Isolation of Quartz Grains for Optically Stimulated Luminescence (OSL) Dating of Quaternary Sediments for Paleoenvironmental Research

NOTE: This section presents the procedures to separate a nearly pure (>99%) quartz fraction from polymineral sediments taken from long (15-20 m) sediment core and are equally applicable to individual tube-like samples collected from outcrops²³. This methodology has been divided into three components: (1) Sediment core opening, description, and interpretation of sedimentary environments to place the resultant OSL age into a paleoenvironmental context, (2) Retrieval of a small OSL sediment sample from a core without exposure to ambient light, and (3) Separation of a mono-mineralogic quartz extract at a specific size fraction (e.g., 150-250 µm). The first step is conducted under ambient light conditions. The second and third components are undertaken with illumination by a sodium vapor bulb, equivalent LEDs, or bulbs with a red to orange filter. Test have shown that these safe light conditions with emissions centered on 589 nm with about 1-0.5 W/m² on the bench surface do not cause inadvertent reset during grain preparations.

1. Open, describe and interpret sediment cores (Figure 3)

NOTE: Use an electric saw at about the quarter diameter (0.5-radian position) of the circumference of the core to open them lengthwise. Perform this "crown" core cut instead of a half-cut to preserve more unlighted-exposed sediment for OSL dating and other analysis without compromising careful visual inspection, sampling, and description of the core.

1. Log and evaluate the sedimentologic and pedologic features of a core.
   1. Evaluate the variation in sedimentologic features such as particle size changes, sedimentary and diagenetic structures, bedding if visible, Munsell colors²⁴, the basis for unit boundaries²⁵ and identifying sequences of strata.
   2. Ascertain macro-pedologic features including carbonate, argillic and cummlic morphologies; rubification and associated horizon designation; and trace fossils.
   3. Take 1-2 g of the sediment with a spatula, put it in a 50 mL acid-resistant beaker to assess the carbonate content gasometrically.
      1. Place the beaker in a well-ventilated box oven (40 °C) for at least 8 h to dry the sample, then weigh on a precision scale and annotate the weight for each sample in the lab book.
      2. Add 30 mL of 15% HCl to the sample, place it uncovered inside a fume hood and let it react for at least 30 min. Add acid until the reaction is complete.
         CAUTION: HCl acid should always be used inside a fume hood, with the sash no more than a quarter open. A lab coat, chemical-resistant gloves, safety goggles, and a shield are required when handling HCl. Place this mixture in a fume hood for 8 h covered by a wax paper sealant. The reaction of HCl with Ca/MgCO₃ is exothermic. Thus, place the beaker in a 300 mL ceramic bowl filled with 100 mL of cold tap water to cool the reaction and capture reaction spillage.
      3. Wash the sample with 100 mL of deionized water (DIW), carefully decant the supernatant in to sink without losing the sediment.
      4. Return the sample to the oven (40 °C) for at least 24 h until dry; weigh and record the value.
      5. Quantify the mass difference between oven-dried samples before and after soaking in 15% HCl to assess carbonate content (%).
   4. Remove 0.5-1.0 g of sediments for particle size analysis every 5-10 cm down the core. Place each sediment sample in a 100 mL acid-resistant beaker. Label the samples in beakers accordingly.
   5. Sieve the sediments through a 2000 µm mesh. Discard the sediment >2000 µm (larger than sand). Continue the process with the remainder sediment <2000 µm.
   6. Add 30 mL of 15% HCl to remove carbonate from the sample. Repeat steps 1.1.3.1-1.1.3.5
   7. Remove the organic matter using 30 mL of 12% H₂O₂ and let it stand for >12 h; do not heat.
      CAUTION: H₂O₂ promotes rapid oxidation, is corrosive, and can be very harmful to the eyes, skin, and respiratory system. A lab coat, chemical-resistant gloves, safety goggles, and a shield are required when handling reagent-grade H₂O₂. The addition of H₂O₂ to sediment containing organic matter is an exothermic reaction. The rapid increase in temperature is proportional to the abundance of organic matter disseminated in the sample. The addition of DIW may be necessary to keep reaction temperature <40 °C. Continue to add H₂O₂ and monitor reaction temperature simultaneously. Let the mixture remain inside a fume hood for 8 h covered by a wax paper sealant. Place the beaker in a 300 mL ceramic bowl filled with 100 mL of cold tap water to cool the reaction and capture reaction spillage.
