Electronic supplementary material

(Online Resource 2)

for

Bulletin of Volcanology:

Terminal Pleistocene Volcanic Eruptions at Zuni Salt Lake,

West-Central New Mexico, USA

Jill Onkena, Steven Formanb

a Department of Geosciences, University of Arizona, 1040 E 4th St., Tucson, AZ 85721, USA;

b Department of Geology, One Bear Place #97354, Baylor University, Waco, Texas 76798, USA

Detailed OSL methods

Single aliquot regeneration (SAR) protocols (Murray and Wintle 2003) were used in this study to estimate the apparent equivalent dose of the quartz fraction for 28 to 39 separate aliquots (Table 3). Each aliquot contained approximately 100 to 500 quartz grains dependent on grain size analyzed (Table 3) and corresponds to a 1.5 to 2.0 millimeter circular diameter of grains adhered (with silicon) to a 1 cm diameter circular aluminum disc. The quartz fraction was isolated by density separations using the heavy liquid Na–polytungstate, and a 40-minute immersion in HF (40%) was applied to etch the outer ~10 µm of grains, which is affected by alpha radiation (Mejdahl and Christiansen 1994). Quartz grains were rinsed finally in HCl (10%) to remove any insoluble fluorides. The purity of each quartz separate was evaluated by petrographic inspection and point counting of a representative aliquot; this procedure was repeated if samples contained >1% of non-quartz minerals. Also, the purity of quartz separates was tested by exposing aliquots to infrared excitation (845 ± 4 nm), which often preferentially excites feldspar minerals. Samples measured showed weak emissions (<200 counts/s), at or close to background counts with infrared excitation, and ratio of emissions from blue to infrared excitation of >20, indicating a spectrally pure quartz extract (Duller et al. 2003).

A series of experiments was performed to evaluate the effect of preheating at 200, 220, 240 and 260° C for isolating the time-sensitive emissions and assessing thermal transfer of the regenerative signal prior to the application of SAR protocols (see Murray and Wintle 2003). These experiments entailed giving a known dose (10 Gy) and evaluating which preheat resulted in recovery of this dose. There was concordance with the known dose (10 Gy) for preheat temperatures above 220° C with an initial preheat temperature used of 240° C for 10 s in the SAR protocols. A “cut heat” at 160°C for 10 s was applied prior to the measurement of the test dose and a final heating at 280° C for 40 s was applied to minimize carryover of luminescence to the succession of regenerative doses (Table S1). A test for dose reproducibility was also performed with the initial and final regenerative dose of ~16 Gy yielding concordant luminescence responses (at one-sigma error) (cf. Murray and Wintle 2003).

Typical OSL shine-down curves for quartz grains are shown in Figure S1. The curve shapes show that OSL signal is probably dominated by a fast component, with the OSL emission decreasing by 90 to 95% during the first 4 seconds of stimulation. The regenerative growth curves are modeled by using the exponential plus linear form. For many aliquots the regenerative growth curves (Fig. S1) show that (1) the recuperation is close to zero; (2) the recycling ratio is consistent with unity at 1σ; (3) the natural Lx/Tx ratio is well below 20% of the saturated level. The few aliquots removed were because of unacceptable recycling ratio, low emissions of the natural (<300 counts/s at peak) and De values at or close to saturation with errors of >10%. Error analysis for equivalent dose calculations assumed measurement error of 1% and Monte Carlo simulation repeats of 2000. Recuperation is lower than 2% for all samples, which indicates insignificant charge transfer during the measurements. These favorable luminescence characteristics for a majority of aliquots indicate that credible equivalent dose values for these sediments can be determined by the SAR protocol.

