenerative dose signals (L_x/T_x), which prevents the establishment of a single common SGC among different grains or small aliquots. Instead, the DRCs for the small aliquots can be divided into a minimum of three groups using the Finite Mixture Model, with the DRC saturating at a different dose level for each group. In order to establish a common DRC, or SGC, for each group, we propose a new normalisation method, the so-called least-squares normalisation procedure (‘LS-normalisation’), which largely reduces the variation between aliquots within the same group and allows the establishment of a common DRC, or SGC, for each group. In order to apply the SGC method for these samples, two regenerative dose points are needed for each aliquot to attribute it to one of the groups based on the ratio of the L_x/T_x signals for two sensitivity-corrected regenerative dose points. Equivalent dose (D_e) values for each aliquot can be estimated using the fitting parameters calculated for the SGC of the relevant group to which it belongs, together with measurements of the natural signal (L_n), one regenerative dose signal (L_x1) and their corresponding test dose signals (T_n, T_x1). For the samples investigated from Haua Fteah, we found that D_e estimates obtained from the SGCs are consistent with those obtained using a full SAR procedure. Our results suggest that small single-aliquot and single-grain D_e values obtained from application of the SGC may be underestimated if there is a significant proportion of early-saturating grains present in the sample; such grains or aliquots are mostly rejected due to saturation when analysed using the full SAR procedure. In this case, it is necessary to calculate the D_e values based only on those grains or aliquots that have relatively high saturation levels.     

EPR investigation of the role of germanium centers in the production of the 110°C thermoluminescence peak in quartz

The identification of the center acting as electron source for the well-known 110 °C thermoluminescence (TL) peak of quartz is of fundamental importance for practical applications in dating and dosimetry. This TL peak was studied in parallel with the electron paramagnetic resonance (EPR) signal of the [GeO₄]⁻ center on natural colourless quartz irradiated at room temperature. Immediately after irradiation, the signals of the 110 °C TL peak and of [GeO₄]⁻ center decay exponentially in the same way, yielding a lifetime of 50.4 ± 0.9 min at room temperature. Besides, we acquired the isothermal decay curves for the 110 °C TL peak and [GeO₄]⁻ center at different temperatures (the samples were held at the selected temperature in the range of 260–308 K). The lifetimes extracted by the isothermal decays were plotted as a function of reciprocal temperature, revealing again the same behavior of the 110 °C TL peak and [GeO₄]⁻ center, both characterized by activation energies very close of 0.76 ± 0.07 eV and 0.77 ± 0.07 eV, respectively. All results of the present work clearly show the role of electron source of [GeO₄]⁻ center in the emission mechanism of the 110 °C TL peak in quartz.     

Maximum age limitation in luminescence dating of Chinese loess using the multiple-aliquot MET-pIRIR signals from K-feldspar

Numerical dating of loess is important for Quaternary studies. Recent progress in post-infrared infrared-stimulated luminescence (pIRIR) signals from potassium-rich feldspar has allowed successful dating of Chinese loess beyond the conventional dating limit based on quartz optically stimulated luminescence (OSL) signals. In this study we tested the multiple-aliquot regenerative-dose (MAR) pre-dose multiple-elevated-temperature post-IR IRSL (pMET-pIRIR) procedure on samples from the palaeosol S5 (∼480 ka) and S8 (∼780 ka) layers from the Luochuan and Jingbian sections, respectively. The results show that (1) compared to sensitivity-corrected signal (L_x/T_x), a higher saturation dose is observed for the sensitivity-uncorrected MET-pIRIR signals (L_x), suggesting that MAR is advantageous for dating old samples; (2) negligible fading component can be achieved using the pMET-pIRIR procedure; (3) for the sample from the top of palaeosol S5, D_e values (1360 + 226/-167 Gy) broadly consistent with expected D_e (1550 ± 72 Gy) can be obtained using the sensitivity-uncorrected 300 °C MET-pIRIR signal. Our study suggests that a D_e value of about 1800 Gy may be the maximum dating limit of Chinese loess using the MAR pMET-pIRIR procedure.     