   8. Determine grain sizes for each sample with a laser diffraction particle size analyzer and classify the range of grain sizes according to the Wentworth scale^26,27.
   9. Assess the data and iteratively resample using finer spacing (2-5 cm) to characterize the unit contacts better or the imprint of pedogenesis (see Figure 4).
2. Interpret the sedimentary and stratigraphic sections.
   1. Use the resultant logs of sedimentology, stratigraphy, pedology, granulometry, and carbonate percentage to define the depositional units and pedosedimentary facies observed in cores.
   2. Draft the respective sedimentary sections for each core (Figure 4).
   3. Interpret the sedimentary and environmental information based on an integrated assessment of the physical core description and granulometry, carbonate content, micromorphology, and facies analysis. Discuss the interpretation of sedimentary environments with others in the research group.
   4. Determine specific depth levels of the cores to be sampled for OSL dating to decipher depositional events⁷.

2. Collect OSL sample (Figure 5)

NOTE: The core sections are transferred to the luminescence lab to sample for OSL dating in safe light conditions.

1. Moisten the core face with DIW using a squeeze bottle to ensure sediment cohesion.
2. Define the sampling area by scoring with a spatula a 2-cm diameter circle from the center point of the core face.
3. Scrape off the upper 1 cm of light exposed sediment with a utility knife. Put this sediment in a labeled ceramic evaporation dish to dry for at least 8 h in a box oven at 40 °C. Pulverize and homogenize this dried sediment sample for U, Th, K, and Rb content for dose rate calculations.
   NOTE: As an example, assign the sample a consecutive laboratory number (e.g., BG4966) to label on each container that harbors any derivative of the original sample (e.g., BG4966 <200 µm). Link this BG number to the electronic laboratory log, co-registered with the sample field or submittal number. Include other information such as the core number, year collected, drive designation (e.g., B drive), and depth. Labeling subsamples in the lab is a critical task and should be done with exactitude to maintain the chain of sample custody.
4. Extract (10-30 g) the light-shielded sediment carefully with a spatula from the circular, scored central area of the core. Place the extract in a labeled 250 mL polyethylene beaker. Clean this sample physically and chemically to isolate a quartz fraction for luminescence dating.
   NOTE: Perform core sampling in one direction (usually top to bottom) and one at a time to avoid sampling errors and contamination. Process the samples individually, in numerical order, to maintain the chain of custody.
5. Fill the remaining sample cavity in the core with a ball of aluminum foil to designate sample position and prevent sidewall collapse of the split core. Moisten the core face with DIW using a spray bottle, wrap in plastic, and seal the core for archiving.

3. Extract mono-mineralogic quartz ( Figure 6)

NOTE: All personnel prior to initiating procedures in the lab are required to wear personal protective equipment (PPE), which includes a heavy and impermeable lab coat, accompanied by nitrile disposable gloves and goggles, and dust masks. This PPE is complemented with heavy PVC gloves and body-long apron, acrylic face shield, and reusable silicone waterproof shoe covers when using solvents at full strength for digestions.

1. Remove organic matter: Add slowly 30 mL of 25% H₂O₂ to 30-60 g of the sediment in a 250 mL polyethylene beaker to remove organic matter. Stir well with a glass rod to facilitate the reaction. Add H₂O₂ until there is no visible effervescence with the release of CO₂; let it sit inside the fume hood for at least 12 h.
   CAUTION: Perform this procedure under a fume hood. H₂O₂ promotes rapid oxidation, is corrosive, and can be very harmful to the eyes, skin, and respiratory system. A lab coat, chemical-resistant gloves, safety goggles, and a shield are required when handling reagent-grade H₂O₂. The addition of H₂O₂ to sediment containing organic matter is an exothermic reaction. The rapid increase in temperature is proportional to the abundance of organic matter disseminated in the sample. The addition of DIW may be necessary to keep reaction temperature <40° C. Continue to add H₂O₂ and monitor reaction temperature simultaneously. Let the mixture remain under a fume hood for 12 h covered by a wax sealant. Place the beaker in a 300 mL ceramic bowl filled with 100 mL of cold tap water to cool the reaction and capture reaction spillage.