The SAR protocols yielded individual OSL ages by averaging 28 to 39 separate, equivalent doses from respective aliquots of quartz grains (Murray and Wintle 2003). Equivalent dose distributions are log normal and the scatter in the data is quantified with overdispersion values (Table 3; Fig. S1). An overdispersion percentage of a De distribution is an estimate of the relative standard deviation from a central De value in context of a statistical estimate of errors (Galbraith et al. 1999; Galbraith and Roberts 2012). A zero overdispersion percentage indicates high internal consistency in De values with 95% of the De values within 2σ errors. However, the lowest reported overdispersion values are about 4 to 6% reflecting inescapable systematic and random errors in equivalent dose determination. Overdispersion values between 5 and 20% are routinely assessed for quartz grains that are well solar reset, like eolian sands (e.g., Olley et al. 2004; Wright et al. 2011) and this value is considered a threshold metric for calculation of a De value using the central age model of Galbraith et al. (1999). However, some studies have concluded that overdispersion values between 20 and 32% may reflect a single De population, particularly if the De distribution is symmetrical, with the dispersion related to variability associated with micro-dosimetry and/or sedimentary processes (e.g., Arnold and Roberts 2009). In this study overdispersion values for equivalent doses were at or below 20% (at one sigma) and considered one population of ages. Computing equivalent dose values with other statistical analysis, such as the minimum age, maximum age and a finite mixture model (Galbraith and Roberts 2012) yielded highly similar equivalent dose values that overlap at one sigma with values by the central age model. Optical ages are reported in years prior to AD 1950 to facilitate comparison with calibrated radiocarbon dates (Table 3).

The environmental dose rate is critical measurement for calculating a luminescence age, which is an estimate of the exposure of quartz grains to ionizing radiation from the decay of the U and Th series, 40K, and cosmic sources during the burial period. The U, Th and K concentrations are determined by inductively coupled plasma mass spectrometry (ICP-MS) by Activation Laboratory LTD, Ontario, Canada. The beta and gamma doses were adjusted according to grain diameter to compensate for mass attenuation for the dose rate (Fain et al. 1999). The U, Th and K2O content was determined for the bulk sediment to calculate the dose rate. A cosmic ray component that accounts for location, elevation and depth of strata sampled is between 0.07 and 0.24 mGy/yr and is included in the estimated dose rate (Prescott and Hutton 1994). There is uncertainty in assessing the moisture content of a sample during burial. We estimated moisture contents taking into account present moisture content values, particle size characteristics, inferred past water table fluctuations, and recent channel entrenchment.

Supplementary references

Arnold LJ, Roberts RG (2009) Stochastic modelling of multi-grain equivalent dose (D-e) distributions: implications for OSL dating of sediment mixtures. Quat Geochronol 4:204–230

Duller GAT, Bøtter-Jensen L, Murray AS (2003) Combining infrared and green-laser stimulation sources in single-grain luminescence measurements of feldspar and quartz. Radiat Meas 37:543–550

Fain J, Soumana S, Montret M, Miallier D, Pilleyre T, Sanzelle S (1999) Luminescence and ESR dating-Beta-dose attenuation for various grain shapes calculated by a Monte-Carlo method. Quaternary Sci Rev 18:231–234

Galbraith RF, Roberts RG (2012) Statistical aspects of equivalent dose and error calculation and display in OSL dating: an overview and some recommendations. Quat Geochronol 11:1–27

Galbraith RF, Roberts RG, Laslett GM, Yoshida H, Olley JM (1999) Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia, part 1, experimental design and statistical models. Archaeometry 41:339–364

Mejdahl V, Christiansen HH (1994) Procedures used for luminescence dating of sediments. Boreas 13:403–406

Murray AS, Wintle AG (2003) The single aliquot regenerative dose protocol: potential for improvements in reliability. Radiat Meas 37:377–381

Olley JM, Pietsch T, Roberts RG (2004) Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz. Geomorphology 60:337–358

Prescott JR, Hutton JT (1994) Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiat Meas 23:497–500

Wright DK, Forman SL, Waters MR, Ravesloot JC (2011) Holocene eolian activation as a proxy for broad-scale landscape change on the Gila River Indian Community, Arizona. Quaternary Res 76:10–21

Table S1

Single aliquot regeneration protocols

Step / Treatment
1 / Natural dose or give beta dose
2 / Preheat 240oC for 10 s
3 / Stimulate with blue light (470 nm) for 40 s at 125oC
4 / Give beta test dose (6.6 Gray)
5 / Preheat 160oC for 10 s
6 / Stimulate with blue light (470 nm) for 40s at 125oC
7 / Stimulate with blue light for 40 s at 280oC
8 / Return to step 1

Fig. S1 (a) Representative regenerative dose growth curves, with inset representative natural shine down curve, and (b) radial plots of equivalent dose values on small aliquots (2-mm plate of 150–250 μm quartz fraction grains)

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