Extending the age limit of luminescence dating using the dose-dependent sensitivity of MET-pIRIR signals from K-feldspar

We investigated the sensitivity change of multiple-elevated-temperature (MET) stimulated post-infrared infrared-stimulated luminescence (MET-pIRIR) signals as a response to irradiation, sunlight bleaching and heating using samples from the Mu Us Desert, central China. A strong dose dependence of MET-pIRIR signal sensitivity was observed. The intensity of the test-dose signals (T_x) increase with the pre-dose received. Furthermore, the signal sensitivity can be reset by sunlight bleaching or heating. This suggests that both the electron traps and hole centres in K-feldspar can be bleached by sunlight, and can, therefore, be used for dating. Using the test-dose signal as a monitor for sensitivity change, it was found that the sensitivity (or hole centres) saturate at a higher dose (D₀ = ∼750 Gy) than the sensitivity-corrected signals (or electron traps) (D₀ = ∼400 Gy). We propose a multi-aliquot regenerative-dose (MAR) MET-pIRIR dating protocol, which utilises the high saturation dose of hole centres. This protocol was tested using aeolian sediments from north China with ages ranging from 0 to 470 ka. It was found that, compared to the dose limit of ∼800–1000 Gy using the normal MET-pIRIR or pIRIR procedure, the new method can measure a natural dose of up to ∼1500 Gy and produce ages consistent with the expected ages for the samples investigated.     

Laboratory and numerical model studies of a negatively-buoyant jet discharged horizontally into a homogeneous rotating fluid

Abstract

The results of laboratory experiments and numerical model simulations are described in which the motion of a round, negatively-buoyant, turbulent jet discharged horizontally above a slope into a rotating homogeneous fluid has been investigated. For the laboratory study, flow visualisation data are presented to show the complex three-dimensional flow fields generated by the discharge. Analysis of the experimental data indicates that the spatial and temporal developments of the flow field are controlled primarily by the lateral and vertical discharge position of the jet (with respect to the bounding surfaces of the container of width W) and the specific momentum (M ₀) and buoyancy (B ₀) fluxes driving the jet. The flow is seen to be characterised by the formation of (i) a primary anticyclonic eddy (PCC) close to the source, (ii) an associated secondary cyclonic eddy (SCE) and (iii) a buoyancy-driven bottom boundary current along the right side boundary wall. For the parameter ranges studied, the size L _{p, s} and spatial location x _{p, s} of the PCC and SCE (and the nose velocity u _N of the boundary current) are shown to be only weakly-dependent upon the value of the mixed parameter M ₀Ω/B ₀, where Ω is the background rotation rate. Both L _p and x _p are shown to scale with the separation distance y?/W of the right side wall (y = 0) from the source (y = y?), both L _s and x _s scale satisfactorily with the length scale l _M (= M ₀ ^3/4/B ₀ ^½) and u _N is determined by the appropriate gravity current speed [(g']₀ H]^½ and the separation distance y?/W.

Numerical model results show good qualitative agreement with the laboratory data with regard to the generation of the PCC, SCE and boundary current as characteristic features of the flow in question. In addition, extension of the numerical model to

diagnose potential vorticity and plume thickness distributions for the laboratory cases allow the differences in momentum-and buoyancy-dominated flows to be clearly delineated. Specifically, the characteristic features of the SCE are shown to be strongly dependent upon the value of M ₀Ω/B ₀ for the buoyant jet flow; not least, the numerical model data are able to confirm the controlling role played by the boundary walls in the laboratory experiments. Quantitative agreement between the numerical and laboratory model data is fair; most significantly, the success of the former model in simulating the dominant flow features from the latter enables the reliable extension of the numerical model to be made to cases of direct oceanic interest.     

Equilibrium approaches to natural water systems—Part 2

The apparent equilibrium constant, K ₁ ^′ , for the first acidity constant of carbonic acid has been determined in a lake water of 1.6 mM ionic strength in the temperature interval 4–18°C. The experimental method used comprises pH measurements in situ (NBS scale) with a probe and an IR-method for the selective determination of [H₂CO₃] and [HCO ₃ ^? ] in water samples. In the temperature interval studied the results can be described by the equation logK ₁ ^′ =126.39?6320.81/T?19.5682 ln T and are in agreement with the values of K ₁ ^′ obtained by the empirical equation presented by Millero [15]. This experimental agreement justifies thermodynamic calculations in the carbonate system based on field data. Also the experimental method described can be used to evaluate the acid-base balance of organic rich natural waters.     