   NOTE: If the organic matter content is >3%, the sample may require 1-3 days of soaking in H₂O₂ to react with organic carbon fully. Monitor the exothermic heat evolved and add DIW to keep it below 40 °C. Do not heat the sample above 40 °C. Higher temperatures may cause partial resetting of the luminescence signal and sensitivity changes detrimental to dosimetric measurements.
2. Wash the sample five times with 100 mL of DIW to remove any remaining H₂O₂ and halides present in the sediment. After settling for 30-60 min, decant the supernatant into the sink with the water running. Take care to preserve the sediment at the beaker bottom during decanting.
3. Slowly add 30 mL of 15% HCl for each 5 g of sediment in a 250 mL beaker to react with the Ca/MgCO₃ disseminated in the sample. Initially add ≤ 1 mL to assess effervescences and modulate further HCl additions to control better reaction. Stir well with a glass rod to facilitate the completion of the reaction. Add more HCl if necessary until there is no visible effervescence with the release of CO₂.
   CAUTION: Use HCl inside a fume hood, with the sash no more than a quarter open. A lab coat, chemical-resistant gloves, safety goggles, and a shield are required when handling this and other acids. The reaction of HCl with Ca/MgCO₃ is exothermic. The addition of DIW may be necessary to keep reaction temperature <40 °C. Continue to add HCl and simultaneously monitor reaction temperature. Let the mixture remain inside a fume hood for 8 h covered by wax paper. Place the beaker in a 300 mL ceramic bowl filled with 100 mL of cold tap water to cool the reaction and capture reaction spillage.
   1. Wash the sample with 100 mL of DIW five times and decant carefully to remove excess (diluted) HCl into a sink with the water running.
   2. Dry the sediment overnight in a box oven at 40 °C.
4. Remove the magnetic, paramagnetic, and diamagnetic minerals.
   NOTE: Most sediments contain <10% magnetic minerals. Perform magnetic mineral removal of the sediment in a dry state using neodymium magnets or wet state using the dispersant Na-pyrophosphate (Na₄P₂O₇·10H₂O) solution (0.3%). Removal of magnetic and associated minerals is necessary as these components compete with HF etching of quartz and dissolution of other silicate minerals.
   1. Wrap a ~2.5 cm long neodymium magnet with a 38 µm nylon mesh sleeve for dry sediment removal of magnetic minerals.
   2. Place the wrapped magnet on the outside wall of the beaker and move in a circular motion to attract magnetic minerals.
   3. Move the magnet slowly to the top of the beaker to extract the minerals into a 20 mL ceramic dish. Remove the magnet and detach the magnetic minerals attached to the nylon sleeve.
   4. Repeat steps 3.4.1-3.4.3 until to remove the magnetic grains completely; usually after 5 to 6 repeats.
   5. To remove the magnetic grains in a water-based solution, place the sediment in a 250 mL glass beaker with ~100 mL of 0.3% Na-pyrophosphate solution and stir thoroughly until the sediment is well disaggregated.
   6. Place the beaker on a hot plate with a built-in magnetic stirrer; set stir rate at 800 RPM at ambient laboratory temperature. Submerge the magnetic rods and stir the sediment for 5 min.
   7. Remove the rods to clean off attracted magnetic grains by rubbing with a cloth or another magnet before returning the magnets to the solution. Repeat until no magnetic minerals are recovered; up to five repeats may be necessary.
      NOTE: A binocular microscopic inspection of the sample is advised to assess the status of magnetic mineral removal. Together, the dry and wet magnetic mineral removal is usually >95% effective.
5. Separate a specific grain-size fraction.
   NOTE: The particle size range of quartz grains to be separated is based on the previously determined particle size distribution for each sample (see step 1.1.5). Common particle size ranges to separate quartz grains are 500-450 µm, 450-355 µm, and 355-250 µm for medium sand, 250-150 µm and 150-100 µm for fine sand and 100-63 µm for very fine sand.
   1. Cut 15 cm x 15 cm squares from rolls of nylon mesh of two sizes (e.g., 150 µm and 250 µm) for particle size isolation using wet sieving with disposable meshes.
   2. Frame the cut mesh in a 10 cm-inner diameter circular plastic guide. For example, to target the fine sand fraction 150-250 µm, use two framing meshes sequentially: 250 µm first and 150 µm second.
   3. Label three beakers with the laboratory sample number (BGXXXX) and sieving limits; >150 >250 µm, and 250-150 µm (Inset Figure 6A).
   4. Place the circular sieving guide tightly with framed mesh, e.g., first, use 250 µm (coarser grain size) over a 1-L beaker rim (10.5 cm diameter).
   5. Sieve sample to the targeted particle size range, e.g., 250-150 µm. Set up 1-L beaker with 250 µm mesh guide on top; ready to sieve.
   6. Add ~100 mL of 0.3% solution of Na-pyrophosphate to a 250 mL beaker that contains the nonmagnetic sediment obtained in step 3.4.7 and stir thoroughly with a glass road to facilitate particle dispersion.
   7. Continue to manually swirl the dispersed sediment mixture, and slowly pour through the 250 µm mesh. The sediment of particles <250 µm size passes through the mesh into the below beaker and is the target for further size separation. Archive the sediment remaining on the mesh (>250 µm) for possible future analysis.
   8. Set up the 150 µm mesh over a new dry 1-L beaker. Take the dispersed sediment mixture of step 3.5.7, continue to swirl in hand, and slowly pour through the 150 µm mesh. The sediment of particles <150 µm size passes through the mesh into the below beaker. Archive the sediment for possible future analysis. The sediment remaining on the 150 mesh is the target size fraction, 150-250 µm, for OSL dating.
   9. Dry the sediments in a box oven overnight at 40 °C.
6. Isolate quartz grains from the 250-150 µm size separately (Inset Figure 6B).
   NOTE: This procedure includes two density separations using the non-toxic heavy liquid Sodium Polytungstate (SPT-Na₆ (H₂W₁₂O₄₀) _H₂O) at densities 2.6 g/cc and 2.7 g/cc. Mix the powder with DIW to constitute this heavy liquid. To prepare 100 mL of the heavy liquid with a density of 2.6 g/cc, add 205.5 g of SPT to 54.5 mL of DIW. Whereas, to prepare 100 mL of the heavier liquid with a density of 2.7 g/cc, add 217.5 g of SPT to 52.7 mL of DIW. Assess the density of the heavy liquid with pre-calibrated density beads and a hydrometer.
   CAUTION: Use only DIW to prepare heavy liquids because tap water contains dissolved ions that react and change the composition of the SPT powder. To generate a homogeneous solution of the desired density, add the SPT powder to the water and not the counter.
   1. Label two 100 mL beakers with the sample number adding "<2.6" to one beaker and ">2.6" to the other beaker. Keep a 1 L beaker ready to collect the heavy liquid washed from the sample with DIW.
   2. Mix thoroughly 80-70 mL of 2.6 g/cm³ heavy liquid with the dry fraction of the sediment obtained in step 3.5.8. Pour the mixture into a well labeled 100 mL graduate cylinder. Cover the top with a wax sealant to avoid evaporation. Place the cylinder inside a fume hood to remain undisturbed and shielded from light. Wait for at least 1 h to allow the sample to separate in two markedly different zones. The higher floating, lighter minerals are often enriched in K-feldspar and Na-rich plagioclases, and the lower heavier grains are rich in quartz and other heavier minerals.
      NOTE: The separation times using the 2.6 g/cc heavy liquid for smaller particle sizes, <100 µm, may take >4 h.
   3. Place a plastic funnel and place a disposable paper filter over a 250 mL beaker. Filter the solution with a tight fit.
   4. Decant the floating sediment of the 2.6 g/cm³ heavy liquid through the filter slowly and carefully, with suspended grains captured on the filter. Preserve the lower zone of settled grains carefully. Let the liquid pass through the filter; wash with DIW as needed.