Environmental Factors Associated With Natural Methane Occurrence in the Appalachian Basin

The recent boom in shale gas development in the Marcellus Shale has increased interest in the methods to distinguish between naturally occurring methane in groundwater and stray methane associated with drilling and production operations. This study evaluates the relationship between natural methane occurrence and three principal environmental factors (groundwater redox state, water type, and topography) using two pre‐drill datasets of 132 samples from western Pennsylvania, Ohio, and West Virginia and 1417 samples from northeastern Pennsylvania. Higher natural methane concentrations in residential wells are strongly associated with reducing conditions characterized by low nitrate and low sulfate ([NO₃^?] < 0.5 mg/L; [SO₄^2?] < 2.5 mg/L). However, no significant relationship exists between methane and iron [Fe(II)], which is traditionally considered an indicator of conditions that have progressed through iron reduction. As shown in previous studies, water type is significantly correlated with natural methane concentrations, where sodium (Na) ‐rich waters exhibit significantly higher (p<0.001) natural methane concentrations than calcium (Ca)‐rich waters. For water wells exhibiting Na‐rich waters and/or low nitrate and low sulfate conditions, valley locations are associated with higher methane concentrations than upland topography. Consequently, we identify three factors (“Low NO₃^? & SO₄^2?” redox condition, Na‐rich water type, and valley location), which, in combination, offer strong predictive power regarding the natural occurrence of high methane concentrations. Samples exhibiting these three factors have a median methane concentration of 10,000 µg/L. These heuristic relationships may facilitate the design of pre‐drill monitoring programs and the subsequent evaluation of post‐drill monitoring results to help distinguish between naturally occurring methane and methane originating from anthropogenic sources or migration pathways.     

Pretreatment of Olive Oil Mill Wastewater by Two Different Applications of Fenton Oxidation Processes

The removal of chemical oxygen demand (COD) and phenol from olive oil mill wastewaters (OOMW) was investigated experimentally by using conventional Fenton (CFP) and Fenton type processes (FTP) with zero valent iron (ZVI). Different operational parameters such as initial pH, Fe²⁺, Fe⁰, and H₂O₂ concentrations were examined. Kinetic studies in terms of COD and phenol removals for both CFP and FTP were performed. The original pH value (4.6) of OOMW for CFP was found as the optimum pH. The determined optimum conditions are [Fe²⁺] = 1500 mg L^?1, [H₂O₂] = 1750 mg L^?1, and pH = 4.6 for CFP; [Fe⁰] = 2000 mg L^?1, [H₂O₂] = 2000 mg L^?1, and pH = 3 for FTP. 82.4% COD and 62% phenol removals were performed under the optimum conditions by CFP, while 82% COD and 63.4% phenol were removed by FTP. According to the results of kinetic studies, it was observed that COD and phenol were removed by FTP more rapidly, compared to CFP. Consequently, it was determined that both CFP and FTP were effective processes for the pretreatment of OOMW.     

The removal of aqueous silica from dilute aqueous solution

The adsorption characteristics of orthosilicate on hydrous γ-Al₂O₃ from dilute aqueous solution (10^?4 to 10^?3 M) at constant ionic strength (0.1M NaCl) was investigated in the laboratory.The extent of adsorption is affected by pH, concentration of silicate, or the relative concentration of silicate and oxide surface. With short reaction time (24 hours), adsorption is the major interfacial reaction and follows an S-type adsorption isotherm. The steepness of the S-curve isotherm is influenced by the degree of equilibrium. The slower the adsorption reaction is, the flatter the S-curve becomes.Two steps, an initial fast followed by a second slow, of silicate removal were observed. The first fast step, which lasts for about one to two hours, depends greatly on pH and has the following expressions: ?d[Si]/dt = 1.3 × 10^?3 [A]²[H⁺]^?0.5[Si]^1.5; pH < 9 and ?d[Si]/dt = 4.0 × 10¹⁰ [A]²[H⁺][Si]^1.5; pH > 9. The second slow step is a first-order form with respect to the concentration of silicate. The specific rate constant depends on pH, concentration of silicate and surface area.Specific chemical energy contributes significantly to the adsorption of silicate and a value of ??12RT was found.     