   5. Transfer the washed light sediment to the beaker labeled as "sample number <2.6", placing the paper filter in the beaker and washing carefully with DIW. Discard the filter after washing off all the grains.
   6. Wash the sample five times with DIW to remove vestiges of heavy liquid.
   7. Dry the sediments in the oven overnight at <40 °C. Store this feldspar-rich fraction for future analyses.
   8. Put a new filter paper on the plastic funnel and place it tightly on a 1 L glass beaker. Decant the lower settled mineral grains in the graduated cylinder with 2.6 g/cm³ solution. Then, wash out the cylinder with DIW using a squirt bottle.
   9. Transfer the washed "heavy" sediment to the beaker labeled with the "sample number >2.6". Place the paper filter in the beaker and wash carefully with DIW. Discard the filter after washing off all the grains.
   10. Wash the sample three times in the sink with DIW.
   11. Dry the sediments in the oven overnight at <40 °C for further density separation using 2.7 g/cc heavy liquid.
   12. Continue with quartz separation with a 2.7 g/cc heavy liquid. Combine the dry "heavy" separate from the beaker labeled "sample number >2.6" with 70-80 mL of 2.7 g/cc heavy liquid.
   13. Decant the floating sediment (quartz-rich) onto a funnel-filter pair over a 1 L beaker slowly and carefully. Wash the floating sample on the filter thoroughly with DIW and collect the wash in the beaker below.
   14. Transfer the washed sediment on the filter to a 250 mL polypropylene beaker labeled with the "sample number + for HF". Place the paper filter in the beaker and wash carefully with DIW; discard the filter after washing off all the grains.
   15. Put a new paper filter on the plastic funnel, and place both on a new 1 L glass beaker. Add DIW to the cylinder where the 2.7 g/cc density separation occurred, decant and wash with DIW until the lower separated grains are transferred completely to the filter. Repeat steps 3.6.10-3.6.12 and archive this heaviest fraction.
7. Etch the quartz grains by immersing in hydrofluoric acid
   NOTE: This procedure has two main goals: 1) to dissolve any remaining minerals other than quartz; 2) To etch the external 10-20 µm of quartz grains, affected by the alpha radiation²⁸.
   CAUTION: Concentrated hydrofluoric acid (HF) is a highly toxic and hazardous liquid. Special training and care are needed to use HF because of the high dermal and pulmonary toxicity. Lab personnel must be familiar with the HF Material Safety Data Sheets. Always handle HF inside an operational laboratory fume hood, near an eyewash and safety shower station. Never work with HF alone. Ensure that non-expired 2.5% calcium gluconate gel antidote is at hand before handling HF. The following PPE must be worn prior to handling HF: Long pants and sleeves, closed-toe shoes, heavy lab coat, acid-resistant apron, thick nitrile gloves (10-20 mil), PVC or neoprene gloves that cover the hands, wrists, and forearms, dust mask, goggles, acrylic face shield, and silicone waterproof shoe covers.
   1. Prepare a timer for 80 min and cut wax paper sealant to cover a 250 mL beaker.
   2. Turn on both the DIW and regular water taps at the sink and have a bottle of DIW at hand as a safety precaution.
   3. Put on the appropriate PPE to use HF acid.
   4. Place a 250 mL heavy-duty polypropylene beaker with the sample obtained in step 3.6.14 inside the fume hood; lower the sash to near closure to be safe and comfortable to work. Add HF to the beaker by pump increments (20 mL) for every 2 g of quartz and cover the beaker with wax paper sealant.
      NOTE: For enhanced safety, use an HF bottle dispenser that delivers set volumes of acid, e.g., 20 mL/pump, to control the amount and direction of acid delivery. High-density plastic containers are used with HF because this acid reacts with and etches glass.
   5. Start the 80 min timer and remove the HF-PPE. Keep in mind to wear the PPE again to clean the sample 5 min before the time is complete.
   6. Wash the sample five times under the hood. Fill the beaker with DIW to dilute the acid and decant it into a satellite container used for HF waste.
   7. Remove the sample from the fume hood and wash the sample three more times with DIW at the sink, keeping both the DIW and regular water taps open to dilute any remaining HF further.
   8. Decant and move the sample into a 250 mL glass beaker, add ~150 mL of 0.3% Na-Pyrophosphate (Na₄P₂O₇ ·10H₂O) solution to the sediment and place the beaker in a sonicator bath for 20 min to fully disaggregate the grains and particles.
   9. Wash the sample five more times with DIW at a sink to remove the Na-pyrophosphate. Decant and label the beaker "Sample Name" for HCl".
8. Immerse the mineral grains remaining after HF digestion (step 3.7.9) in concentrated HCl.
   CAUTION: Concentrated HCl (~36%) is considered a toxic and corrosive fluid that can cause chemical burns upon contact and eye damage if splashed, and injury to the mouth, throat, esophagus, and stomach if ingested. Workers are required to be familiar with the HCl Material Safety Data Sheets. Always handle concentrated HCl inside an operational fume hood, near an eyewash and safety shower station. Never work with HCl alone. Before starting the digestion of the sediment with HCl, be sure to wear the PPE listed in step 3.7.
   NOTE: As with concentrated HF, it is safer to use a bottle dispenser to control the amount and direction of the discharge. Use glass containers when working with HCl. Before removing the PPE, wash the gloves with soap and water, and after removing the PPE, wash hands and forearms.
   1. Prepare the wax sealant to cover the beaker with the sample immersed in the acid.
   2. Turn on both the DIW and regular water taps at the sink and have a bottle of DIW at hand as a safety precaution.
   3. Put on the acid PPE.
   4. Place the 250 mL glass beaker with the sample obtained in step 3.7.9 inside the fume hood. Lower the sash to near closure to be safe and comfortable to work. Add HCl to sample by pump increments (20 mL) for every 5 g of quartz and then cover the beaker with wax sealant paper.
   5. Remove the acid PPE.
   6. Leave the sample for HCl digestion for 8 h in the fume hood.
   7. Put on the acid-PPE before cleaning the HCl.
   8. Wash the sample five times under the hood; decant supernatant into the satellite container to collect HCl waste.
   9. Wash the sample three more times with DIW at the sink, keeping both the DIW and regular water taps open for further dilution. Make sure to continue wearing the necessary PPE.
9. Re-sieve the sediments through the smallest prior mesh (e.g., 150 µm) to remove fractured and broken grains.
10. Decant and label the beaker "Sample Name for OSL" and dry the sediments in the oven for at least 8 h at <40 °C to evaluate the purity of quartz separation of this finished product.
11. Quantify quartz separate purity
    1. Use a dissecting needle to place 200-400 mineral grains on a glass slide and inspect under a 10x or 20x binocular and/or petroscopic microscope to identify grain minerals. Quantify the percentage of quartz grains by point counting and record the mineralogy of 100 individual grains. If a subsample exhibits >1% non-quartz minerals and is an unwanted mineral with high photon output (e.g., K-feldspar) or remains unidentified, cue the sample for Raman spectroscopy.
    2. Use Raman spectroscopy and associated image to confirm the grain mineralogy and identify minerals unrecognized under microscopic inspection. Use a blue beam with a width of 5 µm and 100-grain point counts to assess the percent purity of quartz and identify the unknown grain minerals.
12. Assess the quartz purity spectra by infrared stimulation
    1. Prepare five ultra-small aliquots of quartz separates for IR stimulation by shaking grains onto a circular aluminum disc (1 cm diameter). Each aliquot usually contains approximately 20-100 quartz grains corresponding to a 1 mm or less circular diameter adhered (with silicon) to a disc.
    2. Load the discs on a sample carousel for stimulation by IR LEDs (845 nm ± 4 nm) delivered by an automated TL/OSL reader system and compare it with the blue light excitation (470 nm ± 20 nm), which is preferential for quartz.
    3. Ensure that the ratio between IRSL and blue light emissions of quartz grain aliquots is <5%. If such is the case, the sample is ready for further analysis. Otherwise, the sample requires additional cleaning with HF (step 3.7).