K-feldspar IRSL dating of a Pleistocene river terrace staircase sequence of the Lower Tejo River (Portugal,western Iberia)

We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs, geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced by faults (known from previous studies). Integration of these new ages with those obtained previously in the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley. However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces (with uncorrected D_e values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the natural signals close to luminescence saturation, giving a minimum corrected D_e of about 1000 Gy, and thus minimum terrace ages; this may even be true for those doses >200 Gy. Luminescence dating results suggest that: T3 is older than 300 ka, probably ca. 420–360 ka (～Marine Isotope Stage [MIS]11); T4 is ca. 340–150 ka (～MIS9-6); T5 is 136–75 ka (～MIS5); T6 is 60–30 ka (MIS3); an aeolian sand unit that blankets T6 and some of the older terraces is 30–≥12 ka. Collectively, the luminescence ages seem to indicate that regional river downcutting events may be coincident with periods of low sea level (associated, respectively, with the MIS10, MIS6, MIS4 and MIS2).     

Extraction of Phenol and Chlorophenols Using Ionic Liquid [Bmim]+[BF4]− Dissolved in Tributyl Phosphate

In the present work, experiments have been carried out with a focus to reduce the volume requirement of solvent by mixing with imidazolium based ionic liquids (ILs) for the solvent extraction of phenol, p‐chlorophenol, 2,4‐dichlorophenol, 2,4,6‐trichlorophenol, and pentachlorophenol from aqueous solutions. The effect of aqueous phase pH (2–12), agitation speed (100–450 rpm), solute concentration in feed (2–50 mg/L), temperature (303–333 K), treat ratio (1–11), and 1‐butyl‐3‐methyl imidazolium tetrafluoroborate [Bmim]⁺[BF₄]^? volume in tributyl phosphate (TBP; 0–0.7% v/v) on extraction of phenols has been studied and optimized. Parameters like strip phase pH (3–13) and stripping agent concentration (0.001–0.009 N) have also been studied for stripping of phenols from solvent phase. It has been found that 0.5% v/v of ionic liquid [Bmim]⁺[BF₄]^? in solvent TBP extracts more than 97.5% of phenol and chlorophenols from aqueous solutions with a treat ratio (aqueous to solvent phase ratio) of 5. Transport mechanism for extraction and stripping of phenol and chlorophenols using ionic liquid [Bmim]⁺[BF₄]^? has been discussed. The results show that by appropriate selection of extraction and stripping conditions, it is possible to remove nearly all phenols with a treat ratio of 5.     

Modern and Pleistocene boron isotope composition of the benthic foraminifer Cibicidoides wuellerstorfi

Here we present the first species-specific study of boron isotopes in the epibenthic foraminifer species Cibicidoides wuellerstorfi. Coretop samples from a water depth profile from 1000 to 4500 m on the northern flank of the Walvis Ridge are 4.4‰ lower than the values expected, based on calculations of the δ¹¹B_borate of ambient seawater. Similar values for this foraminifer species are presented from ODP site 668B at the Sierra Leone Rise, in the equatorial Atlantic. The consistency between data of the same species suggests the offsets are primary, rather than diagenetic. Glacial C. wuellerstorfi from ODP 668B and Walvis Ridge have boron isotope compositions only slightly different to interglacial samples, that is no larger than + 0.10 pH units, or + 23 µmol kg^− 1 in [CO₃²⁻] above the reconstructed glacial lysocline, and − 0.07 pH units, or − 14 µmol kg^− 1 in [CO₃²⁻] below. We use these results to suggest that glacial deep water pH in the Atlantic was similar to interglacial pH. The new data resolve the inconsistency between the previously reported high bottom water pH and the lack of significant carbonate preservation of the glacial deep ocean.     

Two types of CO2− radicals threaten the fundamentals of ESR dating of tooth enamel

In ESR dating of tooth enamel, dose values are usually obtained from powdered samples. It has been shown that the qualitative response of enamel powder to environmental and laboratory dosing is closely similar, thus apparently validating ESR protocols for dose estimation. When working on human fossils, their cultural and scientific value prevents the powdering of any samples. Measurements are carried out on enamel fragments instead. In fragments, natural and laboratory irradiations cause significantly different ESR responses. These can be attributed to two distinct CO₂⁻ radicals, one with apparently axial symmetry (orientated) and one that has no preferential orientation (non-orientated). The spectra of the naturally irradiated samples investigated here show a mix of about 90% of the orientated and about 10% of the non-orientated CO₂⁻ radicals. In contrast, laboratory irradiation induces a mix of about 60% orientated and 40% non-orientated CO₂⁻ radicals. Heating experiments show that the non-orientated CO₂⁻ radicals are significantly less stable than the orientated. This fact on its own would have serious implications for dose estimations, implying massive underestimations. It turns out, however, that with the heating induced decline of the non-orientated CO₂⁻ radicals, a larger number of orientated CO₂⁻ radicals is created. It is presently unclear whether these two processes are directly connected. In spite of very large internal reorganisations during these processes, powder spectra are only little affected by heating, because the powder spectra of both CO₂⁻ radicals are closely similar.The maximum differences that are observed on powder spectra in post-irradiation heating experiments, are in the range of 3%, dose values may be affected by a similar amount. However, when using preheating steps, which would occur when applying post-irradiation heating protocols to single aliquots, the thermal behaviour of the irradiated samples is quite different. While the annihilation of the non-orientated CO₂⁻ radicals has about the same decay rate in preheated and non-preheated samples, the increase of the orientated CO₂⁻ radicals is less pronounced in the preheated samples. As a result, overall intensity changes become critically dependent on the preheating times. The observed increase in signal intensity is in contrast to long-term fading experiments that showed that the ESR intensity of irradiated samples actually decreased over 6–12 years after laboratory irradiation. In view of this complexity and the small amount of quantitative information available, it is presently not possible to recommend any pre- or post-irradiation protocols.The good news for ESR dating is that any systematic errors introduced by a different mix of orientated and non-orientated CO₂⁻ radicals in the natural and irradiation spectra seem relatively small (<5%). At present, the most reliable way for the dose estimation of fragments seems to measure the samples rotating around the three principal axes and to merge the spectra to produce a powder-equivalent spectrum and use this for the construction of the dose response curve.     

Titanium solubility in coexisting garnet and clinopyroxene at very high pressure: the significance of exsolved rutile in garnet

Exsolution microstructures including ilmenite±garnet in clinopyroxene and rutile in garnet are common in clinopyroxenite and eclogite from the Sulu ultrahigh-pressure (UHP) terrane. In order to understand the phase relations and Ti solubility in both garnet and clinopyroxene in a natural TiO₂-bearing system, several experiments at 5-15 GPa, 1000-1400°C were carried out using the multianvil high-pressure apparatus. The Hujianlin ilmenite-rich garnet clinopyroxenite showing exsolution microstructure was selected as starting material, because it closely approaches a composition lying in the TiO₂-CaO-MgO-FeO-Al₂O₃-SiO₂ system. Except for minor melt in one experiment at 1400°C and 5 GPa, other run products contain majoritic garnet+clinopyroxene±ilmenite (or rutile) and exhibit neoblastic texture. With increasing pressure, Ti and Ca, Mg and Si contents of neoblastic garnet increase with decreasing Al. The principal coupled substitutions are Ca²⁺Ti⁴⁺→2Al³⁺ and Si⁴⁺Mg²⁺→2Al³⁺ responding to majorite component increase. Titanium solubility (0.8-4.5 wt% as TiO₂) in garnet and Grt_Ti/Cpx_Ti ratio have a pronounced positive correlation with pressure between 5 and 15 GPa. On the other hand, the coexisting clinopyroxene contains low Ti (0.17-0.53 wt% as TiO₂), and shows no significant pressure effect. Rutile exsolution in garnet is coupled to that of pyroxene exsolution; both are exsolved from majoritic garnet on decompression. Therefore, the amount of such exsolved lamellae is a potential indicator of high-pressure metamorphism in exhumed rocks, whereas the TiO₂ content of clinopyroxene coexisting with garnet is not sensitive to pressure